P/C Failure?

[guest]

Anyone seen/heard of many instances where a p/c has failed on deployment? ie:broken center line; blown out mesh or top, etc? Saw the center line break once on a BASE p/c...occurred during an assisted deployment, because, (from what I've heard) the p/c was being held basically by the cap, with the center line taking the beating directly...Have seen skydiving p/c's with blown out mesh...I've also heard that this center line, in most cases is only rated for around 200lbs...Could this "center line scenario" occur on a freefall deployment? If so would the initial "snatch force" be enough to open the container or would we have a p/c in tow with much pain and/or death scheduled soon to follow? In other words, how likely are the odds sayin' that one could have a "total P/C failure" amounting to no canopy, on a BASE jump? Opinions/ideas/occasional flaming welcome...

[guest]

seen video of a pilot chute failure in norway
it was a loooong (10 sec) delay on a 42" PC which had no Load tapes (doh) the nylon ripped from one side to the other
unfortunatly the jumper blamed a certain manufacturer on camera but we know that this is not the case.! :-<

daedalus >(

[crwper]

I believe 3/8" nylon tape has a tensile strength of about 200 lb. IIRC, the pc center line is doubled, so about 400 lb tensile strength.

Intuitively, I would say the pc center line in a freefall situation doesn't take a lot of the force. Most of the force, I would think, is distributed through the fabric and mesh.

I'm curious what the drag on a pilot chute is. I've heard some numbers thrown around before, but I can't find anything on the internet to back them up. Suppose we use the drag equation:

d = 1/2 C p A v^2

C = drag coefficient (about 0.8 for a pc (1))
p = denisty of air (1.275 kg/m^3 at sea level, dry air (2))
A = cross-section area (1.167 m^2 for a 48-inch pc)
v = velocity (terminal is about 50 m/s)

We get a force of about 1487 N, or about 334 lb if we took a 48-inch pc to terminal.

Since I don't have a freefall chart handy, let's say a 3-second delay will get you up to about 25 m/s. Then we're looking at 372 N, or about 83 lb for the 48-inch pc.

These numbers may not have any bearing on this question, though, since the dynamics of the pc opening could produce a higher force than the steady-state equations imply.

A 42-inch pc at terminal would create 256 lb of drag. Depending how it gets distributed throughout the pc opening, it could definitely be enough force to rip even a perfectly good pc.

I'd be interested to hear any experimental results, or if anyone can comment on my calculations that would also be cool.

Michael

(1) http://ffden-2.phys.uaf.edu/211.fall...lfunctions.htm
(2) http://www.usatoday.com/weather/wdensity.htm

[guest]

Uhhhh. Ok, how bout I pop out to MIT for a quick advanced math degree and, uh, get back to you on that. In my personal PC experience, this little piggy and that little piggy says all my pc jumps so far so good...&-)

[guest]

I agree perfectly with the order of magnitude of the results you got, Michael.
The only thing that could differ is because of the value we want to give to the "safety factor" (the coefficient that you called "drag coefficient").
I sent you via private e-mail an Excel table that I created some time ago and which reports all the drag forces for each PC size at any air speed. In that table of mine I created a coefficient, Fd ("dimishing factor") to goes to divide the theoretical value of drag force. You can enjoy yourself in producing as many table as the Fd coefficients are.
Let me know if it makes sense to you.
Stay safe out there
Blue Skies and Soft Walls
Andrea
#689 :7

P.S.: In any case, my humble opinion is that taking a 42" to terminal is way too much!!!!

[guest]

...obviously a 42" is too much for terminal jumps...Hell, I wouldn't take a 38" to terminal if I could help it...my latest terminal jumps involved a 32" and 28"...also, I can appreciate the fact that both of you may very well be mathematical wizards and all, but none of it makes any sense to me...I'm not lookin' to find drag calculations for each p/c...I was more or less wonderin' how often a p/c failure has reared its ugly head, and whether or not it would mean certain dismemberment and/or death.I wanted to calculate some odds on p/c failure and survival of said situation by personal experiences...Thanx for the input, just the same...Let's keep those opinions/ideas and the occasional flame comin'!

[crwper]

I guess what I was trying to establish was that using a 48" or a 42" pc at terminal will definitely put you at the edge of the design envelope, but if you take only a 3 second delay with the same pc, there's plenty of margin for error.

I don't really think pc failure is a statistical thing. Barring manufacturing defects, I would say if you take care of your pc and replace it when it's worn, and if you use the right pc for the job, it should never fail you.

If you pca by holding the cap, or if you take a big pc to terminal, it will fail sooner or later.

Flame away.

Michael

[guest]

I entirely agree with Michael on this one. Tables and physics and formulas are very useful to know order of magnitude of forces under play, they are not "the perfection" (nor they could be). ;-)
Back to PC's: if you regularly check status and integrity of your PC's (sewings, seams, tapes, reinforcement tapes, ripstop fabric, mesh fabric, handle, cap, bridle attachment, center line webbing, etc etc) and if you use the right PC for the right job, PC's will never fail!!!! :-)
Stay safe out there
Blue Skies and Soft Walls
Andrea :7
#689

[guest]

...here's the scene: ...brand new p/c, looks great, seems well built, no obvious defects... ...first jump, center line tapes snap like small twigs under foot in the dead of fall ...Q: do you; A. get a canopy from the deployment? or B. hope and pray for heaven to hold endless delays, and no packing... ...oh yeah...and a bunch of hot chicks that think it's cool that a BASE jumper actually made it into heaven

[guest]

To be honest, to such a question it should answer Space, from the heigth of his experience.
My personal and humble opinion?
In the scenario of a blown up center line tape, I think that the PC cannot stay closed and at the same time PC cannot stay wide open. In a steady state, I think you would get something like a "pulsing PC", the PC would continue to assume shapes between "a little open" and "a little closed", in a continuous dynamic balance of forces that tend to elongate the PC and to open up the PC, instant shape given by the forces that in that particular moment prevail.
In the end: in my humble opinion, in such a scenario you would get, yes, a PC still pulling, but much less than a "not center line failed" PC. I think that such a "blown up" PC could pull between 1/3 to 1/5 (perhaps 1/10 ???) of the force of same "not blown up" PC.
So, yes, it would pull, yes, it would open up the container (???), yes, it would extract the canopy (?????), but with very, very big hesitation :'(
That, of course, would vary a lot depending on the situation: a jump and (mandatory) pull after 2 s, I think you are in the sh!t; a jump at terminal where you pull at a reasonable distance from the ground, you should be able to get an open canopy over your head before hitting the ground.
To end this story: ALWAYS CHECK YOUR BLOODY PC's!!!!!!!!! :D :D :D
Stay safe out there
Blue Skies and Soft Walls
Andrea :7
#689

[crwper]

There have been lots of round parachutes that worked fine with no apex line. For a really simple example, think of one of those GI Joe paratrooper figures. That's about as simple as things can get, a flat sheet of plastic with lines around the circumferance. Most round parachutes work the same way, plus or minus an apex vent and some mods at the back which prevent oscillation.

I'd definitely defer to someone with more experience for a reality check, but I'd say you can bet a pc with no apex line will still provide plenty of pull force, although as Andrea mentioned you will probably experience some hesitation.

I think it's far more likely that the fabric of your pc will rip. In that case, you could be screwed.

Michael