This article is a study of morphing aircrafts, which has attracted many research groups around the globe. The unmanned aerial vehicles (UAV) have some attractive features for aeronautic research. An important factor in morphing systems is the scale of the air vehicle on which they will be incorporated. With the development of more accurate analysis tools and advanced smart materials, researchers are once again investigating compliant morphing aircraft to improve aircraft performance. Such an aircraft would have the potential to adapt and optimize their shape to improve flight performance or to achieve multi-objective mission roles. Low aspect ratio wings provide more manoeuvrability and allow for high flight speeds, but at the cost of efficiency. An aircraft can be fast or efficient, but not both. Compliant control surfaces also lack the discontinuities found in hinged mechanisms, and thus have the potential to reduce drag and noise significantly. Compliant structures are promising solutions because of their low weight and maintenance costs. Large-scale morphing on commercial aircraft may not be practical in the near term. But the application of morphing to secondary structures, such as a compliant control surface, is a realistic goal.
It is one of the world's most famous aircraft, but the 1903 Wright Flyer looks little like the airplanes that we have come to know. It's a biplane, of course, with a canard wing set in front, pusher propellers in back, and skids for landing gear. But it's the wing edges that are both subtly and radically different from what is found on modern planes. Instead of rigid surfaces and discrete aileron flaps, the wing is flexible; the entire surface alters its profile to effect aerodynamic changes.
Within a few years of the Wright brothers’ first flight, those flexible surfaces had disappeared. Due to the need for larger structural rigidity as higher airspeeds were achieved, any form of compliance or flexibility—what we would now call “morphing,” though that term was unknown to the Wrights—was ruled out. By the First World War, little more than a decade removed from the first flight at Kitty Hawk, ailerons were the norm.
The idea of aircraft that change shape to meet changing flying conditions and performance needs never went away completely. It is, after all, the solution found in nature in the form of birds’ wings, and the concept would alleviate a problem in conventional fixed-wing aircraft, which can only be optimized for a limited region of the flight envelope.
With the development of more accurate analysis tools and advanced smart materials, researchers are once again investigating compliant morphing aircraft to improve aircraft performance. Such aircraft would have the potential to adapt and optimize their shape to improve flight performance or to achieve multi-objective mission roles.
In spite of this interest, the technology to accomplish this has rarely been adopted on production aircraft. What is standing in the way?
One can tell a lot about an aircraft by just looking at it. Planes with high aspect ratio wings—that is, wings that are long and narrow—typically have a good range and fuel efficiency, but are slower and less maneuverable. Low aspect ratio wings provide more maneuverability and allow for high flight speeds, but at the cost of efficiency. An aircraft can be fast or efficient, but not both.
Civil and military aircraft are designed to have aerodynamic characteristics that are optimal 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. The hinges and other junctions usually create discontinuities in the surface, resulting in increased drag.
Nature has solved this problem. Biologists have observed birds of prey such as the falcon in flight. The birds are able to loiter above an area in a high aspect ratio configuration, using air currents and thermals to stay aloft. Then, once prey is sighted, the bird changes its profile to strike configuration, pinning its wings back for high speed and great maneuverability. To attack, falcons metamorphose from one flight mode to another.
Among aircraft designers, the word morphing, which is derived from metamorphosis, describes a change in state that's achieved via combinations of advanced materials, actuators, flow controllers, and mechanisms that enable an aircraft to adapt to changing mission environments in mid-flight. One can make the case that established technologies such as flaps or retractable landing gear are a type of morphing—after all, the shape of the aircraft does change when the ailerons move. And a handful of supersonic military aircraft have featured variable-sweep wings that rigidly pivot around a hinge to allow for better performance at take-off and landing.
However, morphing carries the connotation of radical shape changes or shape changes that are only possible with near-term or futuristic technologies. There is neither an exact definition nor an agreement among researchers about the type or the extent of the geometrical changes necessary to qualify an aircraft for the title “shape morphing.”
For instance, the shape of the wing could alter in mid-flight for different parts of the mission, such as high-altitude cruising or landing. The wings might change in plan—span, sweep and chord—or they could bend, twist, or develop a crook like a gull wing, or they could thicken or flatten in cross-section.
Such an adaptable wing would enable 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.
Compliant control surfaces also lack the discontinuities found in hinged mechanisms, and thus have the potential to reduce drag and noise significantly.
The promise of morphing aircraft has attracted the attention of hundreds of research groups worldwide. Since the 1980s, there have been programs in the United States and Europe with names such as Mission Adaptive Wing, Morphing Aircraft Structures, Active Aeroelastic Aircraft Structures, and Smart Intelligent Aircraft Structures, among others.
Sub-optimal performance has more significance for commercial aircraft as they are more flexible than military aircraft and fuel efficiency has far greater importance.
Wind tunnel testing of the FishBAC prototype found it improved the lift-to-drag ratio by 25 percent, compared to traditional trailing edge flaps.
Part of the research interest is due to a technological push: Many 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.
There's also been an application pull, driven by both the demand for greener aircraft and the mission requirements of post-Cold War military aircraft. Military targets today are more distributed and are smaller, but the proliferation of sophisticated air defenses means 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.
But for all the research in the United States and the European Union, very few of the interesting concepts that have been developed have made it to wind tunnel testing, and fewer still have flown. Many of the morphing concepts are not technologically ready, and up to now they have proven to be heavier, more complex, and more costly than conventional mechanisms.
Underlying the surface of an aircraft with distributed actuators, for instance, requires not just a flexible skin robust enough to provide structural integrity under challenging flight conditions, but also a constant supply of power to operate the active mechanisms.
If the complexity of morphing aircraft is a function of scale, then maybe the technology can be tested and perfected in very small planes—ones that are too small to hold a pilot. Such craft are known as unmanned aerial vehicles, or more commonly, UAVs. The explosive growth of satellite services during the past few years has made UAVs the technology of choice for many routine applications, ranging from border patrol and military operations to environmental monitoring, meteorology, and search and rescue.
UAVs also have some attractive features for aeronautic research: they have lower production costs, lower safety and certification requirements, and lower aerodynamic loads. And if they crash, the pilots are not in any danger. As a result, many wing morphing investigations now are focused toward small or radio-controlled aircraft, and UAVs offer 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.
Many morphing concepts have been developed for UAVs with 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.
Potential improvement in air range for planes employing morphing winglets.
An important factor in morphing systems is the scale of the air vehicle on which they will be incorporated. Take, for instance, compliant structures: These usually employ flexible skins (anything from corrugated surfaces to fiber reinforced elastomer) to maintain the aerodynamic shape of the wing before, during, and after morphing.
Compliant structures are promising solutions because of their low weight and maintenance costs. 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.
As such, 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 and weight of the vehicle—and hence the aerodynamic loads—increase, it becomes prohibitive to employ compliant structures due to their relatively low stiffness and strength. It's unlikely we’ll see a large aircraft with compliant primary structures anytime soon.
What morphing concepts might work in large aircraft? We’ve already seen variable sweep wings in supersonic military aircraftbut the weight penalty in these planes was unacceptably large. The next-generation of military vehicles cannot sustain this tradeoff, not if the planes are to meet the levels of performance required of them. And the current trend for highly efficient and green aircraft, as well as the degree of wing variations needed to produce a practical aerodynamic change in small or low-speed vehicles, makes the weight penalty even more unacceptable.
Instead of focusing on significant changes to the primary structure, the emphasis for development should be on secondary structures that can still significantly affect performance. Two possible structures that could see morphing technology are control surfaces 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 take-off and landing to generate very high lift coefficients. To date, the primary means of realizing variable camber has been through the use of discrete trailing edge flaps such as ailerons.
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 improves aerodynamic efficiency. However, one disadvantage of a 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 fixed and optimized for a single point on the flight envelope. A morphing winglet that allows the outboard wing sections to cant and twist during flight could provide as much as a 5 percent improvement in specific air range.
In addition, morphing concepts that have proved impractical for wings could be used for horizontal and vertical stabilizers, rotary wing aircraft blades, and even wind turbine blades. Morphing may also be useful in other areas of an aircraft, such as the fuselage, various fairings, landing gear, and the propulsion system. For instance, adaptive propulsion systems could morph the air intake to improve efficiency or add variable geometry chevrons to reduce noise, both of which are desirable goals.
As an idea, morphing aircraft seems futuristic, but the actual future of the concept is uncertain. 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. But integrating morphing mechanisms onto an aircraft's wings has proved to be more complex than anticipated, and the potential benefits have not exceeded the penalties of this complexity.
UAVs have lower costs, lower safety requirements,and lower aerodynamic loads.
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 and other 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.
Large-scale morphing on commercial aircraft may not be practical in the near term. But the application of morphing to secondary structures, such as a compliant control surface, is a realistic goal.
If we are to see morphing any time soon, it will be on smaller unmanned aircraft or missiles, where current or near-term technology can be applied. The full-scale wing-warping designs of the Wright Flyer will remain in the history books, but 21stcentury UAVs may well nod in the direction of the first manned aircraft.