AeroVironment has launched its solar-powered Pathfinder UAV on its highest climb yet.
Guy Norris/LOS ANGELES
AEROVIRONMENT'S PATHFINDER flying wing, was flown to 50,500ft (15,400m) on 11 September, using solar power alone a new-world record.
The test flight, at NASA's Dryden Research Center in California, was the second for the Pathfinder and marks the beginning of the flight phase of NASA's environmental-research aircraft and sensor technology (ERAST) programme.
For its first flight, on 28 July, the 30m-span unmanned air vehicle (UAV) stayed within the area of the Rogers dry-lake bed and reached a maximum altitude of 250ft. California-based AeroVironment says that the new electronic flight-control system, which is fully redundant, "worked well".
This shakedown flight, which also expanded the airspeed envelope, was the Pathfinder's first since February 1994 when it was offered for the Raptor/Talon anti-ballistic-missile programme. The UAV has since been heavily modified for its new role in the ERAST effort, which NASA has been running since 1994 with four companies: AeroVironment, Aurora Flight Systems, General Atomics and Scaled Composites.
The four share around $5 million between them annually from a NASA budget estimated at $10 million. The project is aimed at developing UAVs and lightweight miniature sensors for high-altitude atmospheric research.
Some of the support for the project comes from NASA's high-speed-research programme because of its potential application to the operating environment of future supersonic transports.
As well as a new flight-control system, the modified Pathfinder now has a much larger solar array. Three times bigger than the original, it generates about 6.5kW compared to 2kW from the initial array. The solar panels cover 75% of the wing surface (compared to 20% before), and are made from Spectralab crystalline-silicon cells which power six 1.5kW brushless DC electric motors (the original had eight).
Although the motors are unchanged from the original, they are worked harder to make up for the loss of two motors, dropped to save almost 14kg (6.8kg per engine, including plastic propeller). The power plants were deliberately over-sized for use on a later 61m-span version.
"It's ready to fly between 60,000ft and 65,000ft, and the first low-altitude flight was one of the steps to it," says NASA Pathfinder project manager John Del Frate. "Weather is critical to this aircraft. It needs a good weather window to climb to high altitudes."
AeroVironment Pathfinder project and flight-test director Bob Curtin says that the plan is to "...go as high as the aircraft will allow us to go, and as high as the Sun will allow us to go. We will climb at 10,000ft increments, checking its health at each".
With a 9.00 take-off, it is up to altitude at around 14.00 and it lands after dark at around 23.30, Curtin says. The Pathfinder took 11h to perform its second mission.
Not surprisingly, solar power reaches its zenith at midday and, by early evening, the panels on the wing stop producing enough energy to power the six motors.
At present, gliding starts at "...about 18.00, and it glides all the way back down to the landing", explains Curtin. A regenerative fuel cell is under development to allow flight to be sustained throughout the night.
Landing and navigation lights have been fitted and were tested in darkness at Edwards AFB, California, on 7 August. The standard light-aircraft navigation light-set weighs almost 7kg, while the 50W spotlights add around 1kg. The lights are played towards the centreline of the Pathfinder, illuminating each engine pod and some of the ground to help the recovery team guide it back for a safe landing. Dual Trimble global-positioning-system receivers are used for navigation.
A Mode 3C transponder is also fitted to help the US Federal Aviation Administration keep track of the UAV's movements, while a radar beacon will be used by NASA's range radar.
The Pathfinder, which has a maximum cruise speed of only 18kt (33km/h) and an estimated true air speed of Mach 0.15 at its potential ceiling of 75,000ft, will be flown to within 5,000ft of this altitude later this year. On its first high-altitude flight it will carry a US Navy communications relay payload while, on later missions, it will carry solar-cell calibration equipment and environmental sensors.
All the other ERAST vehicles, are powered by Rotax piston, 912 engines. Aurora will attempt 70,000ft-plus with its Perseus A, Scaled Composites will try for 65,000ft with its Demonstrator 2 and General Atomics will strive for 50,000ft with its Altus.
NASA hopes that the UAVs will be flown on their first science missions by late 1996, and that missions at the 80,000-85,000ft level may be possible by 1997.
Source: Flight International
