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Researchers from Tohoku University and the Kanazawa Institute of Technology in Japan have found that the stability of UAVs in heavy winds can be improved by analysing the rotor angle and placement (writes Nick Flaherty). The rotors of a multi-rotor craft can flap, and the vehicle can pitch upwards in a headwind, so the researchers set out to evaluate whether angling the rotor blades differently would improve the control of quadrotor vehicles in windy conditions. Pitching is caused by three things: the drag of the body, the asymmetry-induced flow distribution on the rotor with the wind, and rotor thrust difference between the front and rear rotors. The team first estimated the effects of the wake of the front rotors on those at the rear, then isolated the rotors from the vehicle and measured the effect of different angles in a low-speed wind tunnel. This showed that angling the rotors to the outer side by 75 º kept the airflow passing each rotor blade isolated, but increasing it to 90 º or above meant the wake of the front rotors affected the performance of the rear ones. This analysis led to testing various angles of rotor attachment to a prototype quadrotor UAV in a wind tunnel to see the effect on pitching moment generation. The researchers measured the effects of outward and inward tilting of the rotor blades for five different angles, and found that tilting them by 20 º to the outer side degrades the pitch of the vehicle by 26% in a wind of 8 m/s. This is useful for the design of quadcopters used for inspecting wind turbines and disaster sites, or for rescue activities in harsh conditions where cameras need to be stable to provide clear images. It also allows the craft to operate in a wider range of conditions. New angle on UAV rotors Airborne vehicles Tilting the rotor blades by 20 º to the outer side degrades the pitch by 26%