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CORRELATOR / GOVERNORA correlator is a mechanical connection between thecollective lever and the engine throttle. When the collective lever is raised, power is automatically increasedand when lowered, power is decreased. This systemmaintains r.p.m. close to the desired value, but stillrequires adjustment of the throttle for fine tuning.A governor is a sensing device that senses rotor andengine r.p.m. and makes the necessary adjustments inorder to keep rotor r.p.m. constant. In normal operations,once the rotor r.p.m. is set, the governor keeps the r.p.m.constant, and there is no need to make any throttle adjustments. Governors are common on all turbine helicoptersand used on some piston powered helicopters.Some helicopters do not have correlators or governorsand require coordination of all collective and throttlemovements. When the collective is raised, the throttlemust be increased; when the collective is lowered, thethrottle must be decreased. As with any aircraft control,large adjustments of either collective pitch or throttleshould be avoided. All corrections should be madethrough the use of smooth pressure.CYCLIC PITCH CONTROLThe cyclic pitch control tilts the main rotor disc bychanging the pitch angle of the rotor blades in theircycle of rotation. When the main rotor disc is tilted, thehorizontal component of lift moves the helicopter inthe direction of tilt. Figure 4-2. A twist grip throttle is usually mounted on the endof the collective lever. Some turbine helicopters have thethrottles mounted on the overhead panel or on the floor inthe cockpit.IfManifoldPressureisandR.P.M.isSolutionLowLowLowLow HighHighHighHighIncreasing the throttle increases manifoldpressure and r.p.m.Lowering the collective pitch decreasesmanifold pressure and increases r.p.m.Raising the collective pitch increasesmanifold pressure and decreases r.p.m.Reducing the throttle decreases manifoldpressure and r.p.m.Figure 4-3. Relationship between manifold pressure, r.p.m.,collective, and throttle.Figure 4-4. The cyclic pitch control may be mounted vertically between the pilot’s knees or on a teetering bar from asingle cyclic located in the center of the helicopter. The cycliccan pivot in all directions.4-3The rotor disc tilts in the direction that pressure is appliedto the cyclic pitch control. If the cyclic is moved forward,the rotor disc tilts forward; if the cyclic is moved aft, thedisc tilts aft, and so on. Because the rotor disc acts like agyro, the mechanical linkages for the cyclic control rodsare rigged in such a way that they decrease the pitch angleof the rotor blade approximately 90° before it reaches thedirection of cyclic displacement, and increase the pitchangle of the rotor blade approximately 90° after it passesthe direction of displacement. An increase in pitch angleincreases angle of attack; a decrease in pitch angledecreases angle of attack. For example, if the cyclic ismoved forward, the angle of attack decreases as the rotorblade passes the right side of the helicopter and increaseson the left side. This results in maximum downwarddeflection of the rotor blade in front of the helicopter andmaximum upward deflection behind it, causing the rotordisc to tilt forward.ANTITORQUE PEDALSThe antitorque pedals, located on the cabin floor by thepilot’s feet, control the pitch, and therefore the thrust,of the tail rotor blades. . The main purposeof the tail rotor is to counteract the torque effect of themain rotor. Since torque varies with changes in power,the tail rotor thrust must also be varied. The pedals areconnected to the pitch change mechanism on the tailrotor gearbox and allow the pitch angle on the tail rotorblades to be increased or decreased.HEADING CONTROLBesides counteracting torque of the main rotor, the tailrotor is also used to control the heading of the helicopterwhile hovering or when making hovering turns. Hoveringturns are commonly referred to as “pedal turns.”In forward flight, the antitorque pedals are not used tocontrol the heading of the helicopter, except during portions of crosswind takeoffs and approaches. Instead theyare used to compensate for torque to put the helicopter inlongitudinal trim so that coordinated flight can be maintained. The cyclic control is used to change heading bymaking a turn to the desired direction.The thrust of the tail rotor depends on the pitch angle ofthe tail rotor blades. This pitch angle can be positive, negative, or zero. A positive pitch angle tends to move the tailto the right. A negative pitch angle moves the tail to theleft, while no thrust is produced with a zero pitch angle.With the right pedal moved forward of the neutral position, the tail rotor either has a negative pitch angle or asmall positive pitch angle. The farther it is forward, thelarger the negative pitch angle. The nearer it is to neutral, the more positive the pitch angle, and somewherein between, it has a zero pitch angle. As the left pedal ismoved forward of the neutral position, the positive pitch
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发表于 2009-3-20 23:46:24
angle of the tail rotor increases until it becomes maximum with full forward displacement of the left pedal.If the tail rotor has a negative pitch angle, tail rotorthrust is working in the same direction as the torque ofthe main rotor. With a small positive pitch angle, thetail rotor does not produce sufficient thrust to overcomethe torque effect of the main rotor during cruise flight.Therefore, if the right pedal is displaced forward ofneutral during cruising flight, the tail rotor thrust doesnot overcome the torque effect, and the nose yaws tothe right. With the antitorque pedals in the neutral position, the tailrotor has a medium positive pitch angle. In medium positive pitch, the tail rotor thrust approximately equals thetorque of the main rotor during cruise flight, so the helicopter maintains a constant heading in level flight.Figure 4-5. Antitorque pedals compensate for changes intorque and control heading in a hover.Tail Moves Tail MovesNegative or LowPositive PitchMediumPositive PitchHigh PositivePitchFigure 4-6. Tail rotor pitch angle and thrust in relation to pedal positions during cruising flight.4-4If the left pedal is in a forward position, the tail rotorhas a high positive pitch position. In this position, tailrotor thrust exceeds the thrust needed to overcometorque effect during cruising flight so the helicopteryaws to the left.The above explanation is based on cruise power and airspeed. Since the amount of torque is dependent on theamount of engine power being supplied to the main rotor,the relative positions of the pedals required to counteracttorque depend upon the amount of power being used atany time. In general, the less power being used, thegreater the requirement for forward displacement of theright pedal; the greater the power, the greater the forwarddisplacement of the left pedal.The maximum positive pitch angle of the tail rotor isgenerally somewhat greater than the maximum negative pitch angle available. This is because the primarypurpose of the tail rotor is to counteract the torque ofthe main rotor. The capability for tail rotors to producethrust to the left (negative pitch angle) is necessary,because during autorotation the drag of the transmission tends to yaw the nose to the left, or in the samedirection the main rotor is turning.5-1By knowing the various systems on a helicopter, youwill be able to more easily recognize potential problems,and if a problem arises, you will have a better understanding of what to do to correct the situation.ENGINESThe two most common types of engines used in helicopters are the reciprocating engine and the turbineengine. Reciprocating engines, also called pistonengines, are generally used in smaller helicopters. Mosttraining helicopters use reciprocating engines becausethey are relatively simple and inexpensive to operate.Turbine engines are more powerful and are used in awide variety of helicopters. They produce a tremendous amount of power for their size but are generallymore expensive to operate.RECIPROCATING ENGINEThe reciprocating engine consists of a series of pistonsconnected to a rotating crankshaft. As the pistons moveup and down, the crankshaft rotates. The reciprocatingengine gets its name from the back-and-forth movementof its internal parts. The four-stroke engine is the mostcommon type, and refers to the four different cycles theengine undergoes to produce power. When the piston moves away from the cylinder head onthe intake stroke, the intake valve opens and a mixtureof fuel and air is drawn into the combustion chamber.As the cylinder moves back towards the cylinder head,the intake valve closes, and the fuel/air mixture is compressed. When compression is nearly complete, thespark plugs fire and the compressed mixture is ignitedto begin the power stroke. The rapidly expanding gasesfrom the controlled burning of the fuel/air mixturedrive the piston away from the cylinder head, thus providing power to rotate the crankshaft. The piston thenmoves back toward the cylinder head on the exhauststroke where the burned gasses are expelled throughthe opened exhaust valve.Even when the engine is operated at a fairly low speed,the four-stroke cycle takes place several hundred timeseach minute. In a four-cylinder engine, each cylinderoperates on a different stroke. Continuous rotation of acrankshaft is maintained by the precise timing of thepower strokes in each cylinder.TURBINE ENGINEThe gas turbine engine mounted on most helicopters ismade up of a compressor, combustion chamber, turbine,and gearbox assembly. The compressor compresses theair, which is then fed into the combustion chamberwhere atomized fuel is injected into it. The fuel/airmixture is ignited and allowed to expand. This combustion gas is then forced through a series of turbinewheels causing them to turn. These turbine wheelsprovide power to both the engine compressor and themain rotor system through an output shaft. TheFigure 5-1. The arrows in this illustration indicate the direction of motion of the crankshaft and piston during the fourstroke cycle.Intake CompressionPower ExhaustIntakeValveExhaust
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ValveSparkPlugPistonConnectingRodCrankshaft1 23 45-2combustion gas is finally expelled through an exhaustoutlet. COMPRESSORThe compressor may consist of an axial compressor, acentrifugal compressor, or both. An axial compressorconsists of two main elements, the rotor and the stator.The rotor consists of a number of blades fixed on arotating spindle and resembles a fan. As the rotorturns, air is drawn rearwards. Stator vanes are arrangedin fixed rows between the rotor blades and act as adiffuser at each stage to decrease air velocity andincrease air pressure. There may be a number of rowsof rotor blades and stator vanes. Each row constitutesa pressure stage, and the number of stages depends onthe amount of air and pressure rise required for theparticular engine.A centrifugal compressor consists of an impeller, diffuser, and a manifold. The impeller, which is a forgeddisc with integral blades, rotates at a high speed todraw air in and expel it at an accelerated rate. The airthen passes through the diffuser which slows the airdown. When the velocity of the air is slowed, staticpressure increases, resulting in compressed, high-pressure air. The high pressure air then passes through thecompressor manifold where it is distributed to thecombustion chamber.COMBUSTION CHAMBERUnlike a piston engine, the combustion in a turbineengine is continuous. An igniter plug serves only toignite the fuel/air mixture when starting the engine.Once the fuel/air mixture is ignited, it will continue toburn as long as the fuel/air mixture continues to bepresent. If there is an interruption of fuel, air, or both,combustion ceases. This is known as a “flame-out,” andthe engine has to be restarted or re-lit. Some helicoptersare equipped with auto-relight, which automaticallyactivates the igniters to start combustion if the engineflames out.TURBINEThe turbine section consists of a series of turbinewheels that are used to drive the compressor sectionand the rotor system. The first stage, which is usuallyreferred to as the gas producer or N1 may consist ofone or more turbine wheels. This stage drives thecomponents necessary to complete the turbine cyclemaking the engine self-sustaining. Common components driven by the N1 stage are the compressor, oilpump, and fuel pump. The second stage, which mayalso consist of one or more wheels, is dedicated todriving the main rotor system and accessories fromthe engine gearbox. This is referred to as the powerturbine (N2 or Nr).Compressor Discharge Air TubeExhaust Air OutletIgniter PlugFuel NozzleAirInletOutput ShaftGearCompressor Rotor Turbine to Compressor CouplingCombustionLinerN2N1Inlet AirCompressor Discharge AirCombustion GassesExhaust GassesCompression Section Gearbox Section Turbine Section Combustion SectionStatorRotorFigure 5-2. Many helicopters use a turboshaft engine to drive the main transmission and rotor systems. The main differencebetween a turboshaft and a turbojet engine is that most of the energy produced by the expanding gases is used to drive a turbine rather than producing thrust through the expulsion of exhaust gases.5-3If the first and second stage turbines are mechanically coupled to each other, the system is said to be a direct-driveengine or fixed turbine. These engines share a commonshaft, which means the first and second stage turbines, andthus the compressor and output shaft, are connected.On most turbine assemblies used in helicopters, thefirst stage and second stage turbines are not mechanically connected to each other. Rather, they are mountedon independent shafts and can turn freely with respect toeach other. This is referred to as a “free turbine.” Whenthe engine is running, the combustion gases passthrough the first stage turbine to drive the compressorrotor, and then past the independent second stage turbine, which turns the gearbox to drive the output shaft.TRANSMISSION SYSTEMThe transmission system transfers power from theengine to the main rotor, tail rotor, and other accessories. The main components of the transmission system are the main rotor transmission, tail rotor drivesystem, clutch, and freewheeling unit. Helicopter transmissions are normally lubricated and cooled with theirown oil supply. A sight gauge is provided to check theoil level. Some transmissions have chip detectorslocated in the sump. These detectors are wired to warning lights located on the pilot’s instrument panel thatilluminate in the event of an internal problem.MAIN ROTOR TRANSMISSIONThe primary purpose of the main rotor transmission
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is to reduce engine output r.p.m. to optimum rotorr.p.m. This reduction is different for the various helicopters, but as an example, suppose the engine r.p.m. ofa specific helicopter is 2,700. To achieve a rotor speed of450 r.p.m. would require a 6 to 1 reduction. A 9 to 1reduction would mean the rotor would turn at300 r.p.m.Most helicopters use a dual-needle tachometer to showboth engine and rotor r.p.m. or a percentage of engineand rotor r.p.m. The rotor r.p.m. needle normally isused only during clutch engagement to monitor rotoracceleration, and in autorotation to maintain r.p.m.within prescribed limits. Chip Detector—A chip detector isa warning device that alerts you toany abnormal wear in a transmission or engine. It consists of amagnetic plug located within thetransmission. The magnet attractsany ferrous metal particles thathave come loose from the bearingsor other transmission parts. Mostchip detectors send a signal tolights located on the instrumentpanel that illuminate when ferrousmetal particles are picked up.In helicopters with horizontally mounted engines,another purpose of the main rotor transmission is tochange the axis of rotation from the horizontal axis ofthe engine to the vertical axis of the rotor shaft.TAIL ROTOR DRIVE SYSTEMThe tail rotor drive system consists of a tail rotor driveshaft powered from the main transmission and a tailrotor transmission mounted at the end of the tail boom.The drive shaft may consist of one long shaft or a seriesof shorter shafts connected at both ends with flexiblecouplings. This allows the drive shaft to flex with thetail boom. The tail rotor transmission provides a rightangle drive for the tail rotor and may also include gearing to adjust the output to optimum tail rotor r.p.m.Figure 5-3. There are various types of dual-needle tachometers, however, when the needles are superimposed or married,the ratio of the engine r.p.m. is the same as the gear reductionratio.Figure 5-4. The typical components of a tail rotor drive system are shown here.Tail RotorTransmissionTail RotorDrive ShaftMainTransmission5-4CLUTCHIn a conventional airplane, the engine and propeller arepermanently connected. However, in a helicopter thereis a different relationship between the engine and therotor. Because of the greater weight of a rotor in relation to the power of the engine, as compared to theweight of a propeller and the power in an airplane, therotor must be disconnected from the engine when youengage the starter. A clutch allows the engine to bestarted and then gradually pick up the load of the rotor.On free turbine engines, no clutch is required, as thegas producer turbine is essentially disconnected fromthe power turbine. When the engine is started, there islittle resistance from the power turbine. This enablesthe gas producer turbine to accelerate to normal idlespeed without the load of the transmission and rotorsystem dragging it down. As the gas pressure increasesthrough the power turbine, the rotor blades begin toturn, slowly at first and then gradually accelerate tonormal operating r.p.m.On reciprocating helicopters, the two main types ofclutches are the centrifugal clutch and the belt drive clutch.CENTRIFUGAL CLUTCHThe centrifugal clutch is made up of an inner assemblyand a outer drum. The inner assembly, which is connected to the engine driveshaft, consists of shoes linedwith material similar to automotive brake linings. Atlow engine speeds, springs hold the shoes in, so there isno contact with the outer drum, which is attached to thetransmission input shaft. As engine speed increases,centrifugal force causes the clutch shoes to move outward and begin sliding against the outer drum. Thetransmission input shaft begins to rotate, causing therotor to turn, slowly at first, but increasing as the frictionincreases between the clutch shoes and transmissiondrum. As rotor speed increases, the rotor tachometerneedle shows an increase by moving toward the enginetachometer needle. When the two needles are superimposed, the engine and the rotor are synchronized,indicating the clutch is fully engaged and there is nofurther slippage of the clutch shoes.BELT DRIVE CLUTCHSome helicopters utilize a belt drive to transmit powerfrom the engine to the transmission. A belt drive consists of a lower pulley attached to the engine, an upperpulley attached to the transmission input shaft, a beltor a series of V-belts, and some means of applyingtension to the belts. The belts fit loosely over theupper and lower pulley when there is no tension onthe belts. This allows the engine to be started withoutany load from the transmission. Once the engine isrunning, tension on the belts is gradually increased.When the rotor and engine tachometer needles aresuperimposed, the rotor and the engine are synchronized, and the clutch is then fully engaged.Advantages of this system include vibration isolation,simple maintenance, and the ability to start and warmup the engine without engaging the rotor.FREEWHEELING UNITSince lift in a helicopter is provided by rotating airfoils,these airfoils must be free to rotate if the engine fails. Thefreewheeling unit automatically disengages the enginefrom the main rotor when engine r.p.m. is less than main
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rotor r.p.m. This allows the main rotor to continue turningat normal in-flight speeds. The most common freewheeling unit assembly consists of a one-way sprag clutchlocated between the engine and main rotor transmission.This is usually in the upper pulley in a piston helicopteror mounted on the engine gearbox in a turbine helicopter.When the engine is driving the rotor, inclined surfaces inthe spray clutch force rollers against an outer drum. Thisprevents the engine from exceeding transmission r.p.m. Ifthe engine fails, the rollers move inward, allowing theouter drum to exceed the speed of the inner portion. Thetransmission can then exceed the speed of the engine. Inthis condition, engine speed is less than that of the drivesystem, and the helicopter is in an autorotative state.MAIN ROTOR SYSTEMMain rotor systems are classified according to how themain rotor blades move relative to the main rotor hub.As was described in Chapter 1—Introduction to theHelicopter, there are three basic classifications: fullyarticulated, semirigid, or rigid. Some modern rotor systems use a combination of these types.FULLY ARTICULATED ROTOR SYSTEMIn a fully articulated rotor system, each rotor blade isattached to the rotor hub through a series of hinges,which allow the blade to move independently of theothers. These rotor systems usually have three or moreblades. Pitch ChangeAxis(Feathering)FlappingHingeDamperDrag HingePitch HornFigure 5-5. Each blade of a fully articulated rotor system canflap, drag, and feather independently of the other blades.5-5The horizontal hinge, called the flapping hinge, allowsthe blade to move up and down. This movement iscalled flapping and is designed to compensate for dissymetry of lift. The flapping hinge may be located atvarying distances from the rotor hub, and there may bemore than one hinge.The vertical hinge, called the lead-lag or drag hinge,allows the blade to move back and forth. This movement is called lead-lag, dragging, or hunting.Dampers are usually used to prevent excess backand forth movement around the drag hinge. The purpose of the drag hinge and dampers is to compensatefor the acceleration and deceleration caused byCoriolis Effect.Each blade can also be feathered, that is, rotated aroundits spanwise axis. Feathering the blade means changingthe pitch angle of the blade. By changing the pitchangle of the blades you can control the thrust and direction of the main rotor disc.SEMIRIGID ROTOR SYSTEM
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A semirigid rotor system is usually composed of twoblades which are rigidly mounted to the main rotor hub.The main rotor hub is free to tilt with respect to themain rotor shaft on what is known as a teeteringhinge. This allows the blades to flap together as aunit. As one blade flaps up, the other flaps down.Since there is no vertical drag hinge, lead-lag forcesare absorbed through blade bending. RIGID ROTOR SYSTEMIn a rigid rotor system, the blades, hub, and mast arerigid with respect to each other. There are no vertical orhorizontal hinges so the blades cannot flap or drag, butthey can be feathered. Flapping and lead/lag forces areabsorbed by blade bending.COMBINATION ROTOR SYSTEMSModern rotor systems may use the combined principles of the rotor systems mentioned above. Somerotor hubs incorporate a flexible hub, which allowsfor blade bending (flexing) without the need for bearings or hinges. These systems, called flextures, areusually constructed from composite material.Elastomeric bearings may also be used in place ofconventional roller bearings. Elastomeric bearings arebearings constructed from a rubber type material andhave limited movement that is perfectly suited for helicopter applications. Flextures and elastomeric bearings require no lubrication and, therefore, require lessmaintenance. They also absorb vibration, whichmeans less fatigue and longer service life for the helicopter components. SWASH PLATE ASSEMBLYThe purpose of the swash plate is to transmit controlinputs from the collective and cyclic controls to the mainrotor blades. It consists of two main parts: the stationaryTeeteringHingeFeathering HingeStatic StopsPitch HornFigure 5-6. On a semirigid rotor system, a teetering hingeallows the rotor hub and blades to flap as a unit. A static flapping stop located above the hub prevents excess rockingwhen the blades are stopped. As the blades begin to turn,centrifugal force pulls the static stops out of the way.Figure 5-7. Rotor systems, such as Eurocopter’s Starflex orBell’s soft-in-plane, use composite material and elastomericbearings to reduce complexity and maintenance and,thereby, increase reliability.5-6swash plate and the rotating swash plate. The stationary swash plate is mounted around the mainrotor mast and connected to the cyclic and collectivecontrols by a series of pushrods. It is restrained fromrotating but is able to tilt in all directions and move vertically. The rotating swash plate is mounted to the stationary swash plate by means of a bearing and isallowed to rotate with the main rotor mast. Both swashplates tilt and slide up and down as one unit. The rotating swash plate is connected to the pitch horns by thepitch links.FUEL SYSTEMS
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The fuel system in a helicopter is made up of twogroups of components: the fuel supply system and theengine fuel control system.FUEL SUPPLY SYSTEMThe supply system consists of a fuel tank or tanks, fuelquantity gauges, a shut-off valve, fuel filter, a fuel lineto the engine, and possibly a primer and fuel pumps.The fuel tanks are usually mounted to the airframe asclose as possible to the center of gravity. This way, asfuel is burned off, there is a negligible effect on the center of gravity. A drain valve located on the bottom ofthe fuel tank allows the pilot to drain water and sediment that may have collected in the tank. A fuel ventprevents the formation of a vacuum in the tank, and anoverflow drain allows for fuel to expand without rupturing the tank. A fuel quantity gauge located on thepilot’s instrument panel shows the amount of fuelmeasured by a sensing unit inside the tank. Somegauges show tank capacity in both gallons and pounds.The fuel travels from the fuel tank through a shut-offvalve, which provides a means to completely stop fuelflow to the engine in the event of an emergency or fire.The shut-off valve remains in the open position for allnormal operations.Most non-gravity feed fuel systems contain both anelectric pump and a mechanical engine driven pump.The electrical pump is used to maintain positive fuelpressure to the engine pump and also serves as abackup in the event of mechanical pump failure. Theelectrical pump is controlled by a switch in the cockpit.The engine driven pump is the primary pump that supplies fuel to the engine and operates any time theengine is running.A fuel filter removes moisture and other sediment fromthe fuel before it reaches the engine. These contaminants are usually heavier than fuel and settle to the bottom of the fuel filter sump where they can be drainedout by the pilot.Some fuel systems contain a small hand-operated pumpcalled a primer. A primer allows fuel to be pumpeddirectly into the intake port of the cylinders prior toengine start. The primer is useful in cold weather whenfuel in the carburetor is difficult to vaporize.ENGINE FUEL CONTROL SYSTEMThe purpose of the fuel control system is to bring outside air into the engine, mix it with fuel in the properproportion, and deliver it to the combustion chamber.ThrottleLow LevelWarningLightVentFuel QuantityGaugeMixtureControl
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FuelShutoffPrimerTankShut-offValveCarburetorFuelStrainerPrimer Nozzleat CylinderFigure 5-9. A typical gravity feed fuel system, in a helicopterwith a reciprocating engine, contains the componentsshown here.StationarySwashPlatePitchLinkRotatingSwashPlateControlRodFigure 5-8. Collective and cyclic control inputs are transmitted to the stationary swash plate by control rods causing it totilt or to slide vertically. The pitch links attached from therotating swash plate to the pitch horns on the rotor hubtransmit these movements to the blades.5-7RECIPROCATING ENGINESFuel is delivered to the cylinders by either a carburetoror fuel injection system.CARBURETORIn a carburetor system, air is mixed with vaporized fuel asit passes through a venturi in the carburetor. The meteredfuel/air mixture is then delivered to the cylinder intake.Carburetors are calibrated at sea level, and the correctfuel-to-air mixture ratio is established at that altitudewith the mixture control set in the FULL RICH position. However, as altitude increases, the density of airentering the carburetor decreases while the density ofthe fuel remains the same. This means that at higheraltitudes, the mixture becomes progressively richer. Tomaintain the correct fuel/air mixture, you must be ableto adjust the amount of fuel that is mixed with theincoming air. This is the function of the mixture control. This adjustment, often referred to as “leaning themixture,” varies from one aircraft to another. Refer tothe FAA-Approved Rotocraft Flight Manual (RFM) todetermine specific procedures for your helicopter. Notethat most manufacturers do not recommend leaning helicopters in-flight.Most mixture adjustments are required during changes ofaltitude or during operations at airports with field elevations well above sea level. A mixture that is too rich canresult in engine roughness and reduced power. The roughness normally is due to spark plug fouling from excessive carbon buildup on the plugs. This occurs becausethe excessively rich mixture lowers the temperature insidethe cylinder, inhibiting complete combustion of the fuel.This condition may occur during the pretakeoff runup athigh elevation airports and during climbs or cruise flight
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at high altitudes. Usually, you can correct the problem byleaning the mixture according to RFM instructions.If you fail to enrich the mixture during a descent fromhigh altitude, it normally becomes too lean. Highengine temperatures can cause excessive engine wearor even failure. The best way to avoid this type of situation is to monitor the engine temperature gauges regularly and follow the manufacturer’s guidelines formaintaining the proper mixture.CARBURETOR ICEThe effect of fuel vaporization and decreasing air pressure in the venturi causes a sharp drop in temperaturein the carburetor. If the air is moist, the water vapor inthe air may condense. When the temperature in the carburetor is at or below freezing, carburetor ice may formon internal surfaces, including the throttle valve. Because of the sudden cooling that takesplace in the carburetor, icing can occur even on warmdays with temperatures as high as 38°C (100°F) andthe humidity as low as 50 percent. However, it is morelikely to occur when temperatures are below 21°C(70°F) and the relative humidity is above 80 percent.The likelihood of icing increases as temperaturedecreases down to 0°C (32°F), and as relative humidityincreases. Below freezing, the possibility of carburetoricing decreases with decreasing temperatures.Although carburetor ice can occur during any phase offlight, it is particularly dangerous when you are usingreduced power, such as during a descent. You may notnotice it during the descent until you try to add power.Indications of carburetor icing are a decrease in enginer.p.m. or manifold pressure, the carburetor air temperature gauge indicating a temperature outside the safeoperating range, and engine roughness. Since changesin r.p.m. or manifold pressure can occur for a numberof reasons, it is best to closely check the carburetor airtemperature gauge when in possible carburetor icingconditions. Carburetor air temperature gauges aremarked with a yellow caution arc or green operatingarcs. You should refer to the FAA-Approved RotorcraftFlight Manual for the specific procedure as to whenand how to apply carburetor heat. However, in mostcases, you should keep the needle out of the yellow arcor in the green arc. This is accomplished by using a carburetor heat system, which eliminates the ice byTo EngineIncoming AirIceIceVenturiFuel/AirMixtureIceFigure 5-10. Carburetor ice reduces the size of the air passage to the engine. This restricts the flow of the fuel/airmixture, and reduces power.5-8routing air across a heat source, such as an exhaust
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manifold, before it enters the carburetor. .FUEL INJECTIONIn a fuel injection system, fuel and air are metered atthe fuel control unit but are not mixed. The fuel isinjected directly into the intake port of the cylinderwhere it is mixed with the air just before entering thecylinder. This system ensures a more even fuel distribution in the cylinders and better vaporization, whichin turn, promotes more efficient use of fuel. Also, thefuel injection system eliminates the problem of carburetor icing and the need for a carburetor heat system.TURBINE ENGINESThe fuel control system on the turbine engine is fairlycomplex, as it monitors and adjusts many differentparameters on the engine. These adjustments are doneautomatically and no action is required of the pilotother than starting and shutting down. No mixtureadjustment is necessary, and operation is fairly simpleas far as the pilot is concerned. New generation fuelcontrols incorporate the use of a full authority digitalengine control (FADEC) computer to control theengine’s fuel requirements. The FADEC systemsincrease efficiency, reduce engine wear, and alsoreduce pilot workload. The FADEC usually incorporates back-up systems in the event of computer failure.ELECTRICAL SYSTEMSThe electrical systems, in most helicopters, reflect theincreased use of sophisticated avionics and other electrical accessories. More and more operations in today’sflight environment are dependent on the aircraft’s electrical system; however, all helicopters can be safelyflown without any electrical power in the event of anelectrical malfunction or emergency.Helicopters have either a 14- or 28-volt, direct-current electrical system. On small, piston poweredhelicopters, electrical energy is supplied by an engine-driven alternator. These alternators have advantagesover older style generators as they are lighter inweight, require lower maintenance, and maintain auniform electrical output even at low engine r.p.m.Turbine powered helicopters use a starter/generatorsystem. The starter/generator is permanently coupledto the engine gearbox. When starting the engine, electrical power from the battery is supplied to thestarter/generator, which turns the engine over. Once theengine is running, the starter/generator is driven by theengine and is then used as a generator.Current from the alternator or generator is deliveredthrough a voltage regulator to a bus bar. The voltageregulator maintains the constant voltage required bythe electrical system by regulating the output of thealternator or generator. An over-voltage control may beAvionicBusBar
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