Modal Experimental Continuation of an Aircraft During Ground Vibration Test

The phase-locked loop experimental continuation method is extended to the identification of nonlinear dynamics of an aircraft during a large-scale ground vibration test. The proposed framework enables backbone curve tracking in a multi-input–multi-output configuration by using force appropriation and a dedicated phase control strategy. Force appropriation and phase control over multiple excitation points are presented, and particular attention is devoted to introducing weighted mode purity indices to robustly assess nonlinear mode isolation. With appropriate force patterns and a suitable phase control strategy for multi-input–multi-output testing, isolation of closely separated modes is effectively realized along the investigated amplitude points. The phase control strategy follows a least-squares fitting of all driving point signals in the complex plane, hence providing global control to minimize the mean phase deviation at all excitation points. The modified mode purity indices give a better assessment of mode isolation by taking into account higher harmonics and spatial force distribution. A full-scale aircraft is used to illustrate the proposed methods and to demonstrate their effectiveness in identifying nonlinear characteristics.