1

AmorphingAerofoilwithHighlyControllable

AerodynamicPerformance

RuiWu1,CostasSoutis2,ShanZhong1,AntonioFilippone1

1.SchoolofMechanical,AerospaceandCivilEngineering,UniversityofManchester,UK

2.TheUniversityofManchesterAerospaceResearchInstitute,UK

ABSTRACT

Inthispaper,amorphingcarbonfibrecompositeaerofoilconceptwithanactivetrailing

edge is proposed. This aerofoil features of cambermorphingwithmultiple degrees of

freedom. The shape morphing is enabled by an innovative structure driven by an

electrical actuation system that uses linear ultrasonic motors (LUSM) with compliant

runners, enabling a full control ofmultipledegrees of freedom.The compliant runners

also serve as structural components that carry the aerodynamic load and maintain a

smoothskincurvature.Themorphingstructurewithcomplianttrussisshowntoexhibit

a satisfactory flexibility and loading capacity in both numerical simulations and static

loadingtests.Thisdesigniscapableofprovidingapitchingmomentcontrolindependent

of lift and higher L/D ratios within a wider range of angle of attack. Such multiple

morphing configurations could expand the flight envelope of future unmanned aerial

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vehicles.Asmallprototypeisbuilttoillustratetheconceptbutasnooff-the-shelfLUSMs

can be integrated into this bench top model, two servos are employed as actuators

providingtwocontrolleddegreesoffreedom.

Keywords:Morphing,adaptiveaerofoil,multipledegreesoffreedom,linearultrasonic

motor

NOMENCLATURE

CD=dragcoefficient

CL=liftcoefficient

CLmax=maximumliftcoefficient

CM=pitchingmomentcoefficient

CP=pressurecoefficient

CFRP=carbonfibrereinforcedplastic

LUSM=linearultrasonicmotor

FE=finiteelement

SMA=shapememoryalloy

MFC=microfibrecomposite

USM=ultrasonicmotor

x=locationalongchordfromleadingedge

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1.INTRODUCTION

In order to facilitate take-off and landing, and accomplish various manoeuvres, the

aerodynamic forces on the wings of a flying vehicle need to be adjusted. This can be

achievedby changing the camberof thewing. Conventionally,wing camber is changed

usinghingedcontrolsurfaces,whichleadtohighlocalsurfacecurvature,wheretheflow

tendstoseparateandcausesanexcessivedrag.Therefore,largehingedcontrolsurfaces,

suchasflapsarenotlikelytobeusedonaregularbasis,andthisimposesconstraintson

thesizeofflightregimeofthevehicle.

Wing morphing, which enables changes in aerofoil geometry while maintains a

continuous and smooth aerodynamic surface, has the potential to improve aircraft

performance (1-3). Among various morphing design concepts, a continuous change in

camberprovidesawayofimprovingaircraftfuelefficiencyandmissioncapability(1,4,5).

Wind tunnel testshaveshown thatby replacingahinged flapwithamorphing trailing

edge,theL/D(lift/drag)ratioofa2Daerofoilcouldbeincreasedby20%-25%(6,7).

The idea of morphing aerofoil was put forwarded in the 1920s and has attracted

renowned interests in the recent years. However, published aerodynamic data on the

performanceofmorphingaerofoilisstillnotabundant.Moreover,mostresearchershave

focused on investigating the improvement of aerodynamic efficiency via replacing the

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hinged flap with a smoothmorphing structure. In contrast, the potential of morphing

aerofoils to form camber lines with different shapes, which offer multiple morphing

strategies(configurations)areoftenoverlooked.

The structural design for a morphing trailing edge is challenging since contradicting

requirements, such as flexibility, load carrying capacity and lightweight, need to be

satisfied (8,9). Nevertheless, a few practical concepts are available. For example, the

topologysynthesismethodcanbeutilisedtodesigncompliantstructuresthatarecapable

ofperformingdesiredmorphingunderasimpleactuationdisplacementwithoutcausing

asignificantstrain,whilesatisfyingotherdesignconstraints(6,10).Ithasbeenreported

thatamorphingtrailingedgedesignedbysuchamethodiscapableofproviding±10°flap

deflection, 1°/foot twisting along the span, and a satisfactory loading capacity (1-3).

Adaptive “belt rib”concept isdesigned toreplace theconventional ribs. It consistsofa

closed shell (belt) and in-plane spokes that are hinged to the shell (1,4,5). Such a

structure is naturally flexible and allows morphing with a single degree-of-freedom.

Meanwhile, studies have shown that boundary layer can be stabilised to delay flow

separation by replacing a wing’s rigid upper skin in front of the conventional hinged

aileronby an adaptive composite skin that is actuated along theout-of-planedirection

(11-13).Anarrayofactuatorsbeneaththeskinisusedtodeformtheskinintodifferent

shapes optimised for various flight conditions. Similar structures can also be used on

otherpartsofthewingandtoachievedragreduction(14-16).

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Selection of amorphing actuator that is lightweight, small in size, power efficient and

fail-safe, is another challenge. A smart material actuator that can directly convert

electrical, magnetic or thermal stimulation to mechanical displacement, is believed to

offerawaytosatisfysomeoftheserequirements(7,17).Themostwidelystudiedsmart

materials for actuation include shapememoryalloys (SMA)andpiezoelectric ceramics.

However,SMAhasalimitedmaximumstrainof8%,andits2-wayshapememoryeffect

fadesoutasitcycles(8,9).Also,althoughSMAcanbefastreacting,thespeedofactivation

and deactivation relies on the heating rate and cooling conditions, which imposes a

barrier tohigh frequencyoperation (6,10). Furthermore, theirpower consumption can

be increased significantly as a result of cooling (18). Piezoelectric ceramic can output

large forceathighspeedand frequencies,and ithasapowerdensity100 times thatof

SMA(19).Althoughthemaximumstrainofpiezoelectricceramicisonly~0.1%,theyare

usuallyusedasmicrofibrecomposite(MFC)tobendthinstructures(2),andtheycanbe

anattractiveoptioniftheactuationdisplacementcouldbeamplifiedwithoutsacrificing

theactuationforce.

In this paper, a new morphing design concept is presented and it is applied to an

asymmetrical aerofoil (NACA 4418). A compliant truss structure, which has multiple

truss elements hinged to a compliant upper skin, forms the morphing structure. The

actuators between truss elements are controlled independently to allow continuous

camberchangewithmultipledegreesoffreedom.Linearultrasonicmotorisproposedas

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themorphingactuator,which isbasedonpiezoelectric ceramics, and it canpotentially

provideunlimited linearoutputwhileretain thehighactuation force. Italsocopeswell

withthechallengeofmorphingsincetherunnerscanbecompliantandmulti-functional

toserveasthestructuralcomponents.Ournumericalsimulationsandwindtunneltests

show that besides the widely recognised benefits of morphing aerofoil over hinged

controlsurfaces,thevehicleflightregimecanbefurtherextendedbyswitchingbetween

differentmorphingstrategiesandallowingthepitchingmomentcontrolindependentof

Cl.

2.Theproposedmorphingaerofoilconcept

2.1Linearultrasonicmotor(LUSM)

In a typical LUSM, pieces of piezoelectric ceramics are used to generate vibration at

ultrasonicfrequencyandformatravelling-wave.Thetravellingwavecandriveabodyin

contact with the ceramics through friction, and therefore, the high frequency

displacement from the piezoelectric ceramics can be converted into a constant linear

velocity. In thisway, the limitedactuationdisplacementof thepiezoelectricceramics is

amplified to a theoretical infinite without necessarily sacrificing actuation, making a

competitive morphing actuator possible. Also, when LUSM is powered down, the

piezoelectric ceramics could inherently hold the runner and provide blocking force,

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which makes it fail-safe. However, LUSM is rarely reported being used in morphing

structures.

In Table 1, three off-the-shelf LUSMs are set in comparison with revolving ultrasonic

motors(USM)andelectricmotorsthatarereportedinliterature(20).Intermsofpower

density, which is a critical parameter for morphing actuators, LUSM is not attractive.

However, themain reason for this drawback is that LUSM is currently developed as a

positioningdeviceduetoitsintrinsicfastresponseandhighopen-loopprecision,rather

thanaheavy-dutyactuator.Thereisasignificantroomforfurtherimprovements,suchas

in weight reduction and power enhancement. It has been reported that the power of

ultrasonic motors could be increased for instance by fine-tuning the driver frequency

(19,20).

Meanwhile,DCmotorsusuallyneedtransmissionmechanismtoreducethespeed,which

significantly decreases power density. For example, the power density of DCmotor in

Table 1 is reduced from 235 W/kg to 27 W/kg when a harmonic gear transmission

systemisused(20).Thiswillalso leadtoanincreasedspaceoccupationandareduced

reliability.

Therefore,LUSMhasthepotential tobea futuremorphingactuatorsolution,especially

whenspaceisstrictlylimited,suchasinminiunmannedaerialvehicles(MUAV),orwhen

alargenumberofactuatorisneededtocontrolnumerousdegreesoffreedom.

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Table1.Actuatorcomparisons(20)

Actuator Output

force/torque

Holding

force

Velocity

Weight

(g)

Powerdensity

(W/kg)

N216NEXTLINE®(LUSM) 60kg 80kg 1mm/s 1250 0.48

N422LinearActuator(LUSM) 0.7kg 1kg 5mm/s 25 1.4

U-264RodDriveOEM(LUSM) 0.3kg 1.5kg 200mm/s 80 7.5

SPL-801(USM)(20) 10kgcm N/A 210rpm 249 95

MaxonDCmotor(brushless)(20) 0.076kgcm - 25,000rpm 86 235

HarmonicgearwithDCmotor(20) 32kgcm N/A 60rpm 739 27

2.2Theproposedconcept:compliantrunnersdrivenbyLUSM

The LUSM requires a compliant runner that can provide effective contact with the

piezoelectric actuator. Keeping this in mind, an actuation system with multifunctional

compliant runners driven by LUSMs is proposed, as shown in Figure 1. Since a

homogeneouscompliantrunnernaturallymaintainsasmoothcurvatureunderbending,

anditisabletoprovideout-of-planestiffnessandaxialstability,itisanidealactuatorfor

a morphing wing, in which a smooth aerodynamic surface, structural compliance and

loadingcapacityarerequired.

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Figure 1.Schematicviewof theactuationsystemconceptwithacompliantrunnerandLUSMs,

whereLUSMsarepivotedontotheactuatedmorphingstructureandtherunnernaturallyformsa

smoothcurvature.

As shown in Figure 1, multiple LUSMs pivoted to a morphing structure with multiple

degrees of freedom could run along the same runner. Therefore, all the degrees of

freedom can be controlled, and a smooth curvature can be formed. Such an actuation

system can be integrated into various morphing structures as a universal actuator

solution.

Whenusedonthesurfaceofmorphingwing,itisnecessaryforittobeairtight.Thiscan

be achieved by applying morphing skins, such as segmented compliant panels or

elastomers,whichcanpassivelyslidealongtherunner.

Also,besidestheactuationandthestructuralcapacity,morefunctionalitycanpotentially

beembedded into the runnerdue to itsuniqueposition.Forexample, sensors, suchas

opticalfibres,canbeusedtomonitorthecurvatureoftherunner,andelectronicscanbe

embeddedintherunnerhenceconvertingitintoadatalinkbetweenthecontrolleranda

largenumberofactuators.

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2.3TheMorphingStructure

In the morphing aerofoil, the actuation system needs to be supported by a morphing

structurethatcanchangeshapeunderactuation,provideloadingcapacitybyeliminating

theunwanteddegreesoffreedom,andformtheaerodynamicprofile.Inthisresearch,the

baseline NACA 4418 aerofoil was selected because of its relatively high thickness and

near-flat lower surface, which makes it easier to manufacture and implement the

morphingactuationsystem.

Acarbonfibrecompositemorphingstructurewithcomplianttrussisproposed.Asshown

byFigure2,themorphingaerofoilconsistsofarigidwingbox(whichendsat40%chord),

aflexibleupperskin,fourindependentcomplianttrusselementsandarigidtrailingedge.

Theactuationsystemisusedasthelowerskinoftheaerofoil.TheLUSMsarepositioned

at thebottomapexof trusselements,whicharehingedto theupperskin.Oneormore

runnersarefixedtothetrailingedgeandslidethroughtheLUSMsasdescribedin§2.2.

ThetrusselementsshowninFigure2haveaspan-wiseshifttopreventinterferencewith

neighbouringtrusseswithalowervertexanglebetweentrussesof60°.Thisisacompromise

between overall structural stiffness and the number of truss elements; it also reduces the

thickness mismatch between the rigid wing box and the morphing section under large

deflection.Thenumberofactuatorsisequaltothenumberoftrusselements,spacedequally

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alongtherunner.

Whenthetrusselementsareslidingalongtherunner,themorphingstructurewillbend

or straighten and the camber of aerofoil will change.. During morphing, the flexible

compositeupperskinandtherunnersonlowerskinwillmaintainsmoothcurvaturethat

guarantees aerodynamic efficiency. The truss elements will also undergo small elastic

deformationthatcanbeaccommodatedbytheircompliance.

Amajoradvantageofthismorphingstructureisthatitcanmorphwithmultipledegrees

offreedomandthusprovidesawiderangeofcamberlineshapes,whichisenabledbythe

independentactuationofthefourtrusselementsaswellastherigidtrailingedgebythe

fivesetsofLUSMs.Meanwhile,theCFRPtrussstructurenaturallypossesseshighspecific

stiffnessandstrength,andprovidesadequateloadingcapacity.Forthesamereason,low

bendingstiffnessofthestructuralelements isallowed,whichleadsto lowresistanceto

cambermorphing.

Figure2.Schematicviewofthemorphingaerofoilconceptwiththecomplianttrussconfiguration

andtheproposedLUSMactuationsystem.Thesystemprovides5degreesoffreedom.

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2.4SimulationofMorphingProcess

AnonlinearFEsimulationiscarriedouttoshowthemorphingprocessoftheproposed

aerofoilandtoassistlateranalysis.

A3Dmodelofthemorphingaerofoilisconstructedwitha280mmchordand5mmspan.

All materials are assumed to be linear, elastic and isotropic. The upper carbon fibre

reinforcedplastic(CFRP)skinis0.25mmthick(E=100GPa,ρ=1.6g/cm3),thetrussesare

made of 0.5mm thick CFRP plate, the lower joints of trusses are plywood (E=800MPa,

ρ=0.7g/cm3),therunnersareCFRProdsof1mmdiameter(E=100GPa,ρ=1.6g/cm3),and

theLUSMarereplacedbyCFRPplates.Thelowermorphingskinisnotincludedsinceit

should not have significant effect on the morphing process. The model has the same

materialpropertiesanddimensionstotheprototypesdiscussedin§5,exceptthatitisa

near2Dmodelwith5mmspanandonlyonerunnerisincluded.

In the analysis the hinges between truss and upper skin are assumed to provide no

bending resistance. Sections of runners are fixed between adjacent actuators, and

actuationisrepresentedbythermalcontraction/expansionoftherunners.Thisdoesnot

resemble the interface between the piezoelectric ceramic and the runner, but it fully

replicatestheconditionsofthemorphingstructureunderactuation(whileignoringsmall

change of the runner’s diameter), and significantly saves computational time. The

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thermalcontractionofdifferentsectionsofrunnerscanbecontrolledindependently,so

allofthe5degreesoffreedomcanbemanipulated.

The structuralweight (excludingwing box) from the simulation is 22 grams,which is

equivalentto440gramspermeterspan,andthemorphedshape,asshowninFigure3,is

smooth. However, slightwaviness can be seen in the upper and lower skins, which is

capturedinboththeanalysismodelsandwindtunneltestmodels.Accordingtoliterature

(21), liftanddragarebarely influencedbyanuppersurfaceprotuberancelocatedafter

themiddlechordwithaheightofupto0.5%chordlength,thusthiswavinessisexpected

to have minor effect on the morphed aerofoil’s performance. More morphing

configurationsareshownin§3.

Figure 3. Finite-element simulation illustrating amorphing configurationwith near-maximum

trailingedgedeflection.

2.5NumericalSimulationofLoadingCapacity

Achievingsufficientloadcarryingcapacityisanadditionalobjectiveofmorphingaerofoil

design.Asapreliminaryassessment,anonlinearstaticFEsimulationiscarriedouttotest

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themodelaerofoil’sloadingcapacity.

Themodel has the samematerial properties and dimensions to the one used in §2.4,

except that thespan is50mm,and10rodswith5mmspacingare included,making it

identicaltothestaticprototypediscussedin§5.Theaerodynamicloadisassessedwith

the 2D Xfoil code, see §3, then the load distribution on both sides of the aerofoil is

integratedinto6forcesevenlydistributedwithin6adjacentregionsonthelowersurface

of the morphing part, and then the forces are applied to the same regions of the 3D

structuralmodel.The6regionsaredividedbytheactuatorsandtrailingedge,asshown

byFigure4.Thisloadingconditionisidenticaltothestaticloadingtestontheprototype

built in section4.There isnodirect loadon theupper surface since theaimhere is to

evaluate the load carrying capacity of the truss structure, while the behaviour of the

upperskincanbemodifiedasneededifbiggermodelswereconstructed;theobjectiveis

toillustratethemorphingconceptanddemonstrateinasmallphysicalmodel.

Figure4.Chordwiseloaddistributionusedinthestaticloadsimulationwithanoverallloadof70

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N.

Threedifferentloadingconditionswiththewholeaerofoilcarrying70N,130Nand250

Nloadaresimulated.Nolargedeformationisobserved,andtheresultsarelistedinTable

2,whereboth themaximumdeflectionacross theaerofoiland thedeflectionof trailing

edgearereported,andtheratioofmaximumdeflectioninchordlengthisalsoreportedto

showtheextentofdeformation.ThreeequivalentflightconditionsareincludedinTable2

to provide an idea about the possible flight regime. It should be noted, the load here

representstheoverallloadacrossthewholeaerofoil,andthemorphingpartonlycarries

26N,48Nand93Nloadrespectively,sincemostoftheaerodynamicforceisgenerated

neartheleadingedge.

Table2.Resultsofthestaticstructuralsimulation

Load(N) Deflection(mm) Def./chord Equivalentcondition Note

70 0.36(0.2atTE) 0.13% !! = 0.8,v=80m/s,SF=1.5 BaselineAoA=3.6°

130 0.56(0.3atTE) 0.2% !! = 1.5,v=80m/s,SF=1.5 Baselinemax.lift

250 1.17(0.5atTE) 0.42% !! = 1.5,v=111m/s,SF=1.5 Baselinemax.lift

SF=safetyfactor,whichisappliedtoforce;TE=trailingedge;AoA=angleofattack

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3.AerodynamicPerformance

Unlike conventional hinged flap, the morphing aerofoil always maintains a smooth

surface,anditcanbecontrolledwithmultipledegreesoffreedom.Inordertoinvestigate

the aerodynamic impact of these features, aerodynamic analysis andwind tunnel tests

arecarriedouttoevaluatetheperformanceofaerofoil(withabaselineprofileofNACA

4418) at different morphing configurations and flight conditions (different angles of

attack).

3.1MorphingStrategies

The multiple degrees of freedom of the proposed morphing aerofoil can be used to

generatedifferentmorphingconfigurations.Thiscanbedonebycontrollingthefivesets

of LUSMs independently in reality, or assigning the thermal expansion of the five rods

independentlyinthestructuralsimulation(asdiscussedin§2.4).

Themorphingleadstocamberchange,andmultipleactuatorsalongthechordcanalter

thecurvatureofcamberlineatdifferentchord-wiselocations.Ataconstanttrailingedge

deflection,differentmorphingsettingsyieldaerofoilswithdifferentcamberlinesshapes

(Figure5).Thereisaninfinitenumberofpossiblemorphingshapes;foranaerofoilwith

camber increased at one location, that location can be continuously moved along the

chordby adjusting the output of different sets of actuators.As shown in Figure5, two

intuitivelydefinedmorphingstrategiesandaconventionalhingedflapareconsidered:

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• Strategy1:Morphingaerofoilwithcamberintensivelychangednearthemaximum

thicknessandanunchangedtrailingedge;

• Strategy2:Morphingaerofoilwithcamberintensivelychangedneartrailingedge;

• Strategy0:Aerofoilwithahingedflapat0.75chordlength.

Figure5.2Daerofoilmodelsat0˚angleofattackwiththreedifferentmorphingstrategies.

The 2D aerofoils shapes for aerodynamic analysis are generated using the 3Dmodels

resultedfromthestructuralsimulations.Inthestructuralsimulations,theratiobetween

theamountsofrunnerexpansioniskeptconstantforeachmorphingstrategytorealise

differenttrailingedgedeflectionwithasimilarcamberlineshape.

3.2AerodynamicAnalysisSetup

3.2.1XfoilAnalysis

Xfoilisacodewidelyusedinresearchtoanalysetheflowaroundasubsonicaerofoil.Itis

based on a revised 2D panel method that takes into account of the viscosity and

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compressibilityeffects,allowingreasonablepredictionsoflift,dragandpitchingmoment,

as well as flow separation point beyond the stall angle (22,23). In literature (24), a

morphing wing with adaptive upper skin has been tested in wind tunnel at Reynolds

numberssimilartothepresentresearch.Thetestsusefourteentosixteenpressuretaps

to measure the pressure distribution on the aerofoil, the results are then used to

determinethe locationof flowtransition.ThecomparisontoXfoilanalysishasshowna

closeagreementonsurfacepressurecoefficientsandtransitionoccurrence.

ThecomputationalanalysisdiscussedinthefollowingsectionsisperformedwithXfoilat

Reynoldsnumber !" = 1.5 ∙ 10! (basedontheaerofoilchordlength) andMachnumber

! = 0.235.Thisisrepresentativeofa28cmchordlengthminiUAVflyingatsealevel.An

analysis with !" = 0.5 ∙ 10! and ! = 0.078 is carried out to match the condition of

windtunneltests.

3.2.2WindTunnelTests

To support the simulations, wind tunnel tests have been performed at a free-stream

velocityof25m/sandtheReynoldsnumber !" = 0.5 ∙ 10!. Usingmultiplerigidmodels,

sevendifferentaerofoilmorphingstatesaretested,including:baseline(strategy0with0

mm deflection), strategies 0, 1 and 2 with 30 mm downward and 20 mm upward

deflection.

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3.2.2.1ModelConstruction

Rigidmodelswith a 280mm chord (same as in aerodynamic analysis) and a 600mm

span and cross-sections representing differentmorphing states aremade by laying up

glass fibre cloth with epoxy onto a reinforced foam core, forming a skin of ~0.5 mm

thickness.ThefoamcoreisshapedbyaCNChot-wirecuttingmachinewiththetolerance

ofaround0.5mmduetomaterialburn-off.AllthemodelsarepolishedwithP180sand

paperstogiveafairlysmoothsurfacefinish.

Thehingedaerofoilmodelhasamovableflap,whichisattachedtotheforebodyviathree

hinges along the span. The flap has a circular leading edge that can just fit into the

concaveslotintherarepartoftheforebodytoeliminateallgapsbetweentheforebody

and the flap. This ensures the aerofoil surface to be as smooth as possible (with

fluctuationheightnearthejunction<1mm)andnoaircanescapefromthelowersurface

totheuppersurface.

3.2.2.2WindTunnelSetup

TheaerofoilistestedintheProjecttunnelatUniversityofManchester.Thetunnelhasan

octagonaltestsectionmeasuring87cm×111cm(height×breadth).Toensurea2Dflow

around the aerofoil, the aerofoil model is mounted between the two splitter plates

spanningacrossthewholewidthof thetestsection.Thefunctionofsplitterplates is to

isolatethetipsofaerofoilmodelsfromthetunnelwallboundarylayersandtoensurea

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near2Daerodynamicbehaviour.A2mmgapisleftbetweenthesplitterandeachendof

theaerofoilforclearance.Theaerofoilmodelismountedtoasix-componentloadbalance

throughasting,whichpassesthroughaholeintheuppersplitterplate.

The speed measured at the original test section inlet cannot be used since the

complicatedblockageeffectcausedbythesplitterplates,testmodelandtunnelwallwill

alter the flowvelocitybetween the splitters (25).The free streamspeed isdetermined

insteadwithaPitot tube locatedbetweenthesplittersat11cmaway fromthesidewall

and60cmupstreamoftheaerofoilleadingedge.

Thetestpointsarerecordedbyacomputerprogramthataveragesthedatasampledat1

kHzduringasecond.Tofurtherreducethefluctuation,eachdatapointsreportedinthis

paperistheaverageoffiverepeatedtests.

Themaximumtotalblockageeffect,includingthecontributionofsolidblockageandwake

blockage,islowerthan3.5%.Allthetestdatareportedinthispaperhavebeencorrected

forblockageeffectandlifteffect,aswellascirculationeffectthataccountforachangeup

to7%inthemeasureddynamicpressureduetoproximityofthePitottubetotheaerofoil

model(25,26).

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3.3AerodynamicResults

3.3.1ResultsonAerofoilswithanIncreasedCamber

Beforemovingontoadetaileddiscussionoftheresult,itisworthexplainingthefunction

ofthelayoutofFigure7and8.Eachofthesefigureshassetthreesubplotsincomparison.

ThesubplotsrepresenttheresultsfromXfoilanalysisattwodifferentReynoldsnumbers

and thewind tunnel test, respectively.Figure7 (a) shows theresultsat !" = 1.5 ∙ 10!,

whichisrepresentativeofaUAV.Thekeyresultscanbederivedfromthisfigure,butit

still requires experimental validation. Figure 7 (b) shows the results from the wind

tunnel tests,whicharecarriedoutata lowerReynoldsnumberof 0.5 ∙ 10!,due to the

constraint of testmodels and equipment; this included only one deflection, due to the

time limitation.However, it is sufficient to support the analysis results if theReynolds

number effect is not significant. Therefore, Figure 7 (c) is plotted, which shows the

analysisresultsatthetestReynoldsnumber.Bycomparingit to(b), itcanbeseenthat

the curves from the test and the analysis results show very similar trends despite the

difference in magnitudes that is discussed later. The comparison between (c) and (a)

showsthat theReynoldsnumbereffecthascausesnosignificantchange inthetrendof

the curves. Therefore, the trend of the curves shown by Figure 7 (a) is supported by

experimentalresults(andsimilarforFigure8(a)andfortheresultsdiscussedin§3.3.2),

andthekeyconclusionsontheeffectsofdifferentmorphingstrategiescanbemade.

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Thedifferenceintheresults’magnitudes ismostlycausedbythewaythat liftanddrag

aremeasured,whichincludesthetestmodel’s3Deffects.InFigure6,liftanddragresults

of thebaselineaerofoilNACA4418 from thewind tunnel test,Xfoil analysis andNACA

Langleywindtunnel(27)arecompared.Asshowninthefigure,theanalysisshowsgood

agreementwithNACAtestresults,but the testresults in thepresentedresearchshows

differences. For instance, within an angle of attack range of 3°~5°, the CD is

approximately0.02greaterandCLisaround0.2lower.

Figure6.ComparisonsbetweenliftanddragcoefficientofthebaselineNACA4418aerofoilfrom

the presentwind tunnel test (!" = 0.5 ∙ 10!), Xfoil analysis (!" = 0.5 ∙ 10!) andNACA Langleywindtunneltest(!" = 3 ∙ 10!)(27).

In the NACA test, the lift is deduced from the pressure distributionsmeasured on the

aerofoil surfaces, and the drag is evaluated from the pressure in the wake (27). Such

methodsallowthemeasuringdevicestobeplacednearthecentreofwingspan(28),and

hencethe3Deffectcanbesignificantlyreduced.Whereasinthepresentwindtunneltest,

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alltheforcesaremeasureddirectlywiththeforcebalanceandtheforcesonthewingtips

are included. The mechanical clearance between the wingtips and splitters causes 3D

effectsthatincreasedragandreduceliftthatleadstothedifferenceintheresults.

To briefly summarise the observations from the results, when the morphing strategy

changesfromstrategy1to2andfinallytostrategy0,whichistheaerofoilwithahinged

flap,thelocationofmaximumcamberincrementmovestowardsthetrailingedgeandthe

maximumcurvatureof the camber line increases. It is shownby the results thatwhen

such changes aremade, the aerofoil tends to bemore efficient (higher L/D) at higher

anglesofattackandlessefficient(lowerL/D)atloweranglesofattack.

In Figure 7, L/D ratio is plotted against CL, which shows that at each trailing edge

deflection, the morphing strategy 1 yields the highest L/D ratio among the three

configurationswithin the lower rangeofCL tested,but itsL/Ddropsmore rapidy than

otherstrategiesathigherCL,whichisobviouslyassociatedwitharapidincreaseinCD.It

canbeseeninthefigurethatCLcanbeincreasedbyincreasingtrailingedgedeflection.

Morphingstrategy1 is themostefficientuntilCL isover≈1.7withamaximumtrailing

edgedelfectionof30mm.Morphingtrategy2givesthehighestL/DratiountilCL≈1.8.At

higherCLvalues,strategy0,isthebetterperformingmorphingconfiguration.

AsshowninFigure8,whereCLisplottedagainstangleofattack,strategy1startstostall

and lose liftearlier (at~3°angleofattack),whichsuggests that strategy1canonlybe

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advantageousatlowanglesofattack.ThemaximumCLthatcanbeachievedbythethree

strategieswhilekeepingCDbelowacertainvalueispresentedinFigure9.Thebaseline

NACA4418isalsoplottedasareferencewithouttheCDconstraint(CDvaries),Itcanbe

seenthatastheangleofattackincreases,orasthemaximumattainableliftincreases(at

theexpenseofdrag),themostefficientstrategychangesfromstrategy1to2,andfinally

to strategy 0 (hinged flap) when a very low L/D ratio is obtained. It is a sequence in

which the location of maximum camber increment approaches trailing edge and the

maximumcamber linecurvature increases.Thecurves inFigure9areratherzigzagged

sincethedatapointsaregeneratedfromdiscretedatawithseventrailingedgedeflection

valuesandthreestrategies.

Themorphing aerofoils outperform the baseline,while the improvements in efficiency

overhingedflapsatrelativelyhighanglesofattack(seeFigure9)arenotassignificantas

somepreviousstudieshavesuggested.Thismaybebecausethehingedaerofoilusedin

the wind tunnel tests of this research has been made as smooth as possible and the

junctionbetweenwingandflaphasbeenmadeairtight.

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Figure 7. L/D - CL curves of aerofoils with increased camber and different trailing edge

deflections, showing morphing strategies outperforming hinged aerofoil (dotted lines) and

outperformingeachotheratdifferentCLrange,allaerofoilsexceptthebaselinehave0°~10°angle

ofattack.

Figure 8. Lift curvesof aerofoilswith increased camberanddifferent trailingedgedeflections,

showingthatthestrategieswiththehighestefficiencyinFigure7loseliftatrelativelylowangle

ofattack.

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Figure 9.Attainable CL by threemorphing strategieswith variedCD values, generatedbyXfoil

analysis.

Thereasonwhystrategy1 isefficientonlyat lowanglesofattackandsuffersfromlow

CLmax,whilestrategy0(thehingedflap)isthemostefficientathighanglesofattackcan

beexplainedintermsofflowseparation.Flowseparationoccurswhenthespeedofthe

boundary layerdecreasestozerounderanadversepressuregradient.Figure10shows

thepressure coefficient (CP) across aerofoilwithunit chord length at 6°angle of attack

usingan inviscidanalysis (where the flowalways staysattached to show the complete

pressuregradient),aswellastheseparationpointpredictionsfromanalysiswithviscous

effect (where the flow separates in trailing edge region). Focusing on the region with

x≈0.6~0.8 on theupper surfacewhere flow separation starts, strategy1 (aerofoilwith

intensively increased camber near the maximum thickness) forms relatively uniform

variation incurvature,and therefore, consistently increasingadversepressuregradient

27

thattheboundarylatercannotovercome,thustheflowseparatesearlyinthisregionand

causesexcessivedrag.Strategy0(hingedaerofoil)formsintensivelycurvedsurfacenear

thehinge,wherea favourablepressuregradient is formed, thus the flowaccelerates in

this region and separation is delayed to the hinged point. The behaviour of strategy 2

(aerofoilwithmaximumcamberincrementneartrailingedge)isbetweentheothertwo.

Therefore, the efficiency of different strategies can be explained: although strategy 1

(aerofoilwithmaximumcamberincrementnearleadingedge)isthemostefficientatlow

angles of attack, when the location of maximum camber increment moves towards

trailing edge, the flow separationpoints alsomove towards trailing edge and lead to a

higherliftandalowerdragathighanglesofattack.

28

Tobenoted,thehingepivotofstrategy0(hingedaerofoil)islocatedat75%chordlength,

butinFigure10thehingepointisnear80%sincetheflapisdeflectedsothehorizontal

length of the flap is reduced. The three aerofoils have very similar chord length after

morphing,andarenormalizedtounitchordlengthforillustration.

Figure 10. CP-x (x = location along chord from leading edge) curve of aerofoils with 30 mm

downward trailing edge deflection via different morphing strategies generated by inviscid

analysis,andflowseparationpointpredictedbyviscousanalysis.

3.3.2ResultsonAerofoilswithaDecreasedCamber

Aerofoilswithdecreasedcambercangeneratelowornegativeliftforhigh-speedflightor

manoeuvreandcontrolpitchingmomentcoefficientformanoeuvreortrimming.

Asdiscussedin§3.3.1,thekeyconclusionsfromtheanalysisaresupportedbythewind

tunnel tests, so only test data are presented in this section for a clear illustration (see

Figure11).

29

Since thereflexionofcamber linenear trailingedge let theseaerofoilsgenerate lessor

negative lift in the trailing edge region, the pitchingmoment Cm is increased, and the

increment is related to the camber line shape. Therefore, as shown by Figure 11,

strategies 1 and2 yield different near-zero or positive Cm.Meanwhile, similar to 3.3.1,

strategies1and2yieldsimilarCLsincetheyhavethesametrailingedgedeflection.Thus

it is possible to control pitchingmomentwhilemaintaining constant CLbymoving the

locationofmaximumcamberchangetowardstrailingedge.However,dragpenaltyexists

accordingtotheCDcurves,butitisconsiderablylowerthanahingedflap.

AsanexampleofthepitchingmomentcontrolindependentofCL,accordingtheCmcurves

in Figure 11, a Cm increment of approximately 0.04 can be induced by changing the

morphingstrategyfromstrategy1to2,whiletheCLandCDcurvesarebarelychanged.

Figure11.CmCDandCLversusAoAofaerofoilswithdecreasedcamberand20mmtrailingedge

deflections,showingchangesinCmindependentofClwithlittlepenaltyinCD.

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4.Prototyping

Tofurtherunderstandthebehaviourofthemorphingaerofoil,twoprototypesaremade,

including a static prototype aiming to validate the loading capacity, and a moveable

prototypeaimingtotestthemorphingmechanism.

Thestaticprototypeissimilartothesimulationmodel,butwithsomeminordifferences

inmaterials.3DprintednylonreinforcedbyCFRPplatesisusedinsomeparts.Sinceno

changeismadeinthecompliantstructuresincludingthetrussesandtheupperskinand

runners,whicharestillmadeofCFRP,themechanicalbehaviouroftheprototypeshould

beverysimilartothesimulationresults.Inaddition,allthehingesaremadeofcomposite

panel with Kevlar fibre and cyanoacrylate matrix (super-glue). The matrix along the

hingelineiscrackedtoallowfreerotationwithnegligiblerotationalresistance,andthis

issimilartothesimulatedcondition.Theprototypealsoincludesa3Dprintedwingbox,

which is over-designed to sustain the load from static loading test without inducing

perceptible deformation. The finishedmorphing trailing edgeweights 0.22N,which is

closetothesimulationprediction.

Thestaticloadingtestiscarriedoutwith26Nand22Nloadappliedinthewayidentical

to the simulation, which corresponds to the overall load of 70 N and 60 N across the

wholeaerofoil.Thebondingbetweenseveralhingesandupperskinfailsundertheload

31

of70N.Butundertheloadof60N,nofailureisobserved,andatrailingedgedeflectionof

0.2±0.1mm is induced, which is close to the simulation prediction shown in Table 2.

However, the out-of-plane deflection of upper skin is larger than prediction due to

excessiveinitialdeflectioninducedbymanufacturedefects.

Themoveable prototype (Figure 12) is similar to the static prototype, but the runner

diameterisreducedfrom1mmto0.8mmtoreducethebendingstiffnessandtherefore,

theresistancetomorphing.Thetrailingedgeandthesecondtrussfromleadingedgeare

each driven by one digital servo through 5 runners. Those runners slide through the

lowerjointofthetrusselementswhilehavingoneendfixedtothetrussthatisdrivenby

itandtheotherendconnectedtoaservo,thustheconditionsareequivalenttoaLUSM

driven structure and the only difference is the actuators’ location. According to simple

analysis, itwillprovidethesameactuationcapacity(couldsupport250Noverallstatic

loadwhileinducing<1mmout-of-planedeflection)and40%oftheout-of-planeloading

capacityincomparisonwiththestaticprototype,whilethebendingresistanceofrodsis

alsoreducedto40%.

Asnooff-the-shelfLUSMscanbeintegratedintothissmalldemonstrator,servosareused

instead of LUSMs. This also provides a practicalway to drive the presentedmorphing

structure using conventional actuators before the development of LUSM could offer a

viablesolution.Themoveableprototypehastwoservosasactuators,andtherefore,two

32

controlleddegreesoffreedom.TheservosarecontrolledbytwoPWMsignalsgenerated

fromaSTC80C516RDmicrocontroller.ThemorphedshapesareshowninFigure12.

(a)

(b)

Figure12.Moveableprototype,(a)sideviewshowingfivedifferentmorphingstates,(b)oblique

viewshowingtherunnersareconnectedtoeitherthetrailingedgeorthesecondtrussfromthe

left.

Itshouldbenotedthatatthisstagebothoftheprototypesdonothavemorphingskin

onthe lowersurface.Thiswillnothavesignificant influenceonthe testssince theskin

hasaminoreffectonmorphingorstrength.Themorphingskinwillaffect the flowand

will require further examination in the near future. As an example, the segmented

33

morphing lower skin shown in Figure 13 could be introduced; it consists of two

overlappingskinsthatarebondedtotherigidwingboxandthetrailingedge,respectively.

Therearskinbondedtothetrailingedgecanslidealongtherunnersandabovetheskin

partthatisattachedtothewingbox.

Figure13.Anexampleofanoverlappingsegmentedbottommorphingskinthatcanbeutilisedto

formanairtightaerofoilandtransfertheaerodynamicload

Conclusions

In this paper, a new morphing aerofoil design concept, which combines compliant

runners driven by linear ultrasonic motors (LUSM) with an innovative morphing

structure with compliant composite truss, is presented. It is shown to offer fully

controlledmultiple degree-of-freedoms that providemultiplemorphing configurations.

34

The compliant truss structure has shown satisfactory specific loading capacity in both

static finite element simulation and actual static loading test. The prototypemorphing

trailingedgesectionwitha50mmspan,a160mmchordwiselengthanda0.22Nweight

cansupporta22Nloadwithatrailingedgedeflectionof0.2mm.

To validate the aerodynamic benefit of multiple degrees of freedom, the aerodynamic

analysis is comparedwith thewind tunnel test data. It is demonstrated thatwhen the

aerofoil camber is increased, all morphing states show a higher L/D ratio than a

conventionalhingedcontrolsurfaceatmostflightconditionsexceptatveryhighanglesof

attack. It is also shown that different morphing strategies outperform the others at

differentanglesofattack.Forexample,atlargeanglesofattack,theaerofoilaerodynamic

efficiencycanbefurtheroptimisedbyadaptingmorphingstrategiesaccordingtocurrent

flightcondition,i.e.bymovingthelocationofthemaximumcamberincrementalongthe

aerofoilchord.Furthermore,thepitchingmomentcanbecontrolledindependentlyfrom

CLandwithlittledragpenalty;thisisachievedbyadaptingdifferentmorphingstrategies.

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