Ground Pattern Performance of the
Griffith Big Dipper Model 100 Helibucket

The Wildland Fire Chemical Systems (WFCS) program tests a variety of fixed- and rotary-wing tankers to determine the parameters for optimal ground pattern coverage over a wide range of fuel and fire conditions. The Griffith Big Dipper Model 100 helibucket (referred to as the Griffith 100-gallon helibucket) is one of a family of helibuckets with different volumes designed for use with a variety of helicopters. The Griffith 100-gallon helibucket is designed for use with Type III helicopters.

MTDC tested the Griffith 100-gallon helibucket with a series of drops over an array of plastic bowls much like Cool Whip containers. The quantity of material in each bowl was measured and the data were used to determine the drop pattern.

The helibucket is constructed of "double bond" polyurethane mounted to a steel-reinforced rim and bottom (Figure 1). A round door seals the bottom opening from the inside. The bucket is emptied by activating a hydraulic cylinder that pulls the door up. The inside diameter is 31 inches at the top and 25 inches at the bottom. The bucket is 38.5 inches tall. The door is 16 inches in diameter. The bucket’s maximum volume is 100 gallons. Because of helicopter limitations, the bucket’s volume was reduced to 70 gallons for testing.

Figure—1 Griffith 100-gallon helibucket.

The standard Griffith 100-gallon helibucket was tested using a Type III helicopter. Tests included air speeds from 30 to 60 knots (35 to 69 mph) and drop heights from 40 to 99 feet from the bottom of the helibucket to ground. The drops were made with three different materials: water, foam, and gum-thickened retardant.

Drop height and air speed both have an effect on the drop pattern. Increasing drop height gradually widens the drop at the expense of higher coverage levels. This effect is modified by the ambient wind. Increasing wind speed widens the drop and decreases coverage levels. Air speed has a much greater effect on the drop pattern. Figure 2, Figure 3, and Figure 4 show the effect of increasing the air speed from 40 to 60 knots (46 to 69 mph). Drop heights ranged from 42 to 85 feet.

Figure 2—Drop pattern characteristics for the Griffith 100-gallon
helibucket with an air speed of 40 knots and a drop height of 85 feet.
The contour lines are at coverage levels of 0.5, 1, 2, 3, 4, 6, 8, and
10 gallons per 100 square feet.