Morphing Wing Structures for Loitering Air Vehicles

David A. Perkins, John L. Reed, Jr., Ernie HavensCornerstone Research Group, Inc., 2750 Indian Ripple Road, Dayton, OH, 45440

ABSTRACT

Cornerstone Research Group Inc. (CRG) will present current results of ongoing research and development ofadaptive wing structures. The focus of these efforts is to develop and demonstrate viable composite materials andprocess technology to support multiple structural morphing applications. Prototypes under development includeseamless-span morphing wings, chord morphing wings, and folding wings. Chord morphing is the focus of thispaper. Advanced technologies applied include CRG’s shape memory polymers (SMP) Veriflex™, dynamic moduluscomposites (DMC) Veritex™, and dynamic modulus foams (DMF) Verilyte™. The development approach includesinternal structure, seamless skin, thermal activation, and mechanical actuation. Many new composite fabricationtechniques have been investigated. Other technologies under consideration include shape memory alloys,piezoelectric materials, electro- and magneto-rheological materials.Initial design and engineering efforts have focused on integrating new adaptive materials technologies into standardcomposite structures. CRG developed preliminary prototype designs of adaptive wing structures, developed aprocessing approach for fabrication, validated previously undemonstrated fabrication processes, and fabricatedindependently functional subsections of a morphing wing to prove feasibility.

Keywords: Adaptive structures, morphing aircraft, smart materials, shape memory materials

1. INTRODUCTION

Many aircraft are designed solely to operate in a single flight profile, such as a maneuverable fighter or along-range cruise airliner. If an aircraft is designed to be a hybrid and combine several flight profiles, the wingsmust be designed to maximize overall efficiency, causing the efficiency in each individual flight regime to be lessthan the efficiency of an aircraft designed for that regime. Significant improvements can be made to these hybridsby implementing a wing that can change its shape to match the most efficient form for any flight regime. Thechange in wing shape would allow the aircraft to adjust between an aspect ratio suited for long-range cruise and onesuited for maneuvering. The aircraft flies to a point in a cruise-efficient wing shape and upon reaching its targetlocation morphs to a wing shape more efficient for maneuvering.

212 LL v SCρ= (1.1)

CRG investigated an increase in lift by dramatically increasing the planform area (S) and therefore the liftgenerated according to Bernoulli’s Equation (Eq. 1.1), where L is the lift, v is the velocity of the aircraft, ρ is thelocal air density, and CL is the coefficient of lift. CRG began with the goal of increasing lift by 80%.

CRG proposed to accomplish this endeavor with significant effort focused on design and engineering byintegrating new adaptive materials technologies, including shape memory polymers (SMP), dynamic moduluscomposites (DMC), and dynamic modulus foam (DMF), into standard composite structures. The Phase I researcheffort was focused on proving the feasibility of the design and provided the groundwork for full-scale prototypetesting and design in Phase II. During Phase I, CRG developed a preliminary prototype design of an adaptive wingstructure for the morphing aircraft, as well as related actuation and activation components. CRG also developed aprocessing approach for fabrication of the wing and smart material composite skins, validated undemonstratedfabrication processes, investigated scale-up processes, and fabricated and validated sub-scale prototypes of themorphing wing components to prove feasibility. During Phase II, CRG plans to demonstrate the concept byfabricating a full-sized, working prototype ready for integration into existing loitering UAV systems.

45th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials Conference19 - 22 April 2004, Palm Springs, California

AIAA 2004-1888

Copyright © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Smart materials technologies developedThe next sections describe a suite of smart materials CRG has developed for the use in adaptive structures,

such as morphing wings.

Shape memory polymerFirst introduced in the United States in 1984, shape memory polymers (SMPs) are polymers whose

qualities have been altered to give them dynamic shape “memory” properties. Under thermal stimuli, shape memorypolymers can exhibit a radical change from a rigid polymer to a very flexible, elastic state, then back to a rigid stateagain. In its elastic state, SMP will recover its “memory” shape if left unrestrained. The “memory,” or recovery,quality comes from the stored mechanical energy attained during the reconfiguration and cooling of the material.SMP’s ability to change stiffness modulus and shape configuration at will makes it ideal for applications requiringlightweight, dynamic, adaptable materials. Unlike a shape memory alloy (SMA), SMP exhibits a radical changefrom a normal rigid polymer to a very flexible elastic and back on command, a change that can be repeated withoutdegradation of the material. The SMP transition process is a thermo-molecular relaxation rather than a thermally-induced crystalline phase transformation, as with SMA. In addition, SMP demonstrates much broader range andversatility than SMA in shape configuration and manipulation.

SMP is not simply an elastomer, nor simply a plastic. It exhibits characteristics of both materials dependingon its temperature. While rigid, SMP demonstrates the strength-to-weight ratio of a rigid polymer; however, normalrigid polymers under thermal stimulus simply flow or melt into a random new shape, and have no “memorized”shape to which they can return. While heated and pliable, SMP has the flexibility of a high-quality, dynamicelastomer, tolerating up to 200% elongation; however, unlike normal elastomers, SMP can be reshaped or returnedquickly to its memorized shape and subsequently cooled into a rigid plastic. Figure 1.1 shows the elastic modulus ofSMP in relation to temperature. Figure 1.2 shows a chart of storage modulus (stiffness) versus temperature, showingthe initial range of CRG’s patented styrene SMP with activation temperatures customizable to between 110°F and220°F (47°C to 106°C).1

Figure 1.1 – SMP elastic modulus versustemperature

Figure 1.2 - DMA graph of various styrene SMPstorage moduli

There are three types of SMP: 1) partially cured resins, 2) thermoplastics, and 3) fully cured thermosetsystems. In several years of research, CRG has found limitations and drawbacks to the first two types of SMP.Partially cured resins continue to cure during operation and change properties with every cycle. Thermoplastic SMP“creeps,” which means it gradually “forgets” its memory shape over time. With this supporting research and athorough understanding of the chemical mechanisms of SMP, CRG has developed fully cured, high-performancethermoset systems.2 CRG has also demonstrated lab-scale feasibility on many new SMP formulations and areinvestigating triggering mechanisms other than heat such as light, electric field, or magnetic field. CRG has atrademarked SMP material, called VeriflexTM.

As stated, above its transition temperature, in its elastic state, SMP will recover its “memorized” or curedshape very quickly if left unrestrained, a quality useful in deployment applications. In addition, while heated andpliable it can be stretched, folded, rolled, twisted, bent, or otherwise reconfigured or manipulated into other shapes.The SMP can be cooled to maintain its altered shape for as long as necessary until once again it is heated above itstransition temperature. This thermal reconfiguration process can be repeated indefinitely without losing materialintegrity. Both SMP’s memorized shape and its manipulated reconfigurations will maintain shape integrity over timebelow transition temperature. It can be cast and cured into a variety of “memorized” shapes, from thick sheets andconcave dishes to tiny parts or a complicated open honeycomb matrix (Figure 1.3).

Original Stowed Shape 1st Recovery Snapshot 2nd Recovery Snapshot

Fully Recovered

Figure 1.3 - Snapshot series of a shape memory polymer honeycomb structureself-recovering under an IR heat lamp. The memorized shape is the honeycomb.The temporary stowed shape was the result of compressing and rolling thestructure while warm and then cooling to maintain the stowed shape.

Dynamic modulus compositesDynamic modulus composites (DMCs) are like other high-performance composites, except CRG’s

Veriflex™ resin is used as the matrix. Fabrication with Veriflex™ resin allows easy manipulation of the compositeabove the activation temperature and high strength and stiffness at lower temperatures. The composite acquiressome SMP characteristics, making it a unique material for use in dynamic structures and other applications requiringboth load strength and “shape-shifting” modulus flexibility.

CRG’s version of DMC, Veritex™, capitalizes on the ability of the SMP resin to quickly soften and hardenrepeatedly. Because of this property, the dynamic composites can be temporarily softened, reshaped, and rapidlyhardened in real-time to function as structures in a variety of configurations. They can be fabricated with nearly anyfiber type, and creative reinforcements can allow dramatic shape changes in functional structures. Veritex™ is alsomachinable. Figure 1.4 shows VeritexTM tubes that can be flattened and rolled up for storage, then deployed whenneeded.

Figure 1.4 - Tubes made of VeritexTM

Some possible applications capitalizing on the versatility of Veritex™ include rapid manufacturing,dynamic structures that can be stowed flat and subsequently set up, adaptable reinforcements, and portable,lightweight, rigid, deployable structures that can be used as an alternative or enhancement to current inflatablestructures.

Dynamic modulus foamDynamic-modulus foam (DMF) is a low-density, adaptive structural composite foam system fabricated

using SMP resin that can be thermally softened and subsequently hardened. This feature provides the ability tofabricate adaptive composite structures with a foam core, soften a structural foam element, alter its shape, and thenreturn it to a rigid state in its new configuration. The DMF can be reshaped multiple times. As with SMP and DMC,DMF has a very narrow temperature range in which it transitions between hard and pliable. This narrow span allowsDMF to maintain full structural rigidity up to a specifically designed activation temperature, and this activationtemperature can be tailored.3 The DMF concept is depicted in Figure 1.5.

CompressedRigidifiedDMF

Modify Confinement

Activate DMF to allow it to re-conform and provide internalstructural fill

Figure 1.5 – Time-lapse illustration of the conceptof DMF core materials. DMF will conform to fill avoid and become rigid to provide internalstructural support. The void may change shapesindefinitely, and the DMF will re-conform oncommand to maintain, fill, and support.

DMF core materials highlights:• Rapidly fills remote voids on command and becomes rigid to provide lightweight structural support• Functions as deployable/conformable lightweight internal structure, (see Figure 1.6)• Shape is infinitely reconfigurable• Supports adaptive structures concepts

Figure 1.6 – DMF blocks at full size and 400% compression

2. DESIGN METHODOLOGY

CRG developed preliminary prototype designs of adaptive wing structures, developed a processingapproach for fabrication, validated previously undemonstrated fabrication processes, and fabricated independentlyfunctional subsections of a morphing wing to prove feasibility.

An outline of the prototype design requirements is listed.• The wing structure shall be aeroelastically stable throughout the morphing UAV flight envelope.• The wing structure shall withstand a normal load factor of 6 for a 100-pound vehicle.• The wing structure shall withstand, in the carriage configuration, the harsh military environments.• The quarter chord of the wing shall remain at the same station; thus the chord ahead of the quarter chord

must grow at a rate 1/3 that of the chord aft of the quarter chord.• There will be no forward facing or aft facing steps.• The time required to “morph” between configurations shall not exceed 1 second.• Lift must be increased by 80 percent.

The current processing approach involves an internal sliding rib structure, with a styrene-based SMP skinwrapped from the top of the wing at the root, down the span, around the wing tip, and back to the wing root on thebottom. This approach will minimize any effects from necking in the material, because any necking should onlyoccur at the wing root. Fairings can also be used at the root to cover the necking at the root if necessary. This“spanwise” wrap is shown in Figure 2.1. Embedded heating wires will act as the activation system for the SMP.Actuation remains a major issue with the wing. A suitable device that can fit in the volume of the wing and providesufficient force and displacement will be developed. Further work in Phase II will improve the design, fabricate,and test a full-scale prototype of this morphing wing.

Figure 2.1 – The areas marked with red dashed lines depict the location of SMP skin. Each SMP skin areawill be fabricated from a long strip of SMP that will wrap “spanwise” from the top to the bottom around the

wing tip as depicted in the 3-D form on the right.

3. DESIGN ITERATIONS

The composite development approach concentrated on the methods of fabricating the required compositesand smart materials and then integrating the systems together. The critical experiments and demonstrations of thecomponents from the design task pointed to areas that needed improvement. These tasks were repeated in aniterative process to develop the wing from the initial concept to the final Phase I concept.

StructureThe initial concept for the morphing wing can be seen in Figure 3.1. The concept consisted of an SMP or

DMC skin with an internal structure consisting of DMF. Sliding rods within the DMF would stop the expansion atthe required distance. Additional design efforts during the program focused on broadening the acceptability of thetechnology bypassing the original one-way design for a two-way adjustable chord design.

Figure 3.1 - Original UAV wing, which is nearlyoptimal for cruise operations (Top Left). Proposed

adapted wing, which is needed for increasedmaneuverability vital to the ideal mission

performance (Top Right). Cross-section of initialwing concept (Right).

After an initial round of analysis and testing, it was determined that the DMF works very well for one-wayexpansion, but not as well for the two-way expansion and contraction, which is preferred. The design was shiftedtowards an internal structure consisting of either sliding or telescoping ribs with either a neat SMP or corrugatedDMC skin. This concept and subsequent prototype is shown in Figure 3.2. Basic sizing of the structural memberswas accomplished by assuming an elliptical lift distribution across the main spar and assuming that the extra 80%lift must be supported by the expanded chord ribs. First iteration analysis showed that aluminum is an acceptablematerial for the design.

Figure 3.2 – Sliding ribs concept (Top) and prototyped segment (Bottom)

SkinThe accordion folded DMC, shown in Figure 3.3, was fabricated to investigate its applicability as a skin

material. This approach for skin material was interesting because the potential existed to have full compositestrength in the expanded state allowing shear flow through the skin of the wing box as with most traditional wingdesigns. Demonstration of this material showed large forces were required to obtain a perfectly smooth surface,

otherwise small wrinkles were present which would be detrimental to drag performance. Further development ofthis material may yield improved results, but at this time, the neat SMP is showing dramatically better results.

Figure 3.3 – Corrugated DMC skin investigations (Top two diagrams) Conceptual DMC performance,(Bottom two pictures) Corrugated DMC demonstration

A simulator for the skin morphing process was built to test the neat SMP skin concept by inducing 125%strain to match the needed expansion. This rapid deployment simulator (RDS) is shown in Figure 3.4 and used hotair to activate the skin and then stretched and contracted it using two air pistons. Initially, styrene-based SMP skin1/16” thick was used. This material was able to withstand two cycles on average before failure. In order to increasethe failure strength, the skin was increased to 1/8” thick. This material was able to withstand multiple cycles, butwas still too brittle in the hardened state for the expected vibration loads. The SMP used during Phase I wasoriginally developed for a molding application with very unique design criteria. As expected these experimentsdemonstrated feasibility; however, they also demonstrated performance modifications are required for SMP toperform optimally for this application. Original and new design criteria are outlined in Table 3.1.

Figure 3.4 – Rapid deployment simulator (RDS)(Above left) and one stretching cycle of styrene SMP

(Above right; Right)

Table 3.1 – SMP performance design criteria

Original SMP design criteria for moldingapplications

– High cold modulus

– Optical quality thermoforming

– FDA approvable

– Transparent

– Rapid forming

New SMP design criteria for morphing aerospaceapplications

– High toughness

– High tensile strength

– High thermal conductivity

– Highly compatible with fiber

– VARTM compatible

– Chemically resistant

– Abrasion resistant

– UV resistant

Based on these results, CRG added an additional task to begin developing an aerospace grade SMP.Although progress is still underway, significant improvements have been achieved. Based on recent results, CRGexpects to achieve 1/16”, or less, skin thickness. Figure 3.5 shows qualitatively a dramatic improvement in thesenewly specified design parameters. A significant effort is still required to optimize the formulation and fullycharacterize the properties.

Figure 3.5 – Qualitative results showing significant improvement in cold flexure strength. The sample on theright in each picture is CRG’s new aerospace grade SMP (Still under development). The sample on the left ofeach picture is the original SMP formulation developed for molding. (Left) Original samples. (Middle)Results of a 30 degree flexure. (Right) Results of a 90-degree flexure.

ActivationThe initial activation concept involved using the hot gasses from the exhaust as both a source of heat to

activate the skin and DMF structure and provide pressurized air for actuating the wing. Early in the investigation, itwas determined that plumbing issues could complicate the wing, and hot exhaust air was de-selected from the list ofpossible actuation and activation methods. Without access to the hot exhaust air as a heat source for activation, amethod for activating the skin from within the wing needed to be developed. Embedding heating wires into the skinwas successfully demonstrated. Initially, thin nichrome wires were embedded. At 40W, the wires were able to heatthe SMP above its transition temperature, but the heat distribution was not adequately uniform. The wires wereclose to the thickness of the skin causing divots to form along each wire. The second iteration used very thin wiresplaced much closer together. At 40W, the SMP was again heated above the transition temperature, but since the

resistance of the wire was increased, a higher voltage was required. The embedded heating wires and heatdistribution can be seen in Figure 3.6.

Figure 3.6– Heat distribution using liquid crystal thermography for SMP embedded with nichrome wires (26AWG (Right); 36 AWG (Left)

4. CONCLUSIONS

The results of the Phase I work show that the design for an adjustable chord morphing wing is feasible.Plans for Phase II include finalizing the wing structure design, developing a sound actuation design, developing anaerospace grade SMP, fabricating a form, fit, functional morphing wing system, and demonstrating performance inan operationally relevant wind tunnel environment.

Alternative conceptual development and investigation of multiple aspects of morphing technology arebeing actively pursued over the course of this research and development effort. An example of other concepts to beinvestigated include, but are not limited to, expansion of the chord while pivoting at the wing tip, shown in Figure4.1. Since the lift distribution is elliptical, more lift is produced at the wing root. Therefore, an increase in chord atthe wing root will have the largest effect on increasing lift.

Figure 4.1 – Root chord expansion by pivoting at the wing tip

Research into these alternate designs will give CRG a greater insight into the possibilities created bymorphing wings. As the smart materials are improved, more complex designs can be developed and a largeimprovement in air vehicle performance will be possible.

ACKNOWLEDGEMENTS

Dr. Michael Valentino, Air Force Research LaboratoryCapt. Jacob A. Freeman, Air Force Research LaboratoryLockheed Martin

REFERENCES

1. Tat H. Tong et al. “Studies of Shape Memory Behavior of Styrene-Based Network Copolymers”, Proceedingsof The First World Conference on Biomimetics, Albuquerque, NM, Dec, 2002.

2. Sean Cullen, "Thermomechanical Properties of Shape Memory Polymers," Proceedings of SAMPE Conference,Long Beach, CA, May, 2003.

3. Lisa Culver et al. “Thermally Conductive Dynamic Modulus Foam”, Proceedings of SAMPE Conference, LongBeach, CA, May, 2003.

4. David A. Perkins et al. “Adaptive Wing Structures”, Proceedings of SPIE Conference, San Diego, CA, March,2004