Unmanned aerial vehicles redefine the term "nonstop flight."
- By Michael Milstein
- Air & Space magazine, September 2011
Courtesy Aerovironment Inc
(Page 2 of 3)
For early lessons learned on high-altitude, long-endurance flight, O’Neil points to a Boeing program, Condor, that he sees as a jumping-off point. In 1988, the all-composite Condor, with a wingspan greater than that of a B-52, set an altitude record for piston-powered airplanes—67,000 feet—and stayed aloft for two and a half days on its turbo-charged, liquid-cooled, six-cylinder engines. “Condor continues to serve as a strong technical foundation for a lot of what we’re doing today,” O’Neil says. “And that was an unmanned system. So you’re thinking about something 24 years ago—quite an accomplishment.”
NASA’s Nickol points to the late Paul MacCready, founder of AeroVironment, and his human-powered Gossamer Albatross of the late 1970s. “Basically, once they built these super-lightweight vehicles that were very efficient, the next obvious step, instead of having a guy pedal, was to put in solar power and electric motors. So I think the real breakthrough was Solar Challenger. That flew across the English Channel [in July 1981] and actually flew quite a bit farther. At that point they had proof that, Hey, this could work. But then the technology had to catch up.”
It has caught up with manned flight too. Solar Impulse, a piloted airplane, flew from Switzerland to Belgium last May, the first international flight of a manned solar vehicle. Pilot and company CEO André Borschberg reported that he gathered more energy in flight than he needed, and used no more energy than if he had made the trip on “a small scooter.” The goal of Solar Impulse is an around-the-world flight.
SolarEagle will have some catching up to do when it begins flying. Last summer a wispy airplane called Zephyr flew as high as 70,740 feet above the Arizona desert for two weeks, quadrupling the previous record for flight duration by a UAV. Built by the British military firm QinetiQ (pronounced “kinetic”), Zephyr is the descendant of a smaller solar airplane originally dreamed up to capture video of a record-setting balloon flight. The balloon flight never happened, but the British military got curious about Zephyr’s potential. More recently, Boeing has brought QinetiQ on board as a partner to develop SolarEagle.
Zephyr weighs about 110 pounds and carries a payload as heavy as a telephone book, yet its 74-foot wingspan provides so much lift that it can be launched by hand: Five of the company’s employees hoist it to shoulder height and run along until it lifts away. It lands on two four-inch launch handles on each wing, and comes to an almost immediate stop with no damage. Solar cells as large and thin as a sheet of loose-leaf paper blanket Zephyr’s wings, silently powering twin propellers at a cruise speed of about 14 mph at sea level and almost 70 mph at altitude. The cells also charge lithium-sulfur batteries, which supply more than twice the energy per weight of any other battery type. Zephyr descended a few thousand feet at night and relied on the batteries alone to continue, but future flights are intended to maintain altitude overnight. An earlier model of Zephyr bounced hand-held radio transmissions about 350 miles between Phoenix and San Diego, showing its worth as a communications relay. The UAV’s steady flight also makes for prime surveillance. “It doesn’t have the shake and shudder of a jet engine,” says chief designer and pilot Chris Kelleher. The narrow-field-of-view lenses used at extreme altitudes are often compared to looking through a soda straw, which requires that the camera not shake. “You can look down that drinking straw very stably,” he says of Zephyr.
Kelleher has always loved airplanes that push boundaries, and has won titles for aerobatic flying in England. He also spent 20 years planning launch trajectories and flight dynamics for military satellites. “It was all good fun,” he says now. “But you start to think, Is there another way of doing some of these things? ” High-altitude UAVs, he says, provide constant views for far less cost than satellites. “You do it with a telescope from orbit or a pair of binoculars from [a high-altitude aircraft].”
An operational version of Zephyr is about a year away. While satellite instruments may take years to test because they become untouchable after launch, “we can have a problem in the morning and be flying a solution on a subscale test Zephyr in the afternoon,” Kelleher says. “Our design cycle is very short, and that means advances can be very rapid.”
Boeing and DARPA are counting on that for SolarEagle. While Zephyr set its records above sunny Arizona during the long days of midsummer, DARPA has more ambitious expectations for SolarEagle: The operational version will be designed to fly for months on end at latitudes closer to those of Chicago through the winter solstice, when darkness reigns nearly two-thirds of every day.
“Most aeronautical engineers would tear their hair out over this, and I haven’t got much left,” Kelleher says. SolarEagle’s wings will stretch about 400 feet, with solar panels on almost every exposed surface—even the undersides of the wingtips, which are canted upward, to catch extra rays from the low winter sun.
The blessing and the curse of huge wingspans is that they’re much more at home high above most wind and weather than in the lower atmosphere or on the ground, where they’re vulnerable to winds and ground handling. Because of their delicate, lightweight construction they flex in flight, sometimes changing their aerodynamics in unpredictable ways that suddenly alter the behavior of the airplane.
In June 2003, when AeroVironment’s solar-powered Helios, a 247-foot-span flying wing, took off from the Navy’s Pacific Missile Range Facility on the Hawaiian island of Kauai, engineers knew its vulnerabilities. Like today’s designs, Helios was light and flexible and most at risk in the thick air and winds during its initial climb. It maneuvered between clouds to keep its solar panels illuminated as it rose to about 2,800 feet in half an hour. Weather balloons launched preflight provided data that enabled ground controllers to steer the aircraft away from wind shear zones. Then the view from a wingtip camera went haywire. Unknown to the operators, the wiry airframe was bending in unexpected turbulence like a crossbow with the string pulled too far back. Helios began to wobble, and surged to about two and a half times its maximum design airspeed, about 20 mph, for that altitude. Skin and solar cells began to rip away, and Helios disintegrated into the sea.
“This class of vehicle is orders of magnitude more complex than it appears,” concluded the NASA Mishap Investigation Board. Conventional design techniques “did not provide the proper level of complexity to understand” how Helios would handle turbulence. On top of that, NASA’s decision to curtail funding had unexpected fallout: Engineers hoping to attract commercial funding switched to new fuel cells that were 50 percent heavier, and the airframe could not handle the weight.
But there was optimism too. The Helios team “had identified and solved the toughest technical problems,” which would include weight, flexibility of composites, and electric propulsion in high-altitude, long-endurance UAVs, according to the report, giving the United States a strategic advantage in the field. While the board called for better analysis and modeling, the program had affirmed that high-altitude, long-endurance technologies such as solar cells, electric motors, and structural designs had reached new benchmarks that could be applied to future airplanes.
Now, those airplanes are in the air, with more on the drawing board. This year, Boeing hopes to make the first flight of Phantom Eye, an airplane powered by two Ford truck engines that run on hydrogen. Aurora Flight Sciences late last year rolled out Orion, a diesel burner. And AeroVironment had begun testing of Global Observer last year, a hydrogen-powered airplane able to carry 400 pounds for a week at 65,000 feet. Hydrogen supplies about 2.7 times the energy per pound that gasoline does, making it among the most weight-efficient fuels. Global Observer’s four propellers look remarkably small, thanks to their efficiency and the aircraft’s aerodynamics. An accident did occur last April, and is still being investigated, but a second airplane was nearly complete at the time.