Lift and Drift.—Lift is perpendicular to line of flight, drift is parallel.
Angle of Incidence.—Wing in position shown has angle of 5° if moving in direction “A,” 10° if in direction “B;” and a negative angle of 4° if moving in direction “C.” In the last case it is moving along its neutral-lift-line, lift becomes zero.
The propeller push is necessary to overcome the drift or resistance of the wings to forward motion. It appears then that the airplane wing as it moves through the air has two forces on it, one acting straight up and called “lift,” the other acting straight back and called “drift” (see Fig. [19]). The lift is several times greater than the drift, and the situation is quite analogous to that of a kite, which rises upward in the air due to its lift but at the same time drifts backward with the wind due to its drift. In the case of the kite the string takes up an angle which just balances the joint effect of the lift and drift.
The efficiency of an airplane wing is indicated by the ratio of lift to drift, and for a given lift, the efficiency is best, therefore, for small drift. If the lift is 1900 lb. and the wing drift 190 lb.,
Wing efficiency = Lift or weight/Wing drift = 1900/190 = 10
Factors Determining Best Efficiency.—It goes without saying that an airplane wing should attain the best efficiency it can, and there are several ways of doing this.
The first relates to the question of angle of incidence; we have already discussed the effect of angle on lift, but when we come to discuss its effect on efficiency we find that there is only one angle at which we can get the best efficiency. This is a small angle, about 3° to 6°; at this angle the lift is nowhere near as much as it would be at 10° or 15°, but the drift is so small compared to the lift that it is found desirable in airplanes to employ these small angles for normal flight. As the angle increases above this value of maximum efficiency, the efficiency drops off, and when you get up to the stalling angle, the efficiency becomes very low indeed (see Fig. [20]).
Fig. 20.—Wing characteristics.
Curves showing lift, drift, efficiency, and center of pressure travel of typical training-airplane wing, as determined in Aerodynamical Laboratory.