by Marcos Kuhns
Aircraft design has taken huge steps since the time of the Wright brothers' first flight. Modern airplanes range from massive 747 and personnel carriers to sleek, supersonic fighter jets. Yet, all modern planes use the same basic wing design when it comes to providing lift and maneuverability. Now, researchers are trying to perfect new methods of improving the performance and efficiency of airplanes using several new technologies that promise to allow the airplane to come one step closer to flying as gracefully as the birds that inspired their creation.
The foundation of an airplane wing is the airfoil shape which is rounded on top and nearly flat on the bottom. Based on the properties of fluid dynamics, as air passes over the wing's rounded top the air accelerates, creating an area of lower pressure. This low pressure creates lift as the wing is sucked upwards, and once the force provided by lift is great enough to overcome the force of gravity the airplane can fly. In order to control the mobility of the airplane numerous control surfaces and stabilizers are used. Most modern airplanes have vertical and horizontal stabilizers located near the tail of the plane to maintain steady flight. Control surfaces include ailerons: located on the wings of a plane which control roll; elevators: located on the horizontal stabilizer which control pitch; and a rudder: located on the vertical support which provides yaw or side to side movement (Benson). Aside from lift and gravity, the other two major forces effecting an airplane's flight are trust and drag. Thrust is the force provided by the engines, pushing the plane forward, and drag is the force of air resistance decelerating the plane. On an airplane, drag results from several major factors including skin friction: the friction between molecules of air and the airplanes surface, form drag which is fluid resistance of the airplane moving through the air; and induced drag where airflow near the tips of the wings is distorted causing swirling vortexes of air to form near the wing tips causing further drag (Benson). Researchers are now realizing that the current system of control surfaces and stabilizers is actually relatively inefficient in its lift to drag ratio and are now looking for new ways to improve the efficiency of airplanes by eliminating some of these elements of the airplane. One example of a successful modernization of airplane design is NASA's X-36 unmanned jet (Zorpette). Unlike most planes, the X-36 has no tail to provide vertical stability. Instead, this plane uses a trust vectoring, where the trust from the airplane's jet engine can be directed in different directions, to provide stability in flight. This is an important improvement because in a conventional jet fighter as much as 30 to 40 percent of the total drag comes from the tail (Zorpette).
When designing the first airplane, the Wright brothers spent lots of time observing birds for inspiration and modern scientists are again beginning to base much of their research in airplane wing design on the shapes of bird's wings. Birds have an amazing amount of maneuverability when flying and even the most common bird can put a man made fighter jet to shame in its mobility and flexibility. Barry Lazos who works as a fluid aerodynamicist for NASA has been studying birds' flight for years and is still unable to determine how, exactly, birds perform their aerial feats (Hoffman). For example, a common seagull flaps it's wings to make shaper turns, but during steady flight it uses almost imperceptible movements in its wing tips to change direction. In addition, a seagull makes no use of its tail as a rudder during steady flight (Hoffman). Ultimately, researchers at NASA hope to discover some of the secrets that allow birds to fly with such efficiency and maneuverability and apply similar techniques to future aircraft designs. Currently researchers must tackle the enormous task of making an efficient wing one piece at a time.
Barry Lazos is specifically researching the shapes of wings in nature in an attempt to find the most efficient wing shape. Up to this point, most aircraft designers haven't attempted to research more natural wing shapes because they haven't had the ability to make models of the complex shapes for testing in wind tunnels. Now, however, Lazos uses a technology called stereo lithography which is able to sculpt virtually any 3-D shape using computer guided lasers to form the objects out of a polyurethane resin (Hoffman). Lazos designs some of his wings based almost directly from nature and some as hybrids between various animals and airplane wings. Among the models he's tested are seagull wings, shark fins, and a hybrid known as the hyper elliptic, cambered span (Hoffman). In Lazos' studies he found that the a wing with the hyper elliptic shape has a 15% improvement in its lift to drag ratio as compared with a normal airplane wing. Unfortunately, these nature inspired designs are too complex for current computer simulations of fluid dynamics so Lazos in unable to say for certain why this particular shape is so efficient (Hoffman).
Wing shape isn't the only difference between the wings of a bird and those of an aircraft. Another division of NASA is looking at the way birds use their feathers to control the flow of air over their wings. Anna McGowan, the manager of the program, explains that unlike airplane wings which are designed to be as smooth as possible, a bird's wings are covered with feathers which create "flow unsteadiness" over the wing. To achieve flow unsteadiness similar to that caused by feathers, NASA developed micro jets which are tiny flaps of metal that when activated beat up and down, creating tiny air swirls (Hoffman). If several hundred or thousand of these micro jets are attached to airplane wings then they could theoretically be used to improve the aerodynamics of the wing. Wing aerodynamics are crucial because as McGowan puts it, "If you can reduce the drag by 3 percent, you can reduce the amount of trust needed by 49 percent." NASA has even performed a set of wind tunnel test, proving the effectiveness of current micro jets on improving wing aerodynamics (Hoffman). Looking even further into the future, McGowan envisions having thousands of individually controlled micro jets as well as shape changing bumps or blisters which would dynamically change the shape of the wing to manage airflow and increase lift regardless of the airplane's speed. Before this dream can become a reality, however, NASA must first design a system of controlling these blisters (Hoffman). That's where Mark Motter comes in. Motter works for NASA's Small Unmanned Aerial Vehicle Lab and is currently testing different methods by which a computer system could analyze data from sensors on board an airplane to optimize its maneuverability. Currently, Motter is experimenting with a small unmanned aircraft whose ailerons have been divided into ten segments, each of which collect force data and can be adjusted individually. Though ten segments may seem like small amount of data for a computer to analyze keep in mind that the adjustments to the ailerons segments based on this data must be made very quickly in order for the segmentation to improve the wing's aerodynamics. Motter hopes to increase the number of segments to 50 or 100 by the end of the year (Hoffman). If his systems are successful, they will eventually be responsible for collecting data from tens of thousands of blisters and pulse jets covering an airplane and using that data to make the minor shape adjustments necessary for optimum aerodynamic efficiency, just as a bird feels the wind flow over its wings and adjusts a its feathers to fly efficiently (Hoffman).
The Wright brothers' first flier didn't have ailerons or flaps. Instead the Wright brothers used a design which more closely resembles a bird's flight controls. To turn, the pilot would shift in a harness which would twist or warp one of the wings, turning the craft (Wilson). However, as airplanes evolved most engineers began viewing a wing's flexibility as a weakness. For example, when the F-18 was first being designed researchers discovered that excessive twist in the original wing design reduced the ability of the ailerons to roll the jet at the required rate. So engineers added more supports to stiffen the wings and added control surfaces on the tail to assist in turning (Wilson). Now, NASA and Boeing Phantom Works are beginning a project in an attempt to put the flexibility of airplane wings to use by creating what they dubbed the active aeroelastic wing (AAW). They began with one of the original "overly flexible" F-18 wings and modified it so that they the outer and inner portions of the leading edge flaps and ailerons could be moved separately. The flaps on the tail can also be disabled in flight to prove that the plane can be successfully maneuvered using only control surfaces on its wings. Finally, they attached several hundred sensor throughout the wings so that they could constantly monitor the stress on different areas of the wings to ensure that twist doesn't over-strain the wings (Wilson). While in flight the on board computers have been programed to use the outside leading edge flaps to create forces which twist the wings opposite the direction normal wings naturally twist during a turn. Twisting the wings in the opposite direction allows the ailerons to work more effectively while still maintaining a manageable amount of stress on the wings (Wilson). Because AAW wings don't have to be engineered to be super-stiff they can be lighter, reducing the gross weight of fighter jets by 7 to 10 percent while maintaining flight performance. NASA hopes that their results from this experiment will lead to more successful research into the design of morphing wings (Wilson).
Morphing wings are another modification to standard airplanes which promises impressive future improvement in aircraft design. As I mentioned earlier, traditional wings make use of numerous control surfaces to maneuver successfully. These control surfaces, however, are actually quite inefficient when it comes to the amount of drag they produce and, in military applications, the fact that they reduce the stealth of an aircraft (Wall). A morphing wing could change its shape without use of flaps, ailerons, or other standard control surfaces. The United States Defense Advanced Research Projects Agency (Darpa) is currently funding research into several "smart materials" which might be used to morph an aircraft. These candidate materials include piezoelectric materials which change shape when an electric charge is applied, shape-memory alloys which respond to changes in temperature, and magnetostrictive materials which are controlled using magnetic fields (Black). Currently Darpa is focusing on designs for morphing wings to control maneuverability and lift however in the future they hope to incorporate morphing materials into the entire airplane's fuselage so, for example, the intake for engines could be optimized for flight at different speeds, or the fuselage could shrink as fuel is burned (Wall). The primary obstacle faced in the integration of these smart materials into airplane wings is the amount of stress that the materials must be able to withstand. Consider, for example, the wings of an F-16 which under normal flight must support 25,000 pounds (110,000 N) of force. If, however, an F-16 performs a 9-G turn, the force on the wings grows to a whopping 250,000 pounds (1,000,000 N) (Hoffman). In the past, wings that were created to flexible and strong have ended up being too heavy for practical use. To solve this problem Sridhar Kota, president of FlexSys Inc, is working in conjunction Lockheed Martin and the Air Force to create a morphing wing using a technology known as compliant systems (FlexSys). Compliant systems are designed by a computer program which takes inputted requirements for the object and the calculates the shape, number of interconnection, and thickness of components yielding a compliant system to perform the task. Compliant systems are milled out of a single piece of solid material, be it plastic or titanium, and take advantage of the elastic strain of these materials to perform work (FlexSys). FlexSys has used compliant systems design to design a very unique prototype of a morphing wing. The shape of the wing's leading and trailing edges can changed by as much as 20° and at rates fast enough to be useful as control surfaces (FlexSys). Amazingly enough, this is all accomplished without any hinges or gaps in the wing's surface. Additionally, because of the properties of compliant systems, the wing is sturdy enough to withstand the air pressure encountered during flight (Hoffman).
While these airplane concepts are still in their infancy, as far as design and research goes, the future benefits that they might yield are enormous. According to the researchers, most of these technologies won't even show up in military applications until after another 20 to 30 years of research and development. However, once the next generation of morphing airplanes begins to appear it will be fascinating to see whether our testing has resulted in an aircraft that can swoop like a bird, or simply another generation of "clumsy" fighter planes.