aerospace

Article Static and Dynamic Performance of a Morphing Trailing Edge Concept with High-Damping Elastomeric Skin

Maurizio Arena 1,*, Christof Nagel 2, Rosario Pecora 1,* , Oliver Schorsch 2 , Antonio Concilio 3 and Ignazio Dimino 3

1 Department of Industrial Engineering—Aerospace Division, University of Naples “Federico II”, Via Claudio, 21, 80125 Napoli (NA), Italy 2 Fraunhofer Institute for Manufacturing Technology and Advanced Materials, Wiener Straße 12, D-28359 Bremen, Germany; [email protected] (C.N.); [email protected] (O.S.) 3 Smart Structures Division Via Maiorise, The Italian Aerospace Research Centre, CIRA, 81043 Capua (CE), Italy; [email protected] (A.C.); [email protected] (I.D.) * Correspondence: [email protected] (M.A.); [email protected] (R.P.); Tel.: +39-081-768-3573 (M.A. & R.P.)


Received: 18 November 2018; Accepted: 14 February 2019; Published: 19 February 2019


Abstract: Nature has many striking examples of adaptive structures: the emulation of birds’ flight is the true challenge of a morphing wing. The integration of increasingly innovative technologies, such as reliable kinematic mechanisms, embedded servo-actuation and smart materials systems, enables us to realize new structural systems fully compatible with the more and more stringent airworthiness requirements. In this paper, the authors describe the characterization of an adaptive structure, representative of a wing trailing edge, consisting of a finger-like rib mechanism with a highly deformable skin, which comprises both soft and stiff parts. The morphing skin is able to follow the trailing edge movement under repeated cycles, while being stiff enough to preserve its shape under aerodynamic loads and adequately pliable to minimize the actuation power required for morphing. In order to properly characterize the system, a mock-up was manufactured whose structural properties, in particular the ability to carry out loads, were also guaranteed by the elastic skin. A numerical sensitivity analysis with respect to the mechanical properties of the multi-segment skin was performed to investigate their influence on the modal response of the whole system. Experimental dynamic tests were then carried out and the obtained results were critically analysed to prove the adequacy of the adopted design approaches as well as to quantify the dissipative (high-damping) effects induced by the rubber foam on the dynamic response of the morphing architecture.

Keywords: damping; morphing wing; smart system; adaptive structures; adaptive wing trailing edge; compliant skin

1. Introduction Over the next few years, a new generation of air vehicles characterized by high aerodynamic efficiency and low environmental impact could dramatically contribute to achieving the objectives set by the Advisory Council for Aviation Research and Innovation in Europe (ACARE), such as greening and noise reduction. Next-generation aircraft require wings able to reconfigure themselves in multiple shapes in order to match specific flight conditions and reduce fuel waste resulting from non-optimal flight dynamics. Aircraft wings capable of such in-flight reconfiguration are referred to as morphing wings [1,2].

Aerospace 2019, 6, 22; doi:10.3390/aerospace6020022 www.mdpi.com/journal/aerospace Aerospace 2019, 6, 22 2 of 22

One of the key challenges to developing a successful morphing wing is the development of a flexible skin, a continuous layer of material that stretches over a stiff structure, preserving a smooth external surface. Compliant structures are designed to achieve large deformations by relying upon elastic straining of their inner elements. This property would require a high load-carrying capability to sustain aerodynamic pressures, and sufficient flexibility to implement target shapes by the actuation system under the concurrent action of the same external forces. In [3], the authors discuss how compliant deformation and stiffness concepts, albeit contrasting, are important for a morphing structure. Such a study, carried out about 10 years ago, showed that both the structural and material concepts found in the literature at that time did not fulfil all those characteristics. In other words, the level of maturity of morphing skins was still low and the proposed concepts were still unclear. Indeed, combining adequate flexibility and stiffness concepts into a single skeleton is a real design challenge. Kinematic architecture, made of rigid bodies connected to each other, should nevertheless rely on a skin that envelops the skeleton and guarantees the preservation of a regular geometry during shape change. A significant tendency in implementing adaptive surfaces in the rotorcraft field is also taking place. Investigations into the potential of Continuous Trailing-Edge Flaps (CTEF) for enhanced helicopter primary flight control were described in [4]. The CTEF idea encompassed airfoil-embedded active materials to deform the trailing edge section in a static or dynamic way while keeping the target geometry. Solutions to overcome those issues, with a particular focus on the state of the art of morphing technologies for rotor blades, were instead reported in [5]. Over the years, different solutions have been proposed to create smooth surfaces for morphing wings. Anisotropy is essential for corrugated laminates’ suitability. A structure combining a corrugated skin and a honeycomb-reinforced elastomer was suggested in [6]. The skin could stretch and resist vertical aerodynamic loads thanks to differential behaviour along the in-plane and normal directions. Elastomeric-coated composite corrugated panels were proposed in [7], addressing shape optimization for an augmented performance of the morphing skin, even considering manufacturing constraints. A similar concept was reported in [8], where structural elements with in-plane tuneable stiffness were introduced. The design of composite skins with silicone rubber matrix and CFRP rods reinforced by Kevlar layers was studied in [9]; the prototype could attain ±30◦ pure shear morphing. In [10], an Adaptive Aspect Ratio (AdAR) wing was presented, coupling a compliant skin (the dominant component) with a kinematic internal structure consisting of sliding ribs over a telescopic main spar, to create a morphing aerodynamic surface capable of significant changes in span and aspect ratio. Again, it was necessary to balance in-plane and out-of-plane stiffness. In [11], a further camber morphing concept (FishBAC) was introduced. The effects of such a technology on a straight, rectangular wing architecture were discussed with reference to a 25-kg UAV (speed range, weight penalties, max allowable span retraction, and so on). Camber morphing appeared to favour larger weight configurations at high speeds, while combination of camber with span morphing provided the largest aerodynamic efficiency improvements for missions with both low and high-speed phases. In [12], a skin technology based on a combination of an elastomeric outer skin, a flexible honeycomb, and a fiberglass laminate was presented to realize a hybrid morphing trailing edge control surface of a UAV, also addressing camber variation. Currently, many activities aim at implementing “intelligent” performance inside the skin—for instance, in order to measure the deformation state or to monitor structural conditions. In [13], one of the first works on this topic, a distributed sensor system developed at NASA Langley Research Center was placed on a morphing wing surface to collect strain field information. Deformation map was first reduced into a distribution of local slopes over a flat surface under the hypothesis of linearity; then, the morphed curved surface could be approximated by a collection of individual flat surfaces of different slope and used as a reference for an active shape control system. Feasibility studies to realize a carbon nanotubes-based strain sensor system were described in [14,15]. In [16], the authors presented a 3D shape sensing of flexible morphing wing using arrays of Fiber Bragg Grating (FBG) sensors. The maximum error between 3D-visual and FBG measurements was proven to be less than Aerospace 2019, 6, 22 3 of 22

5%, and the proposed method was effective for deformed surface reconstruction. In [17], embedded sensors were introduced to measure the surrounding flow and give a reference to control the actuators’ action. Therein, it was also demonstrated that significant power consumption reductions were possible by exploiting inherent hysteretic properties of SMA and appropriately tailoring the control strategy. In [18], conceptual design and modelling of a distributed sensor system based once more on FBG was reported, aimed at measuring span-wise and chord-wise variations of an adaptive trailing edge. The sensor system was made of two integrated technological solutions for detecting spanwise and chord-wise strains, forming a closed grid network that revealed the complete deformed shape. An extensive review of actuation morphing concepts, applied to unmanned aircraft, may be found in [19]. In the already mentioned kinematic morphing (rigid body architectures), distributed electromechanical actuation have shown significant advantages compared to conventional shafted arrangements, due to their ability to move individual components either synchronously or independently. In [20], a quick-return mechanism for chord-wise camber variations was reported. Motion was transmitted from servo-rotary actuators to adaptive trailing edge device elements. Both inherent rotation sensors associated to each actuator (encoders) and an FBG-based distributed sensor network were used to synchronize the different movements. However, although such a mechanism showed high accuracy and compactness, the potential failure ratio increased because of the larger number of requested motors to withstand external aerodynamic loads. Design, simulation, and control of a miniature linear actuator for morphing wing actuation was proposed in [21]. In that case, the actuator consisted of a miniature brushless direct current (BLDC) motor, a gearing system, and a trapezoidal screw, and was verified as of to managing the high force field appearing under the morphing skin during wind tunnel tests. A wide discussion of the main features of shape memory alloys (SMA) for compliant morphing aircraft was reported in [22]. Particular emphasis was given to camber and twist motions, while the system design tried to maximize the actuation bandwidth and minimize the power consumption. Along the same line, the effectiveness of an SMA thin film actuator to modify an airfoil shape for performance improvement was presented in [23]. Lately, the scientific and technological community is showing growing interest in bi-stable morphing concepts, thanks to the expected benefit of reducing the complexity of servo-activated control systems [24,25]. Studies of morphing control systems have been often referred to as UAV, particularly micro air vehicles (MAV). In [26], the authors worked on the aerodynamic shape optimization to obtain basic wing geometrical properties that would have allowed maximum range and endurance. A detailed study of MAV design is reported in [27]. A design framework was introduced for enhancing roll control authority through morphing wing deployments for non-symmetric twist distribution. A co-simulation strategy for modelling unsteady dynamics of biologically-inspired flapping-wing MAV was developed and reported in [28]. Preliminary control pulse flight-test campaigns were completed for a prototype gull-wing morphing aircraft and described in [29]. The investigated vehicle demonstrated augmented longitudinal and lateral dynamic stability performance. In [30], the same authors proved the lateral dynamics control authority improvement by morphing wings. The present paper deals with an innovative structural concept development, tested on a two-bay trailing edge segment that was made of three articulated finger-like ribs and covered by a multi-material hyper-elastic skin. The goal was to understand and evaluate how morphing skin could affect the adaptive structural system dynamic response, with particular reference to damping increase and modal characteristics’ deviation. Both aspects are essential for aeroelastic assessment, because an augmented DOF system has a reasonably higher modal density than a usual one, leading to more critical behaviour. It should be properly considered that an elastomeric morphing skin has important mass and stiffness contributions. For typical materials, skin may attain almost 30–40% of the global weight, while normal chord-wise stiffness should be in the range of about 10% with respect to the overall value. Skin-added bending rigidity can be neglected with respect to the basic overall structural one, both spanwise and chord-wise. As far as the authors know, the presented analyses are not common in the literature, with some minor exceptions [31]. Moving from those preliminary results, this research Aerospace 2019, 6, 22 4 of 22

Aerospace 2018, 5, x FOR PEER REVIEW 4 of 22 aims at drafting another step for assessing a methodology for defining morphing structure properties and,structure particularly, properties morphing and, particularly, skin specs. Thismorphing work wasskin carriedspecs. This out withinwork was the frameworkcarried out ofwith SARISTUin the (Smartframework Intelligent of SARISTU Aircraft (Smart Structures), Intelligent an EUAircraft FP7 projectStructures), that endedan EU FP7 in 2015. project It involvedthat ended the in design2015. andIt involved integration the of design a full-scale and integrationmorphing wing of a sectionfull-scale including morphing multiple wing section adaptive including structural multiple devices: aadaptive droop nose, structural a morphing devices: winglet a droop and an nose, adaptive a morphing trailing winglet edge, designed and an adaptive on a new-generation trailing edge jet, aircraftdesigned specifications. on a new-generation Prior the ultimate jet aircraft system specifications manufacture,. Prior the the authors ultimate focused system on manufacture, a dummy, a 2-bay the segmentauthors offocused the final on 5-baya dummy, ATED a 2 demonstrator-bay segment (Figureof the final1)[32 5-].bay ATED demonstrator (Figure 1) [32].

FigureFigure 1. 1.Reference Reference wing wing ( a(a)) and and technologytechnology demonstratordemonstrator developed within within SARISTU SARISTU (b (b) )[[18].18].

2.2. Summary Summary and and Investigation Investigation Strategy Strategy TheThe load-bearing load-bearing capability of of a amorphing morphing trailing trailing edge edge,, as well as wellas its asability its ability to reproduce to reproduce target targetshapes shapes under under the action the action of aerodynamic of aerodynamic and inertial and inertial loads, was loads, already was already discussed discussed in previous in previous works works[33,34[].33 Similar,34]. Similar investigations investigations of a more of conventional a more conventional and segmented and segmented skin suitable skin for suitable a morphing for a morphingaileron were aileron detailed were in detailed [35]. Numerical in [35]. simulations Numerical by simulations finite element by finite analysis element were elaborated analysis were to elaboratedsupport these to support investigations; these investigations; numerical models numerical’ reliability models’ as reliability well as the as well overall as the structural overall structuralconcept conceptfunctionality functionality and performances and performances were finally were finallyproven proven through through experimental experimental tests on tests a full on- ascale full-scale test testarticle. article. In In this this work, work, the the influence influence of ofthe the mechanical mechanical properties properties of of a amulti multi-component-component skin skin on on the the modalmodal response response of of a a morphing morphing trailing trailing edgeedge waswas investigated.investigated. Trade Trade-off-off analyses analyses were were carried carried out out to to assessassess the the change change in in the the most most relevant relevant naturalnatural frequenciesfrequencies upon the v variationariation of of the the elastic elastic properties properties ofof the the multi-material multi-material skin.skin. Most Most of of the the issues issues related related to the tothe skin skin integration integration,, suchsuch as temperature as temperature and andenvironment environmentalal effects effects,, fatigue fatigue and chemical and chemical resistance resistance,, were investigated were investigated with reference with reference to the stand to the- stand-alonealone skin skinspecimen specimen.. Moreover, Moreover, vibration vibration tests tests on a ontwo a- two-baybay test testarticle article proved proved the theamount amount of ofdamping damping measured measured on on the the single single sample sample of of ela elastomericstomeric material. material. These These dynamic dynamic tests tests made made it it possiblepossible to to highlight highlight the the damping damping effect effect of the of highlythe highly deformable deformable foam-based foam-based skin, potentially skin, potentially leading toleading improved to improved wing aeroelastic wing aeroelastic stability. stability. The trailing The edge trailing was edge in fact was designed in fact designed for the outboardfor the outboard sections ofsections both wings, of both which wings, are which usually are very usually sensitive very to sensitive vibratory to phenomena vibratory phenomena and prone and to aeroelasticprone to instabilities.aeroelastic Hence, instabilities. a localized Hence, increase a localized of damping increase was of regarded damping as was a valuable regarded contribution as a valuable to the overallcontribution structural to the safety. overall structural safety.

3.3. Morphing Morphing Architecture Architecture Description Description TheThe two-bay two-bay AdaptiveAdaptive TrailingTrailing Edge Device (ATED) (ATED) consist consistss of of a afinger finger-like-like mechanism mechanism and and a amulti multi-box-box arrangement arrangement (see (see Figure Figure 2)2. ).Each Each adaptive adaptive rib rib was was divided divided into into four four sequential sequential blocks blocks (B0, (B0,B1, B1,B2, B2,B3) jointed B3) jointed by hinges by hinges position positioneded along the along aerodynamic the aerodynamic camber line camber (hinge line A, (hingeB, C). Block A, B, B0 C). Blockis assumed B0 is assumed to be constrained to be constrained to the rear to spar the of rear the spar wing of; the the rotation wing; the of the rotation other ofblocks the otheraround blocks the aroundrespective the respectivehinge lines hingephysically lines transforms physically the transforms camber line the into camber an articulated line into layout an articulated of consecutive layout ofparts consecutive (see Figure parts 3). (seeThe Figureblocks 3rotations). The blocks are limited rotations by rigid are limited links (L1, by L2) rigid—hinged links (L1, to non L2)—hinged-adjacent toblocks non-adjacent—which blocks—whichinduce the camber induce line thesegments camber to line rotate segments according to rotateto pre- accordingdefined gear to pre-definedratios. The linking elements make each rib a single-DOF mechanism: if the motion of any one of the blocks is prevented, no shape change can be achieved. On the other hand, if an actuator moves any of the

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Aerospaceblocks, 2018 the, 5, x kinematic FOR PEER REVIEW chain follows the rotation consequently. The segments can move synchronously5 of 22 gearAerospacealong ratios. the 2018 span-wise The, 5, x linkingFOR PEER direction elements REVIEW both make upwards each rib (morphed a single-DOF up mode) mechanism: and downwards if the motion (morphed of any 5down oneof 22 of blocks,themode), blocks the kinematicas is in prevented, Figure chain 4. follow no shapes the change rotation can consequently be achieved.. The On segments the other can hand, move if synchronously an actuator moves alonganyblocks, the of the spanthe blocks,kinematic-wise direction the chain kinematic followsboth upward chain the rotation followss (morphed co thensequently. rotationup mode The consequently.) and segments downward can The smove (morphed segments synchronously down can move modesynchronouslyalong), as the in Figurespan-wise along 4. direction the span-wise both upwards direction (morphed both upwards up mode) (morphed and downwards up mode) (morphed and downwards down (morphedmode), as downin Figure mode), 4. as in Figure4.

Figure 2. Multi-segment structure concept.

Figure 2. Multi-segment structure concept. Figure 2. Multi-segmentMulti-segment structure structure concept. concept. Block 0 Block 1 Block 0 Block 2 BlocBlockk 0 1 Front spar Block 3 BlockBlock 1 2 Front spar BlockBlock 2 3 Front spar Block 3

Spars

Spars Spars

Link 1

Link 1 Link 2 Link 1 Link 2

Link 2 Figure 3. Two-bay trailing edge architecture: hidden skin. Figure 3. Two-bay trailing edge architecture: hidden skin. Figure 3. Two-bay trailing edge architecture: hidden skin. Figure 3. Two-bay trailing edge architecture: hidden skin.

FigureFigure 4.4. TrailingTrailing edgeedge morphingmorphing shapes.

Figure 4. Trailing edge morphing shapes. Figure 4. Trailing edge morphing shapes.

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The finger-like mechanism of the rib thus transforms the camber line into an articulated geometricThe finger-like line, which mechanism can exhibit of thedifferent rib thus configur transformsations the depending camber line on into the anrelative articulated angles geometric between line,the adjacent which canblocks. exhibit The choice different of the configurations number of bloc dependingks is crucial on to the ensure relative that anglesthe shape between is as close the adjacentas possible blocks. to the Thedesign choice targets. of the A larger number number of blocks of small is crucial blocks tocertainly ensure guarantees that the shape a more is accurate as close asreproduction possible to theof designthe camber-morphing targets. A larger numbertrajectories of small. However, blocks certainly this solution guarantees could a more significantly accurate reproductionincrease the manufacturing of the camber-morphing costs (due trajectories. to a higher However,number of this parts) solution and have could detrimental significantly effects increase on thesystem manufacturing reliability and costs safety. (due to Conversely, a higher number a reduced of parts) number and have of detrimentalblocks can lead effects to on a systemstrong reliabilitysimplification and safety.of the Conversely, structural aarrangement—at reduced number ofthe blocks expense, can lead however, to a strong of the simplification shape accuracy of the structuralrequested arrangement—atby high-fidelity aerodynamic the expense, predictions. however, of the shape accuracy requested by high-fidelity aerodynamic predictions. 4. Skeleton Structural Sizing 4. Skeleton Structural Sizing As seen in the previous paragraph, the proposed architecture enables the shape morphing As seen in the previous paragraph, the proposed architecture enables the shape morphing through through an articulated arrangement of consecutive blocks connected to each other by hinges located an articulated arrangement of consecutive blocks connected to each other by hinges located along the along the trailing edge camber. For structural sizing purposes, the presence of the hinges was trailing edge camber. For structural sizing purposes, the presence of the hinges was neglected and neglected and the structural layout of the generic bay was assumed to be an assembly of single-cell the structural layout of the generic bay was assumed to be an assembly of single-cell sectors, rigidly sectors, rigidly interconnected to each other, in a multi-box arrangement (Figure 5). Each cell is interconnected to each other, in a multi-box arrangement (Figure5). Each cell is delimited along the delimited along the span-wise direction by homologue blocks of consecutive ribs, and along the span-wise direction by homologue blocks of consecutive ribs, and along the chord by longitudinal chord by longitudinal stiffening spars. stiffening spars.

Figure 5. Multi-box structural discretizationdiscretization forfor thethe sizingsizing purpose.purpose.

The structural sizing sizing of of each each cell cell was was performed performed by byreferring referring to the to theclassi classicalcal theory theory of the of thin- the thin-walledwalled section section beams beams [36], [unde36], underr the following the following hypotheses: hypotheses: • Al2024-T5Al2024-T5 alloyalloy forfor allall primaryprimary elementselements comprisingcomprising ribrib plates,plates, sparsspars andand skinskin panels;panels; • limitlimit loadload conditioncondition associatedassociated toto aa trimmedtrimmed flightflight symmetricsymmetric manoeuvre,manoeuvre, atat divedive speed, speed, limit loadlimit factorload factor and maximum and maximum take-off take-off weight); weight); • nono couplingcoupling amongamong thethe hingedhinged blocksblocks along along the the chord-wise chord-wise direction direction (in (in more more detail, detail, each each cell of thecell multi-blockof the multi-block arrangement arrangement was analysed was analysed independently); independently);

The three-dimensionalthree-dimensional un-viscid vortex lattice method was used to calculatecalculate the pressurepressure distribution along the wing.wing. The aerodynamic model was obtained by meshing the wing into 16 flatflat panelspanels further subdivided into into 1860 1860 boxes boxes (Figure (Figure 6,6, panels panels boundaries boundaries are are represented represented by by means means of ofthick thick lines); lines); higher higher mesh mesh density density was was used used along along the thetrailing trailing edge edge in order in order to better to better estimate estimate the thepressure pressure trend trend along along the ATED the ATED chord). chord). After some After sensitivity some sensitivity analysis, analysis, the enveloping the enveloping load condition load conditionin terms of in highest terms ofinternal highest loads internal induced loads alon inducedg the ATED along thewas ATEDfound wasto be found a trimmed to be asymmetric trimmed manoeuvre at dive speed (VD = 206 m/s, Mach = 0.65), limit positive load factor (2.5) and maximum

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AerospaceAerospace 20182018,, 55,, xx FORFOR PEERPEER REVIEWREVIEW 77 ofof 2222 symmetric manoeuvre at dive speed (VD = 206 m/s, Mach = 0.65), limit positive load factor (2.5) and taketakemaximum--offoff weightweight take-off (60(60,,000000 weight Kg).Kg). (60,000 PressurePressure Kg). distributionsdistributions Pressure distributions (upper(upper andand lower) (upperlower) evaluatedevaluated and lower) alongalong evaluated thethe centrcentr alongee lineslines the ofofcentre thethe aerodynamicaerodynamic lines of the aerodynamic stripsstrips werewere interpolatedinterpolated strips were alongalong interpolated ATEDATED ribrib along planesplanes ATED ((FigureFigure rib planes 7)7);; atat eacheach (Figure ribrib7 plane,plane,); at each thethe interpolatedinterpolatedrib plane, the distributionsdistributions interpolated werewere distributions convenientlyconveniently were replacedreplaced conveniently byby linearlinear replaced piecewisepiecewise by linear distributionsdistributions piecewise characterizedcharacterized distributions bybycharacterized constantconstant averageaverage by constant valuesvalues ofof average (upper/lower)(upper/lower) values of prpr (upper/lower)essureessure alongalong thethe pressure chordchord ofof along eacheach ribrib the segmentsegment chord of block.block. each TheThe rib piecewisepiecewisesegment block. distributionsdistributions The piecewise,, relatedrelated distributions, toto thethe twotwo baysbays related ofof thethe to dummydummy the two demonstrator baysdemonstrator of the dummy,, areare recappedrecapped demonstrator, inin FigureFigure are 8.8.recapped InIn compliancecompliance in Figure withwith8. In EASAEASA compliance CSCS--2525 airworthinessairworthiness with EASA CS-25 requirements,requirements, airworthiness ultimateultimate requirements, pressurepressure distributionsdistributions ultimate pressure werewere obtaiobtaidistributionsnedned byby multiplyingmultiplying were obtained thethe bylimitlimit multiplying onesones byby aa contingency thecontingency limit ones factorfactor by a contingencyequalequal toto 1.51.5.. factor equal to 1.5.

FigureFigureFigure 6.6. 6. AerodynamicAerodynamicAerodynamic meshmesh mesh adoptedadopted adopted forfor for thethe the referencereference wingwing (( (planplanplan view)view) view)...

FigureFigureFigure 7.7. 7. PressurePressurePressure distributiondistribution distribution alongalong thethe stripsstrips ofof thethe aerodynamicaerodynamic modelmodel (left(left wing)wing) wing)...

The limit (/ultimate) aerodynamic pressure was then integrated along the span-wise and TheThe limitlimit (/ultimate)(/ultimate) aerodynamicaerodynamic pressurepressure waswas thenthen integratedintegrated alongalong thethe spanspan--wisewise andand chordchord-- chord-wise directions to get the principal solicitations components, i.e., torque, bending moment wisewise directionsdirections toto getget thethe principalprincipal solicitationssolicitations componentscomponents,, i.e.i.e.,, torque,torque, bendingbending momentmoment andand shearshear and shear along each cross section of the ATED. alongalong eacheach crosscross sectionsection ofof thethe ATED.ATED.

BayBay 11 ofof thethe DummyDummy DemonstratorDemonstrator BayBay 22 ofof thethe DummyDummy DemonstratorDemonstrator PressurePressure [N/m[N/m22]] PressurePressure [N/m[N/m22]] UpperUpper SurfaceSurface LowerLower SurfaceSurface UpperUpper surfacesurface LowerLower surfacesurface BlockBlock 00 1414,,546546 −−583583 1515,,529529 −−11191119 BlockBlock 11 1313,,716716 −−24232423 1414,,739739 −−387387

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Bay 1 of the Dummy Demonstrator Bay 2 of the Dummy Demonstrator Pressure [N/m2] Pressure [N/m2] Upper Surface Lower Surface Upper surface Lower surface Aerospace Block2018, 5 , 0x FOR PEER 14,546REVIEW −583 15,529 −1119 8 of 22 Block 1 13,716 −2423 14,739 −387 BlockBlock 2 2 12,59512,595 −−42524252 13,896 −−22782278 BlockBlock 3 3 54145414 −−67676767 6457 −−64926492

(a)( aUpper) Upper surface surface ( (b) Lower surfacesurface (negative (negative scale) scale) Figure 8. Pressure distribution values along the trailing edge and relative colormaps. FigureFigure 8. 8.Pressure Pressure distribution distribution valuesvalues along the the trailing trailing edge edge and and relative relative colormaps. colormaps.

The main dimensions of the structural arrangement (namely, spars and skin panel thickness, TheThe main main dimensions dimensions of theof the structural structural arrangement arrangement (namely, (namely, spars spars and and skin skin panel panel thickness, thickness, spar spar caps area) were determined in order to assure adequate margins of safety with respect to capsspar area) caps were area) determined were determined in order in to order assure to adequate assure adequate margins margins of safety of with safety respect with respect to permanent to permanent deformations at limit load (material plasticization) and local failures at ultimate load. deformationspermanent atdeformations limit load (materialat limit load plasticization) (material plas andticization) localfailures and local at failures ultimate at ultimate load. load. 5. Hyperelastic Compliant Skin 5. Hyperelastic5. Hyperelastic Compliant Compliant Skin Skin 5.1. Design Aspects 5.1.5.1. Design Design Aspects Aspects In order to overcome one of the biggest challenges posed by elastomer-based morphing skins In[3]—maintaining orderIn order to to overcome overcome stiffness one oneagainst of of the air the biggest pressure biggest challenges and challenges in-plane posed flexibility posed by elastomer-based at by the elastomer-based same time—a morphing compliant morphing skins skins[3]—maintaining [skin3]—maintaining consisting stiffnessof alternating stiffness against stiff against air and pressure soft air segmen pressure and in-planets was and developed. flexibility in-plane The at flexibility the functi sameonal time—a at principle the samecompliant is that time—a compliantskinsoft consisting segments skin consisting of take alternating up ofthe alternating in-plane stiff and strains stiffsoft andsegmengenera softtedts segments wasby trailing developed. was edge developed. morphing,The functi onal Thewhile functionalprinciple stiff segments is principle that is thatsoftprovide softsegments segments stability take againstup take the up in-planeair thepressure. in-plane strains Soft genera segments strainsted generated were by trailing placed by edgeon trailingthe morphing, top and edge below while morphing, the stiff hinge segments lines, while stiff segmentsprovidewhich provide stability are the stability againstregions airwhere against pressure. the air highest Soft pressure. segments deformation Soft were segmentss occur placed (see wereon Figurethe placed top 9). and Conversely, on below the topthe hingealmost and below lines, no the deformation is generated between the hinge lines due to the relatively stiff rib segments. Therefore, hingewhich lines, are which the regions are the where regions the wherehighest thedeformation highest deformationss occur (see Figure occur 9). (see Conversely, Figure9 ).almost Conversely, no deformationstiff skin segments is generated were betweenplaced above the hinge and below lines thedue ribs. to the relatively stiff rib segments. Therefore, almost no deformation is generated between the hinge lines due to the relatively stiff rib segments. stiff skin segments were placed above and below the ribs. Therefore, stiff skin segments were placed above and below the ribs.

Figure 9. Compliant morphing rib concept: 2D view with detail of hinges and blocks. Figure 9. Compliant morphing rib concept: 2D view with detail of hinges and blocks. The stiffFigure segments 9. Compliant of the morphing compliant rib skin concept: consist 2D of vi anew aluminium with detail alloy.of hinges The and space blocks. between them Thewas stiff filled segments by flexible of foams, the compliant forming the skin soft consist segments. of anFoam aluminium and aluminium alloy. segments The space are between covered them was filledwithThe bya stiffprotective flexible segments foams,layer of of the formingelastomer. compliant the The skin soft use consist segments. of aluminium of an Foam aluminium simplified and aluminium alloy. also the The mechanical space segments between fastening are them covered was filled by flexible foams, forming the soft segments. Foam and aluminium segments are covered with a protective layer of elastomer. The use of aluminium simplified also the mechanical fastening with a protective layer of elastomer. The use of aluminium simplified also the mechanical fastening of the compliant skin to the ribs. The finalized cross section of the upper and lower skins is shown of the compliant skin to the ribs. The finalized cross section of the upper and lower skins is shown in in FigureFigure 1010.. The The thickness thickness of ofthe thesoft softsegments segments is between is between 4 mm and 4 mm 12 mm and and 12 mmthe chord-wise and the chord-wise length lengthis between is between 25 mm 25 and mm 30 andmm, 30depending mm, depending on thickness. on These thickness. values These are due values to different are due amounts to different of amountsspace oflocally space available locally availableon the primary on the primarystructure structure and to the and different to the different air pressure air pressure levels to levels be to be counteractedcounteracted along along the the chord-wise chord-wise direction direction (Figure (Figure 11). 11 ).

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Figure 10. Functional segments of morphing skin: structural layer, elastomer and flexible gapless foam. FigureFigure 10. 10.Functional Functional segments segments of of morphing morphing skin: skin: structural structural layer, layer, elastomer elastomer and and flexible flexible gapless gapless foam. foam.

FigureFigure 11. 11. DetailDetail of of the the gap gap at at the the hinge: hinge: assembly assembly of of the the flexible flexible foam. foam.

5.2.5.2. Damping Damping Properties’ Properties’ Characterization Characterization Figure 11. Detail of the gap at the hinge: assembly of the flexible foam. TheThe soft soft materials materials of of the the morphing morphing skin skin were were designed designed to provide to provide adequate adequate flexibility flexibility across across the operative5.2.the Damping operative temperature Properties’ temperature Characterization range. range. Since Since polydimethylsiloxane polydimethylsiloxane (PDMS) (PDMS) or or polytrifluoropropyl polytrifluoropropyl ◦ methylsiloxanemethylsiloxane lose lose their their flexibility flexibility below below −−4040 °CC due due to to cold cold crystallization, crystallization, the the compliant compliant skin skin The soft materials of the morphing skin were designed to provide adequate flexibility across the materialmaterial was was based based on on polydimethyldiphenylsiloxane polydimethyldiphenylsiloxane (PDMDPS). (PDMDPS). In In this this material, material, crystallization crystallization is is operative temperature range. Since polydimethylsiloxane (PDMS) or polytrifluoropropyl effectivelyeffectively hindered hindered by by bulky bulky phenyl phenyl groups. groups. Since Since unfilled unfilled silicone silicone elastomer elastomer has has poor poor mechanical mechanical methylsiloxane lose their flexibility below −40 °C due to cold crystallization, the compliant skin properties,properties, the the tensile tensile strength, strength, elongation elongation at at brea breakk and and tear tear strength strength were were improved improved by by adding adding a a material was based on polydimethyldiphenylsiloxane (PDMDPS). In this material, crystallization is fillerfiller material material [37]. [37]. Flexible Flexible foam foam was was produced produced from from the the base base material material by by using using a a foaming foaming agent agent that that effectively hindered by bulky phenyl groups. Since unfilled silicone elastomer has poor mechanical releasesreleases gas gas after after reaching reaching a a critical critical temperature. temperature. The The tailoring tailoring of of the the material material was was a a difficult difficult process process properties, the tensile strength, elongation at break and tear strength were improved by adding a becausebecause curing curing reaction reaction and and foaming foaming reactions reactions occurred occurred at the at thesame same time time [37]. [ 37A ].tailored A tailored process process was filler material [37]. Flexible foam was produced from the base material by using a foaming agent that alsowas developed also developed to perform to perform curing, curing, foaming, foaming, and adhesive and adhesive bonding bonding of different of different reactive reactive silicones silicones and releases gas after reaching a critical temperature. The tailoring of the material was a difficult process aluminiumand aluminium profiles profiles at once. at once. The Thewhole whole manufacturing manufacturing process process is isdetailed detailed in in [38]. [38]. The The structural structural because curing reaction and foaming reactions occurred at the same time [37]. A tailored process was robustnessrobustness of of the the skin skin was was appraised appraised through through mechan mechanicalical tests tests and and wind wind tunnel tunnel tests tests [39]. [39]. Within Within the the also developed to perform curing, foaming, and adhesive bonding of different reactive silicones and selectedselected temperature temperature range, range, the the compliant compliant skin skin show showeded excellent excellent fatigue fatigue resistance resistance and and stability stability of of aluminium profiles at once. The whole manufacturing process is detailed in [38]. The structural interfacesinterfaces (polymer (polymer-polymer‒polymer and and polymer polymer-aluminium)‒aluminium) [39,40]. [39,40]. Also, Also, good good stability stability against against moisture moisture robustness of the skin was appraised through mechanical tests and wind tunnel tests [39]. Within the andand hydraulic hydraulic fluid fluid was was confirmed confirmed by by testing testing [39]. [39]. The The measured measured initial initial elastic elastic moduli moduli of of the the flexible flexible selected temperature range, the compliant skin showed excellent fatigue resistance and stability of foamfoam were were between between 2.30 2.30 MPa MPa and and 3.97 3.97 MPa MPa within within the the operational operational temperature temperature range. range. Poisson’s Poisson’s ratio ratio interfaces (polymer‒polymer and polymer‒aluminium) [39,40]. Also, good stability against moisture valuesvalues ranged betweenbetween 0.150.15 and and 0.24 0.24 [38 [38].]. The The given given spans spans represent represent the averagethe average of the of tabulated the tabulated values and hydraulic fluid was confirmed by testing [39]. The measured initial elastic moduli of the flexible valuesplus/minus plus/minus one standard one standard deviation. deviation. The initial The elasticinitial modulielastic moduli of the skin of the were skin between were between 1.83 MPa 1.83 and foam were between 2.30 MPa and 3.97 MPa within the operational temperature range. Poisson’s ratio MPa2.30 MPaand 2.30 [38], MPa with [38], the span with representing the span representing the average the of the average tabulated of the values tabulated plus/minus values oneplus/minus standard values ranged between 0.15 and 0.24 [38]. The given spans represent the average of the tabulated onedeviation. standard Poisson’s deviation. ratio Poisson’s was between ratio 0.46 was and betw 0.47,een representing 0.46 and the0.47, physically representing meaningful the physically range of values plus/minus one standard deviation. The initial elastic moduli of the skin were between 1.83 meaningfulthe measured range values of the given measured in [38]. values The damping given in properties [38]. The damping of the compliant properties skin of were the compliant investigated skin by MPa and 2.30 MPa [38], with the span representing the average of the tabulated values plus/minus wereexposing investigated T-joints samplesby exposing to fully T-joints reversed, samples cyclic to loadsfully reversed, at room temperature. cyclic loads Eachat room T-joint temperature. contained one standard deviation. Poisson’s ratio was between3 0.46 and 0.47, representing the physically Eacha compliant T-joint contained skin material a compliant volume ofskin30 material× 35 × 10 volume mm (chord of 30 × length35 × 10× mmspan3 (chord× thickness) length × and span was × meaningful range of the measured values given in [38]. The damping properties of the compliant skin thickness)loaded in theand chord-wise was loaded direction. in the chord-wise More details directio of then. T-jointMore details geometry of the have T-joint been geometry published have in [39 been,40]. wereA displacement-controlled, investigated by exposing ramped T-joints cyclic samples test with to afully constant reversed, crosshead cyclic speed loads of at 0.3 room mm/s temperature. was chosen published in [39–40]. A displacement-controlled, ramped cyclic test with a constant3 crosshead speed Eachin order T-joint to match contained the specific a compliant operating skin conditionsmaterial volume (see Figure of 30 12 × ).35 Each × 10 testmm was (chord run untillength the × samplespan × thickness) and was loaded in the chord-wise direction. More details of the T-joint geometry have been published in [39–40]. A displacement-controlled, ramped cyclic test with a constant crosshead speed

AerospaceAerospace 2018 2018, 5,, 5x, FORx FOR PEER PEER REVIEW REVIEW 1010 of of22 22 Aerospace 2019, 6, 22 10 of 22 ofof 0.3 0.3 mm/s mm/s was was chosen chosen in in order order to to match match the the specific specific operatin operating gconditions conditions (see (see Figure Figure 1 21)2. )Each. Each test test was run until the sample failed. Load and displacement data were recorded throughout the test. failed.was run Load until and the displacement sample failed. data Load were and recorded displacement throughout data werethe test. recorded Closed throughout load-displacement the test. Closed load-displacement hysteresis loops were obtained for each full cycle. A typical hysteresis loop hysteresisClosed load loops-displacement were obtained hysteresis for each loops full were cycle. obtained A typical for each hysteresis full cycle. loop A istypical shown hysteresis in Figure loop 12. isis shown shown in i nFigure Figure 1 21.2 The. The reason reason for for the the non non-elliptic-elliptic shape shape is isthe the nonlinear nonlinear behavio behaviouru ofr of the the sample, sample, The reason for the non-elliptic shape is the nonlinear behaviour of the sample, which is predominantly whichwhich is ispredominantly predominantly caused caused by by material material non non-linearity.-linearity. caused by material non-linearity. The material damping was calculated on the basis of the mechanical energy dissipated in each TheThe materialmaterial dampingdamping waswas calculatedcalculated onon thethe basisbasis ofof thethe mechanicalmechanical energyenergy dissipateddissipated inin eacheach cycle:cycle: this this energy energy (U (Ud) dis) is equal equal to to the the area area enclosed enclosed by by the the hysteresis hysteresis loop loop (see (see Error! Error! Refer Referenceence source source cycle: this energy (Ud) is equal to the area enclosed by the hysteresis loop (see Figure 13), and was notnot found. found.3)3, )and, and was was calculated calculated by by numerical numerical integration integration [4 [14].1 ].The The elastic elastic part part of of the the deformation deformation calculated by numerical integration [41]. The elastic part of the deformation energy, Uel, was estimated energy,energy, U Uel, elwas, was estimated estimated as as the the area area beneath beneath the the elastic elastic load load-deflection-deflection curve, curve, which which was was assumed assumed as the area beneath the elastic load-deflection curve, which was assumed to be linear, as depicted in toto be be linear, linear, as as depicted depicted in in Figure Figure 12. 12. Tensile Tensile and and compressive compressive half half-cycles-cycles were were considered considered separately. separately. Figure 12. Tensile and compressive half-cycles were considered separately. The material loss factor η TheThe material material loss loss factor factor η ηwas was then then obtained obtained as as a aratio: ratio: was then obtained as a ratio: U = = . d = η. . (1)(1) 2πUel (1) ] ] m m m m [ [

t t n n e e m m e e c c a a l l p p s s i i D D

Figure 12. Displacement vs. time function used in ramped cyclic test. The crosshead speed is ±0.3 FigureFigure 12.12. DisplacementDisplacement vs. vs. time time function function used used in inramped ramped cyclic cyclic test. test. The Thecrosshead crosshead speed speed is ±0.3 is ±mm/s.0.3mm/s. mm/s.

FigureFigureFigure 13.13 1. 3Load-displacementLoad. Load‒displacement‒displacement hysteresis hysteresis hysteresis loop loop loop from from from ramped ramped ramped cyclic cyclic cyclic test test test at atcrosshead at crosshead crosshead speed speed speed± ±0.0.3 ±0.3 mm/s. m3 mm/s.m/s.

Aerospace 2018, 5, x FOR PEER REVIEW 11 of 22

Calculated values of dissipated energy, deformation energy and damping are given in Table 1 Aerospaceas a function2019, 6, 22 of the displacement amplitude. It can be seen that the material damping is high11 of 22and ranges between 1.8% and 3%. Calculated values of dissipated energy, deformation energy and damping are given in Table1 as a Table 1. Material damping properties obtained from ramped cyclic T-joint test. function of the displacement amplitude. It can be seen that the material damping is high and ranges between 1.8% andDissipated 3%. Elastic Loss Nominal Specific Dissipated Specific Amplitude Energy Energy Factor Strain Energy Elastic Energy Table 1. Material damping properties obtained from ramped cyclic T-joint test. u Ud Uel Ud,spec Uel,spec (mm) Dissipated(mJ) Elastic(mJ) Loss (--) Nominal(--) Specific(mJ/mm Dissipated3) Specific(mJ/mm Elastic3) Amplitude 1.0 Energy10 Energy45 Factor0.0354 Strain0.03 Energy9.71 × 10−4 Energy4.29 × 10−3 1.5u Ud24 Uel 86 η0.0444 ε0.05 U2.26d,spec × 10−3 U8.19el,spec × 10−3 (mm)2.0 (mJ)44 (mJ)139 (–)0.0504 (–)0.07 (mJ/mm4.16 ×3 10)−3 (mJ/mm1.32 ×3 )10−2 −3 −2 1.02.5 1068 45197 0.03540.0550 0.030.08 9.716.48× 10 ×− 104 4.291.88× 10 ×− 310 1.53.0 2496 86259 0.04440.0590 0.050.10 2.269.12× 10 ×− 103 −3 8.192.47× 10 ×− 310−2 2.04.0 44157 139412 0.05040.0607 0.070.13 4.161.50× 10 ×− 103 −2 1.323.92× 10 ×− 210−2 −3 −2 2.55.0 68226 197599 0.05500.0601 0.080.17 6.482.15× 10 × 10−2 1.885.70× 10 × 10−2 3.0 96 259 0.0590 0.10 9.12 × 10−3 2.47 × 10−2 6.0 299 813 0.0586 0.20 2.84 × 10−2 7.74 × 10−2 4.0 157 412 0.0607 0.13 1.50 × 10−2 3.92 × 10−2 −2 −1 5.07.0 226377 5991062 0.06010.0565 0.170.23 2.153.59× 10 ×− 102 5.701.01× 10 ×− 210 6.08.0 299462 8131325 0.05860.0555 0.200.27 2.844.40× 10 ×− 102 −2 7.741.26× 10 ×− 210−1 7.09.0 377553 10621605 0.05650.0549 0.230.30 3.595.26× 10 ×− 102 −2 1.011.53× 10 ×− 110−1 −2 −1 10.08.0 462637 13251828 0.05550.0555 0.270.33 4.406.07× 10 × 10−2 1.261.74× 10 × 10−1 9.0 553 1605 0.0549 0.30 5.26 × 10−2 1.53 × 10−1 10.0 637 1828 0.0555 0.33 6.07 × 10−2 1.74 × 10−1 In order to enable the estimation of the material contribution to the overall damping of the structure, specific measures were considered necessary. The specific dissipated energy was calculated In order to enable the estimation of the material contribution to the overall damping of the as the dissipated energy Ud per volume of compliant material contained in the T-joint, V = 10,500 structure,mm3. specific measures were considered necessary. The specific dissipated energy was calculated 3 as the dissipatedIn the same energy way,U thed per specific volume elastic of compliant energy was material determined. contained Furthermore, in the T-joint, theV = nominal 10,500 mm strain. wasIn calculated the same way, by dividingthe specific the elastic displacement energy was amplitude determined. u by Furthermore, the chord length the nominal of 30 strain mm. was These calculatedquantities by are dividing given in the the displacement last three columns amplitude of Tableu by 1. the A graphical chord length representation of 30 mm. Theseis shown quantities in Figure are14 given. These in data the last can three be used columns to estimate of Table the1 .contribution A graphical of representation the compliant is material shown in to Figure the total 14. damping These dataof the can stru be usedcture toby estimate scaling with the contributionthe total volume of the content. compliant material to the total damping of the structure by scaling with the total volume content. ] 3 m m / J m [

y g r e n e

c i f i c e p S

FigureFigure 14. 14. DissipatedDissipated and elastic elastic parts parts of ofthe the specific specific deformation deformation energy energy as function as function on the on nominal the nominalstrain. strain.

The damping ζ, assumed as: η ζ = , (2) 2 Aerospace 2018, 5, x FOR PEER REVIEW 12 of 22

The damping ζ, assumed as: = , Aerospace 2019, 6, 22 12 of( 222) is shown as a function of the nominal strain in Figure 15. It can be seen that the damping is dependent ison shown strain. as It a starts function at 0.23 of the at nominal3% strain strain and ingoes Figure up to 15 a. Itmaximum can be seen of 0.38 that theat 15% damping strain. is The dependent decrease onin strain. damping It starts observed at 0.23 at atstrain 3% strain values and higher goes than up to 15% a maximum, is a consequence of 0.38 at of 15% the strain.linearity The assumption decrease indrawn damping in the observed data evaluation. at strain values Since the higher operational than 15%, strain is a consequencevalues are below of the 15%, linearity the exact assumption damping drawncurve inis only the data needed evaluation. up to this Since value. the operational strain values are below 15%, the exact damping curve is only needed up to this value.

FigureFigure 15. 15Damping. Damping factor, factor, computed computed as as the the ratio ratio between between dissipated dissipated and and elastic elastic deformation deformation energy, energy, asas function function on on the the nominal nominal strain. strain.

6.6. Numerical Numerical Model Model Overview Overview

6.1.6.1. Structural Structural Modelling Modelling TheThe detailed detailed modelling modelling of of a a morphing morphing system system involves involves a a proper proper schematization schematization of of several several componentscomponents and and subassembly subassembly items, items, such such as as the the ribs, ribs connected, connected by by spherical spherical and and cylindrical cylindrical joints, joints, thethe actuator actuator chains, chains, including including motor motor shaft, shaft, the the skin skin and and the the spars, spars among, among the the others. others All. All elements elements are are fastenedfastened and and must must be compliantbe compliant with with the operative the operative and safety and safety loads (Limit loads and (Limit Ultimate and Ultimate Loads) [ 33 Loads),34]. In[3 this3,34 work,]. In this the work, load-bearing the load actuation-bearing system actuation is not system covered is not by thecovered analyses by the in order analyses to characterize in order to thecharacterize single contribution the single of contribution the skin on of the the generalized skin on the stiffness generalized of the stiffness morphing of the structure. morphing FE analysesstructure. ® wereFE analyses all carried were out all in carried MSC Nastran out in MSCenvironment Nastran® environment [42]. The model [42]. includesThe model the includes primary the structure primary (morphablestructure (morphable and un-morphable and un- boxes)morphable and the boxes) skin and segments: the skin aluminium segments: panels aluminium as well panels as hyperelastic as well as sheetshyperelastic and foam sheets segments and foam along segments the morphing along regions. the morphing The internal regions hinges. The (spherical internal hinges and cylindrical) (spherical areand modelled cylindrical) with theare usualmodelled scheme with of rigid the body usual connections scheme of ( rbe2 rigid), with body independent connections nodes (rbe2 located), with toindependent an ideal centre nodes of rotation located [ to42 ].an For ideal each centre hinge, of a rotation local coordinate [42]. For systemeach hinge, defying a local internal coordinate rigid bodysystem is defined. defying The internal hinges rigid kinematics body is is defined. modelled The by hinges coupling kinematics the DOF ofis mastermodelled independent by coupling node the andDOF slave of master nodes: independent pin release node at the and end-nodes slave nodes: of bar- pincbeam releaseelements at the end to- allownodes theof bar relative-cbeam rotationelements betweento allow the the connected relative rotation items has between been imposed the connected [42]. All items the has grids been of theimposed trailing [4 edge2]. All front the grids spar wereof the constrainedtrailing edge in front all six spar degrees were of constrained freedom (see in Figureall six degrees16). of freedom (see Figure 16).

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Constrained ConstrainedFront spar Front spar

Spars Spars

Morphing rib Morphing rib FigureFigure 16 16.. FEMFEM representation representation of ofthe the trailing trailing edge edge segment. segment. Figure 16. FEM representation of the trailing edge segment. TheThe global global finite finite element element model model consist consisteded of of a high a high number number of of nodes nodes and and elements, elements, necessary necessary to to properlyproperlyThe representglobal represent finite the the element trailing trailing modeledge edge geometrical consist geometricaled of layout a layout. high. All number All the the components of components nodes and of ofelements,the the morphing morphing necessary device, device, to apartproperlyapart from from represent the the steel steel thelinks links trailing and and actuation edge actuation geometrical chains, chains, were werelayout made made. All of the of aluminium aluminium components (Al2024 (Al2024-T3). of the-T3). morphing The The morphing morphing device, skinapartskin (shown from (shown the in in Figuresteel Figure links 1 717) andis) iscomposed composedactuation of chains, of a athree three-level were-level made structure: structure: of aluminium (Al2024-T3). The morphing skin (shown in Figure 17) is composed of a three-level structure: • • AnAn aluminium aluminium sheet sheet (plate (plate--cquad4cquad4), connected), connected to tothe the rib; rib; •• • AA hyperelasticnA aluminium hyperelastic sheet sheet sheet (plate (plate (plate--cquad4-cquad4cquad4),), covering ),connected covering all allto the the the structure; rib; structure; •• • FoamA Foamhyperelastic strips strips (solid (solid-sheet-chexa8 (platechexa8), -cquad4 applied), applied), covering in in span span-wise- wiseall the direction structure; direction and and located located close close to to the the hinges hinges to to • guaranteeFoamguarantee strips the the (solidmorphing morphing-chexa8 capability), capability applied to in tothe spanthe skin. skin.-wise direction and located close to the hinges to guarantee the morphing capability to the skin.

Al2024 Foam Al2024 Foam Elastomer Al2024 Elastomer Al2024Figure 17. FEM details: skin modelling. FigureFigure 17 17.. FEMFEM details: details: skin skin modelling. modelling. The mechanical properties assumed for the adopted materials are listed in Table 2, while the modelThe Themesh mechanical mechanical data are recappedproperties properties in assumed Table assumed 3. for for the the adopted adopted materials materials are are listed listed in in Table Table 22,, while while the the modelmodel mesh mesh data data are are recapped recapped in in Table Table 3.3.

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Table 2. FEM materials. Table 2. FEM materials. Material E (MPa) ρ (Kg/m3) ν Items 3 AlMaterial 2024-T3 E (MPa)70,000 ρ (Kg/m2780) ν 0.33 ItemsRibs; Spars HyperelasticAl 2024-T3 layer 70,0001.83–2.30 278010 0.330.46–0.47 Ribs; SparsSkin HyperelasticFoam layer 1.83–2.302.30–3.97 1010 0.46–0.470.15–0.24 SkinSkin Foam 2.30–3.97 10 0.15–0.24 Skin

Table 3. Model data. Table 3. Model data. FEM Entity Number Items FEMNode Entity Number177,548 ItemsAll NodeBar 177,548255 Pins, fasteners All , links BarPlate 177 255,221 Pins,Ribs, fasteners, spars, links skin Plate 177,221 Ribs, spars, skin MPCMPC 226226 Hinges,Hinges, holes holes

6.2. Parametric Normal Modes Analysis 6.2. Parametric Normal Modes Analysis A modal analysis was carried out to predict the dynamic behaviour of the structure for different A modal analysis was carried out to predict the dynamic behaviour of the structure for different values of stiffness and inertial parameters. The modal parameters were extracted by varying the values of stiffness and inertial parameters. The modal parameters were extracted by varying the mechanical properties of the skin in the range between the minimum and the maximum values listed mechanical properties of the skin in the range between the minimum and the maximum values listed in Table 1. Considering the spectral range [0, 200 Hz], the main resonance frequencies of the system in Table1. Considering the spectral range [0, 200 Hz], the main resonance frequencies of the system changed consequently, Figure 18. The (undamped) modal shapes of interest are shown in Figure 19. changed consequently, Figure 18. The (undamped) modal shapes of interest are shown in Figure 19. The compliant skin contribution to the generalized stiffness of the global morphing system was then The compliant skin contribution to the generalized stiffness of the global morphing system was then characterized. The morphing mode, which is predominantly a kinematic rigid movement at zero characterized. The morphing mode, which is predominantly a kinematic rigid movement at zero frequency, is here characterized by a not-null vibrating frequency. In the absence of the actuation frequency, is here characterized by a not-null vibrating frequency. In the absence of the actuation system, the rigid motion of the morphing structure is indeed prevented by the compliant skin, which system, the rigid motion of the morphing structure is indeed prevented by the compliant skin, which makes a unique contribution to the (not-null) modal deformation energy. makes a unique contribution to the (not-null) modal deformation energy.

180 E ; min min 160 E ; 149 Hz max max 140 135 Hz

120

100

80 66.4 Hz 61 Hz 60

36 Hz 40 28.5 Hz 20

0 Morphing Torsional Anti-phase bending

Figure 18. Parametric normal modes analysis: natural frequencies of the first three elastic modes. Figure 18. Parametric normal modes analysis: natural frequencies of the first three elastic modes.

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(a) Morphing mode shape, fmin = 28.5 Hz, fmax = 36 Hz.

(b) Torsional mode shape, fmin = 61 Hz, fmax = 66.4 Hz

(c) Anti-phase bending mode shape, fmin = 135 Hz, fmax = 149 Hz

FigureFigure 1 19.9. FirstFirst three three elastic elastic mode mode shapes shapes..

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6. Experimental Investigation of Dummy Device 7. Experimental Investigation of Dummy Device 6.1. Test Setup Description 7.1. Test Setup Description Frequency response functions (FRFs) were used to measure and characterize the dynamic Frequency response functions (FRFs) were used to measure and characterize the dynamic behaviobehaviourur of the of structure. the structure. It was It was clamped clamped at its at itsfront front spar spar but but free free at at the the other other end end (Figure (Figure 2020).). The excitationThe excitation system systemconsisted consisted of a shaker, of a shaker, which which was was connected connected by by a rigidrigid stringer stringer to to the the structure structure in in correspondencecorrespondence of the of the block block 0 of 0 of the the rib rib 1. 1. A A pure purelyly randomrandom signal signal was was used used in thein the range range 0–200 0– Hz200 Hz for thefor appropriation the appropriation of the of the resonance resonance frequencies. frequencies. AnAn experimentalexperimental grid grid of of 71 71 acquisition acquisition points points was was usedused to reconstruct to reconstruct the the mode mode shapes. shapes. Five Five span span-wise-wise stations were were monitored monitored (corresponding (correspond toing each to each of theof three the three ribs ribs,, and and in inthe the middle middle of of each each bay); bay); at each station,station, three three reference reference points points were were selected selected chordchord-wise,-wise, at the at theedges edges and and at atthe the middle middle of of each foamfoam strip. strip.

(a) Constraint condition in the wall

(b) Detail of the skin and rib

FigureFigure 20 20.. TestTest article article in clampedclamped conditions. conditions.

A schematicA schematic of the of theacquisition acquisition layout layout and and a photo a photo of of the the test test setup setup are are shown shown in in Figure Figuress 2211 and and 22, respectively. The adopted reference system is a Cartesian orthogonal one and was originated 22, respectively. The adopted reference system is a Cartesian orthogonal one and was originated on on the block 0 of rib 3 in the point O. The X-axis was placed along the rib on intermediate surface; the block 0 of rib 3 in the point O. The X-axis was placed along the rib on intermediate surface; the Y- the Y-axis was placed along the foam strip orthogonal to X-axis. The tests were carried out by placing axis thewas accelerometer placed along on the the foam first point strip and orthogonal rowing it step-by-stepto X-axis. The on alltests grid were points, carried recording out by datasets placing at the accelerometereach step. on the first point and rowing it step-by-step on all grid points, recording datasets at each step.

AerospaceAerospaceAerospace 201820182019,, , 556,, , xx 22 FORFOR PEERPEER REVIEWREVIEW 171717 ofof of 2222 22

FigureFigureFigure 221. 21.1. GroundGroundGround vibrationvibration vibration test:test: test: acquisitionacquisition acquisition mapmap map andand and drivingdriving driving pointpoint. point..

Shaker Shaker FrameFrame

AccelerometerAccelerometer

WallWall

TestTest articlearticle

FigureFigureFigure 222. 22.2. GVTGVTGVT setup:setup: setup: boundaryboundary boundary conditionsconditions conditions andand and electromechanicalelectromechanical electromechanical exciter.exciter. exciter.

6.2.7.2.7.2. GroundGround Ground VibrationVibration Vibration TestTest Test ResultsResults TheTheThe experimentalexperimental experimental modalmodal modal analysisanalysis analysis waswas was carriedcarried carried outou outt forfor for twotwo mainmain purposes:purposes: aa a modalmodal modal correlationcorrelation correlation withwithwith thethe the numericalnumerical numerical resultsresults results andand and thethe the estimationestimation estimation ofof of theth thee dampingdamping coefficientcoefficient. coefficient.. TheThe The testtest test resultsresults results confirmedconfirmed confirmed ananan interestinginteresting interesting behaviobehaviour behaviourur ofof of thethe the device:device: device: thethe the dampingdamping damping levelslevels levels werewere were muchmuch much higherhigher higher thanthan than thosethose those typicaltypical typical ofof of metalmetalmetal structuresstructures structures (generally(generally (generally aboutabout about 11–1.5%) 1–1.5%)–1.5%) startingstarting starting fromfrom from thethe the veryvery very firstfirst first modemode mode shapes.shapes. shapes. ThisThis This mechanism,mechanism, mechanism, ininin lineline line wiwith withth whatwhat what hhas hasas alreadyalready already beenbeen been observedobserved observed onon on theth thee singlesingle single sample,sample, sample, increasesincreases increases thethe the disdissipation dissipationsipation ofof of energyenergy, energy,, whichwhichwhich resultedresulted resulted inin in aa highhigher er dampingdamping coefficient.coefficient. coefficient. InIn In parparticular, particular,ticular, modalmodal modal dampingdamping damping increasedincreased increased toto to 3.5%3.5% 3.5% asas as thethethe frequencyfrequency frequency grewgrew grew becausebecause because ofof of thethe the actualactual actual boundaryboundary boundary conditioncondition, condition,, whicwhich whichh affectedaffected affected skinskin skin compresscompression compressionion andand and stretchstretchingstretchinging toto to aa a significantsignificant significant extentextent extent duringduring during thethe the modalmoda modall deformations.deformations. deformations. TheThe The experimentalexperimental experimental modemode mode shapesshapes shapes areare are representedrepresentedrepresented in in Figure Figure 23223.3.. TheseTheseThese remarkable remarkableremarkable outcomes outcomesoutcomes confirmed confirmconfirmeded that thatthat the thethe morphing morphingmorphing skin skinskin increased increaseincreased thed thethe generalizedgeneralized stiffnessstiffness ofof thethe adaptiveadaptive structure,structure, asas alreadyalready foreseenforeseen byby thethe simulations,simulations, byby playingplaying thethe functionfunction ofof aa dynamicdynamic energyenergy dissipatordissipator ((FigureFigure 224).4).

Aerospace 2019, 6, 22 18 of 22 generalized stiffness of the adaptive structure, as already foreseen by the simulations, by playing the Aerospace 2018, 5, x FOR PEER REVIEW 18 of 22 function of a dynamic energy dissipator (Figure 24).

(a) Experimental morphing mode shape, f = 35 Hz.

(b) Experimental torsional mode shape, f = 65 Hz.

(c) Experimental anti-phase bending mode shape, f = 145 Hz.

FigureFigure 2 23.3. ExperimentalExperimental mode mode shapes. shapes.

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Aerospace 2018, 5, x FOR PEER REVIEW 19 of 22 Aerospace 2019, 6, 22 19 of 22

Figure 24. Damping ratio for each modal deformation. Figure 24. Damping ratio for each modal deformation. Figure 25 shows for each mode the deviation between experimentally measured and numerical Figure 24. Damping ratio for each modal deformation. evaluatedFigure 25 frequencies. shows for each It is modeworth thenoting deviation that the between test data experimentally are within the measuredpredicted andnumerical numerical range. evaluated frequencies. It is worth noting that the test data are within the predicted numerical range. Figure 25 shows for each mode the deviation between experimentally measured and numerical evaluated frequencies. It is worth noting that the test data are within the predicted numerical range.

Figure 25. Test—FEM results deviation. Figure 25. Test—FEM results deviation. 8. Conclusions

Adaptive structures are a topic of growing interest in the aviation industry since they can bring Figure 25. Test—FEM results deviation. several benefits to next-generation aircraft. An adaptive trailing edge concept is herein presented by

Aerospace 2019, 6, 22 20 of 22 combining conventional parts and innovative concepts, as for the morphing skin, all assembled into a new structural system. The structure selected for the benchmark problem is a two-bay portion of an outboard wing trailing edge, consisting of three articulated (finger-like) rib covered by a multi-material hyperelastic skin. The authors have investigated some important aspects for the design and validation of the adaptive system. Focus has been given to the integration issues of a multi-block structure with a high-elasticity compliant skin. The skin influences the global structural dynamics in terms of its mass, damping and stiffness. On the one hand, the inclusion of a compliant skin contributes to make the morphing structure more stable, thanks to its stiffness. The rigid motion of the mechanical structure, necessary to morph the shape of the system, is in fact “counteracted” by the multi-segment skin deformation. Both numerical and testing outcomes highlighted that the morphing frequency, typically representative of a multi-body rigid mode close to 0 Hz, is instead about 35 Hz, even in the absence of the actuation system, which generally affects such a mode by introducing torsional stiffness. On the other hand, the increased weight due to the skin, which could also be a relevant percentage of the complete system, leads to a further increment of the modal density at low frequency, thereby introducing more elastic modes in the bandwidth where flutter instabilities are more likely to occur. The outcomes have demonstrated an intrinsically higher damping coefficient than the standard metallic systems: the elastomeric skin increases the system global property to disperse vibrational energy, reducing the resonance amplitudes. In the preliminary experimental investigations, carried out on a simplified mock-up of the final SARISTU demonstrator, this damping trend has been well assessed in several percentage units, in line with the expectations. The outcomes of the dynamic test have de facto confirmed those levels investigated on stand-alone skin samples by means of mechanical tests only. In more detail, the morphing skin contributes decisively to the modal damping of the entire structure by ensuring 3–3.5% damping compared to 1–1.5% of the aluminium structure alone. Further studies are planned to assess the behaviour of the structure by also taking into account the contribution of the actuators and transmission chains to the overall stiffness of the morphing device.

Author Contributions: Conceptualization: M.A., R.P., O.S., and A.C.; Methodology: M.A., R.P., and C.N.; Software: M.A. and C.N.; Validation: M.A., C.N., and I.D.; Formal analysis: All; Investigation: All; Resources: R.P., O.S. and A.C.; Data curation: M.A. and C.N.; Writing—original draft preparation: M.A. and C.N.; Writing—review and editing: R.P., I.D., A.C.; Visualization: M.A. and C.N.; Supervision: R.P., O.S. and A.C.; Project administration: R.P., O.S., A.C. and I.D. Funding: The research outlined in this paper exploits concepts and technologies developed in the framework of the SARISTU project (under Grant Agreement n◦ 284562 of FP7/2007-2013). Acknowledgments: The authors wish to thank all SARISTU partners involved in the design and validation of the morphing trailing edge device. Conflicts of Interest: The authors declare no conflict of interest.

References and Note

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