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 sh.e.l.l forward but equally drives the gun itself backward. The backward energy of the recoil is precisely equal to the forward energy of the sh.e.l.l. 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 pa.s.ses 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 a.s.sumes 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.

CHAPTER XXV

THE AERIAL LIFEBOAT

Balloons had not long been invented when the idea arose of a device by means of which an aeronaut who found himself in difficulties might be able to reach the ground in safety. In other words, the need was felt for something which should play towards the balloon the part which the lifeboat does to the ship.

The original idea of a parachute was even older than that, since we are told of a man away back in the seventeenth century who amused the King of Siam by jumping from a height and steadying his descent by means of a couple of umbrellas. It was not, however, until the very end of the eighteenth century or the beginning of the nineteenth that descents were made from really considerable heights from balloons.

The usual arrangement then was to have the parachute hanging at full length fastened below the basket, or tied to one side of the balloon in such a manner that it could be detached by cutting the cords that held it up. When the parachute was carried below the balloon basket the man was already in the cradle or seat of the parachute ready to be dropped, but when the seat was tied to the side of the car of the balloon the aeronaut, when he wished to make a descent, first got from the car into the seat, and, casting himself adrift from the car, swung out from under the centre of the balloon so that when he was hanging clear another man in the balloon cut the cords or pulled a slip-knot which set the parachute free. There were different ways of doing this and when a man was by himself he had to get into the sling of the parachute and, on finding himself clear of everything, he would give a tug to a cord which would release a catch holding up the parachute and allow it to drop to earth.

The parachute, at the very first, was but a simple affair, being little more than a circular sheet of cotton or similar fabric, but it was very soon found necessary to make it _a bag_ or it would not properly hold the air. Cords were attached at regular intervals all around the edge of this bag, these cords being gathered together and attached to the edge of a basket which carried the man. Sometimes only a sling was used, or a simple light seat after the fashion of the "bosun's chair" upon which a sailor is sometimes hauled to the top of an unclimbable mast, or a steeplejack to the top of a chimney.

Thus, when it was dropped, the weight of the man, pulling upon all the cords simultaneously, drew down the edge of the bag, which, catching the air in its fall, acted as a powerful brake and reduced the rate of falling to such an extent that if all went well the man alighted in safety if not comfort.

As has already been remarked in another chapter, air, which seems to us sometimes to be so exceedingly light as to have practically no weight at all, really has weight and also the property which we call inertia, by virtue of which things at rest prefer to stay at rest.

Now when this open air-bag, of considerable area, is pulled downwards it causes a very considerable disturbance in the air. As it descends the air inside and beneath it is first pushed downwards and compressed a little, then it commences to move outwards, towards the edge, round which it finally escapes to fill the slight vacuum in the s.p.a.ce just above the descending parachute. All this the air objects to do because of its inertia. The parachute has to force it to act thus and in that way it uses up some of the force of gravity which all the time is pulling the man earthwards. In other words, that force, instead of dragging the man downwards at such a speed as to dash him to pieces, is so far employed in churning up the air that what is left only brings him down quite slowly and ends with just a gentle b.u.mp. That is the scientific explanation of what happens, although expressed in somewhat homely language.

To anyone who thinks of this matter it will be clear that a relatively heavy weight like a man, suspended from a parachute, is like a very delicately poised pendulum, and consequently it is not surprising to hear that the early parachutes oscillated very considerably from side to side, so much so, indeed, that this oscillation became a decided danger, for before the proper shape of the air-bag was found out they sometimes skidded and even turned inside out. It was found, however, at quite an early stage that this instability could be to some extent cured by making a hole right in the centre or crown of the parachute through which the air compressed inside could blow upwards in a powerful jet. At first sight it seems as if this would much weaken the parachute and cause it to descend too quickly, but quite a large hole can be safely made, and to make such a hole is only the same thing as slightly reducing the area and that can be easily remedied by slightly increasing the diameter.

Reading of this many years ago, I have often been puzzled as to why the presence of the hole should have this steadying effect, the explanation given in the old scientific textbook from which I learnt it being obviously very unsatisfactory. Of recent years, however, this subject of parachutes has been very deeply studied by an eminent engineer of London, Mr. E. R. Calthrop, the inventor of the "Guardian Angel"

parachute to which these remarks are leading up, and he has. .h.i.t upon what is undoubtedly the explanation. He says that the big jet of air shooting upwards through the crown of the parachute forms in effect a rudder which steers the parachute in a straight downward course, just as the rudder guides a boat upon the surface of the water.

It is quite possible that thus far the impression conveyed to the reader's mind is that the parachute and its use are very simple, straightforward matters. One may be inclined to think that it is only necessary to get a circular sheet of fabric, to fasten the cords to it, to connect them to a suitable seat and then to descend from any height at any time in perfect safety. If you make a model from a flat sheet of cotton, then one made like a bag, and drop them with little weights attached from the top window of your house you will see what funny things the air can do. After having tried these little ones, you will begin to suspect that the big parachute is full of waywardness: and, as a matter of fact, until recent years, it has been very largely a delusion and a snare. By its refusal to act and open at the right moment it has sacrificed many lives. Although apparently so simple, there were conditions existing and forces at work which for a century or more had never been properly considered and investigated, and it is only now that we have arrived at a parachute whose certainty of action and general trustworthiness ent.i.tle it to be called the "lifeboat of the air."

The troubles with the older parachutes were two. First, although often it opened quite quickly, and carried its load as perfectly as could be desired, it sometimes had the habit of delaying its opening, and unless the fall were from a very great height it was unsafe to take the risk, indeed, it sometimes refused to open at all, and the poor parachutist suffered a fearful death. It had to be carried in a more or less folded-up state. Often it was hung up by its centre to the side of a balloon, when it was very like a shut-up umbrella. Consequently the power of opening quickly and certainly was of the first importance, and the lack of that power and the uncertainty of its action were a very serious defect. It has always suffered from an ill reputation as to reliability.

The second fault lay with the cords. They would persist in getting entangled. Everyone knows how a dozen cords hanging near together will get entangled with each other on the slightest provocation. Such cords if blown about by a strong wind would be much worse even than when still, and if, as must often be the case with parachutes, they be coiled up, we all know from our own experience that some of them would be almost sure to get knotted and tangled together when, in a sudden emergency, the attempt was made to pull them all out of their coils in a second or two. Just picture to yourself what it means: a dozen coiled cords all close together, themselves all coiled up in loops, suddenly pulled. Something awkward appears almost inevitable. And the result of even one rope going awry may be fatal, for it may prevent the parachute opening out fully, probably giving it a "lop-sided" form incapable of gripping the air effectually and consequently allowing the unfortunate man to fall with a velocity which means certain death. This second cause of failure to open, through entanglement of cordage, has happened in a number of cases, with fatal results.

So much for the faults of the old primitive parachute. Now let us consider for a moment the urgent need for a parachute which is free from such faults. The man who goes up in a balloon on a Sat.u.r.day afternoon feels so sure of his "craft" that he thinks he needs no "lifeboat," yet men in ordinary free balloons have been killed for want of them. The spectators at country fairs no longer appreciate a parachute descent as a great and extraordinary spectacle. But in warfare, with kite balloons by the dozen, with dirigible balloons by the score and aeroplanes by the hundred, the call for parachutes is urgent and irresistible. At all events, Mr. Calthrop found an irresistible call to devote years of close study, unceasing toil and considerable sums of money to the task of perfecting an improved parachute which would always open and open quickly, and whose cords would never get entangled. He has the satisfaction of knowing that by so doing he has provided an appliance that in the air is as reliable as a lifeboat is at sea, and that at all times, and from every kind of aircraft, can be depended upon in case of accident to save the lives of gallant airmen who but for his work would be dashed to death. The Great War has taught us to regard life somewhat cheaply. For years we were more concerned with taking life than with saving it, yet surely to save the life of one's own men is equivalent to taking the lives of one's opponents, so that even from the point of view of warfare the saving of life may be a help towards victory. This is particularly so when the lives saved are those of the choicest spirits, and among the most highly trained. It has been reckoned that to make a fully-trained pilot costs as much as 1500, so that to save but a few, even in their preparatory nights on the training-grounds where so many accidents happen, makes quite an appreciable difference in the cost of a war, without considering the main question of the men's lives.

Many inventions arise through a man thinking of an idea and then seeking and finding some application for it. Elsewhere in this book, I give examples of such cases. Here we have an instance of the opposite, for Mr. Calthrop found his thoughts strongly directed in this direction by the death of a personal friend, the Hon. C. S. Rolls, one of the early martyrs in the cause of aviation, not to mention others who shared the same risks and in some cases the same fate. His interest thus aroused, he first studied all the records which could be found relating to parachute accidents, so as to ascertain, if possible, what were the causes of failure. Then he commenced a long series of experiments with a view to removing these causes. Improvement after improvement was tried, unexpected difficulties were discovered and grappled with, the kinematograph was called in to record the movements of the falling objects, a task for which it is far better fitted than the human eye, and after years of this there emerged the finished parachute, automatic in its action, perfectly reliable and a true safeguard, which I am about to describe.

The parachute's body consists of the finest quality silk carefully cut into gussets of such a shape that when sewn together somewhat after the manner of the cover of an umbrella, they form a shallow bag, parabolic in section, of that particular shape which the material would a.s.sume naturally were it perfectly elastic when enclosing its resisting body of compressed air.