Propeller Pitch Control
The constant-speed propeller and governor, fine/coarse pitch, feathering and reverse.
Propeller pitch control, summary notes
- 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.
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.
Constant-speed propeller & governor
Pitch control concept map
Propeller Pitch Control
Propeller pitch control quiz
Propeller Pitch Control, quiz
1. A constant-speed propeller holds a selected RPM by:
Varying the blade pitch as power and airspeed changeVarying the engine fuel onlyChanging the reduction gear ratio2. Feathering a propeller means:
Turning the blades edge-on to the airflowSetting the finest pitchStopping the governor3. On a single-acting constant-speed propeller, loss of governor oil pressure usually causes the blades to move towards:
Fine pitchFeatherReverse4. On the rotating blade shown, force X acts radially outward along the blade. It is:
Centrifugal forceThrust bending forceTorque bending force5. On the same blade, moments Y and Z act in opposite senses about the pitch axis. In practice:
The centrifugal twisting moment is the larger, turning the blade to fineThe aerodynamic twisting moment is the larger, turning the blade to coarseThey are always equal and cancel6. A constant-speed propeller holds a set RPM by:
Automatically varying blade pitchChanging fuel flow onlyApplying a brake7. Fine (low) pitch gives:
High RPM, for take-off/climbLow RPM, for cruiseZero thrust8. Coarse (high) pitch gives:
Low RPM, for efficient cruiseHigh RPM, for take-offReverse thrust