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collapsing canopies

 

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steve1  (D 23640)

Dec 4, 2001, 8:50 PM
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I only have about a 100 jumps on a square canopy so I still have lots to learn. I keep hearing about people being hurt and and even killed under a collapsed canopy. A friend of mine broke his back when he hit some dead air on the other side of a building. I was jumping into a tight dz on a windy day a while back. There were trees, a slight hill, and some buildings. I was about 300 feet up, in about half breaks when I experienced this scary falling sensation for a few seconds. (not fun!) I didn't look up because I was close of a goal post and fence, and my attention was focused on missing these. But I think I was experiencing a collapsing canopy. I was using a bigger canopy for a demo type jump. (A falcon 235) At any rate I would like to learn how to avoid this next time. Is your canopy more likely to collapse in half brakes? I assume the danger is greater on smaller canopies. Excuse my ignorance. Most of my jumps are on a para-commander about a hundred years ago. Rounds may have been slow but they didn't collapse. I also know that maybe I shouldn't have been making a demo jump with my limited experience under a square even though I have a D-license. I live a long way from the nearest drop-zone so I really appreciate your input. Be kind. I know there may be some sheer stupidity to this post.



Jimbo  (D License)

Dec 4, 2001, 8:55 PM
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Only 100 jumps on a square canopy _and_ a D license?

I'm guessing that sometime there was a lengthy layoff from the sport?

Anyhow, my limited experience suggests that you're more likely to experience the effects of turbulence under a lightly loaded canopy. Most often this means a larger canopy. Smaller canopies, with greater forward speed do a better job at maintaining pressurization of the individual cells, when the cells remain pressurized we don't seem to notice the turbulence quite as much.

Just my $0.02 - Someone correct me if I'm wrong.

-
Jim



Premier billvon  (D 16479)
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Dec 4, 2001, 11:35 PM
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Generally, 1/4 to 1/2 brakes will help keep your canopy inflated and flying. Smaller canopies are generally more stable but also more dangerous in turbulence - even a slight collapse under a 2:1 loaded canopy can hurt you badly, while it will likely be a non issue at .8 to 1 loading. Avoiding being downwind of obstacles also helps. A good rule of thumb in winds is a 10 to 1 ratio - be 10 feet away from any obstacle 1 foot high, and 100 feet from any obstacle 10 feet high. Also, as you correctly observed, I would try to avoid demos until you have a few hundred jumps on a square, and can land reliably in a very small area.

-bill von


BPO  (D 87411)

Dec 4, 2001, 11:40 PM
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In reply to:
Is your canopy more likely to collapse in half brakes?
I'm no expert, but I was told flying in half brakes is a good thing when hitting turbulence..

Any experts awake?



Dedalus  (D 6018)

Dec 5, 2001, 7:36 AM
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What's the theory behind the idea that 1/4 to 1/2 brakes helps keep the canopy inflated? I might buy the idea that the change in angle of attack/relative wind makes for better presentation of the nose, but slower speed also means less pressurization, and slower speed also means hanging around in the turbulence longer.

Mark Baur



Premier billvon  (D 16479)
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Dec 5, 2001, 11:04 AM
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>What's the theory behind the idea that 1/4 to 1/2 brakes helps keep the canopy inflated?

Two reasons. One, canopies inflate (or reinflate) more quickly and reliablu in brakes - that's one reason brakes are stowed for openings. Two, turbulence is essentially a sudden change in wind direction. The slower you fly, the slower you experience the changes, and the less violence you experience.

-bill von


freeflir29  (D 10000000)

Dec 5, 2001, 11:51 AM
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"The slower you fly, the slower you experience the changes, and the less violence you experience."

So is the theory of using front risers to ensure maximum cell pressure wrong?



"and I'm not easily impressed...Ooohh look...a blue car!" -Homer Simpson


GrumpySmurf  (C 32988)

Dec 5, 2001, 12:36 PM
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Hmm, makes sense - do a deep flare to re-inflate a collapsed end cell - I guess it works a number of collapsed cells too then? (to a certain degree anyways). Just pondering the possible connection between the 1/2 breaks for a collapsed canopy and a held flare for a collapsed end cell.



Premier billvon  (D 16479)
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Dec 5, 2001, 12:49 PM
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>So is the theory of using front risers to ensure maximum cell pressure wrong?

Well, no, it does increase cell pressure. However, no amount of cell pressure will keep your canopy inflated in turbulence beyond a certain point. The air mattress effect of an inflated canopy is nothing compared to the hundred or so pounds of stabilizing force being transmitted through the lines - that's where the majority of the canopy's stability comes from. Pulling on the front risers upsets the balance of those forces a bit.

The other problem with front risering is it distorts the shape of the canopy, and thus makes it fly differently. It may not recover as well from a collapse, and if/when it does, it will be descending faster - not the best thing near the ground.

-bill von


cobaltdan  (D License)

Dec 5, 2001, 3:54 PM
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i have seen a bunch of collapses at the ranch resulting in injury. the landing area is narrow between treelines with typicaly a cross wind. in all but one of the cases i saw, the canopies were lightly loaded and brought in on partial brakes.

additionally many days at the ranch you can watch load after load of canopies and the effect of the tree turbulence on them. (turbulence effects that do not result in accidents)

it has been my observation that canopies (especially lightly loaded) are effected more by turbulence if flown in brakes. maintaining full foward flight speed seems to me to be a better procedure.

one of the events was kind of unique in that i watched 4 jumpers land almost simultaneously in the same area near the tree line. all were lightly loaded (both 7 cell and 9 canopies), two of the jumpers rode out the bumps, maintaining full flight passing by the other 2 jumpers at tree level, landing first . then the 2 other jumpers continued slowly, spooked by the turbulence, in partial brakes sinking down. at about 25' both canopies collapsed. broken ankles resulting.

sincerely,

dan



steve1  (D 23640)

Dec 5, 2001, 8:15 PM
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Cobalt Dan,
I think you are right on as to what happened. There was a wall of very high trees next to a football field and a strong gusty cross wind. And my canopy was lightly loaded with half brakes. I'll no better next time. Thanks for the input from all.



dove  (D 26128)

Dec 5, 2001, 8:20 PM
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Bill -

Wow! Thank you so much for the info and for putting it so eloquently in laymans terms! I was just wondering about this very subject today as I was jumping in some sort of turbulent winds under a demo canopy!

dove



hookit  (D 24838)

Dec 5, 2001, 9:02 PM
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>One, canopies inflate (or reinflate) more quickly and reliablu in brakes - that's one reason brakes are stowed for openings.

Why would a canopy inflate more quickly in brakes? It seems the only thing that's going to inflate a canopy is air flowing into the cells. If a canopy is at full flight it should have more air flowing into the cells than it would if it were in partial brakes and therefore I would think it should inflate more quickly.

I can certainly see how a canopy would inflate 'more reliably' in brakes. I believe the main reason the brakes are stowed when packing is to help with keeping the canopy on heading during the opening. As one side inflates more quickly than the other it won't be at full flight and therefore won't spin up or turn off course as quickly.

I'm having a really hard time seeing how flying in partial brakes could help prevent a canopy from collapsing in turbulence. As you slow your forward flight you also decrease the pressure in the cells and, I would think, make the canopy MORE susceptible to collapse because it would then take a lesser degree of sudden wind change (turbulence) to deflate the canopy.

Isn't the primary reason that a canopy flown at a high wingloading is less susceptible to turbulence due to the fact that the more highly loaded canopy flies with more forward wind speed and therefore maintains a higher degree of pressurization in the cells?

What am I missing?

Blues,
Trey



Premier PhreeZone  (D License)
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Dec 6, 2001, 5:03 AM
Post #14 of 49 (2636 views)
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Re: collapsing canopies [In reply to] Can't Post

In reply to:
Why would a canopy inflate more quickly in brakes? It seems the only thing that's going to inflate a canopy is air flowing into the cells. If a canopy is at full flight it should have more air flowing into the cells than it would if it were in partial brakes and therefore I would think it should inflate more quickly.
I believe that what this does is change the angle of attack of the nose of the cells so they catch more air per foot travleled then leaving it at full flight. With more air being transfred to the cells, the more stable an opening is. By putting the canopy into brakes you are forcing more air into the nose which is keeping the cells over presurized.
And stowing the brakes for opening.... have you ever not stowed them? If not, I'll relay some stuff that happened this summer at the DZ. One of the new packers was learning the propack and was doing his fathers canopy unsupervised. He forgot to stow the brakes on the Heatwave and when his dad opened, it caused his camera to skip frames and throw the camera tracking off for a glitch and made the jumper black out due to the hard opening for a few seconds. It also bruised his dad to the point that he could'nt walk for 3 days. The jumper compaired it to being 3 times worse then a Sabre slam.

Phree
C-31712]


freeflir29  (D 10000000)

Dec 6, 2001, 7:07 AM
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"The other problem with front risering is it distorts the shape of the canopy"

I didn't think this distorted the shape at all.....merely makes for a steeper AOA, more cell pressure, and faster flight. Did I smoke too much crack at the DZ?

"and I'm not easily impressed...Ooohh look...a blue car!" -Homer Simpson


RemiAndKaren  (C 2328)

Dec 6, 2001, 7:13 AM
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In reply to:
Did I smoke too much crack at the DZ?
could explain a few things.. Wink

Clay, unless you can pull differently on the A and B lines, then the shape will be distorted. Think agout it: the tail and C lines dont move, but youre pulling the A and B by, say, 6 inches. If you wanted the shape not to distort, and if the line groups were equaly spaced, you would need to pull the A line by 6 and the B lines by 3.. or am I the one with the crack problem?

Remi
Muff 914


freeflir29  (D 10000000)

Dec 6, 2001, 7:28 AM
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OK...OK...I guess it does distort the shape some. Not like pulling on the brakes or rear risers though....I find that either method tends to smooth out the turbulence. Just don't hold the brakes down too long. Getting too slow causes it's own set of problems. I guess it's all personal opinion and very situational dependent.....*shrug*

"and I'm not easily impressed...Ooohh look...a blue car!" -Homer Simpson


mnischalke  (D 26290)

Dec 6, 2001, 7:39 AM
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Re: collapsing canopies [In reply to] Can't Post

Canopy pilot's handbook
http://www.skydiveaz.com/resources/book_canopy.htm



Premier billvon  (D 16479)
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Dec 6, 2001, 9:43 AM
Post #19 of 49 (2593 views)
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>Why would a canopy inflate more quickly in brakes? It seems the only thing that's going to inflate a canopy is air flowing into the cells.

No, in fact BASE canopies achieve full deployment before any cell pressurization takes place. This can be seen on any BASE video that shows a closeup of canopy deployment. The thing that makes a modern canopy deploy is mainly air pressure on its bottom skin, not air pressure in the cells.

> If a canopy is at full flight it should have more air flowing into the cells than it would if it were in partial brakes and
> therefore I would think it should inflate more quickly.

If cell inflation were the sole source of canopy stability I'd agree. However, the low pressure above the canopy, combined with the restraining force of the lines, are more important in terms of maintaining canopy shape. Keep in mind that there have been several canopy designs that inflated and flew like modern ram-airs but did not have any cells (i.e. were a single surface design.) None of them were very good compared to modern ram-airs, but they flew and maintained their shape without any cell pressurization at all.

One thing that's important to keep in mind is how powerful turbulence is. Even a minor gust can generate a lot of force, as anyone who tries to drag a partially inflated canopy off the field knows. The two things that resist this force are the cell pressurization (minor) and the force on each of the lines pulling the canopy down (major.) Each line attach on a sabre, for example, has an average of 5-6 pounds pulling down on it, and turbulence has to exceed that in order to collapse the canopy downwards.

>I can certainly see how a canopy would inflate 'more reliably' in brakes. I believe the main reason the brakes are stowed when packing is to help
>with keeping the canopy on heading during the opening.

Well, it has an awful lot to do with opening speeds. Brake set points are pretty important when it comes to determining how fast a canopy will open, although there are a huge number of variables to that.

> Isn't the primary reason that a canopy flown at a high wingloading is less susceptible to turbulence due to the fact that the more highly loaded
> canopy flies with more forward wind speed and therefore maintains a higher degree of pressurization in the cells?

Even in aircraft, higher wing loadings equate to more resistance to turbulence. The turbulence has less surface area to act on.

-bill von


Premier billvon  (D 16479)
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Dec 6, 2001, 9:46 AM
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>I didn't think this distorted the shape at all.....merely makes for a steeper AOA, more cell pressure, and faster flight.

If you could pull the lines down proportionally, that would be true. However, the front risers act only on the A and B line groups. If you pull them down hard, A and B will come down as a group, and you will get a "step" in the canopy between C and D.

Interestingly, paragliders have something called a "speed bar" that's designed to pull all the line groups down proportionally. They need this since they generally need every bit of glide they can get, and they can't deal with the extra drag from that 'step' in the canopy. The speed bar increases their speed and therefore wind penetration, but also makes them more prone to collapse.

-bill von


marcin  (D License)

Dec 6, 2001, 11:19 AM
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"He forgot to stow the brakes.... the jumper black out due to the hard opening.."

This is interesting as I have once heard an advice to leave the breaks unstowed on an FX to tame the openings. Maybe it was only a bad joke..

m.



watcher  (D 24876)

Dec 6, 2001, 11:38 AM
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In reply to:
If you could pull the lines down proportionally, that would be true. However, the front risers act only on the A and B line groups. If you pull them down hard, A and B will come down as a group, and you will get a "step" in the canopy between C and D.
Does haveing non cascading lines make a difference? ex: Groundzero canopies.

Jonathan



Premier PhreeZone  (D License)
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Dec 6, 2001, 12:04 PM
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Non-cascaded/Cascaded all react the same when you pull a riser down. they creat a stairstep effect to the canopy. Now if you had 4 risers, one for each line group on a non cascaded canopy, and pulled the A's one length and the B's another, then noncascaded would be different.



cobaltdan  (D License)

Dec 6, 2001, 6:32 PM
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do non cascaded lines make a difference?
yea, double the line drag making the lines the highest source of drag on your canopy.

-dan



Designer  (D 5771)

Dec 7, 2001, 12:33 PM
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After a quick read these people are right!Staying away from obstacles that can cause turbulence is a good Idea.We could debate 1/4 to 1/2 brakes all day on older/larger canopies!A swirlling wind can be very dangerous and has slammed me into the ground with no recovery time in 1/2 brakes.So there ya go.Something faster will eat through most turbulence,(beware)speed kills and the newer canopies are much less forgiving.Later,rob



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