He' Whareleura-tini Kaihautu 0 Aotearoa
THE OPE N P0|.YTE(HNI( OF NEW ZEALAND
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He//copier Gas Turb/ne lnsta/laz‘/ans 555—3—9
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
555/3/9
AIRCRAFT ENGINEERING
TRADE THEORY AND PRACTICE ATRFRAMES
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ASSIGNMENT 9
HELICOPTER GAS TURBINE ENGINE INSTALLATIONS
The usual gas turbine engine in light helicopters is the Model 250»C2O turboeshaft internal—combustion engine, manufactured
by the Detroit Diesel Allison Division of General Motors. As with piston engines, manufacturers may locate the engine at various angles and places in the airframe. The Model 250—C2O engine is usually aft of the mast above the passenger compartment.
This location simplifies the drive system,
improves the inlet exhaust arrangement, reduces cabin noise, and
reduces the danger to the aircraft, crew, and passengers in an accident. The Model 250-C20 is of modular construction. Figure l shows the 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 centrifugal compressor coupled to a two-stage gas—producer turbine,
Cc)
A two~stage power turbine (free turbine), and
Cd)
An accessory gearbox incorporating the.main power and accessory-drive gear—train.
2.
The gear case, which provides the structural support for the engine.
3.
A pneumatic or hydraulic fuelecontrol system comprising the gas producer fuelecontrol unit, power turbine governor, engineadriven fuel pump, and accumulator.
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Components of 250—C2O engine
The power plant installation comprises the following: Engine mounts (Bipod system),
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Engine intake and exhaust system,
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Cowlings,
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Fuel system (external),
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Oil system, ¢
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Electrical system, and
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Engine controls N1 and N2, and antieicing.
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2, the systems related to the turbine engine are
numbered as follows:
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1. ignition exciter, 4. Turbine rightenglne mount,
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3. N, control cable and bracket, 6, |n|etdu¢¢,
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Tail rotor drive shaft ,
Main transmission coupling assembly, Turbine lower engine mount,
N, control cable,
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Anti-ice cable,
11- TQIQUQ llllessuie line» 14. Exhaust ejectors, 17. lgnitor plug lead.
Burner drain valve,
12. Turbine left engine mount 15_ Uppgflifflng pad,
Turbine engine installation 555/3/9
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Engine Mounting As shown in Fig.
3, engine mounts on most turbo engines are
simpler than those of reciprocating engines.
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Typical engine mounts
In a light helicopter, as shown in Fig. H, the engine is
supported 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 in Fig. H.
They are individually ground and bonded in place when
the aircraft is manufactured.
If, for repair purposes, they have
to be removed, index them and replace them in the same position.
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Some helicopters have the engine mount located through machined fittings as shown in Fig. 5.
These are placed between the tubing and machined ste el brackets I that are riveted “ and b olted to th e main ' structure.
All tubu lar mountin gs are mad 6 of H130 steel tubing.
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Drive Coupling Engine to Transmission Figure 6 shows a typical main transmission coupling. The assembly is instal led first to the engine coupling shaft by bolts. The bolts and then to the transmission coupling s haft _ are then torque d an d wire locked or secured with s elf-locking nuts. ;
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Figure 7 shows the m ain transmission coupling (shaft) in position
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Checking Run-out of the Main Transmission Coupling When installing the main transmission coupling, and after vibration is reported, check the run-out of the coupling. Attach a dial test indicator and fixture at a suitab le location, and check run-out at both the engine and the transmission ends of the main transmission coupling, rota t e the shaft against the dial test indicator. limits.
The run-out should be within the manufacturer's
After vibration is reported, also check
l.
Lubrication level of drive, and
2.
The installation of the main transmission coupling
to the drive shafts at both engine and transmission ends.
Engine Air Intake and Exhaust System The air induction system sometimes includes an inlet duct and screen assembly with alternative air doors, as shown in Fig. 8 Any blockage of the inlet duct makes the doors operate automatically thus preventing l oss of air induction to the compressor.
555/3/9
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If the air intake is to be removed, cover the engine compressor inlet immediately afterwards. If the aircraft has the intake incorporated into the cowling, cover the engine compressor intake immediately after removing 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. 2.
Inlet duct and screen assembly, Alternative air doors,
3.
Coupling assembly,
4.
Duct assembly,
5.
Right exhaust ejector,
6.
Left exhaust ejector,
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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 and fairings are numbered 1, 2, 3, and N.‘ Section 3 is the engine
cowl, and the exhaust stacks are shown in place. The construction is of aluminium alloy, fibreglass, and honeycomb material. The engine cowling in Fig.
9 houses the engine air filter,
the inlet bellmouth, and the forward fire wall. is a titanium floor that acts as a drip insulation from heat.
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pan and also gives
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pan is enough to allow the removal of engine accessories. Access panels have snapeopen fasteners.
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readily removable to enable engine and transmission changes.
. Forward fairing Induction fairing . Engine cowl
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SUMMARY The most common gas turbine engine in light helicopters is the Allison 250+C20. The engine is mounted on a bipod to the aircraftls main deck.
tubular frame attached
A drive shaft (main transmission coupling) transmits engine power to the input coupling shaft of the transmission unit. In some installations, cowlings and fairings are fitted over the power plant section.
l PRACTICE EXERCISE A l.
Is the engine usually positioned forward or aft of 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 been replaced, the pilot reports vibration. What checks should you make?
5.
Name the four main sections of the cowling—fairing assembly. (Answers on page 35)
SYSTEMS RELATED TO THE POWER PLANT
Anti-ice System The only engine components with antieicing provision are the compressor inlet guide vanes and the front bearing support, as shown in Fig. lO.
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The antieice system comprises an air shut=off valve, actaptor, piping, and fittings. The shut—off valve and actuator are mounted on top of the engine as shown in Fig. l0.
On the front face of the compressor
scroll, air is tapped from the compressor discharge and is controlled by the shuteoff valve and actuator. When the valve is open, hot air is directed, from the
compressor discharge take~off, through the valve to the compressorinlet guideevanes and front—bearing supportehub, thus preventing icing at the compressor inlet.
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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 fuel system 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 and fuelepressure gauges.
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Oil system: The oil is in a dry—sump system,
As shown in
Fig. l2, the supply tank is externally mounted, and the oil cooler is located on the top aft section of the fuselage‘
555/3/9
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Oil flows from the tank to a gearetype pressure and scavenge pump mounted within the accessory drive gearbox. Return oil is routed from the engine oil outlet port to the oil cooler and from the cooler to the tank. For checking the quantity of oil in the tank a dip stick is mounted on the filler cap, and/or a sight gauge is provided. The tank has outlets and inlet ports for engine supply, return, and venting Cooling air is supplied to the oil cooler by a blower mounted on the tail rotor driveeshaft.
In some aircraft, the fan
and cooler are mounted differently.
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Tank support Scupper drain hose Screw Oil tank External oil system
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Reducer Packing
Bypass valve Drain valve Duct Oil return hose
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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 centre rear cover of the gearbox;
2.
Gaseproducer tachometer generator, located at the front of the gearbox on the rightehand side;
3.
Gas—producer fuelecontrol unit, located at the rear of 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 the gearbox on the upper leftehand side;
2.
Power turbine tachometer generator, located on the leftehand 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 are
given a letter and a number.
Memorise the following letters and
numbers, which we shall often use when discussing engine controls and the fuel system. l.
N1
(sometimes called N 1 denotes compressor and gas
producer turbine speedg 2,
N2 (sometimes called Nfl denotes free power turbine speed.
3.
N3 (sometimes called NP) denotes main rotor speed.
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Turbine speeds
REMEMBER To understand the fuel system and the controls, which we shall discuss next, you must know why a free turbine engine is so called.
The Allison 250—C20 engine has no direct mechanical coupling between l.
The gas producer turbine, which drives the compressor; and
2.
The power turbine, which drives the helicopter rotor through a gear reduction unit.
The two turbines are connected only in terms of gas flow. This fluid coupling arrangement permits the.turbines to operate at different speeds. To manage fuel flow to the engine for all flight and power requirements, the fuel control system must be sensitive to the speeds of bothlturbines. In this engine, the gaseproducereturbine speed is designated as N1 and the power—turbine speed as N2.
PRACTICE EXERCISE B 1.
Name the components for which antieicing is provided.
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 fuel control unit?
555/3/9
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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 automatic system to control rotor speed N3
Cor Nrl in powered flight.
The desired rotor speed is selected before takeeoff.
The
fuel flow is then automatically regulated by the power turbine governor N2. The unit senses power and hence fuel requirements and sends messages to the gaseproducer fuelecontrol unit N1 to maintain
the selected rotor speed within a correct and safe range. This selection is maintained despite changes of collective pitch. The pilotls main selection unit for rotor speed is the twist grip, located on the end of the collective stick.
The
selections on the gaseproducer fuelecontrol unit are as follows:
l.
90° (flight idle) gives lOO% free power turbine speed N2.
2.
30° (ground idle) gives 60e62% gas producer turbine speed N1. ~
3.
5°sQ° is the cuteoff speed.
A special detent button is installed on the end of the collective sticki
The twist grip can not be operated from ground
idle to cuteoff until the button is depressed.
The throttle
cannot, therefore, be inadvertently closed in flight.
Refer to Fig. IQ as you read on.
555/3/9
_ 17 _
The twist grip located at the end of the collective stick is connected through rods, levers, and/or push-pull controls to the lever on the gas-producer fuel-control unit N1. When the pilot raises the collective stick, thus increasing the angle of attack and pitch angle of the main rotor blades, he is also operating a separate engine interconnected control. This control links the collective stick, the linear droop actuator (droop compensater control N2) and hence to the power turbine governor N2. The raising of the collective stick thus resets the power turbine governor.
This increases the fuel flow
to the engine and restores Np rotor speed within the manufacturer’ limits.
Operating the collective stick downwards has the opposite
effect on the power.
L
Figures 1H, l5, and 17 show a fail—safe device in the form of
a 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 operation of the collective stick. O
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555/3/9
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Transient and Static Droop After the pilot increases collective pitch within the normal range, the rotor speed falls, despite the automatic increases in fuel flow and power.
The condition is called transient droop
and is caused by certain design features of the free turbine type of gas turbine engine. Partial recovery takes place due to the power—turbine governor~correction, this restores power to give an improved rotor speed, less than that originally selected.
The difference between
this underspeed and the original rotor speed is called static droop 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 positioned
between the collective stick and the power turbine governor, as shown in Fig. 14. When operated in one direction, the switch increases fuel flow and NP rotor speed, and when operated in the opposite direction, reduces fuel flow and NP rotor speed. When the linear droop actuator is operated to correct static
droop,(a negative drooplthe actuator rod's length alters, adjusting the position of the power-turbine governor-lever, to restore power by increasing fuel flow to the engine.
This
increases NP rotor speed to the original. Positive droop is a condition where NP rotor speed is greater than 100%.
Correct this condition by operating the
governor switch, beeping to reduce NP rotor speed to the original setting.
Control Systems We shall discuss the control systems in a general way. shall not detail any particular system.
We
Either Bendix or
Chandler Evans Company (CECO) fuel—control units and governors are fitted to the Allison 250-C20,
555/3/9
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When rigging controls, you must consult the maker‘s manual. In this assignment, we deal with the controls only in general terms, These maintenance procedures do not constitute a maintenance instruction. 2 1
The collective stick: This must be rigged before the engine controls, and the droop compensator control before the power turbine governor. The throttle control: This may be rigged at any convenient time.
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.
A
Set rod 5 to its correct nominal length, and connect
it to bellcrank 7. 2.
Adjust the stroke of the linear droop actuator 9 by adjusting screw 12 to give correct rod travel, from fully extended to fully retracted.
3.
Operate the governor switch l. (rod fully extended).
Beep to full decrease
Adjust rod end l8 to give
the 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 read OTI-
l.
Set the collective stick to the position fully up or down as specified by the manufacturer.
2.
Operate the governor switch. Beep until the linear droop actuator is in the extreme position specified by the manufacturer.
3.
As shown in Fig. l5(a), position lever 14 to governor shaft 20 in the position specified by the manufacturer.
H.
Back off the stop screws l6 and l8 by the specified amount.
555/3/9
..2[]..
Rotate the lever 1% to the extreme position, as specified
by the manufacturer.
Hold lever 1H in position. the rod end l3 to it.
Adjust and connect '
Select the collective stick fully up to obtain maximum and fully down to obtain minimum stop positions, adjusting the governor switch (beeper) as laid down by the manufacturer, and adjust the stops. Select the collective stick to full up and trim
(beep) the linear droop actuator to full increase (maximum). Check the clearance between the stop arm and screw 18. Select the collective stick to the full down
position, and trim (beep) the linear droop actuator to the full decrease position (minimum).Check the clearance between the stop arm and screw l6.
555/3/9
- 21 ~
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ll. 12. 13. 14.
Rod
15.
Retainer Bellcrank End fitting Linear droop actuator Washer
16. l7. l8. 19. 20.
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Droop compensator control N2 and turbine governor
rhegtwistegrip {throttle-control)
established by mechanical stops.
rigging:
The limits are
When, for example, the twist
grip is moved to the full open position, the fuel control pointer
must contact the maximum throttle stop at or before the limit of twistegrip travel. 555/3/9
_ 22 _
Refer to Fig. 16 as you read on. 1.
Depress the pilot’s flight idle detent button as required to enable full—range operation of the control.
2.
Select the gas producer fuel control to the ground
idle position 30°, by operating the throttle twist grip, and install the rigging pin, locking this selection. 3,
Set Cgntpglex pod to the bellcrank, and set the vertical tube to the operating lever. This lever being previously
positioned
on the gas-producer fuel-control unit—shaft
in the correct relationship as specified by the manufacturer. H.
Remove the rigging pin from the gaseproducer fuel—control unit.
5.
Fully rotate the twist grip, press the detent button as required, and check that the maximum stop is _ contacted at flight idle position, and that the minimum stop is contacted at the cut—off position.
6.
Ensure that the gaseproducer fuel~control~unit pointer
positions at 30°, each time you rotate the twist grip to ground idle from either fully open or closed, even when moving the control rapidly. 7.
The gas—producer fuel-contro1~unit pointer must remain in the same position as selected, irrespective of changes of position of the collective stick. Check this by operating the collective stick to ensure that this is so.
8.
Make final adjustments after engine run.
Remember that the rigging of the linkage must be such that the
stops restrict the travel, so that adjustments of the stops are effective in altering the range of movement and the settings of this control. A'nti‘f-»,icii1_9'p c'0'nt’r'ol :
1.
Operate the actuator to the off position. Ensure that the actuator shaft turns counter—clockwise when viewed 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 actuator shaft.
H.
Check the control for.full free and correct movement, and check all lockings. 555/3/9
-23..
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_ 25 _ Controls N1N2 and Anti-icing To see how the parts fit in a typical system layout, study Fig. l7, which shows the complete control run with engine in
position.
SUMMARY Transient droop is a fall from original rotor speed when the collective pitch lever is operated. Static droop is the difference between a partially recovered rotor speed and the original rotor speed. The three engine controls are l.
Power lever (throttle) to the gas~producer fuel—control unit N1;
2.
Power turbine governor. This control is connected, from the collective stick through the linear droop
actuator (droop compensator N2), to the power turbine governor; and 3.
Antieicingcontrol from the linear actuator to the shut-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—rod mechanically adjusted?
5u
Does correction of static droop (negative droop) by the pilot cause an increase or decrease of rotor speed?
6.
What prevents the pilot from selecting the closed throttle 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 smmysmwtmwmn A typical engine fuel system is the Pratt and Whitney PT6@pneumatic with manual reversion. In this assignment, we shall study another typical system in a general way, the Allison 250~C20 —— pneumatic or hydromechanical.
Think of the system as a series of computers, as shown in
Fig. l8.
To control the Allison 250-C20 engine, the Model MC-H0
system does all the management of the fuel. Fuel Control The fuel control is mechanically connected to the pilot's twist grip and also, through.the accessories gear train, to the
gas producer turbine. through an air line.
It senses compressor discharge pressure
The function of the fuel control is to provide the right amount of fuel to the engine throughout the start cycle, at ground idle, and for all of the requirements of powered flight.
Because
the fuel control is sensitive to and controls the speed of the gas producer turbine it is called the N1 control.
The Governor Remember that the power turbine governor is mechanically connected to the pilot's collective pitch control through a linear
actuator.
The actuator is electrically operated by the governor
trim switch and allows the pilot to make minor adjustments
Qapproximately 5% total) in N2 to compensate for aerodynamic and ambient conditions. The governor is also mechanically coupled, through the output
gear train, to the power turbine and also senses compressor discharge pressure through an air line.
555/3/9
27
A helicopter is designed to operate with a constant rotor
speed in flight. The function of the governor is to maintain the power turbine speed constant (at 100% N2) under varying load
conditions as established by the collective pitch. Because the governor senses and controls powe r turbine speed, it is called the N2 control.
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Fuel system
Separate Units Separate units are required, as shown in Fig. l9.
They must be installed at different locations on the engine gearbox. The fuel control has a splined sh aft that engages in the
accessories gear train and is connected to the gas producer turbine. The splined shaft on the governor engages in the output gear train, which is connected to the power turbine. This arrangement is necessary to pick up and sense the two operating speeds N1 and N2 found in a free turbine engine such as the Allison 250~C20.
555/ 8/9
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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 power turbine governor as valves with builtein computers.
Both the
fuel control and the governor have builtein computers, but these are hydromechanical rather than electronic. In the first stage of fuel management, as shown in Fig. 20,
the gaseproducer fuelecontrol receives the total flow from the fuel pump.
This fuel flow l is greater than the amount the
engine requires under any normal operating condition.
The fuel
control senses or receives data input from the engine compressor discharge pressure PC, the gas
From the data input, the fuel control computes the amount of fuel the engine requires to sustain the gas producer turbine and satisy the maximum requirement of the power turbine. This amount of fuel 2 is sent to the power turbine governor, with the excess Z 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, as shown in Fig.
2l, is based on the gas producer turbine needs and
maximum power turbine requirements.
This fuel flow may be greater
than the amount required to meet actual power requirements at any given moment. The governor senses or receives data input from the engine compressor discharge pressure PC, the power turbine speed N and the collective pitch control setting. From the data input, the governor computes the amount of fuel the engine requires to meet actual power requirements and maintain N2 at 100%, at the same time sustaining gas-producer~turbine operation;
This amount
of 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 from the governor back through the fuel control on its way to the engine is to provide fuel cutoff as the last stage in the fuel
control system. The pilot's twist grip is connected to the fuel control throttle lever, which is in turn linked to the fuel control 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. the unit;
2.
Maximum throttle stop;
3.
Cuteoff stop;
H.
Idle trim adjustment;
5.
Light off; and
6.
Start enrichment.
is adjusted only on replacement of
Other adjustments are made by the manufacturer. Because the adjustment points may look alike, always consult the maker's manual before making adjustments. We shall now consider the adjustments in general terms. Maximum thr0ttle~stOp: A set screw on the gaseproducer fuelcontroleunit 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 before
making an adjustment. Cuteoff stop: A set screw on the gas—producer fuel-control unit limits the travel of the throttle lever in the minimum 0° direction. Its effect is to establish the cuteoff position of the throttle N1 speed. Check the control rigging before making a fuel adjustment. idle trim adjustment; Use the idle trim adjustment to trim idle N1 speed when the fuelecontrol-throttle pointer is at 30° position.
The adjusting screw is located on the gas—producer
fuelecontrol unit. The adjustment is necessary only when N1 idle speed is outside the specified limits, with the generator turned off. Idle speed is based on the 30° fuel control quadrant setting.
Check the control rigging before making a fuel adjustment
by turning a hexagoneheaded screw positioned on the gaseproducer fuel—control unit.
.
Lightsoff adjustment: The light-off adjustment on the gasproducer fuel-control unit is a hexagon screw. If the light-off adjustment is too low, the engine may not have enough fuel to start, and if too high, the starting temperature may be too high. Start enrichment adjustment. During start from approximately 25% Ni to approximately H0% N1, temporary fuel derichment, or a
dip in fuel flow is required to provide starting performance. When fuel flow is plotted against N1, this dip in fuel flow looks like a notch on the graph and so is called a notch. The duration and width of the notch is controlled by the start derichment adjustment. If the notch is too narrow, excessive turbine outlet temperatures result, and this defect must be corrected. The adjustment provided is positioned on the gas—producer fuelecontrol unit.
555/3/9
_-.32..
Large Computerised Fuel Systems In Fig. 23, we again see the need to think of the fuel control and governors as computers. '
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Computerised fuel system
Fuel System Cleanliness Cleanliness is essential in all fuel systems. Figure 2% shows the general procedure for inspecting and cleaning the gaseproducer fuel»control filter.
l.
With the engine installed in the aircraft, you may have to remove the starter generator to gain access to fuel control inlet. '
2.
Disconnect the fuel line from the fuel control inlet fitting l.’ Cut the lockwire, and remove fitting and packing 2,
Discard the packing.
555/3/9
-33-
3
Remove filter 3 and spring H, and check for
contamination. If necessary, clean the filter and spring in an
4.
ultrasonic cleaner with trichlorethylene for approximately l5 minutes. If you cannot clean them in an ultrasonic cleaner, use a saturated solution of l litre of sulphuric acid and 20 grams of sodium dichromate for l minute. Rinse thoroughly in clean water and dry with compressed air. 5
Hold filter up to the light and check for contamination between inside and outside screens. Repeat cleaning
procedure if necessary. 6
Pit spring M into cupped end of filter 3.
Keep parts
together (use a dab of petrolatum if necessary), and install in fuel control. Press in on filter, compressing spring, to check bypass action. 7
Install fitting l with new packing 2. specified figures and lockwide-.
8.
Attach fuel line to inlet fitting and torque tube nut to the specified figures while holding fitting.
9
Purge air from the fuel system as required.
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GASPRODUCERFUELCONTROL Fuel filter
Figure 25 shows the general procedure for inspecting and cleaning the power turbine governor filter. l
Disconnect fuel line from governor inlet and filter fitting l. Cut lockwire and remove inlet and filter fitting and packing 2. Discard packing.
2
Check inlet and filter fitting for contamination.
3
If necessary, clean inlet and filter fitting in an ultrasonic cleaner with trichlorethylene for approximately l5 minutes, or if you cannot use such a cleaner, in a saturated solution of l litre sulphuric acid and 20 grams of sodium dichromate for l minute. Rinse thoroughly in clean water and dry with compressed air.
555/3/9
_ gu _ H.
Check the filter for contamination, and repeat the cleaning procedure if necessary.
5.
Install the inlet and filter fitting l with new packing 2. Torque to the specified figure, and lockwire.
6.
Attach the fuel line to inlet and filter fitting and torque tube nut to the specified figure.
7.
Purge air from the fuel system as required.
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Power turbine governor
Purging the System Purging is required whenever the fuel system is opened up for maintenance or when air gets into the system. ' Purging consists of breaking the system-connections at various places in sequence and pumping fuel through from the booster pump or an external fuel~pump source.
Always pull the
ignition circuit~breaker before starting, and use an auxiliary power unit or battery.
Fire Detection and Prevention Fire detectors are provided in many helicopters in such numbers and locations as to ensure rapid detection of fires.
Where gas turbine engines are installed, detectors and indicator lights show the area for which extinguisher action should be taken. Fire detectors do not operate fire extinguishers The pilot decides when the fire bottles should, under normal flight and ground conditions, be set off. 2
555/3/9
_
35
_
In a crash landing, the bottles are automatically set off on impact by an inertia switch.
SUMMARY Fuel systems are electrohydraulic, mechanical, or pneumatic, and some have manual reversion. Think of the fuel system as computerised, the computers in the Allison 250—C2O engine being the gas-producer fuel—control unit and the power—turbine governor. Fuel flow passes from one unit through the next and back again. You may make some adjustments to fuel flow, but other adjustments are made by the manufacturer and must not be tampered with.
PRACTICE EXERCISE D l.
What does the power—turbine—governor sense?
2.
At what stage does the fuel flow through the cut—off valve?
3.
Are all fuel system adjustments permissible?
4.
How is the drive made between the gas~producer turbine and the power turbine?
5.
What does the term notch mean? (Answers on page
3H
ANSWERS TO PRACTICE EXERCISES EXERCISE A l.
Aft
2.
Ca)
Combustion section,
Cb)
Turbines,
Cc)
Compressor, and
Cd)
Power and accessories gearbox.
3.
SAE H130
555/3/9
,.
4.
5.
(a)
Run-out check of main transmission coupling
(b)
Oil level check of the main transmission coupling
(c)
That the main transmission coupling (shaft) has been correctly installed
Ca)
Forward fairing,
(b)
Induction fairing,
Cc)
Engine cowl, and
(d)
Aft fairing.
EXERCISE B
l.
The COmpP€SSOP inlet guide vanes and front bearing support
2.
Cooling air is supplied to the oil cooler by a blower mounted 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 of the collective stick
2.
Power turbine governor
3.
The droop-compensator and powereturbine-governor control
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 of the collective stick
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EXERCISE D
l.
Compressor discharge pressure PC, collective, pitch control setting, and power turbine speed N2
2.
Just before delivery to the engine fuel nozzle
3.
No, some are bench adjustments made by the manufacturer and 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 notch
on the graph and is so called
TEST PAPER 9 l.
2.
List (al
Pour main components of the engine, and
Cb)
The power plant systems.
Ca)
List four advantages of positioning the engine aft of the mast.
Cb)
Describe two ways by which engine alignment may be achieved.
3.
Showing complete control runs, make a diagrammatic sketch of the throttle control N1 and droop compensator N2 and the power turbine governor controls,
H.
Referring to Fig. l6, briefly describe the rigging checks of the throttle control N1.
5.
List the three items sensed by the gas-producer fuelcontrol unit.
6.
Referring to Fig. l5 briefly describe four steps in rigging the droop compensator N2.
7.
List six fuel-system adjustments, and state the purpose
of each adjustment.
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e 38 e
Briefly describe a fire detection and prevention system and how the fire bottles are operated. NIW
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 not be reproduced without the written authority of the Principal, TOPNZ.
, 555/3/9
