Hi, Im wondering if / how commercial FEA codes take into account additional unbalance forces due to the off-axis displacement of masses. For instance, if I am modeling a jeffcott rotor with an unbalance, I would traditionally represent this unbalance with a force that increases with speed^2. However during the analysis, the jeffcott rotor moves off axis (whirls) and should have an additional unbalance force equal to the mass of the disk times the displacement of the CG times the speed^2. I am not sure how commercial FEA codes account for this force (if they do) - would this come about due to mass matrix being updated? Even if the mass matrix were updated, how would it take into account the new locations (displacements) of the disk into the generation of the new load? Mass matrix is just multiplied by acceleration... unless the mass is fictitiously increased in a non-linear update (a la tangent stiffness matrix for geometric stiffening/softening) Anyone that could shed some light on the situation would be greatly appreciated!! Thanks!!