A Physics-Based Approach to Characterizing Helicopter External Noise Radiation from Ground-Based Noise Measurements
Schmitz, Fredric H
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This thesis describes a new method of characterizing the external noise radiation of a helicopter suitable for use in the generation of ground noise contours for community land use planning and assessing the acoustic observability of helicopter flight plans. This work is an extension of the semi-empirical Rotorcraft Noise Model / Quasi-Static Acoustic Mapping (RNM/Q-SAM) methodology of characterizing helicopter externally radiated noise using acoustic radiation hemispheres. Current methods of interpolation of data on RNM acoustic radiation spheres are found to lead to high levels of inaccuracy when using sparse microphone arrays. A new method of interpolation based on the theory of radial basis functions is developed in this thesis and shown to lead to significantly improved accuracy. This thesis also extends the RNM/Q-SAM methodology to turning flight conditions. New test procedures are developed for steady turning flight conditions and then used in the acoustic flight testing of the Bell 206B helicopter. The extended RNM/Q-SAM method is applied to the resulting data set in order to generate the first acoustic radiation hemispheres for a helicopter in steady turning flight across a range of flight path angles. The results indicate that the extended Quasi-Static Acoustic Mapping technique is valid for steady turning flight Blade-Vortex Interaction noise. Furthermore, steady turning flight alone is shown not to lead to large increases in externally radiated noise compared to similar straight-line flight conditions. This indicates that high BVI noise levels reported during turns in prior research were most likely caused by transient maneuvers and not turning flight alone.