Flight performances of hovering single and multiple rotors inside tunnels are studied. Following the wide spread of applications of the multiple rotor type drones, possible observation and search inside tunnels are under investigations. With constraints of the surrounding walls in a narrow tunnel, there is concern that the downwash caused by the hovering rotors may circulate around the aircraft and the flight performance may be severely deteriorated. In this numerical simulation study, a single rotor and a hexa-rotor UAV are placed inside straight square tunnels with various width. It is found that when the gap between the rotor tip and the wall is less than two diameters of the rotor, the hover performance is severely degraded.
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