  Unlike squared (or alike) quadrotors, elongated bi-copters leverage natural
superiority in crossing tight spaces. To date, extensive works have focused on
the design, modeling, and control of bi-copters. Besides, a proper motion
planner utilizing bi-copters' shape characteristics is essential to efficiently
and safely traverse tight spaces, yet it has rarely been studied. Current
motion planning methods will significantly compromise their ability to traverse
narrow spaces if the map is inflated based on the long dimension of the
bi-copter. In this paper, we propose an efficient motion planning method that
enables the safe navigation of bi-copters through narrow spaces. We first adapt
a dynamic, feasible path-finding algorithm with whole-body collision checks to
generate a collision-free path. Subsequently, we jointly optimize the position
and rotation of the bi-copter to produce a trajectory that is safe, dynamically
feasible, and smooth. Extensive simulations and real-world experiments have
been conducted to verify the reliability and robustness of the proposed method.
