Morphing Aircraft - Michael I Friswell

Proceedings of the ASME 2014 Conference on Smart Materials, Adaptive Structures and Intelligent Systems SMASIS2014 September 8-10, 2014, Newport, Rhode Island, USA

SMASIS2014-7754 MORPHING AIRCRAFT: AN IMPROBABLE DREAM? Michael I. Friswell College of Engineering, Swansea University Singleton Park, Swansea SA2 8PP, UK ABSTRACT Compliant aircraft, with a range of deformations comparable to birds, has been a dream for many years. Earlier aviation pioneers tried to replicate aspects of bird flight, but higher air speeds and larger payloads have required aircraft design to deviate from their biological inspiration. The design of conventional fixed wing aircraft can only be optimized for a limited region of the flight envelope; mechanisms such as deployable flaps and wing sweep are used extensively to enlarge this envelope. The development of more accurate analysis tools, advanced smart materials, and the increasingly demands for improved aircraft performance, are driving research into compliant morphing aircraft. These aircraft have the potential to adapt and optimize their shape to improve flight performance or achieve multi-objective mission roles. However this technology has rarely been adopted on production aircraft. This paper will critically review the progress made to date on compliant morphing aircraft research, and summarize the challenges that need to be addressed before such technology can be adopted widely. In particular the need to demonstrate system level performance benefits for morphing technology is emphasized. INTRODUCTION Flight, and the design of aircraft, has been a great success story. Early pioneers were inspired by the natural world, and early aircraft, such as the Wright Flyer, twisted a compliant wing for roll control. The demand for higher payloads and faster cruise speeds required stiffer wing structures. Controlling the aircraft by deforming the wings was replaced by discrete aerodynamic surfaces. Modern aircraft wings are a compromise that allows the aircraft to fly at a range of flight conditions, but the performance at each condition is inevitably sub-optimal. The ability of a wing surface to change its geometry during flight has interested researchers and designers over the years as this reduces the design compromises required. Concepts based on mechanisms, for example variable sweep, have been

successful, but concepts based on the compliance of the structure have met with limited success. This paper will give a candid and personal opinion of the developments in morphing aircraft, based on answering a series of questions that might sensibly be posed. Barbarino et al. [1] gave an extensive review of morphing technology. WHAT IS A MORPHING AIRCRAFT? There exist various definitions of morphing aircraft. According to Weisshaar [2] morphing are ‘multi-role aircraft that change their external shape substantially to adapt to a changing mission environment during flight’. The NATO RTO Technical Team on Morphing Vehicles suggested that morphing is the real-time adaptation to enable multi-point optimized performance [3]. A more detailed definition was provided by the DARPA Morphing Aircraft Structures (MAS) program. According to Seigler [4], the MAS program defines the morphing aircraft as a multirole platform that changes its state substantially to adapt to changing mission environments, provides superior system capability not possible without reconfiguration, and uses a design that integrates innovative combinations of advanced materials, actuators, flow controllers, and mechanisms to achieve the state change. Using these definitions, established technologies such as flaps or retractable landing gear would be considered morphing technologies. However, morphing carries the connotation of radical shape changes or shape changes only possible with near-term or futuristic technologies. There is neither an exact definition nor an agreement between the researchers about the type or the extent of the geometrical changes necessary to qualify an aircraft for the title ‘shape morphing.’ Since the 1920s aircraft have used devices to increase lift during landing and takeoff. Increases in aircraft weight and cruise speed, and increases in the wing structural stiffness to avoid aero-elastic instabilities, such as divergence and flutter, have led to the use of discrete control surfaces such as ailerons and flaps in place of wing twist. Only since the late 1970s have researchers seriously revisited concepts of variable wing shape. Geometrical parameters that can be affected by morphing

1

Copyright © 2014 by ASME

solutions are planform alteration (span, sweep and chord), outof-plane transformation (twist, dihedral/gull, spanwise bending) and airfoil adjustment (camber and thickness). For each geometrical parameter, Barbarino et al. [1] gave a table summarizing the studies available in literature on that specific topic, together with an overview of the developed work. WHAT ARE WE TRYING TO ACHIEVE? The key requirement for the adoption of morphing technology is the demonstration of the potential performance benefits. This requires a system level analysis that considers not only the aerodynamics of the aircraft, but also the sizing and weight estimation for both the structure and actuators, and how the vehicle will be operated. The objectives are linked to mission requirements of the aircraft, and hence there is a huge range of possibilities. Here we concentrate on improvement in aircraft performance and the expansion of the flight envelope (including multi-mission capabilities) and give some examples. It should be emphasized that the optimum solution, and whether morphing is appropriate or not, will depend on the mission profile and the required objectives; just as there is a wide variety of aircraft designs, particularly linked to flight speed and payload, there will also be a wide variety of appropriate morphing solutions. Historically, morphing solutions always led to penalties in terms of cost, complexity or weight. Without these penalties, morphing would always make sense, although in certain circumstances these were overcome by system level benefits. For most applications there is a cross over point where penalties for not morphing begin to exceed the morphing weight penalty [2]. The current trend for highly efficient and green aircraft makes such compromises less acceptable, calling for innovative morphing designs able to provide more benefits and fewer drawbacks. However, when overall system performance and mission requirements are assessed, large shape change concepts can be a viable approach for some missions, particularly those that combine several requirements in terms of speed, altitude, or take-off and landing. Civil and military aircraft are designed to have optimal aerodynamic characteristics (maximum lift/drag ratio) at one point and fuel condition in the entire flight envelope. However, the fuel loading and distribution changes continuously throughout the flight, and aircraft often have to fly at nonoptimal flight conditions due to air traffic control restrictions. The consequent sub-optimal performance has more significance for commercial aircraft as they are more flexible than military aircraft and also fuel efficiency has far greater importance as a performance measure. Conventional hinged mechanisms are effective in controlling the airflow, but they are not efficient, as the hinges and other junctions usually create discontinuities in the surface, resulting in increased drag. Compliant control surfaces have the potential to reduce drag significantly [5], and thus potentially offset the increased weight to an overall benefit in terms of fuel used or the range. An adaptive wing would allow aircraft to perform multiple missions and enable a single aircraft to have multi-role

capabilities, radically expanding its flight envelope. From a military perspective, a single morphing aircraft could perform different roles within a given mission that otherwise would require different vehicles. Jha and Kudva [6] studied how changing wing parameters affect the performance of an aircraft, and demonstrated that an optimal design requires large geometric changes to satisfy a multi-role mission. Therefore, choosing between high efficiency or high maneuverability at the design stage would not be necessary, and aerodynamic optimization for a single flight condition avoided. Compared to conventional aircraft, morphing aircraft become more competitive as more mission tasks or roles are added to their requirements. Planform morphing (a wing capable of telescoping, chord extension and variable sweep) can significantly improve aircraft performance over that provided by morphing the airfoil alone or by the baseline aircraft [7]. Peters et al. [8] highlighted that morphing benefits are most apparent for missions involving large changes in speed and/or payload. Wings with large spans have good range and fuel efficiency, but lack maneuverability and have relatively low cruise speeds. By contrast, aircraft with low aspect ratio wings can fly faster and become more maneuverable, but show poor aerodynamic efficiency. A variable span wing can potentially integrate into a single aircraft the advantages of both designs, making this emerging technology especially attractive for military unmanned aerial vehicles (UAVs). Increasing the wingspan, increases the aspect ratio and wing area, and decreases the spanwise lift distribution for the same lift. Thus, the drag of the wing decreases, and consequently the range or endurance of the vehicle increases. Unfortunately, the wingroot bending moment can increase considerably due to the larger span. Thus the aerodynamic, structural, aeroelastic, and control characteristics of the vehicle should be investigated in the design of variable-span morphing wings. Asymmetrical span morphing can be used for roll control. Most span morphing concepts are based on a telescopic mechanism with sliding skins. The absence of sharp edges and deflected surfaces on morphing aircraft also has the potential to reduce the radar signature and visibility of the vehicle, thus enhancing its stealth properties. Furthermore, redundancy in conventional aircraft generally requires a conventional flap to be connected to at least one or two additional actuators to provide sufficient flight control in case of airframe damage or actuator failure [9]. In contrast, morphing wings often have distributed actuation that provides sufficient robustness and redundancy to account for actuator failures with only a negligible effect on weight. WHERE ARE THE FLYING MORPHING AIRCRAFT? The idea of morphing aircraft is not new. Even before the official beginning of controlled human flight in 1903, radical shape changing or morphing aircraft appeared and then disappeared, contributing little to aviation [1]. The reason for the disappearance of morphing was the increased need for larger structural rigidity as higher airspeeds were achieved

2


which prohibited any form of compliance or flexibility or morphing. Later on, morphing mechanisms were added to improve the flight performance and control authority of the vehicle during off-design conditions. In the 1980s, NASA launched two research programs dedicated to morphing structures with the Active Flexible Wing program and its Mission Adaptive Wing program. This research effort was followed by several research programs in the 1990s and 2000s in the USA [10]: the Smart Materials and Structures Demonstration program, the Aircraft Morphing program, the Active Aeroelastic Wing program, and the Morphing Aircraft Structures program. Parallel to the research done in the USA, the European Union has also funded several research programs since 2002, including the Active Aeroelastic Aircraft Structures (3AS) project, the Aircraft Wing Advanced Technology Operations (AWIATOR) project, the New Aircraft Concepts Research (NACRE) project, the Smart Fixed Wing Aircraft (SFWA) project, the Smart Intelligent Aircraft Structures (SARISTU) project, the Novel Air Vehicle Configurations (NOVEMOR) project and the CHANGE project. Thus the field of shape morphing aircraft has attracted the attention of hundreds of research groups worldwide. This interested in the last few decades has arisen because of technology push, and application driven pull. Many new, novel materials, material systems, and actuation devices have been developed recently. These developments allow designers to distribute actuation forces and power optimally and more efficiently. Properly used, these devices may reduce weight compared to other, more established designs. The application pull is driven by the demand for greener aircraft and more sophisticated mission requirements of military aircraft. Targets are more distributed and are smaller, but the proliferation of sophisticated air defenses mean that these targets may be very dangerous to attack. Morphing provides mission flexibility and versatility to deal with these kinds of targets in a cost-effective manner. Morphing is a promising enabling technology for future, next generation aircraft. Although many interesting concepts have been synthesized, few have progressed to wing tunnel testing, and even fewer have ever flown. Wing morphing allows the aerodynamic potential of an aircraft wing to be explored, by adapting the wing shape for several flight conditions encountered in a typical mission profile. Moreover, the aero-elastic deformations can increase the performance and maneuverability, and improve the structural efficiency. So why have so many morphing wing concepts been tried in the past with so little impact on production aircraft? Manufacturers and end users are still too skeptical of the benefits to adopt morphing. Many developed concepts have a technology readiness level that is still very low. Apart from the increased complexity of compliant morphing structures, increases in aircraft weight can significantly reduce or eliminate the benefits of the morphing aircraft. Increasing wing structural weight is a serious problem. Skillen and Crossley [11] showed that mechanisms can account for a substantial portion of the weight and are a strong function

of the wing geometry. Moreover, the opportunity to realize a morphing wing requires the availability of materials and implementation solutions that guarantee in all circumstances the necessary deformation of the structure while maintaining structural integrity and load bearing capability. Smart materials may be the solution to realize distributed actuators within the wing structure capable of providing shape modifications. The current challenges of morphing vehicle design are the additional weight and complexity, the power consumption of the required distributed actuation concepts, and the development of structural mechanization concepts covered by flexible skins. Additionally, there is a strong need to understand the scalability of morphing wing concepts to achieve sufficient structural stiffness, robust aero-elastic designs, and an adequate flight control law to handle the changing aerodynamic and inertia characteristics of morphing vehicles [12]. IS BIO-INSPIRATION USEFUL? In spite of the apparent complexity of variable-geometry aircraft, nature has evolved thousands of flying insects and birds that routinely perform difficult missions [13]. Observations by experimental biologists reveal that birds such as falcons are able to loiter on-station in a high-aspect ratio configuration using air currents and thermals until they detect their prey. Upon detection, the bird morphs into a strike configuration to swoop down on an unsuspecting prey. In contrast, most of today’s aircraft are designed according to Cayley’s design paradigm which separates the functions needed for sustained flight. During conventional conceptual design, the geometry of the aircraft is usually optimized for a single flight segment (cruise for transport aircraft), which penalizes behavior at off-design conditions [14]. To overcome the penalty in off-design conditions, discrete surfaces such as flaps and slats are added to allow changes in the wing profile and have better flight characteristics at off-design conditions. Of course the flight regimes between birds and aircraft are usually significantly different, meaning that direct bioinspiration is often not useful. For example, conventional turbojet engines are always going to perform better than flapping wings for aircraft of reasonable size and weight. Indeed rotating machines, bearing and wheels are examples of extremely successful engineering components that do not have widespread counterparts in nature. However, for some specific applications, for example for small scale surveillance UAVs, concepts such as flapping wings [15] or perching [16] may be practicable. ARE UAVS A GOOD VEHICLE FOR DEVELOPMENT? The explosive growth of satellite services during the past few years has made UAVs the technology of choice for many routine applications such as border patrol, environmental monitoring, meteorology, military operations, and search and rescue. For this and other reasons (such as lower production costs, lower safety and certification requirements, and lower aerodynamic loads), many wing morphing investigations are focused toward small or radio-controlled aircraft, i.e. UAVs.

3


This also offers a great opportunity to showcase and test successful designs at an early stage, and to attract industry attention to develop new technologies for large-scale vehicles. However, many UAV morphing concepts involve significant changes to the planform and structure that are likely to be impractical in large aircraft. Many morphing UAVs have been developed for military applications, where a more versatile vehicle compensates for the additional complexity and weight. Except for variable-sweep and swing-wing concepts, most previous morphing concepts were applied to lightly loaded, relatively low-speed airplane designs, typical of the UAV flight regime. An important factor in morphing systems is the scale of the air-vehicle on which they will be incorporated. All the morphing concepts available in literature can be categorized to be either compliant, mechanisms, or hybrid (mixture of compliant and mechanisms). Compliant structures are promising solutions due their low weight and maintenance costs (no rigid body motion). Compliant structures usually employ flexible skins to maintain the aerodynamic shape of the wing while before, during, and after morphing. There is a wide range of flexible skins ranging from corrugated skins to fiber reinforced elastomer. Thill et al. [17] provided an extensive review of the state-of-the-art morphing skins. The drawback of the state-of-the-art flexible skins is that they cannot work as main load carrier members. Their main purpose is to maintain the aerodynamic profile of the wing and transfer the pressure loads to the inner main load carrier structures. Compliant structures seem to work well for small UAVs. In addition, they can be used in wind turbine blades of various sizes due to the relatively lower dynamic pressure. However, as the size/weight of the vehicle increase and hence the aerodynamic loads increase, it becomes prohibitive to employ compliant structures due to their relatively low stiffness and strength. One solution may be to employ highly anisotropic skin structures, such as corrugated skins, which allow some decoupling between high stiffness in one direction to withstand aerodynamic loads, and flexibility in an orthogonal direction to allow morphing deformations. WHAT CONCEPTS FOR LARGE AIRCRAFT MIGHT WORK? Variable sweep has proven successful, particularly to enable military aircraft to fly at supersonic speeds, albeit with a large weight penalty. The current trend for highly efficient and green aircraft makes this compromise less likely to be acceptable, calling for innovative morphing designs able to provide more benefits and fewer drawbacks. For military applications, the current level of performance required by the next-generation vehicles cannot sustain this trade-off. In general, any successful wing morphing system must overcome the weight penalty due to the additional actuation systems. Compared to supersonic aircrafts, small or low-speed vehicles require more dramatic wing variations to produce a noticeable and practical change in their aerodynamic properties.

Any morphing concept that requires significant changes to the primary structure of a large aircraft is unlikely to be practical in the short term. Hence the emphasis for development should be on secondary structures that can still significantly affect performance. Two possible directions will be discussed here: control surfaces / flaps and winglets. Fixed wing aircraft, for example, have used camber variation to control roll, pitch, and yaw motions for over 100 years. Camber variation is also used during takeoff and landing to generate very high lift coefficients [14]. To date, the primary means of realizing variable camber has been through the use of discrete trailing edge flaps. While this approach is conceptually simple and is certainly well proven, it is not without drawbacks. First and foremost, the presence of a sharp, discrete change in camber leads to a significant increase in drag over the baseline airfoil, particularly at high lift coefficients. The sudden change in camber can also lead to early trailing edge flow separation, limiting the maximum lift coefficient. Replacing the discrete trailing edge flap with a compliant control surface avoids the sharp change in camber and potentially reduce improve aerodynamic efficiency [5]. However, one disadvantage of compliant control surface is that actuator requirements can increase, because the aerodynamic balancing used on discrete flaps to reduce the hinge moments is generally not possible. Winglets are used on many commercial aircraft to reduce the induced drag of the aircraft. However these winglets are optimized for a single point on the flight envelope, and 5% improvement in specific air range may be possible by allowing the outboard wing sections to cant and twist during flight [18]. WHY THE EMPHASIS ON MORPHING WINGS? Much of the emphasis has been on morphing wings, since these are the primary lift generator on an aircraft and hence morphing has the most effect. Concepts developed for wings maybe readily utilized for horizontal and vertical stabilizers, rotary wing aircraft blades, wind turbine blades, etc. However, morphing may also be useful in other areas of an aircraft, such as the fuselage, various fairings, landing gear, the propulsion system, etc. Adaptive propulsion perhaps has the most potential, for example by morphing the air intake to improve efficiency, or adding variable geometry chevrons to reduce noise [19, 20]. WHAT ARE THE ACTUATION REQUIREMENTS? Many morphing concepts aim to vary the camber of the airfoil section to achieve performance improvements. At the conceptual design level the deformed airfoil section is required to evaluate the aircraft performance, together with an estimate of the weight of the morphing mechanism, power requirements, and any constraints the mechanism imposes. Thus the mechanisms to implement the concepts for camber change must be investigated in detail to provide this information. The wing camber can vary at specific locations (for example, the leading or trailing edge) or in a global manner, so that the entire wing acts as a control surface. The choice of actuators can be conventional (i.e. electromagnetic motors, hydraulic,

4


pneumatic) or solid-state smart materials (i.e. piezoelectrics, shape memory alloys, rubber muscle actuators, magnetostrictive materials). The actuation (whether it is induced by conventional or smart material systems) can be distributed or localized. The required bandwidth of the morphing deformations has a huge influence on the actuator force and power requirements, and therefore actuator choice; morphing to improve aerodynamic efficiency and range requires low frequency actuation, whereas morphing control surfaces will require high frequency actuation. The wealth of new technologies available to the wing designer provides intriguing design possibilities. Recent developments in smart materials may overcome the actuator limitations and enhance the benefits from similar design solutions. Chopra [21] presented a review of the stateof-the-art of smart structures and integrated systems. The design of a morphing aircraft by means of smart materials is a multidisciplinary problem. The challenge is to design a structure that is capable of withstanding not only the prescribed loads, but also to change its shape in order to withstand several load conditions. In order to reduce the complexity and hence increase the reliability, the actuation system, consisting of active materials, should be embedded in the structure. Ideally there should be no distinction between the structure and the actuation system, so that the system used to produce and carry the loads, is also capable of changing shape. In addition to benefits in terms of complexity, reliability, and production cost, such a concept could also prove to be lighter. However smart materials are not a panacea, and the whole range of possible actuation approaches for a given morphing aircraft concept should be considered. The usual requirements, such as force-displacement characteristics and estimated power, should be considered, alongside the estimated weight as this affects the system level optimization. This estimated weight should be a system level weight, not just the actuator material weight, and should also allow for increased structural weight if actuator material is integrated into load bearing structures. The specific characteristics of smart materials must considered carefully and their use tailored appropriately; for example, piezoelectric materials [22] are low strain and high bandwidth, whereas shape memory alloys [23] are high strain and low bandwidth. Wing shape-changing concepts often require actuators attached to internal mechanisms, covered with flexible aerodynamic surfaces, together with load-transfer attachments between the skin and the skeleton [24]. Mechanism design requirements include the range of motion, concerns about binding and friction, the effects of wing structural deformability under load, and the control of the actuator stroke under load. Actuator performance power and actuator force capability are essential to design success. The size, weight, and volume of the actuators are important metrics, as are the range of motion, bandwidth, and fail-safe behavior. Locking is important when the wing is under load since, without locking features, the actuators must withstand full load unless the actuator works in parallel with a structural element. Woods et al. [25] suggested a

method to better match the force-displacement characteristics of the actuator to that of the load. Morphing designs may also benefit from geometrically flexible structures if the aeroelastic energy from the airstream can be used to activate the shape changing and tabs can maintain the shape using aeroelastic control. HOW SHOULD MORPHING AIRCRAFT BE OPERATED? One drawback with the studies on morphing aircraft is that morphing tends to be added to an existing aircraft that is operated in the same way. This retrofitting approach directly eliminates some major benefits of the morphing technology and constrains any potential for multi-functionality. With morphing systems, the structure is strongly coupled with the actuator(s) and with the sensor(s) and in some morphing systems the structure is the actuator and sensor at the same time. This coupling does not exist in conventional systems. Therefore adding morphing to an existing aircraft generates different outcomes (in terms of benefits) to considering morphing early in the design process of the aircraft. Span morphing can be an effective device to replace conventional ailerons and enhance the flight performance, for example for a medium altitude long endurance (MALE) UAV [26]. The largest increases in wing semi-span required to meet the rolling moment requirement often occurs at the end of the endurance phase. Unlike ailerons, the rolling moment generated by span extension is very sensitive to instantaneous vehicular weight and the instantaneous angle of attack. This indicates that morphing structures should not be operated in the same way conventional control surfaces are operated and the benefits that can be achieved from coupled maneuvers must be exploited in the design of flight control systems. Flight control represents another big challenge for morphing aircraft, as additional terms appear in the governing equations and may require complex control systems. Seigler et al. [27] studied the modeling and flight control of vehicles with large-scale planform changes. The equations of atmospheric flight were derived in a general form, methods to integrate the aerodynamic forces examined, and various approaches and methods of flight control distinguished. Span morphing also induces some additional inertial terms in the roll equation of motion [26], and the bandwidth of any actuation system needs to be checked carefully due to the requirement to move large inertias. Furthermore, conventional control surfaces are often designed to decouple the aircraft dynamics. In contrast, morphing solutions can lead to highly coupled and non-linear equations of motion that require advanced controller designs. Using morphing winglets for flight control is a good example of this [28,29]. Furthermore, distributed actuation and the corresponding deformations can lead to an over-actuated system that is good for redundancy and fault-tolerance, but requires some form of control allocation.

5


WHAT SOFTWARE TOOLS ARE REQUIRED? Design tools are required to model specific components with adaptive structures in a way that enables the simulation and design optimization of the total morphing aircraft system [30]. This requires the development of a framework for the multi-scale modeling of adaptive structures that relies on a library of equivalent physical models for typical structural components. This framework should enable the important physical quantities that are required to be transformed across the length scales. High fidelity models may be used to verify the accuracy of the lower fidelity models, and experiments may be performed to characterize material properties, validate the high fidelity models and to demonstrate the application to morphing aircraft. However, the system level model must be based on low fidelity equivalent models, since the high fidelity aerodynamic or structural models would require excessive computation during optimization. The key is the interaction of the models at the different scales: the equivalent parameters must be chosen as the minimum set of parameters that ensures that the full design space may be explored; and constraints must be placed on these parameters to ensure that the detailed components can be manufactured and will not fail. Equivalent models of corrugated skins are a good example of a structure that has been considered extensively [31, 32]. It should be emphasized that the conceptual design of conventional aircraft uses a range of empirical relationships within the equivalent models employed based on past experience, but such empirical relationships are generally lacking for morphing aircraft. At the top of this hierarchy is a system level model to optimize a range of performance metrics, such as endurance or specific air range [33]. In summary, a set of generic design tools is needed for the conceptual design of morphing aircraft that are at the right level of fidelity, so that the design space can be explored efficiently even with a large number of design variables. WHAT IS THE OUTLOOK FOR MORPHING AIRCRAFT? The future of morphing aircraft is uncertain. Technology programs (beyond basic research) need to be matched to capability gaps to ensure relevance and funding. Morphing does show promise for several types of missions, but often there is not a compelling case for morphing over conventional alternatives. Morphing should be viewed as a design option to be incorporated in a specific vehicle if justified by system-level benefits achieved for the costs and complexity incurred. The examples mentioned in this paper, and in the literature, illustrate the complexity of integrating morphing mechanisms onto an aircraft wing, and thus also the complexity in the analysis, and the potential benefits. Substantial research in the past decades has focused on the aeroelastic modeling of morphing aircraft using models of different level of complexity and the optimization of morphing aircraft by changing the geometry and internal topology. However there is a lack of a transparent way to define the range of morphing aircraft for optimization in a way that results in a sufficiently low number

of design variables for quick sizing, while not constraining the design space a priori. Certainly morphing as a suite of technologies is not flight ready, and the technology readiness level is still very low. Significant work still remains in maturing component technologies such as skins (flexible but load carrying), actuators/mechanisms (distributed and capable of supporting part of the external loads), and control theory (primary flight and actuation) for morphing to be truly realized. Morphing technology needs a transition program for the aerospace community to take the technology seriously and include it as a design option. While large scale morphing on commercial aircraft may not be practical in the near term, opportunities exist on smaller unmanned aircraft or missiles where current or near-term technology can be applied to achieve morphing. On larger aircraft, the application of morphing to secondary structures, for example a compliant control surface, is a realistic goal. ACKNOWLEDGMENTS The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. (247045). The author acknowledges the huge number of researchers, students and collaborators that have significantly influenced his views on morphing aircraft research over the past 10 years or so. REFERENCES [1] Barbarino, S., Bilgen, O., Ajaj, R.M., Friswell, M.I., and Inman, D.J., 2011. “A Review of Morphing Aircraft”. Journal of Intelligent Material Systems and Structures, 22(9), pp. 823-877. [2] Weisshaar, T.A., 2006. “Morphing Aircraft Technology New Shapes for Aircraft Design”. In RTO-MP-AVT-141, Neuilly-sur-Seine, France. [3] McGowan, A.R., Vicroy, D.D., Busan, R.C., and Hahn, A.S., 2009. “Perspectives on Highly Adaptable or Morphing Aircraft”. In RTO Applied Vehicle Technology Panel (AVT) Symposium, Evora, Portugal, 20-24 April 2009, RTO-MP-AVT-168 AC/323(AVT-168)TP/268. [4] Seigler, T.M., 2005. Dynamics and Control of Morphing Aircraft, PhD Thesis, Virginia Polytechnic Institute and State University. [5] Woods, B.K.S., Bilgen, O., and Friswell, M.I., 2014. “Wind Tunnel Testing of the Fish Bone Active Camber Morphing Concept”. Journal of Intelligent Material Systems and Structures, 25(7), May, pp. 772-785. [6] Jha, A.K., and Kudva, J.N., 2004. “Morphing Aircraft Concepts, Classifications, and Challenges”. In SPIE Vol. 5388, Smart Structures and Materials 2004: Industrial and Commercial Applications, San Diego, CA. [7] Joshi, S.P., Tidwell, Z., Crossley, W.A., and Ramakrishnan, S., 2004. “Comparison of Morphing Wing Strategies Based Upon Aircraft Performance Impacts”. In 45th AIAA/ASME/ASCE/AHS/ASC Structures, Structural

6


Dynamics and Materials Conference, Palm Springs, CA, AIAA 2004-1722. [8] Peters. C., Roth, B., Crossley, W.A., and Weisshaar, T.A., 2002. “Use of Design Methods to Generate and Develop Missions for Morphing Aircraft”. In 9th AIAA/ISSMO Symposium of Multidisciplinary Analysis and Optimization, Atlanta, Georgia, paper AIAA-2002-5468. [9] Gern, F.H., Inman, D.J. and Kapania, R.K., 2005. “Computation of Actuation Power Requirements for Smart Wings with Morphing Airfoils”. AIAA Journal, 43, pp. 2481-2486. [10] McGowan, A.R., Vicroy, D.D., Busan, R.C., and Hahn, A.S., 2009. “Perspectives on Highly Adaptable or Morphing Aircraft”. In RTO Applied Vehicle Technology Panel (AVT) Symposium, RTO-MP-AVT-168 AC/323(AVT-168)TP/268, Evora, Portugal, 20-24 April 2009. [11] Skillen, M.D., and Crossley, W.A., 2008. Morphing Wing Weight Predictors and their Application in a TemplateBased Morphing Aircraft Sizing Environment II, Part II: Morphing Aircraft Sizing via Multi-Level Optimization. NASA Report CR-2008- 214903. [12] Moorhouse, D., Sanders, B., von Spakovsky, M., and Butt, J., 2006. “Benefits and Design Challenges of Adaptive Structures for Morphing Aircraft”. The Aeronautical Journal, 110(1105), pp. 157-162. [13] Lentink, D., Muller, U.K., Stamhuis, E.J., de Kat, R., van Gestel, W., Veldhuis, L.L.M., Henningsson, P., Hedenstrom, A., Videler, J.J., and van Leeuwen, J.L., 2007. “How Swifts Control their Glide Performance with Morphing Wings”. Nature, 446(7139), pp. 1082-1085. [14] Raymer, D., 2006. Aircraft Design: A Conceptual Approach. American Institute of Aeronautics and Astronautics (AIAA), 4th edition. [15] Shyy, W., Berg, M., and Ljungqvist, D., 1999. “Flapping and Flexible Wings for Biological and Micro Air Vehicles”. Progress in Aerospace Sciences, 35(5), pp. 455-505. [16] Wickenheiser, A.M., and Garcia, E., 2006. “Longitudinal Dynamics of a Perching Aircraft”. Journal of Aircraft, 43(5), pp. 1386-1392. [17] Thill, C., Etches, J.A., Bond, I.P., Potter, K.D., and Weaver, P.M., 2008. “Morphing Skins - A Review”. The Aeronautical Journal, 112(1129), pp. 117–139. [18] Smith, D.D., Ajaj, R.M., Isikveren, A.T., and Friswell, M.I., 2012. “Multi-Objective Optimization for the Multiphase Design of Active Polymorphing Wings”. Journal of Aircraft, 49(4), July-August, pp. 1153-1160. [19] Mabe, J.H., Calkins, F.T., and Butler, G.W., 2006. “Boeing’s Variable Geometry Chevron: Morphing Aerostructure for Jet Noise Reduction”. AIAA Paper No. AIAA-2006-2142. [20] Calkins, F.T., and Mabe, J.H., 2010. “Shape Memory Alloy Based Morphing Aerostructures”. Journal of Mechanical Design, 132(11), paper 111012.

[21] Chopra, I., 2002. “Review of State of Art of Smart Structures and Integrated Systems”. AIAA Journal, 40, pp. 2145-2187. [22] Bilgen, O., and Friswell, M.I., 2014. “Piezoceramic Composite Actuators for a Solid-State Variable-Camber Wing”. Journal of Intelligent Material Systems and Structures, 25(7), May, pp. 806-817. [23] Barbarino, S., Saavedra Flores, E.I., Ajaj, R.M., Dayyani, I., and Friswell, M.I., 2014. “A Review on Shape Memory Alloys with Applications to Morphing Aircraft”. Smart Materials and Structures, 23(6), June, paper 063001. [24] Inman, D.J., 2001. Wings: Out of the Box. Determining Actuator Requirements for Controlled Morphing Air Vehicles - Aerodynamic Loads. DARPA Technology Interchange Meeting, Wright Patterson Air Force Base, Dayton, OH. [25] Woods, B.K.S., Friswell, M.I., and Wereley, N.M., 2014. “Advanced Kinematics for Morphing Aircraft Actuation”. AIAA Journal, 52(4), April, pp. 788-798. [26] Ajaj, R.M., Friswell, M.I., Saavedra Flores, E.I., Keane, A.J., Isikveren, A.T., Allegri, G. and Adhikari, S., 2014. “An Integrated Conceptual Design Study using Span Morphing Technology”. Journal of Intelligent Material Systems and Structures, 25(8), May, pp. 989-1008. [27] Seigler, T.M., Neal, D.A., Bae, J.S. and Inman, D.J., 2007. “Modeling and Flight Control of Large-Scale Morphing Aircraft”. Journal of Aircraft, 44, pp. 10771087. [28] Bourdin, P., Gatto, A., and Friswell, M.I., 2008. “Aircraft Control via Variable Cant-Angle Winglets”. Journal of Aircraft, 45, pp. 414-423. [29] Bourdin, P., Gatto, A., and Friswell, M.I., 2010. “Performing Co-ordinated Turns with Articulated Wing Tips as a Multi-axis Control Effectors”. The Aeronautical Journal, 114, pp. 35-47. [30] Friswell, M.I., 2012. “Hierarchical Models of Morphing Aircraft”. In 23rd International Conference on Adaptive Structures and Technologies, ICAST, Nanjing, China, 1113 October 2012. [31] Xia, Y., Friswell, M.I., Saavedra Flores, E.I., 2012. “Equivalent Models of Corrugated Panels”. International Journal of Solids and Structures, 49(13), June, pp. 14531462. [32] Dayyani, I., Friswell, M.I., Ziaei-Rad, S., and Saavedra Flores, E.I., 2013. “Equivalent Models of Composite Corrugated Cores with Elastomeric Coatings for Morphing Structures”. Composite Structures, 104, October, pp. 281-292. [33] Beaverstock, C.S., Fincham, J.H.S., Friswell, M.I., Ajaj, R.M., de Breuker ,R., and Werter, N.P.M., 2014. “Effect of Symmetric and Asymmetric Span Morphing on Flight Dynamics”. In AIAA Atmospheric Flight Mechanics Conference, National Harbor, Maryland, USA, paper AIAA-2014-0545.

7
