飞行器概论chapter6英文版

TheRoleoftheControlSystem

TheBasics

Wehaveseeninthepreviouschapterthattheguidancesystemisthebrainsofthewholemissilesystemandprovidestwosignals(steeringcommands)usuallyintheformofanup-downcommandandaleft-rightcommand.ThistypeofcontroliscalledCartesiancontrol.ThetwosteeringsignalscouldbepresentedinPolarform,asRand~,fora'TwistandSteer'missile,aswewillseelater.Itisthetaskofthemissilecontrolsystemtoexecutethesesteeringcommandsefficientlyandconsistently.Theefficiencyofthemissilereflectstheskilloftheairframedesignerandcanbemeasuredintermsoflift-to-dragratio.Theconsistencyofthemissileflightperformanceisthetaskofthemissilecontrolengineer.Letusnowconsidersomeaspectsoftherequirementforconsistency.

Averycommonformofguidanceis'threepointguidance',thatislineofsight(LOS)guidance,wherethetargettrackerisnotinthemissile,butisseparatelysituatedandisusuallyontheground.Thethreepointsarethetarget,thetrackerandthemissile.Themethodofobtainingahitistocon-strainthemissileasnearlyaspossibletothelineofsight,thelinejoiningthetrackerandthetarget.Consideratargetflyingstraightandatapproxi-

matelyconstantspeed,andamissileflyingatadifferentspeedhavingbeen.launched,fromclosetotrackerat0,whenthetargetoccupiesapositionTO,seeFigure6.2.Wecanshowthistwo-dimensionally,whetherthetargrt.isdirectlyoverhead,orinthehorizontalplane,orinanyintermediateposition,sincethedirectionofmotionofthetargetandthetrackertogetherdefineaplane.Atintervalsoftime1,2,3,etc.secondsafterlaunch,theLOSisshownasOTl,OT2,OT3,etc.Sincethemissileideallyalwaysliesontheselines,theflightpathwillbeacurvedoneand,foranapproachingtarget,thecurvaturebecomesincreasinglyseveretowardstheendoftheengagement.Thismeansthatthemissilemustbemanoeuvringthroughouttheentireengagement,eventhoughthetargetmaybeflyingstraight;moreoverthelateralaccelerationofthemissileor'g'requiredwillnotbeconstant.Anexceptiontothisistheuniquecaseofthetargetflyingdirectlytowardsthetracker.

FIG.6.2Interceptgeometry

Themissile'g'requirementsareafunctionoftheproductofthetargftspeedandthemissilespeedandinverselyproportionaltotherangeofti,.target.Themissile'g'isalsomuchgreaterforanglesof0intherange50'90?thanforsmallangles.Forexample,ifweconsideratypicalsurface-t'airengagementwithatargetspeedof240m/s(aboutMachnumber0.7),missilespeedof545m/s(aboutMachnumber1.6)andimpactoccurswhe.0=90?atarangeof1km,themissilewillhavetopull,atimpact,nearl.l27gtokeepintheLOS.If,however,wecanhitthesametargetatarangt'of4km,when53isabout15?thisrequirementisreducedto2.4g.Ifwenowconsiderasituationmoretypicalofananti-tankengagement,andwe

~eattemptingtohitatankcrossingdirectlyinfront,when~is9000,at10IJ;n/swithasubsonicmissilewhosespeedis250m/swefindthatthe'g',:1:equirementatimpactisonly19.The'g'requirementsforhomingmissiles,tendtoberatherless,buthereagain,fasttargets,fastmissilesandshortrangesarethefactorswhichresultinlargemissile'g'.

TheEffectsoftheMissileRole

Howthendotheseconsiderationsaffecttherequirementforaconsistentmissileresponsetoagivendemandforamanoeuvre?WehavealreadyseeninChapter4thatthemanoeuvrabilityofamissileforagivencontrolsurfacemovementis,otherthingsbeingequal,inverselyproportionaltothesizeofthestaticmargin.Inthecaseofananti-tankoranti-shipmissilethe'g'requirementsareverysmallsowecanaffordtohaveareasonablylargepositivestaticmargin,say5-10%ofthemissilelength.If,inaddition,wecanplacethepropellantapproximatelysymmetricallyaboutthemissilecentreofgravity,flymoreorlessatsealevel,withaconstantairdensity,andflyatconstantspeed,thenthemissilewillnotbeexercisedthroughalargerangeofincidence.Hencethepositionofboththeneutralpointandthecentreofgravitywillchangeverylittleandthereforethepercentagechangeinthestaticmarginwillbesmall.Conversely,toproduceaverymanoeuvrablemissilewemusthaveasmallstaticmarginandthenasmallchangeintheneutralpointpositionwillresultinalargepercentagechangeinstaticmargin.Ashasalreadybeenexplained,thepointatwhichtheliftactsontheliftingsurfaceofthemissilemovesastheflightMachnumberchangesandthereforethestaticmarginwillchange.Thiswillaffecttheconsistencyofresponseofthemissile.

If,inaddition,themissileisrequiredtooperateoverarangeofheights(e.g.surface-to-airandair-to-air)andthepropulsionsystemisoftheboost-coasttype,resultinginlargespeedchanges,theresponseofthemissiletoagivencontrolsurfacedeflectionwillbeextremelyvariable.Insuchacasethemissilewillhavearathermorecomplicatedcontrolsystemcalledanautopilot,whichisconsideredlater.

MissileControlConfiguration

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~erodynamicCartesianControl

'I

Manymissilesaresteeredusingaerodynamiccontrol.Foraerodynamic

(~bntrolyoumusthave'aero',i.e.airtopushagainst,and'dynamics',i.e.p.

~ufficlentsteed.

.."

~omeearlymissilesweresteeredusingmovingwings.InFigure6.3we'consideramovingwingconfiguration.Bythiswemeanthatthemainliftin~surfacesthemselvesaremovedinordertomanoeuvrethemissile;

therearefixedstabilisingsurfacesattherear.Inthisfigure,andalsoFigures6.5,Nreferstothenormalforceduetotheincidenceofthebody,wingsandcontrolsurfaces,thelatterbeingregardedasbeinginthecentralpositiqn.Ncistheextraforceduetothedeflectionofthecontrolsurfaces,xistherudderdeflection,bistheincidence,andUrnisthemissilevelocity.

Consideranextremecaseinwhichtheaerodynamiccentreofthewingscoincideswiththecentreofgravityofthemissile.Amovementofthiswingwillproduceanormalforcewhichwillaltertheflightpathofthemissilebutitwillexertnomomentonthebody.Hencethereisnobodyincidence,sowegetnoliftatallfromthebodyorstabilisingsurfaces.Thiswouldbeveryinefficientastheairframe'spotentialisnotbeingutilised.

ConsiderFigure6.4whichshowsaviewofthemediumrangesemi-activehomingmissileBloodhound.Thegeneralconfigurationwasconceivedintheearlynineteen-fiftiesandtworan\ietsmountedexternallyweretoformthesustainingpropulsionsystem.Theseair-breathingenginescaneasilygooutunlessthereisafreeunimpededairflowtotheairintakesoa

FIG.6.3Movingwings

constraintofamaximumofafewdegreesofbodyincidencewasimposed.Thismissilehasmovingwingsslightlyforwardofthecentreofgravitysothatsomeliftfromthebodyisgeneratedasaresultofawingdeflection.ItshouldalsobepointedoutthattomOvelargecontrolsurfacesneededlarge,heavyactuators.Also,ifthesewingshadbeenfixed,theywouldhavebeensupportedstructurallyalongthewholeoftherootchord.Iftheyaremovingtheyaresupportedbyashaftonlyandthismeansthatthecentresectionhastobemuchthicker.Hencetheyareheavierandthedragissignificantlyincreased.Seadart,designedlater,alsousesra~etpropulsionandhastheengineintegralwiththebody,withtheintakeatthefrontofthemissile.However,thismissilehasflownwithabodyincidenceof18?withouttheintakechokingsothatthisisnownotsomuchaproblemtodemandtheuseofmovingwingsanditissteeredbyusingrearrnntrnl.q

FiG.6.4Tt1eBloodhoundmissile

Manymodernmissilesaresteeredbyaerodynamiccontrolsusingfixedliftingsurfaces,andsmallcontrolsurfaces.Thesecontrolsurfaces,orfins,areplacedeitherforwardoftheliftingsurfaces(andcalled'canards')orplacedattherearofthemissile.

SomeoftheargumentsforandagainstcanardcontrolsandrearcontrolshavealreadybeenconsideredinChapter4.Figure6.5showssometypicalplanviewsofdifferentconfigurations.RefertoFigure6.5aandconsiderasteadyturnwherethereisnoresultantmomentonthemissile,otherwisetherewouldbeanangularacceleration.Iftheneutralpoint,thatisthepositionthroughwhichNacts,isatadistancefromthecentreofgravityequalto5%ofthemissilelengthandthedistanceofthecontrolsurfacefromthec.g.is40%ofthelengththen,inthiscaseN=8Ncandsincethesensesofthesetwofactorsareinopposition,thetotalnormalforceavail-ableformanoeuvringis7NC'Ifthereisasmallchangeinthepositionoftheneutralpointorcentreofgravitysothatthestaticmarginis10%ofthemissilelengththenN=4Ncandthetotaleffectivenormalforceis3Nc.

Rearcontrolsareverycommonbecauseofthecomparativeeaseofobtainingcontrolinroll,asalreadymentioned.Also,sinceitisdesirabletoobtainaslargeamomentaspossible,itfollowsthatcanardsmaynotbethebestchoiceforahomingmissile.Forexample,thehomingheadmustbeatthefrontofthemissileandthiswouldresultinashorterleverarmifcanardswereused.Similarlyashaped-chargearmour-piercingwarheadisefficientwhenplacedatthefrontofthemissile.Theuseofsuchawarheadusuallyrulesouttheuseofcanards.

Figure6.5bshowsatypeofconfigurationthathasbeenusedforasubsonicanti-tankmissile.Insubsonicflow,aflapimmediatelybehindaliftingsurfacewillaffecttheairflowallroundthatsurface,whereasinsupersonicflow,acontrolsurfacecanneveraffecttheairflowaheadofitself.A

disadvantageofthisarrangementisthattheservoscontrollingthecontrolsurfaceshavetobemorepowerfultoovercometheconsiderableaerodynamichingemovement.Intheconventionalarrangementtheaero-dynamiccentreofthecontrolsurface,asfaraspossible,liesontheaxisofrotation.

Figure6.5cshowsatypicalarrangementwithcanardcontrols.Ofcourse,ifonemovesthecontrolsurfacestothefront,theoverallneutralpointmovesforwardinordertoretainastablemissile,andthemainliftingsurfacesaremovedaftsomewhat.

Ifoneassumesastaticmarginof10%ofthelengthandacontrolsurfacelevelarmof40%ofthelengththenthetotalnormalforceisNc+N=Nc=5Nc.Thiscompareswith3Ncfor.themissileshowninFigure6.5awiththesamestaticmargin.Ifthestaticmarginis5%themissilelengthinbothconfigurations,thetotalnormalforcewithcanardsisNc+8Nc=9Ncandthiscompareswith7Ncforthemissilewithrearcontrols.Inaddition,ithasalreadybeenpointedoutinChapter4thatwithrearcontrolstheinitiallateralaccelerationisintheoppositesensetothatintended.Itisverydifficulttobespecificconcerningtheseeffectsonsystemefficiencyandaccuracyinthisshortandrathergeneraltreatmentbutitistheauthor'sopinionthat,providedthestaticmargincanbekeptdowntoabout5%orsoofthemissilelength,bothrearcontrolsandcanardscanbeefficientmethodsofmanoeuvringamissile;theadvantagesofcanardscaneasilybeexaggerated.

Beforeleavingthetopicofcanards,consideronceagainatypicallongthinmissileasshowninFigure6.5c.Thisshapereducesthedragandtheairframedesignerwill,ingeneral,aimforthesmallestbodydiameterhecan.Sometimesahomingmissilerequiresalargediameterhominghead;sometimesacertaintypeofwarheadrequiresaminimumdiametertobeeffective.Intheabsenceofsuchconstraintsthereisthisstrongargumentforalongthinmissile.Ifnowthepropulsionmotorissituatedroughlyinthemiddleofthemissile,ablastpipehastobeusedtocarrythehotgastotheexpansionnozzleattherear.Thiswillleaveverylittleroomfortheservodesignertoworkin.Ifrearcontrolsareused,thebodydiametermayhavetobeincreased.Severalmissiles,bothsupersonicandsubsonic,usethe-wholeoftherearhalforsoofthebodyforthepropulsionmotor(ormotors,ifthereareseparateboostsustainermotors).Asthepropellantburns,theoverallcentreofgravitymovesforward,butoftenonlytotheextentofabout10%ofthebodylength.Suchmissilesaregenerallydesignedwithasmallnegativestaticmargininthefullcondition.Iftheboostmotorisatthebackandburnsveryrapidly,themissilebecomesstablewithapositivestaticmarginbeforeitcanseriouslydeviatefromthedesiredflightpath.Suchastrategyisfeasibleonlyifthepropellantweightisnotalargeproportionofthetotalmissileweight.Inmoredifficultcases,

FIG6.5aRearcontrolsurfacessupersonic

I

Ne

FIG.6.5bRearcontrolsurfacessubsonic

'N

FiG.6.5cCanardcontrols

FIG.6.5Typesofmissileconfiguration

-Um

wecandesigninaratherlargerinitialnegativestaticmarginandemployautopilottoassistinoverallstability.Theobjectofallthisistomakethepropulsionsystemdesigner's.tasksimplerandmoreefficient.Itisapackagingconsideration.Weneedcanardcontrolstodothis.

AerodynamicPolarControl

Wehaveonemoreaspectofmissilecontroltoconsider.ThereaderwillhavenoticedthatBloodhound(Figure6.4)canhaveonlyonesetofwingsbecausetheramjetsareinthewayoftheotherpair.Letitbesaidatoncethatthisisanunusualarrangementandthegreatmajorityolmissileshaveacruciformcross-sectionwithtwopairsofwingsandtwopairsofcontrol

surfaces;theymanoeuvreequallywellup-downasleft-right.Indeed,theguidancesystemissuestwocommands,oneup-downandtheotherleft-rightandthesetwocommandsarefedtotheelevatorsandruddersrespectively.Ifthereisonlyonesetofcontrolsurfacesandwings,thecommandshavetobeissuednotincartesianform,butinpolarform.Ifthemissilehastomanoeuvretotheright,andwecallthiscommandx,andup,whichwecally,theninpolarformthecommandsareRand0suchthat:

R=VX2+y2andtan0=X/y

Themissilenowhastorollthroughanangle0andmanoeuvreoutwardsinthiscorrectrollorientation.Thecontrolsurfacesmustbeabletofunctionasaileronandelevators.Withtwosurfacesonlyhowcanthisbedone?Supposethe'R'commandneeds4?ofelevatorandweneedasmalladjustmentinrollangle,say1?ofaileron.Themethodistodemand5?fromonecontrolsurfacewhichis4?ofelevatorplus1?ofaileron,and3?fromtheother.Whentherollorientationiscorrecttheaileronsceasetofunctionandthecontrolsurfacedeflectionsreturneachto4?Theadditionorsubtractionofdemandsisdoneveryeasilyelectrically.

The"obviousquestionisthen,whynotalwaysusepolarcontrol,or'twistandsteer'asitisoftencalled?Afterall,thisishowanaeroplanemanoeuvres,exceptthatacivilaircraftissaidtobankratherthanroll.Usingtwowingsandcontrolsurfacesinsteadoffourleadstoareductioninweightanddrag.Suchashapeisfarmoreconvenienttocarryunderthewingsofanaircraftandwilltakeuplessspacewhencarriedintheverylimitedspacebetweendecksofaship.Theanswerisnotveryclear-cut.First,ifoneattemptstoimplementthe'R'and'0'commandssimultaneously,theresultingmanoeuvreiscomplexandimpure.Itismuchmoredifficulttomodelandpredictthesystembehaviourwiththistypeofguidance-controlphilosophy.Ifonedelaysth~executionofthe'R'commanduntilthe'0'commandisimplementedthereis,bydefinition,adelay;thesystemislessresponsive.Butthismaynotmattermuchwhenthetargetisstationaryorslowmoving.Alsothemissilemusthavesomeonboarddeviceforgivingaspacereferencefromwhichtomeasure0.Nevertheless,thesedaystheaerodynamicistsaresuggestingsomenovelshapesofmissilesforincreasedaerodynamicefficiencyandtheserequirepolarcontrol;wemightwellseemoresystemsusingthisformofcontrolinthefuture.

ThrustVectorControl

Thrustvectorcontrol(TVC)isaformofrearcontrol.Onealtersthedirectionofthrusttosteerthemissile.IfthemagnitudeofthethrustisTanditsdirectionisdeflectedbyanangle0thentheaxialorthrustforceis

Tcos0andtheforceperpendiculartothebodyisTsin0.Itisthelat1forcewhichappliesamomenttothemissile.

FIG.6.6Forcesonamissileduetoathrustdeflectionof0

Inwhatcircumstances,then,isthrustvectorcontrollikelytobeused?Thesimpleansweris,whenyoucannotuseaerodynamiccontrol;i.e.whenthereisno,orinsufficient,airflowingacrosstheaerodynamiccontrolsurfaces.Firstly,allverylongrangemissileshavetobelaunchedgentlytoavoiddestroyingtheairframe,theweightofwhichhastobecuttoanabsoluteminimum.Theyalsohavetobelaunchedvertically.Itisimpossibleinpracticetoensurethatthemotorthrustlinepassespreciselythroughthecentreofgravity;asmalloffsetwillcausethemissiletoturn.Gyroscopesareusedtodetectanydeviationfromthevertical,butitwouldbeuselesstotrytocorrectthisintheearlystagesbymovingaerodynamicsurfaces,Hencealllongrangemissilesusethrustvectorcontroltomaintainthecorrecttrajectory.

Secondly,anumberofshortrangeair-to-airmissilesusethrustvectorcontrol.Suchmissilesneedextremelyhighmanoeuvrabilitytobeabletohitfasttargetsatshortranges,especiallyifthetargetiscrossing.Itisnotapracticalpropositiontofittrainablelaunchersunderthewingsofaircraft,and,initially,themissilemayevenbepointingbehindthetarget.Onejustcannotwaitforthemissiletoattainfullspeedbeforefullmanoeuvrabilityisobtained.

Thethoughtfulreadermaynowseeanapparentflawintheargument.Havewenotsaidthattheobjectofbothrearandcanardcontrolsistoapplyamomenttothemissileinordertoobtainsomeincidencesothatsomenormalaerodynamicforcemaybegeneratedbythewhol~airframe?Ifthemissilevelocityislow,arewenotstillineffective?Ifaerodynamicforcesaretheonlyforcesactingonthemissiletheansweris'yes'.Whenamissileisbeingboostedlongitudinallyatsay5or109'andthethrustisdeflectedbyafewdegrees,themissilebodycanbeturnedthroughalargeanglesay45?ormore.inafractionofasecond.If,however,therearene~ligible

aerodynamicforcesandthebodyisnotturned,theboostmotorwillpropelthemissiledirectlyahead.Ifthebodyisturnedthroughsay45?orwhatevertheboostmotorwilldirectthemissileinthisnewdirection.Indeed,atalltimeswhenthethrustisgreaterthanthedrag,onecanachievesomemanoeuvrabilityfromthepropulsionsystem.Putinanotherway,ifweareboostingat10gandthebodyisturnedthrough45?andsincethecosineandsineof45?arebothequalto0.7approximately,thenthecombinedeffectisequivalenttoboostingintheoriginallaunchdirectionat7gwithanormalaccelerationof7galso.Anyaerodynamicnormalforcecannowberegardedasabonus.Manyshortrangeair-to-airmissilesusethrustvectorcontroltogetherwithanaugmentedboostmotorsothatthemissilecanliterallybepointedalmostanywhere.

AnotherexampleoftheuseofthrustvectorcontrolistheBritishanti-tankmissileSwingfire.Thissystemallowstheoperatortocontrolthemissilewhenthelauncherissituatedupto100metresawayandhiddenorcamouflaged.Thesystemusesline-of-sightguidance.Theuseofaero-dynamiccontrolswouldmeanthatitwouldtakealongtimetobringthemissileintotheoperator'sfieldofviewandhenceresult,inanunacceptablylongminimumoperatingrange.Andfinally,theconceptofverticallaunchevenforshortrangemissilesisbeingactivelypursuedbymanycountries,especiallyforshipbornemissiles;insuchacase,theycanbestoredandfiredfrombelowdeckwithanincreasedrateoffireandwithoutthenecessityofheavytrainablelaunchers.Thrustvectorcontrolmakesitpossibleforthemissiletobeturnedthroughanyangleefficientlyandquickly.

Wenowconsidersomeofthemethodscommonlyused.Allverylongrangemissilesuseliquidpropellantswhicharefedunderpressuretoacombustionchamberthroughflexiblepipes.Thecombustionchamberismountedonaset.ofgimbalsandrotatedbyservos.Figure6.7aindicatesthegeneralmethod.

However,thegreatmajorityofmissilesusesolidpropellantandthiscannotbefedthroughaflexiblepipe!Now,tomaintainthepropulsiveefficiencyofa.convergent-divergentnozzle,thesmoothinternaloutlinemustbemaintained,otherwiseshockwavesaresetupwithanincreaseinpressureandalossofeffluxmomentum.Hencethereisnoproblemwithagimballedcombustionchamber,butwithsolidpropellantsthingsarenotsosimple.TheballandsocketnozzleshowninFigure6.7bmaintainsashockfreegasflowoveradeflectionangleofabout.i:15?Thedisadvantageofthismethodisthataratherheavynozzle,plusthefrictionofthesealsrequiresfairlypowerfulservos.Figure6.7cshowstheendviewofaspoilersystem.Intheunreflectedstateallfourspoilerslieoutsidethegasflow.Movementofoneormoreofthespoilerswilldeflectthedirectionofthethrustattheexpenseofsettingupshockwavesandreducingthethrustby

about1%perdegreeofthrustdeflection.Despitethisdisadvantage,themethodhassmallmovingpartsandthereforesmallservor.

Ifonewishestocontroltherollmotionofthemissileaswell,amorecomplicatedsystemisrequired.OneofthemanytypesofthrustvectorcontrolpackagejointlydevelopedbyIMISummerfieldandSperryGyroscopeisshowninFigure6.7d.Itwasdevelopedforamotordeveloping20,000Newtonsthrustandusestwinnozzleswhichcanbeindependentlymovedintwoplanes,givingcontrolinrollpitchandyaw.Ingeneralithasbeenfoundthat:f:15?nozzledeflection,coupledwithaseparationdistanceofapproximatelyonethirdthemissilediameter,issufficienttocaterforallrollmovementfromahighpressureheliumbottle.ThereaderwillhaveiealisedthatTVCisinoperativeaftermotorburn-out.Inthisarrangement,theservosaresufficientlypowerfultoenableaerodynamiccontrolsurfacesalsotobedirect-coupledduringanycoa.stingphase.

MissileActuators~

Tomovethecontrolsurfaces,jetavatorsorspoilersofamissile,sometypeofactuatorisrequired.Theseactuatorsmayneedtoproduceconsiderableamountsofpowertomovethefinormissilewhenitisflyingsupersonically.Actuatorscaneitherbelinearorrotary,dependingontheapplicationandthespaceavailablewithinthemissile,and,currently,canbeelectrical,hydraulicorpneumatic.

Hydraulicmotors,orrams,usetheenergystoredinpressurisedoilwhichiscontrolledbyaspoolvalve,themovementofwhichisdeterminedbytheerrorsignalinthefincontrolsystem.Hydraulicactuatorshavethefastestresponse(highestbandwidth),oftheactuatorastheoilisincompressible,andalsothelargestpowertoweightratio.Theyalsotendtobethemostexpensive.

Pneumaticactuatorsusetheenergystoredineithercoldgas(pressurisedair),orhotgas,usuallyfromburningsomeformofCorditecharge.Coldgashasthedisadvantageofhavingtocarrywithinthemissiletheheavycontainerinwhichtheairisstored.Ingeneralhotgastakesuplessspace,asthegasisburntinthemissilewhenitisneeded.Itdoes,however,tendtoburnuptheactuatorandthereforecanonlybeusedonce,andforshorttimeofflightmissiles,andprecludesfulltesting.Unlikehydraulics,hotandcoldgasesarecompressibleandthismaycausestabilityproblemsinthefinmovementandtheyarenotasfastashydraulics.

Electricactuators,eitheracordc,aremorelikelytobeusedinthefutureforsmallfinactuators,withthedevelopmentanduseofrareearthmagneticmaterials;Thesemotorsdevelopgreaterpowerforagivensizeofmotorthantheconventionalelectricmotorsofthelastgeneration.Theyalsohavetheadvanta~ethattheycaneasilybetestedinthemissile.

MissileAutopilots

RollAutopilots

Everysystemengineerhastomakethedecisioneithertorollpositioncontrolthemissile,thatisstopitrolling,ortoallowittorollfreely.However,thereisnochoiceifpolarcontrolisused,asinthiscasewehavetodemandacertainrollpositionandthenchangethispositionfromtimetotimeastheengagementproceeds.AshasbeenindicatedinChapter4,thegreatmajorityofmissileshaveasymmetricalcruciformconfigurationsothattheycanmanoeuvrequicklyandaccuratelyinanydirection.Thereisnostronglystablepositioninrollaswithaircraft.

Hence,evenatsubsonicspeeds,thereisatendencytoroll,duetosmallunavoidableairframemisalignmentsandfinbiases.Insupersonicflighttherecanbeappreciableasymmetricpressuresonliftingsurfacesiftheincidenceinpitchandyawarenotequal;thiscanresultinlargeinducedrollingmoments.

Inahomingmissilethereisapparentlynoproblem.Imaginethehomingheadsensesthetargettoohighandasaresultitsendsanupcommandtotheelevators.Ifthetargetisstillhighandthemissilenowrollsclockwisethrough90?say,itwillseethetargettotheleft,soitwillsendacommandtotherudders;buttheruddersalsohaverolledwiththemissileandthehomingheadandarenowreallyelevators.Sowithahomingmissileachangeinrollpositiondoesnotcreatealanguageproblemastowhatis'up'andwhatis'across'.However,therearedangers.

Ahomingmissiletendstoheadoffthetargetbyestablishingaleadangle,sothehominghead,althoughpointingatthetarget,isatanangletothemissilebody.Anysuddenmissilerollmaydisturbthehomingheadtosuchanextentthatitmaylosesightofthetarget.Henceinpracticethemajorityofhomingmissilesarerollpositionstabilised.Althoughthehomingheadservosaredesignedtokeeptheheadpointingatthetarget,itisdifficulttofitreallypowerfulfastactingactuatorswhennearlyallthespaceistakenupbythehomingheaditself.

Inline-of-sightsystemswiththemissileandtargettrackersaccuratelylevelledandontheground,thereisaproblemifthemissileisallowedtorotateabouttheforeandaftaxis.Theguidancesystemisinnodoubtaboutthemeaningofup-downandleft-right.Ifthemissiledoesnotrolla