Remember that the lifting power of the airplane wing is made up of two factors. _First_, there is the resistance or the supporting air pressure created by the weight and speed of the wing; _second_, the arch of the wing creates a vacuum above it which tends to lift the airplane up. Now when for speed the arch is made very slight, the lifting power can still be increased by increasing the _area_ of the wing, thus adding to the upward pressure. Thus for certain war duties an airplane with very large, comparatively flat wings can develop both a very good lifting power and a very high speed.
We have already mentioned the "head resistance" of the airplane wing. If the wing could strike the air in such a way as to sharply divide it into currents flowing above and below, there would be no head resistance. But the very arch of the wing in front gives it a certain amount of thickness where it strikes the air, so that instead of flowing above or below, a portion of the air is pushed along in front, retarding the progress of the airplane. This resistance is called by aviators the "drift." The best wing is the one which has the maximum lifting power with the minimum head resistance, or, to use technical language, the greatest "lift" in proportion to its "drift."
Of course, not only the wing but all parts of the airplane offer resistance to the air. In order to reduce this total head resistance to the minimum, every effort is made to give the body or "fuselage" of the airplane a "streamline" form,--that is, a shape, such as that of a fish or a bird, which allows the air to separate and flow past it with little disturbance. For this purpose the fuselage of the airplane is usually somewhat rounded and tapering toward the ends, often "egg shaped" at the nose.
The method of "wing warping" invented by the Wright brothers is still used on all modern airplanes to preserve lateral stability. The part of the wing which can be warped is called the _aileron_. There are two ailerons on every wing, one on each side at the rear, and they may be raised or drawn down by the action of a lever operated by the pilot.
If the pilot feels that the left side of his machine is falling, he draws down the aileron on that side and raises the right hand aileron. The aileron which is lowered catches the air currents flowing beneath the wing on that side. At the same time the raised aileron on the right lessens the pressure under the wing on that side and so gives it a tendency to fall. In this way, in a fraction of a minute the wings are brought level again and lateral stability is restored.