Category: Research Paper
The design of conventional fixed wing aircraft is constrained by the conflicting requirements of multiple objectives. Mechanisms such as deployable flaps provide the current standard of adaptive aerofoil geometry, although this solution places limitations on manoeuvrability and efficiency, and produces a design that is non-optimal in many flight regimes. The development of new smart materials together with the always present need for better performance is increasingly prompting designers towards the concept of morphing aircraft. These aircraft possess the ability to adapt and optimise their shape to achieve dissimilar, multi-objective mission roles efficiently and effectively. One motivation for such uninhabited aircraft are birds that morph between cruise and attack missions by changing their wing configuration accordingly. Birds also use camber and twist for flight control. The Wright Brothers used wing warping as a seamless flight control in their first flying machine. Morphing wings for flight control bring new challenges to the design of control laws for flight. Because configuration changes move the aerodynamic centre, control of the aircraft during planform morphing requires attention.One primary advantage of a morphing platform would be the increased cost effectiveness of aircraft through eliminating the need for multiple, expensive, mission specific aircraft. However, from current trends in this research area, it is clearly evident that the practical realization of a morphing structure is a particularly demanding goal with substantial effort still required. This is primarily due to the need of any proposed morphing airframe to possess conflicting abilities to be both structurally compliant to allow configuration changes but also be sufficiently rigid to limit aeroelastic divergence. There are typically four applications of morphing:
The structural technologies available to achieve a shape changes in a morphing aircraft fall into two major categories, namely planform changes using rigid mechanisms, and compliance (for example wing twist or compliant mechanisms). Vibration control systems are usually based on directly applying a force to the structure. For shape control, actuators are required to effect the shape change, and sensors are required to measure the actual deflection. Large scale morphing motions for configuration morphing (that is significant planform changes) include wing extension, wing folding, and wing sweep (see figures above for two examples of large scale morphing). Significant aerodynamic performance gains are only really achievable through large overall changes in the aircraft geometry via wing sweep, area and/or span. The application of morphing to flight control usually involves small geometric wing changes such as the use of deployable slats and flaps as well as wing warping techniques to enhance the control authority of the aircraft. At present, in both of these categories, such medium to large scale changes are obtained with complex and sophisticated mechanical devices significantly increasing the installation and maintenance costs as well as the structural weight of the airframe. It is clear therefore, that substantial gains in these areas could be made if alternative methods to enact these changes were found. Basic morphing motions for seamless flight control include wing twist, wing chamber change, and asymmetric wing extension. The use of winglets as control effectors may also yield substantial benefits.
There are many challenges in the design of morphing aircraft: the integrity of compliant structures needs to be ensured, the system should be designed so the required actuation force is realisable, the skin has to be designed to give a smooth aerodynamic surface yet support the aerodynamic loads, the design process should be extended to encompass multiple flight regimes, engines need to be designed for efficient low and high speed operation, and control systems will have to cope with highly coupled control effectors. While many questions remain unanswered regarding the utility of morphing air vehicles, enough evidence of improved performance and new abilities has been established to warrant further consideration of the prospects of morphing aircraft, both for multiple flight regimes and for flight control.The Morphing Aircraft Project (2005-2008) The morphing aircraft project at the University of Bristol was funded by the European Commission through a Marie Curie Excellence Grant. The project took a systems view of morphing aircraft structures and considered the structural design, airflow, structural dynamics, flight control system, aeroservoelasticity, and sensors and actuators. All these areas interact extensively, for example designing how the structure changes shape is critically dependent on the aerodynamic loads and the required flight control. While each topic is a huge area in its own right, a systems approach is the only appropriate way forward. There were five major topics of interest:
Multistable composite structures: For large deformations of morphing aircraft the orthotropic properties of composite materials may be used. This may, for example, enable the elimination of hinges, which reduces the stress concentration around the pivot points, and consequently reduces the weight penalty introduced with morphing. Residual stresses that occur during manufacture are able to produce a structure with multiple stable states of equilibrium. Finite element analysis procedures have been used to model both the cool-down during manufacture and the snap-through during morphing. These predictions have been validated quantitatively for rectangular plates. Morphing aircraft examples, namely wing-tip devices and a variable camber wing profile, have been designed, analysed using finite element analysis, and manufactured. The wing-tip device was successfully tested in the wind tunnel.
Aeroelastic tailoring: Laminated composite materials are commonly employed in the aerospace industry due to their high strength and stiffness ratios. The elastic coupling properties of composite materials may used to morph the structure. Thus, for example, a wing may be designed to passively control the wing twist subject to the lift on the wing. The challenge is to develop an efficient optimisation strategy, based on a suitable choice of objective function and constraints. A novel two level strategy has been developed and applied to panels with T shaped stiffeners. Effects such as the skin-stiffener flange interaction, and variable thickness plates have been included. Approximate closed form solutions for buckling in stiffened panels have been developed that could be included in the optimisation. The extension to global optimisation of a wing structure has been developed, with the objective of minimum wing weight and structural and aerodynamic constraints were imposed. The two level optimisation strategy was used and a reduction in induced drag of 1.4% was achieved, albeit with an increased weight. Current activity will determine the best objective function to use, for example the use of specific aircraft range to optimise the fuel efficiency of the vehicle.
Compliant mechanisms: A compliant mechanism provides the desired shape alterations by elastic deformation as opposed to a mechanised approach. Such methods provide designs that are capable of smooth, conformal deflections together with the prospect of reduced inspection and maintenance and zero backlash. The research has concentrated on airfoil camber change, and the topology optimisation and actuator location problem is formulated based on a truss structure with beam elements, and includes the aerodynamic loads. The optimum location of a series of actuators has been determined for a target deformation with minimum actuator force. As an alternative the dimensions of the truss members have been optimised so that only a single actuator is required to obtain the target deformation. One clear requirement is the use of structural elements which have in-plane flexibility, but are stiff in bending. Elements composed of a cellular substructure and corrugated composite skins with silicone rubber surfaces have been analysed statically, and have been experimentally verified for the skin.
Flight mechanics and control of morphing and flexible aircraft: The flight control of morphing aircraft provides significant challenges such a highly coupled flight dynamics and the optimisation of the control allocation problem. The dynamics and response of a small-scale manned flying wing has been investigated based on an active winglets concept. The flying wing is modelled as a set of hinged rigid bodies, with actuators to change the winglet dihedral angles and provide the aerodynamic moments for maneuvering. The effect of the variation in the dihedral angles on the inertia properties, the centre of gravity location and modal parameters have been assessed. Issues related to the control of such a vehicle are still under investigation. The research has been extended to wings that morph continuously and seamlessly using a distribution of actuators. This requires an understanding of the interaction between the aerodynamics, structures and controls. An aeroelastic tool has been developed using an equivalent plate model of the structure and a vortex lattice model for the aerodynamics. This allows the tailoring of the structural flexibility and optimization of the actuators.European Research Council Project (2010-2015)
Professor Friswell has been awarded a prestigious 2.5M euro grant from the European Research Council to continue codevelop in flight morphing of aircraft wings. The project title is the Optimisation of Multi-scale Structures with Applications to Morphing Aircraft and started on 1 May 2010 for 5 years. Morphing technology using discrete components, for example flaps and slats on commercial aircraft or variable sweep wings on military aircraft, has been very successful. However these systems tend to be very heavy and the airflow is not optimal for different flight conditions. Morphing aircraft integrate distributed actuation within the deforming structure to produce better performance. However the conceptual design of these structures is very challenging and requires the optimisation of low fidelity models of the complete system, combining both the structural response and the aerodynamics. Fundamental to the success of the project is the interplay of models at different length scales and at different fidelities, retaining geometric parameters and constraints from the smaller scales. The potential benefits of novel morphing concepts will be quantified for realistic aircraft systems.Selected References Active winglets as multi-axis effectors and novel aerodynamic concepts
Passive Wing Morphing
Ornithopters, like birds, flap their wings to generate both thrust and static lift. The thrust, in turn, propels the ornithopter forward so that the velocity of the air moving past the wings introduces dynamic lift. When the combined forces of static and dynamic lift are sufficient to overcome gravity, flight is sustainable.
Left: Passive Wing Morphing Concept
This research set off to investigate a method to increase the average static lift of the ornithopter. Current ornithopter designs mechanically drive a pair of wings up and down, but on average, do not produce as much static lift as their biological counter parts. In low speed flight regimes, ornithopters plummet to the ground while birds can hover and perch. Increasing available static lift could make this maneuverability available in flapping wing ornithopters.
The principle flaw of wing flapping is the cyclic way in which the upstroke of the wing undoes the success of the previous downstroke. Negative lift cancels out positive lift almost completely in existing designs.
By allowing the wings to mechanically morph during flight, several ornithopters repeatedly generated approximately 400% as much average static lift as non-morphing wing designs. Unfortunately, this flapping mode took a penalty on the available thrust of the ornithopter, reducing its dynamic lift.
Primary Project Objectives
Ornithopters produce static lift by flapping their wings and dynamic lift by moving through the air, powered by the thrust generated during each flapping cycle. This research project was designed to explore a method of increasing the static lifting force available while the ornithopter was flapping.
The ornithopters tested in this research project flew best when flapping at around 7 Hz. At these speeds, the wings displayed impressive deflections in the wing spars and powerful curves in the sail area. This change in the wings helps generate thrust. By extension a different change in shape might also increase static thrust.
A mechanical spring joint was fabricated to be placed along the main spar of each wing, along the leading edge. This joint was restricted in motion along the top so that the spar would be stiff on downstrokes. On each upstroke, the springs were permitted to deflect, dramatically reducing the area of each wing and thus reducing the negative lift of this portion of the flapping cycle. About the time that each wing reached the top of its stroke, the pair of springs would restore the wings back to their original configuration, ready for the next downward stroke.
Load cell measurements were collected for each ornithopter design over varied frequencies. However, these frequencies were unknown. By applying Fourier transforms of each test result, data could be divided into an integer number of complete flapping cycles. Within each flapping cycle, the ornithopters exhibited a region of positive static lift and of negative static lift. This approach enabled the rapid comparison of several different ornithopter designs over a whole array of flapping frequencies. Results for average static lift and average static thrust are in the charts below.
This plot is a comparison of four differently constructed ornithopter wing sets with the same wing shape and area. Each wing pair was mounted on the same ornithopter, strapped to a load cell. The throttle on the bird was increased to induce an increasing flapping frequency. Naturally, the higher flapping frequencies corresponded to larger static lift readings. These readings were averaged out over an integer number of complete flapping cycles to yield the average static force. The two blue data sets represent the original method of wing construction with rigid, non-morphing spars. The two red data sets describe the force produced with morphing wings.
Average static lift, the force responsible for hovering and perching in bird flight maneuverability, was increased to approximately 400% by passively morphing the wings.
With great success in available static lift, there was an unfortunate drawback: the ornithopters produced far less thrust in the passively morphing wing designs. Flight tests highlighted this behavior. Load cell tests for average static thrust indicated that the morphing spars indeed had this penalty. Five construction models were tested for thrust performance in the flapping wings. The non-morphing wings performed better.
It has been speculated that results could be improved by at least two methods. The first is to purposefully direct the wing tip motion of ornithopters to mimic biological shapes displayed in nature. The second is to actively morph the structure of the wings during flight.
The results were submitted to the 2007 AIAA Regional Student Conference for the (I-MA) Mid-Atlantic Region in the form of a technical paper and a presentation. On merit of the accessibility of the presentation and the original approach of the author, the research project in Passive Wing Morphing was awarded third place.
NASA researchers have completed initial flight tests of a new morphing wing technology that could reduce fuel costs, airframe weight and decrease aircraft noise during take-offs and landings.
Working with the Air Force Research Laboratory (AFRL) and FlexSys Inc, the test team at NASA’s Armstrong Flight Research Center in Edwards, California, flew 22 research flights over six months with experimental Adaptive Compliant Trailing Edge (ACTE) flight control surfaces that are claimed to offer significant improvements over conventional flaps used on existing aircraft.
AFRL began work with FlexSys in 1998 through the Small Business Innovative Research (SBIR) program. AFRL and FlexSys developed and wind tunnel tested several wing leading and trailing edge designs for various aircraft configurations through to 2006. In 2009, AFRL and NASA’s Environmentally Responsible Aviation (ERA) project agreed to equip a Gulfstream III jet with ACTE flaps designed and built by FlexSys, incorporating its proprietary technology.
ACTE technology, which can be retrofitted to existing airplane wings or integrated into entirely new airframes, enables engineers to reduce wing structural weight and to aerodynamically tailor the wings to promote improved fuel economy and more efficient operations while also reducing environmental and noise impacts.
In a statement, Fay Collier, ERA project manager said: “This is the first of eight large-scale integrated technology demonstrations ERA is finishing up this year that are designed to reduce the impact of aviation on the environment.”
According to NASA, the test aircraft was flown with its experimental control surfaces at flap angles ranging from -2 degrees up to 30 degrees. Although the flexible ACTE flaps were designed to morph throughout the entire range of motion, each test was conducted at a single fixed setting in order to collect incremental data with a minimum of risk.
“We are thrilled to have accomplished all of our flight test goals without encountering any significant technical issues,” said Pete Flick, AFRL Program Manager from Wright-Patterson Air Force Base in Ohio. “These flights cap 17 years of technology maturation, beginning with AFRL’s initial Phase 1 SBIR contract with FlexSys, and the technology now is ready to dramatically improve aircraft efficiency for the Air Force and the commercial aviation industry.”
The results of these flight tests will be included in design trade studies performed at NASA’s Langley Research Center in Hampton, Virginia, for designing future large transport aircraft.
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Shaping a New Future for Aerospace Vehicles
NitishDomaleT.E. (Mechanical) padmashri dr. vitthalrao vikhe patil college of engineering ahmednagar.
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SaurabhDhaneshwarT.E. (Mechanical) padmashri dr. vitthalraovikhepatil college of engineering ahmednagar.
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Morphing Wings Technology: Large Commercial Aircraft and Civil Helicopters offers an alternate view to existing topics and provides a fresher look on the research that is currently being conducted on morphing aircraft including coverage of industry design, real manufactured prototypes, and certification. Morphing aircraft are multi-role aircraft that change their external shape substantially to adapt to a changing mission environment during flight. Practical applications of morphing devices are presented—from the challenge of conceptual design incorporating both structural and aerodynamic studies to the most promising and potentially flyable solutions aiming at improving the performance of commercial aircraft and UAVs.
The book consists of eight sections as well as an appendix which contains both updates on main systems evolution (skin, structure, actuator, sensor, and control systems) and a survey on the most significant achievements of integrated systems for large commercial aircraft. This is an invaluable reference for students in aeronautics and aerospace fields who need an introduction to morphing discipline as well as senior professionals desiring a first approach with the morphing potentialities.Key Features
Aeronautics and aerospace PhD students in research and professionals who are interested in this line of research. Anyone involved with the application of morphing onto heavy mechanical sector: ships, trains, automotive (cars, buses or trucks), ground movement, and so onTable of Contents
Section 1: Introduction
Section 2: Requirements and Performance
Section 3: Skins
Section 4: Systems Design
Section 5: Numerical Simulation
Section 6: Morphing Wing Systems
Section 7: Realization and Reliability Aspects
Section 8: Smart Helicopter Technologies
No. of pages: 700 Language: English Copyright: © 2017
Imprint: Butterworth-Heinemann Print ISBN: 9780081009642 Electronic ISBN: 9780081009697
Background on Morphing Systems
The desire for multi-mission capability in military and civil air vehicle systems has created a need for technologies that allow for drastic wing shape changes during flight. Since most current aircraft are fixed-geometry, they represent a design compromise between conflicting mission segment performance requirements, such as high-speed cruise, low-speed loiter, and low turn radius maneuver. If a hybrid aircraft is designed to combine several flight profiles, the wing design must maximize overall efficiency of the anticipated mission. Through morphing, the aerodynamics of the aircraft can be adapted to optimize performance in each segment by changing areas such as the camber of the airfoils and the twist distribution along the wing.
A Veritex morphing wing prototype developed by CRG. In the figure, the process begins at the top and proceeds clockwise around to the center. Under thermal stimulus, the wing quickly unfolds and expands to its appropriate chord and shape.
Adapting the shape of wings in flight allows an air vehicle to perform multiple, radically different tasks by dynamically varying its flight envelope. The wing can be adapted to different mission segments, such as cruise, loitering, and high-speed maneuvering by sweeping, twisting, and changing its span, area, and airfoil shape. Morphing wing technology is considered to be a key component in next-generation unmanned aeronautical vehicles (UAVs) for military and commercial applications.
CRG successfully demonstrated the self-deploying capabilities of its (Veriflex ® -based composite) material in the fabrication and deployment of a sub-scale, carbon fiber reinforced wing. The sub-scale wing was heated, collapsed, and rolled up into a tight package. Once cooled, the structure maintained the rolled up configuration until it was heated and deployed to achieve the memorized wing shape, as shown in the center of the figure below.Adaptive Systems
Adaptive structures allowing drastic wing shape changes have been a long-term goal of the aerospace industry. CRG is developing two methods of allowing wings to change from a slow-flight loitering, high-efficiency configuration to one designed for high speed and maneuverability. This shape change can occur repeatedly with no loss of material integrity.
CRG’s shape memory polymer, Veriflex ®. is being used to create a seamless wing skin that can be heated, reshaped by the internal wing structure, cooled, then reheated and moved back to the original shape. A real-time morphing wing shape increases versatility in the flight capabilities and function of an aircraft.
Veriflex is also being used to develop hinges that allow wings to bend at set points. This will allow planes to change from a loitering configuration to high-speed and high-maneuverability shapes.Wing Structures
CRG has worked with other development partners in developing morphing aircraft technology. The goal was to develop and demonstrate viable composite materials and process technology to support multiple Air Force morphing structural applications. We developed a prototype of a form-fit, improved-function wing ready for simple integration and operational testing. In the process, we applied a unique suite of smart materials technologies, such as CRG’s shape memory polymers (Veriflex ® ), dynamic composites (Veritex™), and dynamic syntactic foams (Verilyte™). We also employed smart materials, engineering design, process development, fabrication, and other supporting technologies to meet goals and requirements.
CRG's smart structures engineering team focuses on integrating multiple smart material technologies with conventional actuation mechanisms and on developing a variety of smart adaptive or morphing structures. We have demonstrated realistic morphing structure concepts for near-term applications. These and other program efforts will help CRG better understand TRL levels, define near-term morphing capabilities, help identify the next enabling materials technologies necessary to round out structural morphing composites capabilities, and predict mid- and far-term morphing capabilities.Biological Inspiration
Understanding how birds perform by making their wings morph, or change shape in flight is one step in CRG research efforts to dramatically increase the efficiency and maneuverability of aircraft. Flight capabilities in nature provide a demonstration of feasibility and proof-of-concept for man-made morphing architecture. In fact, the morphology of a pterodactyl’s wings and body shape provides an excellent model for morphing mechanisms and adaptable air vehicle systems.
Compared with the subtle capabilities of a common bird’s wings, mechanical flaps and slats and pivoting wings are heavy, complex and inefficient. Although these wings are the result of clever ingenuity and years of engineering design, they increase the radar cross-section of a plane and can’t operate at high flight speeds. The ability to substantially change a wing’s shape seamlessly in flight through the use of CRG’s SMP technology will produce aircraft that can fly both fast and slowly, with optimal efficiency at every speed. These vehicles will burn less fuel, run more quietly, fly longer, take off and land in shorter distances, and maneuver more quickly and with greater agility.
In programs with the Air Force Research Laboratories (AFRL), the Defense Advanced Research Projects Agency (DARPA), the Army, Lockheed Martin Skunkworks ®. and other commercial companies as well as through internal R&D, CRG scientists are exploring technologies that could one day liberate aircraft from flaps, slats, and ailerons so that they more closely emulate the astonishing adaptability and control of bird flight.Novel Design Principles
A "sliding rib" concept for the underlying structure of a morphing wing. This represents a new design philosophy for wing structures. Morphing wing design requires the integration of mechanical structure, seamless skin, and actuators.
The ultimate goal of research in these morphing programs is to develop new design principles for fully adaptable systems. These design principles would consist of integrated systems using morphing mechanisms, propulsion systems, control systems, structures, and materials. CRG has demonstrated feasibility in all these areas. For example, the figure below demonstrates one of the company's completely new designs for the underlying structure of a morphing aircraft wing.
In the interest of developing entirely new systems to incorporate shape-changing technologies, CRG's engineering research for morphing wings consists of selection of actuators, designs for morphing mechanisms and skins, integration of these components into a wing structure, experimental verification of aerodynamic and structural performance of a wing segment, and incorporation of the adaptable wing into a complementary morphing air vehicle. CRG’s research in shape memory polymers and morphing structural design has contributed significantly to the development of adaptive wings.The Veriflex Family of Materials and Morphing Technology – An Ideal Match
Shape memory polymer's list of applications continues to grow as researchers and customers experiment with it, and CRG helps integrate the new technology into existing systems. Continued demand for Veriflex as a resin system has prompted its sale to the general public through our spin-off company, CRG Industries .
Morphing applications in particular benefit from the capabilities of shape memory materials. CRG has demonstrated feasibility for adaptable systems in manufacturing, military applications, space systems, aerostructures, and propulsion. An overview of some of those applications is outlined below:Manufacturing
Morphing, adaptable systems increase the usefulness and capabilities of a wide range of applications, and CRG has taken a leadership role in research involving morphing technologies. Through the use of innovative smart materials, process engineering, and integration into real-world systems, the results of this research are already revolutionizing the way we design aircraft, build manufacturing systems, equip multipurpose vehicles, and deploy space mirrors.Tradeshows
ADA Technologies receives contract for morphing UAV wing skin research
ADA Technologies, Inc. received a $100,000 contract from the United States Air Force to conduct early stage research on a new method of creating a wing skin for use on morphing unmanned air vehicles (UAVs).
Much like a bird, morphing UAVs have the ability to dramatically alter their wing shape during flight to maintain optimal aerodynamic efficiency over a wide range of flight conditions. To perform, morphing wings skins must be sufficiently compliant to allow for substantial changes in wing shape, while having sufficient stiffness to carry aerodynamic loads.
Shape Memory Polymers (SMPs) show substantial promise in meeting these conflicting requirements due to their ability to quickly transition between rubber- and rigid-like behaviors through the application of heat. Efficient heat transfer within a timeframe that is consistent with UAV flight control needs is critical to the successful application of SMPs for morphing wing skins. Traditional resistive heating techniques have thus far proven unable to meet this requirement.
ADA's research will utilize the firm's nanotechnology, shape memory polymer and thermal modeling expertise to develop wing skins that are capable of rapid shape change enabled through highly efficient means of heat transfer.
"A key distinction of ADA's approach compared to prior work is the use of a novel approach to transmitting heat within a nanoparticle reinforced SMP wing skin. Specifically, our approach will improve the thermal conductivity at the interface between the nanoparticles and the SMP resin thereby increasing the heat transfer efficiency and ultimately the performance and effectiveness of the morphing UAV," said Steven Arzberger, Ph.D. ADA senior research scientist and project manager.
Press release issued by ADA Technologies, Inc. on February 2, 2010