All quiet on the flightpath estate

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 landings.

In a series of tests conducted at NASA’s Ames Research Centre the sounds have been depicted as coloured images on a computer screen. These new test data are reported to provide critical visual information to aircraft designers concerned about possible enforcement of stricter aircraft noise rules.

‘Some airports are imposing night-time curfews on noisy take-offs and landings, encouraging aircraft manufacturers to make quieter planes,’ said Paul Soderman, leader of the Ames aeroacoustics group. ‘If US 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 Organisation, which makes aeroplane noise rules for much of the non-US aviation community.

‘We can take steps to analyse and eventually reduce airplane noise significantly,’ said Soderman. Airframe parts, including landing gear, flaps and slats, create almost as much noise as the aircraft engines on approach to landing.

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 is said to minimise wind tunnel airflow noise so that landing gear noise sources as small as 6 m can be identified. At full-scale, these sources are 24 mm across, according to engineers who conducted the tests.

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 travelling faster than it should as it approaches the runway,’ said Soderman. ‘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 aeroplane without causing as much noise, he added.

‘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,’ said Soderman.

Ames conducted the landing gear tests in collaboration with researchers at NASA’s Langley Research Centre and Boeing Commercial Airplane Company 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.

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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