Stability analysis of airplane with MIMO control system based on frequency methods

In the framework of frequency analysis various methods of stability analysis of airplane with MIMO control system are considered. They assume that control system is open for selected signal with other loops closed, or that system is open for all signals of selected loops and allow solving stability equation and calculating transfer functions matrix eigenvalues. It is shown that all methods deal with different forms of the same equation, and thus the equivalence of methods considered for calculating closed loop stability domains is proved. The application of methods is demonstrated for calculation of stability domains for airplane equipped with MIMO stability and control augmentation system in lateral motion that uses ailerons and rudder as control surfaces. It is shown that in case of MIMO system open loop transfer function that defines closed loop stability is nonlinear function of control system gains. It introduces uncertainty to concept of amplitude and phase margins.