Aerodynamic interactions between rotors are important factors affecting the performance\nof in-plane multirotor Unmanned Air Vehicles (UAVs) or drones, which are the majority of small\nsize UAVs (or mini-drones). Optimal design requires knowledge of the flow features. The low\nReynolds number of many UAV rotors raises the question of how these features differ from those\nexpected by traditional analytical methods for rotorcraft. Aerodynamics of a set of side-by-side\nrotors in hover over a range of rotor separation and Reynolds number is studied using high-speed\nStereo Particle Image Velocimetry (SPIV) and performance measurements. The instantaneous and\ntime-averaged SPIV data presented here indicate an increase in inter-rotor wake interactions with\ndecrease in rotor spacing and Reynolds number. A dip in rotor efficiency at small rotor spacing at\nlow Reynolds number is observed through thrust and torque measurements. The basic components\nof in-plane multirotor wake and velocity profiles are identified and discussed to help generalize the\nfindings to a wide range of drones. However, the data provide confidence in traditional analysis\ntools, with small modifications.
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