Computer modeling of the behavior of a spray after it is released from an aircraft is intended to reduce the inherent complexity of pesticide application. Two new models developed by the USDA Forest Service seek to predict the effects of the atmosphere and the aircraft wake on sprays, and their predictions compare favorably with field studies. The models can improve application practices and enhance public confidence in the use of chemicals and biorational agents in forestry.

The near-wake portion of the USDA Forest Service aerial application prediction model FSCBG is applied to a sensitivity study of the length of the spray boom relative to the length of the aircraft wingspan or helicopter rotor diameter Building on extensive previous work, this study examines the predictions by the near-wake Lagrangian trajectory model of swath width, mean deposition within the spray block, and drift fraction downwind of the edge of the field Ten aircraft and four BCPC droplet size distributions are used to simulate a typical agricultural application scenario. Findings from this study demonstrate the effects of varying boom length on downwind drift, the reduction of downwind drift with larger droplets, and the inherent ability of certain aircraft type to reduce downwind drift more easily than others. Model results indicate that a broad range of boom length (between 60 and 100% of aircraft wingspan or helicopter rotor diameter) recovers approximately the same levels of downwind di-ift, decreasing levels of mean deposition within the spray block, and increasing swath width between flight lines. The suggestion that boom length should be less than 75% of wingspan or rotor diameter is perhaps based more on the anticipated position of the rolled-up vortices than on solid experimental evidence.

The near-wake portion of the USDA Forest Service aerial application prediction model FSCBG is applied to asensitivity study of the length of the spray boom relative to the length of the aircraft wingspan or helicopter rotordiameter. Building on extensive previous work, this study examines the predictions by the near-wake Lagrangiantrajectory model of swath width, mean deposition within the spray block, and drift fraction downwind of the edge of thefield. Ten aircraft and four BCPC droplet size distributions are used to simulate a typical agricultural applicationscenario. Findings from this study demonstrate the effects of varying boom length on downwind drift, the reduction ofdownwind drift with larger droplets, and the inherent ability of certain aircraft type to reduce downwind drift more easilythan others. Model results indicate that a broad range of boom length (between 60 and 100% of aircraft wingspan orhelicopter rotor diameter) recovers approximately the same levels of downwind drift, decreasing levels of mean depositionwithin the spray block, and increasing swath width between flight lines. The suggestion that boom length should be lessthan 75% of wingspan or rotor diameter is perhaps based more on the anticipated position of the rolled-up vortices thanon solid experimental evidence.