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I can't seem to find much information regarding the size of the Vent (the hole).

Nor can I find a concrete answer to the value of the drag coefficient, $C$. I've read in some websites that $C$ is usually around $1.4$.

If someone could tell me, or, better yet, point me to some sources (books that are readily available on-line, or websites, for example), it would be deeply appreciated.

Thanks.

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Your best information source for this would be to contact your nearest skydive center and ask to speak to a rigger. If you prefer to use the web, you could ask on dropzone.com. The vents in round parachutes were 4 cells removed and replaced with mesh to provide some "drive". Not a lot but some. And the vent at the top was to prevent huge oscillations as air spilled out the bottom.

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  • $\begingroup$ I don't think I have any skydive center near by. I'll ask on that website. Thanks. $\endgroup$ – Rodrigo Dec 3 '17 at 2:47
  • $\begingroup$ "The apex vent is there to prevent over-pressure of the canopy during deployment. Without it the opening would be explosive and would almost always damage the canopy. The smaller the canopy, the larger the apex vent needs to be relative to size. A 24’ flat with an 18” vent will oscillate to the point of making you sick." A lot of people know a lot more about this than I do, but hopefully that points you in the right direction. :) $\endgroup$ – D-- Dec 3 '17 at 22:39
  • $\begingroup$ (Quote is from Michael Owens, senior rigger) $\endgroup$ – D-- Dec 3 '17 at 23:10
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Hoerner's Fluid-Dynamic Drag has an extensive collection of drag data, including a section on the "Characteristics of Parachutes."

It does give data for the effect of a hole in a flat plate normal to the flow, but I couldn't find anything about the effect of the parachute's camber. Nevertheless, there's much that should be of interest to you there.

It is available to download at the following site: http://dl.kashti.ir/ENBOOKS/NEW/FDD.pdf

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