Abstract: Electrostatic spraying has been widely used to enhance the droplet deposition under plant protection using UAV (unmanned aerial vehicles). The plant protection droplets can also move through the intense rotor under downwash and turbulence, when the droplets can carry sufficient electric charge. However, the CMR (charge-to-mass ratio) can attenuate markedly before reaching the crop canopy. Field-scale quantification of CMR has been constrained to deploy over a wide spray footprint, due to the conventional rigid Faraday cylinder collectors. In this study, a flexible AFC (aluminum foil container) was developed to collect the charged droplets under field conditions. The CMR was then quantified in contact charging UAV electrostatic spraying. A coupled attenuation model was further used to interpret the relative contributions of the charge loss and droplet mass loss during flight. A high-voltage generator module was integrated into a quadrotor spray platform and then configured for contact charging. The negative high voltage output was connected to a metal contact electrode inside the liquid tank in charge of the working solution. While the positive terminal was mounted on the landing gear and discharged to air, providing for the weak capacitive coupling that supported charge balance during hovering. The AFC collector followed the Faraday cage principle. But the rigid collectors were replaced with a flexible aluminum foil conductive layer supported by an insulating backing, allowing for the collector geometry to match the spray swath. Repeatability tests under identical spraying showed that the consistent CMR was comparable to a conventional Faraday cylinder in the field deployment. Field experiments were conducted at the charging voltages from 15 to 35 kV and the flight heights from 2.0 m to 5.0 m. Two spray adjuvants were evaluated at the same recommended concentration of 1‰, a surfactant-based product. Momentive Agrospred 910 and a modified plant oil product, Maifei, were used in the field test. Baseline CMR increased with the voltage and then reached 3.50 mC/kg at 35 kV. Field measurements indicated that there was an outstanding CMR loss in the rotor downwash, indicating the height effect. Once the flight height increased from 2.0 m to 5.0 m, the average CMR attenuation rates were 14.09 %, 24.23 %, 39.32 %, and 52.84% at 2.0, 3.0, 4.0, and 5.0 m, respectively. Rotor wind speed during measurements remained close to 14m/s. A shared background served as in the treatments. Model fitting showed that the exponential charge decay dominated the CMR attenuation from 2.0 m to 5.0 m, while there was a weak evaporation-driven mass loss. The charge attenuation coefficient, λ, was fitted approximately 0.16 at 35 kV, whereas the mass loss coefficient, K’ prime, was approximately 4.0×10^-4, corresponding to less than 0.003 correction within 5.0 m. The λ increased from 0.08 to 0.16 over the voltages, as the voltage rose from 15 to 35 kV, with the intermediate values of 0.10, 0.12, and 0.14 at 20, 25, and 30 kV. Despite the high decay per unit distance at the higher initial charge, the higher voltage still produced the higher retained CMR in the typical operating heights. Both adjuvants increased baseline CMR for the retained CMR under UAV operation. Momentive increased the baseline CMR by about 17.71% at 35 kV, with the maximum of 4.12 mC/kg, whereas Maifei increased the baseline CMR by about 45.43 % with the maximum of 5.09 mC/kg. Attenuation rates were broadly comparable with/without adjuvants. The primary benefit of the initial CMR of the more charge remained after in-flight decay. Overall, the droplet charging model was established for contact charging, while the CMR attenuation model was for UAV electrostatic spraying. The AFC effectively captured the CMR variations dominated by flight height and rotor wind in the UAV crop protection spraying. The CMR can play an enhancing role in spray adjuvants. These findings can provide an experimental reference to optimize the UAV electrostatic spraying.