Thrush Aircraft S2RHG-T65 TURBO THRUSH Maintenance Manual page 77

THRUSH AIRCRAFT, INC – MODEL S2RHG-T65 TURBO THRUSH
to lift the pilot valve plunger and the force of the speeder spring tends to push the pilot
valve plunger down. The interaction of these two forces controls the propeller speed.
The lower end of the pilot valve plunger covers the ports in the hollow shaft in the CSU
body. This mechanism directs the servo-oil to the propeller. When the upward force of
the flyweights equals the downward force of the speeder spring, the ports are covered
and no servo-oil flows to or from the propeller. The propeller blades remain at constant
pitch. This is termed "on speed" condition.
The operator may select the propeller RPM at "on speed" condition. He may vary the
downward force on the speed spring by actuating the speed select lever (Fig. 4-2, #9)
which is connected to the propeller control lever on the throttle quadrant.
If the operator selects a low speeder spring force, it follows that only a low flyweight
force is needed to lift the pilot valve plunger into the "on speed" condition. This is
achieved at low flyweight and low propeller RPM. The converse occurs if the operator
selects high speeder spring force.
The CSU maintains selected propeller RPM automatically and compensates for "over
speed" and "under speed". When the propeller RPM is higher than the selected speed,
the "over speed" condition occurs.
propeller RPM is lower than the selected speed. These conditions are described in
detail below.
1. If the propeller RPM drops below the selected speed, the flyweight force
decreases and the force of the speeder spring pushes the pilot valve
plunger down. This process provides oil to the propeller servo-piston. The
servo-piston moves forward, which fines out the blades. The propeller RPM
will then increase. As the propeller RPM reaches the selected speed, the
flyweight force lifts the pilot valve back to the "on speed" condition.
2. If the propeller RPM rises above the selected speed, the flyweight force
increases and overcomes the force of the speeder spring to lift the pilot
valve. The oil is dumped from the propeller, which causes the blades to
coarsen pitch. The propeller RPM will then decrease. As the propeller
RPM reaches the selected speed, the speeder spring force pushes the pilot
valve back to the "on speed" condition.
C. Power Turbine Governing Section
The Nf governor or fuel-topping governor of the power turbine governing section of the
CSU has two functions in the propeller speed control.
1. The first function is during the constant speed operation of takeoff, climb,
and cruise when it acts as a safety in the "over speed" condition only. If a
malfunction occurs which allows the propeller RPM to exceed selected
RPM by 6%, the Nf governor bleeds Py air from the fuel control unit (FCU)
to limit power.
2. The second function is during reverse propeller control when it will start to
bleed Py air from the fuel control unit (FCU) to keep the propeller and
therefore the Nf power turbine from over speeding. This will limit propeller
RPM 4% - 6% below the propeller RPM selected on the speeder spring
which is 1700 RPM, because the propeller control lever is still in full forward
position. This will in turn limit max reverse propeller RPM. During beta
Effective: 05/05/04
AIRCRAFT MAINTENANCE MANUAL
The "under speed" condition results when the
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