Flying qualities assessment using nonlinear frequency response analysis

Current methods of assessing flying qualities are based on linear system theory, which cannot capture the nonlinear phenomena commonly found at high angles of attack. Although bifurcation analysis has been proven to be a powerful tool for numerically analyzing nonlinear flight dynamics, in its standard (unforced) form, the method is unsuitable for flying qualities assessment as no information on the transient response is provided. To address this shortcoming, we propose the use of an extension of bifurcation analysis that treats the aircraft as a harmonically forced system. This facilitates the evaluation of the frequency-domain dynamics where the response is unsteady by definition. It is shown that the method can identify regions with strong nonlinearities that lead to degraded flying qualities. Time simulations of the closed-loop step responses in these undesirable regions show that despite the predictions of desirable behaviors using linear analysis, the closed-loop aircraft can still be attracted to an isola, experience control reversal, or have higher overshoot than anticipated.