Numerical Investigation of the Wind Field Disturbance Around Small Rotorcraft Uncrewed Aerial Vehicles

Accurate in situ wind measurements from rotorcraft uncrewed aerial vehicles (UAVs) can be impacted by the disturbed flow generated by the rotors. However, the extent of this disturbance depends on flight mode, ambient wind, and vehicle configuration, making optimal sensor placement or devising appropriate corrections nontrivial. This study uses steady-state Reynolds-averaged Navier–Stokes (RANS) simulations with an actuator disk model to characterize the flow field around representative quadcopter, hexacopter, and octocopter UAVs under conditions representing hover, ascent, and descent, for different thrust, and with and without crosswind of different magnitude. The results show that the size and shape of the disturbance field vary strongly with flight mode, with descent producing the largest region of disturbed air around the vehicle and ascent the smallest. Crosswinds advect and distort the disturbance region and reduce its vertical extent by sweeping the rotor wash downstream. The disturbance field geometry was found to scale primarily with overall aircraft size and was largely independent of rotor configuration. The effect of differing the rotor thrust was found to approximately scale using a length scale based on the volume flow rate of air through the the rotor plane. Based on these results, to maintain measurement errors below 0.5 m/s, recommended anemometer locations are at least 2.5 aircraft radii from the UAV central axis for hovering conditions when the weight of the aircraft relative to the area swept by the rotors is near 10 kg per square meter. This recommended distance is expected to scale linearly with this ratio, and will reduce under crosswind conditions or when measurements are made during ascent.