Conditional Averaging of Microphone Data from HART II

Abstract

This report is related to the analysis of the HART II (Higher Harmonic Control [=HHC] Aeroacous-tic Rotor Test II) microphone data with respect to the conditional averaging method. Up to now only simple averaging of the instantaneous time histories was used, but without con-sideration of differences caused by fluctuations in rotational speed and varying vortex locations and blade vortex interactions. This so called time jitter has an important influence on the aver-aged time history since by only simple averaging the peak-to-peak magnitude is reduced and the azimuthal difference between minimum and maximum peaks is falsified. This happens even in the case of identical individual time histories in each revolution, just with a scatter in its azi-muthal location caused by fluctuations.