  Unmodeled aerodynamic disturbances pose a key challenge for multirotor flight
when multiple vehicles are in close proximity to each other. However, certain
missions \textit{require} two multirotors to approach each other within 1-2
body-lengths of each other and hold formation -- we consider one such practical
instance: vertically docking two multirotors in the air. In this
leader-follower setting, the follower experiences significant downwash
interference from the leader in its final docking stages. To compensate for
this, we employ a learnt downwash model online within an optimal feedback
controller to accurately track a docking maneuver and then hold formation.
Through real-world flights with different maneuvers, we demonstrate that this
compensation is crucial for reducing the large vertical separation otherwise
required by conventional/naive approaches. Our evaluations show a tracking
error of less than 0.06m for the follower (a 3-4x reduction) when approaching
vertically within two body-lengths of the leader. Finally, we deploy the
complete system to effect a successful physical docking between two airborne
multirotors in a single smooth planned trajectory.
