Constant-Speed Propeller & Governor

The blade angle, or pitch, is the angle between the blade chord line and the plane of rotation. Because a blade section near the tip travels much faster than one near the root, the blade is twisted, coarse at the root and fine at the tip, so that the angle of attack stays efficient along the whole span.

A fixed-pitch propeller is efficient at only one combination of speed and power. A constant-speed propeller adds a governor: engine-driven flyweights are opposed by a speeder spring whose tension the pilot sets with the RPM lever. If the propeller overspeeds, the flyweights fly outwards, lift the pilot valve and port oil to increase blade angle; the coarser blade absorbs more power and the RPM falls back. If it underspeeds, the flyweights fall inwards and the valve ports oil the other way to fine the blade off, unloading the engine so the RPM rises. At the on-speed condition the valve is closed and the angle is held.

The useful positions are fine or low pitch for take-off, coarse or high pitch for cruise, feathered with the blades edge-on to the airflow to minimise drag after an engine failure, and reverse for braking on types so equipped.

• Try this: With the governor running, increase the throttle. What happens to the blade angle, and why does the RPM barely change?
• Try this: Increase the airspeed at constant power. Which way does the governor move the blades, and what does that tell you about the angle of attack seen by the blade?
• Try this: Press Feather. What is the blade angle, and why does this minimise drag and stop the propeller windmilling after an engine failure?