Constant-speed propeller

A constant-speed propeller is a variable-pitch aircraft propeller that automatically changes its blade pitch in order to maintain a chosen rotational speed. The power delivered is proportional to the arithmetic product of rotational speed and torque (radians/second × torque), and the propeller operation places emphasis on torque. The operation better suits modern engines, particularly supercharged and gas turbine types.

An aircraft propeller operates as the source of thrust that moves the plane forward.

When an aircraft is stationary with the propeller spinning (in calm air), air flows past the narrow leading edge of the propeller. This is the most efficient configuration, as the drag forces on the propeller are the lowest. As the airplane starts moving forward, the airflow begins to push against the front, wider cross section of the propeller, creating greater drag.

A constant-speed propeller is able to partly rotate along the longest axis of the blade to take a larger bite of air with respect to the airplane, allowing the propeller to maintain the most efficient orientation to the airflow around it. This balances the tradeoff that fixed-pitch propellers must make between high takeoff performance and high cruise performance.

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. This is similar to a car operating in low gear. When the motorist reaches cruising speed, he will slow down the engine by shifting into a higher gear, while still producing enough power to keep the vehicle moving. This is accomplished in an airplane by increasing the angle of attack of the propeller. This means that the propeller moves more air per revolution and allows the engine to spin slower while moving an equivalent volume of air, thus maintaining velocity.

The first attempts at constant-speed propellers were called counterweight propellers, which were driven by mechanisms that operated on centrifugal force. Their operation is identical to the Watts governor used to limit the speed of steam and large Diesel engines. Eccentric weights were set up near or in the spinner, held in by a spring. When the propeller reached a certain RPM, centrifugal force would cause the weights to swing outwards, which would drive a mechanism that twisted the propeller into a steeper pitch. When the airplane slowed down, the RPM would decrease enough for the spring to push the weights back in, realigning the propeller to the shallower pitch.

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Wikipedia