Mechanical and Industrial Engineering,Texas A&M University-Kingsville,
MSC 91, 700 University Boulevard,Kingsville, TX 78363-8202
Development of a SimpleMorphing Wing UsingElastomeric Composites as Skinsand ActuatorsMorphing wings are desired for their ability to reduce drag, to change flight character-istics, and perhaps to reduce weight by eliminating flap/aileron mechanisms. Develop-ment of two generations of a morphing wing project is documented. The work shows howa relatively low cost but realistic morphing wing test-bed can be fabricated. Wing skin,actuator, and actuator attachment development are discussed, as well as possible auxeticskin behavior. Aerodynamic characterization of the wing will be discussed in anotherpaper. A very simple morphing wing was fabricated in generation one. The nose was ableto elastically camber down approximately 25 deg and the tail 20 deg. Actuation wasprovided by three pneumatic “rubber muscle actuators” that produce high contractive/tensile forces. Upper and lower wing skins were fabricated from carbon fiber/polyurethane elastomer laminates. Lower skin buckling, actuator air leaks, and actuatorattachment problems were resolved in the second generation. A finite element model ofthe second wing was developed and is being used to refine the morphing wing test-bed.The second wing fabrication methodology enabled smooth elastic cambering with nobuckling or waviness in the skins. The nose cambered down 14 deg and the tail cambereddown to 13 deg, and is capable of larger deformations. Improved leak-free biomimeticactuators and attach points now include no metal parts and have higher actuation forcesdue to new braided sheaths and functionally gradient matrix properties.�DOI: 10.1115/1.3159043�
Background and Other Work
There are many morphing wings in development. Perhaps theost well-known is Lockheed Martin’s Z Wing concept �1�. This
ircraft has rigid internal components, hinged at appropriate loca-ions, with the hinged areas initially covered with a heat-activatedhape memory polymer composite skin from Cornerstone Re-earch Group �CRG�. Thermal problems prompted a change to aber-reinforced silicone elastomeric skin �2�. FlexSys has devel-ped a mission adaptive compliant wing �MACW�, fabricatedith aluminum skins where the leading edge can deflect down-ard by 6 deg and the trailing edge can deflect �10 deg �3�.killen and Crossley �4� considered the folding wing and a vari-ble sweep-wing approach in a modeling and optimization study.heir research will be used in future sweep-wing phases of theurrent work. Kikuta �5� outlined a number of requirements for aorphing skin of an aircraft wing with camber variation (a and b
ave been generalized):
�a� very elastic/flexible in the deformation direction to allowlow force actuation
�b� very stiff perpendicular in the deformation direction towithstand aerodynamic and inertial loads
�c� toughness�d� abrasion and chemical resistant�e� resistance to varying weather conditions�f� high strain capability�g� high strain recovery rate�h� environmental longevity and fatigue resistance
Manuscript received November 28, 2008; final manuscript received May 8, 2009;ublished online August 17, 2009. Review conducted by Nancy Johnson. Paperresented at the 2008 ASME Global Conference on Smart Materials, Adaptive Struc-
ures and Intelligent Systems �ASMS2008�, Washington, DC, October 28–30, 2008.
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These requirements are addressed throughout the current work.At first glance the current fiber-reinforced elastomer system ful-fills a–g, however, more study is needed to reach fulfillment of h.In general, any morphing skin must be stiff enough in any direc-tion to maintain the required outer mold line shape under aerody-namic loads. Thill et al. �2� reviewed many types of morphingskins, structures, and actuation methods. On elastomer skins it isnoted that high strain capability is useful but it is “difficult todesign elastomeric skins that can sustain and transfer aerodynamicloads to the underlying structure.” By incorporating oriented fi-brous reinforcement into a suitable elastomer, load transfer ismore easily accomplished. A number of foam and cell-like auxeticmaterials and Alderson’s auxetic composites �6� are reviewed.Thill et al. �2� noted some of the useful properties of auxeticmaterials to be high energy absorption, fracture toughness, andresistance to indentation. “Large inplane Poisson’s ratio skinswould give anticlastic shapes and negative inplane Poisson’s ratioskins would induce synclastic behavior when bent out of plane.”
Peel �7–9� experimentally obtained inplane Poisson’s ratios ashigh as 21 and as low as �5, using fiber-reinforced elastomerlaminates, and has predicted inplane Poisson’s ratios between�60 and 100. In 2004 Rathnam and Peel �10� compared impactresistance and fracture toughness of several fiber-reinforced poly-urethane composites with epoxy composites and baseline metals.The intermediate �RP 6442 polyurethane� and rigid �RP 6444polyurethane� elastomer composites had greater specific impactstrength than equivalent epoxy composites, baseline aluminum,and steel. Their carbon fiber/semirigid polyurethane tubes hadhigher residual compressive strength than carbon fiber/epoxycounterparts. In 2005 Keshavamurthy et al. �11� showed that non-optimized fiber-reinforced elastomer �FRE� laminates, which ex-hibit negative Poisson’s ratios, can produce damping about 100%greater than an equivalent axial stiffness FRE laminate with a
positive Poisson’s ratio.
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Hannaford and co-workers �12–14� worked with pneumaticuscle actuators �McKibben-like� for many years, and explored
heir actuation, fatigue, and control characteristics. In 1997 Peelabricated McKibben-like actuators using a filament winder. Ashe winder laid down the fiber, it was impregnated with a compli-nt elastomer �9�. Hossain �15� conducted linear finite elementnalyses of Peel’s and other pneumatic muscle actuators, and re-iewed many types of actuator attachments. Using a fabricationethod similar to Peel �9�, Shan and Bakis �16� and Peel et al.
17� at Penn State fabricated and characterized pneumatic musclectuators for use in morphing structures.
Research ObjectivesMany research papers discuss the modeling of morphing air-
raft structures, but less has been said on the fabrication of suchtructures. The current work shows a relatively inexpensive ap-roach to the fabrication of simple but realistic morphing struc-ures using conventional composites fabrication techniques. Theork draws heavily on Peel’s work with elastomeric composites.
First Generation WingPeel and two undergraduate students, Krystal Gunter, and
oyce Coons, fabricated and tested the first prototype morphinging during a 2 week “Maymester” undergraduate research expe-
ience program in May 2006. The wing was built with suppliesrom the TAMUK composites fabrication laboratory, using Peel’sxpertise. The two students drew a reasonable airfoil cross sectionnd created a paper model of the wing, as shown in Fig. 1. Theorking paper model allowed the students to simulate camber
hange and to consider possible problems.It was decided that the wing would have a rigid wing box
abricated from carbon/epoxy, and that the upper and lower skinsould be a flexible composite using IM7 carbon tow and a rigidolyurethane RP 6444 elastomer. Actuation would be provided byhree rubber muscle actuators �RMAs�, as discussed in Sec. 3.1elow.
A simple finite element model of the wing was created, andnforced displacements were applied to the nose and tail. Theesultant positive camber change indicated that the current wingonfiguration would indeed camber down.
3.1 Actuator Selection. Table 1 compares the relative perfor-ance characteristics �15� of light-weight push-pull actuators
uch as electroactive polymers �EAPs�, pneumatic rubber musclectuators, shape memory alloy �SMA�, and piezo-electric actua-ors �PEAs�. Hydraulic actuators were not considered due to theireight and support equipment. In terms of most mechanical per-
ormance measures, pneumatic RMAs are comparable or superioro the other choices. Shape memory alloys have higher force/areautput, but are very energy inefficient, produce excess heat, or ifverheated, lose their contractile capability, and like PEAs, havemall contraction ranges.
3.2 Fabrication: Wing Skins and Wing Box. All parts of theing, as shown in Fig. 2, except for the lower wing skin �whichas fabricated earlier� were fabricated from scratch. The upper
omposite skin is a ��10 deg� carbon fiber laminate, impregnated
Fig. 1 Paper model of initial morphed wing
ith Huntsman’s RP6444 semirigid polyurethane elastomer. The
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elastomer has an initial Young’s modulus of 1.8 GPa �26,300 psi�,but can elastically stretch up to 300% �a rubber band has an initialYoung’s modulus of about 1.65 MPa �230 psi� and can stretch toabout 700%�. RP6444 is commonly used as a resilient coating onore-hauling truck beds and is very impact resistant when com-bined with carbon or fiberglass fibers �10�. The bottom skin is a�90 deg� carbon fiber laminate, impregnated with an intermediatestiffness RP6442 elastomer. All elastomer properties are summa-rized in Table 2. The low fiber angle on top makes the upper skinstiffer in bending �about the spar�, and helps the wing to have asmooth curvature as the skin bends downward. The nose sectionwas formed over a male mold. All other skin sections were lami-nated and cured on a flat surface.
The wing box was formed by wrapping epoxy-impregnated car-bon cloth around a shaped Styrofoam core and vacuum-bagging it.After cure, the foam core was dissolved with acetone and the wingbox was trimmed. As this wing was to only demonstrate the mor-phing concept, no effort was made to use aerospace grade/composite-specific fasteners. All skins were attached to the wingbox with small wood screws and washers.
3.3 Fabrication: Actuators. As explored by Hossain �15�and Peel �9�, the rubber muscles actuators expand radially, butcontract axially when filled with compressed air. The magnitudeof force and contraction are functions of fiber angle, actuator di-ameter, and pressure.
These rubber muscles actuators, as shown in Fig. 3, have a 12.7mm �0.5 inches� inside diameter �ID�, and are fabricated usingAS4 carbon tow and Huntsman’s RP6410 very flexible elastomer.Traditional McKibben-like actuators use prebraided sheaths withminimum fiber angles near �20 deg. Using a filament winder theGeneration I actuators were wound at �10 deg, producing highercontractive forces for the same actuation pressure. Each producedapproximately 90.7 kg �200 lb� initial contractive force at 207 kPa�30 psi�.
The actuators are attached to shaped wooden leading and trail-ing edge blocks which were attached to the skins with screws andwashers. One problem noted by Peel in previous works was themethod of attaching the RMAs �9,15�. Early RMAs �9� consisted
aBased on closed-form model at 15 deg, 0.52 MPa �15�.
Fig. 2 Initial morphed wing
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f a filament wound fiber-reinforced elastomer tube, with air inlett one end and a rigid “stopper” at the other end. The metal inlettting and stopper were covered by the wound tube, and a metalose clamp was tightened over it to seal the RMA. To measurectuation force, the metal air inlet and the stopper were held inrips. However, if too much air pressure was applied the stopperr metal air inlet would come off the muscle, possibly causingnjury and loss of actuation force. To partially alleviate this prob-em, the RMAs shown in Fig. 3 did not have elastomer applied toheir whole length. The loose fibers were gathered into twoundles at each end of the muscle. These bundles were passedhrough holes drilled in the wooden blocks, and the fiber endsere then clamped. This method did not place any tension loadsn stoppers or air fittings that are under internal pressure, butaused the fibers to take all tension loads.
To seal the actuators, short wooden dowels were inserted in onend of the RMA, and wire was tightly wrapped around the out-ide. Blue flash tape was wrapped over the wire for protection.imilarly, plastic nonload bearing air inlets were inserted in thether end and had wire and tape wrapped tightly around the ac-uator tube and fitting. This method was less intrusive and just asffective as hose clamps. However, both methods tend to causetress concentrations in the RMA skins at high pressures, and areikely spots for failure if the RMAs are overpressurized.
3.4 Generation I Wing Successes and Problems. With aash outlay of approximately $300 for materials, a working mor-hing wing was developed. The nose would camber down over 20eg and the tail would camber down about 25 deg, considerablyreater than the FlexSys wing �3�. A web page �17� was set up tohowcase the student success and has several videos. The threeMAs provided excellent actuation but developed pinhole leaks
n the fiber-reinforced elastomer RMAs after about 30 cycles. It
aObtained using the rule of mixtures.bFrom Ref. �9�.cFrom Ref. �7�.
ig. 3 High force filament wound „FW… rubber muscle actua-
ors used in the first morphing wing
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was noted that the dry fiber attachments at the end of the RMAswould eventually fray. The lower skin was too compliant in thechordwise direction and also buckled, as predicted by finite ele-ment analysis. The wing had a “typical” airfoil shape and con-sisted of several skin sections. These skin sections showed minorwaviness while being cambered. This wing used simple woodscrews and washers as fasteners. It was understood that if thewing was disassembled too many times, the wood screws would“strip out” the composite laminates.
4 Development of the Generation II WingThe major objectives of the second prototype were to solve the
lower skin buckling problem, to make a prototype small enough tofit in a suitcase, and to improve actuator issues. The main concernwas to provide a structure flexible enough to handle all morphingbehavior while maintaining the rigid characteristics of normalwing structures. That is, the wing skin must maintain its rigiditybut be flexible enough to deflect and either stretch or contract inthe specified direction. A drawing of the Generation II wing �18�,as shown in Fig. 4, and has the common Clark Y airfoil section.The wing has a 51 cm �20 in.� chord and a 33 cm �13 in.� span thatfits nicely in a medium suitcase.
To eliminate lower skin buckling the front lower skin was de-signed to be much stiffer than Generation I and slides against therigid wing box. Simple guides that slide in wing box slots areattached to the skin. The lower rear skin is attached rigidly to thelower wing box and slides against the tail section, as shown inFig. 5. The semirigid lower wing skin slides out beyond the tail,
ffectively increasing wing area, somewhat like a fowler flap.ther possible solutions to the buckling problem included pre-
tretched skins and corrugated skins that would thicken whenompressed. Sliding skins were chosen for their simplicity androven capabilities.
A traditional wing box design was also chosen for the secondeneration. A piano hinge was used to secure the upper wing skino the wing box. The piano hinge allows for a solid axis of flexurend a secure method of attachment using a Hysol 9394 structuraldhesive. The hinge is removable at the wing box using commonood screws. The second generation wing, without actuators, as
hown in Fig. 5, was actuated by “hand” with some effort.The upper wing skin and nose section was molded in a femaleold, again using IM7 fiber and RP6444 polyurethane resin. All
kins had a lay-up of ��10�ns, however, the upper skin has anpproximate thickness of 1.6 mm �0.064 in.�, while the lower skinas an average thickness of 1.3 mm �0.051 in.�. This lay-up meanthat the skins are rather stiff in bending; however, the RMA ac-uators provide adequate force. The stiffer skins will not tend torinkle and will bend in a uniform manner, giving smooth aero-ynamic surfaces. As noted in Sec. 4.5, future skin lay-ups will beptimized.
The ��10�s lay-up was obtained by filament winding fiberhile impregnating with resin, around a large PVC pipe mandrel
overed with peel ply. After winding, the wet laminate was cov-red with another peel ply. Before curing, this laminate was care-ully cut off the mandrel and formed into shape in the femaleold. The lower skins were vacuum bagged and cured on flat
urfaces.The wing box was again fabricated from IM7 cloth and epoxy,
ut was formed in a closed mold with a Styrofoam core that wasater removed. The lower box plate had a molded indentation tollow the flush attachment of a lower rear wing skin.
4.1 Actuator Development. The Generation I RMAs pro-ided sufficient contractive �tensile� force, but developed pinholeeaks and had a less-than-perfect attachment method. To fabricateew actuators, a 12.7 mm �0.5 in.� steel rod was covered with peelly, and a layer of compliant RP6410 polyurethane elastomer waspplied while the rod was rotating in a filament winder. This layerf elastomer was allowed to cure and became the inner bladder ofhe RMA. After curing, carbon tow was wound while being im-regnated with more elastomer. Additional elastomer was appliedver the fiber. The average outside diameter was approximately7.8 mm �0.70 in.�. As noted above, end sections of the RMAber were left dry. A proprietary method was used to remove theMAs from the constant diameter mandrel.This method of fabrication worked but took approximately 3
ays to obtain a set of three RMAs that were wound on the sameandrel. If the inner elastomer layer had any surface irregularities
n it, a weak spot was formed and eventually caused pinhole leakso form when the RMA expanded up to 300% in diameter.
The first attempt to solve this problem involved curing thinheets of degassed elastomer on peel ply. The thin sheets wererapped around a prepared mandrel and bonded in place with
ig. 5 Second generation morphing wing with no lower skinuckling
ore uncured elastomer resin. IM7 6K carbon tow was again
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wound and impregnated with RP6410 resin. Sections of the fiberwere left dry. After curing and removal of the RMAs from themandrel, a plastic air inlet fitting was secured in place with carbonfiber, and the opposite end was also sealed. The dry fiber endswere sandwiched between several layers of carbon cloth, wereimpregnated with a rigid polyurethane resin, and were vacuumbagged and cured. The inspiration for the resultant phase II RMAscame from compliant biological muscles attached to stiff tendons,which thus form an actuator that has functionally gradient matrixproperties. In the RMA, this stiffness gradient is intended to re-duce stress concentrations where the actuator starts to expand ra-dially. These set of actuators are shown in Fig. 6. It is noted thatthe rigid attachment strips have permanent wrinkles resulting froma poor vacuum bag. Despite this, these updated RMAs were easilyattached in the Gen II morphing wing. However, the actuatorssoon developed pinhole leaks and would not produce significantactuation force.
Conversations with an A & P Technology executive led to thedevelopment of a new and better braided sheath with carbon fibersthat would contract down to �10 deg at 12.7 mm �0.5 in.� indiameter, when pulled axially. It was also decided to use a latexrubber tube as an inner liner for the next phase of functionallygradient rubber muscle actuators. The phase 3 braided actuatorsare shown in Figs. 7�a� and 7�b�.
To fabricate these actuators, a braided sheath was drawn over amandrel and pulled down to 12.7 mm �0.5 in.� in diameter. Thecenter portion of the actuator was impregnated with a compliantelastomer to help the braided fibers maintain their configuration.After curing, the braided sheaths were removed while keeping thedry ends from unraveling. A latex tube was sealed at one end, hadthe other end attached to a plastic hose fitting, and was inserted inthe braided sheath. Carbon tow was tightly wound around thebraided sheath over each end of the latex tube; this keeps the latextube on the plastic fitting, and maintains an airtight seal at theother end. All exposed flexible parts and plastic fittings were cov-ered with Teflon tape. The dry tow and braided sheath were im-pregnated with a rigid Polyurethane RP 6444 and vacuum-bagged.When cured flat, the dry braids form a strip at each end of themuscle that is strong enough to carry any tensile load, yet thin andflexible enough to allow insertion into the wing box. FutureRMAs might have fastener holes molded in, but the current RMAwas attached to the nose and tail sections with a series of woodscrews through the flat strips.
4.2 Actuator Test Results. To determine whether the filamentwound or braided sheath actuators produced the highest contrac-tive force and contracted the most, a series of tests were con-ducted. Two previously 1.27 cm �0.5 in.� ID filament wound ac-tuators �FW1 and FW2� were repaired with 9.53 mm �0.375 in.�OD latex tube bladders. Three new “Phase 3” braided sheath ac-tuators �B1, B2, and B3� were also fabricated with a nominal 1.27cm �0.5 in.� ID and 9.53 mm �0.375 in.� OD latex bladders.
Actuator B3 was pressurized to failure to determine maximumsafe operating pressures and unloaded actuator geometry as afunction of pressure. It failed at 379 kPa �55 psi� due to latex
Fig. 6 First functionally gradient rubber muscle actuators,uninflated
bladder rupture.
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FW1 and FW2 have the contractive force versus pressure rela-ionship as shown in Fig. 8. Note that the RMAs do not startroducing much force until about 172 kPa �25 psi�. This is likelyue to the resistance of the inner latex tube, as well as the elas-omer impregnated in the filament wound layer. The actuatorsere fixed in a tensile testing machine with zero tensile load ap-lied to them. Then, air pressure was increased and recordedlong with tensile force. At 345 kPa �50 psi�, the machine cross-ead was lowered, and the actuator was allowed to contract axi-lly and to expand radially until the force readout was approxi-ately zero or did not change. At this point, the crosshead was
aised to its original position. At the original zero displacementosition, the actuators were now in a “prestressed” or “blockedorce” state and typically produced a much higher force. Hence,wo force versus pressure curves are shown for each actuator.
ig. 7 „a… Braided functionally gradient rubber muscle actua-ors, uninflated. „b… Same actuators inflated to 207 kPa „30 psi….
ig. 8 Contractive force versus pressure for filament wound
MAs with latex bladders
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Typically, the higher curve is when the actuator is in a prestressedstate. The exception to this is for the FW2 actuator. Since FW2 isan older repaired actuator; this characteristic may be a function ofthe viscoelastic and/or hysteretic nature of the compliant elas-tomer bladder and skin, and may disappear over time.
B1 and B2 were also tested and found to have the contractiveforce versus pressure relationship shown in Fig. 9. Likewise, theydid not start producing much force until about 138 kPa �20 psi�.The unstressed B1 and B2 curves show a similar maximum forceat 344.7 kPa �50 psi� of approximately 90.7 kg �200 lb�. This issimilar to all of the FW test results. However the prestressed B1and B2 maximum forces are much higher, reaching as high as1806 N �406 lb�.
Representative force versus contraction curves for B1 and FW2are shown in Fig. 10. The braided actuators produced higherforces than the filament wound actuators in contraction, and onthe return stroke. Note also that the braided actuator produced aforce up to a maximum contraction of about 2.8 cm �1.1 in.�,where the filament wound actuator only reached a maximum con-traction of 1.91 cm �0.75 in.� at its minimum force.
Shadow Robotics �19� in the UK, fabricates McKibben-like ac-tuators and has noted similar behavior in its prestressed testing.Preliminary comparisons indicate that the 13 mm B1 and B2 pro-duce higher blocked forces at 207 kPa �30 psi� or 2 bars than the20 mm Shadow Air Muscle.
Fig. 9 Contractive force as a function of pressure for braidedRMAs with latex bladders
Fig. 10 Axial force as a function of contraction for B1 and FW2
at a constant pressure of 276 kPa „40 psi…
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4.3 Performance of the Generation II Morphing Wing. Ahotograph of the Gen II wing, actuated to 276 kPa �40 psi� ishown in Fig. 11. The wing is placed against a grid of 2 in.quares. Little or no difference in deformation was noted between76 kPa and 310 kPa �40 psi and 45 psi� because the actuators hadeached their maximum diameter, so no more contraction wasossible. At 276 kPa �40 psi�, a tracing of the morphed outlineas taken and analyzed. The nose deformed down to 14 deg, and
he tail deformed down to 13 deg. Optimized actuators will enablencreased cambering.
The Generation II wing shows no sign of skin buckling or wrin-ling. The crude guide carbon/epoxy guides enable the lower skino easily slide back and forth. Three phases of rubber musclectuators were developed and refined, with further refinement pos-ible. The wing currently uses standard wood screws as fasteners,ut could easily be modified to use composite-specific fasteners.t a later date, the wing will be modified to allow the tail to
ctuate separately from the nose. Access holes and cut-outs on theext wing box also need to be optimized.
4.4 Discussion of Auxetic Wing Skins. Fiber-reinforced elas-omer laminates can be tailored to produce a large range of Pois-on’s ratios. Figure 12 shows the Poisson’s ratio for a series of� /��s laminates, where � ranges from �90 deg to 90 deg and �
ig. 11 Generation II wing morphed at a pressure of 276 kPa40 psi… on a 51 mm grid
ig. 12 Poisson’s ratios for IM7/RP6444, „Vf=0.4… with a †� /�‡s
ay-up schedule
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ranges from 5 deg to 45 deg. Note that for a balanced or angle-plylaminate, such as ��10�s, the inplane Poisson’s ratio vxy is about8, or extremely positive. A less axially stiff laminate, such as��30�s, has an inplane Poisson’s ratio of 3. On the other hand, anunbalanced laminate, such as �15 /45�s, has a negative inplanePoisson’s ratio of approximately �3. Heracovitch �20� showedthat certain angle-ply laminates can produce negative through-the-thickness vxz Poisson’s ratios. Peel �9� used the definition of cu-bical dilatation to show that for a laminate with an incompressiblematrix, such as an elastomer, that the relation
1 − vxy � vxz �1�
is valid, where vxy is the major inplane Poisson’s ratio and vxz isthe major through-the-thickness Poisson’s ratio. This assumes thatthe cubical dilatation is close to zero and is much smaller than thelaminate axial strain.
This means that the Generation II wing skin should have anapproximate thickness-direction Poisson’s ratio of approximately�7, while a less stiff ��30�s skin would give �2, and a ��45�s
skin should have a vxz approximately zero. Future works willexplore these skin characteristics. As noted by Thill et al. �2�,some of the useful properties of auxetic materials to be high en-ergy absorption, fracture toughness, and resistance to indentation.They also state that “Large inplane Poisson’s ratio skins wouldgive anticlastic shapes and negative inplane Poisson’s ratio skinswould induce synclastic behavior when bent out of plane.” Thesebehaviors have not been noticed in current finite element models,but will be explored in a future work.
4.5 Simulation of Generation II Wing. A finite elementmodel of the second generation wing, as shown in Fig. 13, wasdeveloped by Madhuri Lingala, a graduate student. The wingskins, wing box all have the dimensions, lay-up, and materialproperties of the fabricated Generation II wing. Sliding contactelements were used on the lower skin to simulate the sliding skinson the lower wing box and lower tail section. All mechanicalproperties are found in Table 2.
It appears that laminate orientation has more effect on nosedeformation than tail movement. The following discussion fo-cuses only on nose deformation for simplicity, as this work isprimarily about fabrication. Full modeling details will be pub-lished separately.
Based on the B1 and B2 force versus contraction test results at40 psi, a conservative force of 100 lb was applied to three nodesin the nose and tail of the wing, simulating the three braidedrubber muscle actuators. The nose rotated down 13 deg and thetail rotated down about 12 deg, comparable to the experimentaldeformation shown in Fig. 11. The maximum predicted von Misesstress in any ply is 37,500 psi.
Kikuta �5� and others suggested that a wing skin should behighly compliant in the direction of deformation and stiff in theperpendicular direction. The Gen II prototype is very stiff in thedeformation direction to ensure smooth cambering. This is pos-sible because the RMAs produce high forces for their weight.
Linear and nonlinear simulations for a ��45�s lay-up were es-sentially identical and produced too much skin waviness for thegiven loads. If the upper and lower wing skin lay-ups werechanged to a ��30�s lay-up, the nose rotates down approximately32 deg, as shown in Fig. 14. This would give more camber, butedge effects start to show on the upper skin, and the max vonMises stress in any ply increases to 54,900 psi, which is likely tobe too high.
An inplane auxetic laminate of �10 /30�s for the upper andlower skins was simulated. It showed slightly lower displacementsbut otherwise behaved similar to the model shown in Fig. 14. Alonger chordwise finite element model has been created and willbe used to simulate the effects of various laminates that might be
used in a morphing unmanned air vehicle.
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ConclusionsTwo morphing wing prototypes have been fabricated by under-
raduate student teams and are being characterized, and a finitelement model of the second wing is demonstrated. These wingshow excellent angles of deflection, and the Gen II wing shows nouckling or wrinkling of its wing skins. A series of powerful rub-er muscle actuators were refined. The latest braided actuatorsave functionally gradient properties, produce higher forces thanimilar filament wound actuators, and do not leak. The upper andower skins of Gen II show excellent rigidity but elastically cam-
Fig. 13 Finite element model of
Fig. 14 Finite element model of th
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ber in a smooth manner. The second wing is of high enough qual-ity to be used for physical demonstrations or in a wing tunnel.Extremely tough fiber-reinforced elastomers have enabled boththe morphing wing skins, their internal actuators and can enableauxetic behavior if needed.
6 Future WorkThe Generation II wing will be modified so that the tail and
nose will actuate separately and is intended to be tested in a windtunnel. Aerospace quality fasteners will be used to replace current
Gen II wing with current lay-up
the
e Gen II wing nose with †±30‡s
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ood screws. A shape memory polymer resin system has beenurchased and will be used to fabricate a series of wing skins andctuators that can be tested on the current Gen II system. Theurrent finite element model will continue to be used to optimizeorphing behavior for the current configuration and future com-
liant trusslike configurations. Inplane and through-the-thicknessuxetic skins will be explored.
cknowledgmentWe would like to thank Texas A&M University-Kingsville for
he use of their facilities, especially the Composites Fabricationab; Victor DeLeon for all his great machine work; Krystalunter and Royce Coons for their preliminary Maymester work;
nd the MEEN 4263/64 Senior Design class for their constructiveeedback; also, Juan Rangel for his formatting and Excel plottingxpertise.
eferences�1� Lawlow, M., 2006, “The Shape of Wings to Come,” SIGNAL, www.afcea.org/
�2� Thill, C. L., Etches, J., Bond, I., Potter, K., and Weaver, P., 2008, “MorphingSkins,” Aeronaut. J., paper 3216.
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