"The Sky's No Limit," Input/Output, ME Departments, January 2004

input/output

by Michael
Abrams

The Sky's No Limit

Homo sapiens have wanted to fly like birds for millennia. And they've been killing themselves trying for just as long. For instance, hobbyist lore holds that, between 1930 and 1961, as many as 75 "birdmen" may have leapt from planes wearing wings on their arms. About six dozen of them died.

The chief problem those early winged men faced was that they had no idea what they were doing. With little understanding of wing shape, stabilization, or lift and drag, they neither flew nor lived. Now, Alban Geissler, an aircraft designer in Munich, is marketing the Skyray, a rigid triangular wing that lets modern-day birdmen soar above the clouds at 180 miles per hour, with a glide ratio of 3:1.

"People always told us: 'You will end up like all of them,' " Geissler said. "But those early birdmen missed something— that they should have been thinking first about their security, not about flying. Flying is not so difficult. Security is difficult."

Most of the birdmen of the past died because the wind would catch one wing slightly more than the other and send them into an unstoppable flat spin. And when they tried to open their parachutes, the lines would tangle with their wings.

"That's the biggest problem," Geissler said. "Small areas can develop very big forces and then you can get into trouble really fast."

The Skyray has an angled delta wing—like those found on many fighter jets—that makes a flat spin impossible. A straight-winged aircraft, like a Cessna, will stall when its angle of attack is more than 15 degrees. And as one wing begins to create drag, the other creates lift, putting the whole vehicle into a spin. But the Skyray can maintain a 1:1 glide ratio, even when the angle of attack is as much as 25 degrees. "When you develop an angle of more than 25 degrees, the airstream over the whole wing will detach and the wing creates more drag than lift," Geissler said.

The designer says the swept-back wings of the Skyray let birdmen whip through the air at 180 mph without the risk of going into a flat spin.

Geissler spent three years designing the Skyray. He first used computer simulations to see how a winged human body behaves in the air. "But airstream detachment is very hard to see with code, and the up-to-date codes all showed different outcomes," he said. "When you have detachment 2 or 3 centimeters further downstream, it changes everything."

To see how the Skyray would perform, he took a scaled-down model to a wind tunnel at the University of Applied Science in Munich. There, he tied a dummy (named Ken) to the prototype and discovered that chaotic turbulence at the back of the wing was pushing air back toward the front, just behind the parachute pack.

On another aircraft, this might not be such a problem. But a skydiver flying the Skyray needs a parachute to land. And the parachute is deployed when the skydiver yanks a pilot chute from the base of the pack. This miniature chute catches air and pulls the larger canopy free. The "back-draft" could prevent the pilot chute from doing its job and keep it hovering behind the pack.

So Geissler resculpted the Skyray and "smoothed the airstream" inward to create two controlled vortices.
But computers and wind tunnels didn't make the Skyray's first flight much less of a crapshoot. "Ken is pretty hard, like wood," Geissler said. "But a real human is like jelly with bones. It's pretty different."

One of the safest aspects of the Skyray—for Geissler—is that he doesn't test it himself. That he leaves to his guinea pig, Christoph Aarns, a man with 10,000 jumps under his belt and never a malfunction, (which happen in one of 1,000 parachute openings). When Aarns first attempted to fly the Skyray, he found the ride a bit turbulent—"like being tied to a washing machine," he said.

When he hit the air at some 200-plus mph, he was unable to stop his forward movement and the high speed exploded his pilot parachute. To give Aarns a smoother ride, Geissler reduced the size of the Skyray, added winglets at the ends of the wings, and redirected airflow to avoid turbulence behind the parachute pack. He also added handles so Aarns could hold the Skyray tight against his body.

Since then, Aarns has flown the Skyray nearly 300 times without a hitch. But that doesn't mean making the flight is a breeze. Before he gets on the plane, he goes through a ritual of grabbing each ripcord—main, reserve, and ejection. "The moment you jump," he said, "you are a really lonely man."

The author is a New York-based freelancer who is currently writing a book about birdmen.