Charged Aircraft

Beyond the sound barrier, the main obstacle to commercial supersonic flight is the miles-wide swath of broken windows, cracked plaster, frazzled nerves and aching eardrums that might be left behind by sonic boom. Resigned to it, airlines are planning either routes over water and desert or subsonic speeds over populated areas. Either solution could cut deeply into the time-distance economies that could otherwise be gained by flying huge planes faster than the speed of sound.

Now it appears that operators of supersonic transports may have a happier choice. On the basis of preliminary experiments, two scientists at California's Northrop Corp. believe that the sonic boom may not after all be a necessary evil. Last week, at a meeting of the American Institute of Aeronautics and Astronautics, Aerodynamicists Maurice Cahn and Gustav Andrew suggested that an electric field projected in front of a supersonic plane might eliminate the boom, and lessen drag as well.

Freeway Shock Waves. In subsonic flights, an aircraft exerts a pressure disturbance that travels forward at the speed of sound, parting the air ahead so that it will flow relatively smoothly around the plane. Supersonic planes produce the same kind of pressure wave but actually outrace it, causing the air molecules to pile up. This effect, says Cahn, "produces a shock wave like cars slamming into each other on the freeway." To eliminate or reduce the booms caused by these shock waves, the Northrop scientists decided, SSTs would have to be provided with an artificial forward-moving "pressure wave" that they could not outrace.

Working with small wind tunnels and flowing streams of liquid, they equipped small streamlined models with pointed electrical conductors and applied high, direct-current voltages to them. As a result, a powerful electrical field was projected in front of the models, ionizing the air or liquid molecules ahead of them. Before the charged particles could reach the model, its own electrical charge repulsed them, shoving them out of the way of the model's leading edge. Projected from the leading surfaces of an SST, the scientists hope, a larger and more powerful electrical field will have the same effect, thus preventing or reducing the formation of shock waves.

Visual Dividend. Aerodynamicist Cahn admits that he does not have the answer to one objection raised to his paper at the A.I.A.A. meeting: the creation of a sufficiently strong electrical field might require too much power to be economical. But he points out that there would be less drag or air friction on a charged SST, reducing the power necessary to fly it at a given speed and altitude. He suggests that only further tests with larger models and wind tunnels--now being considered by Northrop, Boeing and NASA--can determine if the system is practical.

Electrically charged SSTs would actually provide a visual dividend. Ionization of the air in front of the planes would produce a corona discharge that would be seen as a bright blue glow in the dark. "When supersonic traffic gets heavy," says Cahn, "this could provide observers on the ground with a spectacular view at night."

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