“…Unmanned vehicles have currently been playing a crucial role in many aspects of our daily lives where many robots can be used to operate and assist with complex tasks. Especially, unmanned aerial vehicles (UAVs) have been widely researched to create many versatile applications for replacing human from impossible missions and dangerous tasks [1], e.g., in agriculture [2], industry [3,4], and military [5,6]. Quadcopters, which are one of the most popular class of UAVs, have only four actuators, while there are six degrees of freedom, in that it is definitely an under-actuated system.…”