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Lehigh University logo
Lehigh University logo

Endless and Effortless Flight


The inspiration was simple: create a small, unmanned glider that once launched would fly on its own…indefinitely.

Nothing, however, can fly forever. The unflagging forces of gravity, drag and air friction all conspire to erode the strength of the airborne and pull them to ground. How close—given the relentless opposing factors—could you get to a plane that would fly unpowered and unassisted for extended periods of time?

While the concept of constant flight might seem outlandish, this is the modest proposition that lehigh professors Joachim Grenestedt and John Spletzer have set for themselves.

No eureka moment has sprung forth to resolve the conundrum; instead, they are devising a host of innovations at every level—from concept to construction to code—in pursuit of their goal.

Migratory birds make herculean seasonal journeys. in 2007, scientists tracked a bar-tailed godwit that flew uninterrupted for nine days, from alaska to new Zealand. Nine days, though, is just a dent in what Grenestedt and Spletzer have in mind, and to complicate things, their plane will have to capture from the air itself the energy it needs to remain aloft indefinitely.

If the endeavor seems outsized, note that neither Grenestedt, professor of mechanical engineering and mechanics, nor Spletzer, associate professor of computer science and engineering, is known for a retiring personality.

In 2009, at the Bonneville salt flats in Utah, Grenestedt set a U.S. land speed record in the 125cc streamline category for gasoline engines, logging just north of 133 mph in a missileshaped motorcycle he drove while encased inside, flat on his back.

Spletzer has been on the forefront in developing autonomous vehicles, including “little Ben,” a Toyota prius that a team of engineers outfitted to navigate the intricacies of city streets. It was one of the few successful finishers in the DARPA Urban challenge in 2007, outperforming cars built by a bevy of bigger and better-funded teams.

The inspiration for their unmanned aerial vehicle (UaV) comes from the flight of the albatross. gliding just above the surface of the ocean, wings locked, the albatross angles gently upward into fastermoving winds to reach higher altitudes, and then coasts downward again. The repetitive cycle, called dynamic soaring, enables the bird to draw energy by exploiting the wind gradient, or difference in velocities between adjacent streams of moving air.

“The albatross can fly a thousand kilometers a day while expending almost no energy,” says Spletzer. “We need to emulate that.” once their UAV is launched into the jet stream, which has layers of air moving at different speeds, it too could exploit these gradients to capture lift and sustain flight.

The project is split into two parts. While Grenestedt focuses on the aircraft design and construction, Spletzer is in charge of the computing features that will guide the plane.

Grenestedt—assisted by Robert Thodal, a doctoral candidate in mechanical engineering, and Jake patterson, a master’s candidate—is constructing the UAV out of sheets of a carbon fiber composite pre-impregnated with an epoxy binder.

Finding the right construction techniques is a laborious process of trial and error, as attested by the dozens of experimental wing sections littering Grenestedt’s lab. Every imperfection can sap precious energy from the craft. Thodal points to tiny striations on a test segment. “This is aerospace grade already. Lockheed would fly with this, but we want to get it as perfect as possible.”

The plane must endure 400 mph speeds, altitudes up to 60,000 feet and 20g of stress on its 50-kg frame- that’s about one ton’s worth of oomph distributed across a plane with a skin only 1/40th of an inch thick in some places. The flashy specifications aren’t for show. If the UAV can tolerate the high speeds and stresses incurred by sharply banked turns and maneuvers, it will increase its odds of following the most energy-efficient flight path.

The recently completed wing— cured together from a pair of molds into a single, contiguous piece of skin and webbed interior reinforcement—spans more than 20 feet but weighs a scant 15 pounds. Ailerons and flaps that will finish the wing assembly are almost done, and the team anticipates having a prototype of the full airframe by next summer.

“Rob and I are getting the chance to do things here that we couldn’t do anywhere else,” says patterson. “We’re able to do higher engineering, and then build something.”

On the command and control side, Spletzer is working with corey Montella, a doctoral student in computer engineering to whom Spletzer has entrusted the code writing for the on-board computer. They are validating software design and planning algorithms using a modified hobby aircraft outfitted with an off-the-shelf automatic pilot device that interfaces with a small on-board computer that will make decisions on flight paths.

Montella describes the computer coding as a reinforcement learning algorithm. “The computer is preloaded with optimal trajectories for particular conditions. If the plane follows the given trajectory and doesn’t get the energy gain expected, it logs that knowledge and adjusts next time.”

“These are big challenges,” Spletzer says. “We need to do things fast, like update wind field and trajectory data as many as 40 times per second. It’s also tough to verify the ground truth—the actual facts regarding local conditions. So the system has to be robust.” To this end, Jack langelaan of penn state is helping in the crucial area of wind field mapping.

Spletzer believes the project is unlike any other of its kind. “To our knowledge, no one has demonstrated sustained, autonomous, dynamic soaring.”

The potential applications of the UAV are myriad. It could be directed to a particular area to gather data, coasting there along an available path amidst the prevailing winds. It could be landed if repairs are required or additional research instrumentation needs to be added. Repairs may indeed be on the docket if one of the most interesting applications of the plane is adopted: flying it directly into the fury of a hurricane.

“Joachim and I have talked about that,” says Spletzer. “You could get an incredible, sustained data set that’s not currently available.”

The UAV might also serve as what Grenestedt calls a “poor man’s satellite,” or for other low-cost aerial tasks.

“We don’t have all the answers yet,” says Spletzer, “but that’s why we call it research.”