b2-13h instruments gyroscopic sr

7/25/2019 B2-13h Instruments Gyroscopic SR

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StudentResource

SubjectB2-13h

InstrumentsGyroscopic

Copyright 2008 Aviation Australia

Allrightsreserved.Nopartofthisdocumentmaybereproduced,transferred,sold,orotherwisedisposedof,withoutthewrittenpermissionofAviationAustralia.


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B2-13h Instruments Gyroscopic

CONTENTS

Topic

Definitions ii

Studyresources iii

Introduction v

Gyroscopes 13.8.2.1

ArtificialHorizons 13.8.2.2.1

SlipIndicators 13.8.2.2.2

DirectionalGyros 13.8.2.2.3


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Tosetforthinwords;declare.

DEFINITIONS

Define

Todescribethenatureorbasicqualitiesof.

Tostatetheprecisemeaningof(awordorsenseofaword).

State

Specifyinwordsorwriting.

Identify

Toestablishtheidentityof.

List

Itemise.

Describe

Representinwordsenablinghearerorreadertoformanideaofanobjectorprocess.

Totellthefacts,details,orparticularsofsomethingverballyorinwriting.

Explain

Makeknownindetail.

Offerreasonforcauseandeffect.


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STUDY RESOURCES

E.H.J.Pallett,AircraftInstruments&IntegratedSystems,Chapter4

JeppesenAircraftInstrumentsandAvionicspp2941

AvionicsFundamentals,IAPInc.Chapter5

B2-13hStudentHandout


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INTRODUCTION

ThepurposeofthissubjectistoallowyoutogainknowledgeofAircraftSystems,Instruments

Gyroscopic.Oncompletionofthefollowingtopicsyouwillbeableto:

Topic 13 8 2 1 Gyroscopic Terminology and Characteristics

DefineGyroscopicrelatedterms.

Explainthefollowing:

Earthrateandcalculateitforvariouspositionsontheearthinrespecttoapparentprecession

Rigidityandlistthefactorswhichaffectit

2and3gimballedgyroscopelayouts Gimballockcondition;

methodsofavoidingand

rectifyingagyrointhiscondition

Realdriftandapparentdriftandlistthefactorswhichaffectthem

Free,tied,earthandrategyros.

Describegyroscopicprecessionanddeterminethedirectionofprecessionresultingfromanappliedforce.

Identifythefollowinggyroscopicinstrumentsystems,statetheirpurposeandexplaintheiroperation:

ArtificialHorizons

SlipIndicators

DirectionalGyros

Describeprecautionsinvolvedwithgyroscopicinstruments/components.

13 8 2 2 1 Gyroscopic Instrument Systems: Artificial Horizons

Identifythefollowinggyroscopicinstrumentsystem,statetheirpurposeandexplaintheiroperation:

ArtificialHorizons;

13 8 2 2 2 Gyroscopic Instrument Systems: Slip Indicators


SlipIndicators(Turn&BankIndicators)and

13 8 2 2 3 Gyroscopic Instrument Systems: Directional Gyros


DirectionalGyros.


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TOPIC 13.8.2.1: GYROSCOPES

Newtons 1st Law of Motion

Inertia

Anobjectinmotionwillremaininmotionandanobjectatrestwillremainatrestunlessactedonbyanunbalanced force.Thismeansthatif therewerenofriction,egspace,youcouldthrow/pushsomethinganditwillcontinueatthatsamespeedforevermore.Inrealityintheatmosphereoftheearthwehaveplenty offrictionfromairandgravitywhichprovides theadditionalforcetoopposetheinitialmotionimpartedbyyou.Buttheconceptisthatamovingmasswillcontinuetomoveinthesamedirectionunlesssomeotherforceactsuponit.

Whenarotorismadetospinathighspeedthedevicebecomesagyroscopepossessingtwoimportantfundamentalproperties:

Gyroscopic Rigidity or Gyroscopic Inertia:

causedbytheinertiaofthemass,keepingtheaxisrigidorpointinginthesamedirection.

Gyroscopic Precession:

describestheapplicationofaforcetothegyroandtheeffectoftheangulardisplacement.

Boththesepropertiesdependontheprincipleofconservationofangularmomentum,whichmeansthattheangularmomentumofabodyaboutagivenpointremainsconstantunlesssomeforceisappliedtochangeit.Angularmomentumistheproductofthemomentofinertia(I)andangularvelocity(w) ofa bodyreferredtoagivenpointthecentreofgravityin thecaseofagyroscope.

Theseratherintriguingpropertiescanbeexhibitedbyanysysteminwhicharotatingmassis

involved.Althoughitwasleftformantodevelopgyroscopesandassociateddevices,itistruetosaythatgyroscopicpropertiesareasoldastheearthitself:ittoorotatesathighspeedand


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Sopossessesrigidity,andalthoughithasnogimbalsystemorframeonwhichexternalforcescanact,itcan,anddoes,precess.Thereare,however,manymechanicalexamplesarounduseverydayandoneofthem,thebicycle,affordsaverysimplemeansofdemonstration.Ifweliftthefrontwheelofftheground,spinitathighspeed,andthenturnthe

handlebars,wefeelrigidityresistingusandwefeelprecessiontryingtotwistthehandlebarsoutofourgrasp.Theflywheelofamotor-carengineisanotherexample.Itsspinaxisisinthedirectionofmotionofthecar,butwhenturningacorneritsrigidityresiststheturningforcessetup,andasthisresistancealwaysresultsinprecession,thereisatendencyforthefrontofthecartomoveupordowndependingonthedirectionoftheturn.Otherfamiliarexamplesareaircraftpropellers,compressorandturbineassembliesofjetengines;gyroscopicpropertiesareexhibitedbyallofthem.

Elements of the Gyroscope

Therotorisaperfectlybalancedmass,mountedonacentralshaft.

Gyro Rotor Construction

Thegyrowheelorrotorunitmustbeperfectlysymmetricalandcircularaboutthespinaxis.Anyothershapewouldcausean imbalanceduringrotation.Togainhighermomentumandthereforestability,theweightisnormallyconcentratedontherim.Toomuchweightcausesexcessivebearingfrictionandconsequentlydrift,soa compromisemustbemadebetweenmomentumandfriction.Becauseinertiadependsuponthesquareoftheradius,therotorsaremadeaslargeaspossiblewiththegreatestmassconcentratedattherim.

Gyroscopic Balance

Thegyroscopemustbeperfectlybalancedtoreducethevibrationfeltduringthehighspeedsatwhichtheyarerotated.Thereforetheyarebothstatically anddynamically balanced.


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Static Balance: tobestaticallybalanced,thecentreofgravitymustbeactinguponthespinaxis.

Dynamic Balance: to be dynamically balanced, the plane of spin must be acting at rightanglestotheaxisofspin.

Constructionoftherotorwilldirectlyaffecttherigidityofthegyro.Theheaviertherotorisandthecloser totheoutsiderim that theweightcanbedistributedwillcontributetothegyrosrigidity.

Weaddaframewithbearingsandwehavecreatedthefirstaxisofspin.Thisframewillsoonbecomeourinnergimbalbutuntoitispivoteditselfweonlyhaveasingleaxisofspin.

Gyroscopic Rigidity or Gyroscopic Inertia:causedbytheinertiaofthemass,keepingtheaxisrigidorpointinginthesamedirection.

Gyroscopic Precession:describestheapplicationofaforcetothegyroandtheeffectoftheangulardisplacement.

Boththesepropertiesdependontheprincipleofconservationofangularmomentum,whichmeansthattheangularmomentumofabodyaboutagivenpointremainsconstantunlesssomeforceisappliedtochangeit.Angularmomentumistheproductofthemomentofinertia(I)andangularvelocity(w) ofa bodyreferredtoagivenpointthecentreofgravityin thecaseofagyroscope.

Theseratherintriguingpropertiescanbeexhibitedbyanysysteminwhicharotatingmassisinvolved.Althoughitwasleftformantodevelopgyroscopesandassociateddevices,itistruetosaythatgyroscopicpropertiesareasoldastheearthitself:Ittoorotatesathighspeedandsopossessesrigidity,andalthoughithasnogimbalsystemorframeonwhichexternalforcescanact,itcan,anddoes,precess.Thereare,however,manymechanicalexamplesarounduseverydayandoneofthem,thebicycle,affordsaverysimplemeansofdemonstration.Ifweliftthefrontwheelofftheground,spinitathighspeed,andthenturnthehandlebars,wefeelrigidityresistingusandwefeelprecessiontryingtotwistthehandlebarsoutofourgrasp.The flywheel ofamotor-car engine isanother example. Its spinaxis is inthe direction ofmotionofthecar,butwhenturningacorneritsrigidityresiststheturningforcessetup,andasthisresistancealwaysresultsinprecession,thereisatendencyforthefrontofthecartomoveupordowndependingonthedirectionoftheturn.Otherfamiliarexamplesareaircraftpropellers, compressor and turbine assemblies of jet engines; gyroscopic properties areexhibitedbyallofthem.


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Asamechanicaldeviceagyroscopemaybedefinedasasystemcontainingaheavymetal

wheel,orrotor,universallymountedsothatithasthreedegreesoffreedom:

Spinningfreedomaboutanaxisperpendicularthroughitscentre(axisofspinXX 1){thismeansthelinefromXtoX1}

Tiltingfreedomaboutahorizontalaxisatrightanglestothespinaxis(axisoftiltYY1)

Veeringfreedomaboutaverticalaxisperpendiculartoboththespinandtiltaxes(axisofveerZZ1).

Axes of Freedom

Engineershaveusedmanyandvariouswaysofdescribingthemountingandaxisreferencesofthegyroscope.Athree frame gyro wassaidtohave three degrees of freedom whichwere

namely: spinningfreedom,whichenabledagyroscopesrotortospin.

tilting freedom, where the gyro case or inner gimbal was free to rotate about thehorizontalplane,atrightanglestothespinaxis.

veeringfreedom,wheretheoutergimbalwasfreetorotateabouttheverticalplane,whichisperpendiculartoboththespinandtiltaxes.

Theoutergimbalissupportedintheframeorcaseofthegyrosystem.Themoderntechnicalterminologyusedtoexpressthedegreesoffreedomofgyroscopestendstowardsacceptingas fact, that a gyromust spin toshow the gyroscopic properties.Therefore, a two framegyroscope has only one degree of freedom, while the three frame gyroscope has two

degrees of freedom.The three degrees of freedom are obtained by mounting the rotor in two concentricallypivoted rings, called inner and outer gimbal rings. The whole assembly is known as the


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gimbalsystemofafreeorspacegyroscope.Thegimbalsystemismountedinaframe,sothatinitsnormaloperatingposition,alltheaxesaremutuallyatrightanglestooneanotherandintersectatthecentreofgravityoftherotor.

Thegyromustbe universallymountedor ingimbalssoastomaintainthetwodegreesoffreedomrequired,thatisverticalandhorizontal(inthisexplanationthespinaxisof freedomisignoredalthoughthetextreferstotwodegreesoffreedom,itmeansfullfreedomofspin,tilt&veer).Theconstructionofthegyrodeterminestheshapeandformofthegimbalswhichinturndependsonhowthegyrowillbeusedandinwhichplaneitwillberequiredtosensemovement.

Gimbalspermitthegyroframe(oranaircraft)tomovearoundthegyrowhileitmaintainsitsoriginalattitudeanddirectionofspinaxis.

Planeofspindoesnotrequireagimbalasthisplaneissimplythefreedomoftherotortospinonitsaxis.Agyrocannotdetectmovementaboutitsplaneofspin,egaDGcannotdetectpitchandanAHcannotdetectyaw.

Eachothergyroaxisrequiresagimbaltoprovideitwithfreedom.

Only1gimbalonlypermitsfreedominonly1axis(inadditiontoplaneofrotationexplainedabove).Asecondgimbalisrequiredtoprovidefreedominbothaxissoftiltandveer.

Wecanlimitthegimbals toouradvantage inmeasuring things,ega rategyroonlyhas1

gimbal,butthatwillbecoveredindepthlater.


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Gyroscopic Inertia or Rigidity

Thepropertyofrigidityofthegyroscopeisitsabilitytoresistanyforcewhichtendstochange

theplaneof rotationof its rotor. Thismeansthat ifa forceis applied totry andmove thegyroscope to another position the rotors axis of spin will try and remain in the constantdirection inspace. This property is the result of its high angular velocity, and the kineticenergypossessedintherotor.Thegyroscopicinertiaorrigiditycanbeincreasedby:

increasingthemassoftherotor

increasingtherotorspeed

concentratingmoremassneartherimoftherotor.Thisiscalledincreasingtheradiusofgyration.


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Precession.Theangularchangeindirectionoftheplaneofrotationundertheinfluenceofanappliedforce.Thechangeindirectiontakesplace,notinlinewiththeappliedforce,butalwaysatapoint90awayinthedirectionofrotation.

Therateofprecessionalsodependsonthreefactors:

Strengthanddirectionoftheappliedforce

Momentofinertiaoftherotor(rigidityofrotor-weight)

Angularvelocityoftherotor(Rigidityofrotorspeed)

Thegreatertheforce,thegreateris therateofprecession,whilethegreaterthemomentofinertiaand the greater the angular velocity, the smaller is the rateofprecession. (greater

rigiditysmallerrateofprecessionforequalamountofappliedforce)

Precessionofarotorwillcontinue,whiletheforceisapplied,untiltheplaneofrotationisinlinewiththeplaneoftheappliedforceanduntilthedirectionsofrotationandappliedforcearecoincident.Atthispoint,sincetheappliedforcewillnolongertendtodisturbtheplaneofrotation,therewillbenofurtherresistancetotheforceandprecessionwillcease.gyrowilleventually gimbal lock or topple if unrestrained a rate gyro functions on the basis ofprecession, but the gyro rotor is restrained by springs so does not gimbal lock rotorcontinuestoprecessagainstspringpressurewhilstturningforceisdetectedbygyrorotormoreonrategyroslater.

Theaxisaboutwhichatorqueisappliedistermedtheinputaxis,andtheoneaboutwhichprecessiontakesplaceintermedtheoutputaxis.


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Theproperty ofprecessiononlybecomesapparentwhenanexternal forceis appliedto aspinningmass.Thiswillcausetheplaneofrotationtochangedirection.Thechangetakesplace,notatthedirectionoftheappliedforce,butatapoint90awayinthedirectionofgyrorotation.Therateofprecessionalsodependsuponthreefactors:

Strengthanddirectionoftheappliedforce

Rigidityoftherotor;(massoftherotor,whereitisconcentratedandspeed)

Thegreatertheforce,thegreateristherateofprecession,whilethegreatertherigidityoftherotor,thesmalleristherateofprecession.

Sinceprecessionistheangularchangeinpositionoftheplaneofrotation(spin)thatoccurswhen the applied force exceeds the rotational force, the direction of the precessionalmovementisdependentuponthedirectionoftheappliedforceandthedirectionofthegyrorotation.


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Unavoidable precession is caused by aircraftmaneuvering and by the internal friction ofattitudeanddirectionalgyros.Thiscausesslow"drifting"andthuserroneousreadings.Whendeflectiveforcesaretoostrongorareappliedveryrapidly,mostoldergyrorotorstoppleover,rather thanmerely precess.This is called "tumbling" or "spilling" the gyro and should be

avoidedbecauseitdamagesbearingsandrenderstheinstrumentuselessuntilthegyroiserectedagain.Someof theoldergyroshavecagingdevicesto hold thegimbals inplace.Eventhoughcagingcausesgreaterthannormalwear,oldergyrosshouldbecagedduringaerobaticmaneuverstoavoiddamagetotheinstrument.Thegyromaybeerectedorresetbyacagingknob.Manygyroinstrumentsmanufacturedtodayhavehigherattitudelimitationsthantheoldertypes.Theseinstrumentsdonot"tumble"whenthegyrolimitsareexceeded,but,however,donot reflectpitchattitudebeyond85degreesnoseupornosedown fromlevelflight.Beyondtheselimitsthenewergyrosgiveincorrectreadings.Thesegyroshaveaself-erectingmechanismthateliminatestheneedforcaging.

Gimbal lock is normally prevented by limiting the movement of the inner gimbal withmechanical stops as shown on the slide. A mechanical stop applied to prevent gimballocking. This physically prevents the inner gimbal and the outer gimbal from becomingaligned.Ifthegimbalsdoreachthesestops,theforcesactingonthegimbalsystemcause

thesystemtoprecessrandomlyandtopple.


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Free or Space Gyros

Anunrestricted,un-referenceddisplacementgyro is calleda spacegyro.Thesearegyrosthathavecompletefreedomaboutthreeaxiswhichareallactingatrightanglestoeachother

(spin,tilt,andveer).Thisenablesthegyrotomaintainitspositionrelativetosomepointinspace for an indefinite time assuming that thereare no bearing imperfectionsorexternalforcessuchasmagneticfieldsorgravity.

Typical gyro training aids and gyro toys are space gyros. They are not referenced toanything,notevengravity.Ifyouweretositandwatchaperfectlybalancedandfrictionlessspacegyro,itwillappeartorotateordriftawayfromtheperpendicular,butinrealitytherotoris remaining rigidly fixed inspace, and as the earthrotates, the framerotatesaround therotor,appearingtothevieweronearthasthoughthegyroisrotating.

Obviouslyanun-referencedspacegyroisofnouseinanaircraft.Forastartiftheaircraftweresittingstillonthegroundthegyrowouldbedriftingoffatarateof15perhourduetotheearthsrotation.Agyroinanaircraftmustbereferencedtothehorizon,ortheearth.Soa

spacegyromustbecontrolledtoremainrigid,butwithrespecttothecentreoftheearth,thisisusuallyachievedbyusinggravityasareferencetomaintainthegyroerect&referencedtothecentreoftheearth.

Free or Space Gyros

Thesearegyrosthathave completefreedomabout threeaxiswhichareall actingatrightangles to each other (spin, tilt, and veer). This enables the gyro to maintain its positionrelative tosome point in space for an indefinite time assuming that thereare nobearingimperfectionsorexternalforcessuchasmagneticfieldsorgravity.


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Tied Gyros

Aspacegyrowouldbeofnousewhateverinestablishingtheneededverticalandhorizontalreferences required for safe instrument flight.Weneed to havegyros that have freedom

about threeaxis at rightangles toeachother, but whichare controlledbysomeexternalforce,sotheyassumeanattitudeinrespecttosomegivenpoint.Bytyingthegyrospinaxistosomefixedpoint,wecreatewhatiscalledatiedgyro.Thedirectionalgyroindicatorusesatiedgyroscope.Thegyrospinaxisbeingtiedhorizontalinrespecttotheindicatorcaseandassuch isparallel to the aircraft lateral and longitudinal axes.This type ofapplication iscalledacasetiedgyro.

Earth Gyros

Thesearetiedgyroswhosespinaxisismaintainedinaverticalpositionwithrespecttotheearthssurfacebyagravitationaldevice.Thesetypesofgyroscopesarecalledverticalgyrosandisthebasicelementofthegyrohorizonorartificialhorizonindicator.

Rate Gyros

Thesearetiedgyros,whicharetiedtoaparticularreferencepointbysprings,creatingagyrowhichhasonlyonedegreeoffreedom.Itisconstructedsoastoindicaterateofmovementaboutaplaneatrightanglestoboththeplaneofrotationandtheplaneoffreedomwhichinthiscaseisthetiltingplane.Thegyroscopeisspring-restrictedaboutthetiltaxissothatwhentheunitisturnedabouttheverticalaxis,theamountofdisplacementduetoprecessionisameasureoftherateofturn.

References Established by Gyroscopes

Theapplicationofthegyroscopeintoaircraftsystemsistoprovidetwoessentialreferencedatums:

vertical flight reference

Againstwhichaircraft attitude changesarenotedandused to indicateboth pitchand rollfunctionsoftheaircraft.

directional flight reference

Againstwhich aircraft heading changesare noted and used to indicatemovement of theaircraftabouttheverticalaxis.

These referencesareestablishedbygyroscopeshaving their spin axisarrangedverticallyand horizontally. The vertical gyro is the sense element of all attitude gyros, whilst thehorizontal gyro is the sense element of the directional gyro providing aircraft headinginformation.

Fromtheabovedescription,itcanbeseenthatthethreegyrocontrolledflightinstrumentsusedbythepilotarealltiedgyros.Ineachcasehowever,adifferentmethodisusedtotiethegyroinstrumenttoitsappropriatereferencepoint.

Earth Gyro

Before a free gyroscope can be ofpractical use asanattitude reference inaircraft flightinstrumentsandotherassociatednavigationalequipment,driftandtransportwandermustbecontrolledsothatthegyroscopesplaneofspinismaintainedrelativetotheearth;inotherwords,itrequiresconversiontowhatistermedanearthgyroscope.

ASpaceGyroreferencedtoearthis thentermedanEarthgyro.AnySpacegyroreferenced

toaparameterisreferredtoasatiedgyro,soanEarthGyro(tiedtocentreoftheearth)isaformofTiedgyro.


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Rate Gyros

The difference between a displacement gyro, and that provided by a rate gyro: where adisplacementgyroutilisesagyrospropertyof rigidity inspace andmeasuredisplacement

around it, a rategyrorelies ona gyrobeingsubjected toprecessiveforcesagainstspringpressuretodeterminerateofmovement.Thehighertherateofmovementthegreatertheinertial force applied to the gyro resulting in precession. The higher the rate of turn, thegreatertheprecessiveforce,thegreaterthemovementagainstspringpressure.


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Displacement Gyros

Aircraftinflightarestillverymuchapartoftheearth,i.e.allreferencesmustbewithrespect

totheearthssurface.Thefreeorspacegyroscopewehavebeenreferringtoinpresentinggyrotheorywouldservenousefulpurposeinanaircraftandwouldhavetobecorrectedfordriftwithrespecttotheearthsrotation,calledapparentdrift,andforwanderasaresultoftransportingthegyroscopefromonepointontheearthtoanother,calledtransportwander.

Itwillalsobenotedthatthepitch,roll,anddirectionalattitudesoftheaircraftaredeterminedbyitsdisplacementwithrespecttoeachappropriategyroscope.Forthisreason,therefore,the gyroscopesare referred toasdisplacement typegyroscopes. Eachonehas the threedegrees of freedom, and consequently three mutual axes, but for the purpose ofattitudesensing,thespinaxisofthegyroisdiscountedsincenousefulattitudereferenceisprovidedwhendisplacementstakeplaceaboutthespinaxisalone(displacementaroundaxisofspin

isnotdetected).Thus,inthepracticalcase,vertical-axisandhorizontal-axisgyroscopesarefurtherclassifiedastwo-axisdisplacementgyroscopes.


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Gyroscope Applications in Aircraft

Foruseinaircraft,gyroscopesmustestablishtwoessentialreferencedatums:

Referenceagainstwhichpitchandrollattitudechangesmaybedetected

Directionalreferenceagainstwhichchangesabouttheverticalaxismaybedetected

Thesereferencesareestablishedbygyroscopeshavingtheirspinaxesarrangedverticallyandhorizontallyrespectively.Bothtypesofgyroscopeutilisethefundamentalpropertiesinthefollowingmanner:

Rigidityestablishesastabilisedreferenceunaffectedbymovementofthesupportingbody

Precessioncontrolstheeffectsofapparentandrealdriftthusmaintainingstabilisedreferencedatums(erectionsystemstoreferencetoearth).

Foruseinaircraft,gyroscopesmustestablishtwoessentialreferencedatums:areferenceagainstwhichpitchandrollattitudechangesmaybedetected,andadirectionalreferenceagainst which changes about the vertical axis may be detected. These references areestablished by gyroscopes having their spin axes arranged vertically and horizontallyrespectively.

Both typesofgyroscopeutilizethe fundamentalpropertiesin thefollowingmanner:Rigidityestablishes a stabilized reference unaffected by movement of the supporting body, andprecessioncontrolstheeffectsofapparentandrealdriftthusmaintainingstabilisedreference

datums.


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Tocompensateaspacegyrotoeliminateearthrate,attheequatorwecouldprecessitat15perhour,sothatitwillcompletelyrotateevery24hours(sameastheearthsrotation)thusappearing to remain erect with respect to earth. If the gyro is not at the equator, theprecessionvaluecanstillbeeasilycalculatedbecauseapparentdriftequals15sin(where

equalsangleoflatitude).Canbeachievedbyelectricaltorquingsignals,orbyunbalancinggimbalstocausegyrotodriftatdesiredrate.

Controlofdriftwhich,relatesonlytohorizontal-axisgyroscopesandcanbeachievedeitherby:

calculatingcorrectionsusingtheearth-rate formulagivenin theprecedingtableandapplyingthemasappropriate;e.g.tothereadingsofadirectionindicator:

applyingfixedtorqueswhichunbalancethegyroscopeandcauseittoprecessatarateequalandoppositetotheearthratewe,

applying torqueshaving a similar effect to that stated inabove, but which can bevariedaccordingtothelatitude.

Agyrocorrectedforearthrateorapparentdriftwillmaintainsitsattitudewithreferencetotheearth,itwillcontinuetopointtothecentreoftheearthevenastheearthrotates.

This is the name given to the apparent drift which becomes evident in the directionalgyroscopeduetotheearthsrotation.Itisacombinationofbothapparenttiltandapparentveer.Apparentprecessionoccursatarateof15degreesperhourxsineofthelatitudeinwhich the gyro is operating. Apparent drift compensation is carried out by causing thegyroscopetobeprecessedintheoppositedirectiontotheearthsrotation.Thisisachievedbyplacingweightsinthespinaxisofthegyrorotortounbalancetheunitsothattheweightforcecauses the gyro toprecess.The rate ofprecession isdeterminedby the latitude inwhichthegyroisbeingoperated.

Real drift results from imperfections in themanufactureof the gyroscope suchasbearingfriction,gimbalimbalances.Imperfectionscancauseunwantedprecessionwhichcanonlybeminimisedbyapplyingprecisionengineeringtechniques.


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Transport Rate

Assumenowthatthegyroscopeistransportedfromonepointontheplanettoanother,withitsspinaxisalignedwiththelocalverticalcomponentofgravity.Itwillhaveappearedtoanobserverontheearththatthespinaxisofthegyroscopehastiltedthisistransportwander

The control of transport wander is normally achievedbyusing gravity-sensing devices toautomatically detect tilting of the gyro scopes spin axis, and to apply the appropriatecorrectivetorques.Examplesofthesedevicesarelaterdescribed.

Transport Rate

IfagyroweretransportedfromtheNorthPoletotheequatoritwillappearasthoughithastilted90.Infactyouhavemovedandnotthegyro.Inthediagrams,theoneontheleftshowsan uncorrected gyro which would display transport rate, the one on the right shows acorrectedgyro.

Transportrateiscorrectedbyreferencingthegyrotothecentreoftheearth,orgravity.


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Gimbal errors

:occurinalldirectionalgyroswhich,asyouwillrecall,haveahorizontalspinaxis.Thegimbalerrorsarecausedduringaircraftmaneuvers.Theyarecausedby lossofgimbal relationshipundercertainconditions, in thatthegyro spin axis andgimbalsarenolongerat90degreestoeachother.Gimbalerrorsarenotcausedbyexternalforces,butby

the output from sensing synchros, as a result of the outer gimbal moving as the aircraftrotates about the spin axis. This output causes heading errors on inter-connectedinstrumentsduringmaneuvershowever,theseerrorsareeliminatedwhentheaircraftreturnstostraightandlevelflight.Iftheaircraftgyroframeisrotatedabouttherotorspinaxis,theoutergimbalmustmovetomaintainthedirectionoftherotorspinaxis.Thismovementoftheoutergimbalwillbedetectedbytheoutergimbalheadingsynchro.


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Handling


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Ring Laser Gyro

Lasergyrosarenowwidelyused inaircraftnavigationapplications.Theyprovideaccurate,independent navigational data with high accuracy and reliability. They are still a dead

reckoningsystem,andrequirenoexternalinputstofunction.ThistypeofInertialReferenceUnitisreferredtoasastrapdownsystembecauseitdoesnotrequire a gyro stabilised platform as described in a conventional INU. Pitch and rollmovementswhichwouldnormally introduceerrorsinanaccelerometer,areprovidedtothecomputer and the accelerometer outputs are modified electronically to compensate forattitudechanges.

Thisformof InertialReferenceUnitnormallyprovidesprimaryAttitudeinformation,andcanalsomeasurealtitude(inertially),rateofascentanddescentandgroundspeed.OutputsfromanIRUaretypicallydistributedoveradigitaldatabustoflightcontrolcomputers,navigationcomputers,multi-functiondisplays,etc.

Fundamentals of Laser Operation

LASER isanacronymforLightAmplificationbyStimulatedEmissionofRadiation.Sinceitsdiscoveryin1960,LASERtechnologyhasexpandedrapidlywithapplicationinmanyfieldsotherthanaviationincludingmedical,agricultureandengineering.ThefirststepinproducingLaseristheionisationofagaswhichmaybehelium,argon,krypton,neonorxenon.Eachgasproducesadifferentcolour(andwavelength)oflight.Amixtureofheliumandneonisused in ring laser gyros. This gas mixture is held at low pressure inside a sealed tubeexposedtoananodeandcathodeplate.Whenahighvoltageisappliedacrosstheseplatesthe gases ionise, producing a glow discharge similar to fluorescent tubes. In lasergyroscopestheappliedvoltageisaround3000volts.

Whatisthedifferencebetweenlaserlightandsayordinarywhitelight?Firstly,whitelightisamixtureofmanywavelengths;laserlightisasinglewavelengthwhichisdependantonthe

typeofgas used.Secondly, ordinary light isscattered inall directionsbut laser light isaparallelbeam.Forexample,recentexperimentsusingapencilsizedlaserlightaimedatthemoon founditspread toadistanceofonly twomilesover the distanceof250,000miles.Laseristermedcoherent light whichmeansitisofaspecificwavelength.


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Construction of the Ring Laser Gyroscope

Ringlasergyrosarenotgyroscopesinthesensethatweknowthem.Theyaresimplytwobeamsoflaserlightrotating inoppositedirectionsengineeredtodetectmotionandbehave

likeagyro.Laserringgyrosareconstructedsothattwolaserbeamsarereflectedaroundatrianglecausingthelighttotravelinanenclosedloop.Thelighttravelsinbothdirectionsatthesametime,sowehaveaclockwisebeamandacounter-clockwisebeam.

So how can this be used to detect motion? Picture a merry-go-round platform which isstationary, with two people walking around it from the same starting point. One walksclockwiseandtheotheranticlockwise,andbothcanwalkentirelyaroundthemerry-go-roundbacktotheirstartingpointbywalking100steps.However,iftheplatformisrotatedslowlyclockwise,thepersonwalkingwiththeplatformwouldneedtotakeshorterstepstocompletethe journey in the same number of steps. Conversely, the person walking against thedirectionofrotationwouldneedtotakelongerstepstocompletethejourneyin100steps(likewalkingonanescalatormorestepstoachievethesamedistance).

ThiscanalsobeexplainedusingtheDopplerprinciple.Asimilarphenomenontakesplaceinourlasergyro.Ifthegyroisturnedclockwise(CW),theCWbeamcompletes the journeyina shorter time.Inordertocompletethe journeyinthesamenumber ofcycles thebeamwavelengthmustbe compressed, that is, the frequencymust be increased. Conversely, the counter-clockwise (CCW) beam wavelength mustincrease(frequencydecreased).


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Fibre Optic Gyro

FibreOpticGyros(FOGs)arealaterexpansionoftheRLGprinciple.SimilartoRLGs,they

operate on exactly the same principle of sending two light beams, in different directionsaroundafibreopticpath.AnymovementoftheFibreOpticcoilineitherdirectionwillresultinthepathoflighttravellingfurtherinonebeamwhiletheotherlightpathtravelslessdistance.Bringingthetwolightbeamsouttosomeformofdetector,whichlooksatthephaseofthelight, will then give an output signal which will relate directly to the amount of rotationencounteredbythecoil.ThisisagainusingtheSagnaceffectasdotheRLGs.


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The reason FOGs are finding greater use than RLGs is that they can be made in muchsmallerandcompactinstallations.Alsothepaththatthebeamsarerequiredtotravelaround,can bemademuch longer thanasimilarRLG lightpath,and this increaseinpath lengthmeans that more accurate and smaller changes of rotation can bemeasured. Typically,

FOGscan have fibreoptic coilswhich canmeasureanywhere from100metres toover5kilometresinlength.Thelongerthepath,meansbasicallythemoreaccuratetheFOGcanbe, which in turn means that the Inertial Measurement Unit will provide far greaternavigationalaccuracy.

Most modern day FOGs, because of their much smaller physical size than RLGs, areincorporatedwithGPS receivers andAir Dataunits.What thisdoes is togive the aircraftdesigneracompact,fullyselfcontained,navigationunitcapableofveryhighaccuracies.

Becauseoftheiroperationandthevirtualabsenceornearlyallmovingparts,themeantimebetween failure (MTBF) for these types of units is nowmeasured in the order of yearsbetween failures. Typical MTBFs are measured in excess of 50,000 hours of continuousoperation, which in laymens terms means nearly over 6 years of continuous operation

betweenreportedfaults.

TheaccuracyoftheseunitsisalsohigherthantypicalRLGunits.Accuraciesintheorderof15metresorless,anywhereintheworld,issomethingthatAirlinesareveryhappytoaccept.

Atypicalunit fromNorthropGrummanasshownaboveiscurrentlyfittedtotheAirbusA380.Itweighs less than8 kilograms,and draws less than 36 watts ofpower from the aircraftelectricalsystem.ThisunitcombinesthefunctionsofGPS,InertialReferenceandAirDatamodulesintoonepackage.Whatthisdoesisgiveasmalllightweightpackagewhichdrawsverylittlepower,yetprovidesextremelyaccuratenavigationalaccuracy.


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Gyroscope Erection Systems

Therearetwomaintypeofairdrivengyroerectionsystems:

wedgeplate

pendulousvane.

Bothtypesrelyonthegyroscopicprecessiontomovethespinningrotorbacktoabalancedorerectposition.

Wedge Plate

Thewedgeplatesystemsimplydeflectstheairfromtherotorsystemacrossawedgeshapedplate.Thisdeflectedairwhenexhaustedevenlyorbalancedacrosstheplatehasnoeffectontheoutergimbal.Whenanunbalanceddeflectionoccursduetothemovementoftherotorfrom the vertical, the air isdistributedmoreonone sideof the plate than the other. Thisdifferential air flow causes a precessioneffect on the outer gimbal and therefore tries toreturnthespinningrotortotheverticalposition.


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A negative pressure is created so that the cabin air enters the filtered inlet and passesthroughthechannelstothejets.Theairfromthejetshitstherotorbuckets,evenlydrivingtherotor at approximately 15,000 rev./min. After spinning the rotor, the air passes through apendulousvaneunitattachedtotheundersideoftherotorcasingandisfinallydrawntothe

vacuumsource.


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Pendulous Vane

Thependulousvaneunitoperatesonanairdrivengyrospinningaroundaverticalaxis,suchasanartificialhorizon.Thistypeoferectionsystemportstheexhaustedairthroughfourports

locatedatrightangles toeachother. Thependulousvanesin thenormal positionportairequallyacrossalloutletsandthereforenoprecessioneffectisgenerated.Inanunbalancedsituation,thegyrorotoristiltedandthegravitationaleffectautomaticallyadjuststhepositionof the pendulous vanes and therefore directs the air differentially. This differential forcecausestherotortoprecessbacktothenormalposition.

Erection Systems for Electrically Driven Gyros

Mechanical

For electrically driven gyros, a mechanical form of erection is to manually cage thegyroscope.Thisactionmechanicallyforcesthegyrogimbalstoapositionatrightanglesto

eachother, that is,pitch, rollandyaw.Caremustbe takenwhencaginganinstrumentasdamagecanresulttogimbalsandbearingsifcagingisundertakenincorrectly.

Ball Type Erection System

Therearetwomaintypesofballstylelevellingorerectionsystemsusedonartificialhorizonsorattitudegyros:

ballcagetype

rollingballtype.


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Ball Cage Type

Theballcagetypeusesanumberofsteelballsusuallybetween5and8suspendedbelowthe gyroand free to roll across a radiused discwith hooks locatedaround the perimeter.Thesehookscapturetheballsastheirmassmovesandapplyaprecessionforcetoerectthegyro.Intheverticalplane,themassofballsiscentredandthereforenoprecessionforceisapplied.Theprecessionalforceappliedwillerectthegyro.

Rolling Ball Type

Therollingballtypesystemwhenusedasalevellingdeviceincorporatesaslotteddiscwithaballfreetotravelinsidetheslot.Theslotisdriveninatauniformspeedapproximately30rpm.Whentheassemblyistiltedtheballexertsaneffectonthedisc.Whentilted,theballrollstothelowerendandwaitsuntilthebottomoftheslotcatchesup.Goinguphilltheballispushedbythedisc.Thismeansthattheaverageweightonthedownhillsideislessthantheuphill and therefore produces a torque effect which is related to tilt angle. This torqueprecessesthegyroassemblyinthedesireddirectiontomovethegyrobacktothevertical

position.


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Erection Systems for Electrically Driven Gyros

Torque Motors and Sensors

The application of torquemotors is themost commonlyused formofgyro correctionandpossiblytheeasiesttouse.Thetorquemotorissimilartoasmallelectricmotorexceptthatthe output shaft applies a twistingmotion or forcewhich is called torque, to the erection

gimbal systemwhenenergised. The power issupplied through levelling switchesand thetorquemotorsaresituatedbetweenthegimbalandthenextsupportingframe.

Becauseoftheforceofprecession,agyrospinningabouttheverticalaxisrequiresthepitchtorquemotortobepositionedbetweentheinnerandoutergimbalandtherolltorquemotorpositionedbetweentheoutergimbalandframe.

Mercury Switches

Alevellingswitchmountedparalleltotheaircraftslongitudinalandlateralaxis(pitchandroll)detect any deviation from leveland therefore control current flow to the torquemotors byapplying a torque opposite to the force creating it. The levelling switches can be eitherstraightorcurved,thecurvedvarietyrequiringagreaterforcetooperate.


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Torque Motors

The erectiontorquemotors are squirrel cagemotors witha laminated iron rotor,mountedconcentricallyaboutthestator.Theironcorestatorhastwowindings,areferencewinding

andacontrolwinding.Thereferencewindingisprovidedwithaconstantfieldandinnormaloperation, that is, nodisplacement, it has current flowing via a capacitor and is thereforephaseshiftedby90 degrees. The controlwinding is intwo partsand isable toprovideareversiblefield.ThiscircuitissuppliedfromthesameACsource,asitisdirectlyconnectedtothesupply there isnophaseshift, leading thereferencewinding.Thecombinedmagneticfield interactswith the field inthe stator and the result controlsthe direction of the torquemotorandprovidescorrectionaltorqueto thegyroassembly.Inthenormalpositionthereisnocurrentflowinginthetorquemotorcircuitsduetothelevellingswitchbeinginthelevelposition.

DC Coils and Permanent Magnets

Both systems use the resultant flux lines to impart a magnetic field which will tend to

magnetically displace the gyro and therefore impart a precessional force. A permanentmagnet isusedwhereaconstantcorrectionforceis required toact,as isthecaseforthecorrectionofapparentdrift.Thesuccessofthistypeofpermanentfieldwilldependontheoperating environment of the aircraft, and the drift rate of the location. DC coilsoffer theabilityofbeingabletobeswitchedonoroffthroughtheuseofcutoutswitches,limitswitches,mercuryswitchesorsimplyasameansoffasterectionofthegyro.


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Erectionsystemsarenecessaryforerectingandmaintainingthegyrospinaxisinaverticalorhorizontalpositionrelativetotheearthssurface.Foranattitudeindicatortobeaccuratethegyrosspinaxismustbekeptvertical,thisisachievedbytheerectiondevicesdetectinganderectingthegyrotothelocalvertical.Duetotheconstructionoferectiondevices,theywillbe

displacedwhenevertheaircraftchangesairspeedoraltersdirection.Unless provision ismade to counter act the acceleration and turning forces, the erectiondeviceswillprecessthegyroaxistoafalseverticalandindoingsowillpresentanincorrect


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indicationoftheaircraftsattitude.Theimportanceoferectionsystemsandtheircorrectionmethodsmustbeclearlyunderstood.

Erection Errors

The erectionerrors caused byacceleration deceleration and turning forces actingon theconducting medium in the electrical leveling switches, or pendulous vanes used in themechanicalerectionsystemscanbecompensatedforbyusingthecharacteristicsofrigidityandprecessiontocorrecttheerrors.

Erection Error Correction

Tocorrectfortheerectionerrorsencounteredduringaccelerationordecelerationandturningforces,acorrecting torque isappliedto theoutergimbal.Thisresults inprecessionoftheinnergimbaltocounteractthefalseverticalerror.


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Caging

Thegyrocanbecagedmanuallybyaleverandcammechanismtoproviderapiderection.When the instrument is not getting sufficient power for normal operation, an "OFF" flagappearsintheupperrightfaceoftheinstrument.

Theinstrumentpermits360ofrotationaboutthepitchandbankaxeswithouttumblingthegyro.Theexpandedmotionofthehorizonbarprovidessensitivepitchindicationsnearthelevelflightposition.

Whentheaircraftexceedsthemaximumof27inpitchupordown,thehorizonbarisheldinextremepositionandthespherebecomesthenewreference.Acontinuedincreaseofclimbordiveangleapproachingtheverticalattitudeisindicatedbygraduationsonthesphere.

When the aircraftnears vertical, the sphere begins torotate 180.Assoonasthe aircraftdeparts from the vertical, the instrument again indicates the attitude of the aircraft. Thismomentary rotation of the sphere is known as controlled precession and should not beconfusedwithgyrotumbling.Theattitudeoftheaircraftabouttherollaxisisshownbythe


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anglebetweenthehorizonbarandtheminiatureaircraft,andalsobythebankindexrelativetothedegreemarkingonthebezelmask(faceplate).

Errors

Followingrecoveryfromunusualattitudes,displacementofthehorizonbarinexcessof5inpitchand/orbankmayresult.Oncetheinstrumentsensesgravitationalforces,theerectionmechanismwillimmediatelybegintocorrecttheprecessionerrorsatarateof3to6persecond. Inanormal turn,centrifugal forceacting on the erectionmechanismwill producenormalprecessionerrorsinpitchand/orbankupto5onreturntostraight-and-levelflight.Accelerationordecelerationwillalsoresultinprecessionerrorsinproportiontothedurationandmagnitudeofthespeedchange.Followingacceleration,theaircraftpitchattitudewillbelowerthantheinstrumentindication;followingdeceleration,theaircraftattitudewillbehigherthanthepitchindicationuntiltheerectionmechanismrealignsthegyro.


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Aircraft Gyro Vacuum Systems

Aircraftgyroinstrumentscanbepoweredbyvacuum(air)orelectricity.ElectricgyroscanrunoffACorDCpower,dependinguponwhattheyweredesignedfor.

Airdrivengyroscanrunonpositivepressure,orvacuumpressure.Airpressureisprovidedbyanenginedrivenvacuumpump,andthevacuum(morepredominantly)isthenplumbedthroughthegyroinstrumentstorunupthegyrorotors.Invacuumgyrosystemsthefiltersarevery important items as any contamination entering the gyro will dramatically shorten itsserviceablelife.Filtersmustberegularlyserviced

Aircraftuseaventurisystemwhentheydonothavethefacilityforanenginedrivenvacuumpumptopowertheirairdrivengyros.Theventuritubeisanopenendedmetaltubetaperingtowardsthecentreorthroat.It isfittedtothefuselageormainplanewiththeinletendinlinewiththedirectionofflight,andusuallylocatedinthepropwasharea.

Inflight,theairisforcedintotheinletendofthetubeandacceleratesthroughthenarrowingsectionorthroatofthetubetoahighervelocity.Thisincreaseinvelocityproducesalowerpressureatthethroatwhichisconnectedthroughtubingtoasuctionreliefvalveandthentothecaseofthegyroinstruments.Thecaseofthegyroisalsoconnectedthroughafilterbacktoatmosphere.As the pressure in the throat of the venturi is lower thanatmosphere, theatmospherecausesaflowofairorsuctionthroughtheinstrumentstotheventurithroatandback toatmosphere as the air leaves the venturi.Venturi tubesasa vacuum source arenormally confined to early light aircraftand some of the later types of simple home builtaircraft.Theventuriisextremelyinefficientandlimitedinitscapacitytodriveinstruments.

Venturitubesareratedbytheamountofvacuumtheywillproduceat120Mphor104Kts.Thetwo-inch/50mmventuriisusedtoproducetwoinchesofmercurysuctiontodriveoneturn and bank indicator, while the larger four-inch tubesare used for the directional andattitudegyros.Onedesignofthelargertubesiscalledthesuper-venturioreight-inchventuri.Thisventurihasanauxiliaryventuriinitsthroatandiscapableofmoresuctionforthesame

speed.


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Problems

Themostcommonproblemwithaventuriisfrom:

beingstruck

thetubeassemblybeingdamaged

beingblockedbyaforeignobject.

Alwaysmake sure that the tubeand fittingsare clean, free fromobstructionsandhaveagoodphysicalappearance.

Withtheintroductionofnewaircraft,theaircraftsystemsandinstrumentationbecamemorecomplex.Higherspeedsandincreasedaltituderequiredamoresophisticatedvacuumsupplysource.Themajorproblemwiththeventurisystemistheformationoficeinthethroatandotherdamagebeingsustainedbythetubeassemblystickingoutintheairflow.Thevacuumpoweredgyroscopicflightinstrumentsfittedtothemanytypesofaircraftvaryinthedemandplaced on the vacuum system. The two main types of positive displacement, vane type

vacuumpumpswhicharedrivenfromtheengineaccessorydrivesare:

wettype

drytype

andareclassifiedaccordingtotheirconstruction.

Wet Pumps

Theearliervacuumpumpswerenearlyallofthesteelvanetypewhichwerelubricatedfromtheenginelowpressureoilsystem.Thisoilhasaone-waypassagethroughthepumpandislostwiththedischargeairoverboard,viatheventtube.Insomedesignsthisoilisreturnedto

theenginecrankcasebyseparatingtheoilfromthedischargeairinanoilseparatorbeforetheairisallowedtoentertheatmospherewhichpreventstheoilcausingstreaksalongthesideofthefuselage.


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Themodern pumpsare dry; that is, theyhave theirwearingpartsmadeofTeflon and orcarbon.Thepumprotorsaremadeofafibrematerialandtherotorbladesareofcarbon.Thepump housing is high grade cast iron finely machined and in some cases the surface isTefloncoated.Thesepumpscannormallybedriveninonlyonedirectionwhichisindicated

byanarrowonthehousingTopreventmechanicaldamagetotheengineaccessorydrivesystem,thepumpshaveaweak-linksheardrivedesignedtofailshouldthepumpsufferaninternalfault.

Problems with Vacuum Pumps

Look for a show of oil or evidence of vibration. The vacuum pump should be smooth inoperationandanerraticoutputischaracterisedbyadifficulttoadjustsuctionreliefvalve.Thevacuumpumpshearlinkmustbecheckedforsignsofstressandthepumpmustappeartobeingoodorder.

Anaircraftvacuumsourcecanbeeitherfromavacuumpumpwhichisenginedrivenorfrom

aventuriwhichislocatedinthepropwash,externaltotheaircraft.Thevacuumsupplyinbothcasesisasourceoflowpressure.

Anaircraftvacuumsourcecanbeeitherfromavacuumpumpwhichisenginedrivenorfromaventuriwhichislocatedinthepropwash,externaltotheaircraft.Thevacuumsupplyinbothcasesisasourceoflowpressure.

Pressurisedairportedovercupsingyrorotor,orvacuumairsuckedacrosscups.Spinsgyrorotor up to speed and is also used for gyro erection system reference gyro to earth toeliminate transport rate.Onlyever lowpressureair used.Only likely tobe incorporated inlightaircraft.


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Athighaltitudesvacuum-drivengyroscopicinstrumentssufferfromtheeffectsofadecreaseinvacuum due to the loweratmosphericpressure; the resulting reduction in rotor speedsaffecting gyroscopic stability. Other disadvantages of vacuumoperation areweight due topipelines, special arrangements to control the vacuum in pressurized cabin aircraft, and,

sinceairmustpassthroughbearings,thepossibilityofcontaminationbycorrosionanddirtparticles

Vacuum-Driven Gyro Horizon

The rotor ispivoted inball bearingswithina case forming the inner ring,which in turn ispivotedinarectangular-shapedouterring.

Intherearendcoveroftheinstrumentcase,aconnectionisprovidedforthecouplingofthevacuumsupply.Withthevacuumsystem inoperation, thesurroundingatmosphereentersthe filteredinletandpasses throughthechannelsto the jets.Theair issuingfromthe jetsimpinges on the rotor buckets, thus imparting even driving forces to spin the rotor atapproximately 15,000 RPM. After spinning the rotor, the air passes through a pendulous

vaneunitattachedtotheundersideoftherotorcasing,andisfinallydrawnoffbythevacuumsource.


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Disadvantages of air driven gyro systems

Dirtanddustareamajorproblemwithairdriveninstrumentsandthereforeinstrumentfiltersandsystemfiltersmustbechecked,cleanedorchangedatregularintervals.

Whencigarettesmokingwasallowedonaircraft,theresiduefromthesmokewasamajorproblemforgyroscopicairdriveninstruments.

Enginedrivenvacuumpumpsmustberegularlycheckedforcorrectoperation.

Incorporationofmechanicalpumpsaddsanadditionalpieceofequipmentrequiringservicing,inadditiontotheaircraftsalternator/generator.

Toovercomethedisadvantagesoftheairdrivengyroscopicinstrumentsinhighperformanceaircraft,gyroscopicinstrumentsweredesignedforoperationonelectricalpowerderivedfromthe aircraft power supplies. This power is generally 115V 400Hz three phase alternatingcurrentassuppliedfromtheaircraftalternatorsorinvertersor28Vdirectcurrent,thelatterbeing requiredfor the operation ofsome turnand bank indicators.The alternatingcurrent

applicationhasbeenusedforthelatertypesofturnandbank,gyrohorizonindicatorsandtheremotely located attitudeand directional gyros associatedwith flight control systems andremote-indicatingcompasssystems.

Electricalgyrosonlyneedasmallamountofpowerfromtheexistingaircraftpowersupplyhenceanadditionalenginedrivencomponent(thevacuumpump)isnolongernecessary.

ACelectricallypoweredgyroscanrunmuchfasterthanairdrivengyrossoprovideamorerigidgyroscopicreference.

Electricallydrivengyrosincorporatemoresolidstatecomponentsandthereforerequirelessmaintenanceeffortcomparedtopneumaticallydrivengyros.

Aparticularlimitationofairdrivengyrosovermostelectricallydrivengyrosis that thegyro

should never beremoved fromtheaircraftuntilat least30minuteshavepassed from thetime the vacuum source was disconnected, or rotor has ceased spinning, as the inertiacontainedwithintherotor,andtherelativeabsenceoffrictionwithinthebearings,mayallowtherotortospinforuptothislengthoftime.Electricallydrivengyrosoftenincorporateaformofelectrical ordynamicbrakingwhichwill slow the gyro rotorvery quicklyoncepower isremoved.


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Thedirectcurrentelectricalgyrousesamethodofconstructionwheretherotorisactuallythearmaturewindingandthestatoristhepermanentmagnet.Thisprincipleofconstructionallowsthegreatermasscontainingthearmaturewindingtospinastherotorgivinggreaterrigidity.

Thisdirectcurrentapplicationutilisesthegyrorotorwhichcontainsthearmaturewindingofasmall permanentmagnet motor. This is supplied with 28 voltsDC via two spring loadedbrushescontactingacommutator,whichismountedontherotorarmatureshaft.Thestatorisatwo-polepermanentmagnetandformspartofthegimbalframe.Inthisapplication(TurnandSlipindicator)therotorspeediskeptatapproximately4,200RPM.


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AC Drive Methods and Motor Types

AC gyros are usually poweredby a three phase115 volt 400Hz squirrel cage inductionmotor, consistingofa rotor andastator.Anormal inductionmotorhas the rotorrevolvinginsidethestator.Inthisinstancethemotorhasbeenredesignedsothattherotorrotateson

theoutsideofthestator,thisincreasesthemassoftherotorinordertoprovidetherequiredinertia.

The115voltsACissuppliedtothestatoranda rotatingmagneticfieldisestablishedinthestator.Thisrotatingfieldcutsthebarsinthesquirrelcagerotorandinducesacurrent,theeffectofwhichproducesamagneticfieldaroundthebarswhichcombinewiththe statorsfieldcausingtherotortospinatapproximately22,500RPM.

AC Drive Methods and Motor Types

AC gyros are usually poweredby a three phase115 volt 400Hz squirrel cage inductionmotor, consistingofa rotor andastator.Anormal inductionmotorhas the rotorrevolvinginsidethestator.Inthisinstancethemotorhasbeenredesignedsothattherotorrotatesontheoutsideofthestator,thisincreasesthemassoftherotorinordertoprovidetherequiredinertia.

The115voltsACissuppliedtothestatoranda rotatingmagneticfieldisestablishedinthestator.Thisrotatingfieldcutsthebarsinthesquirrelcagerotorandinducesacurrent,theeffectofwhichproducesamagneticfieldaroundthebarswhichcombinewiththe statorsfieldcausingtherotortospinatapproximately22,500RPM.


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Electric Gyro Horizon

Madeupofthesamebasicelementsasthevacuum-driventype,withtheexceptionthattheverticalgyroscope isa3-phasesquirrel-cage inductionmotor (consistingofa rotoranda

stator).One of the essential requirements of any gyroscope is to have the mass of the rotorconcentrated as near to the periphery as possible, thus ensuring maximum inertia. Thispresents nodifficulty where solidmetal rotors are concerned, but when adopting electricmotors asgyroscopes some rearrangement of theirbasicdesign isnecessary inorder toachieve the desiredeffect. An induction motor normally has its rotor revolving inside thestator,buttomakeonesmallenoughtobeaccommodatedwithinthespaceavailablewouldmeantoosmallarotormassandinertia.However,bydesigningtherotoranditsbearingssothatitrotatesontheoutsideofthestator,thenforthesamerequiredsizeofmotorthemassoftherotorisconcentratedfurtherfromthecentre,sothattheradiusofgyrationandinertiaareincreased.Thisisthemethodadoptednotonlyingyrohorizonsbutinallinstrumentsandsystemsemployingelectricgyroscopes.

Themotorassemblyiscarriedinahousingwhichformstheinnergimbalringsupportedinbearingsintheoutergimbalring,whichisinturnsupportedonabearingpivotinthefrontcoverglassandintherearcasting.

The115V400Hz3-phasesupplyisfedtothegyrostatorviasliprings,brushesandfingercontactassemblies.Theinstrumentemploysatorque-motorerectionsystem,theoperationofwhichisdescribedinPallettAircraftInstrumentsonpage136,butwillnotbecoveredhere.

Whenpowerisswitchedonarotatingmagneticfieldissetupinthegyrostatorwhichcutsthebarsformingthesquirrel-cageintherotor,andinducesacurrentinthem.Theeffectofthis current is toproducemagnetic fields around the barswhich interactwith the statorsrotatingfieldcausingtherotortoturnataspeedofapproximately20,00023,000rev./min.

FailureofthepowersupplyisindicatedbyaflagmarkedOFFandactuatedbyasolenoid.


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TOPIC 13.8.2.2.1: ARTIFICAL HORIZONS

Inordertomeasureamovement,youneedareference,andinthisinstancethegyrobecomesthereferenceorstablepoint.Theamountofmovementordeflectionsmadebytheaircraftaroundthisstablepointaremeasuredanddisplayedonthecockpitinstruments.

GyroSpinaxisisvertical,soplaneofspinishorizontal.Thispermitsrigidityinlateralandlongitudinalaxisandthedisplacementofthegimbalsfromthestablereferenceiswhatprovidestherollandpitchreadout.

Thegyroisatiedgyroreferencedtotheearthsgravitytomaintaintheverticalspinaxisshouldimperfectionsorerrorscausethegyrotodrift.Theerectionsystemwillre-alignthegyrowithrespecttogravity.

Mostgyrohorizonshaveapulltocageknobtore-alignthegyroinstraightandlevelflightifitisnotedtobedriftingoff,orifittumblesorsuffersgimballock.


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This form of gyro horizon has a fixed back plate or sky plate and the aircraft symbol isattachedtothegimbalsandmoveswithrespecttothebackplatetoindicatepitchandyawattitudes.Thiswasanoldmethodofdisplayingthisinformationandprobablynotveryoftenseeninmoderntimes.

Thepitchrestrictionat85istoavoidgimballock.Thisisnotaconcernintherollaxis.


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Thispictureisamoreexactillustrationofthewaythegyrohorizonactuallyprovidesadisplayofpitchandroll.Thedialisfixedandonceuponatimewouldonlyhavehadahorizonlinedrawnacrossthemiddle.Inmorerecenttimegyrohorizonshavebeencolouredwithalight

colourabove,typicallyblue,torepresenttheskyandadarkercolourbelowtorepresenttheground.

Whenthehorizonpointer isupand intheblueitmeans theaircraftis climbing,andwhendown in thegreen it is diving. Thehorizonbar is restricted in pitchmovement up to 85otherwisegimballockwilloccur,whereastherollingactionisunrestricted.

Thedisplaycanthereforeindicateunrestrictedfullbarrellrollsbutifaloopwereperformedthe indicatorwouldshowaclimbup to85 (when the aircraftnose isalmost vertical, notwhenitsatthetopoftheloop)theassemblywouldthenroll180.Thehorizonpointerwouldindicatestraightandlevelinvertedflightcorrespondingwiththeaircraftbeingupsidedownatthetopoftheloop.Astheaircraftcomesdowntocompletetheloopthehorizonbaragainshowstheaircraftheadingforthegrounduntilit ispointingalmoststraightat theearth(85

nosedown)whenitwillagainspin180.Thismeanstheaircraftsymbolwillcontinuepointingattheearth(indicatingadive).Astheaircraftrecoverstostraightandlevelflightagainatthebottomoftheloopthewholeassemblywillbebackinitsoriginalattitudewiththehorizonbaragainshowingstraightandlevelflight.


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With thevacuumsystem inoperation,anegativepressure iscreatedso that thecabinairentersthefilteredinletandpassesthroughthechannelstothejetswhicharedirectedontothebucketscarvedintothegyrosrotor.Theairfromthejetshitstherotorbuckets,evenlydriving the rotoratapproximately15,000 rev./min. Afterspinning the rotor, the air passes

throughapendulousvaneunit attachedtothe underside of the rotorcasing and isfinallydrawntothevacuumsource.Theairisexhaustedthroughthependulousvaneunit,whichappliesacorrectionalorprecessionforcetkeepthegyroperpendiculartotheearthssurface.

Thisisachievedthroughthemagicofprecessionandasthegyrotiltsofftheverticalanairportisuncoveredpermittingagreaterflowofairfromthatportasitexitsfromthegyrosrotor.Theadditionalairflowexertsaforceonthegyrowhichisfelt90inthedirectionofrotationandwillcausethegyrotoprecessbacktothevertical.

Operation

Theoperationoftheinstrumentisbasicallycontrolledbytheprincipleofgyroscopicinertiaorrigidity. Thegyrospinaxis ismaintained ina verticalposition relativetotheearth.Astheaircraft rollsand pitches in flight, the indication isgivenona two colour dial, the top halfrepresentingtheskyandthebottomhalfwhichisdarker,representstheground.

Thehorizontalgyrospinsabouttheverticalaxisandthereforeitcansenserotationabouttherollandpitchattitudeoftheaircraft.

Disadvantages of air driven gyro systems

Dirtanddustareamajorproblemwithairdriveninstrumentsandthereforeinstrumentfiltersandsystemfiltersmustbechecked,cleanedorchangedatregularintervals.

Whencigarettesmokingwasallowedonaircraft,theresiduefromthesmokewasamajorproblemforgyroscopicairdriveninstruments.

Enginedrivenvacuumpumpsmustberegularlycheckedforcorrectoperation.

Incorporationofmechanicalpumpsaddsanadditionalpieceofequipmentrequiringservicing,inadditiontotheaircraftsalternator/generator.


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Toovercomethedisadvantagesoftheairdrivengyroscopicinstrumentsinhighperformanceaircraft,gyroscopicinstrumentsweredesignedforoperationonelectricalpowerderivedfromthe aircraft power supplies. This power is generally 115V 400Hz three phase alternatingcurrentassuppliedfromtheaircraftalternatorsorinvertersor28Vdirectcurrent,thelatter

beingrequiredfor the operation ofsome turnand bank indicators.The alternatingcurrentapplicationhasbeenusedforthelatertypesofturnandbank,gyrohorizonindicatorsandtheremotely located attitudeand directional gyros associatedwith flight control systems andremote-indicatingcompasssystems.

Electricalgyrosonlyneedasmallamountofpowerfromtheexistingaircraftpowersupplyhenceanadditionalenginedrivencomponent(thevacuumpump)isnolongernecessary.

ACelectricallypoweredgyroscanrunmuchfasterthanairdrivengyrossoprovideamorerigidgyroscopicreference.

Electricallydrivengyrosincorporatemoresolidstatecomponentsandthereforerequirelessmaintenanceeffortcomparedtopneumaticallydrivengyros.

Aparticularlimitationofairdrivengyrosovermostelectricallydrivengyrosis that thegyroshould never beremoved fromtheaircraftuntilat least30minuteshavepassed fromthetime the vacuum source was disconnected, or rotor has ceased spinning, as the inertiacontainedwithintherotor,andtherelativeabsenceoffrictionwithinthebearings,mayallowtherotortospinforuptothislengthoftime.Electricallydrivengyrosoftenincorporateaformofelectrical ordynamic brakingwhichwill slow the gyro rotorvery quicklyoncepower isremoved.


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The need for integrating the functions and indications of certain flight and navigationinstrumentsresultedinthemainfromtheincreasingnumberofspecialisedradioaidslinkingaircraftwithgroundstations.Theseweredevelopedtomeetthedemandsofsafeen-route

navigationandtocopewithincreasingtrafficcongestionintheairspacearoundtheworldsmajorairports.


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The required information isprocessed by black boxes which can bestowed inelectricalcompartments and radio racks, but in order that the necessary precision flying may beexecuted,informationmuststillbepresentedtothepilot.Thisrequiresmoreinstrumentsandmoreinstrumentscouldmeanmorepanelspace.Themethodofeasingtheproblemwasto

combinerelatedinstrumentsinthesamecaseandtocompoundtheirindicationssothatalargeproportionofintermediatementalprocessingonthepartofthepilotcouldbebypassedandtheindicationsmoreeasilyassimilated.

Duringthatphaseofaflightinvolvingtheapproachtoanairportrunway,itisessentialforapilottoknow,amongotherthings,thatheismaintainingthecorrectapproachattitude.Suchinformationcan beobtained from the gyrohorizon and fromaspecial ILS indicatorwhichrespondstoverticalandhorizontalbeamsignalsradiatedbythetransmittersofanInstrumentLandingSystemlocatedattheairport.ItwasthereforealogicalstepinthedevelopmentofintegrationtechniquesinwhataretermedFlightDirectorSystems,toincludetheinformationfromboththegyrohorizonandILSindicator.

Themethodsadoptedfortheintegrationofsuchinformation,andthemannerinwhichitis

presentedvarybetweensystems.Acompletesystemnormallycomprisestwoindicators:

-flightdirector,attitudeflightdirectororanapproachhorizon

-coursedeviationindicator(CDI)orahorizontalsituationindicator(HSI).

Theflightdirectorindicatorhastheappearanceofaconventionalgyrohorizon,butunlikethisinstrument thepitchandroll indicatingelementsareelectricallycontrolled froma remotelylocatedverticalgyrounit.

TheapproachattitudeofanaircraftwithrespecttoitsILSsignalsisindicatedbyindependentpointersmonitoredbytherelevantILSreceiverchannels.Displacementoftheaircrafttothe

leftorrightofthelocaliserbeamisindicatedbydeflectionsofthelocaliserpointer.Glideslopepointerfunctionsinsimilarfashion.

AttitudeDirectorsarebasicallyArtificialHorizonswithcommandsteeringbarsincorporated.Theinstrumentprovidesthepilotwithanindicationofpitchandroll,butalsohascommandbarswhichcanbeusedtoguidethepilotontoaselectedcourse,ortoaselectedaltitude.Thecommandbarsappearandthepilotfliestheaircrafttoaligntheaircraftsymbolwiththe

commandbars.


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Flight Director Indicator FDI)

Thisinstrumentmaybeknownasanattitudedirectorindicator(ADI)oranattitudereferenceindicator(ARI).Theyallhaveslightlydifferentdisplays,buttheyalloperateinthesameway.Thebasic functionof theFDI is tosupply the pilotwith theaircraftsattitudeandsteeringinformation. This represents a view from behind the aircraft looking forward. Steeringcommandandaircraftattitudearedisplayedaroundafixedaircraftsymbol.

Attitude Sphere

Thesphereisfreetomove360inrollanddependingontype,90or360inpitch.Gimballocklimitationminimisedoreliminated

Bank Pointer

Thisdisplays thebankangleof theaircraft,andisreadagainstascaleon thecaseof theinstrument.

Command Bars

Therearetwocommandbars,oneforpitch,andoneforroll.Theyarecalledcommandbarsbecausetheycommandthepilottoflytheaircraftsymboltowardsthecommandbars.The

commands are supplied from the flight director computer, which can receive referencesignalsfromarangeofnavigationaidreceiversorINS

Glideslope Pointer

This is locatedonthe left sideof the FDI and isusedwhen the aircrafthascapturedtherunwayglideslopebeams,whenlanding.Theaircraftsverticalpositionwithinthebeamsisshownbythepointer.Whenthepointerisonthecentreline,theaircraftisinthecentreoftheglideslope.Whenthepointerisonthedotclosesttothecentreline,thepitchcommandbarcomesintoview,andthepilotfliestowardsit.Figure3.13showstheglideslopepointer.


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Localiser Deviation Indicator

Localiser pointershows theaircrafts positionin relationto the localiser beams.When thepointerisinthecentreofthescaletheaircraftispositionedinthecentreofthebeams.


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Theflightdirectorindicatorhastheappearanceofaconventionalgyrohorizon,butunlikethisinstrument thepitchandroll indicatingelementsareelectricallycontrolled froma remotelylocatedverticalgyrounit.

Electricalinterconnectionoftheflightdirectorindicatorcomponentsprimarilyconcernedwithpitch and roll attitude information is shown on the slide. Whenever a change of aircraftattitudeoccurs,signalsflowfrompitchandrollsynchrostothecorrespondingsynchroswithintheindicator.Errorsignalsarethereforeinducedintherotorsandafteramplificationarefedto the servomotors, whichrotate toposition the pitchbar andhorizondisc (or Sphere, or

cylinder)toindicatethechangingattitudeoftheaircraft.Atthesametime,theservomotorsdrivethesynchrorotorstothenullposition.

ThesecondcircuitshowstheinterconnectionoftheglideslopeandlocaliserpointerwiththeILS.DuringanILSapproachthereceiveronboardtheaircraftdetectsthesignalsbeamedfromgroundtransmitters inverticalandhorizontalplanes. If theaircraft isabove theglidepath,signalsarefedtothemetercontrollingtheglideslopepointercausingittobedeflecteddownwardsagainstthescale,thusdirectingthepilottobringtheaircraftdownontotheglidepath.Anupwarddeflectionofthepointerindicatesflightbelowtheglidepathandthereforedirectsthattheaircraftbebroughtuptotheglidepath.Thepointerisalsoreferencedagainstthe pitchbar to indicateany pitchcorrection required tocapture and hold the glidepath.Whenthishasbeenaccomplished,theglideslopepointerandpitchbararematchedatthehorizontalcentreposition.


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If,duringtheapproach,theaircraftistotheleftofthelocaliserbeamandrunwaycentre-line,thelocaliserpointerisdeflectedtotherightdirectingthattheaircraftbebankedtotheright.Flighttotherightofthelocalizerbeamcausespointerdeflectiontotheleft,directingthattheaircraftbebankedtotheleft.Wheneitherofthesedirectionshasbeensatisfied,thepointeris

positionedverticallythroughthecentrepositionofthehorizondisc.

Flight director indicator houses a number of servo/synchro devices. Aircraft pitch & rollinformation from twin gyro platform positions horizon disc & pitch bar. Additionalservo/synchrodevicestodrivecommandbarsdrivenbysignalsfromflightdirectorcomputer.Typical remote indicator housing servo/synchro systems to repeat informationsensed/processedbyaremoteequipmentrackmountedblackbox.


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B:January2008 Revisi 2



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TOPIC 13.8.2.2.2: TURN AND SLIP INDICTORS

Turn Indicators

Theturnandbankindicatororturnandslipindicatorasitismostoftencalled,isoneofthefirst instruments developed for instrument flying. The instrument actually combinesa turnindicatorandaslipindicatorintheoneinstrument.Inearlydaysofflyingtheturn-and-bank,whenusedinconjunctionwiththeaircraftcompassmadeavaluablecontributiontotheartofIFRflying.Itwasthusconsideredtheprimaryblindflyinginstrument.Withdevelopmentsinaircraft instrument technology the turn-and-bank has been replaced as the primary IFRinstrumentbytheAH,althoughinsomelightaircrafttheturn-and-bankisstillconsideredaprimary flight instrument. In larger aircraft the turn-and-bank has become a secondaryinstrumentorisnotused.


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Principle of Operation

The rategyros spinaxis ishorizontaland correspondswith the aircrafts lateral axis, thatmeanstheplaneofspinisthroughthelongitudinalaxisoftheaircraft.Therategyroonlyhas

onegimbalmountedwithinthe frameorcaseofthe instrumentsoit isonlypermittedonedegreeoffreedomwhichis intilt.Thepivotpoint for thegimbal isforeandaftofthegyrorotorsoitispivotedinthelongitudinalaxisoftheaircraft.

Thegyrosensesmovementabouttheyawingaxisoftheaircraft.ItiseffectivelymountedlikeaDG, butdoesnot havethe freedomofaDG.When theaircraftyaws the gyrowants toremaininitscurrentattitudeandalignment,butcannotbecausethereisnogimbaltopermitveer.Becausethegyrocannotremainpointinginthesamedirectiontheturningmotionoftheaircrafthasthesameeffectasifsomeoneappliedaprecessiveforcetothefrontandrearofthegyrorotor,tryingtochangeitsheading.Thisforceisfelt90indirectionofrotation,sowill precess the gyro so itwill tilt over. If the gyrowasnot restrainedbysprings itwouldcontinuetoprecessinthetiltaxiswhileevertheyawingmotionwasfelt.Becausethegyroisheldinplacebysprings,whileevertheyawingmotion(orrateofturn)remainsconstantthe

gyro precession force will remain constant against spring pressure providing a constantindicationoftherateofturn.Iftherateofturnisincreasedtheprecessionforceincreasestiltingthegyrofurtheragainstspringpressure.Whentheturningmotionendstheprecessionforceisremovedsothegyrowillreturntotheoriginalattitude,iespinningintheverticalplanecorrespondingwiththeaircraftslongitudinalaxis.

Rotoraxisparalleltoaircraftslateralaxis

Yawingmotionsensed&duetoprecessionrotortriestolieoveragainstspringpressure

Lieoverangleproportionaltorateofturn&isopposed/restrictedbycalibratedspringtension

2Minuteand4MinuteTurns

Gyrodoesntbeginto layoveruntilaftertheturnhasbegun,iewhentheheadingbeginstochange.ThisstatementwillbereferredbacktowhencoveringTurnCoordinators.

Spinaxisparalleltoaircraftlateralaxis


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Yawingmotionsensedrotortriestolieoveragainstspringpressureduetoprecession

Lieoverangleproportionaltorateofturn

Gyrolaysoverwhenheadingchanges

Rateofprecession(lieoverangle)dependson:

Rateatwhichheadingischanging

Rigidityofrotor(rotorspeed)

Rateofprecessiondependantuponspeedofgyrorotor(gyrorigidity)

RateofTurnindicatorrotorspeedcritical:

toofast&instrumentunderreads

tooslow&instrumentoverreads

Rateofturnindicatorsincorporaterotorspeedgovernortoensureaccuracyofindications


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The Mechanism of a Typical Direct-Current Operated Turn-and-Bank Indicator

Directcurrentisfedtothebrushesandcommutatorviaaradiointerferencesuppressorandflexiblespringswhichpermitmovementoftheinnerring.

The rotor speed is controlled by two identical symmetrically opposed centrifugal cut-outs.Each cut-out consists of a pair of platinum-tipped governor contacts, one fixed and one

movable,whicharenormallyheldclosedbyagovernoradjustingspring.Eachcut-outhasaresistor across its contacts, which are in series with half of the rotor winding. When themaximum rotor speed is attained, centrifugal forceacting on the contacts overcomes thespringrestraintcausingthecontactstoopen.Thearmaturecurrentthereforepassesthroughtheresistors,thusbeingreducedandreducingtherotorspeed.Bothcut-outsoperateatthesamecriticalspeed.

Angularmovementof thegimbalringistransmittedto thepointerthroughageartrain,anddampingisaccomplishedbyaneddy-currentdragsystemmountedattherearofthegyroassembly.Thesystemconsistsofadragcup,whichisrotatedbythegimbalring,betweenafieldmagnetandafieldring.

Apower-failurewarningflagisactuatedbyastirruparmpivotedonthegimbalring.Whenthe

rotor is stationary, the stirrup arm is drawn forward by the attraction between a magnetmounted on it and an extension (flux diverter) of the permanent-magnet stator. In thisconditiontheflag,whichisspring-loadedintheretractedposition,isdepressedbythestirruparm so that the OFF reading appears through an aperture in the dial. As rotor speedincreases, eddy currents are induced in the rotor rim by the stirrup magnet, and at apredeterminedspeed,reactionbetweenthemagnetandinducedcurrentcausesthestirruparmtoliftandtheOFFreadingtodisappearfromview.


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Major Parts

Rotorspeedandrigidityiscrucialforthisinstrumenttoindicateaccurately.Ifspeeddropsoff,egsclogged filter inanair driven system, the gyrowill slowdownandwill thenprecess

furtherwithlessforceapplied(rememberfactorsthatcontributedtoamountofprecession,gyrorigidityandlevelofforceappliedtothegyro).Soaslowergyrowilloverread.Speedcontrolisreliableinelectricalinstrumentsbutinairdrivensystemsthepilotmustensurethevacuumgaugeisreadingcorrectly.Agaugereadinganincreasedvacuum(meaningnearly0psi)would indicate that the system filtersweredirtybecause the pump isevacuating thesystemduetothefactthataircannotcomeinthroughthefiltertoreplacetheairthepumpissuckingout.Thisindicationofagreatervacuum(almost0psia)meansthegyrorotorsofallthe air driven instrumentswouldbe running slower,and hence the turnand slip indicatorreadoutwouldbeinaccurate.LossofrigidityintheDGandAHwouldbecomeapparentduetothemdriftingoffmoreoften,buttheirreadoutswouldstillbereasonablereliable,whereastheturnandbankreadoutwouldbecompromised.Rotorspeediscrucialinaturnandbank.

Noerectionmethodisincorporatedinaturnandbankbecauseitisphysicallypreventedfrom

drifting off. It does have an earth rate correction in that the gimbals would be weightedappropriatetocounteracttheearthsrotation,butthisformofcorrectionisengineeredintotheturnandbankatthetimeofmanufacture.

Limitations of the Turn and Slip Indicator

TurnandSlip indicatorwill not respond toan aircraftbank, itwill only indicatea turn ifayawingmotionissensed.


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b2-13h instruments gyroscopic sr

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