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Solar Flyer

Pathfinder-Plus

If onlookers were astonished when the Wright brothers' ungainly contraption made aviation history as it lifted off from Kitty Hawk, N.C. on December 17, 1903, they should have seen this aerodynamic creation take to the sky on August 6, 1998. As the picture proves, it really does fly. In fact, this plane set a new unofficial record for the highest altitude ever achieved by a propeller aircraft. And it pulled off the feat with energy provided by the solar cells that coat its single, curiously-shaped airfoil.

The flying wing, dubbed Pathfinder Plus, soared to 80,285 feet, surpassing the official record of 71,530 feet set by an earlier version of the Pathfinder last summer. The remotely piloted plane lifted off from the U.S. Navy's Pacific Missile Range Facility (PMRF) at Barking Sands on the Hawaiian island of Kauai and remained aloft for almost 15 hours, spending nearly three hours above its previous record altitude.

Pathfinder is one of a number of experimental aircraft being developed under the National Aeronautics and Space Administration's Environmental Research Aircraft and Sensor Technology (ERAST) program, headquartered at Dryden Flight Research Center, Edwards, Calif. The motto of the effort is to create planes that "fly slower, higher, longer."

Pacific Missile Range
EAGLE EYE. Infrared image of the Pacific Missile Range Facility snapped from 25,000 feet by the Airborne Real-Time Imaging System (ARTIS) on board Pathfinder illustrates the experimental aircraft's potential in remote earth sensing. Red indicates chlorophyll and cellulose typical of healthy plant growth; green is the red dirt typical of Kauai. (The actual images have 10 times the resolution shown here.)

A fleet of these high-altitude, long-duration aircraft are intended to be scientific research platforms in the upper atmosphere. The slower a plane flies, the greater its ability to capture samples of the atmosphere without altering them. Such aircraft could also spend long periods of time over the ocean monitoring storm developments to provide accurate predictions of hurricanes and keeping an eye on forests and other large expanses to provide early warning of crop damage or fires. The test plane has already flown an Airborne Real-Time Imaging System (ARTIS) camera that demonstrated Pathfinder's utility as an Earth-sensing platform. In addition, the unmanned planes could be used as communications relays when satellite coverage is not available.

One key feature not in the mission statement is "cheaper." The planes are being designed to carry payloads of 500 to 1,000 pounds to altitudes between 75,000 and 100,000 feet and to remain aloft for periods ranging from two hours to 96 hours. Cost "on station" is planned to be a mere $5,000 per hour--compared with more than $10,000 a pound to fly an experiment on the space shuttle.

The architects of Pathfinder are at AeroVironment Inc., an innovative aircraft-development firm in Monrovia, Calif., that built the human-powered Gossamer Albatross and Gossamer Condor planes in the 1970s and later made the solar-electric powered Gossamer Penguin and Solar Challenger. The basic configuration and concepts for Pathfinder were first realized with the HALSOL (High Altitude Solar) aircraft, built in 1983 by AeroVironment and the Lawrence Livermore Laboratory.

The present version of Pathfinder has a wingspan of 121 feet and is driven by eight 7.5-foot propellers. It is constructed of advanced composites, plastics and foam and weighs only about 600 pounds. The plane lifts off at a speed of speed of about 17 mph; once airborne, Pathfinder is controlled by a pilot from a control station located in a van, much like the model planes flown by hobbyists. But compared to those hopped-up models, Pathfinder is a sloth--it's cruising speed is only 21 mph.

Pathfinder is a proof-of-concept vehicle for two prototype solar-powered aircraft--Helios and Centurion--which currently are under development at AeroVironment. Both vehicles have a wingspan of about 200 feet. Helios, an ultralong-duration vehicle, would use an energy storage system to power the aircraft at night. The solar cells would power the vehicle during the day and recharge the energy storage system. Duration of such a flight could be several weeks to months. Centurion, an ultrahigh-altitude vehicle that could reach altitudes of 100,000 feet or higher, does not have an energy storage system, and thus would operate only during the day.

The alterations made in Pathfinder for its recent flight were to test components that will be used on Centurion, which is scheduled for low-altitude test flights at Dryden in October and high-altitude solar-powered flights next summer. The changes include more efficient electrical motors--and eight, compared with six on the earlier version extension of the wingspan from 99 feet and the incorporation of higher-efficiency silicon solar cells on the new wing section. The cells, manufactured by SunPower Corp. of Sunnyvale, Calif., convert 19 percent of the solar energy they receive to electrical energy vs. 14.5 percent for those on the original Pathfinder.

In just a few years, Centurion and its fledgling companions may become silent counterparts to the soaring eagles observing Earth from their lofty thermals.

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