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History of the Cup-and-Grid Method
Some of the first drop tests were conducted between 1955 and 1959 in California by the Pacific Southwest Forest and Range Experiment Station and the California Air Attack Coordinating Committee (Davis 1959). These tests were designed to answer six main questions.
These early tests determined that:
Also during 1967 in Alberta, Canada, a University of Montana graduate student conducted a cup and grid test as part of his master’s thesis. Joseph Grigel used a Snow Commander airtanker with a 250-gallon tank to drop Gelgard F, a short-term fire retardant, over an array of cups laid out in a grid format on the ground. Grigel went into more detail regarding methods and design. The objectives of the study as outlined by Grigel were to determine:
Grigel concluded that ground patterns were determined by height, speed, volume, gating system (as it contributes to flow rate), retardant properties, weather, and fuel or vegetation. A grid-within-a-grid layout was used. The inner grid consisted of cups spaced 7.5 feet apart, covering a 270- by 82.5-foot area (figure 8). An outer grid was built around the inner grid with 10- by 10-foot spacing. The outer grid was to catch any retardant that missed the inner grid. The entire grid, including the inner and outer portions, measured 350 by 142.5 feet (figure 9).
It was assumed that each cup on the grid would represent one-half the distance to the adjacent containers. Linear interpolation was used to estimate points. Grigel noted some principal sources of error:
The grid system was evaluated by plotting the observed mean concentration in gpc from the inner grid area against the observed mean concentrations from the table data. The observations were found to be highly correlated. Proportioning points for prediction purposes produced an error of less than ±0.5 gpc. Variation was examined by plotting the standard deviation for the table concentrations against the mean gpc for each table. Levels up to 3 gpc would result in a standard deviation of 0.2 gpc or less.
D. H. Swanson and A. D. Luedecke used ground patterns in their 1978 paper, Tank Design Guide for Fire Retardant Aircraft, to compare the effects of different factors on the drop. Ground patterns obtained from the cup-and-grid method along with film showed that the most efficient drop occurs when the majority of the retardant’s forward momentum has stopped before the retardant reaches the ground. These findings helped determine the proper drop height (figure 12).
In the 1990’s the Forest Service conducted an extensive series of drop tests on a wide variety of retardant delivery systems. The systems tested included a number of helitankers and multiengine and single-engine fixed-wing airtankers. Drop-test data from each system were assembled into drop-test data books.
For these tests, cups were mounted on stakes at 10- by 10-foot intervals for the inner grid and up to 60- by 20-foot intervals for the outer grid. Grid size ranged from 250 by 600 feet up to 250 by 3,000 feet, depending on the volume dropped (figure 13).
Aircraft drop height and drop speed were measured using video analysis, a global positioning system (gps), and a radar altimeter. Video proved to be the most accurate and reliable. Four liquids were used: water, a 0.6-percent foam solution, gum-thickened retardant, and water-like retardant (figures 14 and 15). Flow-rate settings varied depending on the system. For example, the Bambi helibuckets were tested with only one flow-rate setting. Some air-tankers were tested with up to as many as nine selectable flow-rate settings.
Wind speed and direction were measured using a portable weather station adjacent to the drop site. Instrumentation was mounted 20 feet above the ground. The weight of retardant in each cup and the location of each cup within the grid were recorded. Linear interpolation was used to estimate unknown values (values between cups). Contour patterns were made with computer software (figure 16).
As with previous drop tests, ground patterns differed depending on aircraft height and speed, type of retardant or suppressant, flow rate, and wind conditions.
Drop testing over a regularly spaced array of cups has allowed researchers to measure the line length, width, and area, and to calculate the coverage level of the ground pattern. This has improved our knowledge of how height, speed, volume, tank geometry, flow rate, meteorological conditions, and retardant properties influence ground patterns. Yet data regarding the accuracy of the cup and grid method are sparse.
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