Aerodynamic evaluation of wind speed sensor placement on UAVs for meteorological applications

Abstract. Vertical takeoff and landing Unmanned Aerial Vehicles (VTOL-UAVs) can provide accurate, highly resolved, and repeatable atmosphere measurements, especially in scenarios where conventional measurement techniques are inadequate, or impractical. Wind estimation using UAV-mounted sensors is significantly influenced and contaminated by rotor-induced flows, making its placement a critical design consideration for meteorological measurements. This work investigates optimal locations for wind speed sensor placement on various UAV systems by studying the rotor-induced flow field using a free-vortex wake model (FVM) across a single-rotor, quad-rotor, and hexa-rotor configurations under hover, axial descent, and forward-descent flight conditions. This model is validated with an in-house experimental setup for velocity measurements. Care is taken to ensure these results are applicable across a wide range of practical UAV operating conditions through the dimensional disk loading (DL) parameter. The rotor-induced velocity fields are evaluated on multiple planes perpendicular to the rotor disk, and "quiet-zones" for sensor placement are identified based on a threshold of 1% rotor-tip speed. Results reveal that the location and extent of the quiet-zones are strongly dependent on the flight condition. For single-rotor in hover, a well-defined quiet-zone exists above the rotor disk, while viable sensor placement locations are substantially reduced in axial descent. Forward descent introduces asymmetric wake skew, limiting quiet-zones to the upstream, at smaller axial distances. For multi-rotor configurations, the system geometric center is particularly suitable for sensor placement, compared to axial locations about the individual rotor hubs. Overall, by analyzing various rotor systems in different flight conditions, the present work provides a practical guidance for the design of accurate UAV-based atmospheric measurement systems.