The use of a closed loop controller scheme for the validation of nonlinear helicopter models

Abstract

The focus of this study is the tool development for the validation of a nonlinear helicopter flight mechanic model used at the German Aerospace Center (DLR) in its real-time flight simulator by using a controller based approach. Validating a highly coupled, unstable, multi-input multi-output dynamic system is proved to be challenging. Closed-loop validation approach is selected and particular care was taken to ensure that the feedback controller does not induce correlated inputs and thus conceal the model characteristics. However, the closed-loop should be be performant enough to maintain the stable simulation. Model predictive ability is analyzed by root mean square error cost function and by comparing dynamic characteristics in the time domain of the model and the flight test data from DLR's research helicopter ACT/FHS at 60 knots, 3-2-1-1 multi-step inputs in all axes. The results show that the model correctly predicts the trends of the on-axis responses, but the off-axis responses have potential for improvement. Possible technique to enhance model fidelity is proposed in the conclusion.