Low order LFT modelling and generation from the non-linear equations of motion for a fighter aircraft

The development of an LFT representation for the nonlinear HIRMplus research model is presented. The structured singular value μ is applied on the so developed LFT in order to clear critical areas in the flight envelope. μ-analysis allows to determine the combination of uncertain parameters within their respective bounds, for which a performance criterion or stability margin is worst. For a sensible worst-case analysis, it is important that the uncertain parameters are directly related to the physical uncertain/varying parameters in the nonlinear model. First a symbolic nonlinear model of the HIRMplus is developed, which depends on the physical parameters of interest in a rational way. Then the model is linearised symbolically. A low order LFT model is generated and compared with one obtained using an affine modelling approach. Although both model representations are aimed to cover the same flight conditions, the affine model is a function of a large number of artificial parameters. Both models are compared regarding their ease of generation, conservatism, accuracy, and applicability.