Constant Speed Unit (Csu) - Thrush Aircraft S2RHG-T65 TURBO THRUSH Maintenance Manual

THRUSH AIRCRAFT, INC – MODEL S2RHG-T65 TURBO THRUSH
ring rod end (Fig. 4-1) which steadies the low stop rods. As the servo-piston moves
forward, it picks up on the beta nuts at a certain preset blade pitch. From that instant
the propeller feedback ring (Fig. 4-1) moves forward with the servo-piston. As it moves,
the reverse return springs (Fig. 4-1) are compressed. During the return motion, when
the servo-piston moves rearward, the reverse return springs maintain contact between
the beta nuts (Fig. 4-1) and the servo-piston by pushing aft on small plates attached to
each low stop rod.
This forward and reverse movement of the propeller feedback ring is used to monitor
blade pitch change during beta and reverse. The motion is transmitted to the beta
control valve in the CSU via the carbon block (Fig. 4-2, #4) and the propeller reversing
lever (Fig. 4-2, #2).
As the propeller reversing lever pivots back and forth, it opens or closes the beta control
valve (Fig. 4-2, #3) which is attached to the middle of the propeller reversing lever.
The beta feedback mechanism has two uses.
A. It enables the aircraft pilot to select blade angle directly during beta and reverse.
B. It allows provision of a hydraulic low pitch stop during flight.
For clarity and ease of understanding, the CSU is described in five different sections:
A. Servo-oil Supply
B. Constant Speed Section
C. Power Turbine Governing Section
D. Beta Control Valve Section
E. Feathering
A. Servo-oil Supply
The servo-oil that is used to vary the propeller blade angle is supplied by the CSU.
Refer to Figure 4-2. An oil pump in the base of the CSU boosts the engine oil pressure
to approximately 385-PSI. The oil is then routed past a pressure relief valve through the
beta control valve port to a chamber formed by the hollow drive shaft (Fig. 4-1) and the
lower part of the pilot valve plunger. Here it is ready for delivery to the propeller servo-
piston. Excess oil pressure and flow is bypassed via the relief valve back to the pump
inlet.
During normal constant speed operation the beta control valve port is always open. The
beta control valve plays no part on the propeller blade angle control.
B. Constant Speed Section
The constant speed section maintains constant propeller speed during takeoff, climb,
and cruise by controlling the flow of servo-oil to and from the propeller servo-piston.
A hollow drive shaft (Fig. 4-1) is driven by a bevel gear on the engine propeller shaft.
On top of the drive shaft there are two rotating flyweights that pivot outward. This action
provides an upward force proportional to propeller RPM. The feet of the flyweights tend
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AIRCRAFT MAINTENANCE MANUAL

CONSTANT SPEED UNIT (CSU)

Effective: 5/05/04