T-34C AIRCRAFT SYSTEMS FAMILIARIZATION
PRIMARY PROPELLER GOVERNOR
The primary propeller governor, located at the 12 o'clock position on the reduction gearbox, is
driven by gears from the propeller shaft and maintains the propeller speed and condition
(feathered or thrust-producing) as selected by the pilot. The propeller governor performs three
Boosts engine oil pressure to the pressure level required to operate the pitch change
Senses speed of the propeller.
Controls the flow of the high pressure oil to and from the propeller.
These functions are achieved through the operation of four components within the propeller
governor: a gear type pump, flyweight assembly, speeder spring, and pilot valve. High pressure
oil comes from a gear type oil pump which is inside the base of the governor. The pump
receives engine oil from the main oil pump and raises it to the pressure necessary to operate the
pitch change assembly. The flyweight assembly rotates at the same speed as the propeller and
immediately senses any change in propeller speed. A pilot valve controls the flow of high
pressure oil to and from the propeller, and is positioned by the interaction of the speeder spring
and the flyweight assembly. The tension on the spring is adjusted by the condition lever in the
RPM range to establish the desired propeller RPM. The two opposing forces then move the pilot
valve up and down to uncover openings which allow high pressure oil flow from and to the servo
piston in the pitch change assembly.
The propeller governor's purpose during flight is to maintain a desired constant speed. With the
propeller turning at that speed, a state of equilibrium exists between the flyweight assembly and
the speeder spring force and no oil flows to or from the servo piston. If the pilot adds power
with the PCL, an overspeed condition is created, the flyweights move outward and overcome
speeder spring tension to raise the pilot valve and allow high pressure oil to drain back to the
reduction gearbox. The feathering spring and counterweights INCREASE propeller blade angle,
which causes the propeller load to increase and slows the propeller back to an on-speed
condition. If the pilot reduces power with the PCL, an underspeed occurs, the flyweights move
inward and speeder spring tension overcomes the flyweights to lower the pilot valve and increase
the flow of high pressure oil to the servo piston and DECREASE blade angle. The load on the
propeller is reduced and the propeller speeds up back to an on-speed condition.
Moving the condition lever within the RPM range adjusts the speeder spring tension to establish
an on-speed condition from 1800 RPM at DECR to 2200 RPM at INCR. The T-34C NATOPS
prescribes using 2200 RPM for all normal operations in flight.
8-4 POWER PLANT CONTROL SYSTEM