re : Jelly Fish oscillation

**sabre210** · February 1st, 2005, 03:49 AM

I've tried doing a search in the archives but yielded nothing which i find hard to believe.

Anyway. What causes canopies to oscillate violently from front to back upon opening like a shockwave, an occurrence i believe is fondly known as jellyfishing and is there anything that can be done to minimise it on non vented canopies.

I'm trying to find a pattern of sorts as to why it doesn't happen for dozens of jumps and then like buses, three come along at once, all other things being equal.

It it voodoo???? Your answers will be much appreciated......(unless they're sh1te!!!)

ian

Biff · February 1st, 2005, 05:01 AM

All other things weren't equal : there was a crosswind, you had a 45 off-heading opening. Looking from the top, your PC had blown right over to the left when you pitched. I think the crosswind is possibly a reason your opening wasn't pretty (looked pretty **** from the top actually). The wind coming off the lip of the cliff created a downdraft/rotor that affected your opening IMHO. You took a 2 second delay everyone else took 1 second or less - maybe your delay put you right in the 'dirty air' on opening?

Of course it is easy for me to play armchair quarterback because I didn't jump. And naturally your opening wasn't as much fun to watch as the world's slowest '180 resulting in 'cliff strike'.

Will

**sabre210** · February 1st, 2005, 06:39 AM

Biff

i wasn't referring to THAT jump.

I didn't have this specific problem on the last one. It was the 2 prior to that when i had the jellyfishing canopy. One was pretty fu(king wild. Anyway, it's a general question and i'd still be keen to know what causes it and how i might be able to prevent it.

Your answer definitely falls into the (unless they're sh1te!!! category)

ian

**Mac** · February 1st, 2005, 10:34 AM

Pressurization - BASE Jumping:: BLiNC Magazine Forums
Vented Fox safer than standard Fox for low objects? - BASE Jumping:: BLiNC Magazine Forums

**sabre210** · February 1st, 2005, 03:58 PM

Thanks for the links Mac, very helpful.

I'm still curious to know why this occurs only on some jumps. What conditions contribute to this happening. What wind conditions would make this more likely to happen. Why can a non vented canopy open without this pressure wave and then the next jump it happens?

Is it more likely to happen on slider down jumps (i'm guessing yes).Is it a better idea to leave the brakes stowed whilst it occurs or should you pop the brakes fast?

Calling all experienced jumpers. The truth is out there.

Cheers in advance people

ian

**jules** · February 2nd, 2005, 03:53 AM

I think it has something to do with body position as the canopy inflates and starts flying. I have had a couple do this on PCAs or very short delays when I have gone off very flat.

Compare Neil and I jumping the 180' A in DAD and the trailer to DAD II. Neil exits with a near-vertical (deliberately - on most jumps he is almost diving into his track) and his canopy opens very smoothly.

When I jumped this I was my usual calm, composed self - new object, lowest ever jump, so dark that we decided to ditch my SL and go PCA for speed - and I went off very flat.

As the canopy drags me upright, particularly on a PCA or SL where the canopy is deploying behind me, and I started to pendulum underneath the canopy I must be changing the loading on the risers/canopy and therefore changing the way its trying to fly.

Neil went off upright, there is hardly any transition phase for his body from falling to being under canopy and therefore his canopy is loaded reasonably equally throughout.

Or am I spouting crap?

Jules

**sabre210** · February 2nd, 2005, 05:50 AM

Interesting thoughts Jules, but i'm obviously in no position to comment on whether this is the cause of the jellyfishing (having asked the question to start with).

I would imagine that the transfer of the jumpers body weight as they swing under the inflating canopy would indeed have an effect on it, and i know that i am a flat jumper. I tend not to go head high on any freefall jumps but oddly don't have too much problem being very upright for the low(ish) static lines. Anyway, it's definitely something i should think about more. Cheers.

Both Neil's rigs were valved/vented foxes. I assume yours is too or you wouldn't really be comparing like with like.

Regardless, something to think about.

ian

Biff · February 2nd, 2005, 06:34 AM

>>>I would imagine that the transfer of the jumpers bodyweight as they swing under the inflating canopy would indeed have an effect on it

I don't think it does. The canopy is attached to you at two points, as long as those points are level I don't think a swinging body (forward and back NOT side to side) will have much of an effect.

Will

**sabre210** · February 2nd, 2005, 10:27 AM

I don't think it does. The canopy is attached to you at two points, as long as those points are level I don't think a swinging body (forward and back NOT side to side) will have much of an effect.

The key point is what you consider to be "much of an effect". A shockwave ripple through the canopy isn't what you'd consider cataclysmic unless of course you're close to the ground and fighting to turn it to land.

Surely physics dictates that a swinging body under a canopy MUST have an effect, action/reaction, transference of energy and all that.

The energy created by a heavy body penduluming under a piece of nylon, back and forth has to be transferred somewhere and seeing as you're connected to this nylon by bits of string it makes sense to me that this could feasibly be the cause of a shockwave rippling through the wing as it pressurises.

Like i said, i don't know the answer but Jules has hit on something which makes sense of sorts to me.

I'm surprised there hasn't been any input at all from our transatlantic cousins. In the absence of anything else it's the best i've got so far.

Ian

**jules** · February 2nd, 2005, 11:56 AM

[QUOTE=BiffI don't think it does. The canopy is attached to you at two points, as long as those points are level I don't think a swinging body (forward and back NOT side to side) will have much of an effect.[/QUOTE]

I think that the two attachment points is an irrelevance. The following would apply equally well if there was just one (so long as you maintained symmetry)

Go down the local park and kick the kids off the monkey bars. Hang underneath. The force in your arms is a constant.

Start to swing backwards and forwards. You will feel the force in your arms change over each oscillation - it will be greatest at the bottom. The larger the swing, the faster you move through the centre point and the larger the force will be at all points.

You are doing a bad impression of a simple harmonic oscillator - a pendulum. Its a bad impression because (i) your weight is not concentrated at one point and (ii) your body is not rigid so as you increase the size of the oscillations you will get different parts of your body going backwards and forwards at different times.

Now get the kids to rip the monkey bars out of the ground and carry them (and you) to the top of a small cliff over water. Hang underneath the bars and get them to lob you off. Start swinging as you go and notice how the effect is the same as before until you hit the water.

Now you have the additional effect of the vertical deceleration making the forces even more complicated. Factor in the forward acceleration as the canopy starts to move forward (regardless of DBS, it must move forward to fly) and you have a very complex system.

Removing one complication, that of your original swinging under the monkey bars, must make the loading on the canopy more constant.

Jules

Tom Aiello · February 2nd, 2005, 01:42 PM

Forward speed into the relative wind is probably going to have a big effect. So is body position during inflation, and crosswinds.

The list of things that might effect this are huge, but include (assuming same jumper/same canopy/same brake settings) wind and body position as major players.

In general, on really low stuff, I'll launch soft and upright, to minimize the pendulum effect under canopy. I've seen people hit the ground still penduluming (I've done it myself, too) and I prefer to avoid it if at all possible.

I dunno, though. Canopy inflation is a massively chaotic event. Which is a pseudo-scientific way of saying that maybe it is just voodoo and witchcraft.

**Mac** · February 2nd, 2005, 02:02 PM

As for the "swinging body" creating a jellyfish, then I dont feel it would. The swinging body will cause a bad day for a low opening generally, and depending on the break settings and canopy, can also cause the canopy to go from stall to dive to stall to dive (I have seen this from a delay from a 240ft B with incorrect break settings).

I have always thought that the jellyfish has been caused by the air rushing in and then rebounding against a lesser pressure inward due to other (many) factors (change in angle of the canopy, semi stall, wind condition change, blah blah many other factorrs, blah,) and making the top skin "ripple". I cant really say why I think it happens, I just know it does. I am now overly interested in any theories.

I discussed this tonight with someone else and all they could say is "you are throwing up 250+ sq ft of material into the air and trying to describe and understand every single reaction of it to an environment with many variables, you cant understand everything and sometime **** happens"

which in the end, I guess, sometimes is the only thing to say..................

**sabre210** · February 2nd, 2005, 04:53 PM

Tom and Mac

Don't get me wrong, I fully appreciate the massive variables involved in a canopy's inflation and pressurisation, even if i don't understand them all. Just as off-heading openings and pilot chute hesitations can occur without apparant reason we still try to understand to an extent some of the main factors that will certainly increase the likelihood of them happening. Few jumpers would question that a poor pack job can be a contributing factor to an off-heading, or an asymmetrical body position even if they are not the sole reasons.

The jellyfishing of a canopy on opening is an anomaly in the pressurisation of the cells but there are no doubt contributing factors within our control.

If maintaining a slightly more head high body position helps me avoid this nastiness just once in one hundred jumps then i'll do my best to achieve this. It could be the difference between a good and very bad day.

Thanks for your thoughts. More please.