Propellers and propulsion

Preamble

Propulsive systems using atmospheric air include propellers, turbo-jets, ramjets, helicopters and hovercraft. Those which are independent of the atmosphere (if any) through which they move include rocket motors. In every case mentioned above the propulsive force is obtained by increasing the momentum of the working gas in the direction opposite to that of the force, assisted in the case of the hovercraft by a cushioning effect. A simple momentum theory of propulsion is applied to airscrews and rotors that permits performance criteria to be derived. A blade-element theory is also described. For the rocket motor and rocket-propelled body a similar momentum treatment is used. The hovercraft is briefly treated separately.

The forward propulsive force, or thrust, in aeronautics is invariably obtained by increasing the rearward momentum of a quantity of gas. Aircraft propulsion systems may be divided into two classes:

(I) those systems where the gas worked on is wholly or principally atmospheric air; (II) other propulsive systems, in which the gas does not contain atmospheric air in any appreciable quantity.

Class I includes turbo-jets, ram-jets and all systems using airscrews or helicopter rotors. It also includes ornithopters (and, in nature, birds, flying insects, etc.). The only example of the Class II currently used in aviation is the rocket motors.