Re: [billbooth] Deployment Injury - Perris - 23 November 2006

[faulknerwn 36]

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This means a terminal reserve opening only happens every 720,000 jumps...plus, not very many reserves have no-stretch lines. All this means that the chance of terminal opening on a such a reserve are well over a million to one.

Hmmm. My CRW rig has a Tempo with Spectra lines in it and has 8 rides - probably 4 of taken to terminal.. I think my luck must suck! The reserve still opens softer than my CRW main does so it never bothered me!

[billbooth 10]

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A reserve use only happens every 80,000 jumps

where did you get that number???

Sorry. MAJOR TYPO. I meant 800 jumps. Fatalities happen every 80,000 jumps.

[cobaltdan 0]

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I've said it before, again and again...The best "shock loading limiting device" I can think of is Dacron lines. If we all insist on jumping "no stretch" lines, these injuries will just keep happening.

Dacron lines do absorb some shock from opening, which has proved effective on canopies which open quickly e.g. early parafoil canopy designs, or modern canopies which must open quickly such as base parafoils.

where dacron falls short is its application to a modern skydiving parafoil. modern skydiving parafoils limit opening shock instead of generating it and trying to attenuate it with stretchy lines. modern canopies limit max g force by controlling the rate at which they can inflate and hence the drag, generating a force vs time profile that is stretched out compared to canopies of old. the longer you spread out the opening the lower the g force.

much more sophisticated designs exist now than 10 years ago. designs which limit the conductance of air inflating the canopy via specific designed geometry in the nose inlets, crossports, planform, line set and slider. again it is all about not generating high forces at all, instead of generating a high g shock and then trying to attenuate it at the lines before transfering to the jumper.

you might ask, well then why not add dacron lines as a back up, to protect against a fluke opening or improper packjob? dacron is a low strength fiber by todays standards. the specific tenacity is about 1/5 that of spectra. dacron lines must be far bulkier and much higher drag, additionally due to their high stretch it is not possible to use on many modern parafoils, e.g. modern parafoils are higher performance and designed to keep a specific shape to higher tolerances that older canopies. lines that can stretch beyond a certain limit will allow the airfoil to mis-shape beyond its design parameters causing instability.

Daniel Preston <><>
atairaerodynamics.com (sport)
atairaerospace.com (military)

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