Propeller Control for Turbo-Prop Engines
By Dan Ankarlo
Controlling propeller RPM is only one of the functions of a governor installed on the turboprop engine. Indeed, other more complicated functions are performed by the governor as well. Propeller feathering, synchronizing, and beta (reverse power application) must be controlled also. Perhaps the most complicated installation for a turboprop governor is the PT-6 engine. This is due to the fact that the free turbine design of the PT-6 mandates control of the engine and propeller simultaneously. The governor must interface with the propeller, the engine fuel control unit, and the synchronizing system, simultaneously to provide desired outcomes. With so many variables interacting at the same time the potential exists for the governor to exaggerate any problems that might be caused elsewhere in the propeller/governor system. Before examining service difficulties, a basic review of the governor system will help to clarify the troubleshooting techniques given in this article.
At the heart of the system, the PT-6 governor has an internal pump that takes oil from the engine oil system and supplies it at boosted pressure and volume to the propeller servo. Some installations such as the PW100 engine system utilize a separate pump and pitch control unit for this function. In a standard PT-6 propeller engine configuration, operating oil is supplied to the governor at 40-45 psi. The governor pump elevates the pressure to 385 psi at volumes of up to 6 quarts per minute to actuate the propeller toward the low pitch or increase RPM direction. Counteracting this oil are the propeller counterweights and feather spring.
Typical PT-6 governor mounted to the engine.
Propeller speed sensing is performed by a set of spinning flyweights that are in direct drive to the reduction gear box. Speeder spring pressure counteracts the centrifugal force of the spinning flyweights to hold the metering valve (pilot valve) in a predetermined position. The position of the metering valve will determine whether or not oil is ported to or from the propeller servo. In underspeed condition, the valve ports oil to the propeller piston; reducing blade angle and increasing RPM. In overspeed condition, the valve allows oil to drain from the propeller and blade angle is increased due to counterweight and feather spring forces. This will cause a decrease in RPM. In theory, when the governor is at onspeed condition the metering valve is closed; not allowing pump oil to pass to or from the propeller and thus a constant predetermined RPM is maintained. It is important to note that on the engine the governor is always porting oil to the propeller to counteract the internal leakage where the oil transfers between the engine case and propshaft.
Feathering of the propeller, a critical function for multi-engine aircraft, is accomplished by allowing oil to drain from the propeller servo. Feather springs and counterweight forces on the propeller will force the blades into the feather position in the absence of high pressure oil. To do this, the governor makes use of either a feather lift rod or a feather plunger. The feather lift rod is centered in the control shaft on top of the governor. When the control shaft is moved to the minimum RPM position, the lift rod pulls the pilot valve into a simulated overspeed condition which allows oil to drain from the propeller. Some governor models use a feather plunger instead of a lift rod. The feather plunger does not directly contact the pilot valve, rather, it diverts governor pump oil to a feather drain tube. Feather plungers react more quickly than feather lift rods and are thus used on higher pressure operation systems such as those found on PT6-67 series engines. Use of a feather plunger bypasses normal porting and allows for quicker feathering of the prop.
"The heart of the system": Flyweight and pilot valve combination. The feather drain tube allows for quicker feathering of the propeller.
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