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AeroVironment’s Global Observer (in California last year), designed to fly for a week on hydrogen, will triple the endurance of experimental, gas-powered UVAS from the late 1980s. (Courtesy Aerovironment Inc)

Distance Runners

Unmanned aerial vehicles redefine the term "nonstop flight."

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(Continued from page 3)

Designers of winged UAVs believe they have addressed the problems that brought down Helios. New designs include tails to avoid the kind of pitch problems that did Helios in. Operational versions will be ever more autonomous, navigating themselves from one way point to the next, with only occasional oversight. Zephyr, says Kelleher, is a “delight to fly,” requiring only fingertip control once it climbs into the stratosphere. While its aerodynamics improve with altitude, Zephyr’s wings and propellers are optimized for cruising at altitudes from sea level up to those above 60,000 feet, where the air is 1/10th the density at sea level. The altitude also puts the lanky airplanes above winds and threatening weather—Zephyr easily soars over thunderstorms and hurricanes, Kelleher says—and, of more importance for their solar panels, above the clouds. Zephyr’s batteries, manufactured by Sion Power of Tucson, Arizona, have performed very well through hundreds of recharging cycles in ground tests. SolarEagle will carry solid-oxide fuel cells that store power from solar cells during the day for use at night. Boeing identified the highly efficient fuel cells as the only practical means of storing energy, and took the idea to Versa Power Systems of Littleton, Colorado. The company had won federal grants to develop fuel cells as a way to store wind and solar power for the terrestrial grid. They had already developed some concepts for lightweight fuel cells, “so we went back to the drawing board and finally convinced ourselves that we’d be willing to take it on,” says CEO Bob Stokes.

But to do the job on SolarEagle, the fuel cells must be very light and produce more energy per unit of weight. So engineers at Versa are redesigning the cells and thinning their ceramic and metallic layers. The redesign has so far tripled their power output per pound. Versa expects to double even that, which could make fuels cells eventually work for automobiles.

The SolarEagle team hopes to complete a preliminary airplane design in about a year, pushing technology far enough for breakthroughs, but not breakdowns. Other companies competing for DARPA funding proposed even farther-out designs, such as smaller solar-powered airplanes that would take off individually and, once in the stratosphere, link up into a much larger aircraft. As SolarEagle designers push for better solar panels and fuel cells, they’re building reliability into the airplane at every turn. They’re trying to minimize moving parts by controlling the airplane with variable engine speeds rather than control surfaces. “The whole system has to be designed to expect numerous failures and be resilient to that,” Boeing’s O’Neil says. “This is something that’s been done on the satellite side, so the philosophy is there.”

“Whether or not SolarEagle succeeds or fails,” says NASA’s Nickol, “my prediction is, we’re going to have something like it sooner or later. It’s just so attractive. If SolarEagle’s not it, it’ll be something else. I’d say it’s a guaranteed thing that you’re going to have these multi-year-long vehicles in the stratosphere flying around providing all kinds of services at very low cost.”

When Boeing flies SolarEagle in 2014, it will likely do so with a 200-pound payload at the latitude of Albuquerque. The next step would be an operational version designed to carry 1,000 pounds for five years even farther north. By then QinetiQ may have a fleet of Zephyrs roaming the stratosphere. “This is all really a test of how efficient you can make something,” Langford says. “It’s taking sustainability and applying it to aircraft design. Who knows what you can do if you don’t have to stop for gas?”

Michael Milstein wrote about unmanned aircraft being developed for the U.S. Navy in “*Pilot Not Included,” which ran in the June/July 2011 issue.

About Michael Milstein

Michael Milstein is a freelance writer who specializes in science. He lives in Portland, Oregon.

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