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StudentResource
SubjectB1-17:
PropellerSystems
Copyright 2008 Aviation Australia
Allrightsreserved.Nopartofthisdocumentmaybereproduced,transferred,sold,orotherwisedisposedof,withoutthewrittenpermissionofAviationAustralia.
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B1 17 Propeller Systems
CONTENTS
Page
DEFINITIONS 3
STUDY RESOURCES 4
INTRODUCTION 5
PropellerFundamentals 17.1-1
PropellerConstruction 17.2-1
PropellerPitchControl 17.3-1
PropellerSynchronising 17.4-1
PropellerIceProtection 17.5-1
PropellerMaintenance 17.6-1PropellerStorageandPreservation 17.7-1
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B1 17 Propeller Systems
DEFINITIONS
Define
Todescribethenatureorbasicqualitiesof.
Tostatetheprecisemeaningof(awordorsenseofaword).
State
Specifyinwordsorwriting.
Tosetforthinwords;declare.
Identify
Toestablishtheidentityof.
List
Itemise.
Describe
Representinwordsenablinghearerorreadertoformanideaofanobjectorprocess.
Totellthefacts,details,orparticularsofsomethingverballyorinwriting.
Explain
Makeknownindetail.
Offerreasonforcauseandeffect.
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B1 17 Propeller Systems
STUDYRESOURCES
JeppesenSandersonTrainingProducts:
A&PTechnicianPowerplantTextbook.
AircraftGasTurbinePowerplantsTextbook.
B1-17StudentHandout
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B1 17 Propeller Systems
INTRODUCTION
Thepurposeofthissubjectistofamiliariseyouwithconstruction,components,operationand
maintenanceofaircraftpropellersystemsandturbo-propandturbo-shaftengines.
Oncompletionofthefollowingtopicsyouwillbeableto:
Topic 17 1 Propeller Fundamentals
Describebladeelementtheory.
Describethefollowingandexplaintheireffectonpropellerthrust:
High/lowbladeangle
Reverseangle Angleofattack
Rotationalspeed.
Describethefollowinginregardstopropellers:
Propellerslip
Aerodynamicforce
Centrifugalforce
Thrustforce
Torque
Relativeairflowonbladeangleofattack
Vibrationandresonance.
Topic 17 2 Propeller Construction
Describeconstructionmethodsandmaterialsusedinwooden,compositeandmetalpropellers.
Describethefollowingterms:
Bladestation
Bladeface
Bladeshank
Bladeback
Hubassembly.
17.2.3 Describetypicalmountingrequirementsofflanged,taperedandsplinedpropellerinstallations.
17.2.4 Describetheoperationofthefollowingpropellertypesandidentifytheirspinnerinstallation:
Fixedpitch
Controllablepitch
Constantspeeding.
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Topic 17 3 Propeller Pitch Control
Describemethodsusedforpropellerspeedcontrolandpitchchange.
Describetheoperationofcomponentsusedfortocontrolpropellerfeatheringandreversepitch.
DescribeStatethepurposeofpropelleroverspeedprotectiondevices.
Topic 17 4 Propeller Synchronising
Describetheoperationofcomponentsusedforsynchronisingandsynchrophasing.
Topic 17 5 Propeller Ice Protection
Describetheoperationoffluidandelectricalde-icing.
Topic 17 6 Propeller Maintenance
Explainthefollowingpropellermaintenance:
Staticanddynamicbalancing
Bladetracking.
Explainassessmentofthefollowingtypesofpropellerbladedamage:
Erosion
Corrosion
Impactdamage
Delamination.
Explainrepairschemesusedinpropellertreatment.
Explainproceduresandprecautionsforpropellerenginerunning
Topic 17 7 Propeller Storage and Preservation
Describethepreservationanddepreservationofpropellerandpropelleraccessories/systemscomponents.
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TOPIC17.1:PROPELLERFUNDAMENTALS
Lift
Liftistheaerodynamicforcecausedbyairflowingoveranaerofoil(Figure1.1).Theaerofoilshapeofanaircraftwingorpropellerisdesignedtoincreasethevelocityoftheairflowoveritscamberedsurface,therebydecreasingpressureabovetheaerofoil.Thiscombinationofpressuredecreaseabovetheaerofoilandahigherpressurebelowtheaerofoilproducesaforceupward.Thisforceistermedlift,andwithpropellersthisformsthebasisofbladeelementtheorywithabladeelementbeinganyrandomlyselectedareaofthebladeaerofoil.
Figure1-1.Lift
Drag
Dragisaforceopposingthrust,causedbythedisruptionorimpactofairflowover,orontoanaerofoil,(Figure1.2).
Figure1-2.Drag
Thrust
Thrustisaforwardactingforce.Itisthereactiontothemassofairbeingaccelerated
rearwards,(Figure1.2).Thrustisfeltonthebladeface,thisformsthebasisofmomentumtheoryforpropellers(Newtons3rdlawofmotion).
THRUST
Figure1-3.Thrust
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Total Reaction
Totalreactionofabladeistheresultantoftwopairsofforces:
liftanddrag
thrustandtorque
Byplottingthevectorsforliftanddrag,itispossibletoderivethetotalreaction(Figure1.4A).
Itisalsopossibletoderivethetotalreactionbyplottingthevectorsforthrustandtorque,(Figure1.4B).
(Figure1.4C)depictsbothpairsofvectorsarrivingatthesametotalreaction.
Figure1-4.BladeRotationalForces
Anincreaseinrotationalspeedwillincreasethesesforcesequally.
Rotational speedisrestrictedtoapointwherethebladetipspeedmustremainbelowthe
speed of sound.
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EFFECTS ON PROPELLER THRUST
Blade ngle
Ifyoustandsafelytothesideofastationaryaircraftandviewtherotatingpropeller,youwill
seetheplane(path)thatthepropellerisrotatingin.
Theanglebetweenthechordline,whichisanimaginarylinedrawnthroughthebladeandtheplaneofrotation,usuallymeasuredindegrees,istermedthebladeangle,asrepresentedinFigure1.5.
Figure1-5.BladeAngle
ngle of ttack
Theanglebetweenthechordlineandbladepath(angleofrelativewind/airflow)istermedtheangleofattack(Figure1.6).
Forbestresultsthisshouldbe2oto4o.Itiswithinthisangleofattackthattheincomingairiscompressed(shadedarea)thenallowedtoexpandasitleavesthetrailingedgeofthebladeresultinginthrust.
Figure1-6.AngleofAttack
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Pitch
Pitchisthedistancemovedforwardbythepropellerinonerevolution.Thiscanvarywithdifferentbladeangles,asillustratedinFigure1.7.
Figure1-7.Pitch
Blade Twist
Thefurtherawayfromthehubalongthepropellerblade,thefasterthatsectionofthebladeistravellingandifthetipreachesthespeedofsoundthenthatportionwillnotproduceanythrust.Therefore,ifapropellerhadnotwistalongitslengthwhenviewedfromtheside,thenonlypartofthepropellerwouldproduceanyuseablethrust.
Toensureallsectionsofthepropellerbladeproduceequalthrust,thebladeismanufacturedwithagradualtwist,fromhubtotip(Figure1.8).
Maintainingthisgradualtwistalsoensuresthatthecorrectangleofattackismaintainedat2oto4oalongthelengthoftheblade.
Figure1-8.BladeTwist
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Torque Reaction
Ifthepropellerrotatesanti-clockwise,theforceusedtorotatethepropelleristransferredtostationaryitems,eg.bearinghousings.Transferringtheforcetothestationaryitemswilltendtorotatetheaircraftintheoppositedirection(NewtonsThirdLaw)totherotatingpropeller,
ie.clockwise,asinFigure1.9.Thistendencytotryandrolltheaircraftistermedtorquereaction.
Figure1-9.TorqueReaction
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PROPELLER SLIP
Slipisdefinedasthedifferencebetweengeometricpitchandeffectivepitch.
Slip Geometric pitch - Effective pitch
Geometricpitchisacalculateddistancethatapropellerwouldadvanceforwardthroughasolidmedium,inonerevolution.
Effectivepitchisthedistancethatapropelleractuallydoesadvanceforwardinonerevolution.
Figure1.10showsslipasthedifferencebetweengeometricpitchandeffectivepitch.
Figure1-10.Slip
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EFFECTS ON IRCR FT ST BILITY
Propeller Torque
Ifapropellerisbeingdrivenanti-clockwise,thetorquethatisbeingdevelopedtodrivethe
propellerhasaneffectontheaircraftstructureandwilltendtorolltheaircraftclockwiseandviceversa.
Figure1-11.EffectsonAircraftStability
Propeller Slipstream
Arotatingpropellerwillimpartarotationalmotiontotheslip-streaminthesamedirectionasthepropeller.Thisrotatingoftheairhasanadverseeffectontheaircraftsfin.
Figure1.11showstwoairflowsflowingrearwards,onedark,onelight.Thedarkportionfirstlycurlsoverthetopoftheaircraft,thenunderit,priortoarrivingatthetail.Thelightportioninitiallycurlsundertheaircraftuntilitreachesthetrailingedgeofthewing.Itthencommencestorotatebackuphittingtherighthandsideofthetail.Thisforceactingonthe
tailwillcausetheaircrafttoturntotheright.
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Propeller Gyroscopic Effect
Therotatingmassofthepropellermaycauseaslightgyroscopiceffect.Arotatingbody(propeller)tendstoresistanychangeinitsplaneofrotation.Instraightandlevelflight,the
propellerwillresisteitheraturntotheleftorright.Ifsuchachangedoestakeplace,thereisatendencyfortheplaneofrotation(straightandlevel)tochangeinadirectionatrightangles(90o)towheretheforcewasapplied.Ifthepropellerrotatesanti-clockwise,thenosewillyaw(veer)totheright.
Anexampleofgyroscopiceffectistospinabicyclewheelwhileholdingtheaxle,andthentrytotilttheaxleinonedirectionwhileitisspinning.Youwillnotethatitactuallytiltsat90tothedirectionintended.
Contra Rotating Effect
Thefitmentofacontra-rotatingpropellerbasicallyeliminatestheeffectsofpropellertorque,propellerslipstreamandpropellergyroscopiceffect.Thesecondpropellerstraightenstheslipstreamofthefirstandcausesastraighthighspeedflowofairoverthefinandimproves
control.Propellertorqueiscancelledduetothefactthatthepropellersarespinninginoppositedirections,thereforecancellingoutpropellertorquewhilealsoneutralisingthegyroscopiceffect.
Forces cting on a Propeller
Asapropellerisrotating,itisacteduponbycertainforces.Theseforcesare:
centrifugalforce
centrifugaltwistingmoment
aerodynamictwistingmoment
bendingforces-
1. thrustanddrag
Centrifugal Force
Centrifugalforceisaforcethathasatendencytothrowtherotatingpropellerbladesawayfromthepropellerhub(Figure1.12).Thisforcecanamounttomanythousandsofnewtons.
Figure1-12.CentrifugalForce
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Centrifugal Twisting Moment
CentrifugalTwistingMoment(commonlyreferredtoasCTM)isaforcewhichtendstorotatepropellerbladestowardafinebladeangle.ThisisillustratedinFigure1.13.
CTMisaforcethatpropellermanufacturersutiliseonvariablepitchpropellers.Thisforceisusedtoalterbladeanglefromacoarsertoafinerbladeangle.
Figure1-13.CentrifugalTwistingMoment
erodynamic Twisting Moment
AerodynamicTwistingMomentisaforcethattriestomovethepropellerbladestoacoarserbladeangle.AsshowninFigure1.14,thecentreofpressureisforwardoftherotationalaxisoftheblade,whichisatthemidpointofthechordline,thisforcetendstoincreasethebladeangle.
Somepropellerdesignsusethisforcetoaidinthefeatheringthepropeller.
Figure1-14.AerodynamicTwistingMoment
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Bending Forces
Bendingforceisdividedintotwocomponents:
torquebendingforce(causedbydrag)
thrustbendingforce(causedbythrust)
Torque Bending Force
TorqueBendingForceisaresultantforcefromtheloadthatairresistance(drag)placesontheblades.Ithasthetendencytobendthepropellerbladesoppositetothedirectionofrotation.Figure1.15(a)showsanexaggerationoftorquebendingforce.
Figure1-15.(a)Torque Figure1.15(b)Thrust
Thrust ending Force
ThrustBendingForceisaforcewhichhasthetendencytobendthebladesforwardastheaircraftispulledthroughtheair.ThisbendingforwardofthebladesisexertedbythethrustthatpropelstheaircraftforwardasshowninFigure1.15(b).
Force ccentuation
BothAerodynamicandCentrifugalTwistingMoments(TorsionalStresses)areincreasedwithanincreaseinRPM,ie.ifRPMisdoubled,thesestressesarequadrupled.
Force Coupling
Thecouplingofcentrifugalforceandthrustcreateseverestresseswhicharegreaternearthehub.Thebladefaceisexposedtotensionfromcentrifugalforceaswellastensionfrombending.ThereforethepropellerneedstobedesignedtowithstandthesestresseswhichincreaseproportionallywithRPM.Asimplescratchordentinthebladecanhavesevererepercussions.
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EFFECTS ON NGLE OF TT CK
Tounderstandhowapropellersperformancecanvary,youwillneedtounderstandvectors.Youshouldrememberfromyourstudyofvectorsthatwherealineisdrawntoscale,itshows
avelocityorforce.Theselinesaredrawntorepresentspeed,ie.thelongeralineisdrawn,thefasteranitemsspeedisrepresented.
Theperformance(thrust)ofafixedpitchpropellerwillvarywithvariationsineither:
rotationalvelocity
aircraftvelocity
Ifapropellerisdesignedtoproducethecorrectangleofattack(2to4)atsay,1500RPMand50MPHforward,thenitwillproducetherequiredamountofthrustuntileitherrotationalvelocityorforwardvelocityalter(Figure1.16).
Figure1-16.
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Increased Rotational Velocity
Ifforwardvelocityismaintainedbutrotationalvelocityisincreasedto2000RPM,thenitcanbeseenthattheangleofattackisextremelylargeandinefficient.Figure1.17comparesthisincreaseinvelocitytotheefficientrunningofthepropellerblade.
Figure1.17
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Increased Forward Velocity
Ifforwardvelocityisincreased,ie.inadive,androtationalvelocitymaintained,thenitcanbeseenthatthebladepathhasmovedfrombeingbehindthechordline(apositiveangleof
attack)tobeinginfrontofthechordline.ThiscanbeseeninFigure1.18.Thisgivestherotatingbladesanegativeangleofattack,whichproducesnoforwardthrust.Thrustisnowbeingproducedintheoppositedirectionandactslikeabrake.
Figure1.18
Therefore,itcanbeseenthatchangingeitherrotationalvelocityoraircraftforwardvelocitywillalterthebladesangleofattack.Varyingapropellerbladesangleofattackwilllowertheefficiencyofthatbladeandthereforethepropellerasaunit.
Blade Tip Speed Versus Efficiency
Toallowpropellerstoabsorbtheenormouspowerthatenginescandevelop,largerpropellersweremade.Itwasfoundthattheincreaseinpropellerdiameterdidnotnecessarilyincreaseefficiency.
Infact,thelargerpropellerslostperformancethroughtipvibrationorflutter.Thisflutterorvibrationiscausedbyshockwavesasthetipofthepropellerapproachesthespeedof
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sound,whichisapproximately1117ft/s(or660knots)atsealevelonastandarddayof15C.
Itwasthereforenecessarytokeepbladetipspeedbelowthespeedofsound.Thismeantthatthepropellertipshadtobebelowthespeedofsoundandstillbeabletoabsorbthe
availableenginepower.
Thiscanbeachievedinseveralwaysbyincreasingthenumberofblades,orbyincreasingbladeshapeandsection.
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TOPIC17.2:PROPELLERCONSTRUCTION
Hub Assembly
Thehubassembly(Figure2.1)providesameansofattachingthepropellertotheengineand
supportstheblades.Thehubisdividedintoforwardandrearbarrelhalvestoenablefitmentofthebladesontothespiderwhichprovidesbearingsupportfortheblades.
Figure21.HubAssembly
Blade
Thebladeistheaerofoilpartofthepropellerthatconvertsthetorqueoftheengineintothrust.Figure2.2showsapropellerbladeremovedfromthepropellerassembly.
Figure22.Blade
Tip
Thepropellerbladetipistheportionofthebladethatisthefurthestfromthehubassembly.Itisusuallyreferredtoasthelastsixinchesoftheblade.Figure2.2showsthetipsectionofthebladeshadedblack.
Figure23.BladeTip
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Root (Blade Butt)
Theroundbladeroot,whichisalsoknownasthebladebuttispartofthepropellerbladewhichfitsintothepropellerhub(Figure2.4).
Figure24.BladeRoot
Blade Shank
Thisisthecylindricalpartofthebladenearthebladeroot(Figure2.5),itisusuallythickforstrengthandcontributeslittleornothingtothrust.
Figure25.BladeShank
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Cuff
Propellerbladecuffsaredesignedtorestoretheroundsectionofthebladeshanktoanaerofoilshapeandtherebyincreaseairflowtotheengine.Bladecuffsareusuallyconstructedofmetal,woodorplasticandareeitherclampedorbondedtotheblades.Figure2.2showsplasticcuffsbondedtothebladeshanks.
Figure26.BladeCuff
Leading Edge
Theleadingedgeofablade(aerofoilshape)asillustratedinFigure2.7,isthethickedgethatfirstmeetstheairasthepropellerrotates.
Trailing Edge
Afterairhaspassedtheleadingedge,itleavestheaerofoilatthetrailingedge(Figure2.7).Thetrailingedgeofapropellerbladeistherearedgeoftheblade,thepointwherethebladecamberfaceandthebladethrustfacejoin.
Figure27.BladeTrailingEdge
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Blade Back (Blade Camber Face)
Thebladebackistheforwardconvex(outward)curvedfaceofthepropellerbladesaerofoilandjoinstheleadingandtrailingedge,asshowninFigure2.8.
Figure28.BladeBack
Blade Face (Blade Thrust Face)
Theflatsideofapropellerbladeistermedthebladefaceorbladethrustface(Figure2.9).Itisonthisfacethatthethrustproducedbythebladeisfelt.
Figure29.BladeFace
Chord Line
Toassistindeterminingpropellerbladeangles,allaerofoilshaveanimaginarystraightlinedrawnthroughthem.Thisstraightlinecutsthroughthecentreoftheleadingedgeandcentre
ofthetrailingedge,andisknownasthechordline.Figure2.10illustratesthechordlineonapropellerblade.
Figure210.ChordLine
Figure2.11summarisesthetermsrelatingtobladesurface.
Figure211.BladeTerms
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Blade Stations
Toassistmaintenancepersonneltolocaterelevantpositionsonablade,thebladeshavedesignateddistancesalongtheirlengthasmeasuredfromthecentre of the hub,outtothetipofeachblade.
AsdepictedinFigure2.12,these"bladestations"arenormallymeasuredinsixinchintervals.Ifyouweretorefertodamageintheleadingedgeofthepropelleratthe20bladestation,youwouldnormallyrefertoitasbeinglocatedbetweenthe18and24bladestations.
Figure212.BladeStations
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composite(nonmetallicfibre).
CONSTRUCTION
Propellerbladesareusuallymadeofoneofthefollowing:
wood
metal1.
aluminiumalloy2.
steel
Timber
Theearliestpropellersfittedtoaircraftwereconstructedoftimber.Thesepropellersweremadefromanumberoflayersofhardwoodsgluedtogetherwithhighqualitywoodglue.Figure2.13showsatypicalwoodenpropeller.
Figure213.WoodenConstruction
Fabric Covering
Toaidinreinforcingthetipofeachblade,cottonfabricisgluedtothelast12to15(20-28cms).Figure2.14illustratestheareacoveredbyfabriccovering.
Thefabriccoveringnotonlyassistsinreinforcementofthetipbutaidsinprotectingthetipfrommoistureandreducesthetendencyforittosplitorcrack.
Figure214.FabricCovering
Laminating
Timberusedforthemanufactureofpropellersisspeciallyselected,wellseasoned
hardwoods.Thetimbershouldbefreefromimperfectionssuchas:
holes
looseknots
decay
Thetimberislayered,asinFigure2.15,andgivenapreliminaryshapingandfinishing,thenstackedtogetherandglued.
Figure215.TimberLaminatedConstruction
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Thepropelleristhenplacedinakilnwherethepressureandtemperaturearecarefullycontrolledforaprescribedtime.Thepropelleristhenshapedtoitsfinalform(Figure2.16),usingtemplatesandprotractorstoensurethatitmeetsdesignspecifications.
Figure216.BladeShaping
Aftershaping,thepropellerhasvariousprotectivecoatingsappliedtoit(Figure2.17),suchasfabriccovering,varnishandsheathing.Thesemethodswillbediscussedlaterinthistopic.
Figure217.ProtectiveCoating
Varnishing
Wood,duetochangeinmoisturecontent,issubjectto:
swelling
shrinking
warping
Aprotectivecoatingofvarnishisappliedtothefinishedpropellertopreventrapidchangesofmoisturecontent.
Leading Edge Sheathing
Duringtake-offandtaxiing,damagefromsmallstonesandsandcanoccurtotheleadingedgeofthepropeller.Toprotectwoodenpropellerblades,ametalshieldissecuredaroundthetipandalongtheleadingedge.
Thismetalshieldisknownaseitherleadingedgetippingorleadingedgesheathing.Smalldrainholesinthetippingnearthebladetipallowmoisturefromcondensationtodrainaway.
Leadingedgesheathingcanbemadefromeither:
terneplate
monel
brass
stainlesssteel.
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Figure2.18showstheinstallationofmetalsheathingonapropellerblade.
Figure218.MetalSheathing
Metal
Afixed-pitchmetalpropellerisusuallymanufacturedbyforgingasinglebarofaluminiumalloytotherequiredshape.
Thesepropellersincorporateacentreboretoallowfitmentofvarioussteelhubsoradaptorsprovidingfordifferenttypesofinstallations.Figure2.19showsatypicalfixedpitchaluminiumpropeller.
Figure219.FixedPitchMetalpropeller
Aluminium Alloy
Initially,metalpropellersstartoutasasinglebarofaluminiumalloy.Thesebarsarethenshapedandfinishedtothedesiredaerofoilshapebymachineforging,copyingtheshapeofamasterblade(sometimesreferredtoasaprofile)ontothebarofaluminium.
Duetothehighstrengthandmalleabilityofaluminiumalloy,theairfoilextendstothepropellerhub.Thiswillnotincreasethrustastheengineislocatedimmediatelybehindthis
areabutdoesacttoprovideanincreasedflowofcoolingairtotheengine.Shot Peening
Thisprocessisitselfafinishingtreatmentandnormallyrequiresnoothertreatments.
Nicks,gougesandotherminorbladedamagescanquicklyleadtostresscracking.Thisispredominantlyevidentonsteelpropellersduetotheirrelativelybrittlecharacteristic.Shotpeeningofmetalsisdesignedtodistributestressesmoreevenlyinthesurface(eg.aroundthebladeshank)andtoincreasefatiguestrength.Figure2.20showstheareaofametalpropellerwhichisusuallyshotpeened.
Figure220.ShotPeenedAreas
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Shot(beads/ballsofglass,steel,etc.)ofaknownsizearethrownbycentrifugalforceorairblastedthroughanozzleataprescribedpressureontotherequiredarea.
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Theimpactoftheshotcausesplasticdeformationofthesurfacetoadepthofafewthousandsofaninch.Ifthedepthofworkneedstobeincreased,allthatisrequiredisforthevelocityorsizeoftheshottobeincreased.
Varioustypesofshotcanbeused;twocommontypesaresteelandglassbeads.
Anodising
Anodisingisusedtoaddextraprotectiontoalloyblades.Itisanelectroplatingprocessusedtoprovideahardcoatingwhichis:
corrosionresistant
waterproof
airtight
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Composite
Compositebladeconstructioninvolvestheuseofspecialplasticresins.Theseresinsarereinforcedwithfibresorfilamentscomposedofoneofthefollowing:
glass
kevlar
carbon
boron
Therearetwowaysofconstructingacompositeblade.
Figure2.21showshowoneofthematerialslistedaboveisshapedaroundanaluminium-alloysparandfoam.
Figure221.CompositeConstruction
Figure2.22showshowacompositematerialshellisusedtoformthebladeprofileintowhichafoamcoreisplacedtoprovideresistancetodistortion.
Figure222.CompositeConstruction
Fibre Reinforced Plastic (FRP) Moulding
TheFRPmouldingisavariationofthecompositeblade.TheFRPbladeconsistsofalaminatedKevlarshellintowhichisplacedafoamcore.
Toboostthestrengthoftheshell,Kevlarislayerednotonlylengthwisebutalsomulti-directional.TheleadingandtrailingedgesofthebladearereinforcedwithsolidunidirectionalKevlar.
TwounidirectionalKevlarshearwebsareplacedbetweenthecamberandthethrustfacesurfacesoftheshelltoprovideresistancetoflexingandbuckling.
Thepolyurethanefoamfillingsuppliesadditionalresistancetoanydistortioncausedbyoperatingstressesthatthepropellerencounters.
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Figure2.23displaystheconstructionofablademadefromthematerialsdescribedabove.
Figure223.FRPMoulding
Figure2.23CompositeMaterialRetention
Compositematerialsarecommonlyretainedontheshankprimarilybyexternalcompositewindings.Thesecondaryformofretentionistheclampingactionofthehubhalves.ReferFigure2.23
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PROPELLER MOUNTING/INSTALLATION REQUIREMENTS
Correctinstallationofthepropellerontotheenginepropellershaftiscriticaltosafety(somepropshavecomeoffinflight),andtoavoidvibration.
TherearebasicallyTHREEtypesofinstallations:-
flangedshaft
taperedshaftand
splinedshaft.
Generallyspeaking,thesmallerengineshaveeitherofthefirsttwo,whilstthebiggerenginesusuallyhavesplinedshafts.
Flanged Shaft
Flangedshaftdescribesathickcircularflangeatthefrontoftheenginecrankshaft,witharingofholes,eitherplain(dowelpins)orthreaded(Figure2.24).Thepropisattachedbybolts.
Askullcapspinnerisfittedtosmallaircraftasanaerodynamicfairing.
Figure224.FlangedShaft
Preinstallationchecksinclude:
Inspecttheflangefordistortionandsurfacedefects.(doarun-outcheckontheflangeifdistortionissuspected).
Ensureboltholes/threadsareingoodcondition.
Applyalightcoatofoiloranti-seizetotheflangeandpropellermountingsurfacestoaidinthenextremoval.
CloseinspectionofattachmentboltsuseNDTdyepenetrantormagneticparticletobesure.
Ensureretainingnutsarenewandself-lockingnuts.
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Installationchecksinclude:
Offertheproptotheshaftinthecorrectindexingposition.Usually,thereisadowelholeorpintoensurethis.
Mostsplinedshaftshaveamasterspline.Onasmallenginewithoutindexing,fitthepropsothatthebladesareatthe4and10oclockpositiontofacilitatehandstarting.
Insertthebolts,nutsandwasherslightlytightenthenuts.Tightenthenutsprogressively,inthesequencegiveninthemaintenancemanual.
Notethebalancewashersmaybeinstalledundertheboltheadornut.
Correctlytorquethepropretentionnuts,tothetensionspecifiedintheManual.
Forwoodenprops,acircularfaceplateisinstalledatthefrontofthehubbosstospreadthecompressionloadandtherebyprotectthewoodfromcrushing.
Oncompletionoftheinstallation,atracktestwillshowthatbladetipsaredescribingthesametippathplane(seeinlaterchapter).
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Tapered Shaft
Foundmostlyonolderaircraftoflowerhorsepower,theenginecrankshaftisextended,inataperedform,tomatewithasimilarlyshapedprophub.Theinterferencefitofthesetwosurfaceswillprovidetheprimarytransferofpowertothepropeller.Groundthreadsattheend
oftheshaftaccommodatethepropretentionnut.Thesafetyholesallowforlockingofthenut.(SeeFigure2.25)
Thekeywayisalongmilledslotinthetaperedshaft,andthematingkeyindexesthehubtotheshafttopreventrotarymotionbetweenhubandshaftduringinstallation.Inservice,thekeywayissubjecttowearandsmallcracksespeciallyinthesharpcorners.Closeinspectionisessentialusingeitherdye-penetrantormagneticparticlemethods.
Figure225.SafetyHoles
Thekeytoagoodmatingfitbetweenhubandshaftisafullmetal-to-metalcontact,withthepropretentionnutfullytightened.
BeforematingthepartsapplyacoatingofPrussianbluetothecrankshaftend.Carefullymatethetwoandfullytorquetheretentionnut.
Thenseparatethejointandinspecttoseethatthereisatleasta70 transfer of the blue inktothehub.
Ifthereislesstransfer,lappingoftheshaftisallowabletomanufacturersspecifications.Thekeymustbeinsertedintothekeywayeachtimethehubandshaftaremated.
Tapershaftapplicationsgenerallyincorporateasnapringlocatedintheretainingnutandattachedtothehub.Thisitemactsaspulleraidingintheremovalofthehubbyactingtoovercometheinterferencefit.
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Taper Bore
Onvariablepitchapplicationstoprovideabearingsurfaceforthebladestoturnonwhenbladeanglechangesoccur,aremovablebushingisfittedintoaforging(taperbore)atthecentreofthebladebutt.Thisbushingalsoallowsforfitmentofaplugwhichisusedto
initiallybalanceeachbladeandisshowninFigure2.22.
Figure 2 26 TaperBoreForging
Thisforgingalongwiththebushing,permitsfitmentofeachbladeontothespider(Figure2.27),whichislocatedwithinthehubofthepropeller.
Figure227.SpiderForging
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Splined Shaft
Commonlyfoundonthelargerturboprops(Figure2.28).Thesplinesareevenlypitched,andthereisusuallyaMASTER(wider)splinewhichmatestheshafttothehubinonlyoneposition.Atight,butsliding,fitisrequiredtopreventfrettingandsubsequentwear.ThiswearischeckedwithaGONO-GOgauge,andcarefulinspectionforsmallcracksespeciallyinsharpcorners(dyepenetrantormag,particlemethods).
Figure228.SplinedShaft
PropshaftsplinesonAmericanenginesaredescribedbytheirdiametereg.SAE20,40,50,20inascendingordereg.OntheDC2,itsP&WR1820enginesdriveHamStandard22E50modelpropellers.Inthiscode,Edenotesthebladeshanksize,and50denotesthepropshaftsplinesize.
Taperedconesareused,frontandback,tocentrethehubontothepropshaft.Therearconeisofbronze:thefrontofsteel,manufacturedintwomatchedhalveswithmatchingserialnumbers(Figure2.29).
Aswithtaperedshaftinstallations,Prussianblueisusedonconefaces/hubfacestocheckthedegreeofmatingaftertheprop-retainingnuthasbeenfullytorquedtopullthesurfacestogether.
Sometimesthedatarequirestheconestobefitteddry,whilstothersspecifyalightoilcoating.Whenofferingtheproptotheengineitisgoodpracticetofirstfitaprotectortothepropshaftscrewthreads,asitiseasytodamagethemwhilstinstallingthepropeller.
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Figure229.TaperedConeInstallation
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PROPELLER TYPES
Tractor Propeller
Tractorpropellersarethoseconventionallymountedinfrontoftheenginepowerplant.Tractorpropellerspulltheaircraftthroughtheair.Mostaircraftareequippedwiththistype
ofpropeller.RefertoFigure2.30foratractortypearrangement.
Figure230.TractorTypePropellers
Pusher Propeller
Pusherpropellersaremountedonadriveshaftfromtherearoftheengineproducingthrusttopushtheaircraftforward.
Manyseaplanesandamphibiousaircraftusepusherpropellers.
Figure231.PusherTypePropeller
Toreducethechanceofbladesbeingdamaged,manypusherpropellersaremountedaboveandbehindthewings,(Figure2.31).
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Fixed Pitch
Afixedpitchpropellerisonewhosebladeanglecannotbechanged.
Afixedpitchpropellerisdesignedforaspecificpurposeie.cruiseoracceleration.Apropellersperformancewilldropoffrapidlywhenoperatedoutofitsdesignedpurpose.
Figures2.32and2.33showfixedpitchpropellers,beingmetalandwoodenrespectively.
Figure 2 32 MetalFixedPitchPropeller
Figure233.WoodenFixedPitchPropeller
Ground Adjustable
Theearliestadjustablepropellersoperatedasfixedpitchstylepropellers.Thepitchcouldonlybealteredwhenthepropellerwasnotturning.Thiswasachievedbylooseningtheretainingclampsorboltssecuringeachbladeinplace.
Withtheclampsorboltsloosened,thebladescanbeadjustedtotheirrequiredanglewiththeaidofaprotractor.
Aftertheclampshavebeentightened,thepitchofthebladescannotbechangedinflighttomeetvaryingflightconditions.
Figure2.34showstheretainingclampsonagroundadjustablepropeller.
Figure234.GroundClampInstallation
Controllable Pitch
Acontrollablepitchpropellerallowsbladeangletobechangedwhilethepropellerisrotating.Controllablepitchpropellerscanvaryfromatwopositionpropellertoonethatcanbealteredtoanyanglebetweenminimumandmaximumsettings.
Thispermitsthepropellerbladeangle(pitch)tobechangedtogivethebestperformanceforparticularflightconditions.
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Constant Speed
Aircraftfittedwithconstantspeedpropellersallowaselectedenginespeedtobemaintained.IftheengineRPMvaries,thepropellerbladeangleischangedbyaspeedsensitivegovernortobringtheRPMbacktotheselectedspeed.ThistypeofsystemreducespilotworkloadandprotectstheenginefromlargeRPMfluctuations.
Contra Rotating
Contrarotatingpropellersaretwoseparatepropellersmountedinlineontwoconcentricshaftswhichrotateinoppositedirections.
Theprimaryreasonforfitmentofcontrarotatingpropellersistoabsorb(andthereforeefficientlyuse)theoutputofhighpoweredengines.Anadvantageofthistypeofpropelleristhecancellationoftorquereactionandareductionofthespirallingslipstream,ie.muchstraighterairflow.Figure2.35showshowcontrarotatingpropellersaremountedonebehindtheother.
Figure235.ContraTypePropeller
Counter Rotating
Counterrotatingpropellersshouldnotbeconfusedwithcontrarotatingapplications.Thetermcounterrotatingreferstoatwinengineapplicationwherethepropellersoneachengine
turninoppositedirectionsofrotationtocounteracttorquereactionandgyroscopiceffects.Feathering
Afeatheredpropellerisofthecontrollablepitchpropellertype.Onmultiengineaircraft,featheringcapabilitiesmustbeutilisedtopreventdestructionofafailedengine(failuretopreventthisdamagecouldresultinlossofaircraftandorlife).
Thesepropellershaveamechanismtochangethebladeangletosuchapositionthatpropellerrotationstops,ie.thebladechord(atasetdistancefromthehub)isparalleltothedirectionofflight.Thethickedgeofthepropellerfacesinthesamedirectionthattheaircraftisflying,preventingthepropellerfromwindmilling.Featheringthepropelleralsoreducesdragonafailedorshutdownengine.
Shuttingdownanengineandfeatheringthepropellerisamethodusedonmanymulti-enginedaircrafttoconservefuelonlongflightduration.Figure2.36showsacomparisonofpropellerbladeangles.
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Figure236.PropellerBladeAngleComparisons
Reversing
Reversingpermitsanaircrafttoreduce:
landingruns
brakewear
tyrewear.
Reversingalsoassistsingroundhandlingbyallowingtheaircrafttobetaxiedbackwards.Whenreversehasbeenselectedinthecockpit,thepropellerbladesrotatefromapositiveanglethatwillmaintainflight(airflowrearward-forwardthrust)toanegativeanglewhere
thrustisnowbeingproducedrearwards(airflowforward-rearward/negativethrust).Reversecanalsobeusedtoslowtheaircraftdownuponlandingandthereforeshortenthelandingroll.Figure2.37showsacomparisonbetweennegative/reverseangletopositive/forwardangle.
Figure237.NegativeandPositiveAngleComparisons
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PROPELLER EFFECTS ON OPERATION
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Propeller Selection
Somefactorstobeconsideredwhenselectingapropellerare:
Engine power-thepropellerneedstobeabletoabsorbtheavailableenginetorque.
Engine type-themethodofpropellerattachmenttotheengine,i.e.pusher/tractortype,splined/taperedpropellershaft,reciprocating/gasturbineetc.
Aircraft design-clearancesbetweentheground,fuselage,tailplaneandenginenacelleallneedtobeconsideredaswellastheeffectoftheairflowoverthewings,tailplaneandcontrolsurfacesetc.
Aircraft performance-aircraftoperatingaltitude,cruisingspeed,landing,take-offrolletc.
Thesefactorsaswellasotherssuchascostandavailabilityneedtobeconsideredwhenselectingasuitablepropellerforspecificapplications.
Engine Power Requirements/ Performance Factors
Thepropellermustbeabletoabsorbthepowergiventoitbytheengine,otherwisethepropellerwillrace(speedup)andbothpropellerandenginewillbecomeinefficient.
Thefollowingfourfactorsneedtobeconsideredwhenapropelleristobechosenforanenginewithknownpoweroutput:
propellerdiameter
numberofblades(onthepropeller)
propellerbladeshapeandsection
propellermass(solidity).
Propeller Diameter- asmentionedearlier,aspowerincreasedsodidpropellerdiameter.Thediameterofpropellershadtobelimitedduetothetipsreachingthespeedofsound.Thislimitationwasovercomebyusingeithercontrarotatingpropellersorincreasingthenumberofbladesfittedtothepropeller.Fittingofcontrarotatingpropellerstoanengineisineffectputtingtwopropellersontotheoneengine,therebyallowingthediameterofthepropellertobereduced.
Number of Blades- toreducetheoverallsizeofapropelleronemethodusedistoincreasethenumberofbladesfittedtoapropeller.Thisallowsenginepowertobeabsorbedwithoutincreasingthepropellerdiameter.
Ofthefourfactors,increasingthenumberofbladesisthemostefficientmethodofabsorbingincreasingenginepowerasinFigure2.38.
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Figure238.NumberofBladeConfigurations
Blade Shape and Section- anothermethodusedtoabsorbpowerfromanengineistoaltertheshapeorcamberofthepropellerblade;thiseffectivelyincreasesthethrustofapropeller.However,ifcamberisincreasedtoproduceextralift,thendragisalsoincreased.Toachieveabalance,acompromisemustbemadeinrelationtothepropellersshapeandsize.
Figure239.BladeShape
Addingtotheincreaseddragistheextraweightthateachpropellerbladewouldincur.Anyadvantageinliftwouldthereforebelostbythepenaltyoftheincreaseindragandaddedweightofeachblade.Figure2.39illustratesabladewithanincreaseincambershowingtheproportionalincreaseinsizeandthereforeanincreaseinweight.
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Prop Solidity- thesolidityofapropelleristheratiobetweenthepartofthepropellerdiscwhichwhenviewedfromthefront,issolid(blades,dome,etc.)andthatpartwhichisair.
Forexample,inFigure2.40thepropellerareamaybe10%ofthetotalareaofthedisc,thereforeitssolidityis1:10.
Thisratioismeasuredbyaddingupallthebladechordlengthsatacertainbladestation(saythree-quartersofthetipradius)anddividingthissumbythecircumferenceofthatradius.Thegreaterthesolidity,thegreaterthepowerthatcanbeabsorbed.
Figure240.PropellerSolidity
Toincreaseapropellerssolidity:
increasethenumberofblades(takingintoconsiderationpropellerdiameter)
increasethebladeschordlength(width)
fitmentofcontrarotatingpropellers.
Thiswillincreasethepropssolidityandthereforethrust.
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TOPIC17.3:PROPELLERPITCHCONTROL
Pitch Changing Mechanisms
Therearemanyvarioustypesofaircraftoperatingindifferentflyingconditions;noonepropellerwillsuitallaircraftandconditions.Therefore,differentpitchchangingmechanisms/systemsweredevelopedtovarythepropellerbladepitchtosuitaparticularaircraftandoperatingcondition.Fourofthesesystemsare:
aerodynamic
aerodynamic&hydraulic
hydromatic
mechanical
electrical.
Aerodynamic
Aerodynamicpropellersarenormallyreferredtoas"Automatic"pitchchangingprops.Theyareoccasionallyseenonsomelightaircraft.
Agoodexampleisthe"Aeromatic"propellerwhichusesthenaturalforcesactingonthebladestochangebladeangle,assistedbycounterweightsattachedtothebladeshanks.Thebladepivotaxisdoesnotalwaysliealongthesamelineasthebladeaxiscentreline.Duringoperation,theselinesleadandlageachother.
ThedesignoftheAeromaticpropelleractsasfollows:
ThethrottleisopenedandRPMincreases.
AlthoughacourserpitchisrequiredtheRPMriseincreasesCTMandthebladesexperienceahigherangleofattack(Figure3.1a).
Thecentreofpressurepointontheblademovestoapointfurthertendingtowardsafinerpitch.
Thecounterweightsaretryingtocoarsenthepitch-butatthispointareoverwhelmedbytheotherforces.
Astheaircraftacceleratesadecreaseinbladeangleofattackresultsandthebladecentreofliftreversesdirection,thustendingtoincreasepitch(Figure3.1b).
TheriseinairspeedtendstodrivethepropuptohigherRPMandthebladecounterweightscannowcompensatebyforcingthebladestoahigherangle.Thisincreasedpowerabsorptionloadswillallowtheenginetodroptherpmtotheoriginalselectedvalue.
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Figure31.a Figure31.b
TheAeromatichasnocockpitcontrolbutisstillratedasaconstantspeed,variablepitchpropeller.Itdoesnotpossessafeatheringcapability.
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Aerodynamic and Hydraulic Combination
Two Position Propeller
Thisisthemostbasicdesignwhichisnotdependantuponanenginedrivengovernor.The
propellercanbepositionedinafineorcoarsepositionfromthecockpitbyaleverthatcontrolsengineoilpressuretothehub.
Engineoilpressureoverridesthecounterweightsandresultsinafullfinepitch.Thispressureisdumpedbacktotheenginecrankcaseoncoarseselectionandthecounterweightsmovethebladestoafullcoarsepitch.
Thissystemutilises:
CTMtofine
centrifugalforce(onthecounterweights)tocoarse
engineoilpressuretofine.
Figure3.2illustratestheseforcesactingonthetwopositionpropellerandtheirdirections.
PROPELLER BLADE
CTM
ENGINEOIL
PRESSURE
GOVERNOROIL
PRESSURE
CENTRIFUGALFORCE
COUNTERWEIGHT
CYLINDERASSEMBLY
PISTON
Figure32.
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Constant Speed (Bracket Type)
Governor Oil Pressure
Oilfromthegovernorpumpforcesthecylinderout(towardsafinebladeangle)againstthe
centrifugalforceactingonthecounterweightsonarotatingpropeller.Thiskeepsthebladeangleconstant,orcanevenmovethebladestoafineangleifsodesiredbythepilot.
Fine Blade Angle
Acombinationofgovernoroilpressureactingtomovethecylinderout,andCTMtendingtomovethebladestoafineangle,overcomethecentrifugalforceactingonthecounterweights,therebyalteringthebladeangletoafinerpitch(Figure3.3).
Figure33.FinerPitchAngle
Coarse Blade Angle
Aspecialportwithinthegovernorisopened,allowingoiltoflowoutofthecylinder.Thecounterweightsarephysicallyattachedtoeachbladeandthemoveablecylinder.Withtheoilpressuredissipatingfromwithinthecylinder,centrifugalforceactingonthecounterweightsisusedtoovercomeCTMandmovethecylinderrearwards.Theblades,beingattachedtothecounterweightswillaltertoacoarserpitch(Figure3.4).
Figure34.CoarserPitchAngle
Alltheseoperations,oilin/out,arecontrolledbyagovernorwhichinturncontrolsthepositionofthecounterweights.Thegovernorisattachedto,anddrivenbytheengine.
ThisHamiltonStandardcounterweightdesigndoesnotsupportafeatheringcapability.
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McCauley Constant Speed- thisdesignusesgovernoroilpressuretodecreasebladeangle.Theopposingforcesarecounterweightsandaboosterspringlocatedinthehubtoincreasepitch.Themovementoftheinternalpistonistransmittedthroughphenoliclinkstothebladebutt.
Hartzell Constant Speed- Hartzellpropellersutilisetwomajordesigns.TheSteelHubwhichemploysanexposedpitchchangingmechanismandtheCompactwhichcontainsthemechanismwithinthehub.
TheSteelmodelshaveacentralspiderhub,whichallowsthehollowshankbladestobespigottedoverthespiderarms,andretainedbysteeltwopiececlamps.
Thepitchchangingmechanismconsistsofacentrallymountedpistonconnectedtothebladeclampsbysteellinkrods.Steelsinsomeapplicationswillutilisecounterweights.
Steelswithcounterweightsutilise:
counterweightstoincreasebladeangle
governoroilpressuretodecreasebladeangle.
Steelswithoutcounterweightsutilise:
governoroilpressuretoincreasebladeangle
CTMtodecreasebladeangle.
CompactsalwaysuseCTMtodecreasebladeangleandgovernoroilpressuretoincreasebladeangle.Ifcounterweightsareemployedtheywillacttoassistgovernoroilpressure.
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Thepitch-changingmechanismofhydraulic(hydromatic)propellersusesamechanical-hydraulicsystem.Agovernorsensestheenginespeedandcontrolshydraulicflowtoand
fromeithersideofadomepistonlocatedatthefrontofthepropeller(Figure3.5).(Hydraulicflowcanbeacombinationofengineandgovernoroilpressureorjustgovernoroilpressuretoincreaseanddecreasebladeanglesdependingonpropellertype).Thesehydraulicforcesactingontheinternalpistonaretransformedintomechanicalforces.
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Hydromatic/ Hydraulic
ThemechanicalforcesrotatethebladestorequiredanglestomaintainengineRPMbyforeandaftmovementofthepiston,whichhasbeenconvertedtorotarymotionbycamtracksandfollowersinthedome.Abevelgearatthebaseoftherotatingcamengageswiththeblade,andthereforealtersthebladeangle.AlteringbladeangleallowsengineRPMtochangebyalteringtheloadonthepropellerandsotherequiredenginespeedmaintained.
Figure35.PitchChange
Mechanical
Anexampleofamechanicalcontrollablepitchdesignisthe"BeechRoby"forlightaircraftwhichneedonlyasmallpitchrange.Thispropiscontrollablefromthecockpit,allowingthepilottosetthebestbladeangleforvaryingconditionsofflight.
Thereisasmallcrankhandleontheinstrumentpanel.Whenrotated,aconnectingflexiblecablerotatesapiniondrivegear.Thismesheswithalargedrivengearwhichislocatedaroundthecrankshaftandismountedontheenginecrankcase/nosesection.
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Rotarymotionofthedrivengearistranslatedintoaxialpitchchangingviahelicalslotsinthedrivengearflange.Lugpinsintheactuatorflangeslideintheslots.
Thetwoarmsoftheactuatorextendforwardintotheprophubandconnecttoanactuating
pinineachbladebase(Figure3.6).Thus,axialmovementoftheactuatorcausesthebladeangletochange.
Thereisacockpitgaugewhichdisplaysthebladeangle.
Itisnotaconstantspeedingprop.
ThereisnoRPMgovernor.
Onevariationistouseanelectricmotortodrivethepiniongear.Apairofmicroswitchesisusedtostopthemotoratthehighandlowbladeanglepositions.ThisoperationisdescribedundertheElectricsystemfollowing.
Figure36.
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Electric
Theelectricpitchchangingmechanismenableslightaircraft,aslittleas25horsepower,tobefittedwithcontrollablepitchpropellers.Thissystemisusedbecauseitislessexpensiveandcomplexthanaconstantspeedsystem.
Thecontrolforanelectricmotorismanagedbythepilotviaathreepositiontoggleswitchwiththesettingsof:
increaseRPM
decreaseRPM
off.
Theelectricmotorismountedneartherearofthepropellerontoafixedsleeve.Thismotordrivesalargeoutertoothedringgear.Asthisringgearisrotatedbytheelectricmotor,theringgearhasinternalspiralslotsthatengagelugsonthepitch-controlbearing.Thiscausesthebearingtomoveforwardsandbackwardsastheringgearrotates.Theinnerraceofthe
bearinghastwoarmsthatextendforwardintothehub.Thesearmsconnecttoanactuatorpinonthebladebuttandrotatethebladestoeitherahighorlowbladeangle.ThisinturnaltersengineRPMtoeitheralowerorhigherRPMselection.Figure3.6givesadiagrammaticexplanationoftheaboveprocedures.
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PROPELLER AUXILIARY SYSTEMS
NEGATIVE TORQUE SENSING (NTS)
Purpose
Torqueisthetwistingforceimpartedtoashaft.Inapropellerinstallation,whentheengineisdrivingthepropeller,thetorqueisconsideredtobepositive.Negativetorqueisaconditionthatwilloccuriftheengineisnotdevelopingenoughpowerandthewindmillingofthepropellerdrivestheengine.
Figure37.NegativeTorqueSystemSchematic
Components
ThecomponentsoftheNTSsystemarethe:
fixedringgear
planetarygears
ringgearcoupling helicalsplinecoupling
NTSsplinering
NTSplunger
NTSbracket.
Althoughthepropellerwouldgovernonspeed,ahighlevelofdragwouldbepresent.Tominimisedrag,adeviceinthereductiongearboxsensesnegativetorqueandextendsaplungerwhich,throughamechanicallinkage,actuatesthefeathervalve.Thefeatheringsystemoverridesallotherfunctionsandimmediatelyrotatesthebladestowardsincreasepitch.Asthebladeangleincreases,thenegativetorquedecreases.
Whenthenegativetorquesignalisremoved,thepositionofthefeathervalveisreturnedtonormal;increasepitchactionceasesandbladeanglereturnstowardnormal.Iftheconditioncausingnegativetorqueisnotrectified,thenegativetorquesystemwillcausethepropellerto
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operateinawindmillingconditionagain,andtheactionwillberepeated,cyclingaboutabladeanglewhichdevelopsarelativelylowlevelofnegativetorque.
TheresultantdragisfarlessthanthatwhichwouldattendOnspeedgoverninginthewindmillingcondition.Minimumdragcanbeattainedonlybyfeatheringthepropeller.The
abilitytofeatherisnotaffectedbytheexistenceofnegativetorquesignals.
Figure38.
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Operation
Whentheengineisdrivingthepropeller(positivetorque)thetorqueisfeltonthefixedringgear(whichcanturnasmallamount).Thisthenturnstheringgearandhelicalsplinecouplingwhichisattachedtoit.
Thehelicalsplinescausethehelicalsplinecouplingtomoverearwardsandthe14springswillpreventtheplungerfromactuatingtheNTSbracket(Figure3.7).
Whenthetorqueisnegative,thetorquefeltonthefixedringgearisintheoppositedirection.Thehelicalsplinecouplingwillnowbeturnedintheoppositedirectionandthehelicalsplinecouplingwillbeforcedforwardsagainstthe14springs.
Figure39.NegativeTorqueSignalSystem
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TheplungerwillthenbeforcedforwardandwillactuatetheNTSbracketwhichwillmovethefeathervalveandincreasethebladeangleofthepropeller.Asthebladeangleincreases,theloadonthepropelleralsoincreasesandwillslowthepropellerandremovethenegativetorquesituation.
Thetorquehasnowreturnedtonormalandthesystemwillnowreturntonormaloperation.Ifthenegativetorquesituationisstillpresent,thewholeprocesswillberepeated,andwillcontinuetoberepeatedwhileevernegativetorqueispresent.
Figure310.NTSActuator
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Manual Feathering
Manualfeatheringreferstoasysteminitiatedfromthecockpit.
Whetheritbeasystemwhichelectricallyengagesthefeatherpumpasinthehydromatic
propellersystemoremployingaliftrodmethodtooverridethespeederspringandflyweightsinthesmallerMcCauleysystem,aslongasaninputisrequiredthesystemwillbereferredtoasmanual.
Auto Feather
Purpose
Somemulti-engineaircraftuseanautomaticfeatheringsystemtofeatherpropellersautomaticallyiftheengineshouldfail.Thissystemisusuallyturnedoffinnormalcruisingflight,andselectedonforbothtakeoffandlanding.
Components
Inthecockpitthereisaguarded'automaticfeathering'masterswitch,whenthisswitchis
selectedtothe'on'positionalightindicatesthatthesystemisarmed.
Thethrottlewillhaveamicroswitchatapproximately75%offullthrottlemovement(dependingontheaircraft).Whenthethrottleisbelowthissettingtheswitchisopenandtheautofeathersystemwillnotoperate.
Thesystemalsocontainsatorquepressureswitch,whichisusedtosensethetorqueoutputfromtheengine.Whenthetorquedropsbelowaspecifiedleveltheswitchwillcloseandarmthesystem.
Mostcircuitsincorporateatimedelayunittopreventautofeatheringifthereisonlyamomentaryinterruptioninenginepower.Thepowerlossmustthenexceedonetotwosecondsforthesystemtoautofeather(thisdelaymayvarywithaircrafttypes).
Whentheauto-feathersystemisactuated,aredlightinthecockpitisusedtoindicatetothepilotwhichpropellerhasfeathered.Thepilotcanalsooperatethefeathersysteminthenormalmanner.
Thesystemalsousesablockingrelaytopreventmorethanoneenginebeingfeatheredatatimebytheauto-feathersystem.
Atestswitchcanbeusedtobypasspartsofthecircuitsothatthesystemoperationcanbecheckedonthegroundwithoutdevelopinghighpower.
Operation
Priortotakeoffandlanding,thesystemisarmedbyturningonthesystemmasterswitch.Aspowerisadvancedfortakeofforforamissedlandingapproach,thethrottleswitchclosesand
thetorquepressureswitchisarmed,butthetorquepressureswitchcontactsareopen.
Whenalossofenginepoweroccurs,thetorquepressureswitchclosesand,afterasetintervaloftime,thetimedelayunitcompletesthecircuit,energisingthefeathercontrol.
Theblockingrelayisalsoactuatedtopreventotherenginesfromautofeathering.
RefertoFigure3.11forabasicautomaticfeathersystem.
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Figure311.BasicAutomaticFeatherSystemSchematic
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PROPELLER BRAKING
Purpose
Thepropellerbrake(Figure3.12)isdesignedtopreventthepropellerfromwindmillingwhen
itisfeatheredinflightthuscreatingexcessivedragandtodecreasetherundowntimeaftergroundshutdown.
Figure312.PropellerBrakeAssemblyInstallation
Components
Thepropellerbrakeassembly,whichconsistsofthefollowingcomponents,isinstalledinthereductiongearboxassembly(Figure3.13):
1.
innercone
2. outercone
3. outermember
4.
startershaft5. helicalsplines
6.
applysprings.
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Figure313.PropellerBrakeAssemblyComponents
Operation
Thepropellerbrakehasthreepositions.Theseare:
applied-brakeapplied
released-nobrakingaction,and
locked-propellerhasturnedagainstDOR
Applied -WhenengineRPMdropsbelowapproximately21%,theoilpressureinthereductiongearboxthatholdstheinnerandouterconesapartdropsbelowtheappliedspringpressure.Theapplyspringsthenbringstheinnerandouterconestogetherwhichcausesabrakingaction.
Released -Duringstart,thebrakehastomovefromthe"applied"tothe"released"position.Thismovementtakesplacewhenthestarterinputshaftisturnedbythestarter.Thehelicalsplinesmachinedontotheshaftwillcausetheinnerandouterconestoseparateagainstthesprings.Whentheoilpressurerisestoahighenoughpressure(approximately21%engineRPM),theinnerandouterconeswillbeheldapartandthebrakeisreleased.
Locked -Whenthepropelleristurnedagainstthedirectionofrotation,thehelicalsplinescausestheinnerandoutercones(whichareintheappliedposition)tomoveforwardcausingthemtolocktogether.Thespringswillbeovercentredandwilltendtoholdtheconesinthelockedposition.Thepropellerwillnotbeabletobeturnedineitherdirectionuntilthebrakeisreleased.
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SAFETY COUPLING
Purpose
Thesafetycoupling(Figure3.14)isdesignedtodecouplethereductiongearboxfromthe
powersectionshouldtheNTSsystemfailtolimitnegativetorque.
Figure314.SafetyCouplingAssemblyInstallation
Components
ThesafetycouplingconsistsofthefollowingcomponentsasdetailedFigure3.15:
innermember
intermediatemember
outermember
setofbellevillesprings.
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Figure315.SafetyCouplingAssemblyComponents
Thepinioninputgearissplinedtotheinnermemberwhichissplinedtotheintermediate
memberbyhelicalsplineswhichareheldengagedbythebellevillesprings.Theintermediatememberisthensplinedtotheoutermemberwithstraightsplines.Theoutermemberisattachedtothetorqueshaftwithbolts.
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Operation
IftheNTSsystemshouldfailtolimitnegativetorque,thehelicalsplineswillactagainstthebellevillesprings.Oncethenegativetorquereachesapredeterminednegativetorquevalue
thehelicalsplineactionwillovercomethebellevillespringsanddisengage,decouplingthereductiongearboxfromtheengine.
Whentheengineisshutdown,thespringswilltrytore-engagethehelicalteethbetweentheinnerandoutermembers.Thisre-engagementmaycausedamageandoverheatingofthecoupling.
ThesafetycouplingoperationisshowninFigure3.16.
Figure316.SafetyCouplingOperation
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UNFEATHERING ACCUMULATORS
Somepropellershaveaspecialfeaturethatisusedtoincreasethespeedofunfeathering.Innormaloperationtheaccumulatorstoresgovernoroilpressure.Whenthepropellerisfeatheredtheaccumulatorvalveisclosedandtheoilpressureistrappedintheaccumulator.
ThesystemisshowninFigure3.17.
Whenthepropellercontrolisplacedinthenormalpositionthestoredpressureintheaccumulatorisappliedtothepropellertorotatethebladestoalowpitchangle.
Note: Whenthepropellerisinfeathertheengineisstoppedandgovernoroilpressureisunavailable.Thepressurestoredintheaccumulatorisusedinplaceofthepressurethatwouldbenormallysuppliedbythegovernor.
Figure317.UnfeatheringAccumulatorSystem
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TORQUEMETER
Thetorquedevelopedwithinthepowersectionistransmittedtothereductiongearboxviathetorquemeterinnershaft.
Thetorquetransmittedtothereductiongearboxisaccuratelymeasuredbythetorquemeterassembly.Itmaybemeasuredininchpoundsorshafthorsepower.
AttorquemeterassemblyinstallationisdetailedinFigure3.18.
REDUCTIONGEAR ASSEMBLY
TIE STRUT
TORQUEMETERHOUSING
AIR INLETHOUSING
TORQUEMETERASSEMBLY
Figure318.TorquemeterAssemblyInstallation
Components
Thetypicalelectro-mechanicaltorquemeterassemblyconsistsofthefollowingmajorcomponentsasdetailedinFigure3.17:
torquemeterinnershaft
torquemeteroutershaft
torquepickupassembly
torquemeterhousing
phasedetector
indicator.
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Operation
Thetorquemetermeasurestheangulardeflection(twist)betweenthetorqueshaftandthereferenceshaft(Figure3.19).
Thetorqueshaftandreferenceshaftarelockedtogetherandsplinedtothepowersection.Atthereductiongearboxend,eachshafthasatoothedwheelknownasanexciterwheel.
Stock No
Part No
Serial No
CAL A
TOR
CAL SW
S1
J2CAL A
CAL B
PhaseDetector
Indicator
Torquemeter Pickup
TorquemeterHousing
Torquemeter Outer Shaft(Reference Shaft)
Torquemeter Inner Shaft(Torque Shaft)
Figure319.
Onlythetorqueshaftisboltedtothereductiongearboxleavingthereferenceshaftto"freewheel".Whentorqueisappliedtothetorqueshaftitwilltwistinrelationtothereferenceshaft.
Thiswillcausetheteethonthetorqueshaftexciterwheeltolagbehindtheteethonthereferenceshaftexciterwheel.Thetotaldeflectionbetweenexciterwheelteethatfullpowerwouldbeonlyminute.
Thislagismeasuredbythetorquemeterpickupandsenttothephasedetector.Thephasedetectorconvertsthesignaltoavoltage.
Thevoltageisthentransmittedtothecockpitindicator.Thegreaterthetorque,thegreaterwillbethedeflectionbetweenexciterwheelteeth,thegreaterthevoltagethatistransmittedtotheindicator.
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THRUST SENSITIVE SYSTEM
Thethrustsensitivesystemisdesignedtoremovethedragcausedbyawindmillingpropellerbymonitoringenginethrustoutput.
Ifapowerdeclineissensedthesystemwilloperatetomovethepropellertoacoarsebladeangleorthefeatherpositionandallowforamoreslipstreamedcondition.
Onesystemutilisesaplungerswitchrunningonthepropshaftthrustbearingwithinthereductiongearbox.Aspringloadedassemblybetweenthepropellerthrustandaxialbearingsallowsformovementoftheshaftafterpositivethrustisachieved.
Anydropinthrustbelowthepredeterminedpositivethrustvalueoperatestheplungerswitchandbringstheautofeathercircuitonline.
Anotherlesscommonsystemsamplespitot(dynamic)pressurebehindthepropeller.
Adropbelowapredeterminedpressurewillsendasignaltothepropellercontrolsystemtoautofeatherorfullcoarsedependingonthecapabilityofthesystem.
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GOVERNORS
Purpose
Thepurposeofthegovernoronaconstantspeedpropelleristomaintainconstantengine
speed.ItwillmaintainasetengineRPMwithchangesinthrottlepositionandaircraftspeed.
Single Acting Governors
ThepropellergovernorisanRPMsensingdevicethatcontrolsoilflowtothepistonofthepropeller.Themainparts(Figure3.20)ofthesingleactinggovernorare:
governoroilpump
speederspring
pilotvalve
flyweights
rackandpinion.
Governor Oil Pump
ThegovernorsrotatingflyweightsaredrivenviaadriveshaftthatisconnectedtotheenginedrivetrainandisdrivenataspeedproportionaltotheengineRPM.Drivenfromthissameshaftisthegovernorpump(Figure3.20).
Thegovernoroilpumptakesengineoilpressureandboostsittothepressureneededtooperatethepropeller,andisthenknownasgovernoroilpressure.Excesspressurefromthepumpisreturnedtotheinletsideofthepumpbyapressurereliefvalve.
Pilot Valve
Thegovernorboostedoilisdirectedthroughpassagesinthegovernortoapilotvalvewhichsitsinthecentreofthehollowdriveshaft(Figure3.20).
Thepilotvalvemovesupanddowninthehollowdriveshaftundertheinfluenceoftherotatingflyweights.Theupanddownmovementdirectsoilthroughportsinthedriveshafttoorfromthepropeller,toalterthebladeangle.
Thepositionofthepilotvalveisdeterminedbytheactionofthegovernorflyweightsandspeederspring.TherotatingflyweightstiltoutwardundercentrifugalforcewhenRPMincreasesandinwardunderspeederspringpressurewhenRPMdecreases.
Thismovementoftheflyweightsadjuststhepilotvalvetodirectoilflowtoalterbladeangle,therebymaintainingtheselectedRPM.
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Figure320.SingleActingGovernor
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Operation
Theactionoftheflyweightstilting(in-out)toraiseandlowerthepilotvalve,isopposedbyasimplecoilspringcalledthespeederspring,thatislocatedabovetheflyweights(3.20).Thetensionofthespringcanbealteredbythepilotthrougharackandpinionassembly(Figure
3.20).
WhenthepilotrequiresahigherRPM,thepitchcontrolleverinthecockpitismovedtocompressthespeederspring.Thisincreasedspeederspringcompressiontiltstheflyweightsinwardandforcesthepilotvalvedown.
Pushingthepilotvalvedownpermitsgovernoroilpressuretoflowoutoftheinboardsideofthepiston,allowingengineoilpressureandCTMtocombinetomovethebladestoafinerangle.
DecreasingthebladeangleallowstheengineRPMtoincrease,untilthecentrifugalforceontheflyweightsequalstheforceofthespeederspring,stabilisingthepilotvalvetoaneutralposition.
Ifthepilotalterstensionontothespeederspring,thentheenginesresponsewillbetoincreaseordecreaseRPM.
Onlywhenflyweightforceisequaltospeederspringtensionwillthepilotvalvereturntoitsneutralposition(ONSPEED).
Somegovernorsincorporateabalancespringabovetherack,thisspringsetsthegovernortocruiseRPMifthecontrolcableweretobreak.
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On Speed
ONSPEEDiswhentheengineRPMisattherequiredsettingassetonthepropellercontrolbythepilot.NotetheflyweightsintheneutralpositionasinFigure3.21.
Figure321.OnSpeed
Over Speed
OVERSPEEDiswhentheengineRPMisabovetherequiredsettingassetonthepropellercontrolbythepilot.NotetheflyweightsintheoutwardpositionasinFigure3.22.
SPEEDERSPRING
DRIVEGEARSHAFT PILOT
VALVE
Figure322.OverSpeed
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Under Speed
UNDERSPEEDiswhentheengineRPMisbelowtherequiredsettingassetonthepropellercontrolbythepilot.NotetheflyweightsintheinwardpositionasinFigure3.23.
SPEEDERSPRING
DRIVEGEARSHAFT PILOT
VALVE
Figure323.UnderSpeed
Pitch stops
Thepurposeofpropellerpitchstopsistolimitbladeanglemovementtoaknown
specification;theselimitsaresetbythemanufacturer.Thepitchstopsoperatebyprovidingamechanicalmeansoflimitingbladetraveltoaknownbladeangle.
Counterweight Propeller
Toenablemaintenancepersonneltocheckandadjustpropellerbladeangles,alladjustablepropellershaveprovisiontopermithighandlowbladeanglelimitchangestobemade.Onthecounterweightpropeller,stopnutssetthetravelofthepropellercylinderandtherebycontrolthecoarseandfinebladeangles.
AnadjustmentmechanismforuseinacounterweightpropellerisillustratedinFigure3.24.
Figure324.AdjustmentMechanics
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Hydraulic Propellers
Pitchstopsareusedtolimitfine (low), coarse, orifthedesignfeatureisfitted,featherbladeangles.
Low-Pitch Stop-Lever Assembly
Thelow-pitchstop-leverassembly,whichisfittedtoreversingpropellers,providesthemeansformaintainingasetminimumbladeangleforflight.
Accessthroughthedomeplugpermitstheassemblytobescrewedintothepropellerdome(Figure3.25).
Figure325.LowPitchStop
Theassemblyincorporateswedgeswhich,whenengaged,lockthestopleversintheoutwardposition(Figure3.26),preventingthepropellerpistonfromdecreasingbelowasetangle.
Thesetangleistheminimumpositivebladeanglethatiscapableofmaintainingflight.
Figure326.StopLeverPosition
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Feather/Coarse Pitch Stop
Onfeatheringpropellersthecoarsepitchstopisreplacedbythefeatherpitchstop.
Thefeatherstopsonahydromaticpropellerusuallyconsistsofan(indexed)stopringfittedto
therotatingcamatthebaseofthedome,andtwofeatherstopsfixedtothebaseofthedomeassemble.Whenthepropellerbladesreachthefeatherangle,thefeatherstopringcontactsthefeatherstopsthuspreventingfurtherbladeangleincrease.
Figure3.27(A)showsthefeatherstopringatthebaseofthedomeapproachingthefeatherstops.Figure3.27(B)showsthefeatherstopringonthebaseofthedomeatthefeatherposition.
Figure327.FeatherPitchstop
Pitch Stop Settings
Ifapropellerhasitsbladeanglesettoolow(fine)ataworkshop,thenthatenginewillover-revoroverspeedand,iftheengineover-revstoomuch,itmaycausedamagetothatengine.
Ifapropeller'sbladeangleissettoohigh(coarse),thenitmaynotproduceenoughthrusttomaintain/attaintherequiredspeed.
Ifthefeatherangleisincorrectthenthepropellermaywindmill(continuetorotate)whenits
enginehasbeenshutdown.Awindmillingpropeller,ifleftunchecked,cancauseextradamagetotheshutdownengine.
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DOUBLE ACTING GOVERNOR
Thedouble-actinggovernorusedwiththereversingpropeller,issimilarinbasicdesigntothesingleactinggovernor,ie.pump,speederspringandpilotvalve.
Thegovernorcanalsohaveanelectricallydrivenheadwhichregulatesforconstantspeedoperationswithmulti-engineaircraft.
Thedouble-actinggovernordiffersinoperationfromthesingleactingasitcontrolsgovernoroilflowtobothsidesofthepiston(Figure3.28).
GovernorFlyweights
SpeederSpring
Oil Drain
Back toPump
Pilot Valve
CamPiston
Prop Shaft
Oil PumpReliefValve
Oil fromReeservoir
Direction of PropRotation
Direction of PropRotation
Direction of PropRotation
Onspeed
UnderspeedOverspeed
Figure328.DoubleActingGovernorConditions
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GOVERNOR/PROPELLER OPERATING CONDITIONS
Theintroductionofthevariablepitchpropellermeantthatthepropellerbladeanglecouldbeselectedtosuittheflyingconditionsandthusmaintainefficientoperation.Thebasicmodesofoperationofaconstantspeedvariablepitchpropelleraredescribedbelow.
On Speed
ONSPEEDiswhentheengineRPMisattherequiredsettingassetonthepropellercontrolbythepilotfeathering.
Over Speed
OVERSPEEDiswhentheengineRPMisabovetherequiredsettingassetonthepropellercontrolbythepilot.
Under Speed
UNDERSPEEDiswhentheengineRPMisbelowtherequiredsettingassetonthepropellercontrolbythepilot.
Feathering
FEATHERINGistheprocessofmovingthepropellerbladesuntiltheyareapproximatelyparalleltothedirectionofflighttostoptheenginefromwindmillingaftertheengineisshutdowninflight.
Unfeathering
UNFEATHERINGistheprocessofdecreasingthepropellerbladeanglefromthefeathertoananglewherethepropellerwillstartwindmillingandassiststhestartertorestarttheengine.
Reversing
REVERSINGiswherethebladeangleisalteredtoanegativevalueduringoperationsothe
propellerwillproducenegativethrust,actingasabrakeandtherebyreducingaircraftlandingroll.
Alpha Mode
ALPHAMODEcontrolsthepropellergovernorduringairborneoperationbyselectionofaconditionlevertomaintaincorrectproppitchthroughfull fine to full coarse.
Beta Mode
BETAMODEcontrolsthepropellergovernorduringgroundoperationbyselectionofaconditionlevertomaintainselectedproppitchthroughfull fine to full reverse.
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HYDROMATIC PROPELLER
Thebasichydromaticpropellerisafeathering,non-reversingpropeller.Thehydromaticdomeisseparatedintotwochambers.Theoutboardchamberreceivesengineoilpressure
constantlyandassistedbyCTMwillacttomovethebladestoafinepitch.Theinboardchamberreceivesgovernoroilpressureat200200psiandwillacttoovercomeengineoilpressureandCTMtomovethebladestoacoarserpitch.
On speed
IfengineRPMmovesawayfromtherequiredsetting,thegovernorwillalterbladeangletobringtheRPMbacktotherequiredsetting.WhentheengineRPMisattherequiredsettingthenitissaidtobeONSPEED.
Withtheflyweightsstraightupanddown(vertical)andthepilotvalveinaneutralposition,thentheengineisalsosaidtobeONSPEED(Figure3.29).
Fluidisheldinahydrauliclockduetotheneutralpilotvalveposition.
GOVERNORPITCH
LINE
Engine Oil
Return
Figure329.
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Overspeed
IftheengineisoperatingabovetheRPMforwhichthegovernorisset,itisOVERSPEEDING;thebladeswillbeataloweranglethanthatrequiredforconstant-speedoperation.
Duringtheoverspeedconditionthegovernorsflyweightscanbeseentomoveoutwardagainsttheforceofthespeederspring,raisingthepilotvalve(Figure3.30).Thisopensthepropeller-governorport,allowinggovernoroilfromtheboosterpumptoflowthroughinternallinestotheinboardsideofthepiston,movingthebladestoacoarserangleuntilanONSPEEDconditionisrestored.
GOVERNORPITCHLINE
Engine Oil
NilReturn
Figure330.
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Under Speed
UNDERSPEEDINGresultswhenthebladeshavemovedtoahigherbladeanglethanthatrequiredforanONSPEEDcondition.WhentheenginespeeddropsbelowtheRPMforwhichthegovernorisset,thedecreaseincentrifugalforceexertedontheflyweightsallows
thespeederspringtoforcethepilotvalvedown(Figure3.31).
Thisopensthepropeller-governorport,allowinggovernoroilpressuretodrainawayfromtheinboardsideofthepiston.EngineoilpressureontheoutboardsideofthepistonandCTM,pushthepistoninwardandtakethebladestoafinerangle.
GOVERNORPITCHLINE
Engine Oil
Return
Figure331.
AsRPMincreases,thecentrifugalforcefromtheflyweightsliftsthepilotvalveuntiltheforce
ofthespeederspringandthecentrifugalforceoftheflyweightsareinequilibrium.TheenginereturnstotherequiredspeedandisagaininanONSPEEDcondition.
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Feather
ToinitiatethefeatherfacilityofthebasichydromaticpropelleritisonlynecessarytodepresstheFeatherbutton.
RefertoFigure3.32forfeathercircuitoperation.ThePSIfiguresusedwithinthistextisfordescriptiononly.
Whenthefeatherbuttonispressed,aholdingrelayformsacircuitandholdsthebuttonin.Withthebuttonheldin,anelectricalcircuitisactivatedandenergisesthefeatheringpumpmotor.
Figure332.
Thefeatheringpumpsupplieshighpressureoiltothesystem,whichisfeltatthehighpressuretransfervalveinthegovernor.Asthehighpressuretransfervalveislifteditsseat,itisolatesthegovernorfromthesystemsothatittakesnootherpartinproceedings.
Highpressureoilthenpassesthroughthedistributorvalveintotheinboardsideofthepistonanddrivesthebladestoahighangle.Asthebladeangleincreases,thepistonwilltravel
untilthedoglegintherotatingcamsisreached.Pressurefromthefeatheringpumpthenmustbuildtoapproximately200PSItoforcethepistonpastthedoglegandonintofeather.
Whenthepistonhasattainedfulltravel,thepressurebuildsuptoapproximately225PSI,wherethepressurecut-outswitchopens,breakingtheholdingcircuitforthefeatherbuttonwhichpopsout.
Withthebladesinthefeatherposition,thecircuittothefeatherpumpmotorisopen,stoppingthepumpfromsupplyinghighpressureoil.
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Unfeather
Tounfeatherthepropeller,thepilotmustdepressthefeatherbuttonandholditin.Ifitisnotheldin,pressurefromthefeatheringpumpwillquicklyriseto225psiandde-energisethe
circuitviathepressurecut-outswitch.
Withthefeatheringbuttonheldin,pressurefromthefeatheringpumpisstillfeltontheinboardsideofthepiston.Asthepistonisalreadyatfulltravel,thepistondoesnotmoveandthepressurerisesrapidlyto250psi.
At250psi,thespringpackinthedistributorvalveassemble(DVA)isovercome,andthedistributorvalveispushedawaytoopenportstoreversethedirectionofoilflowintothedome(Figure3.33).Highpressureoilisthenportedtotheoutboardsideofthepistonandthepistonisforcedrearwards,bringingthebladestoafinerangle.
Figure333.
Oncethebladeshavemovedfromthefeatherposition,thepilotmustpullthefeatherbuttonout.Thisistoavoidthebladesbeingmotoredbackintofeather,becauseasthepressuredropsfrom250PSItheDVAvalvewillassumeitsnormalposition.
Asthepressuredropsfromunderthehighpressuretransfervalve,thevalveisrelievedandresumesitsseat.Theengineandpropellerarethenagaininthecontrollingmodeandareselfgoverninginthenormalmanner.
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INTEGRAL OIL CONTROL HYDROMATIC PROPELLER
Integraloilcontrol(IOC)propellersareanadvancedversionofthebasichydromaticpropellerandarebothfeatheringandreversing.
TheIOCpropellerisafullyselfcontainedunitwithallnecessaryoil,pumpsandvalvestocontrolbothengineRPMandpropellerbladeangles.
TheIOCpropellerspistonoperatesoppositetothebasichydromaticpropellerinthatitmovesinboardtoincreasepitchandoutboardtodecreasepitch.
On Speed
IfengineRPMstraysfromtherequiredsetting,thegovernorsensesthismovement(viaflyweight-speederspringassembly),anddirectsgovernoroilflowtotherequiredsideofthepiston.
ForOVERSPEED,thepistonwouldbemovedinboardandviceversaforUNDERSPEED,untiltherequiredangleisreachedtobringtheengineRPMbacktoONSPEED.Theoilon
theothersideofthepistonisallowedtodrainthroughadrainlinetotheoilreservoir.
Figure3.34showstheoilflowtothedomeforoverspeed(A)andunderspeed(B).
Figure334.OnSpeedCondition
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Overspeed
Whilegovernoroilpassesthroughtheincreaseline,oilpressurefromtheinboardsideisallowedtodrainaway.Withthegovernoroilpressureactingontotheoutboardside,andreducingoilpressureontheinboardsideofthepiston,thepistonwillmoveinboardtoa
coarserangle(Figure3.35).
Thisplacesaloadontotheengine,slowingtheRPMdown.ThisdecreaseinengineRPMdecreasestherotatingspeed(andthereforecentrifugalforce)ofthegovernorflyweights.Asaresult,theflyweightsmoveinwardbytheforceofthespeederspring.
Thepilotvalveislowered,closingthegovernormeteringport.Withthisportclosed,thepropellerpistonishydraulicallylockedpermittinganONSPEEDconditiontoexist.
Figure335.OverspeedCondition
Underspeed
Whilegovernoroilpassesthroughthedecreasepitchline,oilpressurefromtheoutboardsideisallowedtodrainaway.Withgovernoroilpressurenowbeingdirectedontotheinboardside,andreducingoilpressureontheoutboardsideofthepiston,thepistonwillmoveoutboardtoafinerangle(Figure3.36).Thisreducestheloadontheengine,permittingRPMtoincrease.TheincreaseinengineRPMincreasestherotatingspeed(andthereforethecentrifugalforce)onthegovernorflyweights.Asaresult,theflyweightsmoveoutward,liftingthepilotvalveandclosingthegovernormeteringport.Withthisportclosed,thepropellerpistonishydraulicallylockedpermittinganONSPEEDconditiontoexist.
Figure336.UnderSpeedCondition
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Feather
Featheringapropellerstopsitfromrotating,therebyreducingdrag.Theleadingedgeofthebladesareturnedsotheyfaceintothedirectionofflight,makingthebladeangleapproximately90o.
Atthisangle,airpressureonbothsidesofthebladearesimilarandthereforestoppingthepropellerfromrotating.
ForagraphicalexplanationofthefollowingfeatheroperationrefertoFigure3.37.
FeatheringisinitiatedbythepilotpushingintheFeatherbuttonlocatedinthecockpit(theFeatherbutton,beingpartofthefeathercircuit,remainsin).
Thisenergisesthefeatheringcircuits,allowingthefeatherpumptodeliverhighpressureoiltothepositioningchamber,movingitintowhatthegovernorsensesasanoverspeedcondition.
Withthepilotvalvepositionedintotheoverspeedposition,oilfromthefeatherpumpisthen
directedtotheoutboardsideofthepropellerpiston.Thispressureforcesthepistonrearward,drivingthebladestoacoarserangle.
Whenthebladesreachthefeatheredangle,thefeatherpumpcontinuestosupplyoilpressureuntilapre-setpressureisattained.
Atthispre-setpressure,apressurecut-outswitchopens,cuttingpowertothefeatherpump,andpoppingthefeatherbuttonout,therebycompletingthefeatheringcycle.
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Figure337.FeatheredCondition
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Unfeather
Ifthepilothasfeatheredapropellertoconservefuel,butlaterdecidestorestarttheengine,theunfeatheringproceduremustbeassimpleasthefeatheringprocedure.
ForagraphicalexplanationofthefollowingunfeatheroperationrefertoFigure3.37.Tounfeatherapropeller,thepilotmustpullthepropellersFeatherbuttonOUT.Thisenergisesanelectricrelay,earthedthroughapropellerbladeswitch.Thisrelaycompletesthecircuittothefeatherpumpandenergisestheselectorvalvesolenoid.
Thefeatherpumpthendelivershighpressureoiltothepositioningchamberpositioningitintowhatthegovernorsensesasanunderspeedcondition.
Withthepilotvalvepositionedintheunderspeedcondition,oilfromthefeatherpumpisthendir