Washington DC
THE ear-splitting roar of a supersonic passenger jet at takeoff could be
muffled by surrounding the jet鈥檚 exhaust with a 鈥渧irtual shroud鈥� of air,
according to a researcher in California.
The shroud could be the key to an affordable and politically acceptable
successor to Concorde, says Dimitri Papamoschou, a professor of mechanical and
aerospace engineering at the University of California at Irvine.
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The US government and American aerospace companies are planning a new
supersonic passenger plane, tentatively called the High Speed Civil Transport.
Their goal is to develop the technologies needed for a 300-passenger plane that
could fly at Mach 2.4 and travel as far as 5000 nautical miles (9000
kilometres), by 2001. But one of their problems is meeting the noise limits
imposed by major airports that must avoid disturbing nearby residents.
A supersonic engine鈥檚 exhaust is extremely noisy, because the jet of gas
moves at supersonic speed relative to the surrounding air, even during takeoff,
says Papamoschou. As the exhaust hits the air, it creates shock waves known as
Mach waves, which make the noise.
The traditional solution to the noise problem is to envelop the engines in
metal shrouds. When the exhaust hits the shroud, it should be slowed to subsonic
speed, and thus should not produce shock waves when it is expelled from the
engine. Although this approach works, the shroud adds to the aircraft鈥檚 weight,
which increases the amount of fuel it has to burn.
Papamoschou鈥檚 alternative is to surround the engine鈥檚 exhaust with an
envelope of air rather than metal. The air comes from the fan stream鈥攁ir
that flows around, but not through, the engine鈥檚 compressor. Heating or
accelerating the fan stream would move the envelope of air around the exhaust
jet fast enough for the difference in speeds to be less than supersonic so there
would be no shock waves. At the same time, the air in the shroud would not be
supersonic relative to the surrounding air.
Accelerating or heating the fan stream would require the plane to burn some
extra fuel. But that would only be necessary during the takeoff and first few
minutes of flight. After that, the virtual shroud could be turned off. Over a
typical journey, the plane would need to burn only about 0.5 per cent extra
fuel, says Papamoschou.
Laboratory tests have been promising, and the next step is to build a
full-scale model to see whether the shroud would work as expected. Papamoschou
is looking for a company to fund the test.
鈥淚t鈥檚 a very simple concept,鈥� says John Seiner, head of acoustics research at
NASA鈥檚 Langley Research Center in Virginia, which has provided some grant money
for the project. He says Papamoschou is 鈥渁n extremely promising young scientist
with a very good idea鈥�.
