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11/08/94

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Flying model demonstrates that radical SST design is flyable

STANFORD -- Using a 20-foot flying model, Stanford researchers have shown that a radical design for a supersonic transport can be controlled in flight.

Passing this milestone means that the oblique flying wing, as it is called, may ultimately carry passengers from the United States to Tokyo or London in half the time it takes current subsonic jetliners, for an additional cost of less than 20 percent.

That is, if people are willing to travel in such an unconventional aircraft.

The design is a cross between the old World War II flying wing and an unconventional concept developed by R.T. Jones, an eminent aerodynamicist now retired from NASA Ames Research Center. It looks something like a giant flying surfboard and travels through the air sideways, with one wingtip forward and the other wingtip back.

Theoretically, such a flying wing can achieve nearly optimal performance over a wide range of speeds ranging from below the speed of sound to Mach 2. In several analyses performed by NASA and Stanford researchers, this efficiency could translate into supersonic flight for only pennies more per seat mile than is the case for current subsonic jetliners like the 747.

There has been one major problem, however. Experts didn't know if the oblique flying wing was flyable. Like some advanced military aircraft, the design is intrinsically unstable. That means it can be flown only with a computer system continually correcting its course and attitude. Models and experimental aircraft with oblique wings mounted on conventional fuselages have been designed, flown and proved to be well behaved. But several university research teams that attempted to design and build flyable models of the design couldn't solve the complex control problems.

Stanford researchers now have designed, built and successfully flown a sophisticated 20-foot remote-controlled model of the oblique wing. The tests of the $30,000 model took place at nearby Moffett Field May 10.

"We proved that the unstable wing configuration can be flown. More than that, we showed that its handling characteristics are not nearly as bad as people had thought. In fact, the control laws that we developed are actually fairly simple," said research associate Stephen Morris, who designed and built the model with graduate student Ben Tigner under the supervision of Ilan M. Kroo, associate professor of aeronautics and astronautics.

The control laws that the researchers developed are the rules that the computer follows to automatically adjust the airplane's flaps to keep it flying in a smooth and stable fashion. The model was equipped with computing power equivalent to that of an original Apple Macintosh 2. The aircraft was propelled by two 5-horsepower model airplane engines driving ducted fans to simulate the action of jet engines.

According to Kroo, models like this are a good way to test radical new designs. Miniature sensors allow researchers to gather large amounts of data on a design's performance for a relatively small investment. To develop a model, they must solve many of the same problems that face engineers who design full-scale prototypes. Models also can be changed quickly to correct design problems that arise.

Before the researchers flew their oblique wing model they tested it extensively by mounting it on the top of a moving car. In this fashion, "we were able to solve most of the stability problems until we were confident that it would fly. Without the cartop testing, I have no doubt that we would have crashed like the others," Morris said.

In the course of developing the oblique flying wing model the researchers made a number of important discoveries: Placing vertical stabilizers at each end of the flying wing caused it to twist and bend too much to control, so they mounted the stabilizers on just the trailing end; it is impossible to taxi the plane correctly without adding independently steered landing gear; allowing the engine exhaust to shoot straight back forced the wing into an immediate nose dive, so they added deflection vanes.

Despite its small size and definitely subsonic speeds, the researchers argue that their model tests provide a good indication of how a larger oblique wing will act. "We have good dynamic scaling. So, although we are only flying 65 miles per hour, not at supersonic speeds, the model gives us a good idea of how a prototype will behave," Morris said.

According to analyses performed by NASA and Stanford experts, the aerodynamic superiority of the basic design would allow an oblique flying wing with a 400-foot wingspan to carry 400 people from Los Angeles to Tokyo in seven hours rather than the 10 hours it takes at today's subsonic speeds, for a cost of only a couple of cents per seat mile higher than that of a 747. Comparable studies done for conventional SSTs generally predict substantially higher costs, according to Kroo.

The flying wing's greater fuel efficiency also translates into the generation of lower amounts of pollution that might damage the stratospheric ozone layer, which protects earth dwellers from potentially destructive ultraviolet rays from the sun.

The fact that its wing can generate substantial amounts of lift during takeoff also means that it would not require noisy afterburners like those used on the Concorde, so airport noise would be less of a problem. Although the oblique wing would generate a sonic boom when it travels supersonically, it could fly over inhabited areas at subsonic speeds without incurring a large fuel penalty.

"It may ultimately be necessary to go to radical designs like the oblique flying wing to achieve commercial supersonic flight at reasonable costs," Kroo said.

The research was funded by the NASA Ames Research Center.

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