Propeller Governor

in Propellers

A governor is an engine rpm-sensing device and high pressure oil pump. In a constant-speed propeller system, the governor responds to a change in engine rpm by directing oil under pressure to the propeller hydraulic cylinder or by releasing oil from the hydraulic cylinder. The change in oil volume in the hydraulic cylinder changes the blade angle and maintains the propeller system rpm. The governor is set for a specific rpm via the cockpit propeller control, which compresses or releases the governor speeder spring.

A propeller governor is used to sense propeller and engine speed and normally provides oil to the propeller for low pitch position. [Figure 7-15] There are a couple of nonfeathering propellers that operate opposite to this; they are discussed later in this chapter. Fundamental forces, some already discussed, are used to control blade angle variations required for constant-speed propeller operation. These forces are:


  1. Centrifugal twisting moment—a component of the centrifugal force acting on a rotating blade that tends at all times to move the blade into low pitch.
  2. Propeller-governor oil on the propeller piston side— balances the propeller blade flyweights, which moves the blades toward high pitch.
  3. Propeller blade flyweights—always move the blades toward high pitch.
  4. Air pressure against the propeller piston—pushes toward high pitch.
  5. Large springs—push in the direction of high pitch and feather.
  6. Centrifugal twisting force—moves the blades toward low pitch.
  7. Aerodynamic twisting force—moves the blades toward high pitch.
Figure 7-15. Parts of a governor.

Figure 7-15. Parts of a governor.

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 flyweights 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 flyweight’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 flyweights 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 flyweights to increase blade pitch, which increases the load on the engine, slowing it down or maintaining the set speed.

Figure 7-16. Typical governor.

Figure 7-16. Typical governor.

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 flyweights, which sense propeller speed. If the flyweights 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 flyweights increase the pitch or blade angle slowing the propeller until it reaches an on-speed condition where the force on the governor flyweights 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 flyweights tilt inward because there is not enough centrifugal force on the flyweights 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 flyweights permits the speeder spring to lower the pilot valve (flyweights 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).

Figure 7-17. Underspeed condition.

Figure 7-17. Underspeed condition.

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 flyweights is greater than the speeder spring force. The flyweights 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 flyweights 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.

Figure 7-18. Overspeed condition.

Figure 7-18. Overspeed condition.

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 flyweights 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 the governor flyweights and the tension on the speeder spring are equal; the propeller blades are not moving or changing pitch. If something happens to unbalance these forces, such as if the aircraft dives or climbs, or the pilot selects a new rpm range through the propeller control (changes tension on the speeder spring), then these forces are unequal and an underspeed or overspeed condition would result. A change in rpm comes about in the governing mode by pilot selection of a new position of the propeller control, which changes the tension of the governor speeder spring or by the aircraft changing attitude. The governor, as a speed-sensing device, causes the propeller to maintain a set rpm regardless of the aircraft attitude. The speeder spring propeller governing range is limited to about 200 rpm. Beyond this rpm, the governor cannot maintain the correct rpm.

Figure 7-19. On-speed condition.

Figure 7-19. On-speed condition.

Governor System Operation

If the engine speed drops below the rpm for which the governor is set, the rotational force on the engine-driven governor flyweights becomes less. [Figure 7-17] This allows the speeder spring to move the pilot valve downward. With the pilot valve in the downward position, oil from the gear type pump flows through a passage to the propeller and moves the cylinder outward. This in turn decreases the blade angle and permits the engine to return to the on-speed setting.

If the engine speed increases above the rpm for which the governor is set, the flyweights move against the force of the speeder spring and raise the pilot valve. This permits the oil in the propeller to drain out through the governor drive shaft. As the oil leaves the propeller, the centrifugal force acting on the counterweights turns the blades to a higher angle, which decreases the engine rpm. When the engine is exactly at the rpm set by the governor, the centrifugal reaction of the flyweights balances the force of the speeder spring, positioning the pilot valve so that oil is neither supplied to nor drained from the propeller. With this condition, propeller blade angle does not change. Note that the rpm setting is made by varying the amount of compression in the speeder spring. Positioning of the speeder rack is the only action controlled manually. All others are controlled automatically within the governor.