Abstract: This article discusses about the development of shape altering morphing wings. It includes a brief discussion about the concept of morphing wings or shape changing wings, which can be considered as the future of aircrafts. It also provides information about the materials used for wings along with the use of the Shape Memory Alloy (SMA), or Smart Alloy for actuation in order to reduce costs, space and weight. Classification of morphing wings into planform alternation, out of plane transformation and airfoil adjustment is discussed in detail. In addition, aerodynamic performance of baseline straight wing and that of morphed wing are also compared.
Keywords: Actuator; Airfoil; Classification; Morphing; Wing.
Aircraft wings allow the aircraft to fly in different conditions but performance in each condition is suboptimal. Researchers and designers have been greatly influenced by the ability of wing surface to change its geometry during the flight. The term ‘morphing’ originates from the field of biomimetics. Morphing is the short form for metamorphose 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’ . According to the “Defence Advanced Research Projects Agency” morphing aircraft is ‘adaptable, time variant airframe , whose changes in geometry influence aerodynamic performance’ Weisshaar, T.,(2004). One could also define morphing wings ‘as the ability of the wing to change its shape, seamlessly in order to provide optimal performance to suit different flight conditions.
Conventional aircraft have features to significantly change the geometry of the wings in order to alter their aerodynamic properties, like flaps and slats. However these features, cannot create a seamless airfoil shape when extended as they rely on sliding rails and hinges .Abrupt changes caused by them in the wing surface can decrease the aerodynamic efficiency of the wing. So morphing wing can be a solution of such problems by providing a seem less airflow as the control surfaces are not discrete. Aircraft morphing affect the flight characteristics of aircraft by changing the aircraft wing shape. Many complex aircraft morphing system designs have emerged like rotating, sliding and inflating mechanisms, and numerous more projects rapidly emerging as an opportunity to increase versatility and maximise aircraft efficiency for the duration of a flight. Morphing can cause alteration in geometrical parameters which can be broadly classified into : planform alteration (span, sweep, and chord), out of plane transformation (twist, dihedral/gull, and span-wise bending), and airfoil adjustment (camber and thickness). Changing wing shape or morphing is not new. History provides us with evidences where morphing solutions led to cost, complexity, or system-level benefits. The current trend of efficient and ‘green’ aircraft can see morphing designs as a way provide more benefits and fewer drawbacks. Morphing is a promising technology for the future, next-generation aircraft. But the manufacturers and end users are still too sceptical of the benefits to adopt morphing in the near future. Moreover, many developed concepts have a technology readiness level that is very low.
2. HISTORY OF MORPHING WINGS
The study of bird flight led to the development of the first functional wing. Sir Charles Cayley, in the late 1700s.He realized that the lift function and the thrust function of bird wings were distinct. Birds are able to adapt their wings to the different conditions far better than current aircraft. They can fold their wings tightly when they are going to dive for a prey, or extend their wings completely when they want to glide to save energy. Studies have also shown that Swifts are some of the most efficient birds when it comes to active flying. Researchers have proved how these birds change the shape of their wings to improve performance. The results of the analysis provided clues as to how aircraft wings can be improved. By carrying out wind tunnel testing on dead swifts, Stamhuis et al (2007) found out that low speed flight with extended wings gives swifts maximum flight efficiency. But swept wings have a better aerodynamic performance at higher flight speeds. Swept wings increase manoeuvrability due a smaller aspect ratio which translates into a smaller rolling inertia. They also found that these birds are able to adjust the shape of their wings to increase the efficiency of their glide and/or to make faster turns.
The concept of morphing wing began on December 17, 1903 by the Wright Brothers. They flew the Wright Flyer from Kitty Hawk, North Carolina for 12 seconds and covered a distance of 120 ft. The Wright Brothers twisted the surface of each wing separately and succeeded in changing its orientation with respect to oncoming airflow. Such changes in position led to changes in flight direction. Their theory was initially tested by flying a kite, and later was used to control the Wright Flyer. Wing morphing is a modern day extension of wing warping in which the aerodynamic shape of the wing is modified under computer control. Wing Warping is a technique for rotation around the aircraft’s longitudinal axis employed by many early aircraft and it is the earliest known manmade application of the morphing wings. In this system cables and pulleys were arranged in such a way that they were used to contort the trailing edges of each wing in opposite directions, which caused a difference in the lift properties of each wing and subsequently initiated roll. This technique was implemented on the Wright flyer but the control was problematic as an overly flexible wing could turn corrective inputs into a ‘tab effect’ which would cause further deflection of the wing surface, leading to a roll control reversal. This led to a shift toward rigid wing design, having control inputs which were actuated by hinging flaps on the wings, tail, ailerons and elevators. Idea of altering the geometry of aircraft wings was taken into consideration, resulting in a number of aircraft with variable sweep wings, like the Dassault Mirage G. Although these aircraft demonstrated a variable geometry to their airframe by wing sweep but still they cannot be considered as true morphing wing aircraft because the wing swing mechanism prevents the wings from being seamless.
The concept of seamless wing morphing however with the advances in materials and computer technology in 21st century, made seamless variable wing geometry a possibility. The initial motivation for modern wing morphing resulted largely from birds, like the swift, which can shapes its wings according to the present phase of flight as mentioned earlier. Subsequently, further research has been carried out into various potential opportunities for the use of morphing wing technology. Much of the research is ongoing, but the potential morphing wings has already gained much attention from the various organisations within the aviation industry including the National Aerospace and Space Administration (NASA), who have conducted research with Boeing on an Active Aeroelastic Wing project.
3. WING SKIN MATERIAL
The choice of material for the wing skin for an aircraft with morphing wings is more complex than that for aircraft with rigid wings, this is due to the need for the material to stretch elastically in multiple directions as mentioned by Kikuta, M.T.,(2003),so as to provide little resistance to the change in wing shape and it should also consistently return to its original condition or the position. All of these conditions must be considered in addition to the common aircraft skin requirements, like withstanding a significant temperature range and a high pressure load due to the pressure difference below and above an aerofoil in flight.
The design of skins which are flexible is challenging with many conflicting requirements like the skin must be soft enough so that it allow shape changes but at the same time it must be stiff enough to bear up the aerodynamic loads and keep up the required shape/profile. So this requires thorough design studies between the requirements. In addition, the type of flexible skin required also depends on the loading scenario and the desired change in shape which can be both one-dimensional and multi-dimensional. As a result, the design of flexible skins depends on the specific application.
Thill et al.(2007) performed a wide-ranging review of flexible skins and took into account various novel material systems concepts and technologies. He researched about the use of a composite corrugated structure as morphing skin panels in the trailing edge region to vary the camber and chord of an airfoil. Various skin concepts including elastomeric matrix composites have been investigated for large area changes (Peel et al.(2009); Bubert et al.(2010); Murray et al (2010)). Furthermore, the use of morphing core sandwich structures covered by compliant face sheet has been investigated for both low- and for high-strain utility depending on the cellular arrangement and the material of the core (Joo et al.(2006); Bubert et al.(2010)). In addition, McKnight et al (2010) studied the use of segmented reinforcement in order to create a variable stiffness material that can be adopted as a flexible skin.
4. WING ACTUATION METHOD
A number of different methods of actuation can be utilised for ‘morphing’ morphing wings, these can be hydraulic, electric or pneumatic. Another alternative is the Shape Memory Alloy (SMA), or Smart Alloy that has become increasingly popular because of its simplicity in order to reduce costs, space and weight taken up by the actuator. An example of such a material is Lead Zirconate Titanate (PZT), a memory alloy which displays a piezoelectric effect (an electric current induced will cause a change in shape and vice versa). PZT has another benefit of being very fast in reacting to an induced current, thus allowing very quick precise actuation which is an absolute must in the world of aircraft control as mentioned by Iannucci, L.(2008),.
Let us consider a rigid wing with a standard spar and rib configuration. If we have to add morphing to this wing using hydraulic actuators, pumps and other auxiliary support systems, it would result in higher degree of complexity and weight penalty. So the most efficient approach to add morphing capability to an aircraft is to add multifunctionality to its ribs and spars, which would enhance the degree of freedom of these structures. This multifunctionality is the new trend in aircraft design and would ensure reduced complexity and weight penalty. Materials like the shape memory alloys, shape memory polymers and piezo crystals are just some of the materials that can be used as actuators in smart structures. They each posses a different amount of actuation stress, strain and speed, and depending on the application or the purpose some will be more suited than others. For instance, piezo crystals typically provide very fast high stress actuation, but at the cost of limited displacement. Shape memory polymers provide high actuation strains, but at the cost of very small stress. As stated earlier, wing morphing requires the shape changes to be both seamless and a significant magnitude. Shape memory alloys, such as NiTi, possess a good combination of actuation stress and strain needed for wing morphing. Therefore, SMAs can be used to functionalize wing spars so as to allow shape changes, as well as contribute to load sharing without significantly adding weight to the overall system.
Shape memory alloys (SMA) can be implemented in a structure following a compliant or an antagonistic setup as mentioned by Galantai (2012). Compliant structure makes use of the elastic potential energy stored in its members in order to bring the system to initial shape. As a result, the simplest compliant structure only requires one SMA actuator. Figure 1 shows an actuator connected to an elastic member, while being constrained at the tips. At the initial position, the actuator is pre-strained and its microstructure is 100% martensitic. But when heated, it undergoes phase transformation from martensite to austenite and it contracts. In order to bring the system to its initial shape, the SMA actuator must be cooled such that it will undergo the reverse phase transformation, from austenite back to martensite. At this point, the elastic potential energy which is previously stored in the elastic member would stretch the SMA.
Figure 1: Compliant vs. Antagonistic implementation of SMAs
In an antagonistic setup, at least two SMA actuators are needed. They are arranged in such a way that while one actuator contracts, the other actuators are strained. By changing the order in which the actuators are cooled and heated, a two-way motion can be achieved. While compliant systems have the benefit of requiring fewer SMA actuators, one of their biggest shortfalls is the nonstop requirement of heating in order to maintain their deformed shape. Antagonistic structures, on the other hand, do not suffer from this problem, as they only require energy to change shape and not to maintain it. So the antagonistic setup has a net advantage in situations where both deformed and non-deformed shapes are to be kept for extended periods of time
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