Designed by Lockheed Martin?s Skunk Works division, which is famous for developing the SR-71 Blackbird, the unmanned drone will head to the Air Force Research Laboratory (AFRL) this summer for experiment with lightweight wing design and a dangerous aerodynamic phenomenon.
AFRL engineers will use the X-56A to study the aerodynamic phenomenon known as "flutter"?vibrations in an aircraft?s wings that occur at high airspeeds. Thomas Strganac, professor of aerospace engineering at Texas A&M University, says that flutter can lead to instability, wing failure, or both. "A lot of the time we think of the wing as a static body, but it vibrates," Strganac says. These vibrations change the flow of the air passing over the wing, which in turn increases the vibrations in the wing itself. This back-and-forth can set off a vicious cycle causing the wing to wobble or even snap, all in a matter of seconds.
Every aircraft today has the potential to flutter, Strganac says. To protect against it, aircraft engineers typically will add structural reinforcements to stiffen the wings and dampen the effects of flutter. "To make a structure stiffer, it means we have to make a structure heavier," he says. "The heavier structure, of course, is contrary to what we want to do with airplanes."
The team behind X-56A hopes to change how aircraft designers think about flutter. "We have a better approach, we feel," AFRL project manager Pete Flick says. Rather than building stiffer wings, an approach that aviation pros call passive flutter control, the X-56A will experiment with active flutter control?ditching the heavy structural supports in favor of a system that can actively and autonomously control the aerodynamic forces on the wing.
To do this, the craft will come with wings designed to use a system of sensors and specialized flaps. Flick says the sensors measure the stress in all parts of the wing; if they pick up areas of increasing pressure, the panels will move to deflect the airflow and balance out the load across the wing. There are 10 panels across the 28-foot wing.
If the idea works, then aircraft builders could use lighter and more flexible wings, Flick says. "As we demand more and more of our future aircraft?in terms of being energy-efficient?that leads to aircraft configurations that are inherently very flexible," he says. "We need to learn how to control the flexibility."
Lighter planes with active flutter control could fly faster, higher, more efficiently, and for less, Strganac says. He expects this kind of research to migrate someday from the military to commercial aviation. The Air Force is predictably tight-lipped about the specific applications of X-56A research, though Flick hinted at active flutter suppression being using on aircraft that demand highly efficient flight?such as high-altitude long-endurance (HALE) surveillance drones.
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