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Reduction of aircraft noise by nacelle treatment and active control

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Decibel reduction for aircraft engines

Considerable noise is generated by aircraft in flight, especially on landing approach and takeoff. It is this noise factor that presents one of the greatest growth-limitation factors to the commercial aircraft industry, as communities are reluctant to have low level aircraft flying overhead that generate extensive noise.

Industrial Technologies

In an effort to reduce the noise generated by the powerful engines of jet airliners, the EU has, under the RANNTAC programme, investigated noise reduction methods through advance testing measures both in a theoretical and experimental approach. Reducing jet engine noise is a complex problem for many reasons, not the least of which is that a single rotary engine generates several different noise types from various locations within the engine itself. In order to reduce noise levels to be compliant with noise reduction standards, several factors were investigated. One of the largest noise sources are the compressed, high speed exhaust gasses propelled from turbines as they cut their way through the air. By investigating innovative designs such as the saw-toothed edged exhaust nozzle, considerable noise level reductions followed. Additional studies were conducted in a number of areas such as air-intake, especially since noise generated from the front end of the rotary fan has been found to be the most irritating source of all noise produced by aircraft. Investigations into passive and active absorbers, jet cavity length and shape, hollow spheres and a variety of other concepts, all produced highly favourable results, confirming the potential of active noise control, showing considerable decibel reduction in both aircraft engines as well as in aircraft cabins. A final word, since development costs are of considerable importance to airliner operations, part of the RANNTAC studies involved the risk-cost factor analysis as well as the impact these additional developments would have in manufacturing, development feasibility and the integration of these noise reduction technologies.

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