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Test time for seamless wings - full story

Test Time for Seamless Wings

By Erik Schechter, Aerospace America
April 2014

Seamless Wing

Long hiatus: A flexible wing segment was tested in 2006 by hanging it from Scaled Composites’ White Knight plane. FlexSys plans to resume flights in July by attaching flexible flaps to a NASA Gulfstream 3.

Engineers at NASA’s Armstrong Flight Research Center in California are in the process of replacing the standard mechanical flaps on a Gulfstream 3 business jet with jointless, seamless flaps for a series of flights starting in July.

The flaps were created by the 12- person company FlexSys of Ann Arbor, Mich., using a technology it calls FlexFoil. The flaps will be tested under an Air Force-NASA initiative called the Adaptive Compliant Trailing Edge project, or ACTE.

Proponents say the technology has enormous potential, and not just as add-ons like in the Gulfstream tests. The technology could be incorporated into wing designs from the start to create flexible leading and trailing edges, and it could be used in rotor blades and even windmills.

“If I had to put [its significance] on a scale of 1 to 10, it’s probably an 8 or a 9,” says Fayette Collier, an aerospace engineer who runs the NASA Environmentally Responsible Aviation Project, which is funding the test flights with the Air Force.

Unlike traditional wings, which are rigid affairs with hinged flaps and lots of drag, a wing with FlexFoil flaps will be a seamless structure that can quickly change shape in mid-flight.

Inside each FlexFoil structure is a flexible, jointless lattice made from “an aerospace-grade material” that FlexSys won’t identify for fear of running afoul of the U.S. International Traffic in Arms Regulations. Hydraulic actuators push and pull this compliant lattice to morph the structure into the shape required by flight conditions. A few small strains on the mechanism can bend the wing’s flaps and trailing edge -9 to 40 degrees, according to FlexSys. On each wing of the Gulfstream, a FlexFoil will be connected at the rear wing spar.

The compliant structure distributes stresses through the structure, unlike the focused stress one sees in a “lumped compliance” design like the plastic hinge of a shampoo bottle top, explains FlexSys CEO and founder Sridhar Kota, a professor of mechanical engineering at the University of Michigan. Distributing stress is a key to making the FlexFoil tough, he says.

During product development, company engineers subjected the flexible wing to twice the standard number of cycles a mechanical wing would normally see in its lifetime and at twice the expected load, that is, at 24,000 pounds. The FlexFoil wing was also exposed to harsh chemicals, ultraviolet light and temperatures ranging from -60 to 160 degrees Fahrenheit. “We checked all the boxes,” Kota says.

Elusive goal
Aerospace engineers have long sought to develop shape-changing wings, but success has been elusive. In the 1980s, Air Force Flight Dynamics Laboratory researchers flew a Mission Adaptive Wing on an F-111 Aardvark. This experimental wing proved aerodynamically superior to regular wings, but it relied on a complex set of gears and links that added weight to the plane and made maintenance difficult.

Likewise, in the mid-to-late 1990s, a Northrop Grumman team used piezoelectric motors — which affect tension when an electric current is applied — along with shape memory alloy tubes to twist a wing. This worked to a certain extent, but the technology was not suited for a wide range of motion. “You need a number of such piezo actuators to effectuate a large deformation like the kind you’re looking for in an aircraft wing,” Kota notes, adding that the shape memory material lacked stiffness.



Flight tests in 2006 showed a near constant drag coefficient for a test article flown on the Scaled Composites White Knight plane. With conventional planes, the drag coefficient increases the farther the flap deflects from zero degrees.

There are other wing design efforts as well — for instance, the X-56A Multi-utility Aeroelastic Demonstration. But the X-56A is aimed at testing a technique for actively suppressing wing vibrations called flutter. “FlexFoil is the world’s first seamless, hinge-free wing whose edges morph on demand to adapt to different flight conditions,” says Thomas Rigney, a NASA mechanical engineer and manager of the ACTE project. “It’s a real gamechanger technology.”

If validated in upcoming flight tests, FlexFoil could save operators of transport aircraft hundreds of millions of dollars a year in reduced fuel burn, says mechanical engineer Don Paul, former chief scientist in the Air Force Research Lab’s Air Vehicles Directorate in Ohio and now a consultant for FlexSys. Although it would be pricey to replace old flaps, those who do so would earn back their money within three years, he predicts. Second-tier airlines flying older jets would see an even greater return on their investment, because “old wings were not designed as efficiently as the new wings,” Paul adds.

FlexFoil also could reduce noise levels during landings by as much as 40 percent, because there would be no gap between wing and flap. Kota says that would be particularly important as people build homes closer and closer to airports.

From drawing board to runway
Kota began looking at compliant structures in the early 1990s while working in what was then the new field of micro-electro-mechanical systems, or MEMS. He was addressing an optical switching application for telecommunications, and he needed a very thin structure — only a few microns wide — to lift a microscale mirror off a plane and tilt it in three dimensions. After much trial and error, he developed a one-piece deformable structure that worked better than a jointed mechanism.

Not long afterward, he realized that his compliant structure could be scaled up to the macro level, and he began thinking about airfoils and how at high speed, a little change in shape goes a long way. However, Kota sat on his idea for more than a year, unsure if anyone would be interested in it. Then in 1994, after reading an article about the Mission Adaptive Wing, he finally placed a call to the Air Force Research Lab.

It turned out that the Air Force scientists were very interested in Kota’s idea and called him in to expound on it. “I had a great meeting. They said, ‘Wow! This is great. It’s a refreshingly different idea than what we’ve been working on,’” he recalls.

A couple of years later, the Air Force lab awarded Kota a Small Business Innovation Research grant. In 2001, he formed FlexSys. With backing from the Air Force, company engineers developed a 3-foot-long test article whose performance they tested in wind tunnels at the University of Michigan and Ohio State University. Then, in 2005, they and Air Force lab researchers conducted more advanced wind tunnel tests at the Subsonic Aerodynamic Research Laboratory at Wright-Patterson, says Peter Flick, an aerospace engineer and the Air Force lab’s program manager for supporting FlexSys.

The grant and the wind tunnel tests culminated in multiple flight tests in 2006. The team affixed a 36-inch span of FlexFoil to the bottom of the Scaled Composites Model 318 White Knight test aircraft and tested the compliant structure under more realistic temperature and flight conditions above the Mojave Desert. The test item was able to morph from -10 to 10 degrees. “We were very happy [with the results], but we realized that that was not enough to transition the technology to a real aircraft application, because it was relatively small scale and it wasn’t a critical surface on the aircraft,” Flick says.

A long series of flight tests followed, and in 2009 the team received a Small Business Innovation Research- 3 grant for the activity. The next step will be the ACTE flight test, which will have a modified Gulfstream 3 fly with two FlexFoil-equipped wings at NASA Armstrong. This will determine whether computational fluid dynamics models predicting 5 percent to 12 percent fuel savings will bear out. In general, program officials are optimistic about ACTE, but they are also not taking any chances. They chose the business jet as a test bed because, besides having large flaps and being capable of transonic flight, “the aircraft was capable of taking off and landing without the flap deployed,” Flick says.

Flight testing is set to begin in July and continue through February of next year. In the meantime, test pilots are undergoing training in a simulator at Armstrong in order to get used to the FlexFoil wings. NASA engineers also are putting the ACTE through ground vibration tests to make sure the wings won’t be damaged by flutter. This entails suspending a FlexFoil wing by bungee cords from horizontal beams and vibrating it to “find out what its natural frequencies are,” says NASA Armstrong’s Rigney.

As for the flight testing regime, the Gulfstream 3 wing won’t actually flex in mid-air, because building real-time actuation was deemed too expensive. Instead, officials have decided that the wing will be set at a different angle each time the jet goes up, Collier says.

Looking to the future, FlexSys and Boeing have submitted a joint proposal to the Air Force lab to retrofit a KC-135 Stratotanker with FlexFoil trailing edges to measure fuel savings. But Kota and company are also looking to bring their technology to new aircraft, a move that Paul expects will influence new wing designs. “If you tell a wing designer, ‘I got this technology that lets you droop your leading edge and droop your trailing edge any time you want,’ well, that changes the way you think about your wing,” he says.

Erik Schechter - erik.schechter@gmail.com

A publication of the American Institute of Aeronautics and Astronautics

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