Aeroelastic Loads in Response to Concentrated Excitation Using Fictitious Masses

Abstract

Structural design loads in dynamic response to concentrated excitation forces such as in landing and store-ejection impacts are calculated by state-space aeroelastic equations of motion based on the modal approach where the structural displacements are assumed to be a linear combination of a set of low-frequency normal modes which serve as generalized coordinates. The generalized response parameters are used for recovering the load distributions at selected time points using the mode-displacements (MD) approach. The MD approach is shown to be grossly inaccurate in recovering the loads at and around the excitation points when applied in the conventional way. The reasons for these inaccuracies are analyzed and large fictitious masses that load the excitation degrees of freedom when the normal modes are generated are used to solve the problem. Fictitious masses of magnitudes similar to that of the entire structure are shown to yield dramatic accuracy improvements. The added computational cost is negligible and the method is robust with respect to the fictitious mass magnitudes. The method is demonstrated using numerical examples of landing loads on a generic transport aircraft and on the A400M military transport aircraft currently under development. It is shown that the use of fictitious masses causes substantial improvement in the results accuracy. It is also shown to be very robust and conveniently applicable using common commercial codes.