Pratt & Whitney PT6 Hartzell Propeller System

The Pratt & Whitney PT6 Hartzell propeller system is a hydraulically operated, constant-speed propeller designed to provide efficient thrust and reliable performance throughout all phases of flight. This article explains the construction, operation, and control of the PT6 Hartzell propeller system, including constant-speed governing, beta and reverse operation, feathering, overspeed protection, and autofeather functions.

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 flows through the propeller shaft to the servo piston via the oil transfer sleeve mounted on the shaft. [Figure 1]

Figure 1. Pitch change mechanism

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 flyweights 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.

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 CSU flyweights sense this acceleration and enter an overspeed condition because of the increased centrifugal force. This force causes the control valve to move up and restrict oil flow to the propeller dome. [Figure 2]

Figure 2. Governing mode

The feathering spring increases the propeller blade angle to maintain the selected speed. Reducing power causes an under-speed of the flyweights, downward movement of the control valve, more oil in the propeller dome, resulting in a lower blade 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.

At low power, the propeller and governor flyweights do not turn fast enough to compress the speeder spring. [Figure 3]

Figure 3. Beta mode forward operation

In this condition, the control valve moves down, and high pressure oil pushes the dome forward, moving the blades towards low pitch. Any further movement pulls the beta rod and slip ring forward. The forward motion of the slip ring is transmitted to the beta valve via the beta lever and the carbon block. Forward movement of the beta valve stops the oil supply to the propeller.

This prevents the blade angle from decreasing further. This is the primary blade angle (PBA) and is the minimum blade angle allowed for flight operation. From this point, the propeller is in the beta mode. If the engine power is reduced when the propeller is at the primary blade angle, the propeller speed decreases since the blade angle does not change.

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 move toward reverse pitch. [Figure 4]

Figure 4. Beta mode reverse operation

As the blades move to reverse, the dome pulls the slip ring forward and moves the beta valve outward, restricting the oil flow. This stops the blade movement toward reverse. To obtain more reverse thrust, move the power lever back more to reset the beta valve inward, and repeat the process.

Move the reset arm on the CSU rearward by the interconnecting rod at the same time the blade angle moves toward reverse. This causes the reset lever and reset post to move down in the CSU, bringing the reset lever closer to the speeder spring cup.

As propeller speed increases due to the increase in engine power, the governor flyweights begin to move outwards. Since the reset lever is closer to the speeder spring cup, the cup contacts the reset lever before the flyweights would normally reach the on-speed position (95 percent propeller speed instead of 100 percent).

As the reset lever is pushed up by the flyweights/speeder spring cup, the Py air bleeds from the fuel control unit (FCU), which lowers fuel flow, engine power, and thus propeller speed.

In reverse, propeller speed remains 5 percent below the selected propeller speed so that the control valve remains fully open, and only the beta valve controls the oil flow to the propeller dome.

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 flyweights 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 5]

Figure 5. Nf overspeed governor

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.

The oil overspeed governor houses a set of flyweights connected to a control valve that is driven by a beveled gear mounted on the propeller shaft. [Figure 6]

Figure 6. Propeller overspeed governor

The centrifugal force of the flyweights acts against two springs: a speeder spring and a reset spring. When the propeller speed reaches a specified limit (4 percent over maximum propeller speed), the governor flyweights lift the control valve and bleed off propeller servo oil into the reduction gearbox sump, causing the blade angle to increase. An increase in blade pitch puts more load on the engine and slows down the propeller.

To test the unit, the speed reset solenoid is activated, and servo oil pressure pushes against the reset piston to cancel the effect of the reset spring. With less spring tension acting on the flyweights, the overspeed governor can be tested at speeds lower than maximum.

On twin-engine installations, a second solenoid valve is mounted on the overspeed governor and is used in conjunction with the aircraft autofeather system. The system is switched on for takeoff and, in the event of an engine malfunction, energizes the solenoid valve to dump the propeller servo oil into the reduction gearbox sump. The feathering spring and propeller flyweights move the blade quickly to feather.