76 Engine Control - Prop Governor
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TTP |
B1-L3 |
ATA 76 |
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Beech 90 Series |
B2-L1 |
Engine Control |
ENGINE CONTROLS - DESCRIPTION AND OPERATION
1. General
2. Description
B. Propeller Governor
The propeller governor consists of a single-acting centrifugal governor. In forward thrust operation, the function of the governor is to boost the engine pressure oil to decrease propeller blade pitch, while the centrifugal force of the blade counterweights, assisted by the feathering springs, tends to increase the pitch angle. The propeller governor incorporates an integral gear-type oil pump with pressure relief valve, two pivoted flyweights mounted on a rotating flyweight head, a spring-loaded pilot valve and necessary cored oil passages contained in an aluminum housing. The flyweight head is attached to a hollow driveshaft which protrudes below the housing flange. The shaft is externally splined to mate with the corresponding coupling shaft in the reduction gearbox. The spring-loaded pilot valve is installed in the driveshaft centerbore. Ports in the driveshaft and the position of the pilot valve in the shaft, control the direction of oil flow within the housing. The rotating shaft, and hence rotating flyweights, determine the position of the pilot valve while the opposing spring load on the valve is varied by the speed adjusting lever at the head of the governor. The speed adjusting lever is connected through airframe linkage to the propeller control lever in the cockpit. A speed adjusting lever maximum stop prevents the lever from moving beyond the 100 percent position and enables the propeller to be operated at, or near to, full rated speed and the engine to develop maximum power. Moving the speed adjusting lever towards a preset feathering stop raises the pilot valve and decreases the oil pressure to the propeller servo piston. This decrease in pressure allows the piston to move, under the influence of the feathering and return springs, to rotate the propeller blades to a positive coarse pitch or feathering position regardless of governor flyweight force acting on the pilot valve. For Post-SB1335 Engines: The propeller governor provides asynchronizing function. Magnetic pickups on the propeller shafts provide a signal to an electronic system that determines the difference in speed between the two engines and energizes a solenoid incorporated in the head of the propeller governor on the slave, right hand engine. The solenoid overrides the spring loaded pilot valve thus changing the propeller pitch setting and consequently power turbine speed. In the event of a propeller control (speed) lever linkage failure, a spring attached to the propeller speed adjusting lever holds the lever in its last selected position or moves it against the maximum stop. To provide the propeller governor with a sensing element, the rotating pivoted flyweights are mechanically coupled to the engine by the hollow driveshaft and accessory driveshaft in the reduction gearbox. The rotating flyweights, actuated by centrifugal force, position the pilot valve so as to cover or uncover ports in the drive gearshaft and regulate the oil flow to and from the propeller servo piston. The centrifugal force exerted by the flyweights is opposed by the force of the speeder spring. This determines the engine rpm required to develop sufficient centrifugal force on the flyweights to center the pilot valve, thereby preventing oil flow to the servo piston. The Beta valve incorporated in the propeller governor pump output line to the pilot valve and mechanically connected to the propeller reversing lever, is designed so that forward movement of the valve will initially block off high pressure oil to the propeller servo piston, then, as forward movement continues, will dump pressure oil in the servo back into the reduction gearbox. Axial movement in the reverse direction has no effect on normal propeller control. When the propeller is rotating at a speed lower than selected by the speed adjusting lever, the governor oil pump provides pressure oil to the propeller servo, lessening the propeller pitch until the feedback ring movement pulls back the Beta valve to block off the pressure oil to the servo, thus preventing further pitch change. The pitch angle will coarsen automatically to maintain selected propeller speed as higher engine power (Ng) is selected by the pilot. The Beta control does not forcibly select reverse blade angles; very fine pitch and reverse can only occur when conditions are such that the propeller is in underspeed relative to that selected, and subject to control limitations set by individual airframe manufacturers. The lock pitch solenoid valve (when fitted) is mounted at the front of the propeller governor and is connected by cored passages to the governor pump oil pressure line to the propeller servo piston. The solenoid is energized automatically by airframe-supplied switches at a predetermined propeller blade angle below the normal low pitch setting, and acts as a back-up system. When the solenoid is energized, the valve poppet moves to block the oil pressure line to the servo piston; this action effectively prevents oil flow to the piston chamber and prevents any further movement of the blades. However, if the engine power lever is moved to the reverse position, the lock pitch solenoid valve is de-energized by a microswitch installed in the power lever quadrant enabling the propeller governor to operate in the Beta mode. A test switch is incorporated to functionally test valve operation at maintenance activities. The function of the Nf governor section of the propeller governor, during normal forward thrust operation, is to protect the engine against a possible power turbine overspeed in the event of a propeller governor failure. During reverse thrust operation, the Nf governor is set below the propeller governor selected speed. This acts to control propeller speed via the FCU servo system (Ref. 73-20-00), and thus reduce the power generated by the gas generator to below required setting to maintain approximately five percent less than the selected propeller speed. A yoked bell-crank, operating off the governor flyweight head, opens the Py orifice (Ref. 73-00-00, Fig. 1) as speed is increased to reduce metered fuel flow from the FCU. The speed at which the Nf governor operates is dependent on the speed selected on the governor and the position of the airbleed link reset lever. The airbleed link is normally set so that the Nf governor will control Nf at approximately six percent higher than the selected speed in its maximum position and approximately four percent lower in its minimum position. The Nf governor ‘‘droop’’ is approximately four percent; thus in maximum position the governor will commence governing at 102 percent and will fully govern at 106 percent. It should be noted that to re-set the airbleed link to its minimum position brings the yoked bell-crank into contact with the pilot valve and brings in a spring load in addition to the speeder spring which the governor flyweights must overcome to control propeller speed. This function causes propeller governed speed to increase approximately one percent more than that nominally selected.
C. Propeller Overspeed Governor
The propeller overspeed governor (Ref. Aircraft Manufacturer’s Manual) is installed in parallel with the propeller governor and mounted at the approximate 10 o’clock position on the front case of the reduction gearbox. The governor is incorporated to control any propeller overspeed condition by immediately bypassing pressure oil from the propeller servo to the reduction gearbox sump. The governor consists of conventional type flyweights mounted on a hollow, splined shaft and driven by the accessory drive gearshaft.
The hollow shaft embodies ports which are normally closed by a pilot valve installed in the shaft centerbore and held in position by the governor speeder spring. The spring tension acts in opposition to the centrifugal force of the rotating flyweights. When an engine overspeed condition occurs, the increased centrifugal force sensed by the flyweights overcomes the spring tension, lifts the pilot valve and bypasses the propeller pitch change mechanism oil back to the reduction gearbox through the hollow, splined shaft. This permits the combined forces of the counterweights and return springs to move the blades toward a coarse pitch position, absorbing the engine power and preventing further engine overspeed. A solenoid valve, which resets the governor to a value below its normal overspeed setting, is provided to permit ground testing of the unit
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PTP Beech 90 Series |
B1 |
LOC |
FOT |
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TS |
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B2 |
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