Abstract: Electrostatic spraying for plant protection can enhance droplet deposition when droplets carry sufficient electric charge. Under plant protection UAV operation, droplets move through intense rotor induced downwash and turbulence, and the CMR (charge-to-mass ratio), may attenuate markedly before reaching the crop canopy. Field scale quantification of CMR is constrained because conventional Faraday cylinder collectors are rigid and difficult to deploy across a wide spray footprint. This study developed a flexible AFC (aluminum foil container) method, to collect charged droplets under field conditions and to quantify CMR in a contact charging UAV electrostatic spraying system. A coupled attenuation model is further used to interpret the relative contributions of charge loss and droplet mass loss during flight. A high voltage generator module was integrated into a quadrotor spray platform and configured for contact charging. The negative high voltage output was connected to a metal contact electrode installed inside the liquid tank to charge the working solution, while the positive terminal was mounted on the landing gear and discharged to air to provide weak capacitive coupling that supported charge balance during hovering. The AFC collector follows the Faraday cage principle but replaces rigid collectors with a flexible aluminum foil conductive layer supported by an insulating backing, allowing the collector geometry to match the spray swath. Repeatability tests under identical spraying conditions showed consistent CMR results that were comparable to a conventional Faraday cylinder while enabling practical field deployment. Field experiments were conducted at charging voltages from 15 to 35 kV and flight heights from 2.0 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. Baseline CMR increased with voltage and reached 3.50 mC/kg at 35 kV. Field measurements indicated pronounced CMR loss in rotor downwash and a clear height effect. When flight height increased from 2.0 to 5.0 m, the average CMR attenuation rates were 14.09%, 24.23%, 39.32%, and 52.84%t at 2.0, 3.0, 4.0, and 5.0 m, respectively. Rotor wind speed during measurements remained close to 14m·s ⁻¹ and served as a shared background condition across treatments. Model fitting demonstrated that, within 2.0 to 5.0 m, exponential charge decay dominated CMR attenuation, while evaporation driven mass loss was weak. At 35 kV, the fitted charge attenuation coefficient λ was approximately 0.16, whereas the mass loss coefficient K’ prime was approximately 4.0×10 ⁻⁴, corresponding to less than 0.3% correction within 5.0 m. Across voltages, λ increased from 0.08 to 0.16 as voltage rose from 15 to 35 kV, with intermediate values of 0.10, 0.12, and 0.14 at 20, 25, and 30 kV. Despite faster decay per unit distance at higher initial charge, higher voltage still produced higher retained CMR within typical operating heights. Both adjuvants increased baseline CMR and improved retained CMR under UAV operation. At 35 kV, Momentive increased baseline CMR by about 17.71% and achieved a maximum of 4.12 mC/kg, whereas Maifei increased baseline CMR by about 45.43 % and achieved a maximum of 5.09 mC/kg. Attenuation rates were broadly comparable with and without adjuvants, indicating that the primary benefit was elevating initial CMR so that more charge remained after in flight decay. Overall, this study established a droplet charging model for contact charging and a CMR attenuation model for UAV electrostatic spraying. Using the AFC method, it effectively captured CMR variations dominated by flight height and rotor wind effects in UAV crop protection spraying, bridging the knowledge gap between laboratory research and UAV field operations, and revealing the CMR-enhancing role of spray adjuvants. These findings provide methodological and experimental foundations for optimizing UAV electrostatic spraying technology.