Constant Speed Unit (Csu) - Thrush Aircraft S2R–T660 Maintenance Manual

THRUSH AIRCRAFT INC - T660 TURBO THRUSH
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.
Effective: 12/17/03
AIRCRAFT MAINTENANCE MANUAL

CONSTANT SPEED UNIT (CSU)

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