WHY THE AEROPLANE FLIES
THE aviator must venture in his frail craft upon an unknown and uncharted sea. The great problem is to ride the shifting air currents and keep the machine right side up. Although we cannot see the air currents, we know that they are constantly ebbing and flowing, piling themselves in great heaps, or slipping away in giddy vortices. There is much beautiful scenery, high mountain peaks, deep valleys, and level plains formed by these ever shifting air currents through which the aviator must steer his course blindly as best he may. A great bank of whirling clouds driven before the wind shows how rough and tumbling a sea he must navigate.
The air being a much thinner medium than water is, of course, far more unstable and baffling. Its supporting power is not only very small but constantly varies. The flying machine which will navigate successfully in a perfectly quiet atmosphere may be unseaworthy, or rather, unairworthy, when a wind springs up, or the shifting of the wind may spoil all the air pilot’s plans. To add to his troubles, the aviator must move among air currents which change and change again in a moment’s time. As we study the difficulties of air navigation we will appreciate, more than ever, the wonderful patience, skill, and daring of the successful aviators.
The action of the air currents had first to be carefully studied before flight became possible. Although the air is invisible we now know exactly how the air currents act upon the wings or planes. When a plane surface, such as the wing of an aëroplane, moves horizontally through the air, the air is caught for a moment underneath it and is pressed down slightly and a moment later slips out again from under the other edges at the sides and back. It is this air under pressure which yields a slight support.
It has been proven by many experiments that this supporting power varies with the shape of the plane or surface driven horizontally through the air. A long narrow surface driven sideways gains much more support from the air than the same area in the form of a square or any other shape. In other words, a square surface ten feet square containing 100 square feet will not travel as far as a surface twenty feet long and five feet wide.
The explanation is very simple. As the square surface moves along, the air is momentarily compressed under the front edge, but instantly slips off at the back and sides. As the broad surface of the rectangular plane cuts the air, however, few of the air currents can escape at the sides while the most of them are crowded together and held in place until they slip off at the back. The supporting power of the plane is therefore in direct proportion to the length of the front or, as it is called, the entering edge of the plane.
Here we find one of the secrets of the flight of birds. The spread between the tips of their outstretched wings is much greater than the width of the wings themselves. It also explains why the Wright model, for instance, should be so oddly shaped and should move sideways like a crab. If you study the models of the successful monoplanes with this in mind they have a new meaning. The law of the proportion of the entering edge is very important in designing an aëroplane.
It is so important for the air to be confined as long as possible beneath the gliding plane that many devices have been tried to hold it. Some planes are built with a slight edge running around the sides and back, on the under surface, to hem in the air. Some of the biplanes are built with closed sides, the cellular form they are called, to keep the air from slipping away. The box kite is constructed with this in view. The builder of model aëroplanes will find, however, that the slight edge formed by turning the cloth over the frame of the plane is sufficient to hold the air.
The flight of a kite, by the way, appears a very simple matter once this law is understood. The air currents strike the kite at an angle and are deflected or carrom off at exactly the same angle. A line drawn through the middle of this angle, exactly bisecting it, will give you the direction of the force exerted by the wind. Meanwhile the kite string holds the plane rigidly in position. As the kite darts from side to side it is merely obeying this law and adjusting itself so that its surface will stand at right angles to this thrust of the wind. An aëroplane is simply a kite which makes its own wind or air currents.
The kite is, of course, balanced against the wind currents and kept more or less stable by its cord, but an aëroplane must balance itself. The secret of insuring stability was discovered only after years of experience with gliders in actual flights.
The stability of the aëroplane depends upon the proper adjustment of the pressure of the air on the machine. There is, of course, a center of pressure, just as there is a center of gravity in every aëroplane of whatever form or size. It may be laid down as a general rule that a plane traveling horizontally in a quiet atmosphere is kept horizontal and stable by making the centers of pressure and gravity coincide.
The air currents, as we pointed out, are never entirely at rest but are constantly tilting the plane about. Hold a sheet of stiff paper horizontally and let it fall. It will flutter to the ground or perhaps be twirled away, indicating the presence of a number of unexpected air currents. The aëroplane which would remain stable in a perfectly quiet atmosphere must overcome all these twists and turns. The problem of stability has not yet of course been solved. Having reached this stage in the evolution of the aëroplane the aviator next began to experiment by bending his wings or planes and throwing out lateral or stability planes to help him keep his balance.
It was now found that a very little tilting of the planes upward or downward would serve to right the machine when it leaned over. The secret, like so many others, was gained by watching the flights of birds. You have perhaps seen a great albatross or sea gull soar without the slightest effort and apparently without motion. Look more closely and you will see that the tips of the broad wings move slightly from time to time, while the main body of the wings remains rigid, which is the great secret of stability in flight.
The ends of the planes were next made flexible, very slightly so, and arranged so that they might be moved up and down or flexed at will. The flights made with this adjustment were at once brought under control. New planes were added before and behind, and it was found that the machine could be kept from darting up and down just as well as tilting over at the ends. The aëroplane was now ready for the installation of the motor.
The best curve for the wing of an aëroplane is an irregular curve drawn above the horizontal line. It is not a perfect arc of a circle but reaches its greatest height about one third back of the front edge, with the rest of the line slightly flattened. It is much the same line as is formed by some waves just before they break. The plane thus shaped is driven with the blunt or entering edge forward or against the wind. In building the large aëroplanes this curve is worked out with great accuracy, but the builder of model airships may carry the line in his eye.
As the air strikes the entering edge of this surface it is driven underneath and held there for a moment before it can escape from beneath this hollow. The support of the air is therefore greater than in the case of a flat plane, or in fact, any other form. The air which passes over the top of the entering edge, meanwhile, glides or slips off at a slight upward angle, thus forming a partial vacuum over the greater part of the upper surface. This vacuum, in turn, tends to pull the plane slightly upward thus acting in the same direction as the air which is compressed beneath it.
The planes thus constructed are, besides, much more easily controlled than those of any other shape. When the entering edge of this plane is raised the pressure of the air beneath is increased and the pull of the partial vacuum combines with it to make it rise. The difficult problem of getting the aëroplane aloft was largely solved by this curve. Once aloft, such an airship answers her helm much better than any other form.
This curve is accountable for many of the movements of aëroplanes which seem so mysterious to the mere layman. When an aëroplane turns, its outer end rises, and the more rapid is its flight the greater is this tilt. It must be remembered that the end is moving more rapidly and the increased speed causes the plane to lift. Many photographs of aëroplanes show them balanced at precarious angles while making a turn. If the plane is tilted too high the air currents slip out from beneath, no vacuum is developed above, and it quickly loses speed. On the other hand, if it be inclined downward it soon loses the supporting power of the air and plunges down.
The First Glider Weighted at the Front.
At every stage of this development the aviators are indebted to the birds for information. The successful aëroplanes have great width compared to their depth, they gain stability by flexing the tips of the wings, and their planes are arched upward and forward exactly as are the wings of a bird. The aviator arranges his center of gravity after the same general model, below the planes and well forward. He places his engine forward, just as the bird has its strongest muscles in the chest, and he builds his frame of hollow tubes like the bones of a bird.