Experimental Aircraft to Ditch Control Surfaces

Experimental Aircraft to Ditch Control Surfaces

Are flaps, rudders, and ailerons outdated? With its X-65, DARPA is out to prove that active flow control can maneuver a plane.
The Wright Brothers warped the wings of their famous Flyer to maneuver. Since then we’ve had flaps, ailerons, rudders, and elevators to do our steering in the air. “There’s a lot of maintenance on an aircraft to maintain these moving surfaces,” said Richard Wlezien, program manager for the Control of Revolutionary Aircraft with Novel Effectors (CRANE) program at the Defense Advanced Research Projects Agency, or DARPA. “And if they fail, you’ve got a big problem.”

Now there’s another way. The CRANE program is about to produce an X-65 that will have all the maneuverability of a modern aircraft without all the moving parts. Instead, it will use 14 banks of fixed active flow control actuators (or AFCs). In other words, it will steer by blowing air.

The X-65 won’t be the first aircraft ever to be controlled this way. In 2019, the University of Manchester in England, with BAE Systems, flew what looked like a miniaturized stealth aircraft called MAGMA. It had conventional controls for takeoff but, once it reached altitude, made all its moves with active flow control. But MAGMA was so small—it had a wingspan of 13 feet and weighed roughly 100 pounds—that there was a limit on what it could prove.

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“That’s really the first integrated demonstration flow control, but it’s not a realistic vehicle,” Wlezien said. “The Reynolds numbers are so small, the speeds aren’t realistic, the size isn’t realistic.” The X-65, however, will fly at Mach 0.7, weigh 7,000 pounds, and have a 30-foot wingspan. “It begins to look like a real vehicle.”

It also looks like a weird vehicle. The wings are shaped like sling shots—giant Ys with an empty triangle next to the fuselage—and there’s a giant mouth under the nose and several other inlets. There are also ports for dumping excess air when it’s not being used.

An earlier design looked a little more conventional—with solid triangular planes for wings—but it had the slight disadvantage of being able to pitch up without being able to come down. The forward airfoil on the Y-shape has a bank of AFCs that fixes that. The large inlet is for the engine, while the smaller ones power an APU for compressing air and supplying the air that will be compressed.

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With all that air needing to move to right places with precision, the interior of the aircraft is a complex web of ducts. “It’s been a very challenging effort to design a system that will do all this,” Wlezien said. “To go from a pipe to a distributed array of slots and get the same amount of flow out of each is not a simple task. And frankly, if we didn't have metal 3D printing, we wouldn’t be able to do it.”

Like the MAGMA, the X-65 also has conventional controls, but they’re there for a lot more than backup. The design was, of course, tested and calibrated with a tremendous amount of supercomputing time. (“In modern aircraft design or any aerodynamic design, if you can't compute it, you can't build it. And we can finally compute it,” Wlezien said.) But no matter how much modeling has gone on, no aircraft flies perfectly on its maiden voyage.

“When we first fly this aircraft, we’re going to further calibrate it against the conventional control surfaces. What we’ll do is deflect control surfaces to roll the aircraft. And then we’ll turn the active flow control on and see how much flow control is needed,” Wlezien said. “We’ll calibrate all the controls that way. So eventually we’ll get to the point where we can replace all the conventional moving surfaces with the flow control.”

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Right now, the design has been finalized and drawings have been released to potential bidders who will be making the molds. The plane is scheduled to be completed by September.

The X-65 is a demonstrator, and is by no means as efficient, or as perfectly designed, as a commercial or military plane steering with AFCs might be in the future.

“If we’re successful, efficient vehicles will come after this,” Wlezien said. “What we’re trying to do is step into a new era where we don’t need to use for the control surfaces. That’s the drive for this.”

Michael Abrams is a technology writer in Westfield, N.J.

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