[Illustration: FIG. 62.--FORCES WHICH HOLD UP A KITE] In an aeroplane conditions are just the same as in a kite, except that a propeller drives the plane through the air with a force equal to _CO_. The plane is slightly curved, so that air which is deflected or forced down at the forward edge will continue to press against the plane all the way to the rear edge. The shape of the top of the plane is also important. As the plane is driven through the air a partial vacuum is formed above the plane, so that the difference in pressure between the two sides is increased and there is a greater lifting effect. This partial vacuum is known as “cavitation.” In the early days of the aeroplane it was not realized how important was the effect of cavitation on rapidly moving objects. Every spar and member of the airplane as it plows through the air builds up a head resistance in front and is followed by a wake of rarefied air unless it is given a stream-line construction. This is illustrated in Figure 63, which shows at the left a rectangular body traveling through the air and at the right a stream-lined body. The flow of the air is indicated by the lines and it is evident that head resistance and cavitation are reduced by forming the body with a bulging bow and tapering stern. To-day all the exposed parts of aeroplanes are stream-lined as far as possible. In the early Wright machines wire braces were extensively used. It was not supposed that a wire would offer much resistance to the air, but, upon investigation, it was found that the wire braces would vibrate laterally and present virtually a broad surface which materially increased the head resistance. [Illustration: FIG. 63.--FLOW OF AIR AROUND A RECTANGULAR BODY AND A STREAM-LINED BODY] The angle of the plane determines to a large extent the head resistance of the plane. If the plane is horizontal the head resistance is at a minimum, but the lift is also very slight. If the wing planes were perfectly flat, there would be no lift at all, but because of the curve of the wings there is a certain amount of lift when they are horizontal, and even when they are tipped slightly downward, provided the machine is traveling at high speed. The angle of an aeroplane’s wings is therefore much flatter than that of a kite. MAINTAINING THE EQUILIBRIUM OF AN AEROPLANE