A propeller governor is an engine RPM-sensing device and high-pressure oil pump that automatically maintains the selected propeller RPM in a constant-speed propeller system. It responds to changes in engine RPM by directing pressurized oil to the propeller hydraulic cylinder or releasing oil from it. The resulting change in oil volume adjusts the blade angle, allowing the propeller to maintain the RPM selected by the pilot through the flight deck propeller control, which changes the tension of the governor speeder spring.
A propeller governor normally supplies oil pressure to move the propeller toward the low-pitch position. [Figure 1]
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| Figure 1. Parts of a governor |
Some non-feathering propeller systems operate in the opposite manner; these are discussed in the Constant-Speed Propellers section.
Fundamental Forces
Several forces interact to control propeller blade angle during constant-speed operation:
- Centrifugal twisting moment – A component of centrifugal force acting on the rotating blade that tends to move the blade toward low pitch.
- Governor oil pressure – Oil pressure acting on the propeller piston moves the blades toward high pitch.
- Blade flyweights – Centrifugal force acting on the blade flyweights moves the blades toward high pitch.
- Air pressure on the propeller piston – Pushes the piston toward high pitch.
- Feathering spring – Pushes the blades toward high pitch and feather.
- Centrifugal twisting force – Tends to move the blades toward low pitch.
- Aerodynamic twisting force – Tends to move the blades toward high pitch.
These forces are not equal in magnitude. The most powerful is the governor oil pressure acting on the propeller piston. Since the piston is mechanically connected to the blades, piston movement rotates the blades to change their pitch. When governor oil pressure is reduced or released, the remaining forces move the blades in the opposite direction.
Governor Mechanism
The engine-driven, single-acting propeller governor receives oil from the engine lubrication system and increases its pressure to operate the propeller pitch-changing mechanism. [Figure 2]
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| Figure 2. Typical governor |
The governor consists of:
- A gear pump that increases engine oil pressure.
- A pilot valve controlled by rotating flyweights to regulate oil flow to and from the propeller.
- A relief valve that maintains the correct operating oil pressure.
- A speeder spring that opposes the outward movement of the governor flyweights.
The pilot adjusts the tension of the speeder spring using the propeller control lever. This spring determines the selected governing RPM.
When engine RPM exceeds the selected value, centrifugal force causes the governor flyweights to move outward against the speeder spring. This raises the pilot valve, allowing oil to leave the propeller piston. The blade flyweights then increase blade pitch, increasing the engine load and reducing RPM back to the selected setting.
Conversely, when RPM decreases below the selected value, the flyweights move inward, allowing the speeder spring to lower the pilot valve. Oil is directed into the propeller piston, decreasing blade pitch and reducing engine load so the engine can return to the selected RPM.
Throughout operation, the governor continuously meters the exact amount of oil entering or leaving the propeller hub to maintain the proper blade angle for constant-speed operation.
Underspeed Condition
When engine RPM falls below the RPM selected by the pilot, the governor operates in an underspeed condition. [Figure 3]
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| Figure 3. Underspeed condition |
Because centrifugal force decreases, the governor flyweights move inward and the speeder spring pushes the pilot valve downward. This opens the governor metering port, allowing pressurized oil to flow into the propeller piston. The piston moves the blades to a lower pitch, reducing engine load and allowing engine RPM to increase until the selected speed is restored.
Overspeed Condition
When engine RPM rises above the selected value, the governor operates in an overspeed condition. [Figure 4]
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| Figure 4. Overspeed condition |
The increased centrifugal force causes the governor flyweights to move outward against the speeder spring, raising the pilot valve. Oil is released from the propeller piston, allowing the blade flyweights to increase blade pitch. The higher blade angle increases engine load, reducing RPM until the selected speed is regained.
On-Speed Condition
When engine RPM matches the RPM selected by the pilot, the governor is operating in the on-speed condition. [Figure 5]
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| Figure 5. On-speed condition |
At this point, the centrifugal force acting on the governor flyweights is balanced by the tension of the speeder spring. The pilot valve remains in a neutral position, so no oil flows to or from the propeller hydraulic cylinder. As a result, blade pitch remains constant and engine RPM is maintained.
If engine load changes because of aircraft attitude, airspeed, or the pilot selecting a different RPM with the propeller control, the balance between the flyweights and speeder spring is disturbed. The governor immediately adjusts oil flow to restore the selected RPM. The governing range of the speeder spring is typically about 200 RPM; beyond this range, the governor cannot maintain the selected speed.
Governor Operating Conditions Comparison
| Propeller Governor Operation States | |||
|---|---|---|---|
| Feature | Underspeed | On-Speed | Overspeed |
| Engine RPM | Below selected RPM | At selected RPM | Above selected RPM |
| Governor Flyweights | Move inward | Balanced | Move outward |
| Speeder Spring | Pushes pilot valve downward | Balanced with flyweights | Compressed by flyweights |
| Pilot Valve Position | Down | Neutral | Up |
| Oil Flow | Oil flows to propeller piston | No oil flow | Oil flows from propeller piston |
| Blade Pitch | Decreases (low pitch) | No change | Increases (high pitch) |
| Engine Load | Decreases | Constant | Increases |
| Resulting RPM Change | RPM increases | RPM remains constant | RPM decreases |
Table 1. Operational states and internal mechanics of a propeller governor
Governor System Operation
During normal operation, the governor continuously senses engine RPM and automatically adjusts propeller blade pitch by controlling oil flow to the propeller hub. If RPM decreases, the governor directs oil to the propeller to reduce blade pitch and increase RPM. If RPM increases, the governor releases oil from the propeller to increase blade pitch and decrease RPM. This continuous adjustment enables the propeller to maintain the RPM selected by the pilot.