Application of Ultrasonic Anemometer Array to Field Measurements of the Downwash Flow of an Agricultural Unmanned Helicopter

Unmanned helicopters are widely used in agricultural spray applications; however, there is a lack of high-accuracy field data on the complex downwash flows caused by such helicopters to support efficient spraying. Therefore, we aimed to collect high-accuracy reference data by developing an ultrasonic anemometer array and related software to measure the downwash flow velocity field of a common agricultural unmanned helicopter (Copterworks AF-25B) during outdoor hovering and forward flight. Differential GPS and image scaling were used to determine the exact position and velocity of the helicopter to ensure repeatability of the tests. When the hovering height of the rotor above the ground plane was y/R = 2.5 (where y is the height above ground level and R is the radius of the rotor), the maximum downwash velocity reached 7 m s-1 at x/R = 0.5 (where x is the radial distance from the axis of the rotor). In addition, we obtained the spatial distribution of the turbulence kinetic energy. The attached flow velocity in the near-ground boundary layer increased radially and reached a maximum of 3 m s-1 at x = 3.6 m, after which it decreased. Comparatively, the maximum downwash velocity during forward flight occurred between x/R = 0.5 and 1.5 and was only about 3 m s-1, which was less than half the maximum downwash velocity while hovering. In addition, the circulation flow below the rotor tips had a velocity of 1.5 m s-1. The downwash flow lasted more than 1 s as the helicopter passed over a given point on the ground, the downwash velocity decreased from 3 m s-1 to 1.5 to 2.5 m s-1, and the symmetry of the flow field was not maintained. This study is the first application of an ultrasonic anemometer array to measure the airflow around an unmanned helicopter and provides high-accuracy field data that can be used as a reference for future indoor and simulation studies. Keywords: Agricultural unmanned helicopter, Downwash flow velocity field, Forward flight, Hovering, Ultrasonic anemometer.