AEROPLANES
Of all the great inventions perhaps the most striking because of the
suddenness with which they have come upon us are those relating to the
navigation of the air. Until a few years ago "to fly" was taken to
typify the impossible. Now we see men flying every day and there is
scarcely anyone who has not had a friend or relative in the Flying
Corps.
Recent experience, too, has shown that this one invention has
revolutionized warfare in several important departments, particularly in
the use of very heavy long-range artillery. Huge guns, hidden in a
hollow or behind a hill, have been set to throw shells on to an unseen
target, while a man in an aeroplane above watches the result and signals
back by wireless. Thus by the aid of aircraft the power of artillery has
been immensely increased.
Again, aircraft have superseded cavalry for reconnaissance purposes,
that is to say, for finding out the enemy's strength and preparedness.
Only a few years ago a General who needed information as to his foe
would send forward a screen of cavalrymen who would cautiously creep
forward until, judging by what they could see and by what sort of a
reception they got, they were able to form some idea of the foe's
arrangements. Nowadays, however, the airmen sail over his head and take
photographs of him and his positions. A careful commander to-day not
only screens his men and his guns from view along the land but he also
tries his best to make them invisible from above. And, speaking of
inventions, the soldiers have shown a degree of ingenuity in making
themselves and their guns invisible which almost merits a volume to
itself.
The airman, therefore, goes up and sails over the enemy. He may be
simply observing for some particular unit of artillery, or he may be
sent to find out things generally--nothing in particular, but anything
which seems likely to be of use. He looks out intently and carefully,
moreover he not only looks with his own eyes: as has just been
mentioned, he takes photographs, which can be developed on his return
and studied minutely at leisure. He may, or may not, according to
circumstances, send back reports of an urgent nature by wireless
telegraphy.
In some cases these duties are all carried out by one man, but in others
there are two: one the pilot who looks after the working of the machine,
and the other the observer whose whole attention can thus be devoted to
scrutinizing the enemy.
Of course, when aeroplanes go on scouting expeditions like this they are
apt to be attacked by the enemy both by anti-aircraft guns and also by
other aeroplanes. The former can only be met by high speed and the
steering of a somewhat erratic course so as to confuse the gunners and
prevent them from taking good aim.
The other aeroplanes, however, must be met by actual fighting. The only
way to defeat them is to go for them and attack them, a machine-gun
being the most usual weapon.
Besides those who go up for definite scouting operations or to "spot,"
as it is termed, for the artillery, there are other machines whose sole
duty is fighting. These go up for the purpose of driving off those
machines of the enemy which may come prying, or to keep the ground, so
to speak, for the scouting machines and enable them to do their work
unmolested.
Then there are, of course, still others whose function is to carry out
bombing expeditions.
All these different duties call for different types of machine, but I do
not propose to go into the differences here since changes are so rapid
in this particular field that only the general principles remain
unchanged for any length of time. What has just been hinted, however, as
to the different kinds of work which the aeroplane is called upon to do
will enable the reader to see why different kinds of machines are
needed.
So far we have only spoken of aeroplanes. There is a kind of machine
sometimes called a hydroplane but which we are gradually getting to call
a sea-plane. The latter term is much to be preferred, since the former
is also in use to denote a special kind of high-speed boat.
Now a sea-plane only differs from an aeroplane in that it has floats
instead of wheels. The aeroplane has wheels to enable it to alight upon
and arise from the ground: the sea-plane has floats by which it can
alight upon the water and arise from the water also.
In some instances this float idea is made so pronounced a feature of the
machine that it becomes a flying boat.
Sea-planes are therefore really only aeroplanes specially adapted for a
certain purpose. They are really just as much aeroplanes as those
machines which go by that name. It is somewhat unfortunate, therefore,
that a separate term is used to describe them. But there it is: names
grow in a very curious way, not always in a logical way, and a name
having once stuck to a thing in the mind of the public it is very
difficult to make any alteration.
Aeroplanes, then, may be said to include a subdivision known as
sea-planes, and for the rest of this chapter what is said of aeroplanes
will apply to sea-planes also.
Without doubt, these are the fastest vehicles in existence. Many of them
can exceed a speed of a hundred miles an hour. Consequently, the pilot
lives while he is aloft in the equivalent of a furious gale, and it
would seem as if that must produce such a degree of cold as to be almost
unendurable. Moreover, it appears that this cold is almost as bad in
summer as in winter, for the temperature high up in the air is much the
same all the year round. The consequent muffling up with thick clothes
and gloves, while it mitigates the cold, must add greatly to the pilot's
difficulties in managing his machine. The protection for his eyes and
ears which is made necessary by the same conditions must likewise add to
his difficulties or at any rate to his discomfort. On the other hand,
the effect of gliding at a very high speed over a perfectly smooth
track, for that is in effect what it is, is very exhilarating, which to
some extent compensates for the other drawbacks.
Moreover, the handling of such a machine in the air, particularly if a
fight is included in the programme, appeals strongly to the sporting
instincts of young men, so much so that during the War, in spite of the
dangers and hardships, and the continual loss of life, there was never a
dearth of men anxious to become pilots.
Owing to these considerations, too, it follows that the best aviators
are to be found in those lands where the people are most devoted to
sports. Hence, as we have it on excellent authority, the young men of
Great Britain and the United States, with their love of adventure and
their strong sporting instincts, make better men in the air than the
Germans.
But really we are more concerned here with the machines than with the
men, so let us get back to our subject.
The aeroplane consists of one or more "planes" or surfaces which, on
being held at a certain slant and then pushed forward rise or remain
supported in the air. Therefore the plane or planes need to be
supplemented by first a tail and horizontal rudder to hold them at the
correct slant, and an engine and propeller to drive them forward.
It is not necessary, here, to go over the history of the aeroplane, as
that has been told so often. It is not of much interest, moreover,
except to those who are particularly concerned with small details of
construction, for in a general way the machine of to-day is very little
different from one pictured by Sir George Cayley a hundred years ago. It
is only the perfecting of the details which has transformed a dream into
a very real thing.
So we will look only at the construction of the aeroplane in a general
way, to do which we must first consider why it flies at all. It is due
to the well-established law that action is always accompanied by a
reaction equally strong and in the opposite direction. When a gun is
fired the explosion not only drives the shell forward but equally drives
the gun itself backward. The backward energy of the recoil is precisely
equal to the forward energy of the shell. The two are equal but in
opposite directions. In like manner a rocket ascends because the hot
gases from the paper cylinder blow forcibly downwards, thereby producing
an equal reaction upwards.
Now the plane of a flying machine is held with its forward edge a little
higher than its rear edge, so that as it is pushed along it tends to
catch the air and throw it downwards. Hence the reaction tends to lift
the plane upwards. When the machine starts the reaction is not
sufficient to overcome gravity, which is trying to hold the machine down
upon the ground, but as the speed increases and the air is thrust down
with more and more violence the point is ultimately reached when the
reaction is able to overcome gravity and the machine ascends.
When a sufficient height is reached, the pilot alters the position of
his horizontal rudder or "elevator" so as to make the position of the
plane more flat, with the result that it throws the air downwards to a
less extent, and the reaction is thereby reduced until it is only just
sufficient to keep the machine at the same height. To descend, the
position of the plane is made still flatter, the reaction is reduced
still more and gravity has its way once again, bringing the machine to
earth.
In other words, the machine acts under the influence of two forces: the
downward pull of gravity and the upward reaction due to the action of
the machine in throwing the air downward. The former never varies, the
latter can be varied by the pilot at will: he can increase it by
increasing the speed or by increasing the tilt of his plane or planes:
he can reduce it by diminishing the speed or the tilt. Since generally
speaking the speed of his engine will remain constant, he rises, remains
at the same height or falls, at will, by the simple manipulation of the
elevator through which he can change the tilt or inclination.
Most machines have a fixed tail as well as a horizontal rudder or
elevator, the same being so set that it tends to keep the plane in a
certain normal inclination, the elevator being called in to increase
that or diminish it as may be required.
In addition to the elevator there is also another rudder of the ordinary
kind, such as every ship and boat has, for guiding the machine to right
or left. The elevator steers up and down, the rudder steers to either
hand.
Provision is also made for balancing the machine. This is sometimes in
the form of two small planes hinged to the main plane, one at either
end, connected together and to a controlling lever by wires, so that by
their use the pilot can steer the right-hand side of his machine upwards
and the left-hand downward, or vice versa, if through any cause he finds
a tendency to capsize.
In some machines the same effect is produced not by separate planes but
by pulling the main plane itself somewhat out of shape, but precisely
the same principle is involved.
The planes are usually made with a slight curve in them, so that they
may the better catch the air and "scoop" it downwards, so to speak. They
usually consist of fabric specially made for the purpose, stretched upon
a light wooden framework. The whole framework is usually of wood with
metal fittings frequently made of aluminium for the sake of lightness.
The engines have been mentioned in another chapter. The propeller which
is almost invariably fixed directly upon the shaft of the engine has two
blades only and not three as is usual with those of ships. Precisely why
this should be so is not clear, but experience shows that two-bladed
propellers are preferable for this work. They are made of wood, several
layers being glued together under pressure, the resulting log being then
carved out to the required shape. This makes a stronger thing than it
would be if cut out of a single piece of wood.
All parts, engine, elevator, rudder and balancing arrangement, are
controlled by very simple means from the pilot's seat.
In monoplanes there is but one main plane, resembling a pair of bird's
wings. Or if we care to look upon it as two planes, one each side of the
"body," then we must call it a pair. Since the name "mono" indicates one
it is best to think of it as one plane although it may be in two parts.
The biplane has, as its name implies, two planes, but in that case there
can be no doubt, since they are placed one above the other. Machines
have been made with three planes and even with as many as five, but
monoplanes and biplanes appear to hold the field.
It is not possible for an aeroplane to be in any sense armoured for
protection against bullets: for defence the pilot has to depend upon his
own cunning man[oe]uvres combined with the fast speed at which he can
move. For offensive purposes he usually has a machine gun mounted right
in front of him with which he can pour a stream of bullets into an
opponent or even, by flying low, he can attack a body of infantry. It is
recorded that one German prisoner during the war, speaking of the daring
of the British pilots in thus attacking men on foot, exclaimed, "They
will pull the caps off our heads next."
Some of the aeroplanes have their propeller behind the pilot and some
have it in front. The latter, to distinguish them, are called "Tractor"
machines, since in their case the propeller pulls them along. Now it is
easy to see that a difficulty arises in such cases through the best
position for the gun being such that it throws its bullets right on to
the propeller. But that has been overcome in a most simple yet ingenious
way. The gun is itself operated by the engine with the result that a
bullet can only be shot forth during those intervals when neither blade
of the propeller is in the way. The propeller is moving so fast that it
cannot be seen and the bullets are flying out in a continuous rattle,
yet every bullet passes between the blades and not one ever touches.
It is easy to see that when an aeroplane is manned by a single man, as
is often the case, he must have his hands very full indeed, what with
the machine itself and the gun as well. In fact, he often has to leave
the machine for a short time to look after itself while he busies
himself with the gun.
Now there we see a sign of the wonderful work which has been done in the
course of but a few years in the perfecting of the aeroplane, the result
of a series of improvements in detail which make but a dreary story if
related but which make all the difference between the risky, uncertain
machine of a few years ago and the safe, reliable machine of to-day.
Modern machines are inherently stable. The older ones had the elements
of stability in them but they were so crudely proportioned that these
inherent qualities did not have a chance to come into play.
If one drops a flat card edgewise from a height it seems as if it ought
to fall straight down to the ground. Yet we all know from experience
that it seldom does anything of the kind. Instead, it assumes a position
somewhere near horizontal and then descends in a series of swoops from
side to side. There we see the principle at work which, in a
well-designed aeroplane, causes inherent stability. The explanation is
as follows.
The aeroplane is sustained in the air through the upward pressure of the
air resisting the downward pull of gravity. That has been fully
explained already. Now gravity, as we all know, acts upon every part of
a body whether it be an aeroplane or anything else. But for practical
purposes, we may regard its action as concentrated at one particular
point in that body, called the "centre of gravity." Likewise, the upward
pressure of the air acts upon the whole of the under surface of the
plane or planes, yet we may regard it as concentrated at a certain point
called the "centre of pressure." Further, we all know from experience
that a pendulum or other suspended body is only still when its centre of
gravity is exactly under the point of suspension. If we move it to
either side it will swing back again.
In just the same way, the only position in which an aeroplane will
remain steady is that in which the centre of gravity is exactly under
the point of suspension or, in other words, the centre of pressure. For
the centre of pressure in the aeroplane is precisely similar to the
point of suspension of a pendulum.
Let us, then, picture to ourselves an aeroplane flying along on a
horizontal course with this happy state of things prevailing. Something
we will suppose occurs to upset it with the result that it begins to
dive downwards. It is then in the position of sliding downhill and
instantly its speed increases in consequence. That increase of speed
causes the air to press a little more strongly than it did before upon
the front edge of the planes. In other words, the centre of pressure
shifts forward a little, with the result that the centre of gravity is
then a little to the rear of the centre of pressure.
A moment's reflection will show that with the centre of pressure (or
point of suspension) in advance of the centre of gravity there is a
tendency for the machine to turn upwards again, or, in other words, to
right itself.
If, on the other hand, the initial upset causes it to shoot upwards the
speed instantly falls off and the centre of pressure retreats, turning
the machine downwards once more. And the same principle applies whatever
the disturbance may be. Instantly and automatically a turning force
comes into play which tends to check and ultimately to correct what has
gone wrong.
This principle explains the behaviour of the card dropped from an
upstairs window and, no doubt, as has been said, it operated also in the
early flying machines, but in their case other factors caused disturbing
elements with which the self-righting tendency was not strong enough to
cope. As time went on, however, experience taught the makers how to
avoid these disturbing factors until at last the self-righting tendency
was able to act effectively, thus producing the aeroplane which is
inherently stable and which will, for short periods at all events, fly
safely without attention from its pilot.
Each little improvement in this direction was an invention. Of course,
there were certain men whose names stand out prominently in the history
of the aeroplane, notable among whom are the Wright brothers, but the
final result is due to innumerable inventions, many of them by unknown
men.
But perhaps someone will say, how can you possibly talk about final
results in a matter which is still in its infancy?
The answer to that is that so far as the safe, "flyable" machine is
concerned, it has arrived. Little now remains to be done in that
direction. Further improvements there will, of course, be, but the great
fundamental problems of flight have been solved.