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There are two types of porosity related to parachutes: fabric and geometric porosity. Fabric porosity relates to the inherent permeability of the parachute material. Geometric porosity relates to what percent of the parachute's canopy is "cut out." The equivalent metric between fabric and geometric porosity measures is 27.4 ft3/ft2/min, at 1/2-inch water pressure ≈ 1% geometric porosity (Knacke, 5-71).
Both forms of porosity influence opening forces, drag, and stability by means of the following trends (Knacke, 5-71):
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