飞机设计导论(第六章)

IntroductiontoAircraftDesign,Chapter6Whatsundertheskin?Airframesystems,6.1SecondaryPowerSystems,Secondarypowerisusuallyprovidedbyanaircraftsmainengineswhentheyareoperatingbutcanbesuppliedbyothersourcessuchasanauxiliarypowerunit(APU)intheaircraftoranexternalsourceontheground.Almostallpresenta/cSPSareofthreetypes:electricpower,bleedair/pneumaticpower,andhydraulicpower,seeFig.6.1.theengineshpshaftdrivesanaccessorygearboxonwhicharemountedgeneratorandhydraulicpumpsaswellastheenginesownaccessories.,2020/5/28,2,隐身技术,6.1.1Air-conditioningandpressurization,Bleedairistappedfromoneormorepointsalongtheenginecompressortosupplytheaircraftsair-conditioningandpressurizationsystem.Itisusualtomaintainana/ccabinatapressureequivalenttothatat8000ft(2400m)orlessandthisrequiresaconstantsupplyofpressurizedair.Oxygenusedbytheoccupantsmustbereplacedandthetemperatureandcontaminantlevelscontrolled,requiringasignificantair.,2020/5/28,3,隐身技术,6.1.1Air-conditioningandpressurization,Fig.6.2showsthemajorelementsofanair-conditioningsystem.Thecombinationofair-conditioning,pressurizationandoxygensystemsareusuallytermedtheenvironmentalcontrolsystem(ECS).Inadditiontocoolingthecrewandpassengercompartmentsonhigh-speeda/citmaybenecessarytoremoveheatfromtheundercarriagebays,electricalequipmentandflyingcontrolareas.,2020/5/28,4,隐身技术,6.1.1Air-conditioningandpressurization,Militarya/cuseECSsystemsworkingonthesameprinciplesasthoseforcivila/c,butThepressurizedcabinhasamuchsmallervolumeandworksatalowerpressuredifferential.Oxygensystemsareusedinnormaloperationsathigheraltitudes.Militaryavionicsaremorecomplexandproducecomparativelymoreheatandthusrequiremorecooling.HawkTrainerisshowninFig.6.3.,2020/5/28,5,隐身技术,6.1.2Iceprotectionsystems,Dangerousicingconditionsarerarelyencounteredbymoderna/cbutitisnormallynecessarytoprovideprotection.Engineintakeicingismorecommonduetothespecialflowconditionspresentandmosta/catleasthaveaprovisionforde-icinginthisregionoftheairframe.Protectionagainsticingcanbeobtainedinvariousways.De-icingallowsicetofromtoacertainextentandthemperiodicallyremovesit.Anti-icingisasystemwhichisactivatedwhenicingconditionsarelikelyandpreventsiceformation.,2020/5/28,6,隐身技术,6.1.2Iceprotectionsystems,Bleedairisnotonlyathighpressurebutalsohightemperature,soitthemainformoficeprotectionfortheairframeandengines.Onlargetransports,onlytheouterportionofthewingleadingedgesorslatsandtheenginecowllipsrequireprotection.Smallera/calsorequireprotectionofthetailsurfaceleadingedges,andwhenoperatingiceprotectioncantake3timestheflowforair-conditioning,fortheshorttimethattheiceprotectionisrequired.,2020/5/28,7,隐身技术,6.1.2Iceprotectionsystems,Turbo-proppowereda/chaveinsufficientbleedforthermalanti-icing,soalternativeformsofprotectionarerequired.Someleadingedgesareprotectedusingapneumaticpoweredsystem,butthisusesonlysmallamountsofairtoperiodicallyinflateflexible“boots”thusde-icingthearea.Thesebootsarenottotallyeffectiveinallconditionsandarepronetoerosionduetotheirrelativesoftnature(Fig.6.4).,2020/5/28,8,隐身技术,6.1.2Iceprotectionsystems,Anotherpossibilityistosprayafluidovertheaffectedsurfacetoreducethemeltingpointofthemixtureformedtobelowtheambienttemperature.Localizedanti-icingmaybeprovidedbytheuseofembeddedelectricalheatingelements.Amorerecentdevelopmenthasbeenelectro-inductionde-icing.Wheniceisdetected,thesystemuseselectro-magneticmeanstovibratethewingleading-edgetoshakeiceoff.,2020/5/28,9,隐身技术,6.1.3Electricpowersystems,Engine-drivengeneratorsprovide3-phase400Hz115VlinetoneutralACPower,someofthispowerwillbeconvertedto28VDC.Mosta/chavebothdirectandalternatingcurrentsupplieswiththeformerusedprimarilyforsuchitemsaselectricallydriveninstrumentsandlights.ACpowerforhighenergydevicessuchaselectricalanti-icing.TheusualsourcesofpowersupplyareenginedrivenalternatorswhichsupplyACwhichcanalsobeconvertedtoDCbytransformer/rectifier.,2020/5/28,10,隐身技术,6.1.3Electricpowersystems,Largea/cusesmallturbinesasAPUsforenginestartingandairconditioningwhilstthea/cisontheground.BatteriesareusedtosupplyDCdirectlyandcanbeusedtostartenginesonsmallaircraft.Powersuppliesmustbeduplicatedortriplicatedsothatpowercanbesuppliedevenintheeventoffailuresofindividualcomponents.Fig6.5showsaschematicairlinersystemandFig.6.6describestheBAeHawkssystem.,2020/5/28,11,隐身技术,6.1.4Hydraulicsystems,Themajoruseofhydraulicsona/cistoprovidefortheactuationofdevicessuchasflyingcontrols.Almostallmediumandlargeciviltransportshaveasystemoperatingatapressureof3000psi(210kg/cm2)Undercarriageorlandinggearsareretractedandextendedhydraulically,aswellasbraking.Largedoorsareactuatedbyhydraulicsystems.Highreliabilityisanimportantconsiderationandthisleadstotheduplicationortriplication.,2020/5/28,12,隐身技术,6.1.4Hydraulicsystems,ThisisparticularlyimportantinflyingcontrolactuationasshowninFig.6.7andFig.6.8.Onalargea/cwith3hydraulicchannels,themassofhydraulicpipeworkandfluidcontainedwithinitisconsiderable.Increasingthepressureallowsreducedflowsoffluidandthuspipesize.Actuatorsizescanalsobereducedbutlimitsareimposedbythehigherpressuretobecontainedbythepipewalls.,2020/5/28,13,隐身技术,6.1.4Hydraulicsystems,Reductionof25%inhydraulicsystemmasshavebeenclaimedfor8000psi(560kg/cm2)systemsrelativetopresentstandards.Moderncivilandmilitaryaircraftwithactivecontrolsrequireveryrapidflyingcontrolactuation.Thisisusuallysuppliedbyhydraulicactuatorswhichrequirespeciallydesignedhydraulicsystemswhichwillprovidefortheveryhighfluidflowraterequired.,2020/5/28,14,隐身技术,6.2Thefuelsystem,Onlargea/cwithseveralenginesandnumerousfueltanks,thefuelsystemcanbecomplicated.Alargenumberofdesignrequirementsmustbemetasafuelsystemfailuremayresultinacrash.Safetyriskscomefromfuelstarvationwhichleadstoenginefailureorfireduetouncontrolledfuelignition.Fuelsystemdesignmustpreventeitherofthesefailuresandmustproduceasystemwithhighreliabilityandmaintainability.Itisoftenarequirementthatfuelcanbejettisonedortakenoninflight.,2020/5/28,15,隐身技术,6.2Thefuelsystem,Itisusualtomakefullscalemodelsofthefuelsystemandtestthemunderasrepresentativeconditionsascanbeobtainedontheground.Mostcivilaircraftstorefuelintheirwingsasitsweightactsintheoppositedirectiontowinglift,thusreducingwingbendingmomentandweight.Thistechniquealsohastheadvantageofisolatingfuelawayfromthepassengercompartment.Fig6.9showsafuelsystemwhichistypicalofsubsonicairliner.,2020/5/28,16,隐身技术,6.2Thefuelsystem,Supersonica/cexperienceaerodynamicchangeswhichaffectthelongitudinaltrimofthea/c.Onewaytocombatthisistodeflectcontrolsurfaces,butthiscauseextraaerodynamicdrag.Fig.6-10showstheingeniousmethodusedbyConcordeinwhichfuelispumpedfromtank1and2to10toaltertheaircraftscenterofgravity.Fuelsystemsofmilitarya/cleadtodifferentproblems,themainonebeingvulnerabilitytoenemyfire.,2020/5/28,17,隐身技术,6.2Thefuelsystem,AnalysisofVietnamandMiddleEastwarsshowedthat62%ofsingle-enginea/closseswereattributabletofuelsystemsandtanks.AsimplemeasureistousekerosenefuelratherthanthemoreinflammableJP4.Theuseoffuselagetankswillaffordsomeprotectionbythefuselagestructure.Anothertechniqueistouseself-sealingtanks.Valvesandpumpscanbeplacedwithintheprotectedtankandprotectedbyarmor.Theairvolumeabovethefuelmaybepressurizedwithinertgas.Fig6.10,2020/5/28,18,隐身技术,6.3Furnishings,Apartfromseats,theitemscoveredincluderemovablebulkheads,walltrim,carpets,lightluggageracks,lockers,galleysandtoilets.Incivila/cdesign,furnishingshaveamarkedeffecton:weight,passengerappeal,passengersafety,airconditioning,andelectricalpowerandlighting.Fig6.12showsatypicalcivila/clayout.Oneimportantfactoristheprovisionofparallelseatrailsinthefloor.Thesepermitrapidrelocationsofseats,bulkheads,etc.,2020/5/28,19,隐身技术,6.4Safetyinstallations,Whenana/cisdesignedtoflyoverwideexpansesofwater,provisionmustbemadeforsuchsafetyitemsaslifejacketsanddinghies.Itisalsonecessarytoincorporateemergenceexitchutesonlargea/ctoallowtotalevacuationwithin90seconds.Animportantconsiderationinfuselagedesignistheprovisionofsufficientdoorsandemergencyexitsandadequateaccesstothem.Theprincipalitemofmilitaryaircraftsafetyequipmentistheejectorseat.,2020/5/28,20,隐身技术,6.4Safetyinstallations,Fig6.13showsamodernlightweightejectorseatdevelopedforlighttrainerandgroundattackaircraft.Thereisapoweredharnessretractionmechanismwhichautomaticallypullsthepilotbackintohisseatforasafetyejection.Thelegrestraintlinesalsopullthelegsintoasafeposition.Therocketmotorispowerfulenoughtoejectthepilotsafetyat0forwardspeedandaltitude.Thepilotusuallysitsunderatransparentcanopywhichmustnotblocktheseatsejectionpath.,2020/5/28,21,隐身技术,6.5Landinggearinstallations,Theseformthevitalinterfacebetweenthegroundandaircraftforthetaxi,take-offandlandingphasesofoperation.Asinmanyaeronauticalapplications,adifferentterminologyhasbeenusedontheoppositesidesoftheAtlanticocean.ThetermundercarriagehasbeenusedintheUK,butlandinggearisnowusedmoreuniversally.,2020/5/28,22,隐身技术,6.5.1Wheelarrangements,Manytypeshavebeenusedinthepast,butthethreeimportantarrangementareextensivelyused.Nosewheelortricyclelayout.Tailwheellayout.Bicyclelayout.SeeFig.6.14.,2020/5/28,23,隐身技术,Nosewheellayout,Whilethea/cisontheground,thefuselage,andhencethecabinfloor,isalwaysroughlyhorizontal.Theviewofthepilotwhentaxiingisrelativelygood.Thenosewheelactsasaproptopreventoverturningduringbraking.Theinitialtakeoffattitudehaslowdrag.,2020/5/28,24,隐身技术,Tailwheellayout,Athirdsupport(thetailwheel)issmallandlight.Duringbraking,theaircrafttendstopitchnosedown,increasingthemainwheelreactionandreducingthepossibilityofskidding.Thereisnorequirementforatailbumper(localfuselagereinforcement)asinthetricycleconfiguration,becauseofthetailwheelItisofteneasiertomountthemainundercarriagelegsontosuitablestructure.,2020/5/28,25,隐身技术,Bicyclelayout,Themainload-carryingwheelsarelocatedroughlyequidistantforeandaftoftheCG,therebyleavingasubstantiallengthofthea/cabouttheCGclearofobstructions.Thisisparticularlyusefulforbomberorverticaltake-offaircraft.Thewheelsarelocatedonthecenterlineoftheaircraftandcanbestowedinthefuselagetherebyensuringagoodwingstructure.,2020/5/28,26,隐身技术,Bicyclelayout,Outriggersarenecessary.Thesemustbeabletocastorand,unlesscareistakeninthelayout,theycanbecomelargeandheavy.Theaircraftlandingattitudemustbecarefullycontrolled.Thisisessentialinrollandyawtoavoidoverloadingtheoutriggers,andinpitchtopreventexcessivenosedownpitchingwhichispronetooccurbecauseofthegreatdistanceoftherearmainwheelaftofthecenterofgravity.Considerableelevatorpowerisnecessarytoraisethenoseduringtakeoff.,2020/5/28,27,隐身技术,6.5.2Wheelsandtires,Thenumberoftiresoneachunit,thetirepressureandthenumberofmainwheellegunitsareallverydependentuponthepermissiblerunwayloadingbutcanbeinfluencedbystowagevolumeandlocation.Runwayloadingisdefinedintermsofequivalentsinglewheelloading(ESWL)onagivenmainwheelunit.,2020/5/28,28,隐身技术,6.5.3Landinggearlayout,Itisnecessarytobeinpossessionofcertaininformation.Typeoflayout,includingnumberoflegunits.Tirelayoutoneachunitandtirepressure.Approximatetiresizeandaxletravel.Aircraftanglesatlandingandtakeoff.ExtremevariationsintheverticalandhorizontallocationoftheCG.,2020/5/28,29,隐身技术,Layoutinelevation-Landingcase,Thereshouldnormallybeabout0.15mgroundclearanceunlessabumperisspecificallyincorporated.Thepositionofmainwheel,orcenterofmulti-wheelassembly,isdeterminedbyensuringthatthemostcontactpointisatleast4behindtheperpendiculardrawnfromthegroundlinetothemostadverselandingcenterofgravityposition.SeeFig.6.15.,2020/5/28,30,隐身技术,Layoutinelevation-Landingcase,Excessiveaftlocationofthewheelmayimplyanunacceptablylargeelevatorpowerforliftingthenoseattakeoff,orlargenosedownpitchinlanding.Undulyfarforwardlocationofthewheelimpliespoorgroundstaticstability.Inanycaseitisnecessarytoverifythewheelpositionsubsequentlyinthetakeoffattitude.,2020/5/28,31,隐身技术,Layoutinelevation-staticcase,Itisnecessarytodeterminethestaticclosureofthetireandshockabsorber.Theformerisgivenbytheknowntirecharacteristicsandthelatterbythestaticspringcharacteristics,fromcomparableaircraft.Thepositionofthemainwheelscanbecheckedatthisstagetoensureadequategroundstaticstability.Locationofthenosewheelisinitiallydeterminedbyreferencetothestaticloadwhichitwillreact.Thisshouldbeabout10%oftheall-upweight,2020/5/28,32,隐身技术,Layoutinelevation-staticcase,Pastexperienceshasshownthatlessthanabout8%impliespoorsteeringadhesionandconsequentlydifficultgroundmaneuvering,whilstmoret