I. Introduction
Much research has been done on the control of a vertical takeoff and landing (VTOL) aircraft [1]–[25]. This is due to the fact that the VTOL aircraft model is a simplified model of Harrier (YAV-8B) aircraft and contains the essential characteristics of the original model. In particular, the rolling moment and the lateral acceleration of the aircraft are known to be closely coupled, and this input coupling results in the existence of unstable zero dynamics (i.e., nonminimum phase characteristics). Moreover, the VTOL aircraft is an underactuated system in that 3-D state variables (2-D planar position and 1-D roll angle) should be controlled with 2-D control variables (the thrust and the rolling moment).