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[Illustration: THE FLIGHT FOLK.]
THE AEROPLANE SPEAKS
BY H. BARBER, A.F.Ae.S.
(CAPTAIN, ROYAL FLYING CORPS)
WITH 36 FULL PAGES OF "TYPES OF AEROPLANES" AND 87 SKETCHES AND DIAGRAMS
_FIFTH EDITION_
LONDON
McBRIDE, NAST & CO., LTD.
THE AEROPLANE SPEAKS.
_First edition--December, 1916_
_Second edition--February, 1917_
_Third edition--April, 1917_
_Fourth edition--July, 1917_
_Fifth edition--December, 1917_
FIRST REVIEWS:
=C. G. G. in the AEROPLANE:= "One hopes that the Subaltern
Flying Officer will appreciate the gift which the author has
given him out of his own vast store of experience, for the book
contains the concentrated knowledge of many expensive years in
tabloid form, or perhaps one should say in condensed milk form,
seeing that it is easy to swallow and agreeable to the taste,
as well as wholesome and nourishing. And, besides the young
service aviator, there are thousands of young men, and women
also, now employed in the aircraft industry, who will appreciate
far better the value of the finicky little jobs they are doing
if they will read this book and see how vital is their work to
the man who flies."
=THE FIELD:= "Entirely different from any other text-book on
the subject, not merely in its form, but in its capacity to convey
a knowledge of the principles and practice of flying. Undoubtedly
it is the best book on its subject."
=THE UNITED SERVICE GAZETTE:= "Should be in the hands of every
person interested in aviation."
=THE OUTLOOK:= "As amusing as it is instructive."
=THE MORNING POST:= "Should be read and re-read by the would
be and even the experienced pilot."
PRINTED IN ENGLAND BY
BILLING AND SONS, LIMITED
GUILDFORD
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 "joy-stick" 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_
PAGE
_PART I.--THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES_ 1
_II.--THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES,
FINISH THE JOB_ 15
_III.--THE GREAT TEST_ 27
_IV.--CROSS COUNTRY_ 38
CHAPTER I.--FLIGHT 55
II.--STABILITY AND CONTROL 70
III.--RIGGING 90
IV.--PROPELLERS 115
V.--MAINTENANCE 126
TYPES OF AEROPLANES 130
GLOSSARY 133
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 an illustration thus:
[Illustration]
"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.
[Illustration: The action of the surface upon the air.]
"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 and consequent Force 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."
[Illustration]
"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
or 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 awfully dry."
[Illustration]
"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."
[Illustration]
"That's not practical politics," said the Surface. "The way you talk one
would think you were drawing £400 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:
[Illustration]
"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:
[Illustration]
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.
[Illustration: Maximum Climb. Maximum Speed.]
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-who-takes-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.
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."
[Illustration]
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 "nose-heavy" 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
counter-balance 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:
[Illustration]
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."
[Illustration: The angles shown above are only roughly approximate, as
they vary with different types of aeroplanes.]
"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?"
"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."
[Footnote 1: Propeller Slip: As the propeller screws through the air,
the latter to a certain extent gives back to the thrust of the propeller
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 propeller
will feel the slip as a strong draught of air.]
[Footnote 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.]
[Footnote 3: Pancakes: Pilot's slang for stalling an aeroplane and
dropping like a pancake.]
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.
"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."
[Illustration]
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" (see illustration A,
over-leaf).
"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."
[Illustration]
"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 _rôles_ have changed one with
the other, and I'm then the Elevator and the Elevator is me!"
[Illustration]
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 a certain axis of the
machine, and the Elevator is a Controlling Surface designed to turn the
Aeroplane about another 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:
[Illustration: H.E., Horizontal equivalent.]
"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 total 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.
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.
[Illustration]
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:
[Illustration]
"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."
[Illustration]
"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."
[Illustration: "Wash out" on both sides.]
"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."
[Illustration]
"Well, I hope that's all as it should be," she concluded, "for to-morrow
the Great Test in the air is due."
[Footnote 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.]
[Footnote 5: Skin friction is that part of the drift due to the friction
of the air with roughness upon the surface of the aeroplane.]
[Footnote 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.]
[Footnote 7: An explanation of the way in which the wash-out is combined
with a wash-in to offset propeller torque will be found on p. 82.]
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 "stream-lined" 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.
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
Pitot Air Speed Indicator, and, adjusting it to zero, smiles as he
hears the Pitot-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 Pitot 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--a
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."
"And who on earth are they?" asked the Loops, trembling for their
troublesome little lives.
"On 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.
"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 Triplex 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, crowned 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!"
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]
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!"
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 lower 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.
[Footnote 8: A.M.'s: Air Mechanics.]
[Footnote 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.]
[Footnote 10: Deviation Curve: A curved line indicating any errors in the
compass.]
[Footnote 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.]
[Footnote 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.]
[Footnote 13: Box-kite. The first crude form of biplane.]
PART IV
'CROSS COUNTRY
The Aeroplane had been designed and built, and tested in the air, and
now it 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 too 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 very 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, "Tanks 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
(see p. 40).
"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.
[Illustration:
A--B, 150 miles,
A--C, 50 miles; direction and miles per hour of wind.
C--D, 100 miles; airspeed of aeroplane.
A--D, Distance covered by aeroplane in one hour.
A--E, Compass course.]
"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.
[Illustration: The Pilot's Cock-pit.]
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 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 a 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 place it
appears, nestling down between the hills and 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 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
'cross-country 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 two-thirds 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
wire-cutters. 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 rain-storm," 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. A high tail
means 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:
[Illustration:
The Pilot's Aeroplane.
The Change of Design He Would Like.]
"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," says someone, "and that they haven't had
difficulties with the fog. It rolled up very quickly, you know."
"Never fear," remarks 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," says
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 who 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 walk 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.
[Illustration]
_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.
Flight is secured by driving through the air a surface[15] inclined
upwards and towards the direction of motion.
[Illustration]
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, if, as seems reasonable, we regard the surface from the point
of view of the neutral lift line. 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 criticized adversely
is then applicable. Flat lifting surfaces are, however, never used.
The surface acts upon the air in the following manner:
[Illustration]
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^2.
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 two-fifths 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.
[Illustration]
The direction of the reaction is, at efficient angles of incidence,
approximately at right-angles to the neutral lift line 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 roughness 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.
[Illustration]
_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 wind-tunnel 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 over the top of the surface 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 lift-drift ratio. Also, too great an
angle for the velocity will result in the underside of the surface
tending to compress the air against which it is driven rather than
accelerate it _downwards_, and that will tend to produce drift rather
than the _upwards_ reaction, or lift.
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 lift-drift 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 stream-line 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
thickness 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 higher the
aspect ratio, the greater the reaction. It is obvious, I think, that the
greater the span, the greater the mass of undisturbed air engaged, and,
as already explained, the reaction is partly the result of the mass of
air engaged. I say "undisturbed" advisedly, for otherwise it might be
argued that, whatever the shape of the surface, the same mass of air
would be engaged. The word "undisturbed" makes all the difference, for
it must be remembered that the rear part of the underside of the surface
engages air most of which has been deflected downwards by the surface
in front of it. That being so, the rear part of the surface has not the
same opportunity of forcing; the air downwards (since it is already
flowing downwards) and securing there from an upwards, reaction as has
the surface in front of it. It is therefore of less value for its area
than the front part of the surface, since it does less work and secures
less reaction--_i.e._, lift. Again, the rarefied area over the top of
the surface is most rare towards the front of it, as, owing to eddies,
the rear of such area tends to become denser.
[Illustration]
Thus, you see, the front part of the surface is the most valuable from
the point of view of securing an upwards reaction from the air; and so,
by increasing the proportion of front, or "span," to chord, we increase
the amount of reaction for a given velocity and area of surface. That
means a better proportion of reaction to weight of surface, though the
designer must not forget the drift of struts and wires necessary to
brace up a surface of high aspect ratio.
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 higher 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 wing-tips. 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 about 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.
[Illustration: H.E., Horizontal equivalent. D., Dihedral angle.]
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.
[Illustration]
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 requisite 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.
[Illustration: 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.=
SUMMARY.
_Essentials for Maximum Climb._
1. Low velocity.
2. Large surface.
3. Large angle relative to propeller thrust.
4. Large angle relative to direction of motion.
5. Large camber.
_Essentials for Maximum Velocity._
1. High velocity.
2. Small surface.
3. Small angle relative to propeller thrust.
4. Small angle relative to direction of motion.
5. Small camber.
It is mechanically impossible to construct an aeroplane of reasonable
weight of which it would be possible to vary 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. 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.
[Footnote 14: See Newton's laws in the Glossary at the end of the book.]
[Footnote 15: See "Aerofoil" in the Glossary.]