A flight instructor is teaching the operation of a constant-speed propeller governor. If the pilot advances the propeller control forward (toward high RPM) without changing the throttle, what does the governor do to increase RPM?
- AIt increases oil pressure to the propeller hub, moving the blades toward a coarser (higher) pitch angle
- BIt decreases oil pressure to the propeller hub, allowing counterweights and/or springs to move blades toward a finer (lower) pitch angle
- CIt increases fuel flow to the engine cylinders to raise combustion temperature and RPM
- DIt closes the wastegate to increase manifold pressure and thereby increase RPM
Show answer & explanationAnswer & explanation
Correct answer: B. It decreases oil pressure to the propeller hub, allowing counterweights and/or springs to move blades toward a finer (lower) pitch angle
On a typical single-engine constant-speed propeller (oil-to-increase-pitch design), moving the propeller control toward high RPM causes the governor to decrease oil pressure sent to the propeller hub, allowing the counterweights or springs to move the blades toward a finer (lower) pitch angle, which reduces resistance to rotation and increases RPM for the given power setting.
Why the other options are wrong
- A. This describes the opposite action; increasing oil pressure moves blades toward coarse pitch, decreasing RPM.
- C. The governor controls propeller pitch via oil pressure, not fuel flow, which is throttle/mixture controlled.
- D. The wastegate is part of the turbocharging system and is unrelated to propeller governor operation.
Constant-Speed Propeller Governor
A governor uses engine oil pressure, modulated by a flyweight-driven pilot valve, to adjust propeller blade pitch and maintain a selected RPM regardless of airspeed or power changes.
- Oil-to-increase-pitch is common in single-engine designs
- High oil pressure = coarse pitch = lower RPM
- Low oil pressure = fine pitch = higher RPM (counterweights/springs return blades)
Memory trick: High oil = High(coarse) pitch; Low oil = Low(fine) pitch = high RPM