09 - Helicopter Gas Turbine Installations

He' Whareleura-tiniKaihautu 0 Aotearoa

THE OPE NP0|.YTE(HNI(OF NEW ZEALAND

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He//copier GasTurb/ne lnsta/laz‘/ans

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CONTENTS

Helicopter Gas Turbine Engine Installations

Engine Mounting

Engine Alignment with Transmission

Drive Coupling Engine to Transmission

Checking Run—out of the Main Transmission Coupling

Engine Air Intake and Exhaust System

Cowlings and Pairings

Systems Related to the Power Plant

Anti-ice System

Fuel and Oil Systems

Engine Drives

Designation of Speeds

Engine Controls

Transient and Static Droop

Control Systems

Controls N1 N2 and Anti—icing

Fuel Control System

The Fuel Control

The Governor

Separate Units

Understanding the System

Control Adjustments

Large Computerised Fuel Systems

Fuel System Cleanliness

Purging the System

Fire Detection and Prevention

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AIRCRAFT ENGINEERING

TRADE THEORY AND PRACTICE ATRFRAMES - ASSIGNMENT 9

HELICOPTER GAS TURBINE ENGINE INSTALLATIONS

The usual gas turbine engine in light helicopters is theModel 250»C2O turboeshaft internal—combustion engine, manufacturedby the Detroit Diesel Allison Division of General Motors.

As with piston engines, manufacturers may locate the engine atvarious angles and places in the airframe.

The Model 250—C2O engine is usually aft of the mast above thepassenger compartment. This location simplifies the drive system,improves the inlet exhaust arrangement, reduces cabin noise, andreduces the danger to the aircraft, crew, and passengers in anaccident.

The Model 250-C20 is of modular construction. Figure l showsthe four serialised and easily changeable units of the engine. ‘

The power plant comprises the following:

l. Engine

(al Single combustion chamber,

Cb) A sixastage axial flow and a onesstage centrifugalcompressor coupled to a two-stage gas—producerturbine,

Cc) A two~stage power turbine (free turbine), and

Cd) An accessory gearbox incorporating the.mainpower and accessory-drive gear—train.

2. The gear case, which provides the structural supportfor the engine.

3. A pneumatic or hydraulic fuelecontrol system comprisingthe gas producer fuelecontrol unit, power turbinegovernor, engineadriven fuel pump, and accumulator.

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_FIG.____I_L_ Components of 250—C2O engine

The power plant installation comprises the following:

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Engine mounts (Bipod system),

Engine intake and exhaust system,

Cowlings,

Fuel system (external),

Oil system,

Electrical system, and

Engine controls N1 and N2, and antieicing.

In Figa 2, the systems related to the turbine engine arenumbered as follows:

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1. ignition exciter, 2- $f8f’leFQ6I'l9Ffil0f, 3. N, control cable and bracket,4. Turbine rightenglne mount, 5. Electrical ground cable 6, |n|etdu¢¢,

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Tail rotor drive shaft

Turbine lower engine mount,N, control cable,

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Main transmission coupling assembly, 11- TQIQUQ llllessuie line» 12.. Anti-ice cable, 9. Burner drain valve,

Turbine left engine14. Exhaust ejectors, 15_ Uppgflifflng pad,17. lgnitor plug lead.

FIG. 2 Turbine engine installation

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mount

Engine Mounting

As shown in Fig. 3, engine mounts on most turbo engines aresimpler than those of reciprocating engines.

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In a light helicopter, as shown in Fig. H, the engine issupported on the service deck by three bipod mounts on the.right, left, and lower side of the engine.

Engine Alignment with Transmission

This alignment is by bonded shims as shown in detail A inFig. H. They are individually ground and bonded in place whenthe aircraft is manufactured. If, for repair purposes, they haveto be removed, index them and replace them in the same position.

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Some helicopters have the engine mount located throughmachined fittings as shown in Fig. 5. These arethe tubing and machined ste Ito th '

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All tubular mountings are mad6

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FIG. 5 Engine mount

Drive Coupling Engine to Transmission

Figure 6 shows a typical main

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transmission coupling.

led first to the engine coupling shafthaft by bolts. The boltsand then to the transmission coupling s - _

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FIG. 7 Main transmission coupling in position

Checking Run-out of the Main Transmission Coupling

When installing the main transmission coupling, and afterrun-out of the coupling. Attachvibration is reported, check the

le location, anddial test indicator and fixture at a suitabacheck run-out at both the engine and the transmission ends of the

t the shaft against the dialmain transmission coupling, rota etest indicator. The run-out should be within the manufacturer'slimits.

After vibration is reported, also check

l. Lubrication level of drive, and

2. The installation of the main transmission couplingto the drive shafts at both engine and transmissionends.

Engine Air Intake and Exhaust System

The air induction system sometimes includes an inlet ductand screen assembly with alternative air doors, as shown in Fig. 8

inlet duct makes the doors operateAny blockage of theoss of air induction to theautomatically thus preventing l

compressor.

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If the air intake is to be removed, cover the enginecompressor inlet immediately afterwards.

If the aircraft has the intake incorporated into the

cowling, cover the engine compressor intake immediately afterremoving the engine cowling.

The exhaust system shown in Fig. 8 includes left and right

exhaust ejectors that are clamped to the engine exhaust collector

In Fig. 8, components of the air intake and exhaust system

are numbered as follows:

l l. Inlet duct and screen assembly,2. Alternative air doors,

3. Coupling assembly,

4. Duct assembly,5. Right exhaust ejector,

6. Left exhaust ejector,

7. Exhaust clamps (both sides),and

8. Engine exhaust collectors(both sides).

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Cowlings and Fairings

In Fig. 9, which shows a typical set, the cowling andfairings are numbered 1, 2, 3, and N.‘ Section 3 is the enginecowl, and the exhaust stacks are shown in place.

The construction is of aluminium alloy, fibreglass, andhoneycomb material.

The engine cowling in Fig. 9 houses the engine air filter,the inlet bellmouth, and the forward fire wall. Below the engineis a titanium floor that acts as a drip pan and also givesinsulation from heat. The clearance between the engine and thepan is enough to allow the removal of engine accessories.

Access panels have snapeopen fasteners. The cowling isreadily removable to enable engine and transmission changes.

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FIG. 9 Cowlings and fairings

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SUMMARY

The most common gas turbine engine in light helicoptersis the Allison 250+C20.

The engine is mounted on a bipod tubular frame attachedto the aircraftls main deck.

A drive shaft (main transmission coupling) transmitsengine power to the input coupling shaft of thetransmission unit.

In some installations, cowlings and fairings arefitted over the power plant section.

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PRACTICE EXERCISE A

l. Is the engine usually positioned forward or aftof the mast?

2, Name the main changeable components of the engine.

3, What material is the engine mounting tube made of?

4. After the main transmission coupling has beenreplaced, the pilot reports vibration. Whatchecks should you make?

5. Name the four main sections of the cowling—fairingassembly.

(Answers on page 35)

SYSTEMS RELATED TO THE POWER PLANT

Anti-ice System

The only engine components with antieicing provision arethe compressor inlet guide vanes and the front bearing support,as shown in Fig. lO. ,

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FIG. 10 Antieice actuator and shut—off valve

The antieice system comprises an air shut=off valve, actaptor,piping, and fittings.

The shut—off valve and actuator are mounted on top of theengine as shown in Fig. l0. On the front face of the compressorscroll, air is tapped from the compressor discharge and iscontrolled by the shuteoff valve and actuator.

When the valve is open, hot air is directed, from thecompressor discharge take~off, through the valve to the compressor-inlet guideevanes and front—bearing supportehub, thus preventingicing at the compressor inlet.

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... 12 ._-.

Fuel and Oil Systems

We shall discuss typical systems. In some aircraft, the

components are arranged differently but meet the same requirements

External fuel system: As shown in Pig. ll, the external fuelsystem comprises

l. Engine-driven pump and filter,

2. Shut—off valve,

3. Electrical auxiliary pump,

H. Bladder fuelecell (bag tank), and

5. Instrumentation, which includes fuelequantity andfuelepressure gauges.

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Oil system: The oil is in a dry—sump system, As shown inFig. l2, the supply tank is externally mounted, and the oil cooleris located on the top aft section of the fuselage‘

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External oil system

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Oil flows from the tank to a gearetype pressure andscavenge pump mounted within the accessory drive gearbox.

Return oil is routed from the engine oil outlet port to theoil cooler and from the cooler to the tank.

For checking the quantity of oil in the tank a dip stick ismounted on the filler cap, and/or a sight gauge is provided.

The tank has outlets and inlet ports for engine supply,

Cooling air is supplied to the oil cooler by a blowermounted on the tail rotor driveeshaft. In some aircraft, the fan

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Engine Drives

The following units are coupled to the gear train of the

gas~producer turbine (compressor turbine);

l. Fuel pump and filter assembly, located at the centrerear cover of the gearbox;

2. Gaseproducer tachometer generator, located at the frontof the gearbox on the rightehand side;

3. Gas—producer fuelecontrol unit, located at the rearof the gearbox on the rightehand side; and

H. Oil pumps, located within the gearbox.

The power turbine gear train drives

l. Power turbine governor, located at the rear of thegearbox on the upper leftehand side;

2. Power turbine tachometer generator, located on theleftehand pad at the front of the gearbox;

3. Transmission and rotor system; and

H. Torquemeter.

Designation of Speeds

As shown in Pig. 13, the speedsof the various drives aregiven a letter and a number. Memorise the following letters andnumbers, which we shall often use when discussing engine controls

and the fuel system.

l. N1 (sometimes called N 1 denotes compressor and gasproducer turbine speedg

2, N2 (sometimes called Nfl denotes free power turbinespeed.

3. N3 (sometimes called NP) denotes main rotor speed.

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FIG. l3_ Turbine speeds

REMEMBER

To understand the fuel system and the controls, whichwe shall discuss next, you must know why a free turbineengine is so called.

The Allison 250—C20 engine has no direct mechanicalcoupling between

l. The gas producer turbine, which drives the compressor;and

2. The power turbine, which drives the helicopter rotorthrough a gear reduction unit.

The two turbines are connected only in terms of gas flow.This fluid coupling arrangement permits the.turbines tooperate at different speeds. To manage fuel flow tothe engine for all flight and power requirements, thefuel control system must be sensitive to the speeds ofbothlturbines.

In this engine, the gaseproducereturbine speed isdesignated as N1 and the power—turbine speed as N2.

PRACTICE EXERCISE B

1. Name the components for which antieicing isprovided.

2, How is cooling air supplied to the air cooler?

3. Does the oil system have a separate oil tank?

4. which gear train drives the gas producer fuelcontrol unit?

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5. What does N1 denote?

6. What does Nr denote?

(Answers on page 35)

ENGINE CONTROLS

Helicopters with gas turbine engines usually have an automaticsystem to control rotor speed N3 Cor Nrl in powered flight.

The desired rotor speed is selected before takeeoff. Thefuel flow is then automatically regulated by the power turbinegovernor N2.

The unit senses power and hence fuel requirements and sendsmessages to the gaseproducer fuelecontrol unit N1 to maintainthe selected rotor speed within a correct and safe range.

This selection is maintained despite changes of collectivepitch.

The pilotls main selection unit for rotor speed is thetwist grip, located on the end of the collective stick. Theselections on the gaseproducer fuelecontrol unit are as follows:

l. 90° (flight idle) gives lOO% free power turbinespeed N2.

2. 30° (ground idle) gives 60e62% gas producer turbinespeed N1. ~

3. 5°sQ° is the cuteoff speed.

A special detent button is installed on the end of thecollective sticki The twist grip can not be operated from groundidle to cuteoff until the button is depressed. The throttlecannot, therefore, be inadvertently closed in flight.

Refer to Fig. IQ as you read on.

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Gas producer" ..\ ' fuel control uni

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The twist grip located at the end of the collective stickis connected through rods, levers, and/or push-pull controls tothe lever on the gas-producer fuel-control unit N1.

When the pilot raises the collective stick, thus increasingthe angle of attack and pitch angle of the main rotor blades, heis also operating a separate engine interconnected control.

This control links the collective stick, the linear droopactuator (droop compensater control N2) and hence to the powerturbine governor N2. The raising of the collective stick thusresets the power turbine governor. This increases the fuel flowto the engine and restores Np rotor speed within the manufacturer’limits. Operating the collective stick downwards has the oppositeeffect on the power. L

Figures 1H, l5, and 17 show a fail—safe device in the form ofa weak section of an intermediate bellcrank operating shaft.In the event of the seizure of the power—turbine governor—unit,failure of the shaft at the weak point allows the continued

O I 0 Ioperation of the collective stick.Rotor blades_

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FIG. 14 Engine controls

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Transient and Static Droop

After the pilot increases collective pitch within the normalrange, the rotor speed falls, despite the automatic increases infuel flow and power. The condition is called transient droopand is caused by certain design features of the free turbine typeof gas turbine engine.

Partial recovery takes place due to the power—turbinegovernor~correction, this restores power to give an improved rotorspeed, less than that originally selected. The difference betweenthis underspeed and the original rotor speed is called staticdroop or negative droop.

Static droop is corrected when the pilot operates a governor

switch (beeper), as shown in Fig. l5. '

This action operates the linear droop actuator positionedbetween the collective stick and the power turbine governor,as shown in Fig. 14. When operated in one direction, the switchincreases fuel flow and NP rotor speed, and when operated in theopposite direction, reduces fuel flow and NP rotor speed.

When the linear droop actuator is operated to correct staticdroop,(a negative drooplthe actuator rod's length alters,adjusting the position of the power-turbine governor-lever, torestore power by increasing fuel flow to the engine. Thisincreases NP rotor speed to the original.

Positive droop is a condition where NP rotor speed isgreater than 100%. Correct this condition by operating thegovernor switch, beeping to reduce NP rotor speed to theoriginal setting.

Control Systems

We shall discuss the control systems in a general way. Weshall not detail any particular system. Either Bendix orChandler Evans Company (CECO) fuel—control units and governorsare fitted to the Allison 250-C20,

555/3/9

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When rigging controls, you must consult the maker‘smanual. In this assignment, we deal with thecontrols only in general terms, These maintenanceprocedures do not constitute a maintenanceinstruction.

21

The collective stick: This must be rigged before the enginecontrols, and the droop compensator control before the powerturbine governor.

The throttle control: This may be rigged at any convenienttime. After the engine control rigging has been completed,the fuel adjustments may be made.

Droop compensator N2 rigging: Refer to Fig. 15 as yOu read

on.

l. Set rod 5 to its correct nominal length, and connectA it to bellcrank 7.

2. Adjust the stroke of the linear droop actuator 9by adjusting screw 12 to give correct rod travel,from fully extended to fully retracted.

3. Operate the governor switch l. Beep to full decrease(rod fully extended). Adjust rod end l8 to givethe nominal length specified.

H. Connect the rod end l3 to the governor lever 1%.This completes the rigging of the droop compensator.

gowerjturbine governor rigging: Refer to Fig. l5 as you readOTI-

l. Set the collective stick to the position fully upor down as specified by the manufacturer.

2. Operate the governor switch. Beep until the lineardroop actuator is in the extreme position specifiedby the manufacturer.

3. As shown in Fig. l5(a), position lever 14 togovernor shaft 20 in the position specified by themanufacturer.

H. Back off the stop screws l6 and l8 by the specifiedamount.

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Rotate the lever 1% to the extreme position,as specified by the manufacturer.

Hold lever 1H in position. Adjust and connectthe rod end l3 to it. '

Select the collective stick fully up to obtainmaximum and fully down to obtain minimum stoppositions, adjusting the governor switch (beeper)as laid down by the manufacturer, and adjust thestops.

Select the collective stick to full up and trim(beep) the linear droop actuator to full increase(maximum). Check the clearance between the stoparm and screw 18.

Select the collective stick to the full downposition, and trim (beep) the linear droop actuatorto the full decrease position (minimum).Check theclearance between the stop arm and screw l6.

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FIG. 15 Droop compensator control N2 and turbine governor

rhegtwistegrip {throttle-control) rigging: The limits are

established by mechanical stops. When, for example, the twistgrip is moved to the full open position, the fuel control pointermust contact the maximum throttle stop at or before the limit oftwistegrip travel.

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Refer to Fig. 16 as you read on.

1. Depress the pilot’s flight idle detent button asrequired to enable full—range operation of the control.

2. Select the gas producer fuel control to the groundidle position 30°, by operating the throttle twistgrip, and install the rigging pin, locking thisselection.

3, Set Cgntpglex pod to the bellcrank, and set the verticaltube to the operating lever. This lever being previouslypositioned on the gas-producer fuel-control unit—shaftin the correct relationship as specified by themanufacturer.

H. Remove the rigging pin from the gaseproducerfuel—control unit.

5. Fully rotate the twist grip, press the detent buttonas required, and check that the maximum stop is _contacted at flight idle position, and that the minimumstop is contacted at the cut—off position.

6. Ensure that the gaseproducer fuel~control~unit pointerpositions at 30°, each time you rotate the twist gripto ground idle from either fully open or closed, evenwhen moving the control rapidly.

7. The gas—producer fuel-contro1~unit pointer must remainin the same position as selected, irrespective ofchanges of position of the collective stick. Checkthis by operating the collective stick to ensure thatthis is so.

8. Make final adjustments after engine run.

Remember that the rigging of the linkage must be such that thestops restrict the travel, so that adjustments of the stops areeffective in altering the range of movement and the settings ofthis control.

A'nti‘f-»,icii1_9'p c'0'nt’r'ol :

1. Operate the actuator to the off position. Ensurethat the actuator shaft turns counter—clockwise whenviewed from the end.

2. Move the anti—ice valve lever to the full forward,position.

3. Install the splined end of the lever on the actuatorshaft.

H. Check the control for.full free and correct movement,and check all lockings.

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Controls N1N2 and Anti-icing

To see how the parts fit in a typical system layout, studyFig. l7, which shows the complete control run with engine inposition.

SUMMARY

Transient droop is a fall from original rotor speedwhen the collective pitch lever is operated.

Static droop is the difference between a partiallyrecovered rotor speed and the original rotor speed.

The three engine controls are

l. Power lever (throttle) to the gas~producerfuel—control unit N1;

2. Power turbine governor. This control is connected,from the collective stick through the linear droopactuator (droop compensator N2), to the powerturbine governor; and

3. Antieicingcontrol from the linear actuator to theshut-off valve.

PRACTICE EXERCISE C .

l. Which control is the pilot's main rotor—speed selector?

2. To what fuel control unit is the collective stick linked?

3. which control relates to N2 speed?

4. Is the travel of the linear—droop actuator—rodmechanically adjusted?

5 Does correction of static droop (negative droop) by theu€

pilot cause an increase or decrease of rotor speed?

6. What prevents the pilot from selecting the closedthrottle position in flight?

(Answers on page 35)

555/3/9

v 26 ~

FUEL CONTROL SYSTEM

To understand the fuel system and its adjustments, you must

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A typical engine fuel system is the Pratt and WhitneyPT6@pneumatic with manual reversion.

In this assignment, we shall study another typical systemin a general way, the Allison 250~C20 —— pneumatic or hydromechani-cal. Think of the system as a series of computers, as shown inFig. l8. To control the Allison 250-C20 engine, the Model MC-H0system does all the management of the fuel.

Fuel Control

The fuel control is mechanically connected to the pilot'stwist grip and also, through.the accessories gear train, to thegas producer turbine. It senses compressor discharge pressurethrough an air line.

The function of the fuel control is to provide the rightamount of fuel to the engine throughout the start cycle, at groundidle, and for all of the requirements of powered flight. Becausethe fuel control is sensitive to and controls the speed of thegas producer turbine it is called the N1 control.

The Governor

Remember that the power turbine governor is mechanicallyconnected to the pilot's collective pitch control through a linearactuator. The actuator is electrically operated by the governortrim switch and allows the pilot to make minor adjustments

Qapproximately 5% total) in N2 to compensate for aerodynamic andambient conditions.

The governor is also mechanically coupled, through the outputgear train, to the power turbine and also senses compressordischarge pressure through an air line.

555/3/9

A helicopter is designed to

27

operate with a constant rotorspeed in flight. The function ofpower turbine speed constant (atconditions as established by thegovernor senses and controls powethe N2 control.

the governor is to maintain the100% N2) under varying loadcollective pitch. Because ther turbine speed, it is called

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Separate Units

Separate units are required, as shown in Fig. l9. They mustbe installed at different locations on the engine gearbox.The fuel control has a splined shaft that engages in theaccessories gear train and is connected to the gas producerturbine. The splined shaft on the governor engages in the outputgear train, which is connected to the power turbine. This arrange-ment is necessary to pick up and sense the two operating speedsN1 and N2 found in a free turbine engine such as the Allison250~C20.

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Understanding the System

Think of the'operation of the system in terms of fuel flow,considering both the gaseproducer fuel—control and the powerturbine governor as valves with builtein computers. Both thefuel control and the governor have builtein computers, but theseare hydromechanical rather than electronic.

In the first stage of fuel management, as shown in Fig. 20,the gaseproducer fuelecontrol receives the total flow from thefuel pump. This fuel flow l is greater than the amount theengine requires under any normal operating condition. The fuelcontrol senses or receives data input from the engine compressordischarge pressure PC, the gas<producer—turbine speed N1 and thetwist grip position - cutoff, ground idle, or maximum power.From the data input, the fuel control computes the amount of fuelthe engine requires to sustain the gas producer turbine andsatisy the maximum requirement of the power turbine. This amountof fuel 2 is sent to the power turbine governor, with the excessZ returned to the inlet side of the fuel pump.

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. FIG. 20 Gas producer fuel control unit

The fuel flow from the fuel control to the governor, asshown in Fig. 2l, is based on the gas producer turbine needs andmaximum power turbine requirements. This fuel flow may be greaterthan the amount required to meet actual power requirements at anygiven moment. The governor senses or receives data input from theengine compressor discharge pressure PC, the power turbine speed Nand the collective pitch control setting. From the data input,the governor computes the amount of fuel the engine requires tomeet actual power requirements and maintain N2 at 100%, at thesame time sustaining gas-producer~turbine operation; This amountof fuel 3 is sent to the engine, back through the fuel control.Excess fuel Z is returned to the inlet side of the pump.

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Figure 22 shows that the purpose in directing fuel flow fromthe governor back through the fuel control on its way to theengine is to provide fuel cutoff as the last stage in the fuelcontrol system. The pilot's twist grip is connected to the fuelcontrol throttle lever, which is in turn linked to the fuelcontrol cutoff valve.

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Control Adjustments

The only adjustments you may make are as follows:

l. The maximum PC stop of the gas-producer fuel-control. unit, which. is adjusted only on replacement of

the unit;

2. Maximum throttle stop;

3. Cuteoff stop;

H. Idle trim adjustment;

5. Light off; and

6. Start enrichment.

Other adjustments are made by the manufacturer. Becausethe adjustment points may look alike, always consult the maker'smanual before making adjustments. We shall now consider theadjustments in general terms.

Maximum thr0ttle~stOp: A set screw on the gaseproducer fuel-controleunit is set to give a maximum power setting of l0H% N1.The throttle must be against the stop when the twist grip is

556/3/9

e 31 »

rotated to the maximum. Always check the control rigging beforemaking an adjustment.

Cuteoff stop: A set screw on the gas—producer fuel-controlunit limits the travel of the throttle lever in the minimum 0°direction. Its effect is to establish the cuteoff position ofthe throttle N1 speed. Check the control rigging before makinga fuel adjustment.

idle trim adjustment; Use the idle trim adjustment to trimidle N1 speed when the fuelecontrol-throttle pointer is at 30°position. The adjusting screw is located on the gas—producerfuelecontrol unit. The adjustment is necessary only when N1 idlespeed is outside the specified limits, with the generator turnedoff. Idle speed is based on the 30° fuel control quadrantsetting. Check the control rigging before making a fuel adjustment

by turning a hexagoneheaded screw positioned on the gaseproducerfuel—control unit. .

Lightsoff adjustment: The light-off adjustment on the gas-producer fuel-control unit is a hexagon screw. If the light-offadjustment is too low, the engine may not have enough fuel tostart, and if too high, the starting temperature may be too high.

Start enrichment adjustment. During start from approximately25% Ni to approximately H0% N1, temporary fuel derichment, or adip in fuel flow is required to provide starting performance.When fuel flow is plotted against N1, this dip in fuel flowlooks like a notch on the graph and so is called a notch.

The duration and width of the notch is controlled by thestart derichment adjustment.

If the notch is too narrow, excessive turbine outlettemperatures result, and this defect must be corrected.

The adjustment provided is positioned on the gas—producerfuelecontrol unit.

555/3/9

_-.32..

Large Computerised Fuel Systems

In Fig. 23, we again see the need to think of the fuelcontrol and governors as computers. '

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Fuel System Cleanliness

Cleanliness is essential in all fuel systems.

Figure 2% shows the general procedure for inspecting andcleaning the gaseproducer fuel»control filter.

l. With the engine installed in the aircraft, you mayhave to remove the starter generator to gain access tofuel control inlet. '

2. Disconnect the fuel line from the fuel control inletfitting l.’ Cut the lockwire, and remove fitting andpacking 2, Discard the packing.

555/3/9

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Remove filter 3 and spring H, and check forcontamination.

If necessary, clean the filter and spring in anultrasonic cleaner with trichlorethylene forapproximately l5 minutes. If you cannot clean themin an ultrasonic cleaner, use a saturated solution ofl litre of sulphuric acid and 20 grams of sodiumdichromate for l minute. Rinse thoroughly in cleanwater and dry with compressed air.

Hold filter up to the light and check for contaminationbetween inside and outside screens. Repeat cleaningprocedure if necessary.

Pit spring M into cupped end of filter 3. Keep partstogether (use a dab of petrolatum if necessary), andinstall in fuel control. Press in on filter, compressingspring, to check bypass action.

Install fitting l with new packing 2. Torque to thespecified figures and lockwide-.

Attach fuel line to inlet fitting and torque tube nutto the specified figures while holding fitting.

Purge air from the fuel system as required.

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FIG. 24 Fuel filter

Figure 25 shows the general procedure for inspecting andcleaning the power turbine governor filter.

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Disconnect fuel line from governor inlet and filterfitting l. Cut lockwire and remove inlet and filterfitting and packing 2. Discard packing.

Check inlet and filter fitting for contamination.

If necessary, clean inlet and filter fitting in anultrasonic cleaner with trichlorethylene forapproximately l5 minutes, or if you cannot use sucha cleaner, in a saturated solution of l litre sulphuricacid and 20 grams of sodium dichromate for l minute.Rinse thoroughly in clean water and dry withcompressed air.

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_ gu _

H. Check the filter for contamination, and repeatthe cleaning procedure if necessary.

5. Install the inlet and filter fitting l with newpacking 2. Torque to the specified figure, andlockwire.

6. Attach the fuel line to inlet and filter fittingand torque tube nut to the specified figure.

7. Purge air from the fuel system as required.

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Purging the System

Purging is required whenever the fuel system is opened upfor maintenance or when air gets into the system. '

Purging consists of breaking the system-connections atvarious places in sequence and pumping fuel through from thebooster pump or an external fuel~pump source. Always pull theignition circuit~breaker before starting, and use an auxiliarypower unit or battery.

Fire Detection and Prevention

Fire detectors are provided in many helicopters in suchnumbers and locations as to ensure rapid detection of fires.

Where gas turbine engines are installed, detectors andindicator lights show the area for which extinguisher actionshould be taken. Fire detectors do not operate fire extinguishers

The pilot decides when the fire bottles should, under normalflight and ground conditions, be set off. 2

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In a crash landing, the bottles are automatically set off

_ 35 _

on impact by an inertia switch.

SUMMARY

Fuel systems are electrohydraulic, mechanical, orpneumatic, and some have manual reversion.

Think of the fuel system as computerised, the computersin the Allison 250—C2O engine being the gas-producerfuel—control unit and the power—turbine governor.

Fuel flow passes from one unit through the next andback again.

You may make some adjustments to fuel flow, butother adjustments are made by the manufacturer andmust not be tampered with.

PRACTICE EXERCISE D

EXERCISE

l.

l. What does the power—turbine—governor sense?

2. At what stage does the fuel flow through the cut—offvalve?

3. Are all fuel system adjustments permissible?

4. How is the drive made between the gas~producerturbine and the power turbine?

5. What does the term notch mean?

(Answers on page 3H

ANSWERS TO PRACTICE EXERCISES

A

Aft

2. Ca) Combustion section,

3.

Cb) Turbines,

Cc) Compressor, and

Cd) Power and accessories gearbox.

SAE H130

555/3/9

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4. (a) Run-out check of main transmission coupling

(b) Oil level check of the main transmission coupling

(c) That the main transmission coupling (shaft) hasbeen correctly installed

5. Ca) Forward fairing,

(b) Induction fairing,

Cc) Engine cowl, and

(d) Aft fairing.

EXERCISE B

l. The COmpP€SSOP inlet guide vanes and front bearingsupport

2. Cooling air is supplied to the oil cooler by a blowermounted on the tail rotor shaft.

3. Yes _

4. Gas~producer turbine gear train

5. Gas-producer-turbine speed

6. Rotor speed

EXERCISE C

l. The throttle (twist grip), situated on the end ofthe collective stick

2. Power turbine governor

3. The droop-compensator and powereturbine-governorcontrol

H. Yes. By an adjustment of a screw on the actuator

5. An increase to the originally selected speed

6. The unpressed detent button positioned on the end ofthe collective stick

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_-.37?

EXERCISE D

l. Compressor discharge pressure PC, collective, pitchcontrol setting, and power turbine speed N2

2. Just before delivery to the engine fuel nozzle

3. No, some are bench adjustments made by the manufacturerand are not to be adjusted in the field.

H. By a gas link

5. A derichment to the fuel supply between about 25%N1 and 40% N2, which has the appearance of a notchon the graph and is so called

TEST PAPER 9

l. List

(al Pour main components of the engine, and

Cb) The power plant systems.

2. Ca) List four advantages of positioning the engine aftof the mast.

Cb) Describe two ways by which engine alignment may beachieved.

3. Showing complete control runs, make a diagrammatic sketchof the throttle control N1 and droop compensator N2and the power turbine governor controls,

H. Referring to Fig. l6, briefly describe the rigging checksof the throttle control N1.

5. List the three items sensed by the gas-producer fuel-control unit.

6. Referring to Fig. l5 briefly describe four steps inrigging the droop compensator N2.

7. List six fuel-system adjustments, and state the purposeof each adjustment.

555/3/9

e 38 e

Briefly describe a fire detection and prevention systemand how the fire bottles are operated.

What is meant by the following terms?

(a) Transient droop, and

Cb) Static droop (negative)?

Copyright

This material is for the sole use of enrolled students and may notbe reproduced without the written authority of the Principal, TOPNZ.

,555/3/9

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