TM  55-2840-241-23 engine), a combustion outer case and a combustion liner. A spark igniter and a fuel nozzle are installed in the rear of the outer combustion case. b. Compressor discharge air is ducted from the dif- fuser scroll to the combustion outer case by the two compressor discharge air tubes. Air enters the single combustion liner at the rear through holes in the liner. The air is mixed with fuel sprayed from the fuel nozzle. The fuel-air mixture is then ignited in the combustion liner to provide combustion. Combustion gases move forward out of the combustion liner to the turbine. 1-8. Turbine. a. The turbine consists of a gas producer turbine support,  a  power  turbine  support,  a  turbine  and exhaust  collector  support,  a  two-stage  gas  producer turbine, and a two-stage power turbine. The turbine is mounted between the combustion section and the power and accessory gearbox. The two-stage gas pro- ducer turbine drives the compressor and accessory gear train. The two-stage power turbine furnishes the output power of the engine. b. Combustion  gases  from  the  combustion  section are expanded across the two stages of the gas producer turbine.  Energy  is  extracted  from  the  gas  stream  and converted to shaft torque to drive the compressor and gas producer gear train. c. The combustion gases then move forward from the gas producer turbine and are expanded across the two stages of the power turbine. Additional energy is extracted from the gas stream and converted to shaft torque.  This  shaft  torque  is  then  transmitted  through the power turbine gear train to the output shaft. d. The expanded gas from the power turbine passes through the exhaust collector support and is exhausted upward through the twin ducts The small amount of energy remaining in the gas stream assists in scaveng- ing the exhaust gases from the engine. 1-9. Power and Accessory Gearbox. The main power and accessory drive gear trains are enclosed in a single gear case. The gear case serves as the structural support of the engine. All engine compo- nents, including the engine-mounted accessories, are attached to the case. At 100% engine speed, reduction gearing reduces power turbine speed from 33,290 to 6,016 rpm at the output drive pads. The power turbine gear train has a torquemeter to measure engine output torque. Accessories driven by the power turbine gear train are the power turbine tachometer-generator (N2) and the power turbine governor. The gas producer gear train drives the oil pump, fuel pump, gas producer fuel control, and tachometer-generator (N1). The gearbox has a spare accessory mounting pad which is driven by the  gas  producer  gear  train.  During  starting  the starter-generator cranks the engine through the gas producer gear train. After completion of the starting cycle, the starter-generator functions as a generator. 1-10 Description of Engine Operation a. The engine is a free turbine engine, that is, there is no mechanical connection betweem the gas producer turbine and the power turbine. The power turbine is gas coupled to the gas producer turbine by the combus- tion gases. b. The  helicopter  uses  a  conventional  control  sys- tem. The collective pitch of the helicopter rotor estab- lishes the power output demand on the engine. For all practical  purposes,  helicopter  rotor  speed  is  held  con- stant by the engine and its control system. c. 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 collec- tive pitch sticks through a coordinated system of bell- cranks  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 pro- ducer  turbine  accordingly. d. A  motor-actuated  speed  trimming  device  is installed  in  the  linkage  between  the  collective  pitch sticks and the power turbine governor lever. It is oper- ated by a trim switch on pilots collective pitch stick, and allows engine output speed to be varied over a normal  range  of  approximately  95  to  102%  (5896  to 6196  rpm). 1-11.  Fuel  and  Control  System. a. The gas producer fuel control is located schemat- ically in the fuel system between the fuel pump assem- bly and the fuel nozzle. (See figure 1-3.) A power tur- bine  governor,  also  a  part  of  the  control  system, provides control intelligence to the gas producer fuel control. b. The system controls engine power output by con- trolling gas producer speed. Gas producer speed levels are established by the action of the power turbine gov- ernor which senses power turbine speed. Power turbine speed  is  selected  by  the  operator.  The  power  required  to maintain this speed is automatically maintained by power turbine governor action on the gas producer fuel control. 1-4 Change  14