'Perching' aircraft train for tricky landings

By Tom Simonite Video: Feron’s aircraft lands on steeper slopes by flipping in the air to plant all feet at once (Video: Eric Feron, Georgia Tech University) Video: How and colleagues have found a way for small autonomous aeroplanes to land and take off from a perch fixed to a wall by hovering vertically in a manoeuvre known as a “prop hang” (Video: Jonathan How, MIT) Robot helicopters and aeroplanes capable of perching on steep surfaces are being developed by two separate US researchers. They say such drone aircraft could land on moving vehicles, unsteady ships or even hide inside caves. “Birds and bats can alight in all sorts of positions very well, but [normal] aircraft are limited to level surfaces,” says Eric Feron, an aerospace engineer at the Georgia Institute of Technology, Atlanta, US, who is developing perching helicopters. Feron’s small autonomous helicopters can land on surfaces inclined at up to 60°. In 2002, he developed an autonomous helicopter capable of performing other aerobatic moves, including the first 360° roll performed autonomously. Some military helicopters can land on slopes of 20°, by touching one end of their landing gear down and then lowering the rest to meet the surface. Feron’s aircraft lands on steeper slopes by flipping in the air to plant all of its landing skids at once. Feron and colleague Selcuk Bayraktar worked out a way for their computer-controlled helicopters to perform the trick, using external cameras to track the aircraft from several different angles. The helicopter accelerates towards a Velcro-covered landing pad and, when near, tilts its rotor as far backwards as possible. That rolls it backwards in free fall, bringing its Velcro-covered skids, or feet, parallel to the landing pad to grip on. If the helicopter misses the landing, it performs a recovery manoeuvre to prepare for another attempt. (see video, upper right) Feron plans to do away with the Velcro by having the helicopter reverse its rotors. “That would suck them down onto the pad,” he says. Grabbing onto surfaces using claw-like attachments is another possibility. Jonathan How, at MIT in Boston, US, is also developing small autonomous aircraft capable of unique landing manoeuvres. He suggests Feron’s trick could be useful for landing helicopters on ships. “It could also help land UAV helicopters on moving platforms like jeeps, instead of becoming sitting ducks when they land on a stationary vehicle,” he says. How and colleagues have found a way for small autonomous aeroplanes to land and take off vertically from a perch fixed to a wall (see video, lower right). It does this by hovering vertically in a manoeuvre known as a “prop hang”. How was inspired after seeing a video of a skilled radio-controlled plane “pilot” making his aircraft hover while standing on its tail. He asked a student to perform the same stunt to investigate how a computer-controlled craft might do the same. “We saw it is pretty hard, but at least we knew it could be done,” he told New Scientist. The “prop hang” is particularly challenging because all of the plane’s controls come into play and shifting one requires changing the others to compensate. The propeller provides lift, while the flaps on the wings prevent the body from spinning, and the tail controls the aeroplane’s horizontal position. “It’s like a dolphin standing on its tail,” says How. Next he hopes to develop control software that can learn how to perform such manoeuvres with any aircraft. Switching from hovering back to horizontal flight entails entering freefall momentarily and applying maximum power to move back to normal flight. “We had several instances where it didn’t quite work,” says How. These caused a few broken wings, although the polystyrene planes are fairly robust. Feron suffered a similar incident, when his helicopter missed the landing pad and ploughed straight into a wall. How believes such accidents highlight the value of testing indoors with off-board control systems and sensors. “The bit in the air is the cheapest part of these experiments,” he says. Feron’s work is available online at arXiv:0709.1744 How’s work was presented at the AIAA Guidance, Navigation and Control Conference, held in August 2007. Aviation – Learn more in our comprehensive special report. More on these topics:
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