Primary Propeller Governor

CHAPTER EIGHT

T-34C AIRCRAFT SYSTEMS FAMILIARIZATION

WORKBOOK

803.

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

functions:

Boosts engine oil pressure to the pressure level required to operate the pitch change

assembly.

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