Jet engine propeller

An aircraft propeller is an aerodynamic device which converts rotational energy into propulsive force creating thrust which is approximately perpendicular to its plane of rotation. The rotational energy can be produced by a piston or gas turbine engine.
The cross section of a propeller is similar to that of a low drag wing and is subject to the same aerodynamic issues such as angle of attack, stall, drag and transonic air flow. There is a twist along the length of a propeller blade because the blade speed is much higher at the tip than it is at the root. The twist is necessary to maintain a more or less constant angle of attack along the length of the blade. Like a wing, propeller performance is degraded when it is not at its optimum angle of attack. To overcome this deficiency, many propellers use a variable pitch mechanism to adjust the blade pitch angle as the engine speed and aircraft velocity change.
Propeller design considerations include the number and shape of the blades but compromises are required. As an example, increasing the aspect ratio of the blade will reduce drag. However, as the amount of thrust that is produced by a propeller is proportional to the blade area, increasing the aspect ratio means that either longer blades or more blades are required to maintain equivalent thrust. Longer blades will approach transonic tip speed at a lower RPM than shorter ones and increasing the number of blades also results in an increase in the blade to blade interference effects.
The performance of a propeller diminishes greatly as the blade nears transonic speed. (sky)