Abstract: For a concerned plane , ground modal experiments show that there are 2 order modals whose frequenciesare close ,which correspond to the 1st order bent modal of wing ( FBMW) and symmetric pitching modal of engine(SPME) , respectively. To evaluate whether these two modals cause flutter under design flying speed , the frequencyresponse functions (FRFs) under different flying speeds were measured through a pulsing excitation method. Due tothe constriction , these FRFs were based on single measurement; hence their ratios of signal to noise were very low ,in addition to the dense modals in the concerned frequency bands. The modifying orthogonal polynomial (such as introducing extra weighting factors to reduce the variance of estimation , choosing both bases of the denominator andnumerator as identical orthogonal polynomial) was exploited to modal analysis , and the related apriority informationwas taken use of picking off the pseudo modals. The studies show that , as the fly speed increasing , the frequenciesof FBMW and SPME approach to each other , and their damping ratios descend; however as for the modal parameters under design flying speed , there is still much wide margin to flutter.

Key words: flutter, flight test, modal analysis, pulse excitation, orthogonal polynomial