The design of aerodynamic airfoils is optimized for certain conditions. For instance, the shape of the wings of fixed-wing aircrafts are designed and optimized for a certain flight condition (in terms of altitude, speed, aircraft weight, etc.). However, these flight conditions vary significantly during the flight. Currently, aircraft are provided with control surfaces such as flaps, slat and ailerons, normally governed by powerful but heavy hydraulic mechanisms. These moving parts allow the aircraft to fly under many different flight conditions, although usually with non-optimal performance. Moreover, these mechanisms introduce hinges and surface discontinuities between parts which cause undesirable effects such as turbulences and noise or a decrease of the lift-to-drag ratio.
In contrast, 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 military establishment is looking long and hard at morphing aircraft. One reason for their interest is that military aircraft today are designed to perform one kind of mission. For example, one aircraft might excel at reconnaissance, while another is designed for bombing missions.
Features such as the capability to carry more weight, high speed, and a small turning radius are difficult to combine in one aircraft. Because their designs are so specific, aircraft can’t perform more than one role, and in many cases they are limited to certain airfields or ships to use for takeoff and landing.
The Morphing Aircraft Structures (MAS) was started to design and build these shape-changing aircraft for the military. If the military is successful, that would mean that one aircraft might be able to perform more than one role and be able to take off and land from more types of airfields or ships. This flexibility could result in huge savings and much improved efficiency.
Reich and Sanders listed the major challenges of shape morphing aircraft design to be: the requirement for distributed high-power density actuation, structural mechanization, flexible skins, and control law development. This idea is applicable to any other aerospace applications such as rotorcraft or wind turbines. Morphing applied to aerodynamic airfoils brings along interesting benefits: reduction of mechanical fatigue which has a special importance in wind turbines and rotorcrafts (by minimizing vibrations on the structure), reduction of the wing root bending moment, reduction of fuel consumption of flying machines and increase of the performance of wind turbines by increasing the lift-to-drag ratio of the wings or blades, and the reduction of generated noise.
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. For example, a fighter with folding wings or variable-sweep wings can have good performance at both high speed and low speeds, which can reduce fuel consumption and improve the flight envelope dramatically. The advantages of a morphing aircraft are based on an assumption that the additional weight of the morphing components is acceptable.
Smart materials have the potential in developing morphing aircrafts. “Smart” materials and structures have the advantages of high energy density, ease of control, variable stiffness, and the ability to tolerate large amounts of strain. These characteristics offer researchers and designers new possibilities for designing morphing aircraft.
AFRL camber morphing wing takes flight
The Air Force Research Laboratory recently completed the successful flight demonstration of a game-changing camber morphing wing technology that could significantly increase aircraft range and performance. The AFRL-developed Variable Camber Compliant Wing is capable of changing shape to improve aerodynamic performance and morph itself to various flight conditions and missions. Wing camber, or the shape of a wing surface, is a fundamental element of aerodynamic flight. Conventional wings with discrete hinged control surfaces have greater drag, whereas wings with a smooth camber are efficient and maneuverable. The ability to morph the wing according to aerodynamic conditions would give an aircraft increased lift when needed without a weight penalty—typically at takeoff and landing—and greater fuel-efficiency and maneuverability when in flight.
This flight experiment demonstrated the second iteration of the VCCW, a smaller, more compact version than the first, which was used primarily in wind tunnel experiments. This eight-foot wing was designed to be flown on a commercial-off-the-shelf remotely controlled aircraft, simulating an unmanned air vehicle. During the series of flights, held in September and October 2019, the wing was flown at low speeds, completing a number of maneuvers and demonstrating active shape control for optimized drag reduction and increased agility.
The VCCW features a smooth and continuous skin construction, which not only reduces noise by eliminating sharp surfaces and gaps, but improves aerodynamic performance as well. According to Dr. James Joo, AFRL Advanced Structural Concepts team lead and VCCW program manager, the improved aerodynamics translates into potentially significant fuel savings. “Early estimates show VCCW technology saving aircraft fuel consumption by 10 percent,” said Joo. “This was one of our main goals, and it fits the Air Force’s efforts to reduce overall energy costs.” Jared Neely, AFRL research engineer and designer of the morphing wing, called this demonstration an important step in advancing flexible wing technology for warfighter use.
“The success of this demonstration has given us confidence that this technology can be leveraged to higher-class vehicles, to take advantage of the many benefits this technology can truly offer.” Joo added that although other research organizations have explored the morphing camber concept, AFRL’s version is unique because it is a true flexible wing without any discrete control surfaces to assist in takeoff and landing. This seamless surface can increase overall range, making it ideal for a variety of long-range platforms. He says the team will continue to refine the concept and look into additional ways it can benefit existing aircraft. “We are excited about the success of this demonstration,” said Joo. “We are continuing to explore the opportunities that this technology can offer for future Air Force aircraft development.”
MIT And NASA Engineers Demonstrate A New Kind Of Shape-Changing Airplane Wing
The new approach to wing construction could afford greater flexibility in the design and manufacturing of future aircraft. The new wing design was tested in a NASA wind tunnel and is described today in a paper in the journal Smart Materials and Structures, co-authored by research engineer Nicholas Cramer at NASA Ames in California; MIT alumnus Kenneth Cheung SM ’07 PhD ’12, now at NASA Ames; Benjamin Jenett, a graduate student in MIT’s Center for Bits and Atoms; and eight others.
Instead of requiring separate movable surfaces such as ailerons to control the roll and pitch of the plane, as conventional wings do, the new assembly system makes it possible to deform the whole wing, or parts of it, by incorporating a mix of stiff and flexible components in its structure. The tiny subassemblies, which are bolted together to form an open, lightweight lattice framework, are then covered with a thin layer of similar polymer material as the framework.
The result is a wing that is much lighter, and thus much more energy efficient, than those with conventional designs, whether made from metal or composites, the researchers say. Because the structure, comprising thousands of tiny triangles of matchstick-like struts, is composed mostly of empty space, it forms a mechanical “metamaterial” that combines the structural stiffness of a rubber-like polymer and the extreme lightness and low density of an aerogel. The resulting lattice, he says, has a density of 5.6 kilograms per cubic meter. By way of comparison, rubber has a density of about 1,500 kilograms per cubic meter. “They have the same stiffness, but ours has less than roughly one-thousandth of the density,” Jenett says.
While it would be possible to include motors and cables to produce the forces needed to deform the wings, the team has taken this a step further and designed a system that automatically responds to changes in its aerodynamic loading conditions by shifting its shape — a sort of self-adjusting, passive wing-reconfiguration process. “We’re able to gain efficiency by matching the shape to the loads at different angles of attack,” says Cramer, the paper’s lead author. “We’re able to produce the exact same behavior you would do actively, but we did it passively.” This is all accomplished by the careful design of the relative positions of struts with different amounts of flexibility or stiffness, designed so that the wing, or sections of it, bend in specific ways in response to particular kinds of stresses.
They have also developed an efficient process to manufacture the wing, which is comparable in size to the wing of a real single-seater planec . The new system uses injection molding with polyethylene resin in a complex 3-D mold, and produces each part — essentially a hollow cube made up of matchstick-size struts along each edge — in just 17 seconds, he says, which brings it a long way closer to scalable production levels. “Now we have a manufacturing method,” he says. While there’s an upfront investment in tooling, once that’s done, “the parts are cheap,” he says. “We have boxes and boxes of them, all the same.”
Studies have shown that an integrated body and wing structure could be far more efficient for many applications, he says, and with this system those could be easily built, tested, modified, and retested. “The research shows promise for reducing cost and increasing the performance for large, light weight, stiff structures,” says Daniel Campbell, a structures researcher at Aurora Flight Sciences, a Boeing company, who was not involved in this research. “Most promising near-term applications are structural applications for airships and space-based structures, such as antennas.”
The same system could be used to make other structures as well, Jenett says, including the wing-like blades of wind turbines, where the ability to do on-site assembly could avoid the problems of transporting ever-longer blades. Similar assemblies are being developed to build space structures, and could eventually be useful for bridges and other high performance structures.
Chinese Scientists Developing Bee-Inspired Spacecraft Capable of Changing Shape
The Chinese Academy of Launch Vehicle Technology (CALT) has announced the development of an aerospace vehicle capable of morphing its shape during flight. According to the announcement posted on the academy’s official website, the body of a bee was the prototype for the creation of the spacecraft. The morphing spacecraft is expected to reduce resistance during flight and be more fuel-efficient, CALT said. Designer Hu Guotun, said that, “We have drawn inspiration from the bee’s abdomen structure, which allows the bee to flex freely and control the direction of flight.”
According to him, during a space flight, the aircraft twice crosses through the atmosphere of Earth. “During the second transition, the spacecraft moves for a while due to inertia,” the scientist continued, “modification of the shape and reduction of resistance in this way is of great importance for saving fuel.” The report also says that on the basis of this concept, experts have already developed simulation, allowing them to discover that the morphing nose cone of the craft can reduce aerodynamic drag by more than 20 percent.Hu noted that such a positive result “is of great importance not only for the development of the aerospace industry, but also for the future development in the commercial market of aerospace vehicles.”
Morphing for Stealth
Smart Airfoil, a Collaborative Highly Interdisciplinary Research Project (CHIRP) at ETH:
The CHIRP “Smart Airfoil” project focused on the creation of adaptive structural systems, and successfully led to the design, wind tunnel test, and flight demonstration of a variable camber morphing UAV wing.
This interdisciplinary research project focuses on assessing the potential of wing morphing in improving the characteristics of airplanes. Conventional rigid-wing airplane designs are the result of a tradeoff between different requirements arising from diverse operating points within their typical mission. Morphing wings have the potential of adapting to different flight conditions in an optimal way (e.g. minimal drag at each operating point). The research carried out at the Automatic Control Lab as part of this project consists of parameter identification from free flight data, closed-loop control of Macro Fiber Piezo actuators, attitude stabilization for a flying wing and aims at showcasing closed-loop control of the span-wise lift-distribution on a 3m-span flying wing prototype with smart actuators integrated in a selectively compliant lifting surface.
Active Morphing Winglets
Aerospace composites specialist FACC (Ried im Innkreis, Austria) has developed Active Morphing Winglets technology. The winglets are designed to adapt automatically to changing flight conditions, switching from vertical to horizontal position, offering potential fuel savings of 2.5% and noise reduction of 2 db.The innovation features a control flap that adjusts itself in real time to suit the current conditions. FACC says this also ensures optimal aerodynamics for the fuel-intensive take-off and landing procedures, and helps reduce noise and pollution emissions.
In cruising flight, the aircraft is further stabilized in crosswinds and gusts. A freely warping (morphing) gap covering covers the gap produced when the control flap emerges and ensures aerodynamically optimized geometries in every setting. This variability of the wing geometry compensates for the additional load caused by winglets in the central and outer wing structure, making structural reinforcement of the wing unnecessary when winglets are added. Control unit, sensors, and actuators are accommodated in the smallest of spaces.
DARPA morphing Aircraft Structures program
It is one thing to draw a morphing wing design or to calculate shape changing wing performance. It is quite another to conceive, design, build and operate shape changing designs, particularly when the geometrical shape changes are large. In January 2003 the Defense Advanced Research Projects Agency, DARPA, began a 2 ½ year program whose objective was to design and build active, variable-geometry,
wing structures with the ability to change wing shape and wing area substantially.
U.S. military aircraft may one day mimic the Hollywood special effects of Batman Begins with wings that change from pliable to rigid and back again or that expand and contract on demand. Two approaches for morphing aircraft structures are being considered that would give the armed forces the ability to use the same airplane in multiple roles, from slow-flying reconnaissance missions to high-speed target takedowns. Several enabling technologies are facilitating the development of this capability; however, determining how such aircraft would meet military requirements still remains to be done, said Dr. Terrence A. Weisshaar , the program manager of MAS.
The MAS effort was an extension of activities that began more than a decade ago with DARPA’s development of smart materials and devices; this effort was led by Dr. Robert Crowe, then a Program Manager in the DARPA/Defense Sciences Office (DSO). He followed this with demonstration projects such as the Smart Wing Program, SAMPSON (an advanced inlet morphing program), and the Smart Rotor
Program. The Compact Hybrid Actuator Program (CHAP) was developed by Dr. Ephrahim Garcia during his tenure as a DARPA Program Manager.
Weisshaar explains that creating a truly morphing aircraft requires that changes can be made in the wing area, span, sweep and thickness. While a minimum of wing area is needed to fly at speeds of Mach 2 or Mach 3, a larger wing is necessary for landing. “So you already have a problem. If you want to go really fast, then you can’t land very efficiently. What we said is ‘let’s change the wing area,’ and it turns out that [an increase of] 50 percent is a pretty good number. If you could change the wing area in flight by 50 percent, you can do lots of different things. You can operate efficiently at high speeds and at low speeds,” he relates.
Flying high and slow—an appealing capability for reconnaissance missions—requires a wide wingspan in addition to a larger wing area, so increasing the wingspan by 50 percent to 75 percent also multiplies applications. “The aircraft could hang out at high altitudes, but it could still respond supersonically or at least high subsonically to a threat. This is a big deal for the military,” Weisshaar notes. “Our goals in this morphing program were to get big area changes, as big as 50 percent; big span changes—50 percent to 75 percent—and then also sweep changes because that is something that you want for high speed.”
The ability to change aircraft form and function was the goal of DARPA’s Morphing Aircraft Structures (MAS) program, managed by DARPA’s Defense Sciences Office (DSO).
In January 2003 the Defense Advanced Research Projects Agency, DARPA, began a 2 ½ year program whose objective was to design and build active, variable-geometry, wing structures with the ability to change wing shape and wing area substantially.
The MAS program had two primary technical goals:
1) To develop active wing structures that change shape to provide a wide range of aerodynamic performance and flight control not possible with conventional wings.
2) To enable development of air vehicle systems with fleet operational effectiveness not possible with conventional aircraft. This includes both Navy and Air Force operations.
Three contractors–Lockheed-Martin (Palmdale, California), Hypercomp/NextGen (Torrance, California) and Raytheon Missile Systems (Tucson, Arizona)–were competitively selected in April 2002 for a 12- month Phase 1 effort to develop wing morphing concepts that would lead to radically different, transitionable air vehicles. All three MAS contractors adopted a systems approach and conducted a functional analysis that concluded that changing wing planform area (to allow a wide range of wing loadings in flight) and wing span were the primary enablers of a new class of morphing air vehicles.
The MAS missions considered by the three contractors were structured to provide general system performance for:
• Responsiveness – time critical deployment with the ability to respond to unpredictable crisis situations
• Agility – the ability to attack fast moving air and ground targets
• Persistence – the ability to dominate large operational areas for long time periods
Each of the three contractors used a systems level approach to define combinations of range and loiter requirements to produce a system of Hunter-Killer Unmanned Air Vehicles (UAV’s) or cruise missiles with range/loiter capability far greater than current or planned systems.
Palmdale, California-based Lockheed Martin Aeronautics Company’s Advanced Development Programs section, also known as Skunk Works, designed an aircraft wing that can fold and be locked in two positions. The structural material, called a shape memory polymer, becomes pliable when stimulated with a moderate amount of current, then returns to a solid state when the stimulus is terminated.
Engineers at NextGen Aeronautics Incorporated, Torrance, California, took a different tack. Their in-plane morphing approach increases the wing surface by using innovative materials put together in a proprietary way, multiple actuators and a computer control system that affect the shape shifting. The wings can expand and contract to multiple positions.
Raytheon Missile Systems, Tucson, Arizona, chose to design morphing wings for cruise missiles rather than for manned or unmanned aircraft. One possible application would be in submarines where the missiles would be stored in tubes with folded wings that would be extended once in flight.
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