  This paper addresses the trajectory tracking control problem for
underactuated VTOL UAVs. According to the different actuation mechanisms, the
most common UAV platforms can achieve only a partial decoupling of attitude and
position tasks. Since position tracking is of utmost importance for
applications involving aerial vehicles, we propose a control scheme in which
position tracking is the primary objective. To this end, this work introduces
the concept of attitude planner, a dynamical system through which the desired
attitude reference is processed to guarantee the satisfaction of the primary
objective: the attitude tracking task is considered as a secondary objective
which can be realized as long as the desired trajectory satisfies specific
trackability conditions. Two numerical simulations are performed by applying
the proposed control law to a hexacopter with and without tilted propellers,
which accounts for unmodeled dynamics and external disturbances not included in
the control design model.
