NOISE REDUCTION WITH A TWIN TIP VORTEX CONFIGURATION

A computer program currently used to generate rotor loads for acoustic calculations has been upgraded to handle a twin tip vortex configuration. To check out the program a study has been performed which evaluates the relative influence of various vortex parameters on the potential reduction of the noise signal. Systematic changes were made to vortex separation, relative rotation rate, relative strength and relative coresize. A single flight condition of 70 knots and 6 degrees descent angle has been used to represent typically severe blade/vortex interaction. Various aspects of the results have been presented in some detail. These comprise: wake interaction geometry, azimuthal variation of the local airload, acoustic forcing terms, acoustic waveform and directivity patterns. From the results it was concluded that the most important parameter is the separation of the two vortices and that noise reductions of 3-4 dB may reasonably be anticipated on the flightpath centreline with significantly more on the advancing side of the rotor. The results are in good agreement with a wind tunnel model test on a vane tip.