[aviationweek.com] "The first stable flying wing that does not require recurve, reflex or sweep…" That’s how Defense Research Associates (DRA) describes its DevilRay UAV. Lacking any of the aforementioned features or a tail for stability, the DevilRay instead uses what Dayton, Ohio-based DRA calls an "inverse capped helix winglet" (abbreviated to i-CHeW, I kid you not). In other words, a wingtip that turns downwards and inwards.
I’ll get to how the "i-CHeW" works in a minute, but first the why. Working under a US Air Force Research Laboratory contract, DRA has designed the DevilRay to demonstrate a very specific and unusual thing – the ability for an electrically powered UAV to extend its endurance by hooking onto high-voltage power lines, recharging its batteries, then releasing the wire to keep flying. I kid you not…
Catching the wire, and not missing it or colliding with it, requires a flying machine that is very stable at low speed. Enter the DevilRay with its wacky wingtips.
The capped helix winglet is designed to reuse the wingtip vortex to reduce drag and increase lift. In the original design, the winglet went upwards then inwards, which seemed logical given the wingtip vortex is caused by spanwise airflow wrapping round the tip from the high pressure under the wing to the low pressure on top.
But windtunnel tests showed the result was high pressure on top of the inward section of the winglet, which reduced lift and stability. So DRA did the logical thing; it turned the winglet upside down. As a result, the inboard section of the winglet now produces lift – aft of the centre of gravity, so it generates a stabilising nose-down pitching moment.
As the flying-wing UAV slows down its angle of attack increases, the wingtip vortex increases, and the stabilising pitch moment from the winglet increases. In effect, the wingtips "anchor" the aircraft in the air, making it seemingly ideal for the exacting task of catching a power line on the fly.
DRA has flown its DevilRay prototype, which weighs in at 6 lb, with a 48-in wingspan and an electric motor with pusher propeller. Manually flown wire captures are planned for July, with autonomous hook-ups still a year away. Once it catches the power line, guided first by GPS then a magnetometer, the UAV will recharge its batteries using electromagnetic induction.
Source: http://www.aviationweek.com
