Lift varies as the square of the speed. Thus any increase or decrease of speed results in a great increase or decrease of lift.
Interdependence of Angle of Incidence and Velocity.—The four factors above mentioned all contribute to the lift; if in an airplane wing each factor be given a definite value, the resulting lift is determined according to the formula:
L = KrAV2
| where | L is lift. |
| K is a coefficient referring to the angle. | |
| A is the area. | |
| V is the velocity. | |
| r is the density. |
Two only of these quantities change materially in flight, the angle and the velocity; the lift itself remains substantially the same under most normal circumstances. The angle always changes simultaneously with the velocity, increasing when the velocity decreases. Thus the drop of lift due to velocity decrease is balanced by gain of lift due to angle increase, and the lift remains unchanged when speed changes.
Speed change then requires that the pilot alter the angle of incidence simultaneously with the throttle; so there are two things to do, unlike the case of the automobile where only the throttle is altered.
Minimum Speed.—When, in slowing up an airplane, the angle of incidence reaches the 15° limit, no further decrease of speed is allowable; therefore, the critical angle determines the minimum limit of speed. If for any reason the machine exceeds the 15° limit, it must speed up to gain support; that is, the pilot has to increase angle and speed simultaneously instead of oppositely.
Efficiency of Airplane Wings.—I said at the beginning of this chapter that the airplane was a device for pushing down an enormous quantity of air. A certain amount of force has to be furnished in order to keep the airplane moving, and this force is furnished by the engine and propeller. The propeller by giving a certain amount of push in a horizontal direction to the airplane wing enables this wing to extract from the air ten or twenty times this amount of push in a vertical direction; that is, the airplane wing will give you 10 lb. or more of lifting in exchange for 1 lb. of push.
Fig. 19.