Military have large demand for smart materials and devices including smart self-repair, smart clothing such as cloaking suits, and adaptive hull structures for ships. Morphing aircraft are multi-role aircraft that change their external shape substantially to adapt to a changing mission environment during flight. This creates superior system capabilities not possible without morphing shape changes. The objective of morphing activities is to develop high performance aircraft with wings designed to change shape and performance substantially during flight to create multiple-regime, aerodynamically-efficient, shape-changing aircraft.
The inspiration of morphing wings or aircraft comes from Birds, the wings of a bird can be reshaped to provide optimal performance at all flight conditions. 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.
This issue motivates the research and development of the so-called ‘morphing aircraft’ or ‘morphing wings’. “Shape morphing” has been used to identify those aircraft that undergo certain geometrical changes to enhance or adapt to their mission profiles. Ideally, a morphing aircraft is able to modify quickly the shape of its wings in-flight, thus reaching optimum aerodynamic performance under any flight condition. Morphing technologies offer aerodynamic benefits for an aircraft over a wide range of flight conditions.
The underlying technology for such morphing wings are Smart materials or Active materials or Functional materials, that are designed materials that have diverse, dynamic features that enable them to adapt to the environment. They have one or more properties that can be significantly changed in a controlled fashion by external stimuli, the stimulus and response may be mechanical, electrical, magnetic, optical, thermal, or chemical.
Smart materials are used to construct smart structures. A smart structure (a.k.a. intelligent structure, adaptive structure, and functional structure) is defined as a structure that is able to sense external stimuli such as pressure, velocity, density, or temperature change. It can process the information and respond in a controlled manner in real time. A smart structure is a system containing multifunctional parts that can perform sensing, control, and actuation; it is a primitive analogue of a biological body.
Apart from the use of better functional materials as sensors and actuators, an important part of a “smarter” structure is to develop an optimized control algorithm that could guide the actuators to perform required functions after sensing changes. Many types of actuators and sensors such as piezoelectric materials, shape memory alloys (SMA) (alloys that can remember their original shapes), electrostrictive and magnetostrictive materials, and fiber optics are being considered for various applications.
In aerospace, smart materials could find applications in ‘smart wings’, winglets in aeroplanes that adapt automatically to changing flight conditions health and usage monitoring systems (HUMS), and active vibration control in helicopter blades.Several types of smart materials and structures have been investigated, including piezoelectric materials, electrostrictive materials, magnetostrictive materials, shape memory alloys, and fiber optic sensors.
Shape memory alloys have attracted a great interest by many researchers as a promising morphing wing material because of its shape recovery upon application of voltage. Shape memory concept refers the property of a material or an alloy which regains its original shape when external load or electrical energy is applied. The design possibilities in the field of aerospace engineering are advanced by the unique thermal and mechanical properties of shape memory alloys now a day to improve the aerodynamic efficiency.
Flexible Composite Wings Morph Into Different Shapes
Composite materials, many built around atomic carbon structures like graphene, are revolutionizing manufacturing industries from car frames to robotics to building infrastructure. Thanks to a combination of high strength, heat resistance, and low weight, composites are particularly useful in the aerospace industry. Aviation Partners Inc. (API) in partnership with FlexSys have developed “flexfoil” wings from composite material
Flexfoil is a uniform, jointless mechanism that equally distributes load-bearing to all parts of the structure as each section bends and contributes to the wing morphing. It’s incredibly flexible while retaining its strength and other structural advantages. API and FlexSys claim this seamless alternative to conventional wing modification mechanisms results in low stress across the flexfoil component and provides long service life with low maintenance.
Aviation Week reports that API and FlexSys and working with an undisclosed customer to outfit the first commercial aircraft with flexfoil morphing wings. NASA is also working on adaptive wings, specifically winglets that use actuators to fold up or down mid-flight.
NASA and MIT Are Making Flexible, Morphing Plane Wings using digital materials
Modern aircrafts rely on flaps to boost lift, and on ailerons to change direction. However these actuators require complex hydraulic mechanisms that add weight and reliability problems in addition to loss in efficiency and noise caused by due to generation of turbulent airflow through gaps in the edge of wings. Therefore Researchers have been trying for many years to achieve a reliable way of deforming wings as a substitute for the conventional, separate, moving surfaces, but all those efforts “have had little practical impact,” Gershenfeld says.
The new wing architecture, which could greatly simplify the manufacturing process and reduce fuel consumption by improving the wing’s aerodynamics, as well as improving its agility, is based on a system of just eight tiny, lightweight subunits made of carbon fiber. The wing would be covered by a “skin” made of overlapping pieces that might resemble scales or feathers and ultimately could be used to build the entire airframe.
The basic principle behind the new concept is the use of an array of tiny, lightweight structural pieces, which Gershenfeld calls “digital materials,” that can be assembled into a virtually infinite variety of shapes, much like assembling a structure from Lego blocks. The assembly, performed by hand for this initial experiment, could be done by simple miniature robots that would crawl along or inside the structure as it took shape. The team has already developed prototypes of such robots.“One of the things that we’ve been able to show is that this building block approach can actually achieve better strength and stiffness, at very low weights, than any other material that we build with,” says NASA’S Kenny Cheung, one of the leaders of the project.
The new concept is described in the journal Soft Robotics, in a paper by Neil Gershenfeld, director of MIT’s Center for Bits and Atoms (CBA); Benjamin Jenett, a CBA graduate student; Kenneth Cheung PhD ’12, a CBA alumnus and NASA research scientist; and four others.After their success in the wind tunnel, Cheung and the MIT team pushed their solution further, bolting the wings onto a remote controlled plane. The twist of the wing was almost imperceptible from the ground, making the mechanical system seem more organic. These new building methods would likely get their start on small drones and unmanned aircraft. “It’s hard to make a wing that’s morphable and deformable, and still have the stiffness you need to carry a lot of weight,” says Sensmeier.
“Ultralight, tunable, aeroelastic structures and flight controls open up whole new frontiers for flight,” says Gonzalo Rey, chief technology officer for Moog Inc., a precision aircraft motioncontrols company, who was not directly involved in this work, though he has collaborated with the team. “Digital materials and fabrication are a fundamentally new way to make things and enable the conventionally impossible. The digital morphing wing article demonstrates the ability to resolve in depth the engineering challenges necessary to apply the concept.”
“The broader potential in this concept extends directly to skyscrapers, bridges, and space structures, providing not only improved performance and survivability but also a more sustainable approach by achieving the same strength while using, and reusing, substantially less raw material, ” said Gonzalo Rey, chief technology officer for Moog Inc. And Loubiere, from Airbus, suggests that many other technologies could also benefit from this method, including wind turbines: “Simply enabling the assembly of the windmill blades on the spot, instead of using complex and fuel-consuming transport, would enhance greatly the cost and overall performance,” he says.
Programmable balloons pave the way for new shape-morphing devices
A team of researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has designed materials that can control and mold a balloon into pre-programmed shapes. The system uses kirigami sheets—thin sheets of material with periodic cuts—embedded into an inflatable device. As the balloon expands, the cuts in the kirigami sheet guide the growth, permitting expansion in some places and constricting it in others. The researchers were able to control the expansion not only globally to make large-scale shapes, but locally to generate small features.
The team also developed an inverse design strategy, an algorithm that finds the optimum design for the kirigami inflatable device that will mimic a target shape upon inflation. “This work provides a new platform for shape-morphing devices that could support the design of innovative medical tools, actuators and reconfigurable structures,” said Katia Bertoldi, the William and Ami Kuan Danoff Professor of Applied Mechanics at SEAS and senior author of the study. The research is published in Advanced Materials. An individual cut on a kirigami sheet contributes to the larger shape of the balloon like a pixel helps form an image on a 2-D surface. The researchers found that by tuning the geometric parameters of these cuts, they could control and embed complex shapes.
“By only varying two parameters of the pixels, we can program all different kinds of crazy shapes into the kirigami balloons, including bends, twists and expansions,” said Antonio Elia Forte, a postdoctoral fellow at SEAS and co-first author of the study. “Our strategy allows us to automatically design a morphable balloon starting from the shape that you need. It’s a bottom-up approach that for the first time harnesses the elasticity of the material, not only kinematic.”
Using these parameters, the researchers developed an inverse algorithm that could mix and match pixels of different width and height, or delete certain pixels altogether, to achieve the desired shape. By manipulating the parameters of individual pixels, the researchers were able to tune shapes at a significantly smaller scale. To demonstrate this, they programmed a balloon to mimic the shape of a squash (the experiments took place around Halloween) complete with the characteristic bumps and ridges along the side. “By controlling the expansion at every level of the kirigami balloon, we can reproduce a variety of target shapes,” Lishuai Jin, a graduate student at SEAS and co-first author of the paper.
US Army and MIT connect metamaterials to create dynamic structures
In Nov 2020, it was reported that the US Army and the Massachusetts Institute of Technology (MIT) have developed a new way to connect metamaterials that will enable the creation of dynamic structures. Developed by the scientists from the US Army and MIT’s Center for Bits and Atoms, the method link materials with unique mechanical properties using a discrete lattice system similar to Lego to develop structures with reconfigurable properties.
This technology opens up the possibility of creating military robots made of robots. For instance, a swarm of robots will be able to attach together to form a bridge allowing troops to cross a river. If achieved, the capability will significantly enhance military manoeuvrability and survivability of troops and equipment. The findings were published by researchers in the Sciences Advances cover story.
An army researcher Dr Christopher Cameron said: “Robots rearranging to form a bridge made of robots, similar to ants, is one embodiment of our concept of structural robotics, which blur the line between active and passive elements and feature reconfigurability. “It is still a motivating use case for the system, but we are looking at broader implications for ground robotics which are adaptable, reconfigurable, and resilient. “If a swarm of robots can turn themselves into a bridge, how else can they be rearranged? How do we design and control robots like this?”
These findings, which researchers published in the Sciences Advances cover story are an example of high-risk exploratory science and technology efforts from the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, said Dr. Bryan Glaz, associate chief scientist in the laboratory’s Vehicle Technology Directorate.
Motivated in part by swarms of tiny robots that link together to form any imaginable structure like in the animated movie Big Hero 6, these metamaterials may also enable future high-performance robotics and impact/blast absorbing structures, Glaz said. Futuristic visions from concept developers with the Army Future Command’s Futures and Concepts Center at Fort Eustis, Virginia, directly motivated this work, Glaz said. Researcher pursued this research based on discussions and concepts supported by The U.S. Army Functional Concept for Movement and Maneuver, which describes how Army maneuver forces could generate overmatch across all domains.
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