Two of the most common propeller types are fixed pitch and ground adjustable propellers. Fixed pitch propellers get the job done but are nothing special, while ground-adjustable propellers force the pilot to choose between their performance during climb or during cruise. However, if a pilot wants more than decent performance from their propeller, or doesn’t want to have to pick between phases of flight to optimize, there is a third option: the constant speed propeller.
A constant speed propeller is a type of variable pitch propeller that maintains a constant rotational speed by automatically adjusting the blade’s pitch. The name “constant speed” comes from the propeller’s ability to maintain a steady RPM during operation, not because they only operate at a single, constant speed. The aim of a constant speed propeller is to hold the rotational speed steady at the engine’s optimum speed for a given situation, regardless of factors such as engine torque, altitude, or the aircraft’s airspeed. This stabilization improves fuel efficiency while maximizing aircraft performance. Because the optimum power for most engines is within a very small speed range, it is difficult to keep the engine in that range manually. Through the use of a constant speed propeller, the RPM is automatically kept in the ideal range.
Modern constant speed propellers adjust their pitch automatically through the use of hydraulically powered propeller governors or constant speed units that pump pressurized oil through the propeller shaft. The pilot adjusts the pitch of the blade angle by moving a lever. As the pilot moves the lever, the attached governor and control lever move too. The movement of this shaft either applies or releases pressure on the speeder spring.
Pulling back on the lever increases the pitch of the blades as well as the corresponding torque requirement necessary to hold a constant engine RPM. Without increased power, the engine RPM and propeller speed will both slow. This is known as the high pitch/low RPM propeller blade configuration, and is commonly used once the cruising altitude has been reached. If the propeller lever is forward, the blade pitch and torque requirements needed for constant RPM decreases. This will keep the power the same, but the engine RPM will increase. This configuration is a low pitch/high RPM set-up used during takeoff.
When the propeller level is pulled backward, it causes the threaded shaft to turn left and move up. In turn, this relieves the pressure on the speeder spring, allowing centrifugal force to cause the flyweights to fly outwards and lift up the pilot valve. As the valve lifts, pressurized oil flows into the propeller hub, forcing the piston backwards and increasing the blade angle. Due to the increased angle of attack, more engine torque is required to turn the propeller. The need for increased torque causes the engine’s RPM to decrease as the blade angle increases. Pushing the propeller lever forward forces the shaft down and to the right. This downward motion compresses the speeder spring, applying pressure to the L-shaped flyweights and causing them to fall inward. The weights lower a pilot valve, halting the flow of pressurized oil into the hub and allowing it to drain out the oil sump. Without pressure from the oil, the piston moves forward. This decreases the blade angle, lowering the torque needs and increasing the engine’s RPM.
Once the propeller lever is set to the desired RPM, the governor will make automatic adjustments to maintain that RPM. Should the aircraft enter a climb and no adjustments are made, the engine will have to work harder as the plane pitches up, naturally dropping its RPM. subsequently, fly weights will fall inward, allowing the pilot valve to shift down and oil to flow from the hub. The propeller blade pitch will reduce, less torque will be needed, and the engine’s RPM will increase and re-stabilize. Adversely, if the aircraft descends, the engine will naturally speed up, creating centrifugal force that pushes the flyweights out. The pilot valve will then raise, sending oil into the hub, increasing the propeller blade pitch. The engine will then slow down and stabilize at the set speed.
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