Thrush Aircraft S2R–T660 Maintenance Manual page 88

THRUSH AIRCRAFT INC - T660 TURBO THRUSH
AIRCRAFT MAINTENANCE MANUAL 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 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 flyweight's 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 "overspeed" and
"underspeed". When the propeller RPM is higher than the selected speed, the "overspeed"
condition occurs. The "underspeed" condition results when the 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
4 – 15
Effective: 12/17/03