Perpetual Flight

By Kathy A. Svitil|Friday, November 01, 1996
RELATED TAGS: ROBOTS
Within five years an unusual squad of planes will take off from a runway at Edwards Air Force Base in the Mojave Desert in California. The five planes will have no pilots and will stay aloft for months, flying in a V formation like giant geese. This robotic squad will, its planners hope, provide a low-cost alternative to satellites used for communications and environmental monitoring.

Engineers at the Rockwell Corporation, nasa, and ucla spent this past summer testing a prototype of their unmanned air vehicle, or uav. The craft has a 43-foot wingspan but, because of its aluminum and graphite frame and Mylar covering, weighs a scant 100 pounds. Solar panels cover about 90 percent of its wings and should capture enough energy to run the uav’s small propeller all day and to recharge the batteries that will keep it flying at night. This winter two more uavs will be built with essentially the same design; formation tests with the three planes will begin next summer (ultimately five will fly).

To maintain their V formation, the planes will transmit infrared signals to one another. Each plane will also receive information about its own location from Global Positioning System satellites. Onboard computers will integrate all this data, allowing the fleet to fly in perfect, synchronous formation--without intervention from the ground.

To take off, the planes will be towed by a truck and will then use two-horsepower engines to reach their flying altitude, a flight that will require several hours. Thanks to its light weight and long wingspan, each uav, once aloft, could throttle back to just one-half horsepower without a problem. And by flying in formation, even that modest energy requirement is cut in half because of the way air flows around a group of planes. All plane wings are designed so that air flows more slowly across the bottom of the wing than across the top, resulting in a higher pressure zone below the wing and a lower pressure zone above. The high-pressure air pushes the wing up. At the wing tips the pressure difference still exists, but without any wing in the way, air flows from the high-pressure region up to the low-pressure zone. The result is that a vortex forms around each wing tip and trails off behind the plane.

When planes fly in formation, each plane (except the one in front) flies with its wing tips in the vortex of the plane preceding it, and the vortex acts like an updraft, giving the plane some extra lift. By settling into the vortices, the planes produce an aerodynamic illusion: a single plane with very long wings. If we fly five of them in formation, we have basically a 200-foot-span plane, says Jerry Miller, an engineer at Rockwell and one of the project leaders. And, says Miller, if one plane develops problems, it could be replaced without disrupting an entire mission.

The formation will fly at around 65,000 feet--well above both commercial airlines and the turbulent jet stream. This particular vehicle is very flexible and very fragile, and you are flying in formation, so you don’t want a lot of turbulence, Miller says. You need to fly where the air is calm. The planes would climb to their flying altitude individually and then link up in formation.

NASA plans for these planes to help continuously monitor storms, clouds, and floods with cameras and other instruments and also to serve as low-cost hubs for cellular phone networks. It would work just like a satellite, Miller says, except instead of being several hundred miles away, it is only 12 miles away. For communications, the formation would function as a flying antenna. One plane would pick up microwave signals from the ground and send them to another plane in the formation; that plane would amplify the signals and bounce them back to the ground.

Technologically advanced countries, which already have support systems in place on the ground for cellular communications, may not need the robotic planes. But if you go anywhere in the Third World, says Miller, there is no infrastructure for this, and they can’t afford a satellite. This is a fairly low-cost way to put up cellular phone-type systems for something like a tenth of the cost of a satellite.
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