Research progress in rotor airflow model of plant protection UAV and droplet motion mechanism

Abstract: The usage of aircrafts to perform crop spraying is becoming increasingly prevalent, owing to their high speed and efficiency in operations, compared with the manual crop spraying representing an expensive and time-consuming labor activity in agricultural production. Agriculture drones or unmanned aerial vehicles (UAVs) have attracted increasing attention, due to their small size, easy handling, and without a runway, particularly suitable for small farmlands in Asia. Currently, the agriculture drones have been widely used in modern precision agriculture. The UAVs has produced rapidly in East Asia for their excellent application efficiency and ability to protect densely planted crops, especially in China. A survey that conducted by the National Agricultural Technology Extension Center of Ministry of Agriculture and Rural Affairs showed that there are more than 50 000 plant protection drones, and 311 333.3 hm2 cultivated field spraying in 2019. Nowadays, the spray technology applied on UAVs has developed into the main plant protection approaches. However, the key theory associated with the spraying technology of UAVs remains unknown. Three key theories mainly are: 1) The characters of downwash flow structure and its effect on droplet movement during aerial spraying, as well as plant canopy. Unique complex feature in the downwash flow structure of UAVs influences the law of droplet dispersion, the velocity and direction of droplet movement, as well as the canopy structure and morphology of target plant, particularly on the incidence angle, the impact diameter, and impact velocity of droplet, during the interaction process between droplet and plant. There was a significant influence on the droplet deposition and drift. 2) The flight parameters of UAVs, spray parameters in a spray system applied on UAVs, and physical quality have posed an important effect on the droplet deposition and drift. The flight parameters UAVs also influence the downwash flow structure and droplet transport distance. The unreasonable parameters of UAVs flight can cause some deposition, resulting the drift potential of droplet. The formulation and spray parameters in the UAVs can also influence the droplet size, dispersion, and transport velocity, indicating the interaction process between the droplet and downwash flow of UAVs, as well as the law of droplet deposition and drift. 3) The interaction mechanism between the droplet and plant. There was a significant influence on the retention and drift of final droplet, and the biology efficacy of pesticide, indicating a critical part of plant protection. An investigation on the key theories can contribute to improve the performance of spray technology using UAVs. This review covers the current status related to three key theories, presenting an idea to build the downwash flow model of UAVs, a droplet dispersion model, a droplet evaporation model, the interaction model between downwash flow and droplet, between downwash flow and plant canopy, and between droplet and plant canopy, and a spray retention model applied on UAVs basing on these daughter model. The parameters of flight and spray were optimized by the spray retention model, indicating an excellent spray performance.