📖 The lesson
Why a governor?
A fixed-pitch propeller's rpm changes with airspeed and power. A constant-speed propeller instead holds the rpm selected by the pilot and changes blade angle to absorb whatever power the engine is giving. The device that senses rpm and drives the pitch change is the propeller governor (constant-speed unit, CSU). Pitch can be changed by hydraulic (oil), mechanical (counterweights, springs) or electrical/electronic means; most systems are hydraulic.
The governor
- The governor is mounted on the engine and driven by the engine (so it senses engine rpm).
- Flyweights on a rotating flyweight head are thrown outward by centrifugal force and try to lift the pilot valve.
- The speeder spring pushes the pilot valve down. The pilot's propeller (rpm) lever changes its compression, so it sets the rpm at which the flyweight force balances the spring.
- The pilot valve controls oil flow to and from the propeller hub.
- A booster gear pump in the governor raises engine oil pressure (typically to a few hundred psi) for a fast, positive blade movement. A relief valve limits the pressure and returns excess oil to the pump inlet.
- Oil reaches the hub through the hollow crankshaft or propeller shaft (transfer bearing/oil transfer tube).
On-speed, underspeed and overspeed
- On-speed — rpm equals the selected rpm. Flyweight force balances the speeder spring, flyweights are upright, the pilot valve is centred and oil is trapped in the hub, so blade angle stays constant.
- Underspeed — rpm has fallen (e.g. climbing, throttle reduced, or the pilot raised the rpm lever). Flyweights tilt in, the spring pushes the pilot valve down and the blades move to a lower (finer) pitch, letting the engine speed up.
- Overspeed — rpm has risen (e.g. diving, throttle increased, or rpm lever lowered). Flyweights tilt out, lifting the pilot valve, and the blades move to a higher (coarser) pitch, loading the engine and reducing rpm.
- The governor can only hold rpm while the blades are between the low pitch stop and high pitch stop. At low airspeed and low power (e.g. on the ground at idle), the blades sit on the low pitch stop and the propeller behaves like a fixed-pitch propeller — the rpm is below the governed value.
- Static rpm (full throttle, no airspeed) is set by the low pitch stop and is checked during ground runs.
Pitch-change forces and governor types
- Forces available to move blades: governor oil pressure, engine oil pressure, centrifugal twisting moment (to fine), counterweights (to coarse), springs, and a compressed air or nitrogen charge in the hub dome (to coarse/feather).
- Single-acting governor — sends oil to one side of the hub piston only; an opposing force (CTM, springs, counterweights or air charge) moves the blades the other way when oil is released back to the engine.
- Double-acting governor — sends oil to either side of the piston (e.g. Hamilton Standard Hydromatic), so oil pressure moves the blades in both directions.
Non-feathering constant-speed propellers
- Typical of single-engine aircraft (Hartzell compact and McCauley types). A hydraulic piston in a cylinder at the front of the hub is linked to a pin on each blade butt.
- Governor oil pressure increases pitch. Centrifugal twisting moment (assisted by a spring on some models) decreases pitch.
- If oil pressure is lost, the blades go to low pitch, so full rpm and power remain available — the safe failure mode for a single-engine aircraft.
- In contrast, feathering propellers for multi-engine aircraft use oil to decrease pitch, with counterweights, springs and/or an air charge moving the blades to high pitch and feather when oil pressure is lost (Chapter 6).
Controls and adjustment
- The pilot's propeller lever (blue knob in many cockpits) is connected by cable or rod to the governor control arm. Forward = high rpm (fine pitch), rearward = low rpm (coarse pitch).
- The governor has a high-rpm (maximum rpm) stop screw on the control arm to limit maximum governed rpm. Adjustments are made on the ground per the manual (often a small rpm change per turn of the screw) and checked by engine run. Rigging must allow the control arm to reach both stops.
- Normal operating sequence: to increase power, increase rpm first, then throttle; to reduce power, reduce throttle first, then rpm — this avoids excessive manifold pressure at low rpm.
- Common faults: hunting (rpm oscillates about the selected value — governor, pitch-change mechanism or engine), surging (rpm increases then returns — often air in the system or a sticky pitch change), and rpm varying with airspeed (governor not holding).