Constant Speed Propellers and How They Work

Posted on September 18, 2019 Jack Kartan Aerospace

Some aircraft cockpits feature a large blue lever located next to the throttle. This the propeller controller, and is used to set the propeller and engine speed for an aircraft with a constant speed propeller. Constant speed propellers work by varying the pitch of the propeller blades, which alters the in-flight properties of the propeller. As the propeller blade angle is increased, it produces more thrust, but also requires more torque to spin the propeller, which slows down the engine. Inversely, when the blade angle is decreased, the torque required is decreased, and the engine speeds up.

Constant speed propellers are named such not because they always operate at the same speed all the time, but because the operator can set their RPM, which the propeller then maintains until the operator changes. During takeoff, high RPM is best to achieve maximum power, but during cruising operations pulling RPM back is better for fuel economy.

The propeller’s blade pitch is altered hydraulically by using engine oil. This is the same oil that goes through cylinders to keep them cool and lubricated, and is paired with a spring at the back of the propeller hub assembly that helps the propeller return to a low pitch/high RPM setting. For most single-engine airplanes, there are stops installed so that the blades cannot be fully feathered or flattened.

The job of governing the movement of the propeller falls to an assembly called a governor. The governor moves oil back and forth through the propeller hub to make sure the propeller is at the pitch and speed you want. The governor is comprised of:

  • The governor control lever: Attached to the propeller control lever via cables or linkages, such that when the propeller control lever is moved, the governor control lever moves as well.
  • Threaded shaft: connected to the governor control lever, the shaft functions like a bolt: by turning it left the shaft loosens, and by turning the shaft right it tightens.
  • Speeder spring: the speeder spring sits between the threaded shaft and the flyweights. When the threaded shaft is tightened, the spring is squeezed and forces itself down onto the flyweights, causing them to drop as well. When the threaded shaft moves up, the opposite happens.
  • Flyweights: A set of L-shaped pieces of metal that spin around in a circle, connected to the engine via gears. The flyweights are also connected to pilot valve, which they move up and down. When the engine speeds up, the weights spin faster and fly out due to centrifugal force, lifting the pilot valve. When the engine slows down, the weights fall in from pressure from the speeder spring, lowering the pilot valve.
  • Pilot valve: The pilot valve is connected to the flyweights and is moved up and down by them, allowing oil to flow in and out of the propeller hub.
  • Governor gear pump: The governor gear pump boosts oil pressure before it heads out of the governor and into the propeller hub. With this boosted pressure, the pilot gets a better and faster response from the propeller when they make adjustments. 

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