6.  Centrifugal twisting force—moves the blades toward  low pitch. 7.  Aerodynamic twisting force—moves the blades  toward high pitch. All of the forces listed are not equal in strength. The most  powerful force is the governor oil pressure acting on the  propeller piston. This piston is connected mechanically to  the blades; as the piston moves, the blades are rotated in  proportion. By removing the oil pressure from the governor,  the other forces can force the oil from the piston chamber and  move the propeller blades in the other direction. The engine-driven single-acting propeller governor  (constant-speed control) receives oil from the lubricating  system and boosts its pressure to that required to operate  the pitch-changing mechanism.  [Figure 7-16]  It consists  of a gear pump to increase the pressure of the engine oil,  a pilot valve controlled by counterweights in the governor  to control the flow of oil through the governor to and away  from the propeller, and a relief valve system that regulates the  operating oil pressures in the governor. A spring called the  speeder spring opposes the governor counterweight’s ability  to fly outward when turning. The tension on this spring can  be adjusted by the propeller control on the control quadrant.  The tension of the speeder spring sets the maximum rpm of  the engine in the governor mode. As the engine and propeller  rpm is increased at the maximum set point (maximum speed)  of the governor, the governor counterweights overcome the  tension of the speeder spring and move outward. This action  moves the pilot valve in the governor to release oil from  the propeller piston and allows the blade counterweights to  increase blade pitch, which increases the load on the engine,  slowing it down or maintaining the set speed. In addition to boosting the engine oil pressure to produce one  of the fundamental control forces, the governor maintains the  required balance between control forces by metering to, or  draining from, the propeller piston the exact quantity of oil  necessary to maintain the proper blade angle for constant- speed operation. The position of the pilot valve, with  respect to the propeller-governor metering port, regulates  the quantity of oil that flows through this port to or from  the propeller.  A speeder spring above the rack opposes the action of the  governor counterweights, which sense propeller speed. If the  counterweights turn faster than the tension on the speeder  spring, they fly out; this is an overspeed condition. To slow  the engine propeller combination down, the blade angle  (pitch) must be increased. Oil is allowed to flow away from  the propeller piston and the counterweights increase the  When the engine is operating at the rpm set by the pilot  using the cockpit control, the governor is operating on speed.  [Figure 7-19]  In an on-speed condition, the centrifugal force  acting on the counterweights is balanced by the speeder spring,  and the pilot valve is neither directing oil to nor from the propeller  hydraulic cylinder. In the on-speed condition, the forces of  When the engine is operating above the rpm set by the  pilot using the cockpit control, the governor is operating  in an overspeed condition.  [Figure 7-18]  In an overspeed  condition, the centrifugal force acting on the counterweights  is greater than the speeder spring force. The counterweights  tilt outward and raise the pilot valve. The pilot valve then  meters oil flow to increase propeller pitch and lower engine  rpm. When the engine speed increases above the rpm for  which the governor is set, note that the counterweights move  outward against the force of the speeder spring, raising the  pilot valve. This opens the propeller-governor metering  port, allowing governor oil flow from the propeller piston  allowing counterweights on the blades to increase pitch and  slow the engine. When the engine is operating below the rpm set by the pilot using  the cockpit control, the governor is operating in an underspeed  condition.  [Figure 7-17]  In this condition, the counterweights  tilt inward because there is not enough centrifugal force on the  counterweights to overcome the force of the speeder spring.  The pilot valve, forced down by the speeder spring, meters  oil flow to decrease propeller pitch and raise engine rpm. If  the nose of the aircraft is raised or the blades are moved to a  higher blade angle, this increases the load on the engine and  the propeller tries to slow down. To maintain a constant speed,  the governor senses the decrease in speed and increases oil  flow to the propeller, moving the blades to a lower pitch and  allowing them to maintain the same speed. When the engine  speed starts to drop below the rpm for which the governor is  set, the resulting decrease in centrifugal force exerted by the  counterweights permits the speeder spring to lower the pilot  valve (counterweights inward), thereby opening the propeller- governor metering port. The oil then flows through the valve  port and into the propeller piston causing the blades to move  to a lower pitch (a decrease in load).  pitch or blade angle slowing the propeller until it reaches  an on-speed condition where the force on the governor  counterweights and the tension on the speeder spring are  balanced. This balance of forces can be disturbed by the  aircraft changing attitude (climb or dive) or the pilot changing  the tension on the speeder spring with the propeller control on  the instrument panel (i.e., if the pilot selects a different rpm). Governor Mechanism Underspeed Condition Overspeed Condition 7-10 On-Speed Condition