Since the dawn of aviation, airplane designers and engineers have sought elegant ways to smoothly change the shape of aircraft wings in flight like birds or bats do. Rather than using jointed flaps, moving slats, or other conventional flight surfaces whose individual segments leave drag-causing gaps, they envision wings that morph seamlessly into cambered shapes as needed. Unfortunately, the capability to curve wings at will has proved an elusive goal.
In the 1990s, for example, the U.S. Air Force (USAF) and NASA jointly funded development of the Advanced Fighter Technology Integration/F-111 Mission Adaptive Wing (MAW), which was flapless and gapless. Boeing, the contractor, built a smooth-surfaced, variable-camber wing that could adjust its shape in response to flight conditions and pilot input to maximize aerodynamic efficiency. An internal mechanism flexed the wing skin to produce a highly curved section for subsonic speeds, a supercritical section for transonic velocities, and a symmetrical section for supersonic speeds.
Although the MAW project yielded key design criteria for a future wing system that could maintain peak aerodynamic efficiency throughout the flight envelope, nothing approaching that has yet taken to the skies in part because the necessary structures and actuation systems tend to be heavy and complex.
That should begin to change in July when one of the technical descendants of the MAW and other R&D efforts on jointless wings is scheduled to begin initial flight tests at NASA’s Armstrong Flight Research Center in California. A seamless, variable-geometry trailing-edge control surface called FlexFoil has replaced a standard open-jointed main flap on the wing of an instrumented Gulfstream III business jet.
The Adaptive Compliant Trailing Edge (ACTE), which was developed by Flexsys Inc. of Ann Arbor, MI, is a specially designed 14-ft-long (4.3-m-long) beam that is stiff but flexible. One or two simple actuators can deflect the beam up or down into the oncoming air stream, altering the wing’s camber as needed, explained Sridhar Kota, founder and CEO of the company, and professor of mechanical engineering at the University of Michigan. The beam is made of standard aerospace materials.
The FlexFoil ACTE system results from a design approach that Kota calls distributed compliance, whereby each element along the structure’s length shares the deformation load as the whole uniformly flexes the wing surface into a curved, gap-free geometry that smoothly cuts into the wind. Unlike previous heavy, complicated prototypes, the new control surface technology is lightweight, reliable, cost-effective, and morphs fast enough to respond in real time as well. Two 2-ft-long (0.6-m-long) accordion-like transition surfaces seamlessly span the space between the ends of bending beam to the fixed wing surface, leaving no drag-boosting breeches.
“A clean-sheet wing design using FlexFoil could reduce fuel consumption by 12%,” Kota claimed, “and even a retrofit can lead to a fuel-economy improvement of 4 to 5%.” Exhaust emissions drop a similar amount. “And that is not counting significant potential benefits from being able to twist the wing to reduce span-wise induced drag or to shift the stress loads toward the root of the wing box where it is stronger.”
The lack of gaps also markedly cuts wind noise. During landing, for instance, the surface discontinuities formed by conventional flaps are responsible for 40% of the noise that an airliner produces. NASA is particularly keen to cut noise in the vicinity of airports, according to Fayette S. Collier, Project Manager of the Environmentally Responsible Aviation project at Langley Research Center in Virginia.
“Community noise is the number one impediment to building new or expanding airports,” he said. “The new technology offers a 2- to 4-dB reduction in noise, which is significant.”
Since Kota established the company in 2001 with an SBIR grant, the U.S. Air Force Research Laboratory (AFRL) in Ohio has supported development of FlexFoil technology with about $20 million in funding.
“In the mid-1990s there was a lot of interest in developing morphing aircraft that could smoothly alter shape to adjust to flight conditions and save fuel,” said Pete Flick, AFRL Program Manager, a 30-year veteran of flight research.
Some approaches relied on complex, multi-actuator mechanisms inside wing skins while others tried to exploit electroactive materials that change shape when energized, he explained, but “they delivered either high-rate, small deflections, or low-rate, large deflections.”
The FlexFoil concept, in contrast, “in a simple fashion provides both high rates and large deflections, which is why we think that it was the most promising technologies to come out of those research efforts. This is an elegant, very efficient way to change camber.”
The ACTE is reportedly able to deflect from -9° to +40° at response rates as high as 50°/s, which is quick enough for real-time gust-load alleviation, for example. The device is said to be able to twist up to 1°/ft (0.3 m) of span. In addition, the novel structure is rated at well over 10,000 lb (4536 kg) for a safety factor of 2.4 times the maximum load, and has passed tests for fatigue, temperature, and chemical resistance.
A video of FlexFoil in action: https://www.youtube.com/watch?v=9ZpAHxMj5lU.
The flexible trailing-edge beams, Kota said, can be constructed from standard lightweight materials including aluminum, titanium, or fiber-reinforced polymer composites. No exotic materials are needed.
Many potential benefits
Adaptive compliant wings offer numerous potential benefits, Flick said, including reduced fuel burn, simplified design, finer vehicle-control capabilities, real-time optimization of the airfoil for changing flight conditions and missions, as well as gust- or maneuver-load alleviation.
“Airliner and transport plane wing geometries are optimized for cruise flight, around some theoretical midpoint flight conditions, so the fixed wing shape is almost always nonoptimal,” he explained. With the ACTE, “the wing shape changes in a seamless fashion depending on where you are in the flight envelope.”
“In rapid maneuvers and strong wind gusts the loads get very heavy, making it difficult to counteract and prevent unwanted deflection,” Flick said. The FlexFoil system can respond rapidly, making subtle shape changes to alleviate aerodynamic loads, shedding them to keep them from building up—all without a weight penalty.
“Owners of a lot of older airliners are going to winglets to improve lift as well as reduce drag and fuel burn,” Kota said, “but the downside is that the structural loads go up, which means the wing structure needs to beefed up.”
Such upgrades are often too costly to justify. Retrofitting high-lift flaps systems such as Fowler flaps with FlexFoil Sub-Flap trailing-edge cartridges could control the loads induced by winglets.
“Many of these airliners, such as the MD-80, 737, or A320 could benefit from simply replacing the last 20% of the flap chords with self-contained FlexFoil cartridges,” Kota said. The morphing trailing edges could cut in-flight wing twisting and wing-root bending moments induced by the new winglets. “The trim tabs could get you 4% fuel savings, which could mean payback for the retrofit in as little as two and a half years.” Adding both technologies could thus yield a 10% lower fuel burn.
Diverse applications are possible for FlexFoil technology. They can be the leading and trailing edges of the wings and other flight surfaces of airliners, military transports and tankers, high-altitude aircraft (with thin, high-aspect ratio wings), helicopters—the U.S. Army has tested prototypes on a Bell 222A Jet Ranger—and even wind-power turbines.