THE AEROPLANE SPEAKS BY H. BARBER

THE AEROPLANE SPEAKS
BY H. BARBER
(CAPTAIN, ROYAL FLYING CORPS)
DEDICATED TO THE SUBALTERN FLYING OFFICER
MOTIVE
The reasons impelling me to write this book, the maiden
effort of my pen, are, firstly, a strong desire to help the
ordinary man to understand the Aeroplane and the joys
and troubles of its Pilot; and, secondly, to produce something
of PRACTICAL assistance to the Pilot and his invaluable assistant
the Rigger. Having had some eight years' experience in
designing, building, and flying aeroplanes, I have hopes
that the practical knowledge I have gained may offset the
disadvantage of a hand more used to managing the ``joystick''
than the dreadful haltings, the many side-slips, the
irregular speed, and, in short, the altogether disconcerting
ways of a pen.
The matter contained in the Prologue appeared in the
Field of May 6th, 13th, 20th, and 27th, 1916, and is now
reprinted by the kind permission of the editor, Sir Theodore
Cook.
I have much pleasure in also acknowledging the kindness
of Mr. C. G. Grey, editor of the Aeroplane, to whom I am
indebted for the valuable illustrations reproduced at the
end of this book.
CONTENTS
PROLOGUE
PART
I. THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
II. THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB
III. THE GREAT TEST
IV. CROSS COUNTRY
CHAPTER
I. FLIGHT
II. STABILITY AND CONTROL
III. RIGGING
IV. PROPELLERS
V. MAINTENANCE
TYPES OF AEROPLANES
GLOSSARY
THE AEROPLANE SPEAKS
PROLOGUE
PART I
THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
The Lecture Hall at the Royal Flying Corps School for
Officers was deserted. The pupils had dispersed, and the
Officer Instructor, more fagged than any pupil, was out on
the aerodrome watching the test of a new machine.
Deserted, did I say? But not so. The lecture that day
had been upon the Elementary Principles of Flight, and
they lingered yet. Upon the Blackboard was the illustration
you see in the frontispiece.
``I am the side view of a Surface,'' it said, mimicking
the tones of the lecturer. ``Flight is secured by driving me
through the air at an angle inclined to the direction of
motion.''
``Quite right,'' said the Angle. ``That's me, and I'm
the famous Angle of Incidence.''
``And,'' continued the Surface, ``my action is to deflect
the air downwards, and also, by fleeing from the air behind,
to create a semi-vacuum or rarefied area over most of the
top of my surface.''
``This is where I come in,'' a thick, gruff voice was
heard, and went on: ``I'm the Reaction. You can't have
action without me. I'm a very considerable force, and my
direction is at right-angles to you,'' and he looked heavily
at the Surface. ``Like this,'' said he, picking up the chalk
with his Lift, and drifting to the Blackboard.
``I act in the direction of the arrow R, that is, more or
less, for the direction varies somewhat with the Angle of
Incidence and the curvature of the Surface; and, strange
but true, I'm stronger on the top of the Surface than at
the bottom of it. The Wind Tunnel has proved that by
exhaustive research--and don't forget how quickly I can
grow! As the speed through the air increases my strength
increases more rapidly than you might think--approximately,
as the Square of the Speed; so you see that if the Speed of
the Surface through the air is, for instance, doubled, then
I am a good deal more than doubled. That's because I
am the result of not only the mass of air displaced, but also
the result of the Speed with which the Surface engages
the Air. I am a product of those two factors, and at the
speeds at which Aeroplanes fly to-day, and at the altitudes
and consequent density of air they at present experience,
I increase at about the Square of the Speed.
``Oh, I'm a most complex and interesting personality, I
assure you--in fact, a dual personality, a sort of aeronautical
Dr. Jekyll and Mr. Hyde. There's Lift, my vertical part or
COMPONENT, as those who prefer long words would say; he
always acts vertically upwards, and hates Gravity like poison.
He's the useful and admirable part of me. Then there's Drift,
my horizontal component, sometimes, though rather erroneously,
called Head Resistance; he's a villain of the deepest
dye, and must be overcome before flight can be secured.''
``And I,'' said the Propeller, ``I screw through the air and
produce the Thrust. I thrust the Aeroplane through the air
and overcome the Drift; and the Lift increases with the Speed
and when it equals the Gravity of Weight, then--there you
are--Flight! And nothing mysterious about it at all.''
``I hope you'll excuse me interrupting,'' said a very
beautiful young lady, ``my name is Efficiency, and, while
no doubt, all you have said is quite true, and that, as my
young man the Designer says, `You can make a tea-tray
fly if you slap on Power enough,' I can assure you that I'm
not to be won quite so easily.''
``Well,'' eagerly replied the Lift and the Thrust, ``let's
be friends. Do tell us what we can do to help you to overcome
Gravity and Drift with the least possible Power. That
obviously seems the game to play, for more Power means
heavier engines, and that in a way plays into the hands of
our enemy, Gravity, besides necessitating a larger Surface
or Angle to lift the Weight, and that increases the Drift.''
``Very well,'' from Efficiency, ``I'll do my best, though
I'm so shy, and I've just had such a bad time at the Factory,
and I'm terribly afraid you'll find it awefully dry.''
``Buck up, old dear!'' This from several new-comers,
who had just appeared. ``We'll help you,'' and one of
them, so lean and long that he took up the whole height of
the lecture room, introduced himself.
``I'm the High Aspect Ratio,'' he said, ``and what we
have got to do to help this young lady is to improve the
proportion of Lift to Drift. The more Lift we can get for a
certain area of Surface, the greater the Weight the latter
can carry; and the less the Drift, then the less Thrust and
Power required to overcome it. Now it is a fact that, if
the Surface is shaped to have the greatest possible span,
i.e., distance from wing-tip to wing-tip, it then engages more
air and produces both a maximum Reaction and a better
proportion of Lift to Drift.
``That being so, we can then well afford to lose a little
Reaction by reducing the Angle of Incidence to a degree
giving a still better proportion of Lift to Drift than would
otherwise be the case; for you must understand that the
Lift-Drift Ratio depends very much upon the size of the
Angle of Incidence, which should be as small as possible
within certain limits. So what I say is, make the surface of
Infinite Span with no width or chord, as they call it. That's
all I require, I assure you, to make me quite perfect and of
infinite service to Miss Efficiency.''
``That's not practical politics,'' said the Surface. ``The
way you talk one would think you were drawing L400 a
year at Westminster, and working up a reputation as an
Aeronautical Expert. I must have some depth and chord
to take my Spars and Ribs, and again, I must have a certain
chord to make it possible for my Camber (that's curvature)
to be just right for the Angle of Incidence. If that's not
right the air won't get a nice uniform compression and
downward acceleration from my underside, and the rarefied
`suction' area over the top of me will not be as even and clean
in effect as it might be. That would spoil the Lift-Drift Ratio
more than you can help it. Just thrust that chalk along, will
you? and the Blackboard will show you what I mean.''
``Well,'' said the Aspect Ratio, ``have it your own way,
though I'm sorry to see a pretty young lady like Efficiency
compromised so early in the game.''
``Look here,'' exclaimed a number of Struts, ``we have
got a brilliant idea for improving the Aspect Ratio,'' and
with that they hopped up on to the Spars. ``Now,'' excitedly,
``place another Surface on top of us. Now do you
see? There is double the Surface, and that being so, the
proportion of Weight to Surface area is halved. That's
less burden of work for the Surface, and so the Spars need
not be so strong and so deep, which results in not so thick
a Surface. That means the Chord can be proportionately
decreased without adversely affecting the Camber. With
the Chord decreased, the Span becomes relatively greater,
and so produces a splendid Aspect Ratio, and an excellent
proportion of Lift to Drift.''
``I don't deny that they have rather got me there,''
said the Drift, ``but all the same, don't forget my increase
due to the drift of the Struts and their bracing wires.''
``Yes, I dare say,'' replied the Surface, ``but remember
that my Spars are less deep than before, and consequently I
am not so thick now, and shall for that reason also be able
to go through the air with a less proportion of Drift to Lift.''
``Remember me also, please,'' croaked the Angle of
Incidence. ``Since the Surface has now less weight to carry
for its area, I may be set at a still lesser and finer Angle.
That means less Drift again. We are certainly getting on
splendidly! Show us how it looks now, Blackboard.'' And
the Blackboard obligingly showed them as follows:
``Well, what do you think of that?'' they all cried to the
Drift.
``You think you are very clever,'' sneered the Drift.
``But you are not helping Efficiency as much as you think.
The suction effect on the top of the lower Surface will give
a downward motion to the air above it and the result will
be that the bottom of the top Surface will not secure as good
a Reaction from the air as would otherwise be the case,
and that means loss of Lift; and you can't help matters
by increasing the gap between the surfaces because that
means longer Struts and Wires, and that in itself would
help me, not to speak of increasing the Weight. You see
it's not quite so easy as you thought.''
At this moment a hiccough was heard, and a rather fast
and rakish-looking chap, named Stagger, spoke up. ``How
d'ye do, miss,'' he said politely to Efficiency, with a side
glance out of his wicked old eye. ``I'm a bit of a knut,
and without the slightest trouble I can easily minimize
the disadvantage that old reprobate Drift has been frightening
you with. I just stagger the top Surface a bit forward,
and no longer is that suction effect dead under it. At the
same time I'm sure the top Surface will kindly extend its
Span for such distance as its Spars will support it without
the aid of Struts. Such extension will be quite useful, as
there will be no Surface at all underneath it to interfere
with the Reaction above.'' And the Stagger leaned
forward and picked up the Chalk, and this is the picture
he drew:
Said the Blackboard, ``That's not half bad! It really
begins to look something like the real thing, eh?''
``The real thing, is it?'' grumbled Drift. ``Just consider
that contraption in the light of any one Principle, and I
warrant you will not find one of them applied to perfection.
The whole thing is nothing but a Compromise.'' And he
glared fixedly at poor Efficiency.
``Oh, dear! Oh, dear!'' she cried. ``I'm always getting
into trouble. What WILL the Designer say?''
``Never mind, my dear,'' said the Lift-Drift Ratio,
consolingly. ``You are improving rapidly, and quite useful
enough now to think of doing a job of work.''
``Well, that's good news,'' and Efficiency wiped her eyes
with her Fabric and became almost cheerful. ``Suppose
we think about finishing it now? There will have to be an
Engine and Propeller, won't there? And a body to fix
them in, and tanks for oil and petrol, and a tail, and,'' archly,
``one of those dashing young Pilots, what?''
``Well, we are getting within sight of those interesting
Factors,'' said the Lift-Drift Ratio, ``but first of all we
had better decide upon the Area of the Surfaces, their Angle
of Incidence and Camber. If we are to ascend as quickly
as possible the Aeroplane must be SLOW in order to secure
the best possible Lift-Drift Ratio, for the drift of the struts
wires, body, etc., increases approximately as the square
of the speed, but it carries with it no lift as it does in the
case of the Surface. The less speed then, the less such
drift, and the better the Aeroplane's proportion of lift to
drift; and, being slow, we shall require a LARGE SURFACE in
order to secure a large lift relative to the weight to be carried.
We shall also require a LARGE ANGLE OF INCIDENCE relative to
the horizontal, in order to secure a proper inclination of
the Surface to the direction of motion, for you must remember
that, while we shall fly upon an even keel and with
the propeller thrust horizontal (which is its most efficient
attitude), our flight path, which is our direction of motion,
will be sloping upwards, and it will therefore be necessary
to fix the Surface to the Aeroplane at a very considerable
angle relative to the horizontal Propeller Thrust in order to
secure a proper angle to the upwards direction of motion.
Apart from that, we shall require a larger Angle of Incidence
than in the case of a machine designed purely for speed,
and that means a correspondingly LARGE CAMBER.
``On the other hand, if we are thinking merely of Speed,
then a SMALL SURFACE, just enough to lift the weight off the
ground, will be best, also a SMALL ANGLE to cut the Drift down
and that, of course, means a relatively SMALL CAMBER.
``So you see the essentials for CLIMB or quick ascent and
for SPEED are diametrically opposed. Now which is it to be?''
``Nothing but perfection for me,'' said Efficiency. ``What
I want is Maximum Climb and Maximum Speed for the
Power the Engine produces.''
And each Principle fully agreed with her beautiful
sentiments, but work together they would not.
The Aspect Ratio wanted infinite Span, and hang the
Chord.
The Angle of Incidence would have two Angles and two
Cambers in one, which was manifestly absurd; the Surface
insisted upon no thickness whatever, and would not hear
of such things as Spars and Ribs; and the Thrust objected
to anything at all likely to produce Drift, and very nearly
wiped the whole thing off the Blackboard.
There was, indeed, the makings of a very pretty quarrel
when the Letter arrived. It was about a mile long, and
began to talk at once.
``I'm from the Inventor,'' he said, and hope rose in the
heart of each heated Principle. ``It's really absurdly simple.
All the Pilot has to do is to touch a button, and at his will,
VARY the area of the Surface, the Angle of Incidence,
and the Camber! And there you are--Maximum Climb or
Maximum Speed as required! How does that suit you?''
``That suits us very well,'' said the Surface, ``but, excuse
me asking, how is it done without apparatus increasing the
Drift and the Weight out of all reason? You won't mind
showing us your Calculations, Working Drawings, Stress
Diagrams, etc., will you?''
Said the Letter with dignity, ``I come from an Inventor
so brilliantly clever as to be far above the unimportant
matters you mention. He is no common working man,
sir! He leaves such things to Mechanics. The point is, you
press a button and----''
``Look here,'' said a Strut, rather pointedly, ``where do
you think you are going, anyway?''
``Well,'' from the Letter, ``as a matter of fact, I'm not
addressed yet, but, of course, there's no doubt I shall reach
the very highest quarters and absolutely revolutionize Flight
when I get there.''
Said the Chalk, ``I'll address you, if that's all you want;
now drift along quickly!'' And off went the Letter to The
Technical Editor, ``Daily Mauler,'' London.
And a League was formed, and there were Directors with
Fees, and several out-of-service Tin Hats, and the Man-whotakes-
the-credit, and a fine fat Guinea-pig, and all the rest
of them. And the Inventor paid his Tailor and had a Hair-
Cut, and is now a recognized Press Expert--but he is still
waiting for those Mechanics!
``I'm afraid,'' said the Slide-rule, who had been busy
making those lightning-like automatic calculations for which
he is so famous, ``it's quite impossible to fully satisfy all of
you, and it is perfectly plain to me that we shall have to effect
a Compromise and sacrifice some of the Lift for Speed.''
Thud! What was that?
Efficiency had fainted dead away! The last blow had
been too much for her. And the Principles gathered mournfully
round, but with the aid of the Propeller Slip[[1]] and a
friendly lift from the Surface she was at length revived and
regained a more normal aspect.
[[1]] Propeller Slip: As the propeller screws through the air,
the latter to a certain extent gives back to the thrust of the
propellor blades, just as the shingle on the beach slips back
as you ascend it. Such ``give-back'' is known as ``slip,''
and anyone behind the propellor will feel the slip as a
strong draught of air.
Said the Stagger with a raffish air, ``My dear young lady,
I assure you that from the experiences of a varied career,
I have learned that perfection is impossible, and I am sure
the Designer will be quite satisfied if you become the Most
Efficient Compromise.''
``Well, that sounds so common sense,'' sighed Efficiency,
``I suppose it must be true, and if the Designer is satisfied,
that's all I really care about. Now do let's get on with the job.''
So the Chalk drew a nice long slim body to hold the
Engine and the tanks, etc., with room for the Pilot's and
Passenger's seats, and placed it exactly in the middle of the
Biplane. And he was careful to make its position such that
the Centre of Gravity was a little in advance of the Centre
of Lift, so that when the Engine was not running and there
was consequently no Thrust, the Aeroplane should be ``noseheavy''
just to the right degree, and so take up a natural
glide to Earth--and this was to help the Pilot and relieve
him of work and worry, should he find himself in a fog or
a cloud. And so that this tendency to glide downwards
should not be in evidence when the Engine was running and
descent not desired, the Thrust was placed a little below
the Centre of Drift or Resistance. In this way it would in
a measure pull the nose of the Aeroplane up and counterbalance
the ``nose-heavy'' tendency.
And the Engine was so mounted that when the Propeller-
Thrust was horizontal, which is its most efficient position,
the Angle of Incidence and the Area of the surfaces were
just sufficient to give a Lift a little in excess of the Weight.
And the Camber was such that, as far as it was concerned,
the Lift-Drift Ratio should be the best possible for that Angle
of Incidence. And a beautifully simple under-carriage was
added, the outstanding features of which were simplicity,
strength, light-weight, and minimum drift. And, last of
all, there was the Elevator, of which you will hear more
by-and-by. And this is what it looked like then:
And Efficiency, smiling, thought that it was not such a
bad compromise after all and that the Designer might well
be satisfied.
``Now,'' said she, ``there's just one or two points I'm
a bit hazy about. It appears that when the Propeller shaft
is horizontal and so working in its most efficient attitude,
I shall have a Lift from the Surfaces slightly in excess of the
Weight. That means I shall ascend slightly, at the same time
making nearly maximum speed for the power and thrust.
Can't I do better than that?''
``Yes, indeed,'' spoke up the Propeller, ``though it means
that I must assume a most undignified attitude, for helicopters[[2]]
I never approved of. In order to ascend more
quickly the Pilot will deflect the Elevator, which, by the
way, you see hinged to the Tail. By that means he will
force the whole Aeroplane to assume a greater Angle of
Incidence. And with greater Angle, the Lift will increase,
though I'm sorry to say the Drift will increase also. Owing
to the greater Drift, the Speed through the air will lessen,
and I'm afraid that won't be helpful to the Lift; but I shall
now be pointing upwards, and besides overcoming the Drift
in a forward direction I shall be doing my best to haul
the Aeroplane skywards. At a certain angle known as the
Best Climbing Angle, we shall have our Maximum Margin
of Lift, and I'm hoping that may be as much as almost a
thousand feet altitude a minute.''
[[2]] Helicopter. An air-screw revolving upon a vertical axis.
If driven with sufficient power, it will lift vertically,
but having regard to the mechanical difficulties of such construction,
it is a most inefficient way of securing lift compared with the
arrangement of an inclined surface driven by a propeller
revolving about a horizontal axis.
``Then, if the Pilot is green, my chance will come,'' said
the Maximum Angle of Incidence. ``For if the Angle is
increased over the Best Climbing Angle, the Drift will rush
up; and the Speed, and with it the Lift, will, when my
Angle is reached, drop to a point when the latter will be no
more than the Weight. The Margin of Lift will have
entirely disappeared, and there we shall be, staggering
along at my tremendous angle, and only just maintaining
horizontal flight.''
``And then with luck I'll get my chance,'' said the Drift.
``If he is a bit worse than green, he'll perhaps still further
increase the Angle. Then the Drift, largely increasing, the
Speed, and consequently the Lift, will become still less,
i.e., less than the Weight, and then--what price pancakes,[[3]]
eh?''
[[3]] Pancakes: Pilot's slang for stalling an aeroplane
and dropping like a pancake.
``Thank you,'' from Efficiency, ``that was all most
informing. And now will you tell me, please, how the
greatest Speed may be secured?''
``Certainly, now it's my turn,'' piped the Minimum Angle
of Incidence. ``By means of the Elevator, the Pilot places
the Aeroplane at my small Angle, at which the Lift only
just equals the Weight, and, also, at which we shall make
greater speed with no more Drift than before. Then we get
our greatest Speed, just maintaining horizontal flight.''
``Yes; though I'm out of the horizontal and thrusting
downwards,'' grumbled the Propeller, ``and that's not
efficient, though I suppose it's the best we can do until that
Inventor fellow finds his Mechanics.''
``Thank you so much,'' said Efficiency. ``I think I have
now at any rate an idea of the Elementary Principles of
Flight, and I don't know that I care to delve much deeper,
for sums always give me a headache; but isn't there something
about Stability and Control? Don't you think I ought
to have a glimmering of them too?''
``Well, I should smile,'' said a spruce Spar, who had come
all the way from America. ``And that, as the Lecturer
says, `will be the subject of our next lecture,' so be here
again to-morrow, and you will be glad to hear that it will be
distinctly more lively than the subject we have covered
to-day.''
PART II
THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES,
FINISH THE JOB
Another day had passed, and the Flight Folk had again
gathered together and were awaiting the arrival of Efficiency
who, as usual, was rather late in making an appearance.
The crowd was larger than ever, and among the newcomers
some of the most important were the three Stabilities,
named Directional, Longitudinal, and Lateral, with
their assistants, the Rudder, Elevator, and Ailerons. There
was Centrifugal Force, too, who would not sit still and
created a most unfavourable impression, and Keel-Surface,
the Dihedral Angle, and several other lesser fry.
``Well,'' said Centrifugal Force, ``I wish this Efficiency
I've heard so much about would get a move on. Sitting
still doesn't agree with me at all. Motion I believe in.
There's nothing like motion--the more the better.''
``We are entirely opposed to that,'' objected the three
Stabilities, all in a breath. ``Unless it's in a perfectly
straight line or a perfect circle. Nothing but perfectly
straight lines or, upon occasion, perfect circles satisfy us,
and we are strongly suspicious of your tendencies.''
``Well, we shall see what we shall see,'' said the Force
darkly. ``But who in the name of blue sky is this?''
And in tripped Efficiency, in a beautifully ``doped''
dress of the latest fashionable shade of khaki-coloured
fabric, a perfectly stream-lined bonnet, and a bewitching
little Morane parasol,[[4]] smiling as usual, and airily exclaiming,
``I'm so sorry I'm late, but you see the Designer's
such a funny man. He objects to skin friction,[[5]] and insisted
upon me changing my fabric for one of a smoother
surface, and that delayed me. Dear me, there are a lot
more of us to-day, aren't there? I think I had better meet
one at a time.'' And turning to Directional Stability, she
politely asked him what he preferred to do.
[[4]] Morane parasol: A type of Morane monoplane in which the
lifting surfaces are raised above the pilot in order to afford
him a good view of the earth.
[[5]] Skin friction is that part of the drift due to the friction
of the air with roughnesses upon the surface of the aeroplane.
``My purpose in life, miss,'' said he, ``is to keep the Aeroplane
on its course, and to achieve that there must be, in
effect, more Keel-Surface behind the Vertical Turning Axis
than there is in front of it.''
Efficiency looking a little puzzled, he added: ``Just like
a weathercock, and by Keel-Surface I mean everything
you can see when you view the Aeroplane from the side of
it--the sides of the body, struts, wires, etc.''
``Oh, now I begin to see light,'' said she: ``but just
exactly how does it work?''
``I'll answer that,'' said Momentum. ``When perhaps
by a gust of air the Aeroplane is blown out of its course
and points in another direction, it doesn't immediately
fly off on that new course. I'm so strong I pull it off the
new course to a certain extent, and towards the direction
of the old course. And so it travels, as long as my strength
lasts, in a more or less sideways position.''
``Then,'' said the Keel-Surface, ``I get a pressure of
air all on one side, and as there is, in effect, most of me
towards the tail, the latter gets pressed sideways, and the
Aeroplane thus tends to assume its first position and course.''
``I see,'' said Efficiency, and, daintily holding the Chalk,
she approached the Blackboard. ``Is this what you mean?''
``Yes, that's right enough,'' said the Keel-Surface, ``and
you might remember, too, that I always make the Aeroplane
nose into the gusts rather than away from them.''
``If that was not the case,'' broke in Lateral Stability,
and affecting the fashionable Flying Corps stammer, ``it
would be a h-h-h-o-r-rible affair! If there were too much
Keel-Surface in front, then that gust would blow the Aeroplane
round the other way a very considerable distance.
And the right-hand Surface being on the outside of the turn
would have more speed, and consequently more Lift, than
the Surface on the other side. That means a greater proportion
of the Lift on that side, and before you could say
Warp to the Ailerons over the Aeroplane would go--probable
result a bad side-slip''
``And what can the Pilot do to save such a situation as
that?'' said Efficiency.
``Well,'' replied Lateral Stability, ``he will try to turn
the Aeroplane sideways and back to an even keel by means
of warping the Ailerons or little wings which are hinged
on to the Wing-tips, and about which you will hear more
later on; but if the side-slip is very bad he may not be able
to right the Aeroplane by means of the Ailerons, and then
the only thing for him to do is to use the Rudder and to turn
the nose of the Aeroplane down and head-on to the direction
of motion. The Aeroplane will then be meeting the air in
the direction it is designed to do so, and the Surfaces and
also the controls (the Rudder, Ailerons, and Elevator) will
be working efficiently; but its attitude relative to the earth
will probably be more or less upside-down, for the action
of turning the Aeroplane's nose down results, as you will
see by the illustration B, in the right wing, which is on the
outside of the circle. travelling through the air with greater
speed than the left-hand wing. More Speed means more
Lift, so that results in overturning the Aeroplane still more;
but now it is, at any rate, meeting the air as it is designed
to meet it, and everything is working properly. It is then
only necessary to warp the Elevator, as shown in illustration
C, in order to bring the Aeroplane into a proper attitude
relative to the earth.''
``Ah!'' said the Rudder, looking wise, ``it's in a case
like that when I become the Elevator and the Elevator
becomes me.''
``That's absurd nonsense,'' said the Blackboard, ``due
to looseness of thought and expression.''
``Well,'' replied the Rudder, ``when 'the Aeroplane is
in position A and I am used, then I depress or ELEVATE the nose
of the machine; and, if the Elevator is used, then it turns
the Aeroplane to right or left, which is normally my function.
Surely our roles have changed one with the other, and I'm
then the Elevator and the Elevator is me!''
Said Lateral Stability to the Rudder, ``That's altogether
the wrong way of looking at it, though I admit''--and
this rather sarcastically--``that the way you put it sounds
rather fine when you are talking of your experiences in
the air to those `interested in aviation' but knowing little
about it; but it won't go down here! You are a Controlling
Surface designed to turn the Aeroplane about its vertical
axis, and the Elevator is a Controlling Surface designed to
turn the Aeroplane about its lateral axis. Those are your
respective jobs, and you can't possibly change them about.
Such talk only leads to confusion, and I hope we shall hear
no more of it.''
``Thanks,'' said Efficiency to Lateral Stability. ``And
now, please, will you explain your duties?''
``My duty is to keep the Aeroplane horizontal from
Wing-tip to Wing-tip. First of all, I sometimes arrange
with the Rigger to wash-out, that is decrease, the Angle
of Incidence on one side of the Aeroplane, and to effect
the reverse condition, if it is not too much trouble, on the
other side.''
``But,'' objected Efficiency, ``the Lift varies with the
Angle of Incidence, and surely such a condition will result in
one side of the Aeroplane lifting more than the other side?'
``That's all right,'' said the Propeller, ``it's meant to
off-set the tendency of the Aeroplane to turn over sideways
in the opposite direction to which I revolve.''
``That's quite clear, though rather unexpected; but how
do you counteract the effect of the gusts when they try to
overturn the Aeroplane sideways?'' said she, turning to
Lateral Stability again.
``Well,'' he replied, rather miserably, ``I'm not nearly
so perfect as the Longitudinal and Directional Stabilities.
The Dihedral Angle--that is, the upward inclination of the
Surfaces towards their wing-tips--does what it can for me,
but, in my opinion, it's a more or less futile effort. The
Blackboard will show you the argument.'' And he at once
showed them two Surfaces, each set at a Dihedral Angle
like this:
``Please imagine,'' said the Blackboard, ``that the top
V is the front view of a Surface flying towards you. Now
if a gust blows it into the position of the lower V you see
that the horizontal equivalent of the Surface on one side
becomes larger, and on the other side it becomes smaller.
That results in more Lift on the lower side and less on the
higher side, and if the V is large enough it should produce
such a difference in the Lift of one side to the other as to
quickly turn the Aeroplane back to its former and normal
position.''
``Yes,'' said the Dihedral Angle, ``that's what would
happen if they would only make me large enough; but
they won't do it because it would too greatly decrease the
horizontal equivalent, and therefore the Lift, and incidentally
it would, as Aeroplanes are built to-day, produce
an excess of Keel Surface above the turning axis, and that
in itself would spoil the Lateral Stability. The Keel Surface
should be equally divided above and below the longitudinal
turning axis (upon which the Aeroplane rolls sideways),
or the side upon which there is an excess will get
blown over by the gusts. It strikes me that my future
isn't very promising, and about my only chance is when
the Junior Draughtsman makes a mistake, as he did the
other day. And just think of it, they call him a Designer
now that he's got a job at the Factory! What did he do?
Why, he calculated the weights wrong and got the Centre
of Gravity too high, and they didn't discover it until the
machine was built. Then all they could do was to give
me a larger Angle. That dropped the bottom of the V
lower down, and as that's the centre of the machine, where
all the Weight is, of course that put the Centre of Gravity
in its right place. But now there is too much Keel Surface
above, and the whole thing's a Bad Compromise, not at all
like Our Efficiency.''
And Efficiency, blushing very prettily at the compliment,
then asked, ``And how does the Centre of Gravity affect
matters?''
``That's easy,'' said Grandfather Gravity. ``I'm so
heavy that if I am too low down I act like a pendulum
and cause the Aeroplane to roll about sideways, and if I
am too high I'm like a stick balanced on your finger, and
then if I'm disturbed, over I go and the Aeroplane with
me; and, in addition to that, there are the tricks I play
with the Aeroplane when it's banked up,[[6]] i.e., tilted sideways
for a turn, and Centrifugal Force sets me going the
way I'm not wanted to go. No; I get on best with Lateral
Stability when my Centre is right on the centre of Drift,
or, at any rate, not much below it.'' And with that he
settled back into the Lecturer's Chair and went sound
asleep again, for he was so very, very old, in fact the father
of all the Principles.
[[6]] Banking: When an aeroplane is turned to the left or
the right the centrifugal force of its momentum causes it to
skid sideways and outwards away from the centre of the turn.
To minimize such action the pilot banks, i.e., tilts, the aeroplane
sideways in order to oppose the underside of the planes to the air.
The aeroplane will not then skid outwards beyond the slight skid
necessary to secure a sufficient pressure of air to balance the
centrifugal force.
And the Blackboard had been busy, and now showed
them a picture of the Aeroplane as far as they knew it, and
you will see that there is a slight Dihedral Angle, and
also, fixed to the tail, a vertical Keel Surface or fin, as
is very often the case in order to ensure the greater effect
of such surface being behind the vertical turning axis.
But Efficiency, growing rather critical with her newly
gained knowledge, cried out: ``But where's the horizontal
Tail Surface? It doesn't look right like that!''
``This is when I have the pleasure of meeting you, my
dear,'' said Longitudinal Stability. ``Here's the Tail Surface,''
he said, ``and in order to help me it must be set IN
EFFECT at a much less Angle of Incidence than the Main Surface.
To explain we must trouble the Blackboard again,'' and
this was his effort:
``I have tried to make that as clear as possible,'' he
said. ``It may appear a bit complicated at first, but if
you will take the trouble to look at it for a minute you will find
it quite simple. A is the normal and proper direction of
motion of the Aeroplane, but, owing to a gust of air, it takes
up the new nose-down position. Owing to Momentum,
however, it does not fly straight along in that direction, but
moves more or less in the direction B, which is the resultant
of the two forces, Momentum and Thrust. And so you will
note that the Angle of Incidence, which is the inclination
of the Surfaces to the Direction of Motion, has decreased,
and of course the Lift decreases with it. You will also see,
and this is the point, that the Tail Surface has lost a higher
proportion of its Angle, and consequently its Lift, than has
the Main Surface. Then, such being the case, the Tail must
fall and the Aeroplane assume its normal position again,
though probably at a slightly lower altitude.''
``I'm afraid I'm very stupid,'' said Efficiency, ``but
please tell me why you lay stress upon the words `IN
EFFECT.' ''
``Ah! I was wondering if you would spot that,'' he
replied. ``And there is a very good reason for it. You see,
in some Aeroplanes the Tail Surface may be actually set
at the same Angle on the machine as the Main Surface, but
owing to the air being deflected downwards by the front
Main Surface it meets the Tail Surface at a lesser angle,
and indeed in some cases at no angle at all. The Tail is then
for its surface getting less Lift than the Main Surface, although
set at the same angle on the machine. It may then be
said to have IN EFFECT a less Angle of Incidence. I'll just
show you on the Blackboard.''
``And now,'' said Efficiency, ``I have only to meet the
Ailerons and the Rudder, haven't I?''
``Here we are,'' replied the Ailerons, or little wings.
``Please hinge us on to the back of the Main Surfaces, one
of us at each Wing-tip, and join us up to the Pilot's joystick
by means of the control cables. When the Pilot wishes to
tilt the Aeroplane sideways, he will move the stick and depress
us upon one side, thus giving us a larger Angle of Incidence
and so creating more Lift on that side of the Aeroplane;
and, by means of a cable connecting us with the Ailerons on
the other side of the Aeroplane, we shall, as we are depressed,
pull them up and give them a reverse or negative Angle of
Incidence, and that side will then get a reverse Lift or downward
thrust, and so we are able to tilt the Aeroplane sideways.
``And we work best when the Angle of Incidence of the
Surface in front of us is very small, for which reason it is
sometimes decreased or washed-out towards the Wing-tips.
The reason of that is that by the time the air reaches us
it has been deflected downwards--the greater the Angle
of Incidence the more it is driven downwards--and in order
for us to secure a Reaction from it, we have to take such a
large Angle of Incidence that we produce a poor proportion
of Lift to Drift; but the smaller the Angle of the Surface in
front of us the less the air is deflected downwards, and
consequently the less Angle is required of us, and the better our
proportion of Lift to Drift, which, of course, makes us much
more effective Controls.''
``Yes,'' said the Lateral and Directional Stabilities in
one voice, ``that's so, and the wash-out helps us also, for
then the Surfaces towards their Wing-tips have less Drift
or `Head-Resistance,' and consequently the gusts will affect
them and us less; but such decreased Angle of Incidence
means decreased Lift as well as Drift, and the Designer does
not always care to pay the price.''
``Well,'' said the Ailerons, ``if it's not done it will mean
more work for the Rudder, and that won't please the Pilot.''
``Whatever do you mean?'' asked Efficiency. ``What
can the Rudder have to do with you?''
``It's like this,'' they replied: ``when we are deflected
downwards we gain a larger Angle of Incidence and also
enter an area of compressed air, and so produce more Drift
than those of us on the other side of the Aeroplane, which
are deflected upwards into an area of rarefied air due to
the SUCTION effect (though that term is not academically
correct) on the top of the Surface. If there is more Drift,
i.e., Resistance, on one side of the Aeroplane than on the other
side, then of course it will turn off its course, and if that
difference in Drift is serious, as it will very likely be if there
is no wash-out, then it will mean a good deal of work for the
Rudder in keeping the Aeroplane on its course, besides
creating extra Drift in doing so.''
``I think, then,'' said Efficiency, ``I should prefer to
have that wash-out,[[7]] and my friend the Designer is so clever
at producing strength of construction for light weight, I'm
pretty sure he won't mind paying the price in Lift. And
now let me see if I can sketch the completed Aeroplane.''
[[7]] An explanation of the way in which the wash-out is combined
with a wash-in to offset propellor torque will be found on p. 82.
``Well, I hope that's all as it should be,'' she concluded,
``for to-morrow the Great Test in the air is due.''
PART III
THE GREAT TEST
It is five o'clock of a fine calm morning, when the Aeroplane
is wheeled out of its shed on to the greensward of the Military
Aerodrome. There is every promise of a good flying day,
and, although the sun has not yet risen, it is light enough to
discern the motionless layer of fleecy clouds some five thousand
feet high, and far, far above that a few filmy mottled streaks
of vapour. Just the kind of morning beloved of pilots.
A brand new, rakish, up-to-date machine it is, of highly
polished, beautifully finished wood, fabric as tight as a
drum, polished metal, and every part so perfectly ``streamlined''
to minimize Drift, which is the resistance of the air
to the passage of the machine, that to the veriest tyro the
remark of the Pilot is obviously justified.
``Clean looking 'bus, looks almost alive and impatient
to be off. Ought to have a turn for speed with those
lines.''
``Yes,'' replies the Flight-Commander, ``it's the latest
of its type and looks a beauty. Give it a good test. A
special report is required on this machine.''
The A.M.'s[[8]] have now placed the Aeroplane in position
facing the gentle air that is just beginning to make itself
evident; the engine Fitter, having made sure of a sufficiency
of oil and petrol in the tanks, is standing by the Propeller;
the Rigger, satisfied with a job well done, is critically ``vetting''
the machine by eye, four A.M.'s are at their posts,
ready to hold the Aeroplane from jumping the blocks which
have been placed in front of the wheels; and the Flight-
Sergeant is awaiting the Pilot's orders.
[[8]] A.M.'s: Air Mechanics.
As the Pilot approaches the Aeroplane the Rigger springs
to attention and reports, ``All correct, sir,'' but the Fitter
does not this morning report the condition of the Engine,
for well he knows that this Pilot always personally looks
after the preliminary engine test. The latter, in leathern
kit, warm flying boots and goggled, climbs into his seat,
and now, even more than before, has the Aeroplane an almost
living appearance, as if straining to be off and away. First
he moves the Controls to see that everything is clear, for
sometimes when the Aeroplane is on the ground the control
lever or ``joy-stick'' is lashed fast to prevent the wind
from blowing the controlling surfaces about and possibly
damaging them.
The air of this early dawn is distinctly chilly, and the
A.M.'s are beginning to stamp their cold feet upon the dewy
grass, but very careful and circumspect is the Pilot, as he
mutters to himself, ``Don't worry and flurry, or you'll die
in a hurry.''
At last he fumbles for his safety belt, but with a start
remembers the Pilot Air Speed Indicator, and, adjusting
it to zero, smiles as he hears the Pilot-head's gruff voice,
``Well, I should think so, twenty miles an hour I was registering.
That's likely to cause a green pilot to stall the Aeroplane.
Pancake, they call it.'' And the Pilot, who is an
old hand and has learned a lot of things in the air that mere
earth-dwellers know nothing about, distinctly heard the
Pilot Tube, whose mouth is open to the air to receive its
pressure, stammer. ``Oh Lor! I've got an earwig already--
hope to goodness the Rigger blows me out when I come
down--and this morning air simply fills me with moisture;
I'll never keep the Liquid steady in the Gauge. I'm not
sure of my rubber connections either.''
``Oh, shut up!'' cry all the Wires in unison, ``haven't
we got our troubles too? We're in the most horrible state
of tension. It's simply murdering our Factor of Safety,
and how we can possibly stand it when we get the Lift only
the Designer knows.''
``That's all right,'' squeak all the little Wire loops,
``we're that accommodating, we're sure to elongate a bit
and so relieve your tension.'' For the whole Aeroplane is
braced together with innumerable wires, many of which
are at their ends bent over in the form of loops in order to
connect with the metal fittings on the spars and elsewhere--
cheap and easy way of making connection.
``Elongate, you little devils, would you?'' fairly shout
the Angles of Incidence, Dihedral and Stagger, amid a chorus
of groans from all parts of the Aeroplane. ``What's going
to happen to us then? How are we going to keep our
adjustments upon which good flying depends?''
``Butt us and screw us,''[[9]] wail the Wires. ``Butt us
and screw us, and death to the Loops. That's what we
sang to the Designer, but he only looked sad and scowled
at the Directors.''
[[9]] Butt means to thicken at the end. Screw means to machine a thread
on the butt-end of the wire, and in this way the wire can make connection
with the desired place by being screwed into a metal fitting,
thus eliminating the disadvantage of the unsatisfactory loop.
``And who on earth are they?'' asked the Loops, trembling
for their troublesome little lives.
``Oh earth indeed,'' sniffed Efficiency, who had not
spoken before, having been rendered rather shy by being
badly compromised in the Drawing Office. ``I'd like to
get some of them up between Heaven and Earth, I would.
I'd give 'em something to think of besides their Debits
and Credits--but all the same the Designer will get his
way in the end. I'm his Best Girl, you know, and if we
could only get rid of the Directors, the little Tin god, and
the Man-who-takes-the-credit, we should be quite happy.''
Then she abruptly subsides, feeling that perhaps the less
said the better until she has made a reputation in the Air.
The matter of that Compromise still rankled, and indeed
it does seem hardly fit that a bold bad Tin god should flirt
with Efficiency. You see there was a little Tin god, and he
said ``Boom, Boom BOOM! Nonsense! It MUST be done,''
and things like that in a very loud voice, and the Designer
tore his hair and was furious, but the Directors, who were
thinking of nothing but Orders and Dividends, had the
whip-hand of HIM, and so there you are, and so poor beautiful
Miss Efficiency was compromised.
All this time the Pilot is carefully buckling his belt and
making himself perfectly easy and comfortable, as all good
pilots do. As he straightens himself up from a careful
inspection of the Deviation Curve[[10]] of the Compass and takes
command of the Controls, the Throttle and the Ignition,
the voices grow fainter and fainter until there is nothing
but a trembling of the Lift and Drift wires to indicate to his
understanding eye their state of tension in expectancy of
the Great Test.
[[10]] Deviation curve: A curved line indicating any errors in the compass.
``Petrol on?'' shouts the Fitter to the Pilot.
``Petrol on,'' replies the Pilot.
``Ignition off?''
``Ignition off.''
Round goes the Propeller, the Engine sucking in the
Petrol Vapour with satisfied gulps. And then--
``Contact?'' from the Fitter.
``Contact,'' says the Pilot.
Now one swing of the Propeller by the Fitter, and the
Engine is awake and working. Slowly at first though, and
in a weak voice demanding, ``Not too much Throttle, please.
I'm very cold and mustn't run fast until my Oil has thinned
and is circulating freely. Three minutes slowly, as you love
me, Pilot.''
Faster and faster turn the Engine and Propeller, and
the Aeroplane, trembling in all its parts, strains to jump
the blocks and be off. Carefully the Pilot listens to what the
Engine Revolution Indicator says. At last, ``Steady
at 1,500 revs. and I'll pick up the rest in the Air.'' Then
does he throttle down the Engine, carefully putting the
lever back to the last notch to make sure that in such position
the Throttle is still sufficiently open for the Engine to continue
working, as otherwise it might lead to him ``losing'' his
Engine in the air when throttling down the power for descent.
Then, giving the official signal, he sees the blocks removed
from the wheels, and the Flight-Sergeant saluting he knows
that all is clear to ascend. One more signal, and all the
A.M.'s run clear of the Aeroplane.
Then gently, gently mind you, with none of the ``crashing
on'' bad Pilots think so fine, he opens the Throttle
and, the Propeller Thrust overcoming its enemy the Drift,
the Aeroplane moves forward.
``Ah!'' says the Wind-screen, ``that's Discipline, that
is. Through my little window I see most things, and don't
I just know that poor discipline always results in poor work
in the air, and don't you forget it.''
``Discipline is it?'' complains the Under-carriage, as
its wheels roll swiftly over the rather rough ground. ``I'm
bump getting it; and bump, bump, all I want, bang, bump,
rattle, too!'' But, as the Lift increases with the Speed,
the complaints of the Under-carriage are stilled, and then,
the friendly Lift becoming greater than the Weight, the
Aeroplane swiftly and easily takes to the air.
Below is left the Earth with all its bumps and troubles.
Up into the clean clear Air moves with incredible speed
and steadiness this triumph of the Designer, the result of
how much mental effort, imagination, trials and errors,
failures and successes, and many a life lost in high
endeavour.
Now is the mighty voice of the Engine heard as he turns
the Propeller nine hundred times a minute. Now does the
Thrust fight the Drift for all it's worth, and the Air Speed
Indicator gasps with delight, ``One hundred miles an hour!''
And now does the burden of work fall upon the Lift and
Drift Wires, and they scream to the Turnbuckles whose
business it is to hold them in tension, ``This is the limit!
the Limit! THE LIMIT! Release us, if only a quarter
turn.'' But the Turnbuckles are locked too fast to turn
their eyes or utter a word. Only the Locking Wires thus:
``Ha! ha! the Rigger knew his job. He knew the trick, and
there's no release here.'' For an expert rigger will always
use the locking wire in such a way as to oppose the slightest
tendency of the turnbuckle to unscrew. The other kind of
rigger will often use the wire in such a way as to allow the
turnbuckle, to the ``eyes'' of which the wires are attached,
to unscrew a quarter of a turn or more, with the result that
the correct adjustment of the wires may be lost; and upon
their fine adjustment much depends.
And the Struts and the Spars groan in compression and
pray to keep straight, for once ``out of truth'' there is, in
addition to possible collapse, the certainty that in bending
they will throw many wires out of adjustment.
And the Fabric's quite mixed in its mind, and ejaculates,
``Now, who would have thought I got more Lift from the
top of the Surface than its bottom?'' And then truculently
to the Distance Pieces, which run from rib to rib, ``Just
keep the Ribs from rolling, will you? or you'll see me strip.
I'm an Irishman, I am, and if my coat comes off---- Yes,
Irish, I said. I used to come from Egypt, but I've got
naturalized since the War began.''
Then the Air Speed Indicator catches the eye of the
Pilot. ``Good enough,'' he says as he gently deflects the
Elevator and points the nose of the Aeroplane upwards in
search of the elusive Best Climbing Angle.
``Ha! ha!'' shouts the Drift, growing stronger with the
increased Angle of Incidence. ``Ha! ha!'' he laughs to
the Thrust. ``Now I've got you. Now who's Master?''
And the Propeller shrieks hysterically, ``Oh! look at
me. I'm a helicopter. That's not fair. Where's Efficiency?''
And she can only sadly reply, ``Yes, indeed, but
you see we're a Compromise.''
And the Drift has hopes of reaching the Maximum Angle
of Incidence and vanquishing the Thrust and the Lift. And
he grows very bold as he strangles the Thrust; but the situation
is saved by the Propeller, who is now bravely helicopting
skywards, somewhat to the chagrin of Efficiency.
``Much ado about nothing,'' quotes the Aeroplane
learnedly. ``Compromise or not, I'm climbing a thousand
feet a minute. Ask the Altimeter. He'll confirm it.''
And so indeed it was. The vacuum box of the Altimeter
was steadily expanding under the decreased pressure of
the rarefied air, and by means of its little levers and its
wonderful chain no larger than a hair it was moving the
needle round the gauge and indicating the ascent at the
rate of a thousand feet a minute.
And lo! the Aeroplane has almost reached the clouds!
But what's this? A sudden gust, and down sinks one wing
and up goes the other. ``Oh, my Horizontal Equivalent!''
despairingly call the Planes: ``it's eloping with the Lift,
and what in the name of Gravity will happen? Surely
there was enough scandal in the Factory without this, too!''
For the lift varies with the horizontal equivalent of the
planes, so that if the aeroplane tilts sideways beyond a certain
angle, the lift becomes less than the weight of the machine,
which must then fall. A fall in such a position is known as
a ``side-slip.''
But the ever-watchful Pilot instantly depresses one aileron,
elevating the other, with just a touch of the rudder to keep
on the course, and the Planes welcome back their precious
Lift as the Aeroplane flicks back to its normal position.
``Bit bumpy here under these clouds,'' is all the Pilot
says as he heads for a gap between them, and the next minute
the Aeroplane shoots up into a new world of space.
``My eye!'' ejaculates the Wind-screen, ``talk about a
view!'' And indeed mere words will always fail to express
the wonder of it. Six thousand feet up now, and look!
The sun is rising quicker than ever mortal on earth witnessed
its ascent. Far below is Mother Earth, wrapt in mists and
deep blue shadows, and far above are those light, filmy,
ethereal clouds now faintly tinged with pink And all
about great mountains of cloud, lazily floating in space.
The sun rises and they take on all colours, blending one
with the other, from dazzling white to crimson and deep
violet-blue. Lakes and rivers here and there in the enormous
expanse of country below refract the level rays of the sun
and, like so many immense diamonds, send dazzling shafts
of light far upwards. The tops of the hills now laugh to the
light of the sun, but the valleys are still mysterious dark
blue caverns, clowned with white filmy lace-like streaks of
vapour. And withal the increasing sense with altitude of
vast, clean, silent solitudes of space.
Lives there the man who can adequately describe this
Wonder? ``Never,'' says the Pilot, who has seen it many
times, but to whom it is ever new and more wonderful.
Up, up, up, and still up, unfalteringly speeds the Pilot
and his mount. Sweet the drone of the Engine and steady
the Thrust as the Propeller exultingly battles with the Drift.
And look! What is that bright silver streak all along
the horizon? It puzzled the Pilot when first he saw it,
but now he knows it for the Sea, full fifty miles away!
And on his right is the brightness of the Morn and the
smiling Earth unveiling itself to the ardent rays of the Sun;
and on his left, so high is he, there is yet black Night, hiding
innumerable Cities, Towns, Villages and all those places
where soon teeming multitudes of men shall awake, and by
their unceasing toil and the spirit within them produce
marvels of which the Aeroplane is but the harbinger.
And the Pilot's soul is refreshed, and his vision, now
exalted, sees the Earth a very garden, even as it appears
at that height, with discord banished and a happy time
come, when the Designer shall have at last captured Efficiency,
and the Man-who-takes-the-credit is he who has earned it,
and when kisses are the only things that go by favour.
Now the Pilot anxiously scans the Barograph, which is
an instrument much the same as the Altimeter; but in this
case the expansion of the vacuum box causes a pen to trace
a line upon a roll of paper. This paper is made by clockwork
to pass over the point of the pen, and so a curved line is
made which accurately registers the speed of the ascent in
feet per minute. No longer is the ascent at the rate of a
thousand feet a minute, and the Propeller complains to the
Engine, ``I'm losing my Revs. and the Thrust. Buck up
with the Power, for the Lift is decreasing, though the Weight
remains much the same.''
Quoth the Engine: ``I strangle for Air. A certain proportion,
and that of right density, I must have to one part
of Petrol, in order to give me full power and compression,
and here at an altitude of ten thousand feet the Air is only
two-thirds as dense as at sea-level. Oh, where is he who
will invent a contrivance to keep me supplied with Air of
right density and quality? It should not be impossible
within certain limits.''
``We fully agree,'' said the dying Power and Thrust. ``Only
maintain Us and you shall be surprised at the result. For
our enemy Drift decreases in respect of distance with the increase
of altitude and rarity of air, and there is no limit to the
speed through space if only our strength remains. And
with oxygen for Pilot and Passengers and a steeper pitch[[11]]
for the Propeller we may then circle the Earth in a day!''
[[11]] A propeller screws through the air, and the distance it advances
during one revolution, supposing the air to be solid, is known as the pitch.
The pitch, which depends upon the angle of the propeller blades, must be equal
to the speed of the aeroplane, plus the slip, and if, on account of the rarity
of the air the speed of the aeroplane increases, then the angle and pitch
should be correspondingly increased. Propellers with a pitch capable of being
varied by the pilot are the dream of propeller designers. For explanation of
``slip'' see Chapter IV. on propellers.
Ah, Reader, smile not unbelievingly, as you smiled but
a few years past. There may be greater wonders yet. Consider
that as the speed increases, so does the momentum
or stored-up force in the mass of the aeroplane become
terrific. And, bearing that in mind, remember that with
altitude gravity decreases. There may yet be literally other
worlds to conquer.[[12]]
[[12]] Getting out of my depth? Invading the realms of fancy? Well,
perhaps so, but at any rate it is possible that extraordinary speed through
space may be secured if means are found to maintain the impulse of the engine
and the thrust-drift efficiency of the propeller at great altitude.
Now at fifteen thousand feet the conditions are chilly
and rare, and the Pilot, with thoughts of breakfast far below,
exclaims, ``High enough! I had better get on with the
Test.'' And then, as he depresses the Elevator, the Aeroplane
with relief assumes its normal horizontal position.
Then, almost closing the Throttle, the Thrust dies away.
Now, the nose of the Aeroplane should sink of its own volition,
and the craft glide downward at flying speed, which is in
this case a hundred miles an hour. That is what should
happen if the Designer has carefully calculated the weight
of every part and arranged for the centre of gravity to be just
the right distance in front of the centre of lift. Thus is the
Aeroplane ``nose-heavy'' as a glider, and just so to a degree
ensuring a speed of glide equal to its flying speed. And the
Air Speed Indicator is steady at one hundred miles an hour,
and ``That's all right!'' exclaims the Pilot. ``And very
useful, too, in a fog or a cloud,'' he reflects, for then he can
safely leave the angle of the glide to itself, and give all his
attention, and he will need it all, to keeping the Aeroplane
horizontal from wing-tip to wing-tip, and to keeping it
straight on its course. The latter he will manage with the
rudder, controlled by his feet, and the Compass will tell him
whether a straight course is kept. The former he will control
by the Ailerons, or little wings hinged to the tips of the planes,
and the bubble in the Inclinometer in front of him must be
kept in the middle.
A Pilot, being only human, may be able to do two things
at once, but three is a tall order, so was this Pilot relieved
to find the Design not at fault and his craft a ``natural
glider.'' To correct this nose-heavy tendency when the
Engine is running, and descent not required, the centre
of Thrust is arranged to be a little below the centre of Drift
or Resistance, and thus acts as a counter-balance.
But what is this stream of bad language from the Exhaust
Pipe, accompanied by gouts of smoke and vapour?
The Engine, now revolving at no more than one-tenth its
normal speed, has upset the proportion of petrol to air,
and combustion is taking place intermittently or in the
Exhaust Pipe, where it has no business to be.
``Crash, Bang, Rattle----!----!----!'' and worse than
that, yells the Exhaust, and the Aeroplane, who is a gentleman
and not a box kite,[[13]] remonstrates with the severity
of a Senior Officer. ``See the Medical Officer, you young
Hun. Go and see a doctor. Vocal diarrhoea, that's your
complaint, and a very nasty one too. Bad form, bad for
discipline, and a nuisance in the Mess. What's your Regiment?
Special Reserve, you say? Humph! Sounds like
Secondhand Bicycle Trade to me!''
[[13]] Box-kite. The first crude form of biplane.
Now the Pilot decides to change the straight gliding
descent to a spiral one, and, obedient to the Rudder, the
Aeroplane turns to the left. But the Momentum (two tons
at 100 miles per hour is no small affair) heavily resents this
change of direction, and tries its level best to prevent it
and to pull the machine sideways and outwards from its
spiral course--that is, to make it ``side-skid'' outwards.
But the Pilot deflects the Ailerons and ``banks'' up the planes
to the correct angle, and, the Aeroplane skidding sideways
and outwards, the lowest surfaces of the planes press up against
the air until the pressure equals the centrifugal force of
the Momentum, and the Aeroplane spirals steadily downwards.
Down, down, down, and the air grows denser, and the
Pilot gulps largely, filling his lungs with the heavier air to
counteract the increasing pressure from without. Down
through a gap in the clouds, and the Aerodrome springs
into view, appearing no larger than a saucer, and the Pilot,
having by now got the ``feel'' of the Controls, proceeds
to put the Aeroplane through its paces. First at its Maximum
Angle, staggering along tail-down and just maintaining
horizontal flight; then a dive at far over flying speed, finishing
with a perfect loop; then sharp turns with attendant
vertical ``banks'' and then a wonderful switchback flight,
speeding down at a hundred and fifty miles an hour with
short, exhilarating ascents at the rate of two thousand feet
a minute!
All the parts are now working well together. Such
wires as were before in undue tension have secured relief
by slightly elongating their loops, and each one is now doing
its bit, and all are sharing the burden of work together.
The Struts and the Spars, which felt so awkward at first,
have bedded themselves in their sockets, and are taking
the compression stresses uncomplainingly.
The Control Cables of twisted wire, a bit tight before,
have slightly lengthened by perhaps the eighth of an inch,
and, the Controls instantly responding to the delicate touch
of the Pilot, the Aeroplane, at the will of its Master, darts
this way and that way, dives, loops, spirals, and at last, in
one long, magnificent glide, lands gently in front of its shed.
``Well, what result?'' calls the Flight-Commander to
the Pilot.
``A hundred miles an hour and a thousand feet a minute,''
he briefly replies.
``And a very good result too,'' says the Aeroplane, complacently,
as he is carefully wheeled into his shed.
That is the way Aeroplanes speak to those who love them
and understand them. Lots of Pilots know all about it,
and can spin you wonderful yarns, much better than this
one, if you catch them in a confidential mood--on leave,
for instance, and after a good dinner.
PART IV
'CROSS COUNTRY
The Aeroplane had been designed and built, and tested in
the air, and now stood on the Aerodrome ready for its first
'cross-country flight.
It had run the gauntlet of pseudo-designers, crank inventors,
press ``experts,'' and politicians; of manufacturers
keen on cheap work and large profits; of poor pilots who had
funked it, and good pilots who had expected too much of
it. Thousands of pounds had been wasted on it, many had
gone bankrupt over it, and others it had provided with safe
fat jobs.
Somehow, and despite every conceivable obstacle, it had
managed to muddle through, and now it was ready for its
work. It was not perfect, for there were fifty different
ways in which it might be improved, some of them shamefully
obvious. But it was fairly sound mechanically, had a little
inherent stability, was easily controlled, could climb a thousand
feet a minute, and its speed was a hundred miles an
hour. In short, quite a creditable machine, though of course
the right man had not got the credit.
It is rough, unsettled weather with a thirty mile an
hour wind on the ground, and that means fifty more or
less aloft. Lots of clouds at different altitudes to bother
the Pilot, and the air none to clear for the observation of
landmarks.
As the Pilot and Observer approach the Aeroplane the
former is clearly not in the best of tempers. ``It's rotten
luck,'' he is saying, ``a blank shame that I should have
to take this blessed 'bus and join X Reserve Squadron,
stationed a hundred and fifty miles from anywhere; and
just as I have licked my Flight into shape. Now some
slack blighter will, I suppose, command it and get the credit
of all my work!''
``Shut up, you grouser,'' said the Observer. ``Do you
think you're the only one with troubles? Haven't I been
through it too? Oh! I know all about it! You're from
the Special Reserve and your C.O. doesn't like your style
of beauty, and you won't lick his boots, and you were a bit
of a technical knut in civil life, but now you've jolly well
got to know less than those senior to you. Well! It's a
jolly good experience for most of us. Perhaps conceit won't
be at quite such a premium after this war. And what's
the use of grousing? That never helped anyone. So buck
up, old chap. Your day will come yet. Here's our machine,
and I must say it looks a beauty!''
And, as the Pilot approaches the Aeroplane, his face
brightens and he soon forgets his troubles as he critically
inspects the craft which is to transport him and the Observer
over the hills and far away. Turning to the Flight-Sergeant
he inquires, ``Tank full of petrol and oil?''
``Yes, sir,'' he replies, ``and everything else all correct.
Propeller, engine, and body covers on board, sir; tool kit
checked over and in the locker; engine and Aeroplane logbooks
written up, signed, and under your seat; engine revs.
up to mark, and all the control cables in perfect condition
and tension.''
``Very good,'' said the Pilot; and then turning to the
Observer, ``Before we start you had better have a look
at the course I have mapped out.
``A is where we stand and we have to reach B, a hundred
and fifty miles due North. I judge that, at the altitude
we shall fly, there will be an East wind, for although it is
not quite East on the ground it is probably about twenty
degrees different aloft, the wind usually moving round clockways
to about that extent. I think that it is blowing at the
rate of about fifty miles an hour, and I therefore take a line
on the map to C, fifty miles due West of A. The Aeroplane's
speed is a hundred miles an hour, and so I take a line of one
hundred miles from C to D. Our compass course will then
be in the direction A--E, which is always a line parallel to
C--D. That is, to be exact, it will be fourteen degrees off
the C--D course, as, in this part of the globe, there is that
much difference between the North and South lines on the
map and the magnetic North to which the compass needle
points. If the compass has an error, as it may have of a
few degrees, that, too, must be taken into account, and the
deviation or error curve on the dashboard will indicate it.
``The Aeroplane will then always be pointing in a direction
parallel to A--E, but, owing to the side wind, it will be actually
travelling over the course A--B, though in a rather
sideways attitude to that course.
``The distance we shall travel over the A--B course
in one hour is A--D. That is nearly eighty-seven miles,
so we ought to accomplish our journey of a hundred and
fifty miles in about one and three-quarter hours.
``I hope that's quite clear to you. It's a very simple
way of calculating the compass course, and I always do it
like that.''
``Yes, that's plain enough. You have drafted what
engineers call `a parallelogram of forces'; but suppose you
have miscalculated the velocity of the wind, or that it should
change in velocity or direction?''
``Well, that of course will more or less alter matters,''
replies the Pilot. ``But there are any number of good
landmarks such as lakes, rivers, towns, and railway lines.
They will help to keep us on the right course, and the compass
will, at any rate, prevent us from going far astray when
between them.''
``Well, we'd better be off, old chap. Hop aboard.''
This from the Observer as he climbs into the front seat
from which he will command a good view over the lower
plane; and the Pilot takes his place in the rear seat, and,
after making himself perfectly comfortable, fixing his safety
belt, and moving the control levers to make sure that they
are working freely, he gives the signal to the Engine Fitter
to turn the propeller and so start the engine.
Round buzzes the Propeller, and the Pilot, giving the
official signal, the Aeroplane is released and rolls swiftly
over the ground in the teeth of the gusty wind.
In less than fifty yards it takes to the air and begins
to climb rapidly upwards, but how different are the conditions
to the calm morning of yesterday! If the air were
visible it would be seen to be acting in the most extraordinary
manner; crazily swirling, lifting and dropping, gusts viciously
colliding--a mad phantasmagoria of forces!
Wickedly it seizes and shakes the Aeroplane; then tries
to turn it over sideways; then instantly changes its mind
and in a second drops it into a hole a hundred feet deep,
and if it were not for his safety belt the Pilot might find
his seat sinking away from beneath him.
Gusts strike the front of the craft like so many slaps in
the face; and others, with the motion of mountainous waves,
sometimes lift it hundreds of feet in a few seconds, hoping
to see it plunge over the summit in a death-dive--and so it
goes on, but the Pilot, perfectly at one with his mount and
instantly alert to its slightest motion, is skilfully and naturally
making perhaps fifty movements a minute of hand and feet;
the former lightly grasping the ``joy-stick'' which controls
the Elevator hinged to the tail, and also the Ailerons or little
wings hinged to the wing-tips; and the latter moving the
Rudder control-bar.
A strain on the Pilot? Not a bit of it, for this is his
Work which he loves and excels in; and given a cool head,
alert eye, and a sensitive touch for the controls, what
sport can compare with these ever-changing battles of
the air?
The Aeroplane has all this time been climbing in great
wide circles, and is now some three thousand feet above
the Aerodrome which from such height looks absurdly
small. The buildings below now seem quite squat; the
hills appear to have sunk away into the ground, and the
whole country below, cut up into diminutive fields, has
the appearance of having been lately tidied and thoroughly
spring-cleaned! A doll's country it looks, with tiny horses
and cows ornamenting the fields and little model motor-cars
and carts stuck on the roads, the latter stretching away
across the country like ribbons accidentally dropped.
At three thousand feet altitude the Pilot is satisfied
that he is now sufficiently high to secure, in the event of
engine failure, a long enough glide to earth to enable him
to choose and reach a good landing-place; and, being furthermore
content with the steady running of the engine, he
decides to climb no more but to follow the course he has
mapped out. Consulting the compass, he places the Aeroplane
on the A--E course and, using the Elevator, he gives
his craft its minimum angle of incidence at which it will
just maintain horizontal flight and secure its maximum
speed.
Swiftly he speeds away, and few thoughts he has now
for the changing panorama of country, cloud, and colour.
Ever present in his mind are the three great 'cross-country
queries. ``Am I on my right course? Can I see a good
landing-ground within gliding distance?'' And ``How is
the Engine running?''
Keenly both he and the Observer compare their maps
with the country below. The roads, khaki-coloured ribbons,
are easily seen but are not of much use, for there are so many
of them and they all look alike from such an altitude.
Now where can that lake be which the map shows so
plainly? He feels that surely he should see it by now,
and has an uncomfortable feeling that he is flying too far
West. What pilot is there indeed who has not many times
experienced such unpleasant sensation? Few things in the
air can create greater anxiety. Wisely, however, he sticks
to his compass course, and the next minute he is rewarded
by the sight of the lake, though indeed he now sees that the
direction of his travel will not take him over it, as should
be the case if he were flying over the shortest route to his
destination. He must have slightly miscalculated the velocity
or direction of the side-wind.
``About ten degrees off,'' he mutters, and, using the
Rudder, corrects his course accordingly.
Now he feels happier and that he is well on his way.
The gusts, too, have ceased to trouble him as, at this altitude,
they are not nearly so bad as they were near the ground
the broken surface of which does much to produce them;
and sometimes for miles he makes but a movement or two
of the controls.
The clouds just above race by with dizzy and uniform
speed; the country below slowly unrolls, and the steady
drone of the Engine is almost hypnotic in effect. ``Sleep,
sleep, sleep,'' it insidiously suggests. ``Listen to me and
watch the clouds; there's nothing else to do. Dream,
dream, dream of speeding through space for ever, and ever,
and ever; and rest, rest, rest to the sound of my rhythmical
hum. Droning on and on, nothing whatever matters. All
things now are merged into speed through space and a sleepy
monotonous d-d-r-r-o-o-n-n-e - - - - -.'' But the Pilot pulls
himself together with a start and peers far ahead in search
of the next landmark. This time it is a little country town.
red-roofed his map tells him, and roughly of cruciform shape;
and, sure enough, there in the right direction are the broken
outlines of a few red roofs peeping out from between the trees.
Another minute and he can see this little town, a fairy
town it appears, nestling down between the hills with its
red roofs and picturesque shape, a glowing and lovely contrast
with the dark green of the surrounding moors.
So extraordinarily clean and tidy it looks from such a
height, and laid out in such orderly fashion with perfectly
defined squares, parks, avenues, and public buildings, it
indeed appears hardly real, but rather as if it has this very
day materialized from some delightful children's book!
Every city and town you must know has its distinct
individuality to the Pilot's eye. Some are not fairy places
at all, but great dark ugly blots upon the fair countryside,
and with tall shafts belching forth murky columns of smoke
to defile clean space. Others, melancholy-looking masses
of grey, slate-roofed houses, are always sad and dispirited;
never welcoming the glad sunshine, but ever calling for leaden
skies and a weeping Heaven. Others again, little coquettes
with village green, white palings everywhere, bright gravel
roads, and an irrepressible air of brightness and gaiety.
Then there are the rivers, silvery streaks peacefully
winding far, far away to the distant horizon; they and the
lakes the finest landmarks the Pilot can have. And the
forests. How can I describe them? The trees cannot be
seen separately, but merge altogether into enormous irregular
dark green masses sprawling over the country, and sometimes
with great ungainly arms half encircling some town or village;
and the wind passing over the foliage at times gives the forest
an almost living appearance, as of some great dragon of olden
times rousing itself from slumber to devour the peaceful
villages which its arms encircle.
And the Pilot and Observer fly on and on, seeing these
things and many others which baffle my poor skill to describe--
things, dear Reader, that you shall see, and poets sing of,
and great artists paint in the days to come when the Designer
has captured Efficiency. Then, and the time is near, shall
you see this beautiful world as you have never seen it before,
the garden it is, the peace it breathes, and the wonder of it.
The Pilot, flying on, is now anxiously looking for the
railway line which midway on his journey should point
the course. Ah! There it is at last, but suddenly (and
the map at fault) it plunges into the earth! Well the writer
remembers when that happened to him on a long 'crosscountry
flight in the early days of aviation. Anxiously
he wondered ``Are tunnels always straight?'' and with what
relief, keeping on a straight course, he picked up the line
again some three miles farther on!
Now at last the Pilot sees the sea, just a streak on the
north-eastern horizon, and he knows that his flight is twothirds
over. Indeed, he should have seen it before, but
the air is none too clear, and he is not yet able to discern
the river which soon should cross his path. As he swiftly
speeds on the air becomes denser and denser with what he
fears must be the beginning of a sea-fog, perhaps drifting
inland along the course of the river. Now does he feel real
anxiety, for it is the DUTY of a Pilot to fear fog, his deadliest
enemy. Fog not only hides the landmarks by which he
keeps his course, but makes the control of the Aeroplane
a matter of the greatest difficulty. He may not realize
it, but, in keeping his machine on an even keel, he is
unconsciously balancing it against the horizon, and with the
horizon gone he is lost indeed. Not only that, but it also
prevents him from choosing his landing-place, and the
chances are that, landing in a fog, he will smash into a tree,
hedge, or building, with disastrous results. The best and
boldest pilot 'wares a fog, and so this one, finding the
conditions becoming worse and yet worse, and being forced to
descend lower and lower in order to keep the earth within
view, wisely decides to choose a landing-place while there is
yet time to do so.
Throttling down the power of the engine he spirals downwards,
keenly observing the country below. There are
plenty of green fields to lure him, and his great object is to
avoid one in which the grass is long, for that would bring
his machine to a stop so suddenly as to turn it over; or one
of rough surface likely to break the under-carriage. Now
is perfect eyesight and a cool head indispensable. He sees
and decides upon a field and, knowing his job, he sticks to
that field with no change of mind to confuse him. It is none
too large, and gliding just over the trees and head on to the
wind he skilfully ``stalls'' his machine; that is, the speed
having decreased sufficiently to avoid such a manoeuvre
resulting in ascent, he, by means of the Elevator, gives the
Aeroplane as large an angle of incidence as possible. and the
undersides of the planes meeting the air at such a large
angle act as an air-brake, and the Aeroplane, skimming
over the ground, lessens its speed and finally stops just at
the farther end of the field.
Then, after driving the Aeroplane up to and under the
lee of the hedge, he stops the engine, and quickly lashing
the joy-stick fast in order to prevent the wind from blowing
the controlling surfaces about and possibly damaging them,
he hurriedly alights. Now running to the tail he lifts it up
on to his shoulder, for the wind has become rough indeed
and there is danger of the Aeroplane becoming unmanageable.
By this action he decreases the angle at which the planes
are inclined to the wind and so minimizes the latter's effect
upon them. Then to the Observer, ``Hurry up, old fellow,
and try to find some rope, wire, or anything with which to
picket the machine. The wind is rising and I shan't be able
to hold the 'bus steady for long. Don't forget the wirecutters.
They're in the tool kit.'' And the Observer rushes
off in frantic haste, before long triumphantly returning with
a long length of wire from a neighbouring fence. Blocking
up the tail with some debris at hand, they soon succeed,
with the aid of the wire, in stoutly picketing the Aeroplane
to the roots of the high hedge in front of it; done with much
care, too, so that the wire shall not fray the fabric or set up
dangerous bending-stresses in the woodwork. Their work
is not done yet, for the Observer remarking, ``I don't like
the look of this thick weather and rather fear a heavy rainstorm,''
the Pilot replies, ``Well, it's a fearful bore, but the
first rule of our game is never to take an unnecessary risk,
so out with the engine and body covers.''
Working with a will they soon have the engine and the
open part of the body which contains the seats, controls,
and instruments snugly housed with their waterproof covers,
and the Aeroplane is ready to weather the possible storm.
Says the Observer, ``I'm remarkably peckish, and methinks
I spy the towers of one of England's stately homes
showing themselves just beyond that wood, less than a
quarter of a mile away. What ho! for a raid. What do
you say?''
``All right, you cut along and I'll stop here, for the
Aeroplane must not be left alone. Get back as quickly as
possible.''
And the Observer trots off, leaving the Pilot filling his
pipe and anxiously scrutinizing the weather conditions.
Very thick it is now, but the day is yet young, and he has
hopes of the fog lifting sufficiently to enable the flight to be
resumed. A little impatiently he awaits the return of his
comrade, but with never a doubt of the result, for the hospitality
of the country house is proverbial among pilots!
What old hand among them is there who cannot instance
many a forced landing made pleasant by such hospitality?
Never too late or too early to help with food, petrol, oil,
tools, and assistants. Many a grateful thought has the
writer for such kind help given in the days before the war
(how long ago they seem!), when aeroplanes were still more
imperfect than they are now, and involuntary descents
often a part of 'cross-country flying.
Ah! those early days! How fresh and inspiring they
were! As one started off on one's first 'cross-country flight,
on a machine the first of its design, and with everything
yet to learn, and the wonders of the air yet to explore; then
the joy of accomplishment, the dreams of Efficiency, the
hard work and long hours better than leisure; and what a
field of endeavour--the realms of space to conquer! And
the battle still goes on with ever-increasing success. Who
is bold enough to say what its limits shall be?
So ruminates this Pilot-Designer, as he puffs at his pipe,
until his reverie is abruptly disturbed by the return of the
Observer.
``Wake up, you AIRMAN,'' the latter shouts. ``Here's
the very thing the doctor ordered! A basket of first-class
grub and something to keep the fog out, too.''
``Well, that's splendid, but don't call me newspaper
names or you'll spoil my appetite!''
Then, with hunger such as only flying can produce, they
appreciatively discuss their lunch, and with many a grateful
thought for the donors--and they talk shop. They can't
help it, and even golf is a poor second to flight talk. Says
the Pilot, who must have his grievance, ``Just observe
where I managed to stop the machine. Not twenty feet
from this hedge! A little more and we should have been
through it and into Kingdom Come! I stalled as well as
one could, but the tail touched the ground and so I could
not give the Aeroplane any larger angle of incidence. Could
I have given it a larger angle, then the planes would have
become a much more effective air-brake, and we should
have come to rest in a much shorter distance. It's all the
fault of the tail. There's hardly a type of Aeroplane in
existence in which the tail could not be raised several feet,
and that would make all the difference. High tails mean
a large angle of incidence when the machine touches ground
and, with enough angle, I'll guarantee to safely land the
fastest machine in a five-acre field. You can, I am sure,
imagine what a difference that would make where forced
landings are concerned!'' Then rapidly sketching in his
notebook, he shows the Observer the following illustration:
``That's very pretty,'' said the Observer, ``but how
about Mechanical Difficulties, and Efficiency in respect of
Flight? And, anyway, why hasn't such an obvious thing
been done already?''
``As regards the first part of your question I assure
you that there's nothing in it, and I'll prove it to you as
follows----''
``Oh! That's all right, old chap. I'll take your word
for it,'' hurriedly replies the Observer, whose soul isn't tuned
to a technical key.
``As regards the latter part of your inquiry,'' went on
the Pilot, a little nettled at having such a poor listener,
``it's very simple. Aeroplanes have `just growed' like
Topsy, and they consequently contain this and many another
relic of early day design when Aeroplanes were more or less
thrown together and anything was good enough that could
get off the ground.''
``By Jove,'' interrupts the Observer, ``I do believe the
fog is lifting. Hadn't we better get the engine and body
covers off, just in case it's really so?''
``I believe you're right. I am sure those hills over there
could not be seen a few minutes ago, and look--there's
sunshine over there. We'd better hurry up.''
Ten minutes' hard work and the covers are off, neatly
folded and stowed aboard; the picketing wires are cast adrift,
and the Pilot is once more in his seat. The Aeroplane has
been turned to face the other end of the field, and, the Observer
swinging round the propeller, the engine is awake
again and slowly ticking over. Quickly the Observer climbs
into his seat in front of the Pilot, and, the latter slightly
opening the throttle, the Aeroplane leisurely rolls over the
ground towards the other end of the field, from which the
ascent will be made.
Arriving there the Pilot turns the Aeroplane in order to
face the wind and thus secure a quick ``get-off.'' Then he
opens the throttle fully and the mighty voice of the Engine
roars out ``Now see me clear that hedge!'' and the Aeroplane
races forward at its minimum angle of incidence. Tail
up, and with ever-increasing speed, it rushes towards the
hedge under the lee of which it has lately been at rest; and
then, just as the Observer involuntarily pulls back an imaginary
``joy-stick,'' the Pilot moves the real one and places the
machine at its best climbing angle. Like a living thing it
responds, and instantly leaves the ground, clearing the hedge
like a--well, like an Aeroplane with an excellent margin of
lift. Upwards it climbs with even and powerful lift, and the
familiar scenes below again gladden the eyes of the Pilot.
Smaller and more and more squat grow the houses and hills;
more and more doll-like appear the fields which are clearly
outlined by the hedges; and soon the country below is easily
identified with the map. Now they can see the river before
them and a bay of the sea which must be crossed or skirted.
The fog still lingers along the course of the river and between
the hills, but is fast rolling away in grey, ghost-like masses.
Out to sea it obscures the horizon, making it difficult to be
sure where water ends and fog begins, and creating a strange,
rather weird effect by which ships at a certain distance appear
to be floating in space.
Now the Aeroplane is almost over the river, and the
next instant it suddenly drops into a ``hole in the air.''
With great suddenness it happens, and for some two hundred
feet it drops nose-down and tilted over sideways; but the
Pilot is prepared and has put his craft on an even keel in
less time than it takes to tell you about it; for well he knows
that he must expect such conditions when passing over a
shore or, indeed, any well-defined change in the composition
of the earth's surface. Especially is this so on a hot and
sunny day, for then the warm surface of the earth creates
columns of ascending air, the speed of the ascent depending
upon the composition of the surface. Sandy soil, for instance,
such as borders this river produces a quickly ascending
column of air, whereas water and forests have not such a
marked effect. Thus, when our Aeroplane passed over the
shore of the river, it suddenly lost the lift due to the ascending
air produced by the warm sandy soil, and it consequently
dropped just as if it had fallen into a hole.
Now the Aeroplane is over the bay and, the sea being
calm, the Pilot looks down, down through the water, and
clearly sees the bottom, hundreds of feet below the surface.
Down through the reflection of the blue sky and clouds,
and one might think that is all, but it isn't. Only those
who fly know the beauties of the sea as viewed from above;
its dappled pearly tints; its soft dark blue shadows; the beautiful
contrasts of unusual shades of colour which are always
differing and shifting with the changing sunshine and the
ever moving position of the aerial observer. Ah! for some
better pen than mine to describe these things! One with
glowing words and a magic rhythm to express the wonders
of the air and the beauty of the garden beneath--the immensity
of the sea--the sense of space and of one's littleness
there--the realization of the Power moving the multitudes
below--the exaltation of spirit altitude produces--the joy of
speed. A new world of sensation!
Now the bay is almost crossed and the Aerodrome at B
can be distinguished.
On the Aerodrome is a little crowd waiting and watching
for the arrival of the Aeroplane, for it is of a new and improved
type and its first 'cross-country performance is of
keen interest to these men; men who really know something
about flight.
There is the Squadron Commander who has done some
real flying in his time; several well-seasoned Flight-
Commanders; a dozen or more Flight-Lieutenants; a
knowledgeable Flight-Sergeant; a number of Air Mechanics,
and, a little on one side and almost unnoticed, the
Designer.
``I hope they are all right,'' said someone, ``and that
they haven't had difficulties with the fog. It rolled up very
quickly, you know.''
``Never fear,'' remarked a Flight-Commander. ``I know
the Pilot well and he's a good 'un; far too good to carry on
into a fog.''
``They say the machine is really something out of the
ordinary,'' said another, ``and that, for once, the Designer
has been allowed full play; that he hasn't been forced to
unduly standardize ribs, spars, struts, etc., and has more
or less had his own way. I wonder who he is. It seems
strange we hear so little of him.''
``Ah! my boy. You do a bit more flying and you'll
discover that things are not always as they appear from a
distance!''
``There she is, sir!'' cries the Flight-Sergeant. ``Just a
speck over the silvery corner of that cloud.''
A tiny speck it looks, some six miles distant and three
thousand feet high; but, racing along, it rapidly appears
larger and soon its outlines can be traced and the sunlight
be seen playing upon the whirling propeller.
Now the distant drone of the engine can be heard,
but not for long, for suddenly it ceases and, the nose of
the Aeroplane sinking, the craft commences gliding downwards.
``Surely too far away,'' says a subaltern. It will be
a wonderful machine if, from that distance and height, it
can glide into the Aerodrome.'' And more than one express
the opinion that it cannot be done; but the Designer smiles
to himself, yet with a little anxiety, for his reputation is
at stake, and Efficiency, the main reward he desires, is perhaps,
or perhaps not, at last within his grasp!
Swiftly the machine glides downwards towards them,
and it can now be seen how surprisingly little it is affected
by the rough weather and gusts; so much so that a little
chorus of approval is heard.
``Jolly good gliding angle,'' says someone; and another,
``Beautifully quick controls, what?'' and from yet another,
``By Jove! The Pilot must be sure of the machine. Look,
he's stopped the engine entirely.''
Then the Aeroplane with noiseless engine glides over
the boundary of the Aerodrome, and, with just a soft soughing
sound from the air it cleaves, lands gently not fifty yards from
the onlookers.
``Glad to see you,'' says the Squadron Commander to
the Pilot. ``How do you like the machine?'' And the
Pilot replies:
``I never want a better one, sir. It almost flies itself!''
And the Designer turns his face homewards and towards
his beloved drawing-office; well satisfied, but still dreaming
dreams of the future and . . . looking far ahead whom should
he see but Efficiency at last coming towards him! And to
him she is all things. In her hair is the morning sunshine;
her eyes hold the blue of the sky, and on her cheeks is the
pearly tint of the clouds as seen from above. The passion
of speed, the lure of space, the sense of power, and the
wonder of the future . . . all these things she holds for him.
``Ah!'' he cries. ``You'll never leave me now, when
at last there is no one between us?''
And Efficiency, smiling and blushing, but practical as
ever, says:
``And you will never throw those Compromises in my
face?''
``My dear, I love you for them! Haven't they been
my life ever since I began striving for you ten long years
ago?''
And so they walked off very happily, arm-in-arm together;
and if this hasn't bored you and you'd like some more of the
same sort of thing, I'd just love to tell you some day of the
wonderful things they accomplish together, and of what
they dream the future holds in store.
And that's the end of the Prologue.
CHAPTER I
FLIGHT
Air has weight (about 13 cubic feet = 1 lb.), inertia, and
momentum. It therefore obeys Newton's laws[[14]] and resists
movement. It is that resistance or reaction which makes
flight possible.
[[14]] See Newton's laws in the Glossary at the end of the book.
Flight is secured by driving through the air a surface[[15]]
inclined upwards and towards the direction of motion.
[[15]] See ``Aerofoil'' in the Glossary.
S = Side view of surface.
M = Direction of motion.
CHORD.--The Chord is, for practical purposes, taken to
be a straight line from the leading edge of the surface to its
trailing edge.
N = A line through the surface starting from its trailing
edge. The position of this line, which I call the Neutral
Lift Line, is found by means of wind-tunnel research, and it
varies with differences in the camber (curvature) of surfaces.
In order to secure flight, the inclination of the surface must
be such that the neutral lift line makes an angle with and
ABOVE the line of motion. If it is coincident with M, there is
no lift. If it makes an angle with M and BELOW it, then
there is a pressure tending to force the surface down.
I = Angle of Incidence. This angle is generally defined
as the angle the chord makes with the direction of motion,
but that is a bad definition, as it leads to misconception.
The angle of incidence is best described as the angle the
neutral lift line makes with the direction of motion relative
to the air. You will, however, find that in nearly all rigging
specifications the angle of incidence is taken to mean the
angle the chord makes with a line parallel to the propeller
thrust. This is necessary from the point of view of the
practical mechanic who has to rig the aeroplane, for he could
not find the neutral lift line, whereas he can easily find the
chord. Again, he would certainly be in doubt as to ``the
direction of motion relative to the air,'' whereas he can
easily find a line parallel to the propeller thrust. It is a
pity, however, that these practical considerations have
resulted in a bad definition of the angle of incidence becoming
prevalent, a consequence of which has been the widespread
fallacy that flight may be secured with a negative
inclination of the surface. Flight may conceivably be
secured with a negative angle of chord, but never with a
negative inclination of the surface. All this is only applicable
to cambered surfaces. In the case of flat surfaces the neutral
lift line coincides with the chord and the definition I have
criticised adversely is then applicable. Flat lifting surfaces
are, however, never used.
The surface acts upon the air in the following manner:
As the bottom of the surface meets the air, it compresses
it and accelerates it DOWNWARDS. As a result of this definite
action there is, of course, an equal and opposite reaction
UPWARDS.
The top surface, in moving forward, tends to leave the
air behind it, thus creating a semi-vacuum or rarefied area
over the top of the surface. Consequently the pressure of
air on the top of the surface is decreased, thus assisting the
reaction below to lift the surface UPWARDS.
The reaction increases approximately as the square of
the velocity. It is the result of (1) the mass of air engaged,
and (2) the velocity and consequent force with which the
surface engages the air. If the reaction was produced by
only one of those factors it would increase in direct proportion
to the velocity, but, since it is the product of both factors,
it increases as V<2S>.
Approximately three-fifths of the reaction is due to the
decrease of density (and consequent decrease of downward
pressure) on the top of the surface; and only some twofifths
is due to the upward reaction secured by the action
of the bottom surface upon the air. A practical point in
respect of this is that, in the event of the fabric covering the
surface getting into bad condition, it is more likely to strip
off the top than off the bottom.
The direction of the reaction is approximately at rightangles
to the chord of the surface, as illustrated above; and
it is, in considering flight, convenient to divide it into two
component parts or values, thus:
1. The vertical component of the reaction, i.e., Lift,
which is opposed to Gravity, i.e., the weight of the
aeroplane.
2. The horizontal component, i.e., Drift (sometimes
called Resistance), to which is opposed the thrust of the
propeller.
The direction of the reaction is, of course, the resultant
of the forces Lift and Drift.
The Lift is the useful part of the reaction, for it lifts the
weight of the aeroplane.
The Drift is the villain of the piece, and must be overcome
by the Thrust in order to secure the necessary velocity to
produce the requisite Lift for flight.
DRIFT.--The drift of the whole aeroplane (we have considered
only the lifting surface heretofore) may be conveniently
divided into three parts, as follows:
Active Drift, which is the drift produced by the lifting
surfaces.
Passive Drift, which is the drift produced by all the rest
of the aeroplane--the struts, wires, fuselage, under-carriage,
etc., all of which is known as ``detrimental surface.''
Skin Friction, which is the drift produced by the friction
of the air with roughnesses of surface. The latter is practically
negligible having regard to the smooth surface of the
modern aeroplane, and its comparatively slow velocity
compared with, for instance, the velocity of a propeller
blade.
LIFT-DRIFT RATIO.--The proportion of lift to drift is
known as the lift-drift ratio, and is of paramount importance,
for it expresses the efficiency of the aeroplane (as distinct
from engine and propeller). A knowledge of the factors
governing the lift-drift ratio is, as will be seen later, an
absolute necessity to anyone responsible for the rigging of an
aeroplane, and the maintenance of it in an efficient and safe
condition.
Those factors are as follows:
1. Velocity.--The greater the velocity the greater the
proportion of drift to lift, and consequently the
less the efficiency. Considering the lifting surfaces
alone, both the lift and the (active) drift, being
component parts of the reaction, increase as the
square of the velocity, and the efficiency remains
the same at all speeds. But, considering the
whole aeroplane, we must remember the passive
drift. It also increases as the square of the
velocity (with no attendant lift), and, adding
itself to the active drift, results in increasing
the proportion of total drift (active + passive) to
lift.
But for the increase in passive drift the efficiency
of the aeroplane would not fall with increasing
velocity, and it would be possible, by doubling
the thrust, to approximately double the speed
or lift--a happy state of affairs which can never
be, but which we may, in a measure, approach
by doing everything possible to diminish the passive
drift.
Every effort is then made to decrease it by
``stream-lining,'' i.e., by giving all ``detrimental''
parts of the aeroplane a form by which they will
pass through the air with the least possible drift.
Even the wires bracing the aeroplane together are,
in many cases, stream-lined, and with a markedly
good effect upon the lift-drift ratio. In the case
of a certain well-known type of aeroplane the
replacing of the ordinary wires by stream-lined
wires added over five miles an hour to the flight
speed.
Head-resistance is a term often applied to passive
drift, but it is apt to convey a wrong impression,
as the drift is not nearly so much the result of
the head or forward part of struts, wires, etc.,
as it is of the rarefied area behind.
Above is illustrated the flow of air round two
objects moving in the direction of the arrow M.
In the case of A, you will note that the rarefied
area DD is of very considerable extent; whereas
in the case of B, the air flows round it in such a
way as to meet very closely to the rear of the
object, thus DECREASING DD.
The greater the rarefied area DD. then, the less
the density, and, consequently, the less the pressure
of air upon the rear of the object. The less such
pressure, then, the better is head-resistance D
able to get its work in, and the more thrust will
be required to overcome it.
The ``fineness'' of the stream-line shape, i.e.,
the proportion of length to width, is determined
by the velocity--the greater the velocity, the
greater the fineness. The best degree of fineness
for any given velocity is found by means of windtunnel
research.
The practical application of all this is, from a
rigging point of view, the importance of adjusting
all stream-line parts to be dead-on in the line of
flight, but more of that later on.
2. Angle of Incidence.--The most efficient angle of
incidence varies with the thrust at the disposal
of the designer, the weight to be carried, and the
climb-velocity ratio desired.
The best angles of incidence for these varying
factors are found by means of wind-tunnel research
and practical trial and error. Generally
speaking, the greater the velocity the smaller
should be the angle of incidence, in order to preserve
a clean, stream-line shape of rarefied area
and freedom from eddies. Should the angle be
too great for the velocity, then the rarefied area
becomes of irregular shape with attendant turbulent
eddies. Such eddies possess no lift value,
and since it has taken power to produce them,
they represent drift and adversely affect the liftdrift
ratio.
From a rigging point of view, one must presume
that every standard aeroplane has its lifting
surface set at the most efficient angle, and the
practical application of all this is in taking the
greatest possible care to rig the surface at the
correct angle and to maintain it at such angle.
Any deviation will adversely affect the lift-drift
ratio, i.e., the efficiency.
3. Camber.--(Refer to the second illustration in this
chapter.) The lifting surfaces are cambered, i.e.,
curved, in order to decrease the horizontal component
of the reaction, i.e., the drift.
The bottom camber: If the bottom of the surface
was flat, every particle of air meeting it would do
so with a shock, and such shock would produce a
very considerable horizontal reaction or drift. By
curving it such shock is diminished, and the curve
should be such as to produce a uniform (not
necessarily constant) acceleration and compression
of the air from the leading edge to the trailing
edge. Any unevenness in the acceleration and
compression of the air produces drift.
The top camber: If this was flat it would produce
a rarefied area of irregular shape. I have already
explained the bad effect this has upon the liftdrift
ratio. The top surface is then curved to
produce a rarefied area the shape of which shall
be as stream-line and free from attendant eddies
as possible.
The camber varies with the angle of incidence,
the velocity, and the thickness of the surface.
Generally speaking, the greater the velocity, the
less the camber and angle of incidence. With
infinite velocity the surface would be set at no
angle of incidence (the neutral lift line coincident
with the direction of motion relative to the air),
and would be, top and bottom, of pure streamline
form--i.e., of infinite fineness. This is, of
course, carrying theory to absurdity as the surface
would then cease to exist.
The best cambers for varying velocities, angles
of incidence, and thicknesses of surface, are found
by means of wind-tunnel research. The practical
application of all this is in taking the greatest
care to prevent the surface from becoming distorted
and thus spoiling the camber and consequently
the lift-drift ratio.
4. Aspect Ratio.--This is the proportion of span to
chord. Thus, if the span is, for instance, 50 feet
and the chord 5 feet, the surface would be said to
have an aspect ratio of 10 to 1.
For A GIVEN VELOCITY and A GIVEN AREA of surface,
the greater the aspect ratio, the greater the reaction.
It is obvious, I think, that the greater
the span, the greater the mass of air engaged,
and, as already explained, the reaction is partly
the result of the mass of air engaged.
Not only that, but, PROVIDED the chord is not
decreased to an extent making it impossible to
secure the best camber owing to the thickness of
the surface, the greater the aspect ratio, the better
the lift-drift ratio. The reason of this is rather
obscure. It is sometimes advanced that it is
owing to the ``spill'' of air from under the wingtips.
With a high aspect ratio the chord is less
than would otherwise be the case. Less chord
results in smaller wing-tips and consequently less
``spill.'' This, however, appears to be a rather
inadequate reason for the high aspect ratio producing
the high lift-drift ratio. Other reasons
are also advanced, but they are of such a contentious
nature I do not think it well to go into them
here. They are of interest to designers, but this
is written for the practical pilot and rigger.
5. Stagger.--This is the advancement of the top surface
relative to the bottom surface, and is not, of course,
applicable to a single surface, i.e., a monoplane.
In the case of a biplane having no stagger, there
will be ``interference'' and consequent loss of
Efficiency unless the gap between the top and bottom
surfaces is equal to not less than 1 1/2 times the
chord. If less than that, the air engaged by the
bottom of the top surface will have a tendency
to be drawn into the rarefied area over the top
of the bottom surface, with the result that the
surfaces will not secure as good a reaction as would
otherwise be the case.
It is not practicable to have a gap of much
more than a distance equal to the chord, owing
to the drift produced by the great length of struts
and wires such a large gap would necessitate.
By staggering the top surface forward, however,
it is removed from the action of the lower surface
and engages undisturbed air, with the result that
the efficiency can in this way be increased by
about 5 per cent. Theoretically the top plane
should be staggered forward for a distance equal
to about 30 per cent. of the chord, the exact
distance depending upon the velocity and angle
of incidence; but this is not always possible to
arrange in designing an aeroplane, owing to difficulties
of balance, desired position, and view of
pilot, observer, etc.
6. Horizontal Equivalent.--The vertical component of
the reaction, i.e., lift, varies as the horizontal
equivalent (H.E.) of the surface, but the drift
remains the same. Then it follows that if H.E. grows
less, the ratio of lift to drift must do the same.
A, B, and C are front views of three surfaces.
A has its full H.E., and therefore, from the point
of view from which we are at the moment considering
efficiency, it has its best lift-drift ratio.
B and C both possess the same surface as A,
but one is inclined upwards from its centre and
the other is straight but tilted. For these reasons
their H.E.'s are, as illustrated, less than in the
case of A. That means less vertical lift, and,
the drift remaining the same (for there is the
same amount of surface as in A to produce it),
the lift-drift ratio falls.
THE MARGIN OF POWER is the power available above
that necessary to maintain horizontal flight.
THE MARGIN OF LIFT is the height an aeroplane can gain
in a given time and starting from a given altitude.
As an example, thus: 1,000 feet the first minute,
and starting from an altitude of 500 feet above
sea-level.
The margin of lift decreases with altitude, owing
to the decrease in the density of the air, which
adversely affects the engine. Provided the engine
maintained its impulse with altitude, then, if we
ignore the problem of the propeller, which I will
go into later on, the margin of lift would not
disappear. Moreover, greater velocity for a given
power would be secured at a greater altitude, owing
to the decreased density of air to be overcome.
After reading that, you may like to light your pipe
and indulge in dreams of the wonderful possibilities
which may become realities if some brilliant genius
shows us some day how to secure a constant power
with increasing altitude. I am afraid, however,
that will always remain impossible; but it is probable
that some very interesting steps may be taken in
that direction.
THE MINIMUM ANGLE OF INCIDENCE is the smallest
angle at which, for a given power, surface (including
detrimental surface), and weight, horizontal flight
can be maintained.
THE MAXIMUM ANGLE OF INCIDENCE is the greatest
angle at which, for a given power, surface (including
detrimental surface), and weight, horizontal flight
can be maintained.
THE OPTIMUM ANGLE OF INCIDENCE is the angle at
which the lift-drift ratio is highest. In modern
aeroplanes it is that angle of incidence possessed by the
surface when the axis of the propeller is horizontal.
THE BEST CLIMBING ANGLE is approximately half-way
between the maximum and the optimum angles.
All present-day aeroplanes are a compromise between
Climb and horizontal Velocity. We will compare
the essentials for two aeroplanes, one designed for
maximum climb, and the other for maximum velocity.
ESSENTIALS FOR MAXIMUM CLIMB:
1. Low velocity, in order to secure the best lift-drift
ratio.
2. Having a low velocity, a large surface will be
necessary in order to engage the necessary mass
of air to secure the requisite lift.
3. Since (1) such a climbing machine will move
along an upward sloping path, and (2) will climb
with its propeller thrust horizontal, then a large
angle relative to the direction of the thrust will be
necessary in order to secure the requisite angle
relative to the direction of motion.
The propeller thrust should be always horizontal, because
the most efficient flying-machine (having regard to climb OR
velocity) has, so far, been found to be an arrangement of an
inclined surface driven by a HORIZONTAL thrust--the surface
lifting the weight, and the thrust overcoming the drift.
This is, in practice, a far more efficient arrangement than
the helicopter, i.e., the air-screw revolving about a vertical
axis and producing a thrust opposed to gravity. If, when
climbing, the propeller thrust is at such an angle as to tend
to haul the aeroplane upwards, then it is, in a measure,
acting as a helicopter, and that means inefficiency. The
reason of a helicopter being inefficient in practice is due to
the fact that, owing to mechanical difficulties, it is impossible
to construct within a reasonable weight an air-screw of the
requisite dimensions. That being so, it would be necessary,
in order to absorb the power of the engine, to revolve the
comparatively small-surfaced air screw at an immensely
greater velocity than that of the aeroplane's surface. As
already explained, the lift-drift ratio falls with velocity on
account of the increase in passive drift. This applies to a
blade of a propeller or air-screw, which is nothing but a
revolving surface set at angle of incidence, and which it is
impossible to construct without a good deal of detrimental
surface near the central boss.
4. The velocity being low, then it follows that for
that reason also the angle of incidence should be
comparatively large.
5. Camber.--Since such an aeroplane would be of
low velocity, and therefore possess a large angle
of incidence, a large camber would be necessary.
Let us now consider the essentials for an aeroplane of
maximum velocity for its power, and possessing merely
enough lift to get off the ground, but no margin of lift.
1. Comparatively HIGH VELOCITY.
2. A comparatively SMALL SURFACE, because, being
of greater velocity than the maximum climber,
a greater mass of air will be engaged for a given
surface and time, and therefore a smaller surface
will be sufficient to secure the requisit lift.
3. A small angle relative to the propeller thrust, since
the latter coincides with the direction of motion.
4. A comparatively small angle of incidence by reason
of the high velocity.
5. A comparatively small camber follows as a result
of the small angle of incidence.
SUMMARY.
Essentials for Maximum Essentials for Maximum
Climb. Velocity
1. Low velocity. High velocity.
2. Large surface. Small surface.
3. Large angle relative to Small angle relative to
propeller thrust. propeller thrust.
4. Large angle relative to Small angle relative to direction
direction of motion. of motion.
5. Large camber. Small camber.
It is mechanically impossible to construct an aeroplane
of reasonable weight of which it would be possible to very
the above opposing essentials. Therefore, all aeroplanes
are designed as a compromise between Climb and Velocity.
As a rule aeroplanes are designed to have at low altitude
a slight margin of lift when the propeller thrust is horizontal.
ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE
DESIGNED AS A COMPROMISE BETWEEN
VELOCITY AND CLIMB, AND POSSESSING A SLIGHT MARGIN OF LIFT AT A
LOW
ALTITUDE AND WHEN THE THRUST IS HORIZONTAL
MINIMUM ANGLE.
This gives the greatest velocity
during horizontal flight at a low
altitude. Greater velocity would
be secured if the surface, angle,
and camber were smaller and designed
to just maintain horizontal
flight with a horizontal thrust.
Also, in such case, the propeller
would not be thrusting downwards,
but along a horizontal line
which is obviously a more efficient
arrangement if we regard
the aeroplane merely from one
point of view, i.e., either with
reference to velocity OR climb.
OPTIMUM ANGLE
(Thrust horizontal)
The velocity is less than at the
smaller minimum angle, and, as
aeroplanes are designed to-day, the
area and angle of incidence of the
surface is such as to secure a
slight ascent at a low altitude. The
camber of the surface is designed
for this angle of incidence and
velocity. The lift-drift ratio is
best at this angle.
BEST CLIMBING ANGLE
The velocity is now still less by
reason of the increased angle
producing increase of drift. Less
velocity at A GIVEN ANGLE produces
less lift, but the increased angle
more or less offsets the loss of lift
due to the decreased velocity, and
in addition, the thrust is now hauling
the aeroplane upwards.
MAXIMUM ANGLE
The greater angle has now produced
so much drift as to lessen
the velocity to a point where the
combined lifts from the surface
and from the thrust are only just
able to maintain horizontal flight.
Any greater angle will result in a
still lower lift-drift ratio. The lift
will then become less than the
weight and the aeroplane will
consequently fall. Such a fall is
known as ``stalling'' or ``pancaking.''
NOTE.--The golden rule for beginners: Never exceed the Best Climbing Angle.
Always maintain the flying speed of the aeroplane.
By this means, when the altitude is reached where the margin
of lift disappears (on account of loss of engine power), and
which is, consequently, the altitude where it is just possible
to maintain horizontal flight, the aeroplane is flying with
its thrust horizontal and with maximum efficiency (as distinct
from engine and propeller efficiency).
The margin of lift at low altitude, and when the thrust
is horizontal, should then be such that the higher altitude at
which the margin of lift is lost is that altitude at which most
of the aeroplane's horizontal flight work is done. That
ensures maximum velocity when most required.
Unfortunately, where aeroplanes designed for fighting
are concerned, the altitude where most of the work is done
is that at which both maximum velocity and maximum
margin of lift for power are required.
Perhaps some day a brilliant inventor will design an
aeroplane of reasonable weight and drift of which it will be
possible for the pilot to vary at will the above-mentioned
opposing essentials. Then we shall get maximum velocity,
or maximum margin of lift, for power as required. Until
then the design of the aeroplane must remain a compromise
between Velocity and Climb.
CHAPTER II
STABILITY AND CONTROL
STABILITY is a condition whereby an object disturbed
has a natural tendency to return to its first and normal
position. Example: a weight suspended by a cord.
INSTABILITY is a condition whereby an object disturbed
has a natural tendency to move as far as possible away from
its first position, with no tendency to return. Example:
a stick balanced vertically upon your finger.
NEUTRAL INSTABILITY is a condition whereby an object
disturbed has no tendency to move farther than displaced
by the force of the disturbance, and no tendency to return
to its first position.
In order that an aeroplane may be reasonably controllable,
it is necessary for it to possess some degree of stability
longitudinally, laterally, and directionally.
LONGITUDINAL STABILITY in an aeroplane is its stability
about an axis transverse to the direction of normal horizontal
flight, and without which it would pitch and toss.
LATERAL STABILITY is its stability about its longitudinal
axis, and without which it would roll sideways.
DIRECTIONAL STABILITY is its stability about its vertical
axis, and without which it would have no tendency to keep
its course.
For such directional stability to exist there must be,
in effect,[[16]] more ``keel-surface'' behind the vertical axis
than there is in front of it. By keel-surface I mean everything
to be seen when looking at an aeroplane from the side
of it--the sides of the body, undercarriage, struts, wires, etc.
The same thing applies to a weathercock. You know what
would happen if there was insufficient keel-surface behind
the vertical axis upon which it is pivoted. It would turn
off its proper course, which is opposite to the direction of
the wind. It is very much the same in the case of an aeroplane.
[[16]] ``In effect'' because, although there may be actually the greatest proportion
of keel-surface In front of the vertical axis, such surface may be much nearer to
the axis than is the keel-surface towards the tail. The latter may then be actually
less than the surface in front, but, being farther from the axis, it has a greater
leverage, and consequently is greater in effect than the surface in front.
The above illustration represents an aeroplane (directionally
stable) flying along the course B. A gust striking it
as indicated acts upon the greater proportion of keel-surface
behind the turning axis and throws it into the new course.
It does not, however, travel along the new course, owing to
its momentum in the direction B. It travels, as long as
such momentum lasts, in a direction which is the resultant
of the two forces Thrust and Momentum. But the centre
line of the aeroplane is pointing in the direction of the new
course. Therefore its attitude, relative to the direction of
motion, is more or less sideways, and it consequently receives
an air pressure in the direction C. Such pressure, acting
upon the keel-surface, presses the tail back towards its first
position in which the aeroplane is upon its course B.
What I have described is continually going on during
flight, but in a well-designed aeroplane such stabilizing
movements are, most of the time, so slight as to be imperceptible
to the pilot.
If an aeroplane was not stabilized in this way, it would
not only be continually trying to leave its course, but it would
also possess a dangerous tendency to ``nose away'' from the
direction of the side gusts. In such case the gust shown in
the above illustration would turn the aeroplane round the
opposite way a very considerable distance; and the right
wing, being on the outside of the turn, would travel with
greater velocity than the left wing. Increased velocity
means increased lift; and so, the right wing lifting, the
aeroplane would turn over sideways very quickly.
LONGITUDINAL STABILITY.--Flat surfaces are longitudinally
stable owing to the fact that with decreasing angles of
incidence the centre line of pressure (C.P.) moves forward.
The C.P. is a line taken across the surface, transverse
to the direction of motion, and about which all the air forces
may be said to balance, or through which they may be said
to act.
Imagine A to be a flat surface, attitude vertical, travelling
through the air in the direction of motion M. Its C.P. is
then obviously along the exact centre line of the surface
as illustrated.
In B, C, and D the surfaces are shown with angles of
incidence decreasing to nothing, and you will note that the
C.P. moves forward with the decreasing angle.
Now, should some gust or eddy tend to make the surface
decrease the angle, i.e., dive, then the C.P. moves forward
and pushes the front of the surface up. Should the surface
tend to assume too large an angle, then the reverse
happens--the C.P. moves back and pushes the rear of the
surface up.
Flat surfaces are, then, theoretically stable longitudinally.
They are not, however, used, on account of their poor
lift-drift ratio.
As already explained, cambered surfaces are used, and
these are longitudinally unstable at those angles of incidence
producing a reasonable lift-drift ratio, i.e., at angles below:
about 12 degrees.
A is a cambered surface, attitude approximately vertical,
moving through the air in the direction M. Obviously the C. P.
coincides with the transverse centre line of the surface.
With decreasing angles, down to angles of about 30 degrees,
the C.P. moves forward as in the case of flat surfaces (see B),
but angles above 30 degrees do not interest us, since they produce
a very low ratio of lift to drift.
Below angles of about 30 degrees (see C) the dipping front part
of the surface assumes a negative angle of incidence resulting
in the DOWNWARD air pressure D, and the more the angle of
incidence is decreased, the greater such negative angle and its
resultant pressure D. Since the C.P. is the resultant of all
the air forces, its position is naturally affected by D, which
causes it to move backwards. Now, should some gust or
eddy tend to make the surface decrease its angle of incidence,
i.e., dive, then the C.P. moves backwards, and, pushing up
the rear of the surface, causes it to dive the more. Should
the surface tend to assume too large an angle, then the reverse
happens; the pressure D decreases, with the result
that C.P. moves forward and pushes up the front of the surface,
thus increasing the angle still further, the final result
being a ``tail-slide.''
It is therefore necessary to find a means of stabilizing
the naturally unstable cambered surface. This is usually
secured by means of a stabilizing surface fixed some distance
in the rear of the main surface, and it is a necessary condition
that the neutral lift lines of the two surfaces, when projected
to meet each other, make a dihedral angle. In other words,
the rear stabilizing surface must have a lesser angle of
incidence than the main surface--certainly not more than
one-third of that of the main surface. This is known as the
longitudinal dihedral.
I may add that the tail-plane is sometimes mounted upon
the aeroplane at the same angle as the main surface, but,
in such cases, it attacks air which has received a downward
deflection from the main surface, thus:
{illust.}
The angle at which the tail surface attacks the air (the.
angle of incidence) is therefore less than the angle of incidence
of the main surface.
I will now, by means of the following illustration, try
to explain how the longitudinal dihedral secures stability:
First, imagine the aeroplane travelling in the direction
of motion, which coincides with the direction of thrust T.
The weight is, of course, balanced about a C.P., the resultant
of the C.P. of the main surface and the C.P. of the stabilizing
surface. For the sake of illustration, the stabilizing surface
has been given an angle of incidence, and therefore has a
lift and C.P. In practice the stabilizer is often set at no
angle of incidence. In such case the proposition remains
the same, but it is, perhaps, a little easier to illustrate it
as above.
Now, we will suppose that a gust or eddy throws the
machine into the lower position. It no longer travels in
the direction of T, since the momentum in the old direction
pulls it off that course. M is now the resultant of the Thrust
and the Momentum, and you will note that this results in a
decrease in the angle our old friend the neutral lift line makes
with M, i.e., a decrease in the angle of incidence and therefore
a decrease in lift.
We will suppose that this decrease is 2 degrees. Such decrease
applies to both main surface and stabilizer, since both are
fixed rigidly to the aeroplane.
The main surface, which had 12 degrees angle, has now only
10 degrees, i.e., a loss of ONE-SIXTH.
The stabilizer, which had 4 degrees angle, has now only 2 degrees,
i.e., a loss of ONE-HALF.
The latter has therefore lost a greater PROPORTION of its
angle of incidence, and consequently its lift, than has the
main surface. It must then fall relative to the main surface.
The tail falling, the aeroplane then assumes its first position,
though at a slightly less altitude.
Should a gust throw the nose of the aeroplane up, then
the reverse happens. Both main surface and stabilizer
increase their angles of incidence in the same amount, but
the angle, and therefore the lift, of the stabilizer increases
in greater proportion than does the lift of the main surface,
with the result that it lifts the tail. The aeroplane then
assumes its first position, though at a slightly greater
altitude.
Do not fall into the widespread error that the angle of
incidence varies as the angle of the aeroplane to the horizontal.
It varies with such angle, but not as anything approaching it.
Remember that the stabilizing effect of the longitudinal
dihedral lasts only as long as there is momentum in the direction
of the first course.
These stabilizing movements are taking place all the
time, even though imperceptible to the pilot.
Aeroplanes have, in the past, been built with a stabilizing
surface in front of the main surface instead of at the rear of
it. In such design the main surface (which is then the tail
surface as well as the principal lifting surface) must be set
at a less angle than the forward stabilizing surface, in order
to secure a longitudinal dihedral. The defect of such design
lies in the fact that the main surface must have a certain angle
to lift the weight--say 5 degrees. Then, in order to secure a
sufficiency of longitudinal stability, it is necessary to set the
forward stabilizer at about 15 degrees. Such a large angle of incidence
results in a very poor lift-drift ratio (and consequently great
loss of efficiency), except at very low velocities compared with
the speed of modern aeroplanes. At the time such aeroplanes
were built velocities were comparatively low, and this defect
was; for that reason, not sufficiently appreciated. In the end
it killed the ``canard'' or ``tail-first'' design.
Aeroplanes of the Dunne and similar types possess no
stabilizing surface distinct from the main surface, but they
have a longitudinal dihedral which renders them stable.
The main surface towards the wing-tips is given a
decreasing angle of incidence and corresponding camber. The
wing-tips then act as longitudinal stabilizers.
This design of aeroplane, while very interesting, has
not proved very practicable, owing to the following
disadvantages: (1) The plan design is not, from a mechanical
point of view, so sound as that of the ordinary aeroplane
surface, which is, in plan, a parallelogram. It is, then,
necessary to make the strength of construction greater than
would otherwise be the case. That means extra weight.
(2) The plan of the surface area is such that the aspect ratio
is not so high as if the surface was arranged with its leading
edges at right angles to the direction of motion. The lower
the aspect ratio, then, the less the lift. This design, then,
produces less lift for weight of surface than would the same
surface if arranged as a parallelogram. (3) In order to secure
the longitudinal dihedral, the angle of incidence has to be
very much decreased towards the wing-tips. Then, in order
that the lift-drift ratio may be preserved, there must be a
corresponding decrease in the camber. That calls for surface
ribs of varying cambers, and results in an expensive and
lengthy job for the builder. (4) In order to secure directional
stability, the surface is, in the centre, arranged to dip down
in the form of a V, pointing towards the direction of motion.
Should the aeroplane turn off its course, then its momentum
in the direction of its first course causes it to move in a
direction the resultant of the thrust and the momentum. It
then moves in a more or less sideways attitude, which results
in an air pressure upon one side of the V, and which tends to
turn the aeroplane back to its first course. This arrangement
of the surface results in a bad drift. Vertical surfaces at
the wing-tips may also be set at an angle producing the same
stabilizing effect, but they also increase the drift.
The gyroscopic action of a rotary engine will affect the
longitudinal stability when an aeroplane is turned to right
or left. In the case of a Gnome engine, such gyroscopic
action will tend to depress the nose of the aeroplane when it
is turned to the left, and to elevate it when it is turned to
the right. In modern aeroplanes this tendency is not sufficiently
important to bother about. In the old days of crudely
designed and under-powered aeroplanes this gyroscopic action
was very marked, and led the majority of pilots to dislike
turning an aeroplane to the right, since, in doing so, there
was some danger of ``stalling.''
LATERAL STABILITY is far more difficult for the designer
to secure than is longitudinal or directional stability. Some
degree of lateral stability may be secured by means of the
``lateral dihedral,'' i.e., the upward inclination of the surface
towards its wing-tips thus:
Imagine the top V, illustrated opposite, to be the front
view of a surface flying towards you. The horizontal equivalent
(H.E.) of the left wing is the same as that of the right
wing. Therefore, the lift of one wing is equal to the lift
of the other, and the weight, being situated always in the
centre, is balanced.
If some movement of the air causes the surface to tilt
sideways, as in the lower illustration, then you will note that
the H.E. of the left wing increases, and the H.E. of the right
wing decreases. The left wing then, having the greatest
lift, rises; and the surface assumes its first and normal
position.
Unfortunately however, the righting effect is not proportional
to the difference between the right and left H.E.'s.
In the case of A, the resultant direction of the reaction
of both wings is opposed to the direction of gravity or weight.
The two forces R R and gravity are then evenly balanced,
and the surface is in a state of equilibrium.
In the case of B, you will note that the R R is not directly
opposed to gravity. This results in the appearance of M,
and so the resultant direction of motion of the aeroplane
is no longer directly forward, but is along a line the resultant
of the thrust and M. In other words, it is, while flying
forward, at the same time moving sideways in the direction M.
In moving sideways, the keel-surface receives, of course,
a pressure from the air equal and opposite to M. Since
such surface is greatest in effect towards the tail, then the
latter must be pushed sideways. That causes the aeroplane
to turn; and, the highest wing being on the outside of the
turn, it has a greater velocity than the lower wing. That
produces greater lift, and tends to tilt the aeroplane over
still more. Such tilting tendency is, however, opposed by
the difference in the H.E.'s of the two wings.
It then follows that, for the lateral dihedral angle to
be effective, such angle must be large enough to produce,
when the aeroplane tilts, a difference in the H.E.'s of the
two wings, which difference must be sufficient to not only
oppose the tilting tendency due to the aeroplane turning,
but sufficient to also force the aeroplane back to its original
position of equilibrium.
It is now, I hope, clear to the reader that the lateral
dihedral is not quite so effective as would appear at first
sight. Some designers, indeed, prefer not to use it, since its
effect is not very great, and since it must be paid for in loss
of H.E. and consequently loss of lift, thus decreasing the liftdrift
ratio, i.e., the efficiency. Also, it is sometimes advanced
that the lateral dihedral increases the ``spill'' of air from the
wing-tips and that this adversely affects the lift-drift ratio.
The disposition of the keel-surface affects the lateral
stability. It should be, in effect, equally divided by the
longitudinal turning axis of the aeroplane. If there is an
excess of keel-surface above or below such axis, then a side
gust striking it will tend to turn the aeroplane over sideways.
The position of the centre of gravity affects lateral stability.
If too low, it produces a pendulum effect and causes the
aeroplane to roll sideways.
If too high, it acts as a stick balanced vertically would
act. If disturbed, it tends to travel to a position as far as
possible from its original position. It would then tend,
when moved, to turn the aeroplane over sideways and into
an upside-down position.
From the point of view of lateral stability, the best
position for the centre of gravity is one a little below the
centre of drift.
Propeller torque affects lateral stability. An aeroplane
tends to turn over sideways in the opposite direction to which
the propeller revolves.
This tendency is offset by increasing the angle of incidence
(and consequently the lift) of the side tending to fall; and it
is always advisable, if practical considerations allow it, to
also decrease the angle upon the other side. In that way
it is not necessary to depart so far from the normal angle
of incidence at which the lift-drift ratio is highest.
Wash-in is the term applied to the increased angle.
Wash-out is the term applied to the decreased angle.
Both lateral and directional stability may be improved
by washing out the angle of incidence on both sides of the
surface, thus:
The decreased angle decreases the drift and therefore the
effect of gusts upon the wing-tips which is just where they
have the most effect upon the aeroplane, owing to the distance
from the turning axis.
The wash-out also renders the ailerons (lateral controlling
services) more effective, as, in order to operate them, it is
not then necessary to give them such a large angle of incidence
as would otherwise be required.
The less the angle of incidence of the ailerons, the better
their lift-drift ratio, i.e., their efficiency. You will note
that, while the aileron attached to the surface with washed-out
angle is operated to the same extent as the aileron illustrated
above it, its angle of incidence is considerably less. Its efficiency
is therefore greater.
The advantages of the wash-in must, of course be paid for
in some loss of lift, as the lift decreases with the decreased angle.
In order to secure all the above described advantages,
a combination is sometimes effected, thus:
BANKING.--An aeroplane turned off its course to right
or left does not at once proceed along its new course. Its
momentum in the direction of its first course causes it to
travel along a line the resultant of such momentum and the
thrust. In other words, it more or less skids sideways and
away from the centre of the turn. Its lifting surfaces do
not then meet the air in their correct attitude, and the lift
may fall to such an extent as to become less than the weight,
in which case the aeroplane must fall. This bad effect is
minimized by ``banking,'' i.e., tilting the aeroplane sideways.
The bottom of the lifting surface is in that way opposed to
the air through which it is moving in the direction of the
momentum and receives an opposite air pressure. The
rarefied area over the top of the surface is rendered still more
rare, and this, of course, assists the air pressure in opposing
the momentum.
The velocity of the ``skid,'' or sideways movement, is
then only such as is necessary to secure an air pressure equal
and opposite to the centrifugal force of the turn.
The sharper the turn, the greater the effect of the centrifugal
force, and therefore the steeper should be the ``bank.''
Experentia docet.
The position of the centre of gravity affects banking. A low
C.G. will tend to swing outward from the centre of the turn,
and will cause the aeroplane to bank--perhaps too much, in
which case the pilot must remedy matters by operating the
ailerons.
A high C.G. also tends to swing outward from the centre
of the turn. It will tend to make the aeroplane bank the
wrong way, and such effect must be remedied by means of
the ailerons.
The pleasantest machine from a banking point of view is
one in which the C.G. is a little below the centre of drift.
It tends to bank the aeroplane the right way for the turn,
and the pilot can, if necessary, perfect the bank by means
of the ailerons.
The disposition of the keel-surface affects banking. It
should be, in effect, evenly divided by the longitudinal axis.
An excess of keel-surface above the longitudinal axis will,
when banking, receive an air pressure causing the aeroplane
to bank, perhaps too much. An excess of keel-surface below
the axis has the reverse effect.
SIDE-SLIPPING.--This usually occurs as a result of overbanking.
It is always the result of the aeroplane tilting
sideways and thus decreasing the horizontal equivalent, and
therefore the lift, of the surface. An excessive ``bank,''
or sideways tilt, results in the H.E., and therefore the lift,
becoming less than the weight, when, of course, the aeroplane
must fall, i.e., side-slip.
When making a very sharp turn it is necessary to bank
very steeply indeed. If, at the same time, the longitudinal
axis of the aeroplane remains approximately horizontal,
then there must be a fall, and the direction of motion will be
the resultant of the thrust and the fall as illustrated above
in sketch A. The lifting surfaces and the controlling surfaces
are not then meeting the air in the correct attitude,
with the result that, in addition to falling, the aeroplane
will probably become quite unmanageable.
The Pilot, however, prevents such a state of affairs from
happening by ``nosing-down,'' i.e., by operating the rudder
to turn the nose of the aeroplane downward and towards
the direction of motion as illustrated in sketch B. This
results in the higher wing, which is on the outside of the turn,
travelling with greater velocity, and therefore securing a
greater reaction than the lower wing, thus tending to tilt
the aeroplane over still more. The aeroplane is now almost
upside-down, but its attitude relative to the direction of
motion is correct and the controlling surfaces are all of them
working efficiently. The recovery of a normal attitude
relative to the Earth is then made as illustrated in sketch C.
The Pilot must then learn to know just the angle of bank
at which the margin of lift is lost, and, if a sharp turn
necessitates banking beyond that angle, he must ``nose-down.''
In this matter of banking and nosing-down, and, indeed,
regarding stability and control generally, the golden rule
for all but very experienced pilots should be: Keep the
aeroplane in such an attitude that the air pressure is always
directly in the pilot's face. The aeroplane is then always
engaging the air as designed to do so, and both lifting and
controlling surfaces are acting efficiently. The only exception
to this rule is a vertical dive, and I think that is
obviously not an attitude for any but very experienced
pilots to hanker after.
SPINNING.--This is the worst of all predicaments the
pilot can find himself in. Fortunately it rarely happens.
It is due to the combination of (1) a very steep spiral
descent of small radius, and (2) insufficiency of keel-surface
behind the vertical axis, or the jamming of the rudder
end or elevator into a position by which the aeroplane is forced
into an increasingly steep and small spiral.
Owing to the small radius of such a spiral, the mass of
the aeroplane may gain a rotary momentum greater, in effect,
than the air pressure of the keel-surface or controlling surfaces
opposed to it; and, when once such a condition occurs,
it is difficult to see what can be done by the pilot to remedy
it. The sensible pilot will not go beyond reasonable limits
of steepness and radius when executing spiral descents.
GLIDING DESCENT WITHOUT PROPELLER THRUST.--All
aeroplanes are, or should be, designed to assume their gliding
angle when the power and thrust is cut off. This relieves
the pilot of work, worry, and danger should he find himself
in a fog or cloud. The Pilot, although he may not realize
it, maintains the correct attitude of the aeroplane by observing
its position relative to the horizon. Flying into a
fog or cloud the horizon is lost to view, and he must then rely
upon his instruments--(1) the compass for direction; (2) an
inclinometer (arched spirit-level) mounted transversely to
the longitudinal axis, for lateral stability; and (3) an inclinometer
mounted parallel to the longitudinal axis, or the airspeed
indicator, which will indicate a nose-down position
by increase in air speed, and a tail-down position by decrease
in air speed.
The pilot is then under the necessity of watching three
instruments and manipulating his three controls to keep the
instruments indicating longitudinal, lateral, and directional
stability. That is a feat beyond the capacity of the ordinary
man. If, however, by the simple movement of throttling
down the power and thrust, he can be relieved of looking
after the longitudinal stability, he then has only two instruments
to watch. That is no small job in itself, but it is,
at any rate, fairly practicable.
Aeroplanes are, then, designed, or should be, so that the
centre of gravity is slightly forward of centre of lift. The
aeroplane is then, as a glider, nose-heavy--and the distance
the C.G. is placed in advance of the C.L. should be such as
to ensure a gliding angle producing a velocity the same as
the normal flying speed (for which the strength of construction
has been designed).
In order that this nose-heavy tendency should not exist
when the thrust is working and descent not required, the
centre of thrust is placed a little below the centre of drift
or resistance, and thus tends to pull up the nose of the
aeroplane.
The distance the centre of thrust is placed below the
centre of drift should be such as to produce a force equal
and opposite to that due to the C.G. being forward of the
C.L.
LOOPING AND UPSIDE DOWN FLYING.--If a loop is desired,
it is best to throttle the engine down at point A. The C.G.
being forward of the C.P., then causes the aeroplane to nosedown,
and assists the pilot in making a reasonably small
loop along the course C and in securing a quick recovery.
If the engine is not throttled down, then the aeroplane may
be expected to follow the course D, which results in a longer
nose dive than in the case of the course C.
A steady, gentle movement of the elevator is necessary.
A jerky movement may change the direction of motion so
suddenly as to produce dangerous air stresses upon the surfaces,
in which case there is a possibility of collapse.
If an upside-down flight is desired, the engine may, or
may not, be throttled down at point A. If not throttled
down, then the elevator must be operated to secure a course
approximately in the direction B. If it is throttled down,
then the course must be one of a steeper angle than B, or
there will be danger of stalling.
Diagram p. 88.--This is not set at quite
the correct angle. Path B should slope
slightly downwards from Position A.
CHAPTER III
RIGGING
In order to rig an aeroplane intelligently, and to maintain
it in an efficient and safe condition, it is necessary to possess
a knowledge of the stresses it is called upon to endure, and
the strains likely to appear.
STRESS is the load or burden a body is called upon to
bear. It is usually expressed by the result found by dividing
the load by the number of superficial square inches contained
in the cross-sectional area of the body.
Thus, if, for instance, the object illustrated above contains
4 square inches of cross-sectional area, and the total load
it is called upon to endure is 10 tons, the stress would be
expressed as 2 1/2 tons.
STRAIN is the deformation produced by stress.
THE FACTOR OF SAFETY is usually expressed by the result
found by dividing the stress at which it is known the body
will collapse, by the maximum stress it will be called upon to
endure. For instance, if a control wire be called upon to endure
a maximum stress of 2 cwts., and the known stress at which
it will collapse is 10 cwts., the factor of safety is then 5.
[cwts. = centerweights = 100 pound units as in cent & century.
Interestinly enough, this word only exists today in abbreviation
form, probably of centreweights, but the dictionary entries, even
from a hundred years ago do not list this as a word, but do list
c. or C. as the previous popular abbreviation as in Roman Numerals]
The word listed is "hundredweight. Michael S. Hart, 1997]
COMPRESSION.--The simple stress of compression tends
to produce a crushing strain. Example: the interplane and
fuselage struts.
TENSION.--The simple stress of tension tends to produce
the strain of elongation. Example: all the wires.
BENDING.--The compound stress of bending is a combination
of compression and tension.
The above sketch illustrates a straight piece of wood of
which the top, centre, and bottom lines are of equal length.
We will now imagine it bent to form a circle, thus:
The centre line is still the same length as before being
bent; but the top line, being farther from the centre of the
circle, is now longer than the centre line. That can be due
only to the strain of elongation produced by the stress of
tension. The wood between the centre line and the top
line is then in tension; and the farther from the centre,
the greater the strain, and consequently the greater the
tension.
The bottom line, being nearest to the centre of the circle,
is now shorter than the centre line. That can be due only
to the strain of crushing produced by the stress of compression.
The wood between the centre and bottom lines is
then in compression; and the nearer the centre of the circle,
the greater the strain, and consequently the greater the
compression.
It then follows that there is neither tension nor compression,
i.e., no stress, at the centre line, and that the wood
immediately surrounding it is under considerably less stress
than the wood farther away. This being so, the wood in
the centre may be hollowed out without unduly weakening
struts and spars. In this way 25 to 33 per cent. is saved in
the weight of wood in an aeroplane.
The strength of wood is in its fibres, which should, as far
as possible, run without break from one end of a strut or
spar to the other end. A point to remember is that the
outside fibres, being farthest removed from the centre line,
are doing by far the greatest work.
SHEAR STRESS IS such that, when material collapses under it,
one part slides over the other. Example: all the locking pins.
Some of the bolts are also in shear or ``sideways'' stress,
owing to lugs under their heads and from which wires are
taken. Such a wire, exerting a sideways pull upon a bolt,
tries to break it in such a way as to make one piece of the bolt
slide over the other piece.
TORSION.--This is a twisting stress compounded of compression,
tension, and shear stresses. Example: the propeller shaft.
NATURE OF WOOD UNDER STRESS.--Wood, for its weight,
takes the stress of compression far better than any other
stress. For instance: a walking-stick of less than 1 lb. in
weight will, if kept perfectly straight, probably stand up to
a compression stress of a ton or more before crushing; whereas,
if the same stick is put under a bending stress, it will probably
collapse to a stress of not more than about 50 lb. That is
a very great difference, and, since weight is of the greatest
importance, the design of an aeroplane is always such as to,
as far as possible, keep the various wooden parts of its
construction in direct compression. Weight being of such vital
importance, and designers all trying to outdo each other in
saving weight, it follows that the factor of safety is rather
low in an aeroplane. The parts in direct compression will,
however, take the stresses safely provided the following
conditions are carefully observed.
CONDITIONS TO BE OBSERVED:
1. All the spars and struts must be perfectly straight.
The above sketch illustrates a section through an
interplane strut. If the strut is to be kept straight,
i.e., prevented from bending, then the stress of
compression must be equally disposed about the
centre of strength. If it is not straight, then
there will be more compression on one side of the
centre of strength than on the other side. That
is a step towards getting compression on one side
and tension on the other side, in which case it
may be forced to take a bending stress for which
it is not designed. Even if it does not collapse
it will, in effect, become shorter, and thus throw
out of adjustment the gap and all the wires attached
to the top and bottom of the strut, with the result
that the flight efficiency of the aeroplane will be
spoiled.
The only exception to the above condition is
what is known as the Arch. For instance, in the
case of the Maurice Farman, the spars of the centresection
plane, which have to take the weight of
the nacelle, are arched upwards. If this was not
done, it is possible that rough landings might
result in the weight causing the spars to become
slightly distorted downwards. That would produce
a dangerous bending stress, but, as long as
the wood is arched, or, at any rate, kept from
bending downwards, it will remain in direct
compression and no danger can result.
2. Struts and spars must be symmetrical. By that I mean
that the cross-sectional dimensions must be correct,
as otherwise there will be bulging places on the
outside, with the result that the stress will not be
evenly disposed about the centre of strength, and
a bending stress may be produced.
3. Struts, spars, etc., must be undamaged. Remember
that, from what I have already explained about
bending stresses, the outside fibres of the wood are
doing by far the most work. If these get bruised
or scored, then the strut or spar suffers in strength
much more than one might think at first sight;
and, if it ever gets a tendency to bend, it is likely
to collapse at that point.
4. The wood must have a good, clear grain with no crossgrain,
knots, or shakes. Such blemishes produce
weak places and, if a tendency to bend appears,
then it may collapse at such a point.
5. The struts, spars, etc., must be properly bedded into
their sockets or fittings. To begin with, they must
be of good pushing or gentle tapping fit. They
must never be driven in with a heavy hammer.
Then again, a strut must bed well down all over its
cross-sectional area as illustrated above; otherwise
the stress of compression will not be evenly disposed
about the centre of strength, and that may
produce a bending stress. The bottom of the strut
or spar should be covered with some sort of
paint, bedded into the socket or fitting, and then
withdrawn to see if the paint has stuck all over the
bed.
6. The atmosphere is sometimes much damper than at
other times, and this causes wood to expand and
contract appreciably. This would not matter but
for the fact that it does not expand and contract
uniformly, but becomes unsymmetrical, i.e., distorted.
I have already explained the danger of that in
condition 2. This should be minimized by WELL
VARNISHING THE WOOD to keep the moisture out of it.
FUNCTION OF INTERPLANE STRUTS.--These struts have to
keep the lifting surfaces or ``planes'' apart, but this is only
part of their work. They must keep the planes apart, so
that the latter are in their correct attitude. That is only so
when the spars of the bottom plane are parallel with those of
the top plane. Also, the chord of the top plane must be
parallel with the chord of the bottom plane. If that is not
so, then one plane will not have the same angle of incidence
as the other one. At first sight one might think that all
that is necessary is to cut all the struts to be the same length,
but that is not the case.
Sometimes, as illustrated above, the rear spar is not so
thick as the main spar, and it is then necessary to make
up for that difference by making the rear struts correspondingly
longer. If that is not done, then the top and
bottom chords will not be parallel, and the top and bottom
planes will have different angles of incidence. Also, the
sockets or fittings, or even the spars upon which they are
placed, sometimes vary in thickness owing to faulty manufacture.
This must be offset by altering the length of the
struts. The best way to proceed is to measure the distance
between the top and bottom spars by the side of each strut,
and if that distance, or ``gap'' as it is called, is not as stated
in the aeroplane's specifications, then make it correct by
changing the length of the strut. This applies to both front
and rear interplane struts. When measuring the gap, always
be careful to measure from the centre of the spar, as it may
be set at an angle, and the rear of it may be considerably
lower than its front.
BORING HOLES IN WOOD.--It should be a strict rule that
no spar be used which has an unnecessary hole in it. Before
boring a hole, its position should be confirmed by whoever
is in charge of the workshop. A bolt-hole should be of a size
to enable the bolt to be pushed in, or, at any rate, not more
than gently tapped in. Bolts should not be hammered in, as
that may split the spar. On the other hand, a bolt should not
be slack in its hole, as, in such a case, it may work sideways and
split the spar, not to speak of throwing out of adjustment
the wires leading from the lug or socket under the bolt-head.
WASHERS.--Under the bolt-head, and also under the nut,
a washer must be placed--a very large washer compared
with the size which would be used in all-metal construction.
This is to disperse the stress over a large area; otherwise
the washer may be pulled into the wood and weaken it,
besides possibly throwing out of adjustment the wires
attached to the bolt or the fitting it is holding to the spar.
LOCKING.--Now as regards locking the bolts. If split
pins are used, be sure to see that they are used in such a way
that the nut cannot possibly unscrew at all. The split pin
should be passed through the bolt as near as possible to the
nut. It should not be passed through both nut and bolt.
If it is locked by burring over the edge of the bolt, do not
use a heavy hammer and try to spread the whole head of
the bolt. That might damage the woodwork inside the
fabric-covered surface. Use a small, light hammer, and gently
tap round the edge of the bolt until it is burred over.
TURNBUCKLES.--A turnbuckle is composed of a central
barrel into each end of which is screwed an eye-bolt. Wires
are taken from the eyes of the eye-bolt, and so, by turning
the barrel, they can be adjusted to their proper tension.
Eye-bolts must be a good fit in the barrel; that is to say,
not slack and not very tight. Theoretically it is not necessary
to screw the eye-bolt into the barrel for a distance
greater than the diameter of the bolt, but, in practice, it is
better to screw it in for a considerably greater distance than
that if a reasonable degree of safety is to be secured.
Now about turning the barrel to secure the right adjustment.
The barrel looks solid, but, as a matter of fact, it
is hollow and much more frail than it appears. For that
reason it should not be turned by seizing it with pliers, as
that may distort it and spoil the bore within it. The best
method is to pass a piece of wire through the hole in its centre,
and to use that as a lever. When the correct adjustment
has been secured, the turnbuckle must be locked to prevent
it from unscrewing. It is quite possible to lock it in such a
way as to allow it to unscrew a quarter or a half turn, and
that would throw the wires out of the very fine adjustment
necessary. The proper way is to use the locking wire so
that its direction is such as to oppose the tendency of the
barrel to unscrew, thus:
WIRES.--The following points should be carefully observed
where wire is concerned:
1. Quality.--It must not be too hard or too soft. An
easy practical way of learning to know the approximate
quality of wire is as follows:
Take three pieces, all of the same gauge, and each about a
foot in length. One piece should be too soft, another too hard,
and the third piece of the right quality. Fix them in a vice,
about an inch apart and in a vertical position, and with the light
from a window shining upon them. Burnish them if necessary,
and you will see a band of light reflected from each
wire.
Now bend the wires over as far as possible and away from
the light. Where the soft wire is concerned, it will squash
out at the bend, and this will be indicated by the band of
light, which will broaden at that point. In the case of the
wire which is too hard, the band of light will broaden very
little at the turn, but, if you look carefully, you will see some
little roughnesses of surface. In the case of the wire of the
right quality, the band of light may broaden a very little
at the turn, but there will be no roughnesses of surface.
By making this experiment two or three times one can
soon learn to know really bad wire from good, and also learn
to know the strength of hand necessary to bend the right
quality.
2. It must not be damaged. That is to say, it must be
unkinked, rustless, and unscored.
3. Now as regards keeping wire in good condition. Where
outside wires are concerned, they should be kept WELL GREASED
OR OILED, especially where bent over at the ends. Internal
bracing wires cannot be reached for the purpose of regreasing
them, as they are inside fabric-covered surfaces. They should
be prevented from rusting by being painted with an anti-rust
mixture. Great care should be taken to see that the wire
is perfectly clean and dry before being painted. A greasy
finger-mark is sufficient to stop the paint from sticking to
the wire. In such a case there will be a little space between
the paint and the wire. Air may enter there and cause the
wire to rust.
4. Tension of Wires.--The tension to which the wires are
adjusted is of the greatest importance. All the wires should
be of the same tension when the aeroplane is supported in
such a way as to throw no stress upon them. If some wires
are in greater tension than others, the aeroplane will quickly
become distorted and lose its efficiency.
In order to secure the same tension of all wires, the aeroplane,
when being rigged, should be supported by packing
underneath the lower surfaces as well as by packing underneath
the fuselage or nacelle. In this way the anti-lift wires
are relieved of the weight, and there is no stress upon any
of the wires.
As a general rule the wires of an aeroplane are tensioned
too much. The tension should be sufficient to keep the
framework rigid. Anything more than that lowers the factor
of safety, throws various parts of the framework into undue
compression, pulls the fittings into the wood, and will, in
the end, distort the whole framework of the aeroplane.
Only experience will teach the rigger what tension to
employ. Much may be done by learning the construction
of the various types of aeroplanes, the work the various
parts do, and in cultivating a touch for tensioning wires by
constantly handling them.
5. Wires with no Opposition Wires.--In some few cases
wires will be found which have no opposition wires pulling
in the opposite direction. For instance, an auxiliary lift
wire may run from the bottom of a strut to a spar in the top
plane at a point between struts. In such a case great care
should be taken not to tighten the wire beyond barely taking
up the slack.
Such a wire must be a little slack, or, as illustrated above,
it will distort the framework. That, in the example given,
will spoil the camber (curvature) of the surface, and result
in changing both the lift and the drift at that part of the surface.
Such a condition will cause the aeroplane to lose its
directional stability and also to fly one wing down.
I cannot impress this matter of tension upon the reader
too strongly. It is of the utmost importance. When this,
and also accuracy in securing the various adjustments, has
been learned, one is on the way to becoming a good
rigger.
6. Wire Loops.--Wire is often bent over at its end in the
form of a loop, in order to connect with a turnbuckle or
fitting. These loops, even when made as perfectly as possible,
have a tendency to elongate, thus spoiling the adjustment
of the wires Great care should be taken to minimize this
as far as possible. The rules to be observed are as
follows:
(a) The size of the loop should be as small as possible
within reason. By that I mean it should not be
so small as to create the possibility of the wire
breaking.
(b) The shape of the loop should be symmetrical.
(c) It should have well-defined shoulders in order to
prevent the ferrule from slipping up. At the same
time, a shoulder should not have an angular place.
(d) When the loop is finished it should be undamaged,
and it should not be, as is often the case, badly scored.
7. Stranded Wire Cable.--No splice should be served with
twine until it has been inspected by whoever is in charge of
the workshop. The serving may cover bad work.
Should a strand become broken, then the cable should be
replaced at once by another one.
Control cables have a way of wearing out and fraying
wherever they pass round pulleys. Every time an aeroplane
comes down from flight the rigger should carefully examine
the cables, especially where they pass round pulleys. If
he finds a strand broken, he should replace the cable.
The ailerons' balance cable on the top of the top plane
is often forgotten, since it is necessary to fetch a high pair
of steps in order to examine it. Don't slack this, or some
gusty day the pilot may unexpectedly find himself minus the
aileron control.
CONTROLLING SURFACES.--The greatest care should be
exercised in rigging the aileron, rudder, and elevator properly,
for the pilot entirely depends upon them in managing the
aeroplane.
The ailerons and elevator should be rigged so that, when
the aeroplane is in flight, they are in a fair true line with the
surface in front and to which they are hinged.
If the surface to which they are hinged is not a lifting
surface, then they should be rigged to be in a fair true line
with it as illustrated above.
If the controlling surface is, as illustrated, hinged to the
back of a lifting surface, then it should be rigged a little below
the position it would occupy if in a fair true line with the
surface in front. This is because, in such a case, it is set
at an angle of incidence. This angle will, during flight,
cause it to lift a little above the position in which it has been
rigged. It is able to lift owing to a certain amount of slack
in the control wire holding it--and one cannot adjust the
control wire to have no slack, because that would cause it
to bind against the pulleys and make the operation of it too
hard for the pilot. It is therefore necessary to rig it a little
below the position it would occupy if it was rigged in a fair
true line with the surface in front. Remember that this
only applies when it is hinged to a lifting surface. The
greater the angle of incidence (and therefore the lift) of the
surface in front, then the more the controlling surface will
have to be rigged down.
As a general rule it is safe to rig it down so that its trailing
edge is 1/2 to 3/4 inch below the position it would occupy if in
a fair line with the surface in front; or about 1/2 inch down for
every 18 inches of chord of the controlling surface.
When making these adjustments the pilot's control levers
should be in their neutral positions. It is not sufficient
to lash them. They should be rigidly blocked into position
with wood packing.
The surfaces must not be distorted in any way. If
they are held true by bracing wires, then such wires must be
carefully adjusted. If they are distorted and there are no
bracing wires with which to true them, then some of the
internal framework will probably have to be replaced.
The controlling surfaces should never be adjusted with
a view to altering the stability of the aeroplane. Nothing
can be accomplished in that way. The only result will be
to spoil the control of the aeroplane.
FABRIC-COVERED SURFACES.--First of all make sure
that there is no distortion of spars or ribs, and that they are
perfectly sound. Then adjust the internal bracing wires
so that the ribs are parallel to the direction of flight. The
ribs usually cause the fabric to make a ridge where they occur,
and, if such ridge is not parallel to the direction of flight,
it will produce excessive drift. As a rule the ribs are at
right angles to both main and rear spars.
The tension of the internal bracing wires should be just
sufficient to give rigidity to the framework. They should
not be tensioned above that unless the wires are, at their
ends, bent to form loops. In that case a little extra tension
may be given to offset the probable elongation of the
loops.
The turnbuckles must now be generously greased, and
served round with adhesive tape. The wires must be rendered
perfectly dry and clean, and then painted with an anti-rust
mixture. The woodwork must be well varnished.
If it is necessary to bore holes in the spars for the purpose
of receiving, for instance, socket bolts, then their places
should be marked before being bored and their positions
confirmed by whoever is in charge of the workshop. All is
now ready for the sail-maker to cover the surface with
fabric.
ADJUSTMENT OF CONTROL CABLES.--The adjustment of
the control cables is quite an art, and upon it will depend to
a large degree the quick and easy control of the aeroplane
by the pilot.
The method is as follows:
After having rigged the controlling surfaces, and as far
as possible secured the correct adjustment of the control
cables, then remove the packing which has kept the control
levers rigid. Then, sitting in the pilot's seat, move the
control levers SMARTLY. Tension the control cables so that
when the levers are smartly moved there is no perceptible
snatch or lag. Be careful not to tension the cables more than
necessary to take out the snatch. If tensioned too much
they will (1) bind round the pulleys and result in hard work
for the pilot; (2) throw dangerous stresses upon the controlling
surfaces, which are of rather flimsy construction; and (3)
cause the cables to fray round the pulleys quicker than would
otherwise be the case.
Now, after having tensioned the cables sufficiently to
take out the snatch, place the levers in their neutral positions,
and move them to and fro about 1/8 inch either side of such
positions. If the adjustment is correct, it should be possible
to see the controlling surfaces move. If they do not move,
then the control cables are too slack.
FLYING POSITION.--Before rigging an aeroplane or making
any adjustments it is necessary to place it in what is known
as its ``flying position.'' I may add that it would be better
termed its ``rigging position.''
In the case of an aeroplane fitted with a stationary engine
this is secured by packing up the machine so that the engine
foundations are perfectly horizontal both longitudinally and
laterally. This position is found by placing a straight-edge
and a spirit-level across the engine foundations (both
longitudinally and laterally), and great care should be taken to
see that the bubble is exactly in the centre of the level. The
slightest error will assume magnitude towards the extremities
of the aeroplane. Great care should be taken to block up
the aeroplane rigidly. In case it gets accidentally disturbed
while the work is going on, it is well to constantly verify the
flying position by running the straight-edge and spirit-level
over the engine foundations. The straight-edge should be
carefully tested before being used, as, being generally made of
wood, it will not remain true long. Place it lightly in a vice,
and in such a position that a spirit-level on top shows the
bubble exactly in the centre. Now slowly move the level
along the straight-edge, and the bubble should remain exactly
in the centre. If it does not do so, then the straight-edge
is not true and must be corrected. THIS SHOULD NEVER BE
OMITTED.
In the case of aeroplanes fitted with engines of the rotary
type, the ``flying position'' is some special attitude laid
down in the aeroplane's specifications, and great care should
be taken to secure accuracy.
ANGLE OF INCIDENCE.--One method of finding the angle
of incidence is as follows:
First place the aeroplane in its flying position. The
corner of the straight-edge must be placed underneath and
against the CENTRE of the rear spar, and held in a horizontal
position parallel to the ribs. This is secured by using a
spirit-level. The set measurement will then be from the
top of the straight-edge to the centre of the bottom surface
of the main spar, or it may be from the top of the straightedge
to the lowest part of the leading edge. Care should be
taken to measure from the centre of the spar and to see that
the bubble is exactly in the centre of the level. Remember
that all this will be useless if the aeroplane has not been placed
accurately in its flying position.
This method of finding the angle of incidence must be
used under every part of the lower surface where struts
occur. It should not be used between the struts, because,
in such places, the spars may have taken a slight permanent
set up or down; not, perhaps, sufficiently bad to make any
material difference to the flying of the machine, but quite bad
enough to throw out the angle of incidence, which cannot
be corrected at such a place.
If the angle is wrong, it should then be corrected as follows:
If it is too great, then the rear spar must be warped up
until it is right, and this is done by slackening ALL the wires
going to the top of the strut, and then tightening ALL the
wires going to the bottom of the strut.
If the angle is too small, then slacken ALL the wires going
to the bottom of the strut, and tighten ALL the wires going to
the top of the strut, until the correct adjustment is secured.
Never attempt to adjust the angle by warping the main spar.
The set measurement, which is of course stated in the
aeroplane's specifications, should be accurate to 1/16 inch.
LATERAL DIHEDRAL ANGLE.--One method of securing
this is as follows, and this method will, at the same time,
secure the correct angle of incidence:
The strings, drawn very tight, must be taken over both
the main and rear spars of the top surface. They must run
between points on the spars just inside the outer struts.
The set measurement (which should be accurate to 1/16 inch
or less) is then from the strings down to four points on the
main and rear spars of the centre-section surface. These
points should be just inside the four centre-section struts;
that is to say, as far as possible away from the centre of the
centre-section. Do not attempt to take the set measurement
near the centre of the centre-section.
The strings should be as tight as possible, and, if it can
be arranged, the best way to accomplish that is as shown in
the above illustration, i.e., by weighting the strings down to
the spars by means of weights and tying each end of the strings
to a strut. This will give a tight and motionless string.
However carefully the above adjustment is made, there is
sure to be some slight error. This is of no great importance,
provided it is divided equally between the left- and righthand
wings. In order to make sure of this, certain check
measurements should be taken as follows:
Each bay must be diagonally measured, and such measurements
must be the same to within 1/16 inch on each side of
the aeroplane. As a rule such diagonal measurements are
taken from the bottom socket of one strut to the top socket
of another strut, but this is bad practice, because of possible
inaccuracies due to faulty manufacture.
The points between which the diagonal measurements
are taken should be at fixed distances from the butts of the
spars, such distances being the same on each side of the
aeroplane, thus:
It would be better to use the centre line of the aeroplane
rather than the butts of the spars. It is not practicable
to do so, however, as the centre line probably runs through
the petrol tanks, etc.
THE DIHEDRAL BOARD.--Another method of securing
the dihedral angle, and also the angle of incidence, is by
means of the dihedral board. It is a light handy thing to
use, but leads to many errors, and should not be used unless
necessary. The reasons are as follows:
The dihedral board is probably not true. If it must be
used, then it should be very carefully tested for truth beforehand.
Another reason against its use is that it has to be
placed on the spars in a position between the struts, and
that is just where the spars may have a little permanent
set up or down, or some inaccuracy of surface which will,
of course, throw out the accuracy of the adjustment. The
method of using it is as follows:
The board is cut to the same angle as that specified for
the upward inclination of the surface towards its wingtips.
It is placed on the spar as indicated above, and it
is provided with two short legs to raise it above the flanges
of the ribs (which cross over the spars), as they may vary
in depth. A spirit-level is then placed on the board, and the
wires must be adjusted to give the surface such an inclination
as to result in the bubble being in the centre of the level.
This operation must be performed in respect of each bay
both front and rear. The bays must then be diagonally
measured as already explained.
YET ANOTHER METHOD of finding the dihedral angle,
and at the same time the angle of incidence, is as follows:
A horizontal line is taken from underneath the butt of
each spar, and the set measurement is either the angle it makes
with the spar, or a fixed measurement from the line to the
spar taken at a specified distance from the butt. This operation
must be performed in respect of both main and rear
spars, and all the bays must be measured diagonally afterwards.
Whichever method is used, be sure that after the job is
done the spars are perfectly straight.
STAGGER.--The stagger is the distance the top surface
is in advance of the bottom surface when the aeroplane
is in flying position. The set measurement is obtained as
follows:
Plumb-lines must be dropped over the leading edge of
the top surface wherever struts occur, and also near the
fuselage. The set measurement is taken from the front of the
lower leading edge to the plumb-lines. It makes a difference
whether the measurement is taken along a horizontal line
(which can be found by using a straight-edge and a spiritlevel)
or along a projection of the chord. The line along
which the measurement should be taken is laid down in the
aeroplane's specifications.
If a mistake is made and the measurement taken along
the wrong line, it may result in a difference of perhaps 1/4
will, in flight, be nose-heavy or tail-heavy.
After the adjustments of the angles of incidence, dihedral,
and stagger have been secured, it is as well to confirm all of
them, as, in making the last adjustment, the first one may
have been spoiled.
OVER-ALL ADJUSTMENTS.--The following over-all check
measurements should now be taken.
The straight lines AC and BC should be equal to within
1/8 inch. The point C is the centre of the propeller, or, in the
case of a ``pusher'' aeroplane, the centre of the nacelle.
The points A and B are marked on the main spar, and must
in each case be the same distance from the butt of the spar.
The rigger should not attempt to make A and B merely the
sockets of the outer struts, as they may not have been placed
quite accurately by the manufacturer. The lines AC and BC
must be taken from both top and bottom spars--two measurements
on each side of the aeroplane.
The two measurements FD and FE should be equal to
within 1/8 inch. F is the centre of the fuselage or rudderpost.
D and E are points marked on both top and bottom
rear spars, and each must be the same fixed distance from
the butt of the spar. Two measurements on each side of the
aeroplane.
If these over-all measurements are not correct, then it
is probably due to some of the drift or anti-drift wires being
too tight or too slack. It may possibly be due to the fuselage
being out of truth, but of course the rigger should have made
quite sure that the fuselage was true before rigging the rest
of the machine. Again, it may be due to the internal bracing
wires within the lifting surfaces not being accurately adjusted,
but of course this should have been seen to before covering the
surfaces with fabric.
FUSELAGE.--The method of truing the fuselage is laid
down in the aeroplane's specifications. After it has been
adjusted according to the specified directions, it should then
be arranged on trestles in such a way as to make about threequarters
of it towards the tail stick out unsupported. In
this way it will assume a condition as near as possible to
flying conditions, and when it is in this position the set
measurements should be confirmed. If this is not done it
may be out of truth, but perhaps appear all right when
supported by trestles at both ends, as, in such case, its
weight may keep it true as long as it is resting upon the
trestles.
THE TAIL-PLANE (EMPENNAGE).--The exact angle of
incidence of the tail-plane is laid down in the aeroplane's
specifications. It is necessary to make sure that the spars
are horizontal when the aeroplane is in flying position and
the tail unsupported as explained above under the heading
of Fuselage. If the spars are tapered, then make sure that
their centre lines are horizontal.
UNDERCARRIAGE.--The undercarriage must be very carefully
aligned as laid down in the specifications.
1. The aeroplane must be placed in its flying position
and sufficiently high to ensure the wheels being off the ground
when rigged. When in this position the axle must be horinontal
and the bracing wires adjusted to secure the various
set measurements stated in the specifications.
2. Make sure that the struts bed well down into their
sockets.
3. Make sure that the shock absorbers are of equal
tension. In the case of rubber shock absorbers, both the
number of turns and the lengths must be equal.
HOW TO DIAGNOSE FAULTS IN FLIGHT, STABILITY, AND CONTROL.
DIRECTIONAL STABILITY will be badly affected if there is
more drift (i.e., resistance) on one side of the aeroplane than
there is on the other side. The aeroplane will tend to turn
towards the side having the most drift. This may be caused
as follows:
1. The angle of incidence of the main surface or the tail
surface may be wrong. The greater the angle of incidence,
the greater the drift. The less the angle, the less the drift.
2. If the alignment of the fuselage, fin in front of the
rudder, the struts or stream-line wires, or, in the case of
the Maurice Farman, the front outriggers, are not absolutely
correct--that is to say, if they are turned a little to the
left or to the right instead of being in line with the direction
of flight--then they will act as a rudder and cause the aeroplane
to turn off its course.
3. If any part of the surface is distorted, it will cause
the aeroplane to turn off its course. The surface is cambered,
i.e., curved, to pass through the air with the least possible
drift. If, owing perhaps to the leading edge, spars, or trailing
edge becoming bent, the curvature is spoiled, that will
result in changing the amount of drift on one side of the aeroplane,
which will then have a tendency to turn off its course.
LATERAL INSTABILITY (FLYING ONE WING DOWN).--The only possible
reason for such a condition is a difference in the lifts
of right and left wings. That may be caused as follows:
1. The angle of incidence may be wrong. If it is too
great, it will produce more lift than on the other side of the
aeroplane; and if too small, it will produce less lift than on
the other side--the result being that, in either case, the aeroplane
will try to fly one wing down.
2. Distorted Surfaces.--If some part of the surface is
distorted, then its camber is spoiled, and the lift will not be
the same on both sides of the aeroplane, and that, of course,
will cause it to fly one wing down.
LONGITUDINAL INSTABILITY may be due to the following reasons:
1. The stagger may be wrong. The top surface may have
drifted back a little owing to some of the wires, probably
the incidence wires, having elongated their loops or having
pulled the fittings into the wood. If the top surface is not
staggered forward to the correct degree, then consequently
the whole of its lift is too far back, and it will then have a
tendency to lift up the tail of the machine too much. The
aeroplane would then be said to be ``nose-heavy.''
A 1/4-inch area in the stagger will make a very considerable
difference to the longitudinal stability.
2. If the angle of incidence of the main surface is not right,
it will have a bad effect, especially in the case of an aeroplane
with a lifting tail-plane.
If the angle is too great, it will produce an excess of lift,
and that may lift up the nose of the aeroplane and result in
a tendency to fly ``tail-down.'' If the angle is too small,
it will produce a decreased lift, and the aeroplane may have a
tendency to fly ``nose-down.''
3. The fuselage may have become warped upward or
downward, thus giving the tail-plane an incorrect angle of
incidence. If it has too much angle, it will lift too much,
and the aeroplane will be ``nose-heavy.'' If it has too little
angle, then it will not lift enough, and the aeroplane will be
``tail-heavy.''
4. (The least likely reason.) The tail-plane may be
mounted upon the fuselage at a wrong angle of incidence,
in which case it must be corrected. If nose-heavy, it should
be given a smaller angle of incidence. If tail-heavy, it should
be given a larger angle; but care should be taken not to give
it too great an angle, because the longitudinal stability
entirely depends upon the tail-plane being set at a much
smaller angle of incidence than is the main surface, and if
that difference is decreased too much, the aeroplane will
become uncontrollable longitudinally. Sometimes the tailplane
is mounted on the aeroplane at the same angle as the
main surface, but it actually engages the air at a lesser angle,
owing to the air being deflected downwards by the main
surface. There is then, in effect, a longitudinal dihedral
as explained and illustrated in Chapter I.
CLIMBS BADLY.--Such a condition is, apart from engine
or propeller trouble, probably due to (1) distorted surfaces,
or (2) too small an angle of incidence.
FLIGHT SPEED POOR.--Such a condition is, apart from
engine or propeller trouble, probably due to (1) distorted
surfaces, (2) too great an angle of incidence, or (3) dirt or
mud, and consequently excessive skin-friction.
INEFFICIENT CONTROL is probably due to (1) wrong setting
of control surfaces, (2) distortion of control surfaces, or
(3) control cables being badly tensioned.
WILL NOT TAXI STRAIGHT.--If the aeroplane is uncontrollable
on the ground, it is probably due to (1) alignment
of undercarriage being wrong, or (2) unequal tension of shock
absorbers.
CHAPTER IV
THE PROPELLER, OR ``AIR-SCREW''
The sole object of the propeller is to translate the power
of the engine into thrust.
The propeller screws through the air, and its blades, being
set at an angle inclined to the direction of motion, secure
a reaction, as in the case of the aeroplane's lifting surface.
This reaction may be conveniently divided into two
component parts or values, namely, Thrust and Drift.
The Thrust is opposed to the Drift of the aeroplane, and
must be equal and opposite to it at flying speed. If it falls
off in power, then the flying speed must decrease to a velocity,
at which the aeroplane drift equals the decreased thrust.
The Drift of the propeller may be conveniently divided
into the following component values:
Active Drift, produced by the useful thrusting part of the propeller.
Passive Drift, produced by all the rest of the propeller,
i.e., by its detrimental surface.
Skin Friction, produced by the friction of the air with
roughnesses of surface.
Eddies attending the movement of the air caused by
the action of the propeller.
Cavitation (very marked at excessive speed of revolution).
A tendency of the propeller to produce a
cavity or semi-vacuum in which it revolves, the
thrust decreasing with increase of speed and
cavitation.
THRUST-DRIFT RATIO.--The proportion of thrust to drift
is of paramount importance, for it expresses the efficiency
of the propeller. It is affected by the following factors:
Speed of Revolution.--The greater the speed, the greater
the proportion of drift to thrust. This is due to
the increase with speed of the passive drift, which
carries with it no increase in thrust. For this
reason propellers are often geared down to revolve
at a lower speed than that of the engine.
Angle of Incidence.--The same reasons as in the case of
the aeroplane surface.
Surface Area.--Ditto.
Aspect Ratio.--Ditto.
Camber.--Ditto.
In addition to the above factors there are, when it comes
to actually designing a propeller, mechanical difficulties to
consider. For instance, the blades must be of a certain
strength and consequent thickness. That, in itself, limits
the aspect ratio, for it will necessitate a chord long enough
in proportion to the thickness to make a good camber possible.
Again, the diameter of the propeller must be limited, having
regard to the fact that greater diameters than those used
to-day would not only result in excessive weight of construction,
but would also necessitate a very high undercarriage
to keep the propeller off the ground, and such undercarriage
would not only produce excessive drift, but would also tend
to make the aeroplane stand on its nose when alighting.
The latter difficulty cannot be overcome by mounting the
propeller higher, as the centre of its thrust must be approximately
coincident with the centre of aeroplane drift.
MAINTENANCE OF EFFICIENCY.
The following conditions must be observed:
1. PITCH ANGLE.--The angle, at any given point on the
propeller, at which the blade is set is known as the pitch
angle, and it must be correct to half a degree if reasonable
efficiency is to be maintained.
This angle secures the ``pitch,'' which is the distance the
propeller advances during one revolution, supposing the air
to be solid. The air, as a matter of fact, gives back to the
thrust of the blades just as the pebbles slip back as one
ascends a shingle beach. Such ``give-back'' is known as
Slip. If a propeller has a pitch of, say, 10 feet, but actually
advances, say, only 8 feet owing to slip, then it will be said
to possess 20 per cent. slip.
Thus, the pitch must equal the flying speed of the
aeroplane plus the slip of the propeller. For example,
let us find the pitch of a propeller, given the following
conditions:
Flying speed .............. 70 miles per hour.
Propeller revolutions ..... 1,200 per minute.
Slip ...................... 15 per cent.
First find the distance in feet the aeroplane will travel
forward in one minute. That is--
369,600 feet (70 miles)
------------------------ = 6,160 feet per minute.
60 `` (minutes)
Now divide the feet per minute by the propeller revolutions
per minute, add 15 per cent. for the slip, and the result
will be the propeller pitch:
6,160
----- + 15 per cent. = 5 feet 1 3/5 inches.
1,200
In order to secure a constant pitch from root to tip of
blade, the pitch angle decreases towards the tip. This is
necessary, since the end of the blade travels faster than its
root, and yet must advance forward at the same speed as
the rest of the propeller. For example, two men ascending
a hill. One prefers to walk fast and the other slowly, but they
wish to arrive at the top of the hill simultaneously. Then
the fast walker must travel a farther distance than the slow
one, and his angle of path (pitch angle) must be smaller
than the angle of path taken by the slow walker. Their
pitch angles are different, but their pitch (in this case altitude
reached in a given time) is the same.
In order to test the pitch angle, the propeller must be
mounted upon a shaft at right angles to a beam the face of
which must be perfectly level, thus:
First select a point on the blade at some distance (say
about 2 feet) from the centre of the propeller. At that
point find, by means of a protractor, the angle a projection
of the chord makes with the face of the beam. That angle
is the pitch angle of the blade at that point.
Now lay out the angle on paper, thus:
The line above and parallel to the circumference line must
be placed in a position making the distance between the
two lines equal to the specified pitch, which is, or should be,
marked upon the boss of the propeller.
Now find the circumference of the propeller where the
pitch angle is being tested. For example, if that place is
2 feet radius from the centre, then the circumference will
be 2 feet X 2 = 4 feet diameter, which, if multiplied by
3.1416 = 15.56 feet circumference.
Now mark off the circumference distance, which is
represented above by A-B, and reduce it in scale for convenience.
The distance a vertical line makes between B and the
chord dine is the pitch at the point where the angle is being
tested, and it should coincide with the specified pitch. You
will note, from the above illustration, that the actual pitch
line should meet the junction of the chord line and top
line.
The propeller should be tested at several points, about
a foot apart, on each blade; and the diagram, provided the
propeller is not faulty, will then look like this:
At each point tested the actual pitch coincides with the
specified pitch: a satisfactory condition.
A faulty propeller will produce a diagram something
like this:
At every point tested the pitch angle is wrong, for nowhere
does the actual pitch coincide with the specified pitch.
Angles A, C, and D, are too large, and B is too small. The
angle should be correct to half a degree if reasonable efficiency
is to be maintained.
A fault in the pitch angle may be due to (1) faulty manufacture,
(2) distortion, or (3) the shaft hole through the boss
being out of position.
2. STRAIGHTNESS.--To test for straightness the propeller
must be mounted upon a shaft. Now bring the tip of one
blade round to graze some fixed object. Mark the point it
grazes. Now bring the other tip round, and it should come
within 1/8 inch of the mark. If it does not do so, it is due to
(1) faulty manufacture, (2) distortion, or (3) to the hole
through the boss being out of position.
3. LENGTH.--The blades should be of equal length to
inch.
4. BALANCE.--The usual method of testing a propeller
for balance is as follows: Mount it upon a shaft, which must
be on ball-bearings. Place the propeller in a horizontal
position, and it should remain in that position. If a weight
of a trifle over an ounce placed in a bolt-hole on one side of
the boss fails to disturb the balance, then the propeller is
usually regarded as unfit for use.
The above method is rather futile, as it does not test for
the balance of centrifugal force, which comes into play as
soon as the propeller revolves. It can be tested as follows:
The propeller must be in a horizontal position, and then
weighed at fixed points, such as A, B, C, D, E, and F, and
the weights noted. The points A, B, and C must, of course,
be at the same fixed distances from the centre of the propeller
as the points D, E, and F. Now reverse the propeller and
weigh at each point again. Note the results. The first
series of weights should correspond to the second series,
thus:
Weight A should equal weight F.
`` B `` `` `` E.
`` C `` `` `` D.
There is no standard practice as to the degree of error
permissible, but if there are any appreciable differences the
propeller is unfit for use.
5. SURFACE AREA.--The surface area of the blades should
be equal. Test with callipers thus:
The points between which the distances are taken must,
of course, be at the same distance from the centre in the
case of each blade.
There is no standard practice as to the degree of error
permissible. If, however, there is an error of over 1/8 inch,
the propeller is really unfit for use.
6. CAMBER.--The camber (curvature) of the blades should
be (1) equal, (2) decrease evenly towards the tips of the blades,
and (3) the greatest depth of the curve should, at any point
of the blade, be approximately at the same percentage of
the chord from the leading edge as at other points.
It is difficult to test the top camber without a set of
templates, but a fairly accurate idea of the concave camber
can be secured by slowly passing a straight-edge along the
blade, thus:
The camber can now be easily seen, and as the straightedge
is passed along the blade, the observer should look for
any irregularities of the curvature, which should gradually
and evenly decrease towards the tip of the blade.
7. THE JOINTS.--The usual method for testing the glued
joints is by revolving the propeller at greater speed than it
will be called upon to make during flight, and then carefully
examining the joints to see if they have opened. It is not
likely, however, that the reader will have the opportunity
of making this test. He should, however, examine all the
joints very carefully, trying by hand to see if they are quite
sound. Suspect a propeller of which the joints appear to
hold any thickness of glue. Sometimes the joints in the
boss open a little, but this is not dangerous unless they extend
to the blades, as the bolts will hold the laminations together.
8. CONDITION OF SURFACE.--The surface should be very
smooth, especially towards the tips of the blades. Some
propeller tips have a speed of over 30,000 feet a minute,
and any roughness will produce a bad drift or resistance
and lower the efficiency.
9. MOUNTING.--Great care should be taken to see that
the propeller is mounted quite straight on its shaft. Test in
the same way as for straightness. If it is not straight, it
is possibly due to some of the propeller bolts being too slack
or to others having been pulled up too tightly.
FLUTTER.--Propeller ``flutter,'' or vibration, may be due
to faulty pitch angle, balance, camber, or surface area. It
causes a condition sometimes mistaken for engine trouble,
and one which may easily lead to the collapse of the propeller.
CARE OF PROPELLERS.--The care of propellers is of the
greatest importance, as they become distorted very easily.
1. Do not store them in a very damp or a very dry place.
2. Do not store them where the sun will shine upon them.
3. Never leave them long in a horizontal position or
leaning up against a wall.
4. They should be hung on horizontal pegs, and the
position of the propellers should be vertical.
If the points I have impressed upon you in these notes
are not attended to, you may be sure of the following results:
1. Lack of efficiency, resulting in less aeroplane speed
and climb than would otherwise be the case.
2. Propeller ``flutter'' and possible collapse.
3. A bad stress upon the propeller shaft and its bearings.
TRACTOR.--A propeller mounted in front of the main
surface.
PUSHER.--A propeller mounted behind the main surface.
FOUR-BLADED PROPELLERS.--Four- bladed propellers are
suitable only when the pitch is comparatively large.
For a given pitch, and having regard to ``interference,''
they are not so efficient as two-bladed propellers.
The smaller the pitch, the less the ``gap,'' i.e., the distance,
measured in the direction of the thrust, between the
spiral courses of the blades.
If the gap is too small, then the following blade will
engage air which the preceding blade has put into motion,
with the result that the following blade will not secure as
good a reaction as would otherwise be the case. It is very
much the same as in the case of the aeroplane gap.
For a given pitch, the gap of a four-bladed propeller is
only half that of a two-bladed one. Therefore the fourbladed
propeller is only suitable for large pitch, as such
pitch produces spirals with a large gap, thus offsetting the
decrease in gap caused by the numerous blades.
The greater the speed of rotation, the less the pitch for
a given aeroplane speed. Then, in order to secure a large
pitch and consequently a good gap, the four-bladed propeller
is usually geared to rotate at a lower speed than would be
the case if directly attached to the engine crank-shaft.
CHAPTER V
MAINTENANCE
CLEANLINESS.--The fabric must be kept clean and free
from oil, as that will rot it. To take out dirt or oily patches,
try acetone. If that will not remedy matters, then try
petrol, but use it sparingly, as otherwise it will take off an
unnecessary amount of dope. If that will not remove the
dirt, then hot water and soap will do so, but, in that case,
be sure to use soap having no alkali in it, as otherwise it may
injure the fabric. Use the water sparingly, or it may get
inside the planes and rust the internal bracing wires, or cause
some of the wooden framework to swell.
The wheels of the undercarriage have a way of throwing
up mud on to the lower surface. This should, if possible, be
taken off while wet. It should never be scraped off when
dry, as that may injure the fabric. If dry, then it should
be moistened before being removed.
Measures should be taken to prevent dirt from collecting
upon any part of the aeroplane, as, otherwise, excessive skinfriction
will be produced with resultant loss of flight speed.
The wires, being greasy, collect dirt very easily.
CONTROL CABLES.--After every flight the rigger should
pass his hand over the control cables and carefully examine
them near pulleys. Removal of grease may be necessary
to make a close inspection possible. If only one strand is
broken the wire should be replaced. Do not forget the aileron
balance wire on the top surface.
Once a day try the tension of the control cables by smartly
moving the control levers about as explained elsewhere.
WIRES.--All the wires should be kept well greased or
oiled, and in the correct tension. When examining the wires,
it is necessary to place the aeroplane on level ground, as
otherwise it may be twisted, thus throwing some wires into
undue tension and slackening others. The best way, if there
is time, is to pack the machine up into its ``flying position.''
If you see a slack wire, do not jump to the conclusion
that it must be tensioned. Perhaps its opposition wire is
too tight, in which case slacken it, and possibly you will
find that will tighten the slack wire.
Carefully examine all wires and their connections near
the propeller, and be sure that they are snaked round with
safety wire, so that the latter may keep them out of the way
of the propeller if they come adrift.
The wires inside the fuselage should be cleaned and regreased
about once a fortnight.
STRUTS AND SOCKETS.--These should be carefully examined
to see if any splitting has occurred.
DISTORTION.--Carefully examine all surfaces, including
the controlling surfaces, to see whether any distortion has
occurred. If distortion can be corrected by the adjustment
of wires, well and good; but if not, then some of the internal
framework probably requires replacement.
ADJUSTMENTS.--Verify the angles of incidence; dihedral,
and stagger, and the rigging position of the controllingsurfaces,
as often as possible.
UNDERCARRIAGE.--Constantly examine the alignment and
fittings of the undercarriage, and the condition of tyres and
shock absorbers. The latter, when made of rubber, wear
quickest underneath. Inspect axles and skids to see if
there are any signs of them becoming bent. The wheels
should be taken off occasionally and greased.
LOCKING ARRANGEMENTS.--Constantly inspect the locking
arrangements of turnbuckles, bolts, etc. Pay particular
attention to the control cable connections, and to all moving
parts in respect of the controls.
LUBRICATION.--Keep all moving parts, such as pulleys,
control levers, and hinges of controlling surfaces, well greased.
SPECIAL INSPECTION.--Apart from constantly examining
the aeroplane with reference to the above points I have made,
I think that, in the case of an aeroplane in constant use
it is an excellent thing to make a special inspection of every
part, say once a week. This will take from two to three
hours, according to the type of aeroplane. In order to carry
it out methodically, the rigger should have a list of every part
down to the smallest split-pin. He can then check the parts
as he examines them, and nothing will be passed over. This,
I know from experience, greatly increases the confidence of
the pilot, and tends to produce good work in the air.
WINDY WEATHER.--The aeroplane, when on the ground,
should face the wind; and it is advisable to lash the control
lever fast, so that the controlling surfaces may not be blown
about and possibly damaged.
``VETTING'' BY EYE.--This should be practiced at every
opportunity, and, if persevered in, it is possible to become
quite expert in diagnosing by eye faults in flight efficiency,
stability and control.
The aeroplane should be standing upon level ground, or,
better than that, packed up into its ``flying position.''
Now stand in front of it and line up the leading edge
with the main spar, rear spar, and trailing edge. Their
shadows can usually be seen through the fabric. Allowance
must, of course, be made for wash-in and wash-out; otherwise,
the parts I have specified should be parallel with each other.
Now line up the centre part of the main-plane with the
tail-plane. The latter should be horizontal.
Next, sight each interplane front strut with its rear
strut. They should be parallel.
Then, standing on one side of the aeroplane, sight all
the front struts. The one nearest to you should cover all
the others. This applies to the rear struts also.
Look for distortion of leading edges, main and rear spars,
trailing edges, tail-plane and controlling surfaces.
This sort of thing, if practiced constantly, will not only
develop an expert eye for diagnosis of faults, but will also
greatly assist in impressing upon the memory the characteristics
and possible troubles of the various types of aeroplanes.
MISHANDLING OF THE GROUND.--This is the cause of a
lot of unnecessary damage. The golden rule to observe is:
PRODUCE NO BENDING STRESSES.
Nearly all the wood in an aeroplane is designed to take
merely the stress of direct compression, and it cannot be bent
safely. Therefore, in packing an aeroplane up from the
ground, or in pulling or pushing it about, be careful to stress
it in such a way as to produce, as far as possible, only direct
compression stresses. For instance, if it is necessary to
support the lifting surface, then the packing should be
arranged to come directly under the struts so that they may
take the stress in the form of compression for which they are
designed. Such supports should be covered with soft packing
in order to prevent the fabric from becoming damaged.
When pulling an aeroplane along, if possible, pull from
the top of the undercarriage struts. If necessary to pull
from elsewhere, then do so by grasping the interplane struts
as low down as possible.
Never lay fabric-covered parts upon a concrete floor.
Any slight movement will cause the fabric to scrape over the
floor with resultant damage.
Struts, spars, etc., should never be left about the floor,
as in such position they are likely to become scored. I
have already explained the importance of protecting the outside
fibres of the wood. Remember also that wood becomes
distorted easily. This particularly applies to interplane
struts. If there are no proper racks to stand them in, then
the best plan is to lean them up against the wall in as near a
vertical position as possible.
TIME.--Learn to know the time necessary to complete
any of the various rigging jobs. This is really important.
Ignorance of this will lead to bitter disappointments in civil
life; and, where Service flying is concerned, it will, to say the
least of it, earn unpopularity with senior officers, and fail to
develop respect and good work where men are concerned.
THE AEROPLANE SHED.--This should be kept as clean and
orderly as possible. A clean, smart shed produces briskness,
energy, and pride of work. A dirty, disorderly shed nearly
always produces slackness and poor quality of work, lost
tools and mislaid material.
GLOSSARY
Aeronautics--The science of aerial navigation.
Aerofoil--A rigid structure, of large superficial area relative to its
thickness, designed to obtain, when driven through the air at an
angle inclined to the direction of motion, a reaction from the air
approximately at right angles to its surface. Always cambered
when intended to secure a reaction in one direction only. As the
term ``aerofoil'' is hardly ever used in practical aeronautics,
I have, throughout this book, used the term SURFACE, which,
while academically incorrect, since it does not indicate thickness,
is a term usually used to describe the cambered lifting surfaces,
i.e., the ``planes'' or ``wings,'' and the stabilizers and the
controlling aerofoils.
Aerodrome--The name usually applied to a ground used for the
practice of aviation. It really means ``flying machine,'' but is
never used in that sense nowadays.
Aeroplane--A power-driven aerofoil with stabilizing and controlling
surfaces.
Acceleration--The rate of change of velocity.
Angle of Incidence--The angle at which the ``neutral lift line'' of
a surface attacks the air.
Angle of Incidence, Rigger's--The angle the chord of a surface makes
with a line parallel to the axis of the propeller.
Angle of Incidence, Maximum--The greatest angle of incidence at
which, for a given power, surface (including detrimental surface),
and weight, horizontal flight can be maintained.
Angle of Incidence, Minimum--The smallest angle of incidence at
which, for a given power, surface (including detrimental surface),
and weight, horizontal flight can be maintained.
Angle of Incidence, Best Climbing--That angle of incidence at which
an aeroplane ascends quickest. An angle approximately halfway
between the maximum and optimum angles.
Angle of Incidence, Optimum--The angle of incidence at which the
lift-drift ratio is the highest.
Angle, Gliding--The angle between the horizontal and the path along
which an aeroplane at normal flying speed, but not under engine
power, descends in still air.
Angle, Dihedral--The angle between two planes.
Angle, Lateral Dihedral--The lifting surface of an aeroplane is said to
be at a lateral dihedral angle when it is inclined upward towards
its wing-tips.
Angle, Longitudinal Dihedral--The main surface and tail surface are
said to be at a longitudinal dihedral angle when the projections
of their neutral lift lines meet and produce an angle above them.
Angle, Rigger's Longitudinal Dihedral--Ditto, but substituting
``chords'' for ``neutral life lines.''
Angle, Pitch--The angle at any given point of a propeller, at which
the blade is inclined to the direction of motion when the propeller
is revolving but the aeroplane stationary.
Altimeter--An instrument used for measuring height.
Air-Speed Indicator--An instrument used for measuring air pressures
or velocities. It consequently indicates whether the surface is
securing the requisite reaction for flight. Usually calibrated in
miles per hour, in which case it indicates the correct number of
miles per hour at only one altitude. This is owing to the density
of the air decreasing with increase of altitude and necessitating
a greater speed through space to secure the same air pressure
as would be secured by less speed at a lower altitude. It would
be more correct to calibrate it in units of air pressure.
Air Pocket--A local movement or condition of the air causing an
aeroplane to drop or lose its correct attitude.
Aspect-Ratio--The proportion of span to chord of a surface.
Air-Screw (Propeller)--A surface so shaped that its rotation about
an axis produces a force (thrust) in the direction of its axis.
Aileron--A controlling surface, usually situated at the wing-tip, the
operation of which turns an aeroplane about its longitudinal axis;
causes an aeroplane to tilt sideways.
Aviation--The art of driving an aeroplane.
Aviator--The driver of an aeroplane.
Barograph--A recording barometer, the charts of which can be calibrated
for showing air density or height.
Barometer--An instrument used for indicating the density of air.
Bank, to--To turn an aeroplane about its longitudinal axis (to tilt
sideways) when turning to left or right.
Biplane--An aeroplane of which the main lifting surface consists
of a surface or pair of wings mounted above another surface or
pair of wings.
Bay--The space enclosed by two struts and whatever they are fixed to.
Boom--A term usually applied to the long spars joining the tail of a
``pusher'' aeroplane to its main lifting surface.
Bracing--A system of struts and tie wires to transfer a force from
one point to another.
Canard--Literally ``duck.'' The name which was given to a type of
aeroplane of which the longitudinal stabilizing surface (empennage)
was mounted in front of the main lifting surface. Sometimes
termed ``tail-first'' aeroplanes, but such term is erroneous,
as in such a design the main lifting surface acts as, and is, the
empennage.
Cabre--To fly or glide at an excessive angle of incidence; tail down.
Camber--Curvature.
Chord--Usually taken to be a straight line between the trailing and
leading edges of a surface.
Cell--The whole of the lower surface, that part of the upper surface
directly over it, together with the struts and wires holding them
together.
Centre (Line) of Pressure--A line running from wing-tip to wing-tip,
and through which all the air forces acting upon the surface may
be said to act, or about which they may be said to balance.
Centre (Line) of Pressure, Resultant--A line transverse to the
longitudinal axis, and the position of which is the resultant of the
centres of pressure of two or more surfaces.
Centre of Gravity--The centre of weight.
Cabane--A combination of two pylons, situated over the fuselage,
and from which anti-lift wires are suspended.
Cloche--Literally ``bell.'' Is applied to the bell-shaped construction
which forms the lower part of the pilot's control lever in
a Bleriot monoplane, and to which the control cables are
attached.
Centrifugal Force--Every body which moves in a curved path is
urged outwards from the centre of the curve by a force termed
``centrifugal.''
Control Lever--A lever by means of which the controlling surfaces
are operated. It usually operates the ailerons and elevator. The
``joy-stick".
Cavitation, Propeller--The tendency to produce a cavity in the air.
Distance Piece--A long, thin piece of wood (sometimes tape) passing
through and attached to all the ribs in order to prevent them from
rolling over sideways.
Displacement--Change of position.
Drift (of an aeroplane as distinct from the propeller)--The horizontal
component of the reaction produced by the action of driving
through the air a surface inclined upwards and towards its direction
of motion PLUS the horizontal component of the reaction produced
by the ``detrimental'' surface PLUS resistance due to
``skin-friction.'' Sometimes termed ``head-resistance.''
Drift, Active--Drift produced by the lifting surface.
Drift, Passive--Drift produced by the detrimental surface.
Drift (of a propeller)--Analogous to the drift of an aeroplane. It is
convenient to include ``cavitation'' within this term.
Drift, to--To be carried by a current of air; to make leeway.
Dive, to--To descend so steeply as to produce a speed greater than the
normal flying speed.
Dope, to--To paint a fabric with a special fluid for the purpose of
tightening and protecting it.
Density--Mass of unit volume, for instance, pounds per cubic foot.
Efficiency--Output
Input
Efficiency (of an aeroplane as distinct from engine and propeller)--
Lift and Velocity
Thrust (= aeroplane drift)
Efficiency, Engine--Brake horse-power
Indicated horse-power
Efficiency, Propeller-- Thrust horse-power
Horse-power received from engine
(= propeller drift)
NOTE.--The above terms can, of course, be expressed in footpounds.
It is then only necessary to divide the upper term by
the lower one to find the measure of efficiency.
Elevator--A controlling surface, usually hinged to the rear of the tailplane,
the operation of which turns an aeroplane about an axis
which is transverse to the direction of normal horizontal flight.
Empennage--See ``Tail-plane.''
Energy--Stored work. For instance, a given weight of coal or petroleum
stores a given quantity of energy which may be expressed
in foot-pounds.
Extension--That part of the upper surface extending beyond the
span of the lower surface.
Edge, Leading--The front edge of a surface relative to its normal
direction of motion.
Edge, Trailing--The rear edge of a surface relative to its normal
direction of motion.
Factor of Safety--Usually taken to mean the result found by dividing
the stress at which a body will collapse by the maximum stress
it will be called upon to bear.
Fineness (of stream-line)--The proportion of length to maximum width.
Flying Position--A special position in which an aeroplane must be
placed when rigging it or making adjustments. It varies with
different types of aeroplanes. Would be more correctly described
as ``rigging position.''
Fuselage--That part of an aeroplane containing the pilot, and to which
is fixed the tail-plane.
Fin--Additional keel-surface, usually mounted at the rear of an
aeroplane.
Flange (of a rib)--That horizontal part of a rib which prevents it
from bending sideways.
Flight--The sustenance of a body heavier than air by means of its
action upon the air.
Foot-pound--A measure of work representing the weight of 1 lb.
raised 1 foot.
Fairing--Usually made of thin sheet aluminum, wood, or a light
construction of wood and fabric; and bent round detrimental
surface in order to give it a ``fair'' or ``stream-like'' shape.
Gravity--Is the force of the Earth's attraction upon a body. It
decreases with increase of distance from the Earth. See ``Weight.''
Gravity, Specific--Density of substance
Density of water.
Thus, if the density of water is 10 lb. per unit volume, the same
unit volume of petrol, if weighing 7 lb., would be said to have a
specific gravity of 7/10, i.e., 0.7.
Gap (of an aeroplane)--The distance between the upper and lower
surfaces of a biplane. In a triplane or multiplane, the distance
between a surface and the one first above it.
Gap, Propeller--The distance, measured in the direction of the thrust,
between the spiral courses of the blades.
Girder--A structure designed to resist bending, and to combine lightness
and strength.
Gyroscope--A heavy circular wheel revolving at high speed, the effect
of which is a tendency to maintain its plane of rotation against
disturbing forces.
Hangar--An aeroplane shed.
Head-Resistance--Drift. The resistance of the air to the passage of
a body.
Helicopter--An air-screw revolving about a vertical axis, the direction
of its thrust being opposed to gravity.
Horizontal Equivalent--The plan view of a body whatever its attitude
may be.
Impulse--A force causing a body to gain or lose momentum.
Inclinometer--A curved form of spirit-level used for indicating the
attitude of a body relative to the horizontal.
Instability--An inherent tendency of a body, which, if the body is
disturbed, causes it to move into a position as far as possible away
from its first position.
Instability, Neutral--An inherent tendency of a body to remain in the
position given it by the force of a disturbance, with no tendency
to move farther or to return to its first position.
Inertia--The inherent resistance to displacement of a body as distinct
from resistance the result of an external force.
Joy-Stick--See ``Control Lever.''
Keel-Surface--Everything to be seen when viewing an aeroplane from
the side of it.
King-Post--A bracing strut; in an aeroplane, usually passing through
a surface and attached to the main spar, and from the end or ends
of which wires are taken to spar, surface, or other part of the
construction in order to prevent distortion. When used in connection
with a controlling surface, it usually performs the additional
function of a lever, control cables connecting its ends with the
pilot's control lever.
Lift--The vertical component of the reaction produced by the action
of driving through the air a surface inclined upwards and towards
its direction of motion.
Lift, Margin of--The height an aeroplane can gain in a given time and
starting from a given altitude.
Lift-Drift Ratio--The proportion of lift to drift.
Loading--The weight carried by an aerofoil. Usually expressed in
pounds per square foot of superficial area.
Longeron--The term usually applied to any long spar running lengthways
of a fuselage.
Mass--The mass of a body is a measure of the quantity of material
in it.
Momentum--The product of the mass and velocity of a body is known
as ``momentum.''
Monoplane--An aeroplane of which the main lifting surface consists
of one surface or one pair of wings.
Multiplane--An aeroplane of which the main lifting surface consists
of numerous surfaces or pairs of wings mounted one above the
other.
Montant--Fuselage strut.
Nacelle--That part of an aeroplane containing the engine and
pilot and passenger, and to which the tail plane is not fixed.
Neutral Lift Line--A line taken through a surface in a forward direction
relative to its direction of motion, and starting from its
trailing edge. If the attitude of the surface is such as to make
the said line coincident with the direction of motion, it results
in no lift, the reaction then consisting solely of drift. The position
of the neutral lift line, i.e., the angle it makes with the chord,
varies with differences of camber, and it is found by means of
wind-tunnel research.
Newton's Laws of Motion--1. If a body be at rest, it will remain at
rest; or, if in motion, it will move uniformly in a straight line
until acted upon by some force.
2. The rate of change of the quantity of motion (momentum) is
proportional to the force which causes it, and takes place in the
direction of the straight line in which the force acts. If a body
be acted upon by several forces, it will obey each as though the
others did not exist, and this whether the body be at rest or in
motion.
3. To every action there is opposed an equal and opposite
reaction.
Ornithopter (or Orthopter)--A flapping wing design of aircraft intended
to imitate the flight of a bird.
Outrigger--This term is usually applied to the framework connecting
the main surface with an elevator placed in advance of it. Sometimes
applied to the ``tail-boom'' framework connecting the
tail-plane with the main lifting surface.
Pancake, to--To ``stall ''
Plane--This term is often applied to a lifting surface. Such application
is not quite correct, since ``plane'' indicates a flat surface,
and the lifting surfaces are always cambered.
Propeller--See ``Air-Screw.''
Propeller, Tractor--An air-screw mounted in front of the main lifting
surface.
Propeller, Pusher--An air-screw mounted behind the main lifting surface.
Pusher--An aeroplane of which the propeller is mounted behind the
main lifting surface.
Pylon--Any V-shaped construction from the point of which wires
are taken.
Power--Rate of working.
Power, Horse--One horse-power represents a force sufficient to raise
33,000 lbs. 1 foot in a minute.
Power, Indicated Horse--The I.H.P. of an engine is a measure of the
rate at which work is done by the pressure upon the piston or
pistons, as distinct from the rate at which the engine does work.
The latter is usually termed ``brake horse-power,'' since it may be
measured by an absorption brake.
Power, Margin of--The available quantity of power above that necessary
to maintain horizontal flight at the optimum angle.
Pitot Tube--A form of air-speed indicator consisting of a tube with
open end facing the wind, which, combined with a static pressure
or suction tube, is used in conjunction with a gauge for measuring
air pressures or velocities. (No. 1 in diagram.)
Pitch, Propeller--The distance a propeller advances during one revolution
supposing the air to be solid.
Pitch, to--To plunge nose-down.
Reaction--A force, equal and opposite to the force of the action producing
it.
Rudder--A controlling surface, usually hinged to the tail, the operation
of which turns an aeroplane about an axis which is vertical in
normal horizontal flight; causes an aeroplane to turn to left or
right of the pilot.
Roll, to--To turn about the longitudinal axis.
Rib, Ordinary--A light curved wooden part mounted in a fore and aft
direction within a surface. The ordinary ribs give the surface
its camber, carry the fabric, and transfer the lift from the fabric
to the spars.
Rib, Compression--Acts as an ordinary rib, besides bearing the stress
of compression produced by the tension of the internal bracing
wires.
Rib, False--A subsidiary rib, usually used to improve the camber of
the front part of the surface.
Right and Left Hand--Always used relative to the position of the
pilot. When observing an aeroplane from the front of it, the
right hand side of it is then on the left hand of the observer.
Remou--A local movement or condition of the air which may cause
displacement of an aeroplane.
Rudder-Bar--A control lever moved by the pilot's feet, and operating
the rudder.
Surface--See ``Aerofoil.''
Surface, Detrimental--All exterior parts of an aeroplane including
the propeller, but excluding the (aeroplane) lifting and (propeller)
thrusting surfaces.
Surface, Controlling--A surface the operation of which turns an aeroplane
about one of its axes.
Skin-Friction--The friction of the air with roughness of surface. A
form of drift.
Span---The distance from wing-tip to wing-tip.
Stagger--The distance the upper surface is forward of the lower surface
when the axis of the propeller is horizontal.
Stability--The inherent tendency of a body, when disturbed, to return
to its normal position.
Stability, Directional--The stability about an axis which is vertical
during normal horizontal flight, and without which an aeroplane
has no natural tendency to remain upon its course.
Stability, Longitudinal--The stability of an aeroplane about an axis
transverse to the direction of normal horizontal flight, and without
which it has no tendency to oppose pitching and tossing.
Stability, Lateral--The stability of an aeroplane about its longitudinal
axis, and without which it has no tendency to oppose sideways
rolling.
Stabilizer--A surface, such as fin or tail-plane, designed to give an
aeroplane inherent stability.
Stall, to--To give or allow an aeroplane an angle of incidence greater
than the ``maximum'' angle, the result being a fall in the liftdrift
ratio, the lift consequently becoming less than the weight of
the aeroplane, which must then fall, i.e., ``stall'' or ``pancake.''
Stress--Burden or load.
Strain--Deformation produced by stress.
Side-Slip, to--To fall as a result of an excessive ``bank'' or ``roll.''
Skid, to--To be carried sideways by centrifugal force when turning
to left or right.
Skid, Undercarriage--A spar, mounted in a fore and aft direction, and
to which the wheels of the undercarriage are sometimes attached.
Should a wheel give way the skid is then supposed to act like the
runner of a sleigh and to support the aeroplane.
Skid, Tail--A piece of wood or other material, orientable, and fitted
with shock absorbers, situated under the tail of an aeroplane in
order to support it upon the ground and to absorb the shock of
alighting.
Section--Any separate part of the top surface, that part of the bottom
surface immediately underneath it, with their struts and wires.
Spar--Any long piece of wood or other material.
Spar, Main--A spar within a surface and to which all the ribs are
attached, such spar being the one situated nearest to the centre
of pressure. It transfers more than half the lift from the ribs
to the bracing.
Spar, Rear--A spar within a surface, and to which all the ribs are
attached, such spar being situated at the rear of the centre of
pressure and at a greater distance from it than is the main spar.
It transfers less than half of the lift from the ribs to the bracing.
Strut--Any wooden member intended to take merely the stress of
direct compression.
Strut, Interplane--A strut holding the top and bottom surfaces apart.
Strut, Fuselage--A strut holding the fuselage longerons apart. It
should be stated whether top, bottom, or side. If side, then it
should be stated whether right or left hand. Montant.
Strut, Extension--A strut supporting an ``extension'' when not in
flight. It may also prevent the extension from collapsing upwards
during flight.
Strut, Undercarriage--
Strut, Dope--A strut within a surface, so placed as to prevent the
tension of the doped fabric from distorting the framework.
Serving--To bind round with wire, cord, or similar material. Usually
used in connection with wood joints and wire cable splices.
Slip, Propeller--The pitch less the distance the propeller advances
during one revolution.
Stream-Line--A form or shape of detrimental surface designed to
produce minimum drift.
Toss, to--To plunge tail-down.
Torque, Propeller--The tendency of a propeller to turn an aeroplane
about its longitudinal axis in a direction opposite to that in which
the propeller revolves.
Tail-Slide--A fall whereby the tail of an aeroplane leads.
Tractor--An aeroplane of which the propeller is mounted in front of
the main lifting surface.
Triplane--An aeroplane of which the main lifting surface consists of
three surfaces or pairs of wings mounted one above the other.
Tail-Plane--A horizontal stabilizing surface mounted at some distance
behind the main lifting surface. Empennage.
Turnbuckle--A form of wire-tightener, consisting of a barrel into each
end of which is screwed an eyebolt. Wires are attached to the
eyebolts and the required degree of tension is secured by means
of rotating the barrel.
Thrust, Propeller--See ``Air-Screw.''
Undercarriage--That part of an aeroplane beneath the fuselage or
nacelle, and intended to support the aeroplane when at rest, and
to absorb the shock of alighting.
Velocity--Rate of displacement; speed.
Volplane--A gliding descent.
Weight--Is a measure of the force of the Earth's attraction (gravity)
upon a body. The standard unit of weight in this country is
1 lb., and is the force of the Earth's attraction on a piece of platinum
called the standard pound, deposited with the Board of Trade
in London. At the centre of the Earth a body will be attracted
with equal force in every direction. It will therefore have no
weight, though its mass is unchanged. Gravity, of which weight
is a measure, decreases with increase of altitude.
Web (of a rib)--That vertical part of a rib which prevents it from
bending upwards.
Warp, to--To distort a surface in order to vary its angle of incidence.
To vary the angle of incidence of a controlling surface.
Wash--The disturbance of air produced by the flight of an aeroplane.
Wash-in--An increasing angle of incidence of a surface towards its
wing-tip.
Wash-out--A decreasing angle of incidence of a surface towards its
wing-tip.
Wing-tip--The right- or left-hand extremity of a surface.
Wire--A wire is, in Aeronautics, always known by the name of its
function.
Wire, Lift or Flying--A wire opposed to the direction of lift, and used
to prevent a surface from collapsing upward during flight.
Wire, Anti-lift or Landing--A wire opposed to the direction of gravity,
and used to sustain a surface when it is at rest.
Wire, Drift--A wire opposed to the direction of drift, and used to
prevent a surface from collapsing backwards during flight.
Wire, Anti-drift--A wire opposed to the tension of a drift wire, and
used to prevent such tension from distorting the framework.
Wire, Incidence--A wire running from the top of an interplane strut to
the bottom of the interplane strut in front of or behind it. It
maintains the ``stagger'' and assists in maintaining the angle
of incidence. Sometimes termed ``stagger wire.''
Wire, Bracing--Any wire holding together the framework of any part
of an aeroplane. It is not, however, usually applied to the wires
described above unless the function performed includes a function
additional to those described above. Thus, a lift wire, while
strictly speaking a bracing wire, is not usually described as one
unless it performs the additional function of bracing some welldefined
part such as the undercarriage. It will then be said to
be an ``undercarriage bracing lift wire.'' It might, perhaps,
be acting as a drift wire also, in which case it will then be described
as an ``undercarriage bracing lift-drift wire.'' It should
always be stated whether a bracing wire is (1) top, (2) bottom,
(3) cross, or (4) side. If a ``side bracing wire,'' then it should be
stated whether right- or left-hand.
Wire, Internal Bracing--A bracing wire (usually drift or anti-drift)
within a surface.
Wire, Top Bracing--A bracing wire, approximately horizontal and
situated between the top longerons of fuselate, between top tail
booms, or at the top of similar construction.
Wire, Bottom Bracing--Ditto, substituting ``bottom'' for ``top.''
Wire, Side Bracing--A bracing wire crossing diagonally a side bay
of fuselage, tail boom bay, undercarriage side bay or centre-section
side bay. This term is not usually used with reference to incidence
wires, although they cross diagonally the side bays of the
cell. It should be stated whether right- or left-hand.
Wire, Cross Bracing--A bracing wire, the position of which is diagonal
from right to left when viewing it from the front of an aeroplane.
Wire, Control Bracing--A wire preventing distortion of a controlling
surface.
Wire, Control--A wire connecting a controlling surface with the pilot's
control lever, wheel, or rudder-bar.
Wire, Aileron Gap--A wire connecting top and bottom ailerons.
Wire, Aileron Balance--A wire connecting the right- and left-hand top
ailerons. Sometimes termed the ``aileron compensating wire.''
Wire, Snaking--A wire, usually of soft metal, wound spirally or tied
round another wire, and attached at each end to the framework.
Used to prevent the wire round which it is ``snaked'' from becoming,
in the event of its displacement, entangled with the
propeller.
Wire, Locking--A wire used to prevent a turnbuckle barrel or other
fitting from losing its adjustment.
Wing--Strictly speaking, a wing is one of the surfaces of an ornithopter.
The term is, however, often applied to the lifting surface of
an aeroplane when such surface is divided into two parts, one being
the left-hand ``wing,'' and the other the right-hand ``wing.''
Wind-Tunnel--A large tube used for experimenting with surfaces and
models, and through which a current of air is made to flow by
artificial means.
Work--Force X displacement.
Wind-Screen--A small transparent screen mounted in front of the
pilot to protect his face from the air pressure.