Constant Constant Speed Propellers
In automatically controllable pitch propeller systems, a control device adjusts the blade angle to maintain a specific preset engine RPM without constant attention by the pilot (Fig. 2-16). For example, if engine RPM increases as the result of a decreased load on the engine, the system automatically increases the propeller's blade angle (increasing the air load) until the RPM has returned to the preset speed. A good automatic control system will respond to such small variations in RPM that, for all practical purposes, a constant RPM will be maintained. These automatic propellers are termed "constant speed" propellers.
An automatic system consists of a governor unit which controls the pitch angle of the blades so that the engine speed remains constant. The propeller governor can be regulated by the pilot with controls in the cockpit, so that any desired blade angle setting (within its limits) and engine operating RPM can be obtained, thereby increasing the airplane's operational efficiency in various flight conditions. A low pitch, high RPM setting, for example, can be utilized to obtain maximum power for takeoff; then after the airplane is airborne, a higher pitch and lower RPM setting can be used to provide adequate thrust for maintaining the proper airspeed. This may be compared to the use of low gear in an automobile to accelerate until high speed is attained, then shifting into high gear for the cruising speed (Fig. 2-17).