Variable-pitch propeller (aeronautics)

In aeronautics, a variable-pitch propeller is a type of propeller (airscrew) with blades that can be rotated around their long axis to change the blade pitch. A controllable-pitch propeller is one where the pitch is controlled manually by the pilot. Alternatively, a constant-speed propeller is one where the pilot sets the desired engine speed (RPM), and the blade pitch is controlled automatically without the pilot's intervention so that the rotational speed remains constant. The device which controls the propeller pitch and thus speed is called a propeller governor or constant speed unit. Reversible propellers are those where the pitch can be set to negative values. This creates reverse thrust for braking or going backwards without the need to change the direction of shaft revolution. Some aircraft have ground-adjustable propellers, however these are not considered variable-pitch. These are typically found only on light aircraft and microlights. When an aircraft is stationary with the propeller spinning (in calm air), the relative wind vector for each propeller blade is from the side. However, as the aircraft starts to move forward, the relative wind vector comes increasingly from the front. The propeller blade pitch must be increased to maintain optimum angle of attack to the relative wind. The first propellers were fixed-pitch, but these propellers are not efficient over a range of conditions. If the propeller blade angle is set to give good takeoff and climb performance, the propeller will be inefficient in cruising flight because the blade will be at too low an angle of attack. In contrast, a propeller set for good cruise performance may stall at low speeds, because the angle of attack is too high. A propeller with adjustable blade angle is more efficient over a range of conditions. A propeller with variable pitch can have a nearly constant efficiency over a range of airspeeds. A shallower angle of attack requires the least torque, but the highest RPM, because the propeller is not moving very much air with each revolution.