Turbulent end for Pathfinder

The Pathfinder-Plus solar-electric flying wing recently completed a short series of research flights from NASA's Dryden Flight Research Center to investigate the effects of turbulence on lightweight flexible wing structures. The flights marked the end of an era in solar-powered flight research for the 23-year-old craft, which is due for retirement shortly.

Flown by crews from AeroVironment, owner and builder of the experimental aircraft, the Pathfinder-Plus made two low-altitude flights over the northern portion of Rogers Dry Lake at Edwards Air Force Base, California. The first was a three-hour flight on August 31, followed by a two-and-a-quarter hour mission on September 14. Both missions were flown on a combination of solar and battery power.

"The primary goal for these research flights of the Pathfinder-Plus was to gather aeroelastic data to really begin understanding how these vehicles perform in flight," said Robert Navarro, NASA Dryden's project manager for the test flights. "Being so flexible, they constantly change shape and we need to understand how the airplanes behave.

"We gathered some really good datasets on turbulence, and also on aircraft dynamics," Navarro added. "The plan is to use these datasets to correlate some of the models that we are going to be developing for this class of vehicles, and also to validate some of the analytical tools that need to be developed to do the analysis on them. The data would then be available to aircraft developers to reduce design risk for future high-altitude, long-endurance uninhabited aircraft systems."

The lightweight solar craft was instrumented with an atmospheric turbulence measurement system (ATMS) on seven long booms mounted across the leading edge of the wing. Data collected by the instrumentation was telemetered to flight controllers in a ground station at NASA Dryden several miles away.

"The idea is that you fly into a gust, and the gust is measured by the booms out in front," said AeroVironment flight director Jim Daley. "Then just fractions of a second later, you fly into that gust with the (leading) edge of the wing, the wing starts to respond, and the instrumentation inside the wing measures what happens when that gust measured out (front) hits the wing."

As an experimental technology demonstration aircraft, the slow-flying Pathfinder-Plus was required to conduct its flights over a work area for uninhabited aerial vehicles (UAVs) over the northern portion of the dry lake. The flights originally were scheduled for September 2004, but were delayed a year due to record-breaking rainfall that flooded the lakebed and left it unusable for flight operations until recently.

Bob Curtin, vice-president and director of AeroVironment's Design Development Center in Simi Valley, California, indicated the solar aircraft will be offered to a major aerospace museum for preservation and public display following its retirement.

"Pathfinder was built in the early 1980s to experiment with an airplane that could potentially stay up for months at very high altitudes," Curtin reflected. "Through its career, we gradually flew it higher and higher on solar power, culminating in a flight to over 80,000 feet in 1998. It's getting harder to fly, because it's getting very old. So we are retiring the airplane and this was probably its last flight.”

Curtin added that data collected from the recent Pathfinder-Plus flights would be used during the design of AeroVironment’s next stratospheric unmanned aircraft, the Global Observer.