TM 55-2840-231-23
1-9.
DESCRIPTION OF ENGINE OPERATION. The engine is a free turbine engine (no mechanical
connection between the gas producer turbine and the power turbine). The power turbine is gas coupled
to the gas producer turbine by the combustion gases.
The helicopter uses a conventional control system. The collective pitch of the helicopter rotor
establishes the power output demand on the engine. For all practical purposes, helicopter rotor speed is
held constant by the engine and its control system.
The fuel control is connected to the twist grip on the pilot’s and copilot’s collective pitch sticks. The
power turbine governor is interconnected to the collective-pitch sticks through a coordinated system of
bellcranks and linkages. Any change in collective pitch resets the-governor to a new power demand. This
demand is transmitted to the gas producer fuel control, which resets and varies the N1 speed of the gas
producer turbine accordingly.
A motor-actuated speed trimming device is installed in the linkage between the collective pitch sticks and
the power turbine governor lever. It is operated by a trim switch on each collective pitch stick, and
allows engine output speed to be varied over a normal range of approximately 98 to 103 percent (5880 to
6180 rpm).
1-10.
Fuel and Control System.
The gas producer fuel control (figs. 1-2 and 1-3) is located
schematically in the fuel system between the fuel pump assembly and the fuel nozzle. A power turbine
governor, also apart of the control system, provides control intelligence to the gas producer fuel system.
The system controls engine power output by controlling gas producer speed. Gas producer speed levels
are established by the action of the power turbine governor which senses power turbine speed. Power
turbine speed is selected by the operator. The power required to maintain this speed is automatically main-
tained by power turbine governor action on the gas fuel control.
The power turbine governor lever schedules the governor requirements. The power turbine governor in
turn, schedules the gas producer speed to a changed power output to maintain output shaft speed.
Fuel flow for engine control depends on compressor discharge pressure (Pc), engine speed (gas producer -
N1 and/or power turbine - N2), and lever angle. Fuel flow is a function of Pc as sensed in the fuel control.
Variations of fuel flow schedules are obtained by modulating the Pc to Px and Py pressures in the control
through the action of a bleeddown circuit actuated by the governors.
Change 7
1-7
