As tickets for Concorde’s final flight go on sale this week, an American aerospace company has demonstrated a way to modify a supersonic jet to dramatically reduce its sonic boom.

The work could pave the way for a new generation of business jets quiet enough to fly at supersonic speed over populated areas.

Sonic booms are one of the biggest drawbacks of supersonic flight. They are the thunderclaps caused when shock waves created at the nose and tail of an aircraft meet as they travel to the ground. Where the shock waves overlap they reinforce each other, creating the boom. Concorde’s boom is so loud that the plane is forbidden from flying at supersonic speeds over land.

In the 1970s, Richard Seebass and Albert George at Cornell University in Ithaca, New York, came up with a straightforward way to counter the problem. They reasoned that a shock wave would be weaker if it were spread out over a larger area.

This could be achieved by replacing a plane’s sharp nose with a blunter shape and redesigning parts of the wings, for example where the base of the wing meets the fuselage, so that the angles between surfaces do not change so abruptly.

The idea was to force the shock waves to fan out more rapidly as they move away from these curves, spreading out their energy. Years of computer modelling and wind tunnel tests have validated the concept, but it had never been tried in flight.

Now the American aerospace company Northrop Grumman has tested the idea with a modified F-5E jet fighter at Edwards Air Force Base in California. “In this business, you don’t know what you think you know until you fly it,” says Charles Broccadoro, the test programme’s manager.

Distant rumble

The team replaced the aircraft’s ordinary cone-like nose with a bulging design reminiscent of a pelican’s throat.

The first results matched the computer models so well it was breathtaking, says Broccadoro. Ed Haering, a physicist at NASA’s Dryden Flight Research Center and an expert on sonic booms, monitored the test flights of the F-5E with an array of microphones and pressure sensors on the ground and on chase planes. He says the intensity of the shock wave was reduced to about 38 pascals, less than half the force created by Concorde’s boom.

These tests were not intended to produce a boomless flight – that would require a much more drastic modification of the aircraft’s shape than could be accomplished with the bolted-on structure of aluminium panels used in these tests, referred to as a “glove” because of the perfect fit required.

But he added that the work could form the basis of a prototype in which the boom could be reduced to 15 pascals. This may lead to a virtually boomless jet that produces merely a distant rumble.

Next month, the team hopes to fly the modified plane at higher speeds and see a more pronounced reduction in the shock wave. It will also test the plane for a variety of weather conditions and flight paths. And Haering hopes to use a photographic technique involving pinpoint alignment of the test plane, chase plane and the sun to provide a high-resolution image of the pressure waves.

He believes these tests will provide enough data to allow engineers to begin work on more advanced prototype.
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