Team Design Projects
Hermes Disk -Gap Band Gap Drogue
Materials
Porosity
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).
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This outdated material burns easily and has poor performance when wet.
Tapes
Treated Fabrics
A note on treated fabrics from Poynter (4.021): coating/treating a fabric may decrease tear strength by preventing the fibers from adjusting around a tear.
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| Spec ID | Title |
|---|---|
| MIl-C-7020 | Cloth, Parachute, Nylon |
Procedures
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Gores
Block vs. Bias Construction relates to at what angle you cut the gores out the fabric. Advantages of block are: simpler fabrication. Advantages of bias are: better canopy strength, less line loading and less required slack. (Poynter 8.1.5 and 8.1.9)
Shroud Lines
From Knacke (6-84), : "all lines and tapes are [traditionally] measured under a preload. A preload of about 1% of the ultimate material strength is required to align all fibers in the woven or braided material before the individual fibers are stretched." Furthermore, "[it] is practical to let all fabrics relax for a short period after rewinding from the storage spool before measuring the material."
From Poynter (8.1.5): "Because of the uncertainty of canopy loading due to non-uniformity, suspension line systems should have a designed margin of safety of 2.0. Lines lose about 35% of their strength due to sewing and the attachment of fittings."
Vent
Using a vent helps stabilize a parachute and eases construction.
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Full details on sewing standards can be found here.
Terminology
take-up: the shortening of two pieces of fabric around the seam. Ideal parachute design takes this into account.
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Heinrich and Haak, Stability and Drag of Parachutes With Varying Effective Porosity
Dan Poynter, The Parachute Manual: A Technical Treatise on Aerodynamic Decelerators, Third Ed.