Supersonic Yaw

The designer and the pilot of the world's fastest airplane, the rocket-pushed Douglas Skyrocket, loosened up a little last week and told a few new facts about how the plane behaves. High above the speed of sound, said Designer Ed Heinemann and Pilot Bill Bridgeman, there is a new peril of the sky: "supersonic yaw."

As designers and test pilots pushed their planes up toward the speed of sound, the danger they feared most was the beating they took in the "transsonic zone." When an airplane is moving close to sonic speed, shock waves'(powerful sound waves) form on its wings and control surfaces. They come and go, shift irregularly and sometimes exert enormous forces on the plane's structure. Many early airplanes that trespassed too far into the transsonic range were destroyed by galloping shock waves. The remedy is now understood: thinner wings and tail surfaces, and a quick passage through the danger zone. Above the transsonic, the designers hoped, the air would be easier to cope with. Shock waves would still form, but they would act predictably, like the bow waves of a ship. When the Bell X-1 flew faster than sound in 1947, much was written about the smoothness and peacefulness of supersonic flight.

Little Queen. In reality, supersonic flights proved anything but peaceful. Both the X-1 and the Skyrocket, says Heinemann, met the strange and terrible phenomenon of supersonic yaw.

A rocket flight in the Skyrocket, says Pilot Bridgeman, starts out peacefully enough. When the plane is dropped from its mother B-29 at 35,000 ft., there is a gentle sensation like going down in an elevator. When Bill "kicks on" his rocket motors, he feels a great push of acceleration but no sensation of speed. Below the speed of sound, the Skyrocket "flies like a little queen," responds sensitively to his lightest touch.

He turns the nose upward for a steep climb. This keeps the speed below Mach 1, and takes him up toward the thin upper atmosphere where really high speed is possible. Bill finally reaches a point where the air is so thin that it can no longer support the Skyrocket below the speed of sound. Then he "bends over," flies at a flatter climb, and lets the speed build up.

Swooping Plunge. In the thin, high air (probably close to 80.000 ft.), there is not much kickback in passing the speed of sound. The Skyrocket was designed to minimize transsonic buffeting, and the rockets push it quickly to high supersonic speed.

One day, when Bill really let her out, he felt a slight yawing (turning from side to side). "Then I was in for it. Suddenly the yawing began to get violent. If I had thought it would get as bad as it did, I would have cut the power. But things happened too fast, and I was too late."

With a dreadful swooping, plunging motion the plane swung all over the sky. Its oscillations were so quick that Bridgeman's trained reactions could not keep up with them. If he tried to correct one of the violent swings, he might act a trifle too late and make the next one worse. While the ship zigzagged out of control almost 15 miles above the earth, Bill timed himself to catch every third or fourth oscillation. "It didn't take muscle, he says, "it took concentration. I never concentrated so hard in my life."

The coasting Skyrocket slowed at last and its yawing died out. The plane did not come apart. "I was scared as hell," says Bill, "but not until I got back on the ground."

The reason for this frightening behavior, explains Designer Heinemann, is that the Skyrocket's controls were designed for somewhat slower flight in somewhat thicker air. The rudder, elevators and ailerons are comparatively small and are mounted on the trailing edges of the wings and tail surfaces. They do not get enough grip on the air to damp out the yawing motion. One reason (much oversimplified) is that the disturbance they make in the air passing over them cannot have any effect on the air's behavior upstream. No disturbance in the air can propagate itself above the speed of sound.

Black Box. The Skyrocket can be equipped with a black-boxed gyroscopic "yaw damper" that can detect an incipient yaw much faster than the pilot can and correct it through an automatic pilot. But neither Heinemann nor Bill Bridgeman likes this "black box" approach. Heinemann believes that supersonic airplanes will have to be radically redesigned. One possibility is to make whole tail surfaces, or even whole wings, movable. Only by getting a firm grip on the thin fast-moving air, he believes, can a plane avoid the wild oscillations of supersonic yaw.

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