Constant Speed Propeller
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A constant speed propeller is a propeller that is designed to automatically change its blade pitch to allow it to maintain a constant RPM, irrespective of the amount of engine torque being produced or the airspeed or altitude at which the aircraft is flying. This is accomplished by means of a Constant Speed Unit, or governor, integrated into the propeller design.
Constant Speed Unit
A constant speed unit, or propeller governor, is the mechanism which allows a constant speed propeller to work. Most constant speed units work on the principle of centrifugal force and incorporate a speeder spring and a set of fly weights. The speeder spring is tensioned to balance the fly weights at a specific propeller RPM and, in some installations, is pilot adjustable allowing more than one target RPM to be selected. Should the propeller exceed the preselected RPM, the fly weights will be forced outward whereas a propeller under-speed would cause the fly weights to swing inward. In both cases, this changes the tension on the speeder spring. In early constant speed propellers, the movement of the weights would drive a mechanism to mechanically change the pitch of the propeller - increasing it in response to an over-speed and decreasing it in response to an under-speed. In newer propeller models, the blade pitch change is accomplished by porting oil, under pressure, through a pilot valve in response to an under or over speed condition. The oil, which might either be from the engine or integral to the propeller itself, causes the propeller blade angle to change as required to maintain the selected RPM. Some manufacturers have elected to incorporate electronic governing mechanisms to replace the speeder spring and flyweights.
Most engines produce their maximum power in a narrow speed band. This is especially true for a turboprop engine. A constant-speed propeller system permits the manufacturer or, in some installations, the pilot, to select the propeller speed appropriate to the situation and then automatically maintain that RPM under varying conditions of aircraft altitude, airspeed, phase of flight and engine power (as selected by power lever position. This allows operation of propeller and engine at the most efficient RPM and torque for the phase of flight. RPM is controlled automatically by varying the pitch of the propeller blades – that is, the angle of the blades with relation to the plane of rotation.
For any given power setting, as the blade angle is reduced, the torque required to spin the propeller is reduced and the airspeed and RPM of the engine will tend to increase. Conversely, if the blade angle is increased, the torque requirement to maintain a constant RPM increases. If the power is not changed, then the engine and the propeller will tend to slow down. In essence, this relationship allows both the propeller and the engine to be set to maintain their respective optimum RPM. Adding fuel (by moving the power lever forward) increases the power output of the engine and would tend to increase the engine RPM - however, to maintain the selected propeller RPM, the blade angle increases to absorb the additional torque that the engine is now producing allowing the engine RPM to remain at its original value. The converse is true for a reduction in power lever position where, as less torque is available, the propeller RPM decreases allowing the engine RPM to remain more or less constant.
- Propeller operation and malfunctions - basic familiarization for flight crews; informal note produced by CAST.