Section 17.3

Propeller Pitch Control

The constant-speed propeller and governor, fine/coarse pitch, feathering and reverse.

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Main ideas

A constant-speed propeller holds a selected RPM by automatically varying blade pitch: FINE (low) pitch gives high RPM for take-off and climb, COARSE (high) pitch gives low RPM for efficient cruise — like changing gear.
A governor controls it: spring-loaded flyweights sense engine RPM and meter (boosted) engine oil to or from the pitch-change piston in the hub. On-speed holds pitch; over-speed coarsens pitch; under-speed fines it — always driving RPM back to the set value.
FEATHERING turns the blades to about 90° (edge-on to the airflow) after an engine failure so the propeller stops windmilling and its drag is minimised — essential on multi-engine aircraft; springs/counterweights and accumulators drive feather/unfeather.
REVERSE pitch (a negative blade angle) is used on turboprops for aerodynamic braking and reverse thrust on landing; the low-pitch 'beta' range is used for ground handling.
Mechanisms are single-acting (oil pressure one way, a spring or counterweight the other) or double-acting (oil on both sides of the piston); a pitch-lock or feathering stop prevents unwanted movement.
⚠ Exam trap: FINE pitch = high RPM (take-off), COARSE pitch = low RPM (cruise); feathering streamlines the blades (~90°) on a failed engine to cut drag; the governor's flyweights meter oil to the pitch-change piston.

Solved examples

On a constant-speed prop the aircraft enters a descent and speeds up, yet the RPM stays constant. How does the governor achieve this?
As the aircraft accelerates, the blades tend to speed up (over-speed). The governor's flyweights fly out, porting oil to increase blade pitch (coarser). The coarser blades take a bigger 'bite' of air, loading the engine more and bringing RPM back to the selected value — so RPM stays constant while pitch changes automatically.

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