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Auxiliary Power Plant Systems

S64F Model Helicopter

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Auxiliary Power Unit Auxiliary Power Unit SystemSystem The APU system drives the main gearbox accessory

section which powers the generators, utility and first stage hydraulic pumps, and transmission oil pump, without operating the engines.

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APU SpecificationsAPU Specifications

254

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APU ComponentsAPU Components The T-62T-16A2 gas turbine engine is a centrifugal flow single

shaft design in which the compressor and turbine are back to back on a common shaft. Power produced by the turbine rotor is transmitted by the shaft to the reduction drive assembly. The gas turbine is made up of three major sections.

1. Reduction drive and accessory drive assembly

2. Turbine assembly

3. Combustor assembly

11

22

33

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APU DescriptionAPU Description Reduction drive and accessory drive assembly: The reduction

drive is mounted on the front of the turbine assembly and drives the drive shaft and the accessory drive. Mounted on the accessory drive is the start motor, fuel pump and fuel control unit, overspeed switch, tach generator. The oil filter and oil pressure switch are also on the reduction drive housing.

Turbine Assembly: The turbine assembly housing is mounted to the compressor assembly and is where the air intake is located.

Combustor Assembly: The combustor assembly is an annular type, and consists of a combustor housing, combustor liner, nozzle shield and fuel manifold assembly. Air enters the combustor from the diffuser, flows between the walls of the combustor housing and the combustor liner and reverses direction to enter the burner. The inner walls of the housing mate with the turbine to form a circular duct for exhaust gasses to flow out of the combustor.

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APU SectionsAPU Sections

Reduction Drive

Turbine

CombustorAccessory Drive

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APU THEORYAPU THEORY Air is drawn into the compressor portion of the engine by

external cranking power during initial start cycle, after which air continues to be drawn into the compressor by the power produced at the turbine.

The air is compressed and directed into the combustor, fuel is introduced through six main fuel injectors into six vaporizer tubes, and is burned.

During the start cycle, fuel from the start fuel nozzle is ignited by the igniter. Initial combustion stabilizes, and engine attains 90 percent rated speed: a speed switch then de-energizes the start fuel solenoid valve, shutting off fuel flow to the start fuel nozzle.

Combustion is maintained by the six main fuel injectors. The resultant hot gases flow through a turbine nozzle and expand in the turbine assembly. The power produced in the turbine assembly is the transmitted to the reduction drive assembly.

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APU Wiring DiagramAPU Wiring Diagram

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APU Fuel SystemAPU Fuel System The APU fuel system is supplied from the pressure line

between the aft main tank and the #2 engine. APU fuel system

components:a) airframe fuel

shutoff valveb) Pressure relief

valve c) Check valve

a

b

c

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APU Fuel SystemAPU Fuel System

d

f

e

d) Fuel pump fuel control

e) Main fuel valve

f) Start valve

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a) Airframe fuel shut off valve: The shutoff valve, in the hoist well, shuts off fuel supplied to the APU fuel control. The APU master switch on the over head control panel controls the operation of the shutoff valve.

b) Pressure Relief Valve: The pressure relief valve opens at 20 psi (138 kPa) to prevent high pressures in the fuel lines.

c) Check Valve: The check valve directs fuel flow to the APU and prevents back flow to the fuel tank.

APU Fuel SystemAPU Fuel System

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APU Fuel SystemAPU Fuel Systemd) Fuel Pump Fuel Control:

The Fuel control is mounted on the APU fuel pump. The fuel pump fuel control is driven by the APU component drive. The START & MAIN fuel valves and fuel filter are mounted on the fuel control. The valves are controlled by the APU fuel switch on the OVERHEAD MISC CONTROL PANEL.Main Fuel Valve:The main fuel valve is mounted in the fuel control assembly. The main fuel valve directs fuel to six fuel nozzles for primary operation of APU. The valve is controlled by the Main Fuel Switch on the OVERHEAD MISC. CONTROL PANEL.

e) Start Fuel Valve: The start fuel valve is mounted in the fuel control assembly. The start fuel valve directs fuel from the fuel control to the APU during start. The valve is controlled by the 20% SWITCH on the OVERHEAD MISC CONTROL PANEL.

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APU Fuel SystemAPU Fuel System The Fuel system consists of a fuel control and pump combination that

has a set of bellows for altitude compensation and a flyweight controlled governor to maintain near constant speed.

Electrically activated start and main fuel valves are installed on the housing.

The start and main fuel valves are controlled by a switch on the APU control panel in the cockpit.

Fuel ControlFuel Control

Start ValveStart Valve

Main ValveMain Valve

Fuel FilterFuel Filter Fuel PumpFuel Control

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Start and Main Fuel NozzlesStart and Main Fuel Nozzles

Start Fuel Nozzle (1)

Main Fuel Nozzle (6)

Fuel is pumped to the nozzles at 265-335 psig (1827-2310 kPa)

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APU ControlsAPU Controlsa) Controls for the APU are located on the APU and overhead

misc. control panels, mounted in the overhead control panel. The APU controls consist of the following switches.

b) APU master switch

c) APU start switch

d) APU stop switch

e) 20% switch

f) Start fuel/Main fuel switch

a

b

c

d

e

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APU ControlsAPU Controlsa) The APU master switch is located on the APU control panel. The

switch controls the operation of the airframe fuel shutoff valve and arms the APU circuit.

b) The APU start switch arms the holding circuits for the start fuel valve and the ignition exciter and energizes the start relay located on the winch well relay panel.

c) The stop switch is pushed closed to abort a start or turn off the APU.

d) The 20% switch controls the start fuel valve. The 20% switch is spring loaded, and is held in the HOLD FOR START until the APU reaches 20% speed. The switch is then let go activating the start fuel and igniter.

e) The start/main fuel switch controls the start fuel valve and main fuel valve. In the start position the main fuel valve will be closed and the start fuel valve will be open. At 40% rpm on the indicator the start/main fuel switch is manually moved to the main fuel position.

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APU InstrumentsAPU Instruments The instrumentation for the APU is located on the APU

control panel. The panel is located on the overhead control panel. It contains the following indicator:

a) RPM gageb) Pyrometerc) Warning Lights(4)

1) Overspeed2) Overtemp3) Low Oil Press4) Fire

b

a

1 2 3 4

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APU InstrumentsAPU Instruments The RPM gauge is located on the APU control panel. The RPM

gauge displays the speed of the APU in percent. The dial markings are graduated from 0 to 100 percent with each increment on the dial representing 2 percent.

The pyrometer is in the APU control panel. It indicates the exhaust temperature of the APU turbine engine and is connected to the high exhaust temperature relay to cut electrical power to the APU in case of an overheat condition.

The pyrometer is a thermocouple type sensing device using one chromel and one alumel lead.

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APU InstrumentsAPU Instruments The warning lights for the APU system are located on the APU

control panel. The warning lights warn of an APU overspeed(110%), high exhaust temp (1070º-1090ºF) 577º-588º C) or low oil pressure (6±1 psi) (87 +/- 7 kPa) condition. The overspeed light receives its input from the speed switch

mounted on the APU accessory drive. The high exhaust temp light receives input form the

pyrometer relay. The low oil pressure light receives input from the oil

pressure switch. The fire warning light receives its signal from a fire eye

mounted by the APU.

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APU InstrumentsAPU Instruments Oil Pressure Switch

When the oil pressure drops below 5 to 7 psig (34-48 kPa) the pressure switch opens, turning the oil pressure light on. It also closes the main fuel solenoid valve which shuts down the APU.

ThermocoupleThe thermocouple is mounted in the top, aft end of the combustor housing; the temperature probe projects into the exhaust stream and senses engine exhaust temperature.

Speed Switch

The speed switch is mounted on the rear accessory drive pad in tandem with the tach generator, it contains two switches. One set at 90 % for fuel and ignition sequencing and the other at 110 % for overspeed.

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APU Component LocationsAPU Component Locations

APU Exciter Box

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APU Starter The APU had a hydraulic start system. Erickson modified it by

installing an Allison 250 series starter/generator. The starter is mounted to the APU accessory case. It is

energized when the start button is pushed, supplying 28 vdc from the battery.

The starter is de-energized at the same time the ignition and start fuel is shut off, through the speed switch at 90%.

APU Starter

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Speed Switch

Tach Generator

Pressure Switch

Thermocouple Probe

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APU DriveshaftAPU Driveshaft The drive shaft consists of a flange, shaft, thomas couplings and

attaching hardware. It extends from the clutch flange to the APU flange and transmits power from the APU to the clutch.

Thomas couplings are made up of steel discs. The discs are stacked with the grain of one disc 90º to the grain of the next disc.

Thomas Coupling

Flange

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APU ClutchAPU Clutch The clutch is located between the MGB accessory section

and the APU drive shaft. The clutch transmits power from the APU turbine to the

MGB accessories. The clutch engages automatically through centrifugal force

at 76% to 90% APU speed. The centrifugal force of the clutch causes the brake shoe to

engage with the clutch drive surface. The clutch is lubricated with oil from the MGB through the

drilled clutch output shaft to the clutch bearings.

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APU Clutch

APU Driveshaft

MGB Accessory Section

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APU ClutchAPU Clutch Adjustments to the clutch should be made anytime the

engagement is too early or too late. Adjustments on the clutch are made by adding or removing

tension on the brake shoe spring. Turning the adjustment bolts counterclockwise decreases engagement speed, turning the adjustment bolts clockwise increases engagement speed.

One turn of adjustment screw will change RPM about 6%. Turning the adjustment screw clockwise increases engagement

RPM, turning it counterclockwise decreases engagement RPM.

Adjustment Screw (3)

Access Holefor Adjustment

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APU ClutchAPU Clutch

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APU Fuel Control APU Fuel Control AdjustmentAdjustment

To adjust acceleration turn adjustment screw clockwise to increase acceleration and counterclockwise to decrease it.

Adjustments should be made in 2º increments.

The time it takes the APU to go from start to 100% is called the Acceleration Schedule.The acceleration schedule adjustments are made if acceleration is less than 6 seconds or more than 12 seconds.

Acceleration ScheduleAdjustment Screw

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APU Fuel Control APU Fuel Control AdjustmentAdjustment

Adjust the RATED SPEED by turning the adjustment screw clockwise for increased speed and counterclockwise for decreased speed.

If the RATED SPEED is below 98% or above 100% an adjustment must be made.

Adjust the RATED SPEED by turning the adjustment screw clockwise for increased speed and counterclockwise for decreased speed.

The speed that the APU is operating at is called the Rated Speed. The Rated Speed is displayed on the RPM indicator on APU control panel.

RATED SPEEDADJUSTMENT SCREW