APPLICATION OF A LOW-FREQUENCY AEROELASTIC ELEMENT METHOD TO THE HARMONIC GUST RESPONSE ANALYSIS OF A FLEXIBLE AIRPLANE

Abstract

The low-frequency aeroelastic element method developed recently at Helsinki University of Technology, Finland, is briefly reviewed. In this method, the unsteady aerodynamic forces are calculated by using a low-frequency vortex panel method. The state of motion of the flexible airplane is given in terms of rigid-body modes and measured elastic modes. For comparison, some published experimental and calculated results obtained by this method for the B-47A sweptwing bomber are shown. The method is applied to the harrnonic gust responce analysis of the PIK-20D sailplane. Calculaced plunge, pitch, and wing bending responses in terms of the gust frequency are given. Elastic gust amplification factors and zero-crossing frequencies at four spanwise wing stations are shown in different flight conditions and compared with classical plunge-only values. In some cases, the elastic gust amplification factors are seen to be over 70 per cent in excess of the rigid plunge-only values, which is much more than the common design practice. Possible causes of this are briefly discussed.