366 FORMS OF ENERGY An airplane propeller acts like a boat propeller, but it has to be much larger relative to the size of the ship because the density of the air is much less than the density of water. Propellers act like wood screws, bits, or augers, in the way in which they force their way through matter. The denser the air is, the greater will be the forward motion produced by a revolution of the propeller, because a greater mass of material has to be moved by the propeller and therefore a greater force is exerted. Take-ofTs require longer runs, and the rate of climbing is lower on damp days than on dry days, because the density of damp air is than that of dry air. Metal propellers are more efficient than wood propellers because of their thinner sections. less A change from a wood to a metal propeller may result in an increase of air speed (speed of the airplane relative to air) as high as 5 per cent. The pitch of a propeller determines the distance which the propeller advances per revolution. It is important that all portions of the propeller have equal pitch, and for that reason the propeller is so designed that it has increasingly greater angles toward the hub (center), so that each portion of the blade will perform an equal share of the propelling action. Inasmuch as the blade travels faster at the tip than at the hub the blade is tapered. When taking off and climbing, the pitch of the propeller should be least, thus obtaining maximum power by allowing the number of revolutions per minute of the engine to increase; but when flying at a cruising-altitude, the propeller pitch should be increased in order to permit it to take the maximum "bites" out of the air. The maximum amount of power is required when taking off and climbing. When flying at a level, the highest efficiency is obtained by maintaining the manufacturer's recommended number of revolutions per minute by increasing the pitch of the propeller. Large, high-speed airplanes have devices by which the propeller pitch may be varied automatically so as to keep the number of revolutions per second constant. This is of special importance in multi-engined aircraft, as it is a means of keeping the engines accurately synchronized. The Force of Gravity Must Be Considered in Designing an Airplane. The performance of an airplane depends upon the power and the weight of the airplane. The speed of an airplane is not greatly increased by increasing the power of the engine because the horsepower varies as the cube of the speed — doubling the speed requires eight times as many horsepower.
AERODYNAMICS 367 Weight, i.e.^ a measure of the force of gravity, has a pronounced effect on the performance of an airplane, as would be expected ; and designers go to great lengths to obtain light materials of great strength and to design structures of great strength for the weight of materials used. In World War II German aviators towed strings of five gliders having long slender fuselages and tremendous wings, by means of large transport planes. Each glider was said to be capable of carrying fifteen to twenty men. These gliders were used in the attack on the island of Crete. The gliders acted as the equivalent of a very greatly increased wing area for the transport planes, giving them much greater lifting power. Such a train of planes would naturally have to sacrifice speed for lifting power. The Drag Opposes the Thrust. The drag is the resistance of the motion of the airplane relative to the air. The drag is the result of the displacing of air masses, surface friction, and turbulence. Turbulence is the irregular motion of the atmosphere produced when air flows over an uneven surface. That portion of the frictional drag which is produced by nonliftlng surfaces is called the parasitic drag. The drag of the aircraft is determined by computing the equivalent fiat-plate area of the wing and other parts of the aircraft. The drag is decreased by streamlining. Landing gears are made retractable in large planes so as to reduce the drag. The earlier airplanes had most of their struts and braces exposed, but modern planes enclose these within the airfoils in order to decrease the drag. An important principle to keep in mind is that the drag increases with increased speeds just as the lift does, and for that reason streamlining is very important for high-speed airplanes. The greater the area of the airfoil is, the greater will be the drag. Inasmuch as the lifting power of an airfoil is increased by increasing the speed of an airplane, the tendency in modern airplane design is to decrease the wing area so as to decrease the drag' and use more powerful engines so as to increase the speed, especially to give the airplane performance or climbing ability. The same results are obtained by moving the air relative to the airplane that are accomplished when the airplane moves relative to the air. Wind tunnels enable engineers to test small-scale models and even full-scale airplanes. Lift and drag measurements made in wind tunnels enable engineers to calculate the number of feet of wing area required to support an airplane of a given weight carrying a maximum load at a given speed.