October 26, 1995 in Nation/World

Nasa Surfs The Sky Working On Jet Design That Would Travel At Mach 5

Newport News Daily Press
 

You can’t get much more cutting-edge than this: NASA engineers are designing an aircraft that actually rides its own shock wave while traveling at five times the speed of sound.

Only astronauts know the feeling of traveling that fast. If you took off from New York City and accelerated to Mach 5, for instance, you’d zoom past Paris in 2-plus hours.

Don’t bother calling your travel agent. For one, the “waverider” is more likely to be a combat jet than a high-speed airliner.

For another, it’s not built. And it might never be, although the engineers who explore the super-fast world of hypersonics have successfully tested an 8-foot model of such an aircraft at Langley Research Center in Hampton.

The goal, however, isn’t necessarily to build a waverider prototype. Instead, their efforts are aimed at developing the technology for aircraft that represent a quantum leap over those flying today.

“Waverider” conjures an image of a tanned surfer shooting a curl at Waikiki Beach. And that’s pretty close to how a waverider would operate. The waverider - potentially a triangular, needle-nosed aircraft powered by an air-breathing jet engine - would:

travel fast enough to catch up to its own shock wave;

be designed so perfectly that the sweep of its wings “fits” into that wave at a given speed, and, consequently; and

use less fuel to fly more efficiently.

Riding the shock wave is the key. That can’t be done, researchers say, unless they can keep high air pressure on the bottom of the airplane, where it belongs.

Birds and airplanes fly because the curved upper surface of their wings forces air to travel faster across the top than it does underneath. Because of a phenomenon scientists know as the Bernoulli effect, pressure is reduced in the faster-moving air. The difference between the pressure above and below the wing creates “lift.”

All wing designs, however, allow some of the high pressure underneath to swirl upward at the wing tips. In other words, the designs aren’t a perfect fit for the air pressure. That pushes down on the wings, causing a decrease in lift - better known as “drag.”

Pilots and birds attain the best combination of lift and drag by correctly matching the airspeed and the angle flown through the air.

But in a perfect waverider design, there is no upward swirl.

Here’s how it works.

Everything that moves through air creates turbulence, as anyone who’s ever been blown aside by the wind from a passing truck can attest. As it moves, the truck compresses the air ahead and creates a low-pressure wake that it pulls along. The result, at both ends of the truck, is drag.

As an aircraft flies through the atmosphere, it creates this same sort of drag, which intensifies with speed. At supersonic or greater speeds, a “bow wave,” or shock wave, is formed. We can’t see it, but we can hear the edge of the wave when the aircraft exceeds the speed of sound, smashing air molecules together with such force that they create a sonic boom.

The wave, which contains high-pressure fields, intensifies with speed and takes on a narrower, more conical shape.

As an aircraft approaches five times the speed of sound, the wave narrows to the point where an aircraft can begin to actually “fit” into this conical shape, given the proper design and flight angle - also called the angle of attack.

The NASA waverider design is sort of an elongated triangle, its wings set in a dramatic rearward sweep. It is the leading edges of these wings, which run all the way forward to the tip of the aircraft’s nose, that would touch the edge of the shock wave.

At this point, the shock on the upper surface disappears, and the plane “rides” the wave on its lower surface.

“Why row your way through the surf, when you can sit on a surfboard?” asked Charlie Morris, NASA’s manager for hypersonics. Morris said the effect is somewhat similar to the way porpoises swim beside ships. “They’re benefiting from the flow field,” Morris said. “The ship curves the flow of the water, and the porpoise does something in the water very much like a surfboard on top of the water.”

Planners say the waverider technology can be applied to designs for future long-range strike fighters, high-speed transport planes or even cruise missiles. Each of those aircraft would share some common technology and one overpowering military advantage: for enemy air defense crews, the waverider would be little more than an unhittable blur.


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