Structural dynamic loads in response to impulsive excitation

The modal approach is used to analyze the dynamic loads on a flexible structure due to local impulsive excitations such as that caused by store ejection from a flight vehicle. First-order, time-domain equations of motion in generalized coordinates are constructed for restrained and free-free structures, without and with unsteady aerodynamic effects. The dynamic loads associated with the structural response are expressed by the mode displacement (MD) and by the summation-of-forces methods. The MD approach is simpler and easier to apply, but requires the inclusion of more modes for obtaining results of acceptable accuracy. A rigorous comparison between the resulting loads shows that the performance of the MD method is especially poor when the excitation is local and impulsive. A dramatic improvement is obtained when the generalized coordinates are based on normal modes calculated with fictitious masses at the excitation points. Fictitious masses are also used to generate artificial load modes that yield simple and efficient expressions for integrated shear forces, bending moments, and torsion moments at various structural sections.