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Imagine a long narrow rag lying on the floor. You are holding the narrow end of it and trying to make a 180 (turn it upside down).

If you just twist the end, the whole rag won't turn upside down. If you jitter it up, down, left, right randomly, it won't turn. But if you shake and twist just right and send the twisting wave along the rag, it will turn upside down easily and quickly.

Mikki's 180 video, in my opinion, demonstrates just this scenario. A gust or some asymmetry in PC sends it orbiting and this triggers this "death wave" through the extracting packjob.

Oscillating PC does not automatically mean an offheading, as the whole mechanics of coupling betwen PC and canopy has chaos built in (just as a well-known doulbe pendulum problem in mathematics). Everything needs to be "just wrong" for it to happen. Sometimes this "just wrong" is so perfect that all you can see in slow-mo is that the canopy is apparently leaving the tray already with 180 as if it was packed this way.

But now imagine that the rag is rather heavy and it bends down between your hands so its center of gravity is lower than the supports. Now the rag will be much more resistant to "death wave".

This is, I think, the mechanism behind the [generally] great on-heading performance of WS BASE pulls from full flight. As the canopy is extracted at ~20-30 degrees to horizon, it bows between risers and PC - it actually has time to "fall" below the imaginary straight line between risers and PC - and it's this low center of gravity that makes it more resistant to twist. (This also explains why in skydiving pulling from full flight generally gives worse heading performance: deployment bag, unlike long "rag"/BASE packjob, is not stabilized by this bow effect and it actually has a negative effect as the rubber stows are released one by one on either side, making it easy for the bag in this limbo position to spin.)

Now, I just measured my canopy and the bridle attachment point is located almost smack in the middle of the wing chord: 45% from the nose, 55% from the tail. It's right in the middle between B and C lines. So it's pretty much attached to the center of gravity.

What if it was attached much closer to nose? Will it improve the on-heading performance? Will it negatively affect the openings so much that it's too dangerous to jump such a setup?

By the argument above, it may improve onheading performance, especially if there's a horizontal component of airspeed (e.g. strong/running exit on very short delays, or sub- and terminal tracking, headwind on bridge jumps), as the center of gravity will be offset from the bridle attachment point. Also, nose will be loaded first and will expand more quickly, setting the heading and reducing the chance of lineovers.

It's the negative effects that are much harder to predict. Will the openings be funky? Will it increase the chance of PC getting under the front of the canopy? (not that it can't do this with centered attachment: bridle is still ~4-5ft longer than half the chord.) How it will affect slider up openings?

Anyway, just throwing another idea how to creatively kill yourself.

I was thinking the same thing about Mikki's video, but I couldn't think of a way to explain it.

Have you ever heard about that bridge that collapsed because it was exposed to just the 'wrong' winds? They were at just the right speed/direction/"frequency" to create waves in the steel and caused it to collapse (and made for a really cool video). They call it 'resonance' (or something to that effect)

A pilot chute could orbit on 100 jumps, but only occasionally do it 'just right' and cause an offheading.

The forward bridle attachment point idea, by your argument, does sound like it might help in cases where there is forward speed... but in vertical deployments (2-3 sec slider down all the way to terminal with no tracking) I think it would be far more likely to contribute to offheadings.

Has anyone ever just tried putting a container on the ground and lifting the packjob all the way to linestretch (via stairs, elevetor, platform, whatever) repeatedly and subjected it to different things like wind (or different bridle attachment points) to see what it did? And maybe caught it all on video to potentially uncover some offheading-causing mechanism that we haven't discovered yet?

Do I need to go to sleep before my brain hurts any more than it already does at 5:30 in.... the.................

..........zzzzzzzzzzzzzzzzzzzz

i have always wantd to deploy a rig in a re-circulating wind tunnel. that would be fun.

We did some early "poorman" testing, not so much on pilot chutes, but on rig designs, by replacing the pilot chute with a small lead weight and renting a lifting bucket. We were looking to see how much the angle of the container (at the moment of flap release) contributed to off heading openings.

We attached the rigs (flaps to earth) to the bottom of the bucket at various angles to simulate head-low, head-up, and sideways tilted deployments. Then we dropped the weights and filmed the drops. When viewed upside down these deployments looked pretty real up to full line stretch when the canopy was affected by the uneven risers. At that point they would always pivot somewhat on the loaded riser.

Of course this test doesn't model burbles and things of that nature, but it did lead us to start believing, that behind a totally screwed up pack job, heavy pilot chutes thrown too hard to the side, and uneven shoulders at line stretch, that crosswinds were probably the main culprit in off heading openings when everything else is right . . .

NickD
BASE 194

   Curious as to what the winds were doing on that jump, as there looks to be a fair amount on the lake below ?
Also,(IMO) pilot chute forces are not consistent throughout the extraction process and I think it's when the forces are reduced, after pins open and canopy is first lifted from container ,that momentarily unloads the PC and subjects it to outside conditions.
Was Mikki Ok on that jump ?

 
I'm not sure you're going to gain that much. I think you'll very quickly run into problems with the extraction. Free packed canopies already go to shit when they lift out of the containior. That's one of the things I like about the multi is that it does a much better job of lifting the weight of the lines in the tail pocket. Squares are not like a round where you can get even tention from one point. Frankly it amazes me that free packing works at all. The curent location is at least more or less in the center of the canopy. There is still a lot of slop between the mid B/C attachment point and the tail pocket, where most of the weight is. The further you move forwards the more slack you have between there and the pocket. The pocket is being lifted by the center b and c lines as they go into the pocket. The snatch force is trying to pull these lines out of the stowes in the middle of the tail pocket fucking up the line stoes and potentally caussing knoting and line deploment problems. One very good reason to have a good tight locking stow at the mouth of the pocket. This is a very good argument against jumping with out a stow. As the bridal attachment moves farther forwards you are now lifting by only the b lines and there is even more slack in the top skin.
For the record CRW dogs once put pull out puds on the noses of there canopies. They would just drag the center cell of the nose out and let it catch air like a pc. These canopies did however have a diaper to controle and lift the rest of the pack job. That's the most extream example I can think of. It aparently worked but fell out of favior.

Lee

how did you came to this conclusion ?