Unmanned aerial vehicles redefine the term "nonstop flight."
- By Michael Milstein
- Air & Space magazine, September 2011
Courtesy Aerovironment Inc
AT SOME POINT in early 2014, Pat O’Neil and Carol Wilke expect to launch a new airplane they hope they will never see again—at least not for a long time. The longer, the better. Initially, the two engineers from Boeing’s Phantom Works want the unmanned aerial vehicle (UAV), named SolarEagle, to fly at twice the altitude of cruising airliners for 30 days straight. Ultimately they want it to stay up for five years, long enough to outlast a presidential administration and who knows how many generations of iPods. They’re so unconcerned about seeing the airplane again, they may have it jettison its wheels after takeoff.
“We don’t want to take any unnecessary weight to altitude,” explains Wilke, flight test lead for SolarEagle, underscoring the key challenge of keeping an airplane in the sky as long as possible: Make it light so it needs very little energy to stay aloft. Energy, then, becomes the limiting factor. SolarEagle will draw its power from the sun, with solar panels carpeting a wingspan almost twice that of a 747. Such endurance will require solar cells and an energy storage system so light and efficient that they have not yet been fully developed.
In September 2010, the design became the winner of the Vulture II program, a competition run by the Defense Advanced Research Projects Agency (DARPA), the U.S. military’s group that funds concepts bordering on the impossible. DARPA has now committed $89 million to the prototype. Everyone involved hopes to see SolarEagle, now under construction in St. Louis and near Seattle, combine the durability of satellites with the lower cost and flexibility of airplanes, and, of course, meet the central challenge from DARPA: fly for five years without landing.
SolarEagle and similar high-altitude, long-endurance prototypes are stretching our notions of airplanes. Their spindly, giant wings push the limits of aerodynamic efficiency. They fly high and slow without refueling, and shrug off maintenance schedules and ground crews. An autopilot flies a preplanned route while ground-based operators make sure it stays on course.
In the 1990s, NASA sought to develop a fleet of such UAVs for environmental research. The program ended in 2003, when unpredictable winds tore up Helios, a prototype built for the program by a pioneering company in unmanned flight, AeroVironment. The technology has since evolved. Advances in solar cells from the green energy industry, more efficient batteries and fuel cells, more durable engine parts, and new aerodynamic designs may now offer the tools to keep an airplane aloft for years.
“You’ve had a convergence of technology at the same time you’ve had a real need emerge,” says John Langford, chairman and president of Aurora Flight Sciences, which specializes in UAVs. In November his company unveiled a mid-altitude UAV that burns diesel fuel and will stay aloft for five days. “There’s no other place in aviation where you can be talking about this kind of potential,” Langford says.
The dream of indefinite endurance is now closer than ever, says Craig Nickol, a NASA researcher who led a concept study on high-altitude, long-endurance UAVs in 2007. What does he mean by indefinite? “Well, something will always break on you,” he reasons. But he points to SolarEagle. “Five years—that’s, I would say, pretty much indefinite.”
The key challenge, says Nickol, is efficiency. “You basically need state-of-the-art, revolutionary...breakthroughs in each of these areas to enable the goals of some of these programs,” he says.
Engineers are squeezing out every ounce of unneeded material to the point where “now you get into very flexible structures,” says Nickol. “So a lot of the challenge is analysis and design. That’s a new area to deal with, because there are not a lot of good analytical tools that can handle this.”
“In a normal airplane, you can make up for shortcomings in aerodynamics by getting more power,” says Langford, who helped pioneer human-powered flight with the record-setting, human-pedaled aircraft Daedalus, which a pilot flew 71 miles across the Aegean Sea in 1988. “In these airplanes, you don’t have that luxury.”
The efficiency of solar cells, the power source for many high-altitude, long-endurance UAVs, has generally doubled over the last 10 years. Fuel cells, which convert hydrogen to water and can charge onboard batteries for flying at night, have continued to shed pounds. With no moving parts, fuel cells mean reliability.
O’Neil, the SolarEagle program manager at Boeing’s Phantom Works, calls the UAV a “pseudo-satellite,” which would fly at about 65,000 feet, circling combat zones or other targets. “The key technologies have now emerged enough that we can talk about designs that inhabit the upper atmosphere,” he says. “Ten years ago the technologies were not ripe.
“Your candidates are undoubtedly the composite family of materials,” O’Neil continues, “whether you’re talking soft skin over a composite frame or honeycomb, or very lightweight composite plies [sheets or surfaces], where you might have just a couple of plies in key areas just to keep the weight down.” Plastics and carbon fiber are attractive to engineers because of their strength and light weight.
But prolonged exposure to extreme altitude means dealing with ultraviolet radiation and even the occasional gamma particle from space. On the interior, some metals may be needed to shield avionics from these threats.