Propeller Overspeed Governor; Engine Power And Propeller Controls (Fcu) - Thrush Aircraft S2R–T660 Maintenance Manual

THRUSH AIRCRAFT INC - T660 TURBO THRUSH
2. The second function of the beta control valve is to enable direct control of the propeller
blade angle in beta and reverse. After the hydraulic low pitch stop is reached, finer
blade angles through flat pitch to reverse can be selected by the aircraft operator
after landing. If the beta control valve is opened again by rearward movement, the
servo-oil flows to the propeller and moves the blades to a finer angle. This can be
continued to the maximum reverse blade angle. The beta feedback mechanism will
limit the blade angle reached in beta or reverse to that desired by the aircraft
operator. It does this by reclosing the beta control valve.
E. Feathering
Feathering is accomplished by raising the override rod. This pulls the pilot valve plunger up to dump
the servo-oil from the propeller. The blades feather automatically under the action of the counter-
weights and feather springs.
The propeller overspeed governor 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 a propeller overspeed condition occurs, the increased centrifugal force sensed by the
governor flyweights overcomes the speeder spring tension and lifts the pilot valve to bypass
propeller servo oil back to the reduction gearbox sump via the governor hollow drive shaft. This
allows the combined forces of the blade counterweights and the return springs to move the
propeller blades toward a coarse pitch position, thereby absorbing engine power and reducing
propeller rpm. A solenoid-operated valve is incorporated to facilitate functional testing of the
overspeed governor. When operated, the valve resets the governor below its normal overspeeds
setting. (See overspeed governor check later in this section.)

ENGINE POWER AND PROPELLER CONTROLS (FCU)

The cockpit power lever is connected to the engine power lever (Fig. 4-3, #3). This operates the
cam follower pin as shown in Figure 4-3, #12.
Effective: 12/17/03
AIRCRAFT MAINTENANCE MANUAL

PROPELLER OVERSPEED GOVERNOR

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