Varying pitch

The purpose of varying pitch angle is to maintain an optimal angle of attack for the propeller blades, giving maximum efficiency throughout the flight regime.

Variable pitch 

Cut-away view of a Hamilton Standard propeller. This type of constant-speed propeller was used on many American fighters, bombers and transport aircraft of World War II

Early pitch control settings were pilot operated, either with a small number of preset positions or continuously variable.

Following World War I, automatic propellers were developed to maintain an optimum angle of attack. This was done by balancing the centripetal twisting moment on the blades and a set of counterweights against a spring and the aerodynamic forces on the blade. Automatic props had the advantage of being simple, lightweight, and requiring no external control, but a particular propeller's performance was difficult to match with that of the aircraft's power plant.

Modern light aircraft and advanced homebuilt aircraft sometimes have variable pitch (VP) propellers. These tend to be electrically operated and controlled manually or by computer. The V-Prop is self-powering and self-governing.

A simpler version was the spring-loaded "two-speed" VP prop, which was set to fine for takeoff, and then triggered to coarse once in cruise, the propeller then staying in coarse for the remainder of the flight. An even simpler version is the ground-adjustable propeller, which may be adjusted on the ground, but is effectively a fixed-pitch prop once airborne.

Constant speed

An improvement on the automatic type was the constant-speed propeller. This type automatically adjusts the blade pitch according to the engine speed, thereby maintaining a constant engine speed for any given manual control setting Constant-speed propellers allow the pilot to set a rotational speed according to the need for maximum engine power or maximum efficiency, and a propeller governor acts as a closed-loop controller to vary propeller pitch angle as required to maintain the selected engine speed. In most aircraft this system is hydraulic, with engine oil serving as the hydraulic fluid. However, electrically controlled propellers were developed during World War II and saw extensive use on military aircraft, and have recently seen a revival in use on homebuilt aircraft.

Feathering

Feathered propeller on the outboard TP400 turboprop of an Airbus A400M

On some variable-pitch propellers, the blades can be rotated parallel to the airflow to reduce drag in case of an engine failure. This is called feathering, a term borrowed from rowing. On single-engined aircraft, whether a powered glider or turbine-powered aircraft, the effect is to increase the gliding distance. On a multi-engine aircraft, feathering the propeller on a failed engine helps the aircraft maintain altitude with the reduced power from the remaining engines.

Most feathering systems for reciprocating engines sense a drop in oil pressure and move the blades toward the feather position, and require the pilot to pull the propeller control back to disengage the high-pitch stop pins before the engine reaches idle RPM. Turboprop control systems usually utilize a negative torque sensor in the reduction gearbox which moves the blades toward feather when the engine is no longer providing power to the propeller. Depending on design, the pilot may have to push a button to override the high-pitch stops and complete the feathering process, or the feathering process may be totally automatic.

Reverse pitch

In some aircraft, such as the C-130 Hercules, the pilot can manually override the constant-speed mechanism to reverse the blade pitch angle, and thus reverse the thrust of the engine (although the rotation of the engine itself does not reverse). This is used to help slow the plane down after landing in order to save wear on the brakes and tires, but in some cases also allows the aircraft to back up on its own - this is particularly useful for getting floatplanes out of confined docks. See also Thrust reversal.