Bright, computer images of landing gear wind noise are enabling NASA
engineers to pinpoint loud and preventable aircraft flight sounds more
easily than in the past, raising the prospect of quieter take-off and

In a series of tests conducted at NASA’s Ames Research Center, Moffett
Field, CA, the sounds have been depicted as colored images on a computer
screen. These new test data provide critical visual information to aircraft
designers concerned about possible enforcement of stricter aircraft noise

“Some airports are imposing nighttime curfews on noisy take-offs and
landings, encouraging aircraft manufacturers to make quieter planes,” said
Paul Soderman, leader of the Ames aeroacoustics group. “If U.S. airplane
makers can’t meet the new noise rules, those manufacturers may well have
difficulty selling their aircraft, both domestically and in foreign
markets,” he speculated.

Engineers anticipate that lower aircraft noise limits also will be issued
by the International Civil Aviation Organization, which makes airplane
noise rules for much of the non-U.S. aviation community.

“Now that we can easily see the causes of the annoying ‘whooshing’ wind
noises that come from airliner landing gear, we can take steps to analyze
and eventually reduce the noise significantly,” Soderman said. Airframe
parts, including landing gear, flaps and slats, create almost as much noise
as the aircraft engines on approach to landing.

“We are pleased to see a lot of detail in the sound pictures. The sound
images of the quarter-scale landing gear model we constructed are the first
of their kind generated in the United States at this scale, to the best of
our knowledge,” Soderman said. Using an array of 70 microphones inside a
wind tunnel wall and linked to a computer, engineers can see the vivid
images of landing gear wind sounds that normally occur during aircraft
take-off and landings.

The microphone array minimizes wind tunnel airflow noise so that landing
gear noise sources as small as 6 mm (about a quarter of an inch) can be
identified. At full-scale, these sources are 24 mm across, or about an
inch, according to engineers who conducted the tests in the Ames 7-by
10-foot wind tunnel that the U.S. Army operates for NASA.

Researchers reduced noise significantly as they removed various
combinations of landing gear parts from the test model in the wind tunnel.

“A landing gear slows an airplane as it comes in for a landing, and if we
reduce the drag too much, the plane would be traveling faster than it
should as it approaches the runway,” Soderman explained. “Removing pieces,
or altering part shapes, is not as easy as it sounds because many of the
changes would greatly affect how the landing gear and plane operate.” The
results from this test will enable researchers to decide how to create air
drag, or friction, to slow the airplane without causing as much noise, he

“Preliminary data analysis indicates that a faired landing gear generates
considerably less noise than an unmodified landing gear and, though full
fairings may not be commercially practical, the data represent a probable
lower limit of landing gear noise,” Soderman said. A fairing is a
tear-drop-shaped shield airplane designers use to reduce air drag from wind
flowing around odd-shaped surfaces.

Ames conducted the landing gear tests in collaboration with researchers at
NASA’s Langley Research Center, Hampton, VA, and Boeing Commercial Airplane
Co., Seattle, WA, as part of NASA’s quiet aircraft technology program.

In June 2001, engineers plan to attach the quarter-scale landing gear model
to a model of a quarter-scale commercial transport wing and to conduct more
tests. These are slated to take place in Ames’ larger 40-by 80-foot wind
tunnel. Researchers will measure airframe fly-over noise and surface wing
pressures with and without the landing gear extended during simulated
landing approaches. Researchers also will evaluate noise control devices.

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