LFT representation of a longitudinal perturbed aircraft model by flatness approach

In this paper, an linear fractional transformations (LFT) model representing, the dynamic of the gap between a nominal model and a perturbed model of a longitudinal aircraft fighter along a trajectory is given. The proposed approach is based on a simplified model used to determine a nominal trajectory and corresponding input. This simplified nonlinear model is a flat nonlinear system. As the measures on the so-called flat outputs, used in the linearizing feedback, are corrupted with noise and effects of unmodelled dynamics or parametric errors (model simplifications), the state space trajectory of the actual plant is not identical to the reference state space trajectory. Then the exact linearization is not achieved, but, as long as the tracking error remains small enough, the actual quasi-linearised plant can be modelled as a disturbed linear plant. The gap behaviour is modelled by a set of linear models described by a LFT model.