Department of Agricultural and Biological Engineering
Agricultural aircraft play a critical role in the application of pesticides. The setup and operation of an agricultural aircraft, however, is different from a ground sprayer. The boom of an agricultural aircraft is mounted below and behind the aircrafts wings, keeping the spray out of the turbulence caused by the air moving over the wings. The length of the boom is limited to about 75 percent the length of the wing span. This keeps the spray out of the turbulence caused by vortices formed at the wing tips. The nozzles underneath the fuselage of the aircraft are often mounted on nozzle drops to lower them further, or removed altogether. This is done to prevent spray from being displaced by the turbulence associated with prop wash.
Droplet size for aerial applications is controlled by the nozzle type, orifice size, operating pressure, and the angle of deflection or the nozzle. The two types of nozzles most commonly used by aerial applicators are flat-fans and what are commonly called CP nozzles. The orientation of flat-fan nozzles on the boom can be adjusted to change the angle at which the spray enters the high-speed air. CP nozzles are designed specifically for aerial applicators, and have two adjustable parts. One changes the orifice size and the other changes the deflector angle, which changes the angle at which the spray enters the high-speed air.
The high-speed air caused by the flight of the aircraft creates a shearing effect on the spray as it leaves the nozzle, and this is the primary factor influencing droplet size for an aerial application. The amount of shearing determines droplet size – more shear creates smaller droplets and less shear creates larger droplets. By changing the degree of shearing on the spray, droplet size can be controlled.
Aircraft height is also important. An ideal height for aircraft to fly is often between 10 and 14 feet above the canopy. Any higher and the droplets are excessively exposed to wind. Flying too low can actually increase drift and reduce deposition for aerial applicators because droplets can become trapped and carried off in air turbulence caused by the aircraft flying so close to the crop canopy.