  Nowadays, multirotors are playing important roles in abundant types of
missions. During these missions, entering confined and narrow tunnels that are
barely accessible to humans is desirable yet extremely challenging for
multirotors. The restricted space and significant ego airflow disturbances
induce control issues at both fast and slow flight speeds, meanwhile bringing
about problems in state estimation and perception. Thus, a smooth trajectory at
a proper speed is necessary for safe tunnel flights. To address these
challenges, in this letter, a complete autonomous aerial system that can fly
smoothly through tunnels with dimensions narrow to 0.6 m is presented. The
system contains a motion planner that generates smooth mini-jerk trajectories
along the tunnel center lines, which are extracted according to the map and
Euclidean Distance Field (EDF), and its practical speed range is obtained
through computational fluid dynamics (CFD) and flight data analyses. Extensive
flight experiments on the quadrotor are conducted inside multiple narrow
tunnels to validate the planning framework as well as the robustness of the
whole system.
