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.
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
In order to maintain itself in equilibrium the center of gravity must coincide with the center of pressure, otherwise there will be a turning action about the center of gravity and the machine will upset. The location of the center of pressure depends upon the angle of the plane. The greater the angle the farther it is from the forward edge of the plane and the pilot can maintain fore-and-aft equilibrium by tilting the horizontal planes of his rudder so as to change the angle of the machine, and hence of the main or forward planes. Unfortunately the air is a turbulent ocean filled with invisible air currents and the aeroplane must be capable of adjustment to meet the variations of pressure due to sudden gusts of wind. This is particularly true of lateral balance. A gust coming from the side will put a greater pressure on one side of the aeroplane than on the other. To meet this the angle of the plane at one side must be less than that at the other. This was discovered by the Wright brothers and their method of overcoming the lateral variations of pressure was the key to their early success. They warped their wings or twisted them so that the angle of the wing was reduced on the side from which the gust came and was increased on the other, and thus the center of pressure over the whole wing was kept on the line of the center of gravity. At the same time there was a variation in head resistance which had to be corrected by moving the rudder, and the Wright brothers used a single lever to warp the wings and at the same time to operate the rudder so as to keep the aeroplane on its course.
Before the Wright brothers made public their invention other aeronauts had great difficulty in making turns. When turning, one side naturally has to move through the air faster than the other. This produces an increase of pressure on one side over the other which may be counterbalanced by warping the wings. The same effect is produced by the use of small wing planes, called “ailerons,” at each side of the main planes. An automatic means of stabilizing an aeroplane has also been devised, a description of which will be found in Chapter XXI.