The Drift of the propeller may be conveniently divided into the following component values:
Active Drift, produced by the useful thrusting part of the propeller.
Passive Drift, produced by all the rest of the propeller, i.e., by its detrimental surface.
Skin Friction, produced by the friction of the air with roughnesses of surface.
Eddies attending the movement of the air caused by the action of the propeller.
Cavitation (very marked at excessive speed of revolution). A tendency of the propeller to produce a cavity or semi-vacuum in which it revolves, the thrust decreasing with increase of speed and cavitation.
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.
Surface Area.—Ditto.
Aspect Ratio.—Ditto.