Flexible Flap Test Program Looks to Take “Next Step”

A team of government and private researchers is poised to take the “next step” in an ambitious effort to prove the merits of new morphing control surface technology, technology that promises to cut aircraft weight, noise, and fuel consumption.

Twenty-two recent research flights flown over six months at NASA’s Armstrong Flight Research Center in Edwards, CA, tested – in fixed configurations – the performance of Adaptive Compliant Trailing Edge (ACTE) flight control surfaces, surfaces that aim to replace (among other things) conventional flaps. In a prepared release, Air Force Research Laboratory program manager Peter Flick said, “We’re thrilled to have accomplished all our goals without encountering any significant technical issues.”

In the trials, a specially fitted Gulfstream G-III was flown with fixed ACTE configurations ranging from minus two degrees up to a full 30 degrees.

Web-based promotional material from ACTE developer FlexSys Inc. of Ann Arbor, MI, contends the company’s FlexFoil™ system “is able to produce large camber changes (-9 to +40 degrees).”

During the recent round of trials, “The flaps were tested … at a certain flap angle,” says FlexSys president and founder Sridhar Kota. After landing, “we would set [them] to a new position.” ACTE “was not actuated in flight.”

Now that researchers have successfully demonstrated the fundamental feasibility of the morphing ACTE technology, Kota says the “next step” is clear: “We’re going to actuate the flaps in flight. We’re studying incorporating actuators. The design and integration of that is just beginning.”

The beginnings of this specific flexible, variable geometry technology stretch back 20 years when Kota says he showed AFRL engineers the design. The effort was subsequently funded through SBIR, the Small Business Innovative Research program. That support has lasted for 17 years.

Managing the effort for the past 14 years is Pete Flick. He says AFRL’s investment in the current G-III flight demonstration phase is “on the order of $13 million” since 2009. Since 1998, total investment on AFRL’s part is some $20 million.

NASA ACTE project manager Thomas Rigney says his agency has “also contributed money toward flight testing. Since 2009 that’s amounted to about $25 million. NASA’s investment lay largely in “preparing the [G-III] test-bed,” taking a luxury business jet and fitting it with instrumentation and a new power system. “There was a significant [number] of upgrades that were required.”

ACTE’s Promise

The benefits of any technology are always predicated on how that technology is used. “When applied to a new airplane design,” Flick asserts there are considerable benefits to be had. “Some studies have [projected] a 12 percent fuel savings.” Retrofits to existing airframes envision a more modest 3 to 4 percent reduction in drag.

Then there’s noise. Compared to conventional flaps, ACTE produces less of it. Rigney says NASA estimated “a four to six decibel noise reduction,” this largely by virtue of eliminating the gap between wing and flap. “There’s lots of noise associated with that gap.” The airflow over conventional snaggletooth configurations on takeoff and landing can create a loud and continued noise. ACTE technology is seamless and hingeless.

How It Happen

“You need to think of this not so much as a flap, but as a multi-functional surface,” says Flick. “It can serve as a vehicle control surface, or as a load-alleviation device.”

Kota picks up the train of thought: “The FlexFoil variable geometry surface uses the natural structural flexibility of existing aerospace-grade materials (he declines to say just what materials) arranged in a jointless, skeletal configuration to continuously re-shape its external form. Driven by one or more aerospace-grade actuators the flexible structure is designed to achieve desired aerodynamic shape on demand, while sustaining tens of thousands of pounds of aerodynamic forces.”

FlexSys says since the internal mechanism has no joints to wear out the control surface’s profile “can be crafted directly to the fixed portion of the wing, enabling the wing to maintain attached flow in all configurations.” As for controllability, the company says, “The design is also able to twist span-wise along the trailing edge at the rate of up to 30 degrees [per] second.”

So, just how does this variable-geometry setup affect lift, that most fundamental of necessities? FlexSys says, “Even with all this flexibility these control surfaces are able to generate 11,000 pounds of lift, handling up to 24,000 pounds of load.” That, contends the company, renders the structure “as strong or stronger than any conventional design.”

Maintainability of any system, especially a new one, is critical. How will they hold up against the elements, endure the rigors of the flight envelope? FlexSys says by employing “aerospace-grade materials” the shape-changing surfaces, including adjoining fairings, are designed to withstand temperatures ranging from -65 F to 180 F. The company says FlexFoil is similarly fashioned to handle the harsh chemicals employed in the aerospace industry.

When you do have to pull maintenance on the control surfaces Kota says access is had to things such as actuators via panels, “like you do now.”

ACTE’s Next Acts?

Aerospace research moves at a steady, oft-times settled space – the better to identify gremlins which can gather in the shadows. NASA’s Tom Rigney says, “We’re taking the results and data from those flight tests and incorporating [them] into design trade studies.” Boeing’s involved in the studies, with the aim of looking into ACTE’s application into large, twin-aisle designs that might emerge about the 2025 timeframe.

But for the time being, it’s retrofitting that’s of immediate practical import. Pete Flick approaches the matter from the perspective of the AFRL. “We’re ready to start looking at applying [ACTE] to [United States] Air Force aircraft,” he says.

Although AFRL’s Adaptive Compliant Trailing Edge program manager declines to identify the specific airframe just now, he does say, “we have one in mind.” He can’t disclose it because, as of this writing, “we have yet to award a contract to do the work.” Still, the thrust is clear: “We anticipate actually applying this technology to one of our aircraft to demonstrate drag reduction on an actual Air Force operational vehicle.”

This single-aircraft demonstration approach is designed to wring out as much data as possible. “Then,” he says, “the plan is to get the benefits we measure in flight and build a business case for the Air Force to consider retrofitting a number of that type of aircraft.”

How much will a retrofit run? Flick says cost is going to vary widely, that it’s dependent on aircraft type. “I don’t think that any of us can really talk about the cost of retrofitting without understanding the details of a particular aircraft.” He cautions, “It’s going to depend on the flight control system and the existing hardware,” that’s been fitted to a particular flying machine.

In an age in which R&D budgets are at the mercy of the prevailing political winds, it’s refreshing to examine the alliance among NASA, AFRL and FelxSys Inc. for a moment. In a prepared statement, Jaiwon Shin, associate administrator for NASA’s Aeronautics Research Mission Directorate, says, “[The Armstrong Flight Research Center’s] work with ACTE is a great example of how NASA works with our government and industry partners to develop innovative technologies that make big leaps in efficiency and environmental performance.”