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Comment: Added relationship with porosity and performance factors

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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." 

Both forms of porosity influence opening forces, drag, and stability by means of the following trends (Knacke, 5-71):

Mathinline
bodyPorosity \uparrow: C_{D} \downarrow, F_{opening} \downarrow, Oscillation \downarrow

Resources

T.W. Knacke, Parachute Recovery Systems: Design Manual

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