PC Oscillation Theory

[d_goldsmith 1]

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 1]

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 0]

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 7]

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 26]

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).

-- Tom Aiello

Tom@SnakeRiverBASE.com
SnakeRiverBASE.com

[base283 0]

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 7]

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 7]

Quote

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 1]

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 1]

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 0]

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?


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"

[pope 0]

short PC hesitation/oscillation video for those that want to see a pretty good one.

pope

[Calvin19 0]

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 1]

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.

I've got this really hardcore group of gaurdian angels that need a free paid vacation.
~Dan Osman

[yuri_base 1]

Quote

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?!?

Android+Wear/iOS/Windows apps:
L/D Vario, Smart Altimeter, Rockdrop Pro, Wingsuit FAP
iOS only: L/D Magic
Windows only: WS Studio

[yuri_base 1]

Quote

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!!!










PS. Let's fuckin' rock this world.

Android+Wear/iOS/Windows apps:
L/D Vario, Smart Altimeter, Rockdrop Pro, Wingsuit FAP
iOS only: L/D Magic
Windows only: WS Studio

[Calvin19 0]

your right, it was a lie.

[hookitt 1]

Who ever said that?

My grammar sometimes resembles that of magnetic refrigerator poetry... Ghetto

[obi 0]

Quote

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

[Lokky_luke 0]

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?

[BASEjumper 0]

Quote

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.

Life is like a box of chocolates.

[Calvin19 0]

Quote

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.

[hookitt 1]

Quote

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.

My grammar sometimes resembles that of magnetic refrigerator poetry... Ghetto

[BASEjumper 0]

Quote

Quote

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

Life is like a box of chocolates.

[mindtrick 0]

pcs wont oscilliate if u let Jesus into your heart