I. Introduction
Recently, increasing attention has been paid to vertical take-off and landing (VTOL) unmanned aerial vehicles (UAVs) pertaining to a wide area of vital applications, including patrolling for forest fires, traffic monitoring and surveillance rescue via hovering, tracking and coordination [1]–[4]. Recently, flapping-wing flying robotics have also attracted much attention by devising novel neuro-adaptive methods [5], [6]. Compared with fixed-wing aircrafts, the rotary-wing UAV possesses the significant advantage that it can take-off and land vertically in limited spaces and is easy to hover over the target. Note that the quadrotor UAV (QUAV) is a typical VTOL-UAV with simple mechanical structure and favorable maneuverability. In this context, as a remarkable platform of the UAV, the QUAV has attracted numerous research [7]–[10].