Controllable pitch propeller

A propeller whose blade pitch angle can be changed in flight. Takeoff is made with a low blade angle (low pitch) that allows the engine to develop its maximum power. When the airplane is set up for cruise flight, the pitch of the blades is increased so the propeller advances farther through the air each revolution of the engine. This allows the engine to operate at a slower, more economical speed. A controllable-pitch propeller and a constant-speed propeller are similar, the main difference being the control. The pitch of a constant-speed propeller is controlled by a governor, and the pitch of a controllable-pitch propeller is controlled by a manually actuated valve or switch.