NASA's Helios solar-powered remotely operated aircraft crashed into the Pacific in June last year because available analysis methods were unable to predict the vehicle's increased sensitivity to atmospheric turbulence after modification with a fuel-cell system for an attempted 40h long-duration flight.
The AeroVironment-built Helios prototype broke up soon after take-off from Kauai, Hawaii on 26 June, 2003. The highly flexible flying wing encountered normal turbulence that produced unexpected, persistent upward bowing of the wing, which caused the aircraft to become unstable in pitch. The propeller-driven Helios exceeded its design speed, causing the leading-edge secondary structure on the outer wing panels to fail and upper-surface skin and solar cells to rip off.
In its original configuration, in which Helios set an altitude record of just under 97,000ft (29,500m) in 2001, the aircraft was a spanloader, says NASA, with loads distributed along its span. For long-endurance flights, however, a non-regenerative fuel-cell power system weighing 520lb (235kg) was installed in the centreline pod and two 82kg hydrogen tanks mounted under the outboard wing panels. This concentrated loads at three points along the span.
Behaviour of the modified configuration in turbulence was not adequately modelled, says NASA, and a check flight of the modified Helios two weeks earlier encountered none of the normal turbulence. Flight tests of the smaller Pathfinder Plus at NASA Dryden later this month will include low-altitude flights in turbulence to gather data on aircraft response for use in developing better analysis tools.
NASA does not plan to replace Helios, but expects to build a follow-on vehicle under a new five-year, $100 million UAV technology programme.