"On Target Application with Superior Drift Control"
A. Droplet Formation: A combination of smooth bore nozzle tubes, low pressure, surface tension of the liquid and non-turbulent air flow all play a part in forming uniform droplets. These droplets must be large enough to fall predictably.
B. Droplet Characteristics: Droplet uniformity, size and trajectory are all maintained and perform best when kept in clean air. Things that can cause droplet degradation are fixed wing prop wash, rotor wash, air disturbances from pump, fixed landing gear and excessive speed. Drop down tubes can aid in placing the nozzles in clean air, further reducing drift potential. (See photo examples at bottom of this page.)
Median uniform droplets fall at varying speeds depending on size. Larger micron droplets are less subjective to turbulence because they fall faster. Below is a chart depicting the rate of a 10 foot drop for a specific particle size.
Liquid droplets emitted from a nozzle with a specific gravity of 1.0 would take the following time to fall 10 feet in still air as a function of droplet size:
For example, for helicopters, a short period elapses from the moment the droplets are ejected into the free air stream until the droplets meet the rotor wake. A boom located directly under and approximately 10 feet below the main rotor would put the droplets roughly 60 to 80 feet forward of the rotor wake. At 60 mph, the closure rate that the rotor wake is catching up to the particles is 88 feet per second. This should allow approximately 0.8 seconds for distribution in the free air stream prior to making contact with the rotor wake. Note that a particle larger than 400 microns would be on the ground before being caught by the rotor wake if boom height above the ground is less than 10 feet and out of the rotor wake disturbance if flight is at higher altitudes. Conversely, particles of lesser size than approximately 400 microns will enter the rotor wake and will be redistributed within the rotor wake and may result in off target application. (In part from Helicopter Techniques for Aerial Applications, p. 2-25, Bell Helicopter Textron Inc., 1998
C. Weather conditions also play an important role in pattern and droplet behavior. Wind direction and speed must be carefully evaluated. Generally Speaking, 5-10 MPH is tolerable. In critical border areas, 3 MPH may be the limit.
Drop Downs are Recommended in Certain Circumstances
It's Important to Keep your Nozzles in the Cleanest Air Possible and Spraying Parallel to the Wind