飞机设计导论(第五章)

IntroductiontoAircraftDesignChapter5What’sundertheskin?Structureandpropulsion2020年3月7日星期六IntroductiontoAircraftDesign第2页5.1GeneralTherearemanysystemsinana/c,eachofthemisimportant,buteachmayhaveconflictingrequirements.Agooddesignermustweighuptheconflicts,userelevantanalysesandsynthesizethea/cintoanefficientwhole.Outsideofthea/chastobebiggerthanthespacerequiredinsidethea/c.Itisoftennecessarytomodifytheexternalshapetoaccommodatetheinterior,withconsequentaerodynamicchanges.2020年3月7日星期六IntroductiontoAircraftDesign第3页5.2ThestructureLowweight.Acceptablematerialandmanufacturingcosts.Adequatestrengthtomeetthemax.expectedloads,withasuitablesafetyfactor.Adequatestiffnesssothatdistortionsarekeptwithinacceptablelimits.Goodin-servicepropertiessuchasfatigueandcorrosionresistancetogetherwithtoleranceofexpectedtemperaturesandotheratmosphericconditions.2020年3月7日星期六IntroductiontoAircraftDesign第4页5.2.1MaterialsLightalloys:Aluminumalloy.Highstrengthandstiffness-weightratios,relativelylowcost,easyofhandling.max.temp.forcontinuousoperationis130ºC，equivalenttoaMachno.2.2duetokineticheating.Aircraftwhichdonotuselightalloysastheprimestructuralmaterialarerare.2020年3月7日星期六IntroductiontoAircraftDesign第5页5.2.1MaterialsTitanium:Higherstrength-weight,stiffness-weightratio,density.Morecostanddifficulttohandleintheprocessofmanufacture.Titaniumalloysareusedwhenheatresistantpropertiesarerequired,suchasfirewalls,mainframesandstructureclosetoengineexhaustandgunbarrels.MaybeusedincloseproximitytoCFCs.(thermalexpansionandcorrosion)2020年3月7日星期六IntroductiontoAircraftDesign第6页5.2.1MaterialsSteel:1stmetala/cusedsteelratherthanlightalloys.Evenhigherstrength-weightandstiffness-weightratios,densityisas3timesasthelightalloys.Highlystressedcomponents,suchaslandinggearandenginepylons,makeextensiveuseofsteel.Casesofmanyearliersolidrocketmotorswereweldedsteelunits.Ticostishigh.2020年3月7日星期六IntroductiontoAircraftDesign第7页5.2.1MaterialsMagnesiumalloys:Themainuseofwhichislargecastingsandtheverylowdensityresultsisaweightsaving.Lowresistancetocorrosionanddifficultofmakingjointswithoutweldingduetocrackpropagation.Thebiggestdrawbackisunfortunatepastexperienceswithcorrosionofthematerial.Itscostisnotveryhigh.2020年3月7日星期六IntroductiontoAircraftDesign第8页5.2.1MaterialsNon-metallicmaterials:reinforcedplastics,rubber,sealantsandcockpittransparencies.Resin-bondedglassfiber,poorerstiffness,temperaturelimitationduetoresinanddifficultyofattachmenttometalparts.Carbonfibercomposite.Carbon-fiberhoneycombwingskinsandfuselagepanelsgivegoodstiffnessandminimizethenumberofribsandframes.2020年3月7日星期六IntroductiontoAircraftDesign第9页5.2.2Load-carryingmethodsAstructureisadevicefortransferringmechanicalloadsfromonepointtoanotherusingthefollowingmechanisms.SeeFig.5.2.Tension:thesimplestwayofcarryingaloadistouseatensionmember.Itisinherentlystable.Afailureimpliesseveranceofthestructure,normallywithcatastrophicresultsunlessalternativeloadpathsareprovided.Compression:allcompressionmembersmustbedesignedwithconsiderationofthequestionofinstability.2020年3月7日星期六IntroductiontoAircraftDesign第10页5.2.2Load-carryingmethodsShear:ashearmemberreactsloadcanbeconsideredasacombinationofdiagonaltensionandcompression.Bending:Inrealityabeamisnotasingletypeofloadcarryingmemberbutasimplestructure.Theloadarecarriedbyacombinationoftension,compressionandshear.Thestrengthofabeamvariesaccordingtotheshapeofthecross-section.Torsion:aspecialformofshear.Torsionusuallyoccursinstructurewhentheloadpathchangesdirection.2020年3月7日星期六IntroductiontoAircraftDesign第11页5.2.3ThemainstructuremembersFlyingsurfaces:Theflyingsurfacesastypifiedbythewingareessentiallybeams.Inadditiontoactingasabeamtoresistspanwiseairloads,thewingmustalsocarryconsiderabletorsionloadsandprovidesufficientstiffnesstopreventexcessivetwist.Thetorsionloadarisesprimarilyfromflapsandcontrolsurfaces,andthetorsionalstiffnessisnecessarytopreventdivergence,flutterandcontrolreversal.2020年3月7日星期六IntroductiontoAircraftDesign第12页5.2.3.1FlyingsurfacesAreductionofthickness/chordratiodirectlyreducestheenclosedarea,andthustheeffectontwistisvariedasthesquareofthedepth.Fig.5.3showsthespanwiseandchordwiseloads.Reducedaspectratioreducesbothbendingmomentandtwistandthisisthusstructurallyadvantageous.Increasedtaperhastheeffectofmovingtheairloadtowardstheroot,thusreducingthebendingmoment.Sweepbackisthusadisadvantageinthispoint.2020年3月7日星期六IntroductiontoAircraftDesign第13页5.2.3.1FlyingsurfacesRecentadvancesinmanoeuvreloadcontrol(MLC)andgustloadalleviation(GLA)activelymodifytheairloaddistribution.InGLA,theflightcontrolsystemsensesanup-comingsymmetricgustandsendsasignaltotheailerons.Theserapidlydeflectsymmetricallytoreducewing-tipcamber,thenreducewing-tipliftandthespanwisecenterofpressuremovesinboard.Thusthewingshearforceandbendingmomentarereduced.2020年3月7日星期六IntroductiontoAircraftDesign第14页5.2.3.1FlyingsurfacesTheAirbusIndustriesA320usedGLAtoreducethefatiguedamagetothewing,thusleadingtoreducedmaterialandweight.Thewingfunctionsasabeamandatorsionbox(withskins).Fig