  In this letter, we present a novel bi-modal bi-copter robot called Skater,
which is adaptable to air and various ground surfaces. Skater consists of a
bi-copter moving along its longitudinal direction with two passive wheels on
both sides. Using a longitudinally arranged bi-copter as the unified actuation
system for both aerial and ground modes, this robot not only keeps a concise
and lightweight mechanism but also possesses exceptional terrain traversing
capability and strong steering capacity. Moreover, leveraging the vectored
thrust characteristic of bi-copters, the Skater can actively generate the
centripetal force needed for steering, enabling it to achieve stable movement
even on slippery surfaces. Furthermore, we model the comprehensive dynamics of
the Skater, analyze its differential flatness, and introduce a controller using
nonlinear model predictive control for trajectory tracking. The outstanding
performance of the system is verified by extensive real-world experiments and
benchmark comparisons.
