Thrush Aircraft S2RHG-T65 TURBO THRUSH Maintenance Manual page 78

THRUSH AIRCRAFT, INC – MODEL S2RHG-T65 TURBO THRUSH
operations the propeller control lever on the throttle quadrant is at the max
RPM (full forward position). The speeder spring is exerting its maximum
downward force so that it will always exceed the upward force of the
flyweights in order to keep the pilot valve plunger down at all times during
the beta and reverse. The oil passages to the propeller will then be wide
open, and only the beta control valve now controls the oil flow, which is
upstream of the pilot valve plunger.
3. The components used in the Nf governor include the reset arm (Fig. 4-2,
#13), the under speed adjustment eccentric (Fig. 4-2, #15), and the fuel
governor interconnect rod (Fig. 4-2, #14).
4. If a malfunction causes propeller "over speed" that cannot be controlled by
the CSU constant speed section during the constant speed operation of
takeoff, climb, and cruise; then the top of the pilot valve plunger lifts the air
bleed lever. The air bleed lever tilts and allows the orifice lever to bleed Py
air from the FCU. The governing action begins when the propeller RPM is
approximately 106% Np (1802 Np Maximum) or 6% above that selected on
the speeder spring. 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
over speed 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
4-14
AIRCRAFT MAINTENANCE MANUAL
Effective: 5/05/04