We study the problem of flatness of two-input driftless control systems. Although acharacterization of flat systems of that class is known, the problems of describing allflat outputs and of calculating them is open and we solve it in the paper. We show thatall x-flat outputs are parameterized by an arbitrary function of threecanonically defined variables. We also construct a system of 1st order PDE’s whosesolutions give all x-flat outputs of two-input driftless systems. Weillustrate our results by describing all x-flat outputs of models of anonholonomic car and the n-trailer system.
