A quantitative analysis method was put forward

concentrating on couplings among state variables in helicopter motional modes. In the method

based on motional modes resulting from a decoupling procedure

couplings among state variables in motional modes were quantitatively described in combination with the matrix of aerodynamic derivatives. And all couplings were distinguished quantitatively by instantaneous characteristics and integrated characteristics. With the application into helicopters

Lynx

Bo105

and Puma

coupling characteristics of hovering longitudinal motional modes were analyzed. It showed that the coupling on pitching rate from longitudinal velocity by speed stability played a vital role in the unsteady hovering longitudinal oscillatory mode. Based on the quantitative analysis of examples

simplified models were obtained with the selection of main couplings

and the resulted deviation of natural frequency and damping ratio of motional modes was less than 2%

superior to simplified models in references. So it is feasible to obtain simplified models on the basis of the quantitative analysis.