A propeller is the unit which provides thrust to propel the airplane through the air. It consists of two or more blades and a central hub to which the blades are attached. Each blade of a propeller is an airfoil, and, therefore, is essentially a rotating wing.
The power needed to rotate the propeller blades is furnished by the engine. The propeller is mounted on a shaft, which may be an extension of the crankshaft on low horsepower engines or, on high horsepower engines it may be mounted on a propeller shaft which is geared to the engine crankshaft. In either case, the engine rotates the airfoils of the blades through the air at relatively high velocity, and in this way the propeller transforms the rotary power of the engine into thrust.