Thrush Aircraft S2R–T660 Maintenance Manual page 89

THRUSH AIRCRAFT INC - T660 TURBO THRUSH
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 "overspeed" 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 overspeeding. 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 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
underspeed adjustment eccentric (Fig. 4-2, #15), and the fuel governor
interconnect rod (Fig. 4-2, #14).
4. If a malfunction causes propeller "overspeed" 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 104% [(1768
RPM Np) or 1802 RPM maximum propeller speed] above that selected on the
speeder spring.
Effective: 12/17/03
AIRCRAFT MAINTENANCE MANUAL
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