GUST RESPONSE ANALYSIS BY MODAL DECOMPOSITION OF THE GUST IMPINGEMENT DISTRIBUTION

A new formulation referred to as the mode impingement method has been developed which achieves a radical improvement in computational efficiency over the current panel method of multi-dimensional (2-d and 3-d) gust response analysis. The mode impingement method employs an orthogonal decomposition of the gust impingement distribution to reduce a large number of statistically interdependent panel inputs to a small number of statistically independent impingement patterns or gust modes. Since the normalized gust modes are slowly varying functions of frequency, orthogonal decomposition need be performed only at the chosen interpolation frequencies for which aerodynamic input data are furnished. Modal accuracy is then maintained throughout the interpolation interval by simple interpolation procedures. The small deficit in local gust amplitude resulting from exclusion of the higher modes is eliminated by a compensating adjustment in the local amplitude of the included gust modes. Computational techniques are furnished to deal with the anisotropic turbulence inherent in measured gust input spectra. Formulations are provided to obtain transfer functions and six-component spectral tensors for loads and accelerations, as well as cross transfer functions and coherence functions for responses relative to gust components measured by a probe. Results are compared with corresponding dynamic response flight test measurements from the C-5A aircraft, and also with a 2-d analysis and with the conventional 1-d gust response analysis. The computational efficiency of the mode impingement method relative to the 3-d panel method and the 1-d analysis is also examined.