The influence of rotor downwash on spray distribution under a quadrotor unmanned aerial system

•Computational Fluid Dynamics model of sprays interaction with propeller airflow.•CFD model for multi nozzle/propeller crop sprayer, crosswind and forward flight.•Experimental validation of CFD model using a spray pattonator.•CFD model can be used to model the distribution of the spray’s impact with the crop.•Modelled distributions can be used for spray mission planners.•Spray distribution heavily effected by propeller downwash.•Higher propeller thrust leads to a peakier and less uniform distribution.