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General BASE

d_goldsmith
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PC Oscillation Theory
After watching a lot of videos and seeing occasional oscillation, it seems that oscillation might be partly due to pendulum force (not talking about double pendulum).

If you pitch right and hit bridle strech, the upward force causes a pendulum force, since the PC is attached to a line and has to arc to the left to be able to move up completely. When it does get to the top, that pendulum force might drive it slightly farther left (if straight right is 0 degrees and straight up is 90, the pendulum force could push the PC over to 100 degrees). It seems like that could cause it to begin orbiting the center.

Thoughts?

Edited to fixed "begin orbiting the center." it said being orbiting.
d_goldsmith
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Re: [d_goldsmith] PC Oscillation Theory
I should clarify, in a perfect enviromet if it went to 100 degrees in a perfectly straight line, it would just bounce back and forth until is centered. But since air is chaotic, and there are other axis involed, it could oscilate.

Edited to fix "it could oscilate." I had said "it could pendulum"
ultraviolet
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Re: [d_goldsmith] PC Oscillation Theory
From some of the vids I've watched it seems the PC's that ocillate are the ones that are starting to strip the center cell away from the packjob.
crwper
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Re: [d_goldsmith] PC Oscillation Theory
My feeling is that the pilot chute doesn't really have enough momentum to pendulum as a result of "overshooting". The oscillation that we see is, I think, a minimum-energy mode for the pilot chute.

Think of a pot of water which is heated from the bottom. Rather than maintain a simple temperature gradient from bottom to top, it will tend to form current loops which transport the heat. This has the net effect of lowering the energy of the system.

In two dimensions, an asymmetric pilot chute would just tend to be pushed off to one side, until an equillibrium is reached between the force of the air being spilled out the side and the force trying to restore the pilot chute to the "vertical" position.

However, in three dimensions, we have extra "modes" which allow a more efficient release of the energy. In particular, it's possible for the pilot chute to move in a circle horizontally. This dynamic mode actually allows the pilot chute to reach a better trade-off between the two forces. Unfortunately for us, it also means the canopy is being pulled around in a circle.

Michael

Edited for clarity
TomAiello
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Re: [d_goldsmith] PC Oscillation Theory
I generally agree with Dave. I think the oscillation often starts because of the pitch--mostly because the initial PC inflation is way off to one side, which means that the PC is "sideways" a little bit because of the angle of the bridle pull. The PC bounces into the middle, and almost always past, starting a "pendulum" motion.

In my experience F-111 PC's do not have this problem (they reach bridle extension, then move up above the jumper, and only then inflate while centered over the jumpers back).
base283
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Re: [d_goldsmith] PC Oscillation Theory
As the first to annouce the PC oscillation theory. And having worked on other methods of deployment in the field of BASE, I can say this: Extraction speed is what matters. With low extraction speeds, momentum plays a major role in the heading. Granted that I was dealing with much lower extraction speeds than are normal for common BASE jumps. I personally don't think one would get this on a common BASE jump. Because the PC must move into the windline to get the drag to deploy the canopy from the container. Factors that affect one at this moment are the windline relationship to the shoulderline perpendicularity to the windline or lack thereof.
Hope this helps.
take care,
space
crwper
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Re: [TomAiello] PC Oscillation Theory
The motion of a pendulum could also be described as the sort of mode I'm talking about, so in a way we're talking about the same thing. However, I think when we compare pilot chute motion to a pendulum, we are iimplicitly saying that the pilot chute's momentum is important, which I think it is not.

If we imagine a mass hanging from a thread, and a jet of air shooting from the side of the mass, we are much closer to the problem of asymmetric pilot chutes. In this case, the pendulum will start out being pushed off to one side by the jet, but its being balanced way out there on the jet is an unstable situation. It will rotate slightly, and now the jet pushes it in a circular motion. That's a very stable situation, and very similar to an orbiting pilot chute. But the motion is primarily caused by the force of the jet and the restoring force. It has little to do with the momentum of the suspeded mass.

With a zero-p pilot chute, we can add other forces to the equation. When the pilot chute swings "left", it spills air out the right side, and generates a force which pushes it further in that direction. Like the suspended mass with a jet of air, it will eventually reach what would be an unstable equllibrium point, where small perturbations will cause it to move in some other direction. A similar phenomenon can be seen in Karman vortex streets, another periodic solution.

This may seem like nit-picking, but I think the difference is important. If momentum was a big player, then the key would be to reduce the mass of the pilot chute. As noted above, an asymmetric pilot chute, or a zero-p pilot chute, introduce problems not related to momentum. As you point out, an F-111 pilot chute is much less succeptible to the second mode. A symmetric F-111 pilot chute is a pretty good solution to both problems.

Michael
crwper
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Re: [base283] PC Oscillation Theory
In reply to:
As the first to annouce the PC oscillation theory. And having worked on other methods of deployment in the field of BASE, I can say this: Extraction speed is what matters. With low extraction speeds, momentum plays a major role in the heading.

When you say "low extraction speed", are you referring to airspeed when the pilot chute is extracted, or to the strength of the pilot chute toss?

Michael
d_goldsmith
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Re: [crwper] PC Oscillation Theory
Have you been talking to Jason about this, or are you just reading his text books. j/k

Check out this video. The main jumper in view pitches forward (blue PC) and you see it bounce (pendulum) backward and if you watch close you see it start to extract the canopy backward. It then bounces back forward and straightens the canopy out pretty well.

http://www.skydivingmovies.com/ver2/pafiledb.php?action=file&id=3060&string=low%20pull

Same thing in this one.

http://www.skydivingmovies.com/ver2/pafiledb.php?action=file&id=1187&string=jinx

The first one on this video is a staight out pitch, that goes left of center, back right and then starts clockwise. It seems like it might have been feet side of the attachment to the container, and it goes slightly forward as it is going left, and then slightly backward when it bounces right, and the backward and right might be what causes the clockwize spin.

http://www.skydivingmovies.com/ver2/pafiledb.php?action=file&id=1551&string=pbasegainers

This could all be the chaos of air and trim of PC, but it seems like pendulum force to me.
d_goldsmith
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Re: [d_goldsmith] PC Oscillation Theory
I just realized if you are really head strong on the points you made you might have taken offense to me joking about you getting that from Jason. I was just messing with you.

And I forgot to make my main point. I don't think decreasing mass of the PC is the way, I was thinking that pitching straight behind you instead of to the side would stop that pendulum and make all the force from one direction. Then the only way it could oscilate is wierd air or bad trim. However I'm sure there are down sides to this (first one to mind is PC trapped in burble).

I watched Baseclimb and when Nick Feteris gwent handheld he stowed the bridle in his container and held the PC by the bridle attachment point, and then just lets go of it in freefall. Then again this was in the 90's
Bernt
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Re: [d_goldsmith] PC Oscillation Theory
Summarizing the discussion sofar, parts of the problem are:
1) PC inflation before PC is centered above the jumper
2) Air escaping from the sides of the PC, non-symmetrically.

The 'forces' working on the PC are:
3) the weight of the main parachute tries to pull the PC towards the center
4) Air spilling from the sides of the PC tries to push the PC away from the center, if the PC is not in the center already, or non-symmetrically mounted/fabricated.

I agree with you crwper that the mass of the PC is probably not the main cause. It has mass, so some effect yes, but I think the other forces are more dominant.

It hasn't been mentioned yet, maybe it's common knowledge, but I realised not so long ago the the PC does not inflate before it is loaded, meaning not before bridle stretch. Until then it's just a piece of cloth...

For 2), F111 and vents seem to help, since less air escapes from the sides of the PC.

For 1), we can put a small slider on the PC so that it has time to stabilize in the center before it starts inflating… he,he, anyone who wants to try this? Wink


How about more vertical surface on the PC (increasing the drag for movement sideways), like on a tandem drogue, see the attached picture (looks more like an ice cone though…). Or would this have minimal effect?

Could the next PC be F111, vented and with more vertical surface?

Has anyone experimented with vented F111 PC's? Or is vents not giving any benefits here?

Any problems with more vertical surface on a PC?

Why is ZP used instead of F111? More durable?

Bernt
"If you jump from a bridge in Paris, you are in Seine"
PC mod.JPG
pope
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PC Oscillation Theory
short PC hesitation/oscillation video for those that want to see a pretty good one.

pope
Calvin19
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Re: [Bernt] PC Oscillation Theory
i have to test the final PCs i have been making, and so far, the PCs i have made seem to COMPLETELY get rid of oscilation, then i will start to ship a few to some super experienced, well educated jumper to see what they think. then, if they sign it off, i will offer them for sale.

i would post pics, but that would not be very competitive, would it?
d_goldsmith
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Re: [pope] PC Oscillation Theory
Wow that's crazy. It looks like there was a rubber band on it and after it inflated it was stretching like one of the pieces of frabric was put on with the block and bias off.
yuri_base
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Re: [pope] PC Oscillation Theory
In reply to:
short PC hesitation/oscillation video for those that want to see a pretty good one.

There's a distinct 2-second PC snivel (!) on that one. WTF?!?
yuri_base
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Re: [Calvin19] PC Oscillation Theory
In reply to:
the PCs i have made seem to COMPLETELY get rid of oscilation

Wait a minute... but we were told that PCs CANNOT be improved! Absolutely, positively not!

PCs - cannot be improved.
Canopies - cannot be improved.
Rigs - cannot be improved.
Wingsuits - cannot be improved.

You just can't outsmart the generations before you!!!

Wink








PS. Let's fuckin' rock this world.
Calvin19
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Re: [yuri_base] PC Oscillation Theory
your right, it was a lie.
hookitt
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Re: [yuri_base] PC Oscillation Theory
Who ever said that?
obi
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Re: [pope] PC Oscillation Theory
In reply to:
short PC hesitation/oscillation video for those that want to see a pretty good one.

pope

Looks like old skydiving-gear to me, incl pod and real noice asymetric pc Crazy
Lokky_luke
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Re: [Calvin19] PC Oscillation Theory
i have to test the final PCs i have been making, and so far, the PCs i have made seem to COMPLETELY get rid of oscilation, then i will start to ship a few to some super experienced, well educated jumper to see what they think. then, if they sign it off, i will offer them for sale.

i would post pics, but that would not be very competitive, would it?
Hi.
Ship some of them to VKB, and we will test them during summer.
100_00402.jpg
BASEjumper
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Re: [pope] PC Oscillation Theory
In reply to:
short PC hesitation/oscillation video for those that want to see a pretty good one.

pope

In this video it is clear to see that the pilot chute was centered above the jumpers back and was not inflated yet (it was hesitating to inflate rather badly). Only when the pilot chute inflated did it start to orbit, and the orbit became more and more violent.

The jumper didn't throw it to the side much at all. His lazy throw may or may not have contributed to the PC hesitation. Throwing the pilot chute to the side is a good practice because it keeps the pilot chute away from your burble. The most important part of the deployment process is that the pilot chute executes its most important job of inflating on time and pulling out the canopy. Throwing the PC to the side will get it to bridle-stretch quicker which will allow it to start to inflate quicker. That is a good thing on lower objects.

It was concluded that Shannon doing a soft throw is what caused her tragic death (#99 on the BASE Fatality List). Other BASE jumpers might want to learn from this. Doing a soft throw with the idea of not putting your PC as far to the right is asking for trouble in my opinion.

The PC orbit from pope's video was most likely caused by an asymmetrical pilot chute. It may have been attached asymmetrically or built asymmetrically or both.

The trade off for keeping the pilot chute closer to your burble in order to try and keep it to stay directly obove your back hoping to reduce the chance of it orbiting is not worth the risk of it getting pulled into your burble. Also if you have the habit of not throwing it to the side and you deploy unstable, there is more chance of getting entangled with the bridle.

The manufacturers have addressed the PC orbit issue to some degree by making vented PCs which do seem to reduce the occurance of orbit. Martin Tilley at Asylum has a new PC out with the vents moved away from the apex and an attachment point that will only attach symetrycally. I think it cost $175 though but if it works much better, then that would be an extra $85 well spent. I saw a PC with an attachment point like that a few years back and I thought it was brilliant but I forget which manufacturer makes it.

Since Martin has been developing this new PC, perhaps he can share his thoughts on this subject.
Calvin19
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Re: [Lokky_luke] PC Oscillation Theory
In reply to:
i have to test the final PCs i have been making, and so far, the PCs i have made seem to COMPLETELY get rid of oscilation, then i will start to ship a few to some super experienced, well educated jumper to see what they think. then, if they sign it off, i will offer them for sale.

i would post pics, but that would not be very competitive, would it?

Hi.
Ship some of them to VKB, and we will test them during summer.
i thnk i will do that.

what sizes do you want?

I see no need to improve on 44, and 46.

I have made super-vented36 and 41/42

PS- i think you need a human body in your tunnel, to make the simulated wake of a jumper (retard word being burble)
hookitt
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Re: [BASEjumper] PC Oscillation Theory
In reply to:
It was concluded that Shannon doing a soft throw is what caused her tragic death

It is beyond a doubt, not from a soft throw. It was missed grip, followed by pushing it onto her butt going for a regrab.

While a soft throw is not good, missing a grip is worse.
BASEjumper
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Re: [hookitt] PC Oscillation Theory
In reply to:
In reply to:
It was concluded that Shannon doing a soft throw is what caused her tragic death

It is beyond a doubt, not from a soft throw. It was missed grip, followed by pushing it onto her butt going for a regrab.

While a soft throw is not good, missing a grip is worse.

Thanks for the clarification.

Point is: pilot chute in burble = really bad
mindtrick
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Re: [d_goldsmith] PC Oscillation Theory
pcs wont oscilliate if u let Jesus into your heart
pope
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Re: [mindtrick] PC Oscillation Theory
In reply to:
pcs wont oscilliate if u let Jesus into your heart


Jesus won't get into your heart if you can keep your PC's from oscillating.
leroydb
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Re: [pope] PC Oscillation Theory
What if Jesus created the perfect PC? Then would you let him in your heart?
yuri_base
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Re: [leroydb] PC Oscillation Theory
Jesus was offered a chance to do a BASE jump, but he rejected it. Apparently, he was afraid his PC would oscillate and cause 180.

Unlike him, we follow Satan's words without hesitation!










(ok, written after a couple of beers, take it easy Wink)
leroydb
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Re: [yuri_base] PC Oscillation Theory
I wonder how many BASE Jumpers are athiest, agnostisc, or otherwise?
badenhop
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Re: [leroydb] do i smell fire?
I am a Christian.
hollyhjb
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Re: [badenhop] do i smell fire?
ME TOO!!!
d_goldsmith
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Re: [hollyhjb] do i smell fire?
I'm an agnostic atheist. I think all the mass and energy in the universe is tied togethor and is something special and even spiritual like a god, but I don't think that "god" has a conscience.
RayLosli
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Re: [d_goldsmith] do i smell fire?
yuri base : ... " Unlike him, we follow Satan's words without hesitation ! "
D goldsmith : ... " spiritual like a god, but I don't think that "god" has a conscience. "
_____

Yes but chances are, Even God knows to at Least make a ' Conscience' effort to throw a Pilot chute without ' Hesitation' .

When I go-in BASE jumping and God lets me into Heaven. First thing I'm going to do is run over to Buda and ask him the age old philosophical question of.
If a BASE jumper Crates-In leaving body parts all over the Talus and No One is there to hear it. Does it make a sound ?
.
.
luv2fly
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PC Oscillation
GIVE IT UP TO GOD!!!

The PC Oscillation problem must be solved to reduce the injury and death we see in BASE. I've put lots of energy into this problem. Given that I have not BASEd in 3 years 4 months..since losing my friend DW at RG in 2003. I may be off a bit in my explanation so please correct me if you see a mistake in my understanding here...but it amuses me that nothing has advanced in this area in years! And it keeps coming back as a problem...Every so often I say my .02 cents again and then it goes away until some new observation comes from someone different...

To explain where I'm coming from...I'll start at the beginning (as far as I can remember it). Anne Helliwell and Todd Shoebothom from BR were the first to introduce me to the idea of production and use of F111 PCs in BASE. I used the F111 for a couple years and later discarded it as not as "bullet proof" as ZP. I got some PC insight from SpACE whilst hanging out in Norway but still did not understand the whole concept. Then came the vented ZP and this supposedly was the "Godsend" of PC safety...and everyone tried to grab a bit of the market share of the ultimate PC - a vented ZP. BUT, I began to see this as a micromental improvement over regular ZP because the damn PC was still oscillating and killing.

It was at this point that Anne and I looked for an explanation to the problem and we searched for a solution. As soon as a ZP PC reached full bridle stretch in non-terminal jumps (mostly sub-terminal slider up), it was more likely to meet the air with a reaction to the action and begin the oscillation. We tested and researched the vented ZP PC on balloon drops with a weight to replicate a 4 second delay and the results were as expected. The vented ZP PC was a hoax...a great way to market a "revolutionary idea." But, without the desired outcome - a significant reduction in oscillation. It was only a minor one - maybe 10% - but not enough testing was done.

Anne did great camera work in Malaysia - recording more than 800 jumps made over 2 years. From this work we saw that of my 23 jumps in 2002/2003, I did not have one PC oscillation beyond about 5 degrees from center while the regular ZP PC and vented ZP PC saw about a 20% chance of severe oscillation leading to offheadings - sometimes up to 45 degrees - swirling around and around. This convinced me that F111 was the true "Godsend" - whatever that means - and ZP PCs with or without vents was a hoax.

Think of a F111 PC as a PC with thousands of vents distributed equally. That's the best way to look at it and it's truly the best way to eliminate offheading openings attributed to PC action (with current technology).

That is all. Be smart and live longer without injury - stop using a ZP PC for subterminal slider up jumping!

Good luck!
JJ
d_goldsmith
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Re: [luv2fly] PC Oscillation
Does that only apply to sub-terminal slider up? If so, why?

Edit to add: I'm guessing you don't have as much data on sub-terminal slider down and terminal, but does anyone else have data about those types of jumps with f111 compared to zp vented?
BASEjumper
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Re: [luv2fly] PC Oscillation
In reply to:
GIVE IT UP TO GOD!!!

The PC Oscillation problem must be solved to reduce the injury and death we see in BASE. I've put lots of energy into this problem. Given that I have not BASEd in 3 years 4 months..since losing my friend DW at RG in 2003. I may be off a bit in my explanation so please correct me if you see a mistake in my understanding here...but it amuses me that nothing has advanced in this area in years! And it keeps coming back as a problem...Every so often I say my .02 cents again and then it goes away until some new observation comes from someone different...

To explain where I'm coming from...I'll start at the beginning (as far as I can remember it). Anne Helliwell and Todd Shoebothom from BR were the first to introduce me to the idea of production and use of F111 PCs in BASE. I used the F111 for a couple years and later discarded it as not as "bullet proof" as ZP. I got some PC insight from SpACE whilst hanging out in Norway but still did not understand the whole concept. Then came the vented ZP and this supposedly was the "Godsend" of PC safety...and everyone tried to grab a bit of the market share of the ultimate PC - a vented ZP. BUT, I began to see this as a micromental improvement over regular ZP because the damn PC was still oscillating and killing.

It was at this point that Anne and I looked for an explanation to the problem and we searched for a solution. As soon as a ZP PC reached full bridle stretch in non-terminal jumps (mostly sub-terminal slider up), it was more likely to meet the air with a reaction to the action and begin the oscillation. We tested and researched the vented ZP PC on balloon drops with a weight to replicate a 4 second delay and the results were as expected. The vented ZP PC was a hoax...a great way to market a "revolutionary idea." But, without the desired outcome - a significant reduction in oscillation. It was only a minor one - maybe 10% - but not enough testing was done.

Anne did great camera work in Malaysia - recording more than 800 jumps made over 2 years. From this work we saw that of my 23 jumps in 2002/2003, I did not have one PC oscillation beyond about 5 degrees from center while the regular ZP PC and vented ZP PC saw about a 20% chance of severe oscillation leading to offheadings - sometimes up to 45 degrees - swirling around and around. This convinced me that F111 was the true "Godsend" - whatever that means - and ZP PCs with or without vents was a hoax.

Think of a F111 PC as a PC with thousands of vents distributed equally. That's the best way to look at it and it's truly the best way to eliminate offheading openings attributed to PC action (with current technology).

That is all. Be smart and live longer without injury - stop using a ZP PC for subterminal slider up jumping!

Good luck!
JJ
good post.

I also prefer an F-111 for not only slider up in the 4-6 sec delay range but also terminal jumps as well for the very reason you explained. How do you feel about using F-111 on slider up jumps at the idaho bridge? How would you feel about everyone else using F-111 there?

One thing jumpers should keep in mind if using F-111 is that the porosity will degrade much faster than with ZP. Perhaps that's what you meant by "not as 'bullet proof' as ZP." As much as I do like F-111 PCs for some types of jumps I personally would not want to jump a clapped out one. What are your experiences and thoughs about a tired F-111 PC? Is there an exit altitude where you would not want to use F-111?
Carpediem
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Re: [BASEjumper] PC Oscillation
In reply to:
One thing jumpers should keep in mind if using F-111 is that the porosity will degrade much faster than with ZP. Perhaps that's what you meant by "not as 'bullet proof' as ZP."

I prefer F111 also and will use a new PC after 100 jumps. How many jumps would you kids put on a F111 PC before replacing it?
TomAiello
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Re: [BASEjumper] PC Oscillation
In reply to:
How do you feel about using F-111 on slider up jumps at the idaho bridge? How would you feel about everyone else using F-111 there?

I have several F-111 PC's in service for FJC's here. I've been pleased with their performance (I think they oscillate noticeably less than even the best vented ZP PC's I'm using). I replace the F-111 PC's approximately every 100 jumps.

At some point I'll try to sit down and edit out all the video from my FJC's, to compare PC's, as Annie did at the events in Malaysia. The short version, though, is that I agree with her conclusions. F-111 PC's are an excellent tool, and are usually vastly underrated by modern BASE jumpers.

I don't think I'd mind if everyone used F-111 here, provided they were careful about maintenance (and replacement).
TomAiello
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Re: [luv2fly] PC Oscillation
Just an additional observation.

In addition to damping PC oscillation better than ZP, the F-111 PC's actually start with less oscillation. This is because of the typical inflation patterns. From observation, the ZP PC's will inflate when they reach bridle stretch out to one side of the jumper (which means that they have a "pendulum" swing to start the oscillation). The F-111 will actually swing up and over (directly above) the jumper, and _then_ inflate (eliminating that initial "pendulum" into position).
Calvin19
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Re: [TomAiello] PC Oscillation
Im sure it has been brought up before... and maybe not.

I dont think pc swings are caused by 'air spilling out one side', granted, that is what is happening to some extent, but i think the PC is acting like a foil itself, and actualy 'gliding' in the circle above the jumper, much like a wing on a kite.

but, i dont know.
just a thought.
luv2fly
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Re: [BASEjumper] PC Oscillation
TO answer your questions BASEjumper I had about 150 jumps on a 36" F111 - mostly terminal and it still worked fine...(but I gave it to someone when I sold him a canopy - and he continued to use it - not sure how many jumps tho). I have used F111 (not clapped out) on 160' freefall jumps with no noticeable problems or differences. But yes, F111 when exposed to water and more then 100 jumps will degrade to the point where I wouldn't want to personally use one but wouldn't mind if you did...clearly for testing purposes only tho...

I would ALWAYS use F111 on sub-terminal and terminal jumps. Sometimes I used ZP on slider down mostly because I don't think in no wind conditions (what I mostly liked to jump in) there was clearly not a difference between F111 and ZP.

I can't stress this enough! Use F111 - especially for sub-terminal slider up! That is, if you want to live injury/fatality free...
Hope this helps!
JJ
Mahle
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Re: [luv2fly] PC Oscillation
Hi,

just want to post this attachement again. It contains some thoughts about pcs, too.

I wrote this a while ago after years of own research. I checked it once in a while and updated new thoughts. So I don't know if it is a wise document. It's just a point of view.
I'm this way not overly concerned but it seems basic stuff gets lost quicker and quicker.

feel free to read it

my 2 cents
M.
Off Heading english.doc
mbondvegas
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Re: [TomAiello] PC Oscillation
This is a good thread. I thought Will Ox's PC hesitation was the reason that F-111 went out of fashion as it showed the downside (potential to not unfold quickly) of F-111. That was on a super short delay...so maybe the answer is ZP for big PC's and short delays and F-111 for 5+ seconds?

On a related note, has anyone observed that smaller PC's oscillate more at a given airspeed? For instance, a 38 at 5 seconds versus a 42?

It would seem to me that the bigger PC (purely based upon the time it takes to do its job) would oscillate less...

I have BR's old PC chart...but what is the real downside (i.e. danger) of using a 42" in the 5-8 second range? I know a very experienced jumper who uses a 42" almost exclusively and has a ton of jumps from big A's. Just interested in other's thoughts!
SBCDave
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Re: [mbondvegas] PC Oscillation
I've heard people say that it messes up your pack job by extracting it too fast. That seems pretty ridiculous to me. I could see it possibly damaging your canopy though at the bridle attachment point.
mbondvegas
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Re: [SBCDave] PC Oscillation
SBCDave wrote:
I've heard people say that it messes up your pack job by extracting it too fast. That seems pretty ridiculous to me. I could see it possibly damaging your canopy though at the bridle attachment point.

I could see this at the extremes...like a 46" at 10+ seconds...but would not expect it with a 42" at 5ish seconds. Has anyone actually seen a canopy damaged by a big PC and long delay?
UberChris
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Re: [mbondvegas] PC Oscillation
if you throw your pilot chute almost straight out in front of you, wont you be eliminating oscillation while still being able to grab good air real quick no matter what fabric your PC is made of?

this is assuming winds are completely calm
mbondvegas
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Re: [UberChris] PC Oscillation
UberChris wrote:
if you throw your pilot chute almost straight out in front of you, wont you be eliminating oscillation while still being able to grab good air real quick no matter what fabric your PC is made of?

this is assuming winds are completely calm

Your PC doesn't know front to back from right to left. IF a hard right toss causes oscillation, so would a hard forward toss. Make sense?
Stut
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Re: [mbondvegas] PC Oscillation
I can't speak for the oscillation issue but i have been on some jumps with a buddy that has around 500 jumps and he used a 42" toxic on 10sec and 8sec delays. Says that he typically does that and has no problem with damage.
tr027
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Re: [mbondvegas] PC Oscillation
mbondvegas wrote:
Your PC doesn't know front to back from right to left. IF a hard right toss causes oscillation, so would a hard forward toss. Make sense?
It's not the PC knowing front to back or right to left, it's the canopy knowing when it gets extracted off-axis, especially right/left.
mbondvegas
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Re: [tr027] PC Oscillation
tr027 wrote:
mbondvegas wrote:
Your PC doesn't know front to back from right to left. IF a hard right toss causes oscillation, so would a hard forward toss. Make sense?
It's not the PC knowing front to back or right to left, it's the canopy knowing when it gets extracted off-axis, especially right/left.

I get that...but if you see an oscillation...it is a very fast circular motion...not just right to left.
UberChris
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Re: [mbondvegas] PC Oscillation
well i have not seen this in the field yet, but the few videos ive watched it seems that when a PC is oscillating its more of a right/left violent motion as opposed to a circular clock motion. </green>

while the topic has been brought up, anyone who owns a toxic please speak about why you like that PC. ive been thinking that because the 42 is the workhorse, i may invest 175 bux in one but does it really make that much of a difference for someone who doesnt have much money, as opposed to just a regular morpheus ZP 42?
i would like to hear your opinion.
mfnren
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Re: [UberChris] PC Oscillation
I have over a hundred jumps using various 42 toxics. Anywhere from go and thro's to 5 sec. slider up. Some of my theories from using them.

The vents are closer to the true apex of the pc, resulting in more even venting and reduction in air spilling out the bottom randomly.

faster inflation and higher drag force. People debate if they are faster than a normal vented pc, in my opinion, yes. I notice a difference when switching back and forth. It is just tenths of a second, but that is how fast events are occuring in base- the faster the inflation the less time for things to go wrong.

I don't use a 46 anymore, have used to freefall down to 270' and get comparable results to people using larger pc's

watching video of the pc, it is much more stable than normal pc's, it parks above the jumper much faster.

On size and longer delays, my experience has been that larger pc's oscilate more on longer delays, more force creates more erratic movement, and degrades heading performance.

Is the cost is worth it? Thats up to you to decide...
GreenMachine
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Re: [UberChris] PC Oscillation
You wrote:
</green>

Um, why? I did not post in this thread?

I agree with the observation stated that increased air
speed increases oscillations. Drogues and SKY PC's
are always twisted. Plus you can see more rotation
on videos from BD than The Perrine.
SBCDave
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Re: [UberChris] PC Oscillation
UberChris wrote:
well i have not seen this in the field yet, but the few videos ive watched it seems that when a PC is oscillating its more of a right/left violent motion as opposed to a circular clock motion.

Do you have any links to these videos? All the oscillation I've seen has either been a circular motion at around the same radius throughout the circle, or a totally chaotic (random) motion still, roughly the same radius, but moving in all directions.

As far as damage. The way I see it, the pull force of your PC should be strong enough to open your container and get you to line stretch quickly, and not much stronger than that, ideally. Anything weaker obviously won't get you to line stretch quickly, anything stronger does unnecessary wear to your gear (PC, bridle, bridle attachment point, center cell top skin around attachment point, the seems where the center cell ribs attach to the top skin [probably mainly between b and c lines]). You wouldn't rap your bridle around a tree branch and use your rig as a rope swing. There's not a ton of info available on this (that I know of) but all I'm saying is whenever possible do what you can to avoid wear on your gear.

If a 36 or 38 will get you quick line stretch after 5 seconds, why use a 42?

/End rant Tongue
crwper
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Re: [mbondvegas] PC Oscillation
mbondvegas wrote:
I have BR's old PC chart...but what is the real downside (i.e. danger) of using a 42" in the 5-8 second range? I know a very experienced jumper who uses a 42" almost exclusively and has a ton of jumps from big A's. Just interested in other's thoughts!

As I understand it, one of the biggest dangers using too big a PC on long delays is line dump. When the canopy is lifted off a jumper's back, there's a bit of give and take between the canopy and the PC--i.e., the PC speeds up a bit, and the canopy slows down a bit. With a larger PC (at high airspeeds), there's less give, so the canopy can be lifted very abruptly off the jumper's back. Particularly as the Velcro on the tail pocket gets weak, this can cause line dump, and all the problems it comes with.

Michael