An Assessment of the Importance of the Residual Flexibility of Neglected Modes in the Dynamical Analysis of Deformable Aircraft

SUMMARY The mathematical framework for a unified approach to the dynamical problems of deformable aircraft, set up by Taylor in his contribution to R & M 3776, is used as the basis for a limited numerical investigation of the usefulness of the residual flexibility concept in truncated modal analyses. A finite-element model of a supersonic transport aircraft of slender-delta configuration is the subject of stability and response calculations, in which various representations of the structural deformability are used. These comprise up to four natural modes both with and without the residual flexibility of the remaining modes. It is concluded that the addition of residual flexibility to a structural model which comprises only one or two modes significantly improves the accuracy of estimates of low-frequency characteristics. However, if a single model is to be used in an integrated approach to the aeroelastic problems of an aircraft, it must incorporate a fairly large number of modes in order to deal with the higher-frequency problems. In these circumstances the residual flexibility concept would seem to have little practical value.