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.
CHAPTER III
HOW TO BUILD A “GLIDER”