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Constant-speed propellers
Both reciprocating and gas-turbine engines generate maximum power at a rotational speed that is close to the limit imposed by mechanical and temperature considerations. Maximum efficiency is also usually obtained at a high rotational
speed. Changing the engine speed wastes fuel, and thus, it is desirable to run the engine at an optimum constant speed, independent of flight speed. The actual engine speed selected depends on the flight conditions required. Normally, near-maximum speed is required to produce high power for take-off and initial climb, with lower settings being used for cruising or other flight conditions, in order to prevent overheating or overstressing the engine.
With propeller propulsion, such constant-speed operation can be obtained by employing a mechanism that automatically adjusts the blade pitch angle to alter the aerodynamic resistance torque. If an increase in speed is sensed, the pitch is made more coarse to increase the resistance torque. The speed (rev/min) setting can be altered by the pilot by means of a selector level. Nowadays, such constant-speed propellers are fitted even to quite unsophisticated aircraft.
Further improvements in efficiency can be obtained by linking the pitch control mechanism to the engine control system, so as to give a carefully programmed match.
