NASA and AeroVironment, makers of the huge and unconventional Helios flying wing, are investigating the inadvertent deployment of the flight termination system parachute. It brought to a premature end a successful first flight of Helios at the NASA Dryden Flight Research Center, California, on 8 September.

The 75.3m (247ft)-span remotely piloted flying wing was 1h 45min into its maiden flight over the dry lakebed at Edwards AFB when the parachute system suddenly deployed. The braking effect of the parachute, normally manually deployed by the range safety officer in the event of a loss of control, put the giant vehicle into a helical descending turn. The ground-based pilot was able to maintain some control as the Helios descended and the vehicle was undamaged in the landing.

Before the incident, the Helios handled well during its initial check-out flight, says NASA programme manager John del Frate. The flight was the first test of the revised configuration, which is 12.5m greater in span than its predecessor, the Centurion, flight tested at Dryden late last year. Powered by 14DC-brushless 1.5kW electric motors driving 2m-diameter propellers, the Helios is aimed at achieving altitudes of 100,000ft and continuous flights of up to four days above 50,000ft. Power for the motors is obtained from electricity generated by silicon crystal solar cells mounted on the upper surfaces of the wing.

Conceived as an "atmospheric" satellite, or airborne relay station, the Helios prototype is being developed under NASA's Environmental Research Aircraft and Sensor Technology programme. NASA believes that production versions of the Helios, equipped with rechargeable energy storage systems, could be capable of flights lasting up to six months.

As part of its efforts to reduce the number of moving parts, and to improve the chances of achieving long duration operations, NASA plans to conduct Helios flight tests in which the motors will be used for pitch control. If this is successful, the team believes it may be able to remove some or all of the aircraft's 72 elevators and their individual servos and associated wiring.

Test flights will also evaluate the behaviour of the flexible airframe structure in turbulence. In previous tests, the smaller versions such as the Pathfinder and Centurion were "literally tossed around", says del Frate. "It's [Helios] like an inflatable mattress on the ocean and, because it is so flexible, it makes up for its fragility," he adds.

Source: Flight International