re : Jelly Fish oscillation

[sabre210]

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]

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]

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]

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

[sabre210]

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]

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]

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]

>>>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

[SideSwooper]

I'm making an assumption that the jump that you're referring to was possibly the first jump that we made last Sunday by the seaside, before I disappeared off to work.

The canopy certainly did jellyfish, but what concerned me more was the complete end cell closure that required not only releasing the brakes, but a 180 degree turn to inflate.

I suspect that on that 330' cliff, an upright body position would not be a big factor. That is a more appropriate consideration on much lower jumps, with considerably less canopy time (even taking a nice delay, you had loads of canopy time).

I think it may have already been suggested by one considerably more knowledgeable than myself, but I believe what you need for a much fuller flavour on those jumps is a vented canopy.

[Mac]

but I believe what you need for a much fuller flavour on those jumps is a vented canopy.

I have most of my jumps on an unvented canopy and found that there was some days it would jellyfish and others it would not, I cant say I ever really thought alot about it as I always put it down to the winds, brake settings and pulling off the brakes quickly (as most of these jumps were quite low) - I have since got a vented and valved (MDV) canopy and with the few jumps I have with this (only 5 before the **** hit the fan) have not experienced the jellyfishing I was so used to on my unvented - BUT I only took this from 315ft with 2+ sec delays and once from 210ft with a result I am sure you are aware of....... I can certainly rule out jellyfishing as a reason on that result!

With the minimal experience I have on a vented canopy, it only took one jump for me to notice the difference in what I had above my head (although the canopy I had before was not rated as something special) - I can only say that for the types of jumps I was doing (mainly sub300ft, but generally around the 300ft mark) then I would certainly prefer the vented canopy option.........

Saying that, I saw sabre210 have an interesting jump off that cheesy little tike one dark evening (after a perfect jump from myself), and was impressed by his canopy performance that evening, but this may have been down to skill of his fight too........... cheese in the dark anyone? not for me anymore ta!

[jupija]

i got it again... out of last 15 jumps or so i had 3 oscilations. Im getting worried with it... It happend on jumps with and without slider....
What do u think, could lines shrink - some say it can, any additional experoance?

here is the last jelly fish... from one of our newest jumping stuff http://www.skydivingmovies.com/ver2/...n=file&id=2014 and u can also c the second one i had on mexbase-death day boggie (u cant miss it - its the one from another e in mex.)

[Tom Aiello]

Jupija,

What canopy are you using and where are your brakes set?

It looks to me like you are using overly deep brakes on a vented canopy and experiencing a backsurge on opening, which stalls your canopy. The canopy then dives forward to recover from the stall. This could happen erratically if you are right on the edge of stall (especially if you are jumping at a variety of altitudes or temperatures, or in different wind conditions).

You could also be seeing this result from popping the toggles while the canopy is still pressurizing (more likely on an unvented canopy, but still possible with a vented canopy), initiating that forward dive.


Note that this video isn't, at all, what I've previously seen described as "jellyfishing".

[jupija]

[QUOTE=Tom Aiello]
What canopy are you using -- MOJO 220(no vents, and valves)

till this year i really didnt have any probl. with my brake setting, i had it made shorter, and i didnt get any weight, well maybe 2 kg.

and i poped the togles when i realised im falling bacwards, maybe i should not reacted as fast as i did, couse after deblockinthe brakes i imidiatly lift my arms ahead...

/QUOTE]
so what do u say about possibility that lines would get shorter