Thrust-Drift Ratio.—The proportion of thrust to drift is of paramount importance, for it expresses the efficiency of the propeller. It is affected by the following factors:
Speed of Revolution.—The greater the speed, the greater the proportion of drift to thrust. This is due to the increase with speed of the passive drift, which carries with it no increase in thrust. For this reason propellers are often geared down to revolve at a lower speed than that of the engine.
Angle of Incidence.—The same reasons as in the case of the aeroplane surface.
Aspect Ratio.—Ditto.
Camber.—Ditto.
M, Direction of motion of propeller (rotary).
R, Direction of reaction.
T, Direction of thrust.
AD, Direction of the resistance of the air to the passage of the aeroplane, i.e., aeroplane drift.
D, Direction of propeller drift (rotary).
P, Engine power, opposed to propeller drift and transmitted to the propeller through the propeller shaft.