LEARNING FROM BIRD WINGS

Wing morphing experiment takes flight

USAF and NASA Adaptive Compliant Trailing Edge program revisits the concepts of nature and early pioneers of flight.

By Stuart Lau
ATP/FE/CFII Boeing 747, 747-400, 757/767, CRJ and Saab 340


Borrowing a chapter from Orville and Wilbur Wright, USAF and NASA are experimenting with FlexSys wing warping system that holds promise to reduce the environmental impact of an aircraft by aerodynamically improving efficiency and performance while reducing the noise signature.

On Nov 6, 2014, NASA launched a flight test campaign to evaluate trailing edge wing surfaces that can change shape while flying. The adaptive compliant trailing edge (ACTE) program—a joint effort between NASA and the US Air Force Research Laboratory (AFRL)—explores the use of smoothly moving or morphing surfaces to mimic the flight of birds, replacing hinged flight control surfaces to make future aircraft much more fuel efficient and quieter.

ACTE is a direct result of over 15 years of collaboration between FlexSys and AFRL. In 2009, NASA became involved in the ACTE program as one of its 8 initiatives of environmentally responsible aviation (ERA) project to be integrated into civil fleets by 2025. Projections from the ERA project call for a reduction in fuel burn and associated carbon emissions by 43 to 49%.

The ACTE flight test program is based at NASA's Armstrong Research Center (Edwards AFB CA) and utilizes a Gulfstream III as the test article. Based on its patented compliant structures technology, FlexSys designed and fabricated the FlexFoil variable geometry control surfaces and shipped them to NASA Armstrong in Feb 2014.

Engineers at NASA Armstrong replaced the GIII's conventional aluminum trailing edge flaps with FlexFoil advanced shape-changing components that form seamless, bendable and twistable surfaces. The new flap system includes 18-ft span-wise FlexFoil sections and 2-ft wide compliant fairings at each end to provide a smooth surface, eliminating the noise-generating gaps in the airframe.

Following the success of the 1st test flight, NASA ACTE Project Manager Thomas Rigney said, "We validated many key elements of the experimental trailing edges. We expect this technology to make future aircraft lighter and quieter. It also has the potential of saving hundreds of millions of dollars annually in fuel costs."

According to AFRL Program Manager Peter Flick, "ACTE can undergo large deflections at high rates. This enables the ACTE to serve as a multifunctional surface used for vehicle control, load alleviation, optimum trim during cruise and high lift conditions such as takeoff and landing. ACTE enables drag reduction throughout the envelope, reduces critical structural design loads through maneuver load control and/or gust load alleviation, and will enable the aerodynamic efficiencies associated with higher aspect ratio wings to be realized without paying a high structural weight penalty."

ACTE promises to solve complex aerodynamic issues by altering the camber of the trailing edge of the wing in all phases of flight. Future projects could focus on the leading edge to provide additional benefits. Interestingly, the concept of morphing a wing is not new. Throughout the history of flight, pioneers looked to the sky and watched the birds to solve basic issues such as controlled flight.

Back to the future

FlexSys FlexFoil trailing edge flap system optimizes the wing for any given phase of flight by bending and twisting the airfoil. Benefits reported include fuel savings of 3 to 5% on retrofit applications and up to 12% on new aircraft designs.

Historian Tom Crouch wrote a book on the Wright Brothers published by The National Geographic Society. According to Crouch, over 110 years ago the Wright Brothers believed sufficient solutions existed for basic wing and engine design, but controlled flight was the greatest challenge.

At the time, lateral control was a subject of debate and there were many different theories. Wilbur Wright concluded—on the basis of his observations—that birds changed the angle of the ends of their wings to roll right or left.

During their experiments in 1902, the Wright Brothers successfully achieved in simultaneous control of a glider in all 3 axes of flight. Lateral control was accomplished by changing the shape of the wing through "wing warping." On December 17, 1903, the concept of lateral roll control through wing warping and coordinated pitch and yaw led to the world's first successful example of powered manned flight. Orville's first flight covered 120 ft—about 10 yards greater than the length of a GIII—at a height of less than 10 ft.

Like Wilbur Wright and others, FlexSys Founder & CEO Dr Sridhar Kota, a mechanical engineer and inventor of FlexFoil, has long had an appreciation for nature. According to Dr. Kota, "Nature is filled with strong, elegant, shape-shifting mechanisms." In nature, examples of designs that are flexible and yet strong include the wings of birds, the trunk of an elephant or tree branches. By contrast, in human technology most artifacts that are strong are also rigid and stiff with multiple parts and complex assemblies.

As an example, aircraft flaps must morph to change their camber and twist depending on the phase of flight. So by design, traditional flap systems are a complex mix of rigid parts such as hinges, tracks, actuators, panels and fairings, and often these complex systems are inefficient. While current aircraft flap systems are an excellent source of lift at slow speeds during takeoff and landing, they create undesirable drag and noise.

Dr Kota, who is the Herrick professor of engineering at the University of Michigan, has a decades-long obsession with jointless mechanisms, compliant mechanisms and microelectromechanical (MEMS) systems. A keen interest for adaptive structures led him to research mechanisms without joints that could change shapes, exploring opportunities for improvement. Recalling the NACA airfoil profiles from earlier studies, Kota began his journey to improve aircraft flight control systems.

The FlexFoil variable geometry surface uses the natural flexibility of existing aerospace-grade materials to continuously reshape its external form. The jointless skeletal configuration of the system is optimized using FlexSys' proprietary design tools to achieve flexibility without sacrificing the strength required to support thousands of pounds of flight loads.

Driven by one or more actuators, the flexible structure is designed to achieve desired aerodynamic shapes on demand. Each section of the flexible structural system shares the load and undergoes the specified deformation without overstressing any part of it. This concept of distributed compliance, pioneered by Kota in the 1990s, enables large deformations with very low stresses so that the system can be cycled thousands of times without failure—meaning it achieves a high fatigue life.

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