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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
24
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.
25
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.
26
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.
30
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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