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It has been suggested that Constant speed unit be merged into this article or section. (Discuss)

Constant speed propellers can change their pitch to take better advantage of the power supplied by an engine in much the same way that a transmission in a car takes better advantage of its power source. The mechanism varies depending on the aircraft, but the desired effect is to change the angle of attack of the propeller blades to take a smaller or larger "bite" of air as it rotates.

An airplane propeller operates on the same principle as a wing, with faster air moving over the top surface of the blade causing a low pressure area on top and high pressure area behind the blade, which in turn accelerates the air over the front of the propeller which pulls the airplane forward.

When an airplane 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 rotate along the longest axis of the blade to take a sharper 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 take-off performance and high cruise performance.

A shallower angle of attack requires the greatest horsepower, as well as 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 you get up to speed you want to slow down the engine while still putting out enough energy 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 which operated on centripetal force. A counterbalance was set up near or in the spinner, held in by a spring. When the propeller reached a certain RPM, centripetal force would cause these counterbalances 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 counterweights back in, realigning the propeller to the shallower pitch.

In newer models of constant-speed propellers, oil is pumped through the propeller shaft to push on a piston which drives the mechanism to change pitch. The flow of oil and the pitch is controlled by a governor. This system is fairly straight forward, except that sealing the rotating propeller shaft requires tight manufacturing tolerances.

All high performance aircraft have constant-speed propellers as they vastly improve fuel efficiency and performance, especially at high altitude.

• Variable pitch propeller - where the pitch is manually selected by the pilot
• Constant speed unit - with which this article should be merged

• http://www.pilotfriend.com/aero_engines/aero_eng_dvmt.htm, Kimble D. McCutcheon