A Guide to Constant Speed Propellers
A constant speed propeller is a variable pitch propeller that automatically adjusts the pitch of the propeller to maintain a desired RPM. Unlike fixed-pitch propellers, which only operate at maximum efficiency during a single stage of flight, constant speed propellers are able to adjust to provide peak performance during each phase. Excessive pitch in a propeller creates drag and hinders power, while too little pitch causes the engine to work harder to reach the desired speed. Constant speed propellers eliminate this problem by partially rotating along their longest axis to alter its pitch, thereby increasing or decreasing its exposure to the air depending on airspeed. The vast majority of high-performance propeller-driven aircraft use constant speed propellers to help protect their powerful engines from excessive wear.
So, how does a constant speed propeller work? By moving pressurized oil back and forth through the propeller shaft, the propeller governor, also known as the constant speed unit, is responsible for controlling the propeller. The oil is directed to the propeller where it pushes a piston to alter the blades’ pitch. Once set, the governor automatically controls the flow of oil and adjusts the pitch of the blades throughout the flight, as necessary. This allows the engine to operate at optimum speed regardless of the aircraft’s current airspeed or altitude.
Constant speed does not mean that the propeller is always operating at one speed. Instead, it means the pilot has the ability to select the appropriate engine speed for each stage of flight. This ensures that the ideal amount of power is being put out in any situation, improving the performance of the aircraft in addition to saving fuel. For example, during takeoff, the pilot can set a low pitch. This will provide higher RPM and maximum power. Then, once the aircraft reaches cruising altitude, the pilot can set a high pitch (lower RPM) to increase the engine's torque. The increased torque helps slow the engine down, resulting in better fuel efficiency, particularly in high altitudes. By adjusting the angle of the blades, constant speed propellers control RPM levels and provide the appropriate amount of power throughout all stages of flight, significantly increasing the performance characteristics and fuel efficiency of an aircraft.
Most engines, especially turboprop engines, produce their maximum power within a narrow speed band. Constant speed propellers allow the manufacturer or pilot to select the appropriate speed to the situation and maintain that RPM through a range of conditions such as altitude, airspeed, phase of flight, and engine power. For a given power setting, as the blade angle is reduced, the torque needed to spin the propeller is reduced and the airspeed and RPM of the engine tends to increase. Adversely, increasing the blade angle increases the torque requirement to maintain a constant RPM. If the power is not changed, the engine and propeller will slow down. This relationship essentially allows both the propeller and engine to maintain a constant optimal RPM. adding fuel increases the power output of the engine and would normally increase the engine RPM. However, a constant speed propeller is designed to increase and absorb the additional torque to keep the RPM at its original value.
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