[Illustration: DIAGRAM SHOWING THE ESSENTIAL PARTS OF AN AIRPLANE]
The modern airplane is constructed with the most painstaking attention to this principle of _balance_. Next to it in importance is that of _wing construction_: that is, the size, shape and proper curve of the supporting planes. Here again the construction of the airplane follows very closely the general form of the bird. A large bird which flew very high would be found to have his wings arched high in front, where they would have considerable thickness, and sloping down very rapidly toward the rear, while their thickness rapidly diminished. This sort of wing has great lifting power, and it is the sort that is used on an airplane which is built to "climb" rather than to develop speed.
As the arched wing cuts through the air it leaves above it a partial vacuum. Nature always tends to fill a vacuum, and so the airplane is drawn upward to fill this space. As the wings cut through the air a new vacuum is constantly created and so the airplane mounts higher and higher. The airplane is being carried upward by two forces: the air pressure beneath it and the vacuum above it which draws it up. The air pressure beneath it increases with the speed at which the airplane is traveling, and it has a tendency to press the wing into a more horizontal position, thus destroying its climbing properties. At the same time, when this happens, the thick front section of the wing presents a great "head resistance" which retards progress, and a very high speed becomes impossible.
Wings of this type can never be used on an airplane which is intended to travel at high speed. They were used on the heavy bombing and battle planes of the Great War, for they are capable of lifting a very great weight. But on the scouting planes, where speed is essential, a totally different sort of surface was employed. Here the plane is very little arched and of almost even thickness, tapering only very slightly to the rear edge. It also tapers somewhat at the front, so as to lessen its "head resistance" as it cuts through the air.
Such a surface creates little vacuum above it, and consequently has not a great lifting power. On the other hand it offers little "head resistance" and so permits a high speed. And right here it should be mentioned that a powerful motor does not in itself make a swift airplane, unless the wings are right,--for if the wings create a strong resistance _in front of the airplane_ they destroy speed as fast as the motor generates it.