Interpolation of aircraft source noise directivity patterns modeled by spherical harmonics

Abstract

Time and funding constraints reduced the total number of measured power settings for aircraft source noise models. These constraints resulted in a need to estimate the source characteristics at power settings other than those measured. Interpolation was one method of estimating the source characteristics at power settings not measured. Acoustic measurements of an Unmanned Aircraft System (UAS) (Silver Fox) operated at one percent throttle setting increments over the entire power range were conducted on a test fixture at Owens-Corning in Columbus Ohio. The results demonstrated the relationship between throttle setting and noise level was linear over a large range. Source noise directivity patterns were constructed for each one percent increment in throttle using a spherical harmonic series expansion and single value decomposition (SVD); directivity patterns were also constructed using a proposed interpolation methodology. The proposed interpolation methodology employed directivity patterns, constructed with SVD, which bounded the linear region of the noise level - throttle setting function. Predictions from the SVD and interpolated directivity patterns were compared to validation measurements and showed that the interpolation method had similar accuracy to the spherical harmonic series expansion.