Constant Speed Propellers

Governor cut-away view

With a typical constant speed propeller the engine RPM is set by the propeller control lever in the cockpit which is connected to the propeller governor that is mounted on the engine. The governor along with the propeller hub can change the propeller blade pitch (the angle of the blade with the plane of rotation).

With all other things equal, the higher the pitch (coarse pitch), the more load will be placed on the engine as it requires more torque to turn and will decrease the RPM. With a decrease in pitch (flat or fine pitch) the engine will have an increase in RPM.

Some of the benefits of a constant speed propeller:

Optimum RPM is set with the propeller control, allowing the propeller to adjust automatically to the most efficient pitch angle. As airspeed increases, such as in a descent, propeller pitch is automatically increased keeping the RPM the same. As airspeed decreases propeller pitch decreases and the RPM also stays the same. Increasing the manifold pressure or throttle will increase engine torque and the governor will increase propeller pitch to maintain RPM.

The ability to choose different power setting combinations. Check your owner's manual for approved RPM/manifold pressure combinations. Using a lower RPM decreases engine friction and noise levels.

The engine can develop redline RPM (such as at takeoff to achieve the manufacturer rated engine horsepower). A fixed pitch propeller is a compromise on propeller pitch, you typically aren't able to reach redline RPM for takeoff.



The Propeller Governor

The governor uses engine oil to hydraulically direct oil pressure toward the propeller hub to increase, decrease, or maintain the same propeller blade angle. It has a gear pump which boosts oil pressure for faster propeller response. The governor has a few key components:

Bottom view of a governor

Flyweights, Speeder Spring & Pilot Valve Flyweights are L-shaped, with the bottom part of the "L" under the speeder spring. As they spin faster, they overcome the speeder spring tension that is pressing on the bottom of the "L" and start to lean outwards from centrifugal force. The pilot valve is lifted or lowered by the bottom of the flyweights. The flyweights have have 3 positions: Under-speed when the flyweights lean inwards (see photos), lowering the pilot valve as the inward part of the "L" tilts downward from spring pressure, directing oil pressure to decrease propeller pitch. On speed when the flyweights are straight up and centrifugal force acting on the spinning flyweights is equal to the spring pressure. The pilot valve no longer directs oil pressure to or from the propeller hub (the piston in the propeller hub is stationary) and the desired RPM has been obtained. Over-speed when the flyweights are leaning outwards from centrifugal force, raising the pilot valve as the inward part of the "L" lifts it upwards, directing pressure to increase propeller pitch.



Cut-away view of a propeller hub

The Propeller Hub

At the front end of the propeller in the propeller hub there is a piston which moves back and forth in response to the hydraulic oil pressure from the governor. It is connected with arms and linkages to the propeller blades to change their blade angle. The propeller naturally wants to decrease in pitch because of a centrifugal twisting moment acting on it.

Single engine airplanes use oil pressure from the governor to increase propeller pitch.

Multiengine airplanes use oil pressure from the governor to decrease propeller pitch.

A major difference between constant speed props on single engine versus multi-engine airplanes are how they behave when oil pressure is lost. Since single engine airplanes use oil pressure in the hub to increase propeller pitch, they will return to a low pitch (high RPM) condition. On multi-engine airplanes the opposite is true, so the propeller will be forced into feather typically with the help of large springs, nitrogen pressure in the hub, and propeller counterweights. This will allow the blades to increase in pitch until they align with the relative wind ("feathered"). The windmilling propeller drag is decreased so that you can continue flying on the remaining good engine.

Cut-away view of governor attached to an engine (right side of image)

Fixed Pitch Propellers vs. Constant Speed

A fixed pitch propeller must offer a high enough pitch to not exceed engine RPM limitations during high speed cruise flight and also low enough pitch to be able to achieve high enough RPM for takeoff and climb performance. A constant speed propeller provides greater performance without having to compromise on a single pitch angle but also requires more complexity and cost.