Thrush Aircraft S2R–T660 Maintenance Manual page 90

THRUSH AIRCRAFT INC - T660 TURBO THRUSH
In this phase of the operation the fuel governor reset arm (Fig. 4-2, #13) is against
the maximum stop. (Figure 4-2, #12)
5. During beta and reverse the pilot valve plunger is always in a lowered position.
Therefore, in order that the air bleed lever can contact the pilot valve plunger, the
fulcrum point of the air bleed lever is lowered by lowering the rest. This action is
performed by the NF governor reset arm. As the aircraft operator commands the
beta operation, the fuel governor reset arm moves off the maximum stop by the
fuel governor interconnection rod. This action continually lowers the reset post to
lower the RPM from its normal overspeed protection duty of being set at 106% Np to
a setting of 96% Np. This will keep the propeller from never exceeding 96% Np
(1632RPM) as the aircraft operator chooses beta and reverse operations by
bleeding Py pressure (pneumatic governor servo pressure). This causes a
decrease in Py pressure at the computing section of the FCU (fuel control unit),
causing the fuel metering valve to move in a closing direction, thus reducing fuel
flow and consequently Ng and Nf speeds.
D. Beta Control Valve Section
The beta control valve (Fig. 4-2, #3) performs two functions in the propeller control.
1. The first function during takeoff, climb, and cruise is to act as a hydraulic low pitch stop
by limiting the finest blade angle possible in flight to the low blade angle. As power
is reduced, the constant speed section maintains selected propeller speed by fining
the propeller blade angle until the servo-piston picks up the beta nuts. The beta
feedback mechanism starts to close the beta control valve by moving it forward. As
the blades fine out further, the valve closes completely at the low blade angle.
Because the beta control valve is upstream of the pilot valve plunger, the constant
speed section can no longer select finer blade angles because its supply is cut off.
Except for a malfunction, the hydraulic low pitch stop is normally achieved in
descent only. It is available only as a safety during takeoff, climb, and cruise.
Normally in those configurations the blades are much coarser than the angle at
which the servo-piston picks up the beta nuts.
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AIRCRAFT MAINTENANCE MANUAL
Effective: 12/17/03