Constant-Speed Propellers Feathering Propellers The propeller has a natural tendency to slow down as the  aircraft climbs and to speed up as the aircraft dives because  the load on the engine varies. To provide an efficient  propeller, the speed is kept as constant as possible. By using  propeller governors to increase or decrease propeller pitch,  the engine speed is held constant. When the airplane goes  into a climb, the blade angle of the propeller decreases just  enough to prevent the engine speed from decreasing. The  engine can maintain its power output if the throttle setting  is not changed. When the airplane goes into a dive, the blade  angle increases sufficiently to prevent overspeeding and,  with the same throttle setting, the power output remains  unchanged. If the throttle setting is changed instead of  changing the speed of the airplane by climbing or diving, the  blade angle increases or decreases as required to maintain  a constant engine rpm. The power output (not the rpm)  changes in accordance with changes in the throttle setting.  The governor-controlled, constant-speed propeller changes  the blade angle automatically, keeping engine rpm constant. One type of pitch-changing mechanism is operated by oil  pressure (hydraulically) and uses a piston-and-cylinder  arrangement. The piston may move in the cylinder, or the  cylinder may move over a stationary piston. The linear  motion of the piston is converted by several different types  of mechanical linkage into the rotary motion necessary to  change the blade angle. The mechanical connection may be  through gears, the pitch-changing mechanism that turns the  butt of each blade. Each blade is mounted with a bearing  that allows the blade to rotate to change pitch.  [Figure 7-13] In most cases, the oil pressure for operating the different types  of hydraulic pitch-changing mechanisms comes directly from  the engine lubricating system. When the engine lubricating  system is used, the engine oil pressure is usually boosted  by a pump that is integral with the governor to operate the  propeller. The higher oil pressure (approximately 300 pounds  per square inch (psi)) provides a quicker blade-angle change.  The governors direct the pressurized oil for operation of the  hydraulic pitch-changing mechanisms. The governors used to control hydraulic pitch-changing  mechanisms are geared to the engine crankshaft and are  sensitive to changes in rpm. When rpm increases above the  value for which a governor is set, the governor causes the  propeller pitch-changing mechanism to turn the blades to a  higher angle. This angle increases the load on the engine,  and rpm decreases. When rpm decreases below the value  for which a governor is set, the governor causes the pitch- changing mechanism to turn the blades to a lower angle; the  load on the engine is decreased, and rpm increases. Thus, a  propeller governor tends to keep engine rpm constant. In constant-speed propeller systems, the control system adjusts  pitch through the use of a governor, without attention by the  pilot, to maintain a specific preset engine rpm within the set  range of the propeller. For example, if engine speed increases,  an overspeed condition occurs and the propeller needs to slow  down. The controls automatically increase the blade angle  until desired rpm has been reestablished. A good constant- speed control system responds to such small variations of rpm  that for all practical purposes, a constant rpm is maintained. Each constant-speed propeller has an opposing force  that operates against the oil pressure from the governor.  Counterweights mounted to the blades move the blades in the  high pitch direction as the propeller turns.  [Figure 7-13]  Other  forces used to move the blades toward the high pitch direction  include air pressure (contained in the front dome), springs,  and aerodynamic twisting moment. Feathering propellers must be used on multi-engine aircraft to  reduce propeller drag to a minimum under one or more engine  failure conditions. A feathering propeller is a constant-speed  propeller used on multi-engine aircraft that has a mechanism  to change the pitch to an angle of approximately 90°. A  propeller is usually feathered when the engine fails to develop  power to turn the propeller. By rotating the propeller blade  angle parallel to the line of flight, the drag on the aircraft is  greatly reduced. With the blades parallel to the airstream, the  propeller stops turning and minimum windmilling, if any,  occurs. The blades are held in feather by aerodynamic forces.  Almost all small feathering propellers use oil pressure to  take the propeller to low pitch and blade counterweights,  springs, and compressed air to take the blades to high pitch.  Since the blades would go to the feather position during  shutdown, latches lock the propeller in the low pitch position  Blade bearing support area for each blade Figure 7-13.  Blade bearing areas in hub.  7-8