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Quiet flights for turboprops.

Future airliners may sport properllers--pinwheels of eight or more thin, swept-back blades that spin rapidly yet are more fuel-efficient than jet engines. But because the blade tips will be able to whip through the air faster than the speed of sound, some researchers are concerned that noise inside such aircraft may be unbearably loud, especially at low frequencies for which conventional damping techniques don't work well.

The turbulence caused by the propellers shakes the aircraft fuselage, which in turn sets in motion the air inside the aircraft, generating low-frequency interior noise. One answer would be to find a way to limit fuselage vibration. Mechanical engineer Chris R. Fuller of the Virginia Polytechnic Institute and State University in Blacksburg suggests that deliberately vibrating the fuselage may at least partly cancel out the sound-induced vibrations and reduce interior noise levels.

In a preliminary investigation, Fuller studied the sound inside a closed metal cylinders meant to represent an aircraft fuselage. An external sound source excited the shell while an interior "minishaker" added its own vibrations. Microphones inside and outside the cylinder mapped the resulting sound patterns. Fuller found that even with just one minishaker set at an appropriate frequency, the interior noise level is reduced almost everywhere. "the new method shows much potential for reduction of propeller interior noise in aircraft," Fuller concludes, "without the penalty of large added weight."
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Title Annotation:research on reducing propeller interior noise in aircraft
Author:Peterson, Ivars
Publication:Science News
Date:Nov 23, 1985
Words:227
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