NEXT time you see something flapping in the breeze on an overhead power line, squint a little harder. It may not be an old pair of sneakers or a plastic bag. It could be a miniature aircraft in disguise, stealing power from the line to recharge its batteries.
The US Air Force Research Lab (AFRL) in Dayton, Ohio, has come up with this bizarre idea in order to keep surveillance planes in the air for much longer than is possible today.
Very small spy planes known as (MAVs), with a wingspan of around 1 metre, already exist. They can be carried in a soldier’s backpack, assembled on the battlefield and hand-launched towards areas of interest, from where they are supposed to beam back photos. The trouble is that the planes run out of power after about 45 minutes.
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Now the AFRL is developing an electric-powered MAV that can “harvest” energy by attaching itself to a power line. The hope is to design one that keeps going for days or even weeks.
To avoid arousing suspicion when recharging, the spy plane will need to collapse its wings and hang limp on the cable, looking like a piece of wind-blown detritus. The Pentagon’s Defense Advanced Research Projects Agency (DARPA) has already developed much of the morphing technology to do this, including telescopic wings that can be folded away and “sliding skins” made of carbon composite, which allow the fuselage to change shape.
The AFRL’s most immediate challenge is working out how to get an MAV flying at 40 knots to latch onto a power line without destroying itself or the line. Originally it had hoped that by flying close to a line electromagnetic induction would provide enough power. But during tests, AFRL researchers found that this technique limited the amount of power harvested to microwatts – not nearly enough to fly an MAV.
Landing on the cable is likely to provide more power, and the AFRL will now have to prove that the idea is viable. It has succeeded in landing two out of the six MAV designs it tested on power lines, but has yet to devise an efficient latching mechanism. It hopes to come up with ideas and test them for latching this year.
Even if this is successful, challenges abound, says Zac Richardson, a power line engineer with National Grid in the UK. He points out that if the MAV touched a pair of 11-kilovolt local power lines at the same time the short circuit would disconnect the very power the plane seeks. On a 400-kilovolt inter-city power line, it risks “fizzing and banging and giving its position away anyway”. The sparks could also interfere with the plane’s wireless communications system and prevent it from beaming back footage in real time.
“Even kites falling across power lines cause breakdowns,” says Ian Fells, an expert in electricity transmission based at Newcastle University in the UK. “It’s an utterly bizarre idea to try to land a plane on one.”
“It is an utterly bizarre idea to try to land a plane on a power line”
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