Fly Like a Bird: FlexSys, Inc.’s Adaptive Compliant Trailing Edge
High over the Mojave Desert, a Gulfstream III flew with its shape-shifting wings flexed like a hawk on the hunt. When the jet smoothly touched down at the end of that final test flight, it rolled straight into aviation history.
Aircraft designers have dreamt of adaptive wings since the Wright Brothers experimented with “wing warping” in their first flying machine. Soon after that inaugural flight, hinged flaps replaced the twisting wing—and aircraft designers have been trying to find their way back to a seamless control surface ever since. Flaps provide excellent lift, but the resulting aerodynamic drag creates fuel inefficiency and noise. For decades, engineers pursued the quest by designing increasingly complicated systems with hundreds of rigid moving parts and dozens of motors. They were able to attain controlled wing flex, but the systems were too heavy and complex to be practical.
Then in 1994, Sridhar Kota, at the time an associate professor of mechanical engineering at the University of Michigan, took a wild idea to a group of engineers at Wright-Patterson Air Force Base. Instead of hundreds of parts, how about one? Instead of hardened strength, why not flexible brawn? He walked them through his ideas, sketched out on big sheets of paper. Kota’s profoundly radical approach was based on a concept he was pioneering called “distributed compliance.” Rare in human-built systems, distributed compliance is abundant in natural structures like bird wings, crab legs, and elephant trunks. Instead of gears, hinges, joints, sliders, tracks, and springs, nature’s biomechanical component parts grow out of each other or bond together with strong, self-regenerating surfaces. Kota’s idea for a morphing wing combined the natural elasticity of existing aerospace-grade materials with a jointless skeleton to achieve flexibility without sacrificing strength.
“I thought they would say, ‘Okay, thank you, good-bye,’” recalls Kota. “But they said, ‘Wow. This could work.’ It was one of the best meetings of my life.”
Kota received an Air Force Phase I SBIR to pursue the design in 1998, and recruited some help to convert his garage into a lab and fabrication workshop and get going. About a year later, he took his wind-tunnel-tested prototype back to Wright-Patterson, where the engineers were equal parts surprised and excited by the progress he had made. Kota secured a Phase II SBIR in 2000 and started FlexSys, Inc., headquartered in Ann Arbor, Michigan. Pete Flick, a senior aerospace engineer at the Air Force Research Lab (AFRL), joined as the project manager soon after. Flick collaborated closely with FlexSys throughout Phase II, providing relevant challenges to help steer the developing technology into real-world application.
“The Air Force aligned our Phase II work with specific mission-related goals,” says Kota. “They asked us to focus on the trailing edge, and consider how it might be integrated into different types of aircraft.” Each time FlexSys nailed a challenge, the Air Force set and funded the next one. By the end of Phase II in 2008, FlexSys and AFRL had successfully matured the Adaptive Compliant Trailing Edge (ACTE) technology with wind tunnel tests, structural tests, and a flight test in which a prototype wing section was attached to the underside of a White Knight aircraft. Results of the White Knight-assisted flight test were spectacular, and the ACTE was ready for its full-scale fledging.
NASA joined the team in 2009 under a Phase III collaboration to advance the flight testing stage. Recognizing the significant “green” potential of the technology, NASA incorporated the ACTE into its Environmentally Responsible Aviation (ERA) initiative, which was created to explore aircraft designs that might reduce noise, emissions, and fuel consumption. The team acquired a used Gulfstream III, replaced its existing flaps with the ACTE, and rigorous NASA-led flight testing began. In twenty-two test flights between November 2014 and April 2015, the trailing edge was subjected to speeds greater than 0.75 Mach and hard banking maneuvers up to 1.7G. The ACTE exceeded all expectations, with test pilots describing the wing as very smooth, and reporting zero concerns about its performance or material condition.
“The aviation community had been trying to achieve this capability for decades,” Flick says. “FlexSys had an excellent, innovative idea, but the SBIR program was absolutely critical to the technology development. The partnership worked extremely well, and FlexSys' passion for maturing its technology was always evident in their exceptional effort.”
“I’m grateful to the Air Force for making it all possible,” Kota says. “They had the vision to see the application for this technology from day one.”
With flight tests successfully completed, the Air Force is currently considering integrating the ACTE into its KC-135 Stratotanker fleet—which could save an estimated tens of millions of dollars in fuel costs every year. And a number of major aerospace industry suppliers have all expressed interest in working with FlexSys on a variety of applications. Retrofitting the ACTE into an existing aircraft wing can boost fuel efficiency by four percent or more, and building the ACTE into a “clean sheet” wing designed to take advantage of the technology could result in as much as a twelve percent increase in fuel efficiency. No wonder the ACTE is attracting the attention of an industry that spends over $200 billion a year on jet fuel and considers a mere one percent savings to be significant. In addition to fuel savings and the associated reduction in carbon emissions, the technology is also expected to cut airframe noise by about 40 percent. That’s some environmentally responsible aviation.
With its small but strong and flexible team of twelve employees and the full power of the Air Force SBIR program, FlexSys had captured the Holy Grail of seamless wing control surfaces and become the world leader in adaptive structures technology.
“It’s easy to get lost in the daily technical and programmatic issues associated with running a technology development program,” says Flick. But on the day of the final flight test it hit him: they had done it. After twenty years of work, they had given new wings to an old idea and sparked the next revolution in aviation design.