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Reduction of aircraft noise by nacelle treatment and active control

Exploitable results

The RANNTAC programme has allowed to investigate new liners, the negatively scarfed intake and Active Noise Control technologies from both theoretical and experimental points of view. From results obtained during the project, the following conclusions can be drawn up. For liners, the following technologies have been investigated: - Hollow spheres developed by METRAVIB and SNECMA; - Variable cavity depth sequence liner developed by University of Salford; - Non locally reacting liners developed by NLR; - Three degree of freedom developed by SNECMA and Hispano Suiza Aerostructures; - Passive/active absorbers developed by DORNIER and EADS CRC Germany; - Hybrid liner developed by Ecole Centrale de Lyon and METRAVIB. Significant reduction of the fan-forward noise with respect to current technologies has been measured thanks to these technologies. These improvements come from a much better control of reactance that remains finite on a broader frequency range. Furthermore, A/C noise computations have shown that additional benefit could be achieved with a slight increase of liner's resistance. The Negatively Scarfed Intake has been investigated by Rolls-Royce who carried out back to back scale model tests between 0° and -15° scarfed intake. Significant reductions of noise have been pointed out in the fly-over plane, at both approach and take off. Active noise control has been addressed from both algorithm and actuator technology points of view. Algorithms were studied investigating both in-duct and far-field controls. From actuator points of view, various driver technologies associated to various sound generation mechanisms were investigated and very promising level to size ratios were obtained. Scale model test together with predictions have confirmed the potential of active control approaches but have also pointed out the need to work on its integration in the nacelle.