  This letter presents a novel and retractable ring-shaped quadrotor called
Ring-Rotor that can adjust the vehicle's length and width simultaneously.
Unlike other morphing quadrotors with high platform complexity and poor
controllability, Ring-Rotor uses only one servo motor for morphing but reduces
the largest dimension of the vehicle by approximately 31.4\%. It can guarantee
passibility while flying through small spaces in its compact form and energy
saving in its standard form. Meanwhile, the vehicle breaks the cross
configuration of general quadrotors with four arms connected to the central
body and innovates a ring-shaped mechanical structure with spare central space.
Based on this, an ingenious whole-body aerial grasping and transportation
scheme is designed to carry various shapes of objects without the external
manipulator mechanism. Moreover, we exploit a nonlinear model predictive
control (NMPC) strategy that uses a time-variant physical parameter model to
adapt to the quadrotor morphology. Above mentioned applications are performed
in real-world experiments to demonstrate the system's high versatility.
