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Helicopter Piston Engine Installations 555—3—4
CONTENTS Piston Engine Installations
1
Position of the Engine
1
Adaption of a Horizontal Engine to a Vertical Position
3
Installation of a Vertical Engine
Q §~
Installation of a Horizontal Engine
10
Components in Common
15
Fuel Systems
15
Gravity—flow System
17
A Non—gravity System
20
Oil Systems
23
A Wet—sump System
23
A Dry-sump System
25
Engine Controls and Instruments
25
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/H
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AI RCRAFT ENGll\iEERi%G hELICGPTERS
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PESTON ENGINE §NST§LLAI§@fiS
The helicopter engine provides power for flight and for the various systems needed for the operation of the helicopter; The piston engines used in helicopters are adaptions of fixed“
wing aircraft engines.
These adapted engines provide a reliable
source of power that is cheaper than would be an engine developed specially for helicopter use.
Piston engines are now used only in the small helicopters. This is because for all but the small helicopters, small, reliable gas turbines are readily available. Furthermore, turbine fuel is cheaper and easier to produce than the Avgas needed by the piston engine.
In future, the production of piston-engined helicopters
may well be phased out.
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eesition at the Engine Because of the compact mass of the piston engineg it is usually located immediately beneath the main rotor mast and thus approxi~
mately in the middle of the centre—of~gravity range for the helicoptere
Its installed attitude is either pointing straight
upward, with the original lower half facing rearwardp {Hiller l2E§ or lying horizontally, with the drive shaft facing rearward
€Hughes 269}. Figure l shows a rear view of a Lycoming vo-see engine installation in the Hiller l2E.
Cylinder numbering and references
to engine parts are the same as if the engine were installed in a
fixed~wing aircraft.
For instance? the left~hand side of the engine
is still referred to as such, although in this installationfi it is
on the right~hand side of the aircraft.
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1. Engine Strut Assembly 3. Engine 5. Earth Strap
FIG. l
2. Right-Hand Engine Frame Assembly 4. Engine Air Induction System
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Vertical engine installation
Figure 2 shows the installation of a Lycoming HIO~360 engine for a Hughes 269 helicopter.
Note that the engine manufacturer's
engine locations are still used.
For example, the right—hand side
of the engine is on the left~hand side of the helicopter, and the right-hand magneto is now on the left—hand side of the helicopter. Note, too, that the engine front mounts are now at the rear of the airframe, and the rear mounts are at the front of the airframe.
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2. Engine Mounting installation Front Mounts A .2
3. Cooling Scroll Assembly 4 Engine
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1. Fuel System Installation
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Horizontal engine installation
Adaption of a Horizontal Engine to a Vertical Position /é
Because the engine now has to run in a vertical position, the oil system must be modified so that the splash lubricating oil that drains to the bottom of the engine can be picked up by the scaveng G pump and returned to a separate oil tank. This is done by a sump that bolts directly on to the engine accessorj\ hou"ing. ts Used oil from the modified cylinder-head rocker box covers in carried in the new sump through two external oil pipes. The sump may be used as the oil tank. or the oil may be pumped from the sump to a separate oil tank.
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The eng ine is attached to the airframe by modifying the
forward ends of the crankcase right- and left—hand halves so that a substantial casing designed by the helicopter manufacturer can be bolted to th e two halves,
{This casing, in turnt is attached to
another heav ily made airframe component, which is bolted to the engine mount assembly,) The eng ine is now suspended from its
shaft end and must be tethered at its base to keep it steady and upright. inis is ~
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achieved by snubbing rods joining attachment points on the special
sump to the helicopter structurei
A minimum of two rods will be
fitted, one in a fore—and~aft direction, and one in a lateral
direction,
In some installationsa four rods may be used, two in
each directi Oils
The induction system is also redesigned so that the carburettor Q?
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ll be in a level position when the engine is installed, ii
The oil pressure system of the engine may be altered to provide 0 I 1 n 0 u for an oil s upply to the main transmission. This is simply done by taking an external oil line from the oil pressure pump housing directly to a connection at the top of the transmission. Finally , and most important, is the fact that, although all models of a vertical engine may look alikeg there can be very -
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important di fferences in their internal construction to cope with
If rtU1 Do wrong engine model is installed in a helicopter (as can be done quite easily), the result will possibly be a mis—match between the engine and transmission, the
damping of torsional vibrationsi
which will rapidly lead to an engine and/or transmission failure.
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at a Vertical engine
Figure 3 shows the installation of a vertical engine into its
support mounts, The engine is_bolted to the inner gimbal (1% which is located and secured in the outer gimbal /'\ I\3 M,’ by two bonded t o xible mounts r~\ C1.) \,../ and bolts /"\ 43' -../ The two mounts damp and U
restrict engine movement in a clockwise and anti~clockwise direction when viewed from above“
This allows for changes in engine torque
and lets the engine be tilted in a fore~and~aft direction,
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The outer gimbal <2} is secured in the engine frame assembly(5) which is bolted to the airframe by two bearings and bolts /'"-. O‘) \./ and(7) The two bearings (6) allow the engine to rock laterally. Attached to the sump (8), at the base of the engine, are the aft and lateral engine snubber assemblies <9} and (l0).
The other ends of these
assemblies are attached to the engine snubber brackets (ll) an d flexible mounts (12), which are bolted to the airframe structure.
The engine§ as it is now installed? is flexibly but firmly held by the mounts (32 and (l2}@ fore—and—aft and lateral
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The engine can be adjusted for it S
tilting by the snubber assemblies (9) and
This adjustment is made during the rigging of the helicopter. The engine is coupled to the driven member of the clutch by a
torsion coupling bolted to the driving end of the crankshafte In w H m Q, the torsion coupling lower section (2) is bolted directly on to the crankshaft mounting flange (1).
To the top of the torsion
coupling lower section is bolted the torsion coupling upper section (3), and to this is bolted the main drive clutch (4): which is encircled by the clutch housing (5). Finally, the main transmission is splined directly to the clutch housing. The engine drive torsion coupling (2) and (3) is a two-section, interlocking steel and rubber assembly positioned between the engine crankshaft and the main drive clutch. The coupling reduces engine torsional vibration loads imposed by the engine on the main drive clutch and transmission,
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Crankshaft Mount Flange Torsion Coupling Lower Section Torsion Coupling Upper Section Main Drive Clutch Clutch Housing Bearing Transmission To Clutch Spline
Torsion coupling
The compact assembly of the torsional coupling, main drive clutch, and inner gimbal are shown in part section in Fig
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Engine to transmission coupling assembly
Because the helicopter can hover and fly sideways and backwards the airflow over the fuselage cannot be used for engine cooling T effect engine cooling, a fan driven by a shaft from the main trans~ mission is mounted on the front face of the engine Tn's fan will always turn when the main rotor is turning Figure 6 shows an exploded view of the engine cooling fan and shroud assemblies Secured to the engine are the rightehand enc left-hand shroud assemblies £1) and
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tog ther "i E ft%Q3 the inter-cyl_nder baffles crevice CL by the engine a..J
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To these two snroocs are secured the loser
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cooling air to the generator and warm air to tne ceoin neeter system
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2. Len-Hand Shroud Assembly 3. Lower Transition Assembly 4. Right-Hand Upper ‘transition Assembly
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8, Access Door 9. Oil Cooier Duct Assembly
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Engine cooling fan and shroud assemblies
Because of the rubberlike flexible mounts (3) and (12) of 8, the engine and transmission are not in good electrical
contact with the airframe,
To prevent problems with the generator,
the engine starter motor, and the magneto switch circuits that will
arise because of this lack of contact, a heavy earthing strap is f't i ted between the engine ' and the engine mount frame (5).
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To protect the pilot and passengers in the event of an engine ~F
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the cabin enclosure.
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The Lycoming HID» (.0 0') CD engine used in the Hughes 269 helicopter is an adaption of a fixed~wing 10-350 engine, the changes made being
mainly to the cylinders and pistons to enable the engine to develop more power than its fixed~wing counterpart.
ln Fig, 7 the rear of the engine is attached to the helicopter centre frame (3) by two bolt, spacer, and rubber bushings assemblies, using the top two rear mounting lugs {M} of the engine. front of the engine, two special hangers
side of the engine crankcase.
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are bolted, one on each
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ment points for two rod—and~bushings assemblies, whose top attachment points are lugs (2) on the main transmission. Thus, the rear of the engine is supported by the helicopter centre frame, and the front of the engine is supported by the main transmission.
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2. Main Trasmission Engine Mtg Lug 4. Engine Rear Mtg Lug
Engine mounting
Figure 8 shows the coupling of the engine to the transmission. An internally splined engine lower coupling adaptor (1) is bolted to the engine starter ring gear support. Mating with these internal splines are the front splines of the lower coupling drive shaft (l2L whose rear splines engage with internal splines in the lower pulley assembly (8). The lower coupling drive shaft is made of a highquality steel, and its splined ends are slightly convex in section to permit small lateral and vertical movements of the engine on its rubber mounts. Fore-and-aft movement is catered for by the freely floating attitude of the shaft in the two sets of splines. Rubber bumper plugs (6) and (ll) are fitted inside the engine lower
coupling adaptor and inside the aft end of the lower pulley assembly to cushion the floating drive shaft.
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to the airframe, as is the upper and lower pulley and Vee-belt assembly, and the engine is mounted on flexible rubber mounts. The engine is aligned with the lower coupling by raising or lowering the engine with the length-adj ustable mounts between the main transmission and the engine front hangers (1) of fig. 7 until the alignment ring (9) is exactly centred in the hollow lower pulley assembly. This adjustment is made with the main drive Vee belts tensioned. Cooling air for the engine and
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oil cooler is supplied by an impeller assembly bolted to the engine starter ring gear support.
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This assembly comprises an aluminium
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outer edge of the wheel. The steel weight increases the mass of the
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assembly, which helps to maintain smooth engine operation. Figure 9 shows the placement of the impeller in relation to the scroll assembly.
1. 3. 5.
Scroll Assembly Scroll lniet Ring Starter Ring Gear Support 7. Scroii Forward Lip
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2. inlet Surface 4. kmpeiler Assembiy 6. Fiy Wheei Weight.Assembiy
Impeller and scroll
The scroll assembly direc ts a cooling airflow through the engine shroud assembly and thr ough the oil cooler or coolers fitted In Fig. l0, the scroll and shroud to the shroud and the scroll. assembly is shown in an explod ed view. The oil cooler —-not shown Various - is mounted on the left~hand side of the scroll assembly. "‘.. -1 air to the generator/ tubes from the shroud panels c onvey cooling alternator and to the accessor ies mounted on the rear of the engine. ‘ The engine is separated from t he cabin by;a firewall that is designed to give protection to the pilot and passengers should the engine catch fire.
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_ 15 i All earth straps and bonding jumpers fitted between the engine and the airframe must be kept in good repair if easy engine starting, efficient electrical generation, accurate instrumentation, and a safe ignition system are to be had.
Components in Common In both the vertical and horizontal installations, the exhaust 1»
system fitted must be the correct one for the helicopter model.
In
both installations, the system is very simple, the object being to s
get the exhaust gases safely clear of the aircraft without any back pressure imposed on the pistons. The exhaust system is often used as a source of heat to prevent carburettor icing and to heat the cabin enclosure. In both cases, the heat is taken from the exhaust gases by a simple pilot-controlled exhaust gas-to-air heat exchanger. The carburettor air intake systems are similar for both installations, comprising a forward-facing air intake connected by ducting to the engine. Installed in the ducting is a chamber housing one or two paper cartridge air filters, a bypass to admit air should the filters become blocked and, if the engine is fitted with a carburettor, a duct to bring hot air from a heat exchanger fitted to the engine exhaust assembly. All flexible ducting in an engine air induction system will be made from a wire reinforced petrol-, oil-, and flame~resistant material. Ducting that does not have these qualities must ngt be used as a substitute.
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Only the correct engine model for a particular helicopter may be fitted to that helicopter.
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A heavy earth strap between the engine and engine mount »frame (earth) is essential for the proper and troublefree operation of the electrical components fitted to the engine and transmission. .»-~ »m-.i,-_s:-
555/3/M
_ 15 _ PRACTICE EXERCISE A State whether each of the following is true or false. l.
Engine—mount assemblies may be used to adjust the alignment of the engine to the transmission.
2.
In a helicopter with a centrifugal clutch, the cooling fan will turn only when the engine is running.
3.
Any type of flexible ducting may be used in the carburettor air induction system.
4..
The exhaust system is often used as a heat source for the carburettor and for cabin air.
5.
Because the engine is aft (downwind) of the cabin, no firewall is needed between the two.
6.
The engine earth strap is needed only to take static electricity to earth in the airframe.
7.
Some engine cooling air is distributed through ducting to cool magnetos, the generator/alternator, and the oil cooler(s).
8O
9.
l0.
The engine-to-transmission torsional coupling takes up acceleration and deceleration loads when the throttle is opened and closed. Engine cylinder numbering remains the same whatever its installed attitude. A fixed—wing aircraft engine with an unmodified wet~sump oil system cannot be installed in a vertical position. (Answers on page 23;
FQEL SYS?E I3 6/J The fuel systems used in helicopters, as in fixed-wing aircraft, can be divided into two types: l.
Gravity~flow systems, and
2.
Non-gravity systems. 555/3/Q
_ 17 _ The fuel tanks in the gravity~flow system are mounted aboye, and in the non-gravity system, below, the level of the carburettor or fuel injector unit. A gravity-flow system may or may not need a fuel pump, but a non-gravity system must have one or more pumps. Generally, only the smaller of the helicopters and fixed-wing aircraft utilise gravity—flow systems.
Gravity-flow System Figure 11 shows a gravity~f1ow system from the tank to the engine, and Fig. 12 shows a rear view of the tank. Because the engine has a fuel injector or fuel metering, an electrically driven fuel boost pump has to be fitted to the system so that the fuel injector and the engine cylinders can be primed with fuel to enable the engine to be started. To ensure that the necessary fuel pressure exists at the fuel injector all the time the engine is running, an engine—driven fuel pump is also fitted, with the fuel boost pump being available should the engine—driven pump fail. The fuel boost pump is controlled by the pilot through an on/off switch in the cockpit.
l
Referring to Fig. 11, fuel is housed in the fuel tank (1) mounted on the right-hand side of the helicopter, aft of the cabin. It is attached to the airframe by two strap assemblies (2) and (17) and separated from the engine by a firewall pan (7). Mounted on the rear-sloping face of the tank is a filler cap neck (10) of Fig. 12, a fuel quantity sender (8) of fig. 12, and a low-level switch (3), of Fig. 12. At the top of the tank is a tank vent (2) o W w |-I. m 12, to let air out of the tank when the fuel expands and to let air into the tank as fuel is consumed by the engine. At the bottom of the tank is the tank sump (Q). The fuel outlets to the engine and to the fuel drain valve (16) are taken from this sump. The fuel drain valve is readily accessible, and from its outlet, a pipe (12) —-a carries the drained fuel clear of the helicopter. fitted to the inside of the tank is a baffle assembly (Q) of Fig. 12 that prevents fuel from "sloshing". The tank is bonded electrically to the airframe. The filler cap neck is sited below the top of the tank, thus providing an airspace for expansion of the fuel when it becomes heated. 555/3/Q
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Fuel tank
Fuel from the tank passes through an elbow/strainer assembly (5), whose coarse mesh stops large pieces of foreign matter from entering the fuel system, and passes through the pilot-controlled fuel shut—off valve (15) to the external fuel boost pump (13) and then to the fuel strainer assembly (ll). The fuel strainer houses a fine—mesh filter inside a large filter bowl, to which is attached a fuel drain valve (not shown). The filter bowl and filter are easily removed for inspection and cleaning, and the fuel drain valve is easily reached to obtain a fuel sample after refuelling. from the fuel strainer assembly, the fuel goes through the engine—driven fuel pump (8) (which operates whenever the engine is running) and then into the fuel injector (10), where it has to pass through another fine—mesh strainer before being metered to the engine, . The resistor quantity operates
fuel quantity sander unit, which consists of a variable controlled by an attached float arm, is connected to a fuel gauge on the instrument panel. The fuel low—level switch a fuel low warning light on the instrument panel. This
sss/3/Q
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light comes on when the fuel remaining in the tank falls below l§ to 2 gallons.
Injector fuel pressure is measured at the injector
by an electrical fuel pressure sender and displayed on a gauge on the instrument panel.
A son-gravity System Figure l3 shows a nonegravity fuel system, where a bladder~type, rubber nylon reinforced, fuel cell is installed and secuzifi by tabs in a specially prepared cavity in the lower section of the fuselage. lnstalled immediately above the fuel tank are the engine, transmission, and main rotor, so the fuel is carried almost exactly at the c.g. position of the helicoptert Thus, as fuel is burned? there is very little change in the c.g. position, 1. 4. 7. 10. 13.
Fuel Cell Assembly 2. Drain Valve 5. Supply Hose 8. Supply Hose To Engine Driven Fuel Pump 11. Overboard Drain 14.
Filler Cap 3. Fuel Strainer 6. Auxlllary Pump 9. Fuel Quantity Tank Unit 12. Shutfoflvalvo _(§gnt_r_ol' _ _ A _
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fuel is drawn through the coarse mesh strainer (5)5 the fuel shutoff valve (6), and the fine meshed strainer assembly (9) by the auxiliary pump (8) and then sent under pressure to the engine—driven fuel pump £2) of Fig. ls. Both the fuel shutoff valve and the auxiliary pump are controlled by the pilot from the cockpit, 555/3/H
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_ 21 _ When the engine is running, the engine~driven fuel pump is used to supply fuel to the engine, with the auxiliary pump being used for engine starting and during take—off and landing as a precautionary measure. The auxiliary fuel pump has its own relief valve and an internal bypass so that it will offer no restriction to the fuel flow when it is not in use. Fuel is added to the tank through a conventional filler cap sited in a drained scupper. The tank assembly is vented by forward and aft vent pipes (12), which join with a drain from the auxiliary pump seals and go overboard through the drain (l3). A drain valve (4) is fitted to the bottom of the fuel tank sump immediately below the strainer (5). Another fuel drain valve (not shown) is fitted to the auxiliary pump (8) and fuel strainer assembly (9), and an air relief valve (not
shown) is fitted to the top of the fuel strainer (9).
The fuel drain
valves are used to obtain fuel samples after each re—fuelling and before the first flight of each day. The air relief valve is opened to release air that may be trapped in the strainer assembly. Fuel quantity gauging is done by either two capacitance units or two float-operated resistance units sited at position (ll). The fuel quantity is shown by an indicator mounted on the instrument panel. t Figure ls shows the continuation of the fuel system from Figg l3. The fuel arrives through the hose (l) at the engine-driven pump (2), where its pressure is increased. From the pump, the fuel is taken to the carburettor(s) and to the solenoid priming valve (4), which is normally closed, being used only for engine startup. This valve is controlled by a switch on the cabin console and, when energised, allows fuel under auxiliary pump pressure to pass through the priming line (5) and into the inlet manifold of the engine, where it is readily available for the cylinders. A drain valve and drain line are fitted to the base of the carburettor(s) for taking fuel samples. Drain lines (6) are fitted to the base of the air filter assembly and to the engine—driven fuel pump seal chamber to drain away any fuel and oil that may have accumulated. 555/3/4
-22-
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Figure l5 shows this fuel system in schematic form.
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FIG. l5
Schematic fuel system
OIL SYSTEMS Aircraft piston engines have either a wet sump or a drg sump. The wet—sump engine carries its oil supply in its sump, and the dry—sump engine uses a separate oil tank to hold its oil. We shall discuss the helicopter airframe part of the lubrication system for an engine of each type.
A Wet-sump System _Figure l6 shows a wet-sump engine and its external oil system. The system is a simple one and consists of an oil cooler inlet line (6), an oil cooler (7), an oil cooler return line (5), a crankcase breather (4), an oil temperature sender unit (2), and, not shown, an electrical oil pressure sender unit clamped to the engine mount and connected to an oil pressure tapping on the engine with a flexible hose.
The oil temperature and pressure are
displayed on the instrument panel in the cockpit. The oil system is vented to atmosphere through the crankcase breather tube (Q).
555/3/4
....2l.,L..
Cooling air for the oil cooler is ducted from the scroll surrounding the engine~driven fan assembly. In later versions of, the installation shown, another oil cooler is added, in series, with the existing oil cooler.
This cooler also takes its air supply
from the engine cooling air. Temperature control and filtration of the oil are done by the thermostatic oil cooler bypass valve (3) and an oil pressure screen
(l). This combined unit is part of the engine assembly, not part of the airframe. .
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Wet~sump engine external oil system
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A Dry-sump System The dry-sump system shown in Fig. 17 comprises an oil tank, a cooler, and connecting hoses between the engine, the cooler, and the oil tank.
The cooling air for the oil cooler is tapped from the
engine cooling air supplied by the engine cooling fan.
A breather
hose between the oil tank and the engine allows the oil in the tank to expand or contract without bursting the tank. The engine breathes to atmosphere through a breather line attached to the top of the engine. Oil under pressure is taken from a tapping on the engine oil pump by small tubing to an indicating gauge on the instrument panel. Engine oil temperature is measured electrically and displayed on the same gauge. This gauge, sometimes known as a triple gauge, is a combined fuel-pressure, oil-pressure, and oil—temperature gauge. TRAQSMISSIOK Oil. UNES 1:3
ENGINE OIL LINES
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REMEMBER The oil system is vented to the atmosphere. A thermostatic valve controls the flow of oil through the cooler.
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The three basic engine controls used are 1
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A hand—operated twist grip mounted on the end of the collective control lever to open and close the carburettor/injector throttle valve;
2.
A lever-operated mixture/idle cutoff control sited near the instrument panel and used for adjusting the strength of the fuel/air mixture engering the engine and for stopping the engine; an
3.
A lever~operated hot air control to regulate the temperature of the air in the air intake. This control is fitted only when a carburettor is used to meter the fuel.
Other controls needed for controlling and starting the engine <3.I"€
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A lever- or knob—operated fuel supply valve On/Off selector;
2.
A starter button used to energise, through a relay, the engine starter motor and the starter vibrator;
3.
An on/off switch for each magneto, for testing the
magnetos only when ’ the helicopter is on the ground
and for making the engine dead when it is not running.
In some aircraft, two separate switches may be used, but a combined magneto/starter switch is usually fitted;
A.
An on/off switch to operate the fuel priming valve. This switch is not needed if the engine is fitted with a fuel injector and not a carburettor;
S.
An on/off switch to operate the fuel booster pump; and
6.
An on/off switch to bring the generator/alternator on to line.
S55/3/Q
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The engine and engine-related instruments used are 1.
An oil pressure gauge, which measures the regulated oil pressure as it leaves the engine oil pressure PUmP5
2.
An engine tachometer, which is usually housed in the same case as the rotor tachometer, with the needles of the two instruments lying one above the other when the drive from the engine to the rotor is made and is not Slipping;
3.
An oil temperature gauge that shows the temperature of the oil as it enters the engine oil pressure pump;
H.
A cylinder head temperature gauge, which shows the temperature of the hottest running cylinder (the hottest running cylinder is determined by the aircraft manufacturer);
5,
A carburettor air temperature gauge that gives the
temperature of the induction air at or close to the carburettor venturi; 6. 7.
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An induction manifold gauge, which shows the pressure of the air/fuel mixture in the induction system; A fuel pressure gauge, which shows the pressure of 'the fuel at the inlet to the carburettor or fuel injector;
8.
A voltameter to show the voltage and current output of the generator/alternator, or
9.
A warning light that will light up when the generator/ alternator output falls below that of the battery;
l0.
A fuel quantity gauge to indicate the amount of fuel held in the fuel tank; and
ll.
A fuel low—level light, which will light up when the
fuel quantity remaining in the fuel tank(s) falls below a certain minimum.
5
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Fuel drain valves must be opened, and the fuel must be checked for contamination, after every refuelling and
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before the first flight of the day.
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Magnetos must never be tested while the helicopter is airborne.
555/3/A
i 28 _ ERACTECE EXERCISE B State whether each of the following is true or false. l.
To prevent flooding of the carburettor in a gravity—flow system. the fuel tanks are sited just below the level of the carburettor.
2.
A fuel booster pump must be running whenever the engine is running.
3.
A fuel injector houses a fine—mesh fuel filter.
4.
The fuel outlet from a tank houses a fine~mesh fuel filter.
5.
A fuel tank must be bonded to the airframe.
6.
The main purpose of a baffle is to strengthen the fuel tank.
7.
The fuel tank vent incorporates a one-way check valve. '
8.
An engine oil system breathes to the atmosphere.
9.
The oil temperature gauge shows the temperature of the oil as it leaves the engine.
l0.
A hot air supply is needed only when a carburettor is used to meter the fuel to the engine.
(Answers on page 29)
ANSWEQS T9 PRACTECE EXE%CZSE$ EXERCISE A
Statements l, 4, 7, 9, and l0 are true.
2.
False: Because the cooling fan, in this installation, is driven by the transmission, the fan will turn whenever the rotors are turning.
3.
False: The flexible ducting used must be ....-..._.,...._....-.-.-_......-._-._---.-----fire-resistant and impervious to oil and fuel.
555/3/M
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5.
False:
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The position of the engine makes no difference.
A firewall must be installed between the engine and the
cabin. 6.
False:
The engine earth strap must be capable of
carrying, with little resistance, the heavy return
current from the starter motor. 8.
False: The torsional coupling damps the torsional vibrations of the engine, due to the power impulses of the cylinders, to an acceptable level for the transmission.
EXERCISE B Statements 3, 5, 8, and 10 are true. l.
False: If the tanks were mounted below the level of the carburettor by even a small amount, the carburettor would never fill with fuel and would not function properly. In practice, the carburettor manufacturer gives a minimum fuel pressure for the carburettor. This pressure translates to a minimum height for the tank above the carburettor.
2.
False: The fuel booster pump is needed to start the engine and as a safety precaution during take-off and landing. In normal flight, this pump is not usually required.
H.
False:
The fuel outlet from a tank houses a coarse-
mesh filter.
Fine filtration is done by the main
fuel strainer/filter assembly. 6.
False: The main purpose of a baffle is to stop the fuel from sloshing around in the tank during changes in flight attitude.
7.
False: Air must be free to enter and leave the tank during changes of atmospheric pressure and fuel temperature, when fuel is taken from the tank during engine operation, and during fuel draining. This means that the vent must be unobstructed, and so no one-way check valve is fitted.
9.
False: The oil temperature gauge shows the temperature of the oil as it enters the engine.
555/3/Q
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TEST PAPER /-I
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a
Give reasons for the small helicopter having its engine
located immediately under the main rotor mast. How is fore-and—aft and lateral alignment of the vertical engine and transmission installation achieved? Make a schematic sketch, including the fuel tank and fuel
injector, of the fuel system shown in Fig. ll and l2. Label all the components.
i .5
3 :9
Why must the magnetos not be tested while the helicopter is airborne? (Hint: the main rotor can never drive the engine.) What are the three basic engine controls needed for an
engine fitted with a carburettor? Which of these controls is not needed for an injected engine, and why? Suggest how a horizontally opposed engine may be modified so that it can be installed and run in a vertical position. State why a heavy earthing strap is used to electrically bond the engine to the airframe. If the strap becomes disconnected and the starter motor is energised, what could happen to
(a)
The shielded ignition switch leads, and
(b)
The fuel lines to the carburettor/injector.
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