The Wildland Fire Chemical Systems (WFCS) program tests a variety of fixed- and rotary-wing tankers to determine the parameters for optimal coverage over a wide range of fuel and fire conditions. The Sikorsky S-64 Skycrane (military version identified as Tarhe CH-54) owned, operated, and modified by Erickson Air Crane, Inc. is one of a family of helitankers designed for fire suppression. The Erickson Air Crane is designated as a Type I Helitanker.
The aluminum tank is a constant flow system. The door opening is actuated by an electrically operated hydraulic system using 28 volts dc aircraft power. The tank consists of one compartment that is certified by the Interagency Airtanker Board to drop 2000 gallons. The tank is divided into bays by bulkhead dividers to minimize fluid movement within the tank. The doors are controlled by an Erickson designed controller, which interprets fluid height information and adjusts door opening to produce a nearly constant rate of flow for a selected flow rate. Partial loads can be delivered by closing the doors before the tank has emptied. Tests included airspeeds from 35 to 91 knots (40 to 105 mph), drop heights from 144 to 262 feet (measured from the bottom of the tank to ground), and flow rates of 75, 300, and 700 gallons per second (gal/sec). The drops were made with three different materials: water, foam, and gum-thickened retardant.
The Missoula Technology and Development Center tested the Erickson Air Crane (Figure 1) 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.
|
| Figure 1-The Erickson Air Crane. |
Flow rate, drop height, and airspeed affect the drop pattern. Increasing drop height gradually widens the drop while decreasing coverage levels. This effect is modified by the ambient wind. Increasing windspeed widens the drop while decreasing coverage levels. Airspeed has amuch greater effect on the drop pattern. Increased airspeed increases the line length while reducing the coverage level. Different flow rates affect the amount of retardant that is dropped over a given period of time from the tank. Variation in flow rate also has a great effect on drop pattern.
Flow rate, drop height, and airspeed affect the drop pattern. Increasing drop height gradually widens the drop while decreasing coverage levels. This effect is modified by the ambient wind.
Increasing windspeed widens the drop while decreasing coverage levels. Airspeed has a much greater effect on the drop pattern. Increased airspeed increases the line length while reducing the coverage level. Different flow rates affect the amount of retardant that is dropped over a given period of time from the tank. Variation in flow rate also has a great effect on drop pattern. Figures 2, 3, and 4 show the effect of increasing the flow rate from 75 to 700 gal/sec with airspeeds ranging from 47 to 68 knots (54 to 78 mph) and drop heights ranging from 177 to 199 feet.
 |
| Figure 2-Drop pattern characteristics for the Erickson Air Crane using water with a constant flow system at a flow rate of 75 gal/sec, an airspeed of 47 knots (54 mph), and a drop height of 199 feet. The contour lines are at coverage levels of 0.5, 1, 2, 3, 4, 6, 8, and 10 gallons per square feet. |
 |
| Figure 3-Drop pattern characteristics for the Erickson Air Crane using foam with a constant flow system at a flow rate of 300 gal/sec, an airspeed of 59 knots (68 mph), and a drop height of 184 feet. The contour lines are at coverage levels of 0.5, 1, 2, 3, 4, 6, 8, and 10 gallons per square feet. |
 |
| Figure 4-Drop pattern characteristics for the Erickson Air Crane using gum-thickened retardant with a constant flow system, a flow rate of 700 gal/sec, an airspeed of 68 knots (78 mph), and a drop height of 177 feet. The contour lines are at coverage levels of 0.5, 1, 2, 3, 4, 6, 8, and 10 gallons per square feet. |
The proper amount of fire-retarding material (expressed as coverage levels in gallons per 100 square feet) depends on the fuel model. Table 1 shows the coverage needed for specific fuel models using both the National Fire Danger Rating System (NFDRS) and the Fire Behavior Fuel Model descriptions.
The results of drop tests allow managers to estimate the length of line a specific airtanker produces at various coverage levels. Table 2 or Figure 5 can be used to estimate the flow rate of a water drop required for the longest line of the desired coverage level. Table 3 or Figure 6 can be used to estimate the flow rate of a foam drop for the longest line of the desired coverage level. Table 4 or Figure 7 can be used to estimate the airspeed of a gum-thickened retardant drop to obtain the maximum line length of the desired coverage level.
Table 2-Test drops producing the longest line at various coverage levels using water.
| Coverage Level (gal/100 sq. ft) |
Door Opening (percent) |
Line Length (feet) |
|---|---|---|
| 0.5 | 75 | 2455 |
| 1 | 75 | 2112 |
| 2 | 300 | 1329 |
| 3 | 300 | 615 |
| 4 | 300 | 519 |
| 6 | 300 | 396 |
| 8 | 700 | 173 |
| 10 | 700 | 128 |
 |
| Figure 5-Use this graph to estimate the flow rate needed to produce the longest line of water at various coverage levels. |
Table 3-Test drops producing the longest line at various coverage levels using foam.
| Coverage Level (gal/100 sq. ft) |
Door Opening (percent) |
Line Length (feet) |
|---|---|---|
| 0.5 | 75 | 2341 |
| 1 | 75 | 1179 |
| 2 | 300 | 674 |
| 3 | 300 | 324 |
| 4 | 700 | 131 |
| 6 | 300 | 168 |
| 8 | 300 | 4 |
| 10 | - | - |
 |
| Figure 6-Use this graph to estimate the flow rate needed to produce the longest line of foam at various coverage levels. |
Table 4-Test drops producing the longest line at various coverage levels using gum-thickened retardant.
| Coverage Level (gal/100 sq. ft) |
Flow Rate (gal/sec) |
Line Length (feet) |
|---|---|---|
| 0.5 | 75 | 2482 |
| 1 | 75 | 2315 |
| 2 | 75 | 1739 |
| 3 | 75 | 612 |
| 4 | 300 | 545 |
| 6 | 300 | 466 |
| 8 | 300 | 356 |
| 10 | 700 | 179 |
 |
| Figure 7-Use this graph to estimate the flow rate needed to produce the longest line of gum-thickened retardant at various coverage levels. |
The graphs predict line length (in feet) as a function of flow rate (in gallons per second). The tables are constructed by selecting the drop producing the longest line (on the ground) at each coverage level. Either the graphs or tables may be used to estimate the flow rate required to produce the longest line for a given coverage level. The tables show an ideal case, while the graphs represent an average.
To select the proper helitanker flow rate, first use Table 1 to determine the coverage level required by the NFDRS or Fire Behavior Fuel Model. The coverage levels in Table 1 represent the coverage level required for average fire intensity for each fuel model. The required coverage level can be adjusted up or down depending on the actual fire intensity. Once the required coverage level is determined, the flow rate can be found. Use the graph for the material dropped (water, foam, or gum-thickened retardant) to find the flow rate that produces the longest line for the desired coverage level. The same information can be found in the appropriate drop table.
 |
| Figure 7-An Erickson Air Crane drops foam from the constant flow tank. |
Cammie Jordan is a Project Assistant for the Wildland Fire Chemical Systems Program at MTDC. She is an elementary education student at the University of Montana and has worked for MTDC since 1998.
Paul Solarz is Program Leader for the Wildland Fire Chemical Systems Group. He received his bachelor's degree from Eastern Oregon State College in 1986. Paul has worked in Aviation and Fire Management since 1973, serving at seven Ranger Districts and in two Forest Supervisor's offices. He has an extensive operational background in fire, fuels, and aviation.
Additional single copies of this document may be ordered from:
USDA Forest Service
Missoula Technology and Development Center
5785 Highway 10 West
Missoula, MT 59808
Phone: 406-329-3978
Fax: 406-329-4811
E-mail: wo_mtdc_pubs@fs.fed.us
For additional Information contact:
Greg Lovellette, Project Leader
Missoula Technology & Development Center
5785 Highway 10 West
Missoula, MT 59808
Phone: 406-329-4719
Fax: 406-329-4811
E-mail: glovellette@fs.fed.us
Lotus Notes: Greg Lovellette/WO/USDAFS
This page last modified June 14, 2002
Visitor
since June 14, 2002