A constant speed propeller allows a pilot to delegate specific engine speed (RPM) per stage of flight. The ability to maintain “on” speed functioning is due to a component called the aircraft propeller governor— Ello, Guv’nor! It’s the governor's job to maintain a pilot specified RPM when the blades are moving over speed or under speed. Though turbofan and turbojet aircraft require constant speed capabilities as well, the use of a governor to achieve this is most often seen in a turboprop engine.
A governor system controls RPM by shifting pressurized oil to and from the propeller shaft, where a piston is connected to outer blades. Most constant speed governor systems will include a speeder spring, flyweights, a pilot valve, a gear driven pump, a fine pitch tube, and a coarse pitch tube. Once the governor is set to a definitive RPM, it will adjust the angle and position of the propeller blades whenever necessary during the flight to maintain constant speed. A pilot can usually set takeoff and cruise speed using a propeller control knob or a manual lever near the throttle.
A governor is shaped like a jack-hammer positioned on top of a gear driven pump. The speeder spring sits at the top, with flyweights attached directly underneath. Both of these components sit above the pilot valve, which is linked to a high-pressure oil tube extending from the engine. In the event of an overspeed propeller condition, the propeller begins to move faster than the selected speed. This causes the engine gear to turn at a faster rate as well, which will rotate the governor components. If the propeller is spinning too fast, the engine gear will spin faster, and in tandem, it will spin the pilot valve, flyweights, and speeder spring. The centrifugal force will cause the flyweights to push up on the speeder spring, adjusting the pilot valve. Its new raised position allows high pressure oil to flow into the coarse pitch tube and into one side of the piston, causing the piston to adjust, and therefore adjusting the propeller blades to a coarse position.
When a propeller is underspeed, a governor achieves “on” speed by employing centrifugal force like it does with overspeed, but this time the flyweights will collapse and lower the speeder spring, causing the pilot valve to drop instead of pull up. The governor then allows pressurized oil to flow only into the fine pitch tube, the pressurized oil pushes the piston against existing oil in the cylinder. This oil is then moved by the piston and transferred out through the coarse pitch tube to the engine sump as the propeller blades adjust to a fine position— propeller blades in a fine position raise RPM. Now remember, the propeller speed will adjust the speed of the engine gear as a direct result. As the engine gear speeds up, it creates centrifugal force with the flyweights, and pushes the speeder spring up. The pilot valve is raised again, and hydraulic lock is achieved. With its ability to customize RPM based on varying conditions, a governor can help a constant speed propeller aircraft to achieve optimized fuel efficiency and performance.
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