b1-17 propeller systems sr

7/25/2019 B1-17 Propeller Systems SR

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StudentResource

SubjectB1-17:

PropellerSystems

Copyright 2008 Aviation Australia

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


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IssueB:January2008 Revision2 Page1of6

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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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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STUDYRESOURCES

JeppesenSandersonTrainingProducts:

A&PTechnicianPowerplantTextbook.

AircraftGasTurbinePowerplantsTextbook.

B1-17StudentHandout


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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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B:January2008 Revisi 2

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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B1-17.1:PropellerFundamentalsIssueB:January2008 Revision1 Page1of14

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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B1-17.2:PropellerConstruction

Issue B: January 2008 Revision 2 Page 1of 24

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.



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



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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Issue B: January 2008 Revision 2 Page B1-17.2:PropellerConstruction

24of 24

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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B1-17.3:PropellerPitchControlIssueB:January2008 Revision1 Page1of50

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.


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



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

b1-17 propeller systems sr

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