The PT6 Hartzell propeller system incorporates three-, four‑, or six-bladed propellers made of aluminum or composite materials. It is a constant-speed, feathering, reversing propeller system using a single-acting governor. Oil from the propeller governor feeds into the propeller shaft and to the servo piston via the oil transfer sleeve mounted on the propeller shaft. [Figure 7-49] As oil pressure increases, the servo piston is pushed forward and the feather spring is compressed. Servo piston movement is transmitted to the propeller blade collars via a system of levers. When oil pressure is decreased, the return spring and counterweights force the oil out of the servo piston and change the blade pitch to a high pitch position. An increase in oil pressure drives the blades towards low pitch.
![Figure 7-49. Pitch change mechanism. [click image to enlarge]](https://www.flight-mechanic.com/wp-content/uploads/2017/07/7-49.jpg)
Engine oil is supplied to the governor from the engine oil supply. A gear pump, mounted at the base of the governor, increases the flow of oil going to the constant speed unit (CSU) relief valve. When the oil pressure reaches the desired level, the relief valve opens to maintain the governor oil pressure. When the speed selected by the pilot is reached, the flyweight force equals the spring tension of the speeder spring. The governor flyweights are then on speed. When the engine output power is increased, the power turbines tend to increase speed. The flyweights in the CSU sense this acceleration and the flyweights go into an overspeed condition because of the increase centrifugal force. This force causes the control valve to move up and restrict oil flow to the propeller dome. [Figure 7-50] The feathering spring increases the propeller pitch to maintain the selected speed. Reducing power causes an under-speed of the flyweights, downward movement of the control valve, more oil in propeller dome, resulting in a lower pitch to control propeller speed. The propeller governor houses an electro-magnetic coil, which is used to match the rpm of both propellers during cruise. An aircraft supplied synchrophaser unit controls this function.
![Figure 7-50. Governing mode. [click image to enlarge]](https://www.flight-mechanic.com/wp-content/uploads/2017/07/7-50.jpg)
![Figure 7-51. Beta mode forward operation. [click image to enlarge]](https://www.flight-mechanic.com/wp-content/uploads/2017/07/7-51.jpg)
The lock-pitch solenoid valve prevents the propeller from going into reverse or below the primary blade angle in the event of a beta system malfunction in flight. The solenoid is energized by a switch (airframe supplied) mechanically connected to the propeller slip-ring linkage via a second carbon block. As oil pressure leaks off around the propeller shaft oil transfer sleeve, the blade angle slowly drifts back toward high pitch. This deactivates the low pitch solenoid valve and restores the oil supply to the propeller servo. The low pitch solenoid valve cycles (close/open) as backup to the beta valve function. Moving the power lever backwards causes the reversing cam and cable to move the beta valve backward, allowing more oil to flow into the propeller dome, and causing the blades to go towards reverse pitch. [Figure 7-52]
In this mode, the propeller speed is no longer controlled by changing the blade angle. It is now controlled by limiting engine power. Bringing the propeller lever to the feather position causes the speed selection lever on the CSU to push the feathering valve plunger and allows propeller servo oil to dump into the reduction gearbox sump. The pressure loss in the propeller hub causes the feathering spring and the propeller counterweights to feather the propeller. In the event of a propeller overspeed not controlled by the propeller overspeed governor (oil governor), the flyweights in the propeller governor move outward until the speeder spring cup contacts the reset lever. [Figure 7-53] The movement of the reset lever around its pivot point opens the Py air passage. Py bleeds into the reduction gearbox limiting the fuel supply to the engine. This prevents the propeller/power turbines from accelerating beyond 106 percent rpm.

![Figure 7-54. Propeller overspeed governor. [click image to enlarge]](https://www.flight-mechanic.com/wp-content/uploads/2017/07/7-54.jpg)
Flight Mechanic Recommends
