In 2009, the B-2 Spirit of Kansas crashed just after takeoff from Andersen Air Force Base in Guam. The crew survived but the total loss of the plane cost the Air Force an estimated $1.4 billion.
The cause? Condensation had fouled the instruments. That kind of confusion never happens to bats or insects, creatures that rely on super-fine hair-like structures to guide them in flight.
Air Force Research Lab engineers say they are taking their cues from the animal world to avoid incidents like the Guam crash. They’re developing artificial hair sensors that they say will may aircraft more agile and more reliable.
“We modeled this off of bats and birds, which can fly by feel. They feel the air, and that allows them to rapidly and in an agile way take advantage of the current air conditions,” said Jeff Baur, a principal engineer in AFRL’s materials and manufacturing directorate.
Researchers began basic research and development around the idea four years ago and last year demonstrated that micro sensors could read the condition of the air with great accuracy. This spring they plan to review results from a recent “pitch-and-plunge” test, designed to gauge the effectiveness of hair sensors in gusty conditions.
While actual implementation on an aircraft may be a decade away, researchers say the evaluations presently under way could have a dramatic impact, especially on the operation of UAVs, which may need to be especially nimble if they are to perform ISR missions in urban terrain.
“A lot of this is based on the idea of smaller unmanned vehicles, where they are more prone to issues with gust, or where they may be operating in an urban canyon where they turn a corner and the wind changes suddenly,” Baur said. “That’s where these things pay off: Any place where you need to maneuver more quickly.”
The “hair” here is comprised of a single glass fiber, 10 times smaller than a human hair, 10 microns in diameter. “We have a small glass capillary, a sort of soda straw, and then we have a special way of growing carbon nanotubes inside that glass fiber. When the glass fiber gets pushed right or left, the carbon nanotubes gets squashed like a sponge and we can correlate that to a force on that area,” Baur said.
A hairy challenge
Researchers who have looked at such mechanisms in detail report a host of hurdles that must be overcome in order to cull meaningful data from these glass fiber mechanisms.
“Challenges include difficulty in determining the electromechanical properties of the sensors, limited working knowledge of the boundary layer, low sensitivity to small hair deflections, and lack of models for large deflections,” according to a November 2016 research paper in Advanced Materials Technology.
The Air Force reports it is making progress on all these issues, as it seeks out ways to translate an insect’s ability “fly by feel” capability into the realm of human flight.
AFRL says pilots need sophisticated sensors of this type to overcome shortcomings in the methodology. In order to gauge current and other wind effects today, most aircraft rely on a clunky, externally mounted boom. Not only does a boom add drag, it also measures air flow in just one spot on the plane, rather than giving a holistic picture of conditions around the aircraft.
As a result, “you don’t quite know what’s happening quickly enough,” Baur said.
Experiments so far have combined sensor input from about 20 artificial hairs at a time, but researchers envision deploying hundreds or perhaps thousands of the microscopic devices in order to get a rich picture of prevailing forces.
The long-term goal is to process this sensor input through a neural net, as a means to building real-time intuitive response capability into the very fabric of an aircraft.
“You can have a skin of these and if you know what a 50-mile-an-hour wind feels like, they can use an algorithm and self-learning to interpret that,” Baur said. “So rather than going through a detailed calibration, they can simply remember and recognize. It makes it quicker, easier and less expensive.”