How a Constant-Speed Propeller Works

A fixed-pitch propeller is a compromise — efficient at one airspeed and mediocre everywhere else. A constant-speed propeller removes the compromise by automatically changing blade pitch to hold a selected RPM across a wide range of conditions, much like the gears of a car.

The governor

A governor senses engine RPM with spinning flyweights and meters oil pressure to a piston in the propeller hub:

• If RPM rises above the setting, the governor increases pitch (coarser blades take a bigger “bite”), which loads the engine and brings RPM back down.
• If RPM falls, it decreases pitch (finer blades), unloading the engine so RPM rises.

The pilot selects the target RPM with the propeller lever; the governor does the rest.

Two controls, two gauges

Constant-speed installations add a control and a gauge:

• The throttle sets manifold pressure (engine power / how hard it’s working).
• The propeller lever sets RPM (how fast it spins).

A common rule is to avoid high manifold pressure with low RPM (“over-square” beyond POH limits) because it stresses the engine. On takeoff you set high RPM (fine pitch) for maximum power; in cruise you reduce RPM (coarser pitch) for efficiency and lower noise.

Why it matters

By keeping the engine at an efficient speed while the propeller adapts, a constant-speed system delivers better climb, cruise and fuel economy — at the cost of an extra control to manage and an extra failure mode (a governor or oil-supply failure typically drives the prop to a default pitch). See the glossary on manifold pressure and governor.

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*Reference and training only — not for navigation. Always consult the current AIM, your aircraft’s POH/AFM and a certificated flight instructor.*