Flight control design for a nonlinear non-minimum phase VTOL aircraft via two-step linearization

In this paper, we offer a new design method for the flight control of a nonlinear nonminimum phase VTOL aircraft. The nonminimum phase property is caused by the small coupling between rolling moments and lateral acceleration, and a few researchers have derived control laws by neglecting the coupling. We show that the true model can be transformed equivalently into a model with zero coupling by changing the plant output from the center of mass to the Huygens center of oscillation. Then, we design a controller by applying a linear high gain approximation of backstepping to the model; we call this method a two-step linearization. Computer simulation shows that the control law robustly stabilizes the true model with good tracking for both small and large couplings. Experiments using a twin rotor helicopter model also show equally good performance.