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. 

## Governor Mechanism 

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 

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). 

## Underspeed Condition 

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). 

## Overspeed Condition 

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. 

## On-Speed Condition 

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 

