During 1998, the Pathfinder was modified into the longer-winged Pathfinder-Plus configuration. It used four of the five sections from the original Pathfinder wing, but substituted a new 44 feet (13 m) long center wing section that incorporated a high-altitude airfoil designed for the follow-on Centurion/Helios. The new section was twice as long as the original, and increased the overall wingspan of the craft from 98.4 feet (30.0 m) to 121 feet (37 m). The new center section was topped by more-efficient silicon solar cells developed by SunPower Corporation of Sunnyvale, California, which could convert almost 19 percent of the solar energy they receive to useful electrical energy to power the craft’s motors, avionics and communication systems. That compared with about 14 percent efficiency for the older solar arrays that cover most of the surface of the mid- and outer wing panels from the original Pathfinder. Maximum potential power was boosted from about 7,500 watts on Pathfinder to about 12,500 watts on Pathfinder-Plus. The number of electric motors was increased to eight, and the motors used were more powerful units, designed for the follow-on aircraft.
The Pathfinder-Plus development flights flown at PMRF in the summer of 1998 validated power, aerodynamic, and systems technologies for its successor, the Centurion. On August 6, 1998, Pathfinder-Plus proved its design by raising the national altitude record to 80,201 feet (24,445 m) for solar-powered and propeller-driven aircraft.
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