THE PROPELLER, OR "AIR-SCREW"
The sole object of the propeller is to translate the power of the engine into thrust.
The propeller screws through the air, and its blades, being set at an angle inclined to the direction of motion, secure a reaction, as in the case of the aeroplane's lifting surface.
This reaction may be conveniently divided into two component parts or values, namely, Thrust and Drift (see illustration overleaf).
The Thrust is opposed to the Drift of the aeroplane, and must be equal and opposite to it at flying speed. If it falls off in power, then the flying speed must decrease to a velocity, at which the aeroplane drift equals the decreased thrust. 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 roughness 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.