MINIMUM-STATE UNSTEADY AERODYNAMIC APPROXIMATIONS WITH FLEXIBLE CONSTRAINTS

Minimum-state (MS) rational-function approximations of unsteady aerody namic force coefficient matrices for time-domain state-space aeroelast ic analysis are discussed, The MS approximation procedure was original ly developed with three approximation constraints assigned to each ter m, The constraints were used for a significant reduction of the number of unknowns in the iterative nonlinear least-square solution. While s ome constraints may be requested anyway by the analyst, the removal of others leads to more accurate approximations, A new formulation that starts with the unconstrained MS problem is presented, Regular solutio ns of unconstrained cases increase the computation time per iteration by factors of about 10 compared to typical three-constraint solutions. a modified solution method, which takes advantage of the special stru cture of the interim matrix equations, reduces the computation time of unconstrained cases by factors of about 7. The new formulation allows maximum flexibility with the number and type of zero to three constra ints.