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We consider flat differential control systems for which there exist flat outputs that are part of the state variables and study them using Jacobi bound. We introduce a notion of saddle Jacobi bound for an ordinary differential system of nequations in n+mvariables. Systems with saddle Jacobi number equal to 0 generalize various notions of chained and diagonal systems and form the widest class of systems admitting subsets of state variables as flat output, for which flat parametrization may be computed without differentiating the initial equations. We investigate apparent and intrinsic flat singularities of such systems. As an illustration, we consider the case of a simplified aircraft model, providing new flat outputs and showing that it is flat at all points except possibly in stalling conditions. Finally, we present numerical simulations showing that a feedback using those flat outputs is robust to perturbations and can also compensate model errors, when using a more realistic aerodynamic model.

We introduce a concept of differential flatnessfor systems described by nonlinear partial differential equations. It generalizes the now classical notion of differential flatness for finite differential systems and its recent extensions to linear partial differential equations. We apply it to the motion planning of a very flexible rod, outside of the linear approximation range.

According to a theorem first stated by Clifford, Noether [8] and Rosanes [12], which later received a complete proof by Castelnuovo [2], the Cremona group of the plane is generated by de Jonquières transformations. J.F. Ritt [10] asked whether such a result could be generalized to the differential plane. We give a negative answer, relying on a flat system of Rouchon.