An aircraft propeller (noun /prəˈpel·ər/) or airscrew converts rotary motion from an engine or other mechanical power source, to provide propulsive force. It comprises a rotating power-driven hub, to which are attached several radial airfoil-section blades such that the whole assembly rotates about a longitudinal axis. The blade pitch may be fixed, manually variable to a few set positions, or of the automatically-variable "constant-speed" type.