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A NOT so lightly loaded Sabre (2) and turbulence

 


peek  (D 8884)

Jan 28, 2007, 6:32 PM
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A NOT so lightly loaded Sabre (2) and turbulence Can't Post

A NOT so lightly loaded Sabre (2) and turbulence

Some of you may remember postings I made about lightly loaded high performance canopies in turbulence, and how sometimes their end cells fold under a bit and then snap back out, sometimes without the person even realizing it happened.

Well, again, yesterday, but this time the jumper was 190 pounds under a Sabre 2, 150 square feet. And this guy had some drive even for the wind conditions, perhaps 5 MPH.

It was 20-25 MPH with some gusts, and I'm rather sure that the turbulence was generated by the same thing I mentioned last posting on this subject, that is, a ditch providing a "ramp" to guide the wind up and cause a rotor.

He didn't land all that hard, in fact, keeping the canopy under control after landing was the most troublesome thing, because it seemed to me like the wind increased about 5 MPH right after he landed.

(But maybe it was right before he landed, huh?)

As always, be careful out there. It ain't just happenin' to the lightweights.


(This post was edited by peek on Jan 28, 2007, 6:39 PM)


AggieDave  (D License)

Jan 28, 2007, 6:42 PM
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Re: [peek] A NOT so lightly loaded Sabre (2) and turbulence [In reply to] Can't Post

Canopy design really doesn't matter when you're flying in turbulence conditions. Even an airlocked canopy won't keep you "safe" in turbulence.

The only difference that wingloading really plays on turbulence is that the canopy flies through it quicker. It doesn't mean that the canopy is less effected. On the "up" side you're flying MUCH faster at a high wingloading when your canopy folds up...if that's an upside.

Shouldn't your advice as a "wisened old jumper" be "do your best not to jump in turbulent conditions or fly in areas that are prone to turbulence?"


BIGUN  (D 23385)

Jan 28, 2007, 6:53 PM
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Re: [AggieDave] A NOT so lightly loaded Sabre (2) and turbulence [In reply to] Can't Post

Quote:
do your best not to jump in turbulent conditions or fly in areas that are prone to turbulence?"

I'll second that.


murps2000  (D 23114)

Jan 28, 2007, 7:08 PM
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Re: [AggieDave] A NOT so lightly loaded Sabre (2) and turbulence [In reply to] Can't Post

The only difference that wingloading really plays on turbulence is that the canopy flies through it quicker. It doesn't mean that the canopy is less affected.
Quote:

You sure?


AggieDave  (D License)

Jan 28, 2007, 7:18 PM
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Re: [murps2000] A NOT so lightly loaded Sabre (2) and turbulence [In reply to] Can't Post

Think of turbulence is like a pocket of air. Sort of like a pothole on your "road" in your air. So like driving down a bumpy road, a lot of bumpy air will act similarly. Moving quickly it doesn't seem as bumpy, taking less input to keep "in your lane." Moving slowly it'll take more input to keep "in your lane."

That's an over simplification for the sake of argument, but it touches the major points. I also stole that analogy from a well known and respected canopy coach.

When dealing with things other then simply turbulent air from thermal conditions, things change. Wind rotors, wind shear, and other types of turbulence can do some crazy things that no wingloading, no canopy design, nothing will help. If you're jumping in it you're going to simply get what you get. Then hopefully you're smart enough that when you're lucky enough to land safely that you sit out the next few loads to see if the conditions change.

That or if you know that when the wind blows from a certain direction that a certain part of the landing area gets rotors, turbulence, etc then don't freak'n land in that part of the landing area. That only takes the A-license basic ability of landing accuracy.


tdog  (D 28800)

Jan 28, 2007, 8:13 PM
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Re: [AggieDave] A NOT so lightly loaded Sabre (2) and turbulence [In reply to] Can't Post

Quote:
The only difference that wingloading really plays on turbulence is that the canopy flies through it quicker. It doesn't mean that the canopy is less effected.

AggieD, I really respect your opinions a lot of the time...

But, could one not argue that the "ram air" design of a canopy has a lot more air "ramming in" at higher speeds, and thus collapses will occur less often and/or inflate quicker because of the increased pressurization when a canopy has more airspeed.

I know they are completely different beasts, but I also know my paraglider loaded .5 ish was prone to very dynamic asymmetric collapses (like half the canopy). My current canopy loaded 1.45 to 1 bounces around a lot, but feels much more rigid. My 1 to 1 canopy was somewhere in between, but still more "pressurized" than the paraglider.

A downdraft or thermal will take any canopy up or down with it. No canopy will fly thru a tornado well. But, I think your comment is a little to blanketed for the occasional rough spot where pressurization is going to play a factor.


(This post was edited by tdog on Jan 28, 2007, 8:15 PM)


fallingfaster  (B 29136)

Jan 28, 2007, 8:22 PM
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Re: [AggieDave] A NOT so lightly loaded Sabre (2) and turbulence [In reply to] Can't Post

Quote:
Canopy design really doesn't matter when you're flying in turbulence conditions. Even an airlocked canopy won't keep you "safe" in turbulence.

"There is a value judgment implied here that is if you are not going fast you are being safe. Many of us that have really thought about parachute stability choose to come in fast and jump smaller parachutes, a big part of that decision is that parachutes are stable at high speed, now a stable parachute is stable at high speed I should say. So for me it's a safety issue, jumping a 190 in turbulence for me would be a terrifying thing I don't do it, it scares me." - Brian Germain, Skydive Radio #75

As I understand it, a canopy that is flying faster through the air maintains higher pressurization of it's cells. A more highly pressurized wing means the wing is less likely to collapse.

Quote:
Moving quickly it doesn't seem as bumpy, taking less input to keep "in your lane." Moving slowly it'll take more input to keep "in your lane."

I've seen a lightly loaded canopy (less than 1 to 1) collapse at 30 feet in turbulent conditions (winds 15 knots and thermal activity). The collapse was precipitated by the low experience jumper making several large opposite toggle inputs in response to the turbulence. This slowed the canopy down further leading to its collapse, line twists and impact.


Premier billvon  (D 16479)
Moderator
Jan 28, 2007, 11:03 PM
Post #8 of 11 (1115 views)
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Re: [fallingfaster] A NOT so lightly loaded Sabre (2) and turbulence [In reply to] Can't Post

>a canopy that is flying faster through the air maintains higher
>pressurization of it's cells.

Correct. However, it also sees more turbulence. Aircraft slow down to something called "structural cruise" (or even further, to Va or Vb) in turbulence - slowing down reduces the effective turbulence they see, and so reduces stress on the airframe.

Turbulence is not a specific "pocket" of air that causes you trouble. It's just air moving in different directions. If you cross an area where the wind changes 20kts in 20 feet, you will perceive that as turbulence. As you go faster, you will traverse that 20 feet faster, and the turbulence will seem more severe.

Pressurization is overrated as a turbulence mitigator. Pump up a pool raft and try to hold it up vertically on a windy day. It won't take much wind to fold it over - and that mattress is way more pressurized than your canopy will ever be. Or imagine you losing the outer two line sets on your left side. Will the "air mattress" effect keep the canopy rigid? Nope, it will fold up almost instantly.

Most of the structural integrity of your canopy comes from the strength of the lines opposing the aerodynamic forces on the canopy. Reverse those forces and load the _top_ of the canopy, and any canopy will collapse. Pressurization plays a minimal role in keeping a canopy stable, although it does have some effect.


kelpdiver  (B 7)

Jan 29, 2007, 5:48 PM
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Re: [fallingfaster] A NOT so lightly loaded Sabre (2) and turbulence [In reply to] Can't Post

In reply to:
"There is a value judgment implied here that is if you are not going fast you are being safe. Many of us that have really thought about parachute stability choose to come in fast and jump smaller parachutes, a big part of that decision is that parachutes are stable at high speed, now a stable parachute is stable at high speed I should say. So for me it's a safety issue, jumping a 190 in turbulence for me would be a terrifying thing I don't do it, it scares me." - Brian Germain, Skydive Radio #75

Are we missing some context here? We're talking about the guy who suggested the most conservative mandatory wing loadings ever, and who stressed the importance of getting comfortable in slow flight modes in his class.


Pulse  (D 16387)

Jan 29, 2007, 6:48 PM
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Re: [kelpdiver] A NOT so lightly loaded Sabre (2) and turbulence [In reply to] Can't Post

I guess I've always seen it as such.

Perhaps high-pressure does help some. But that's not to say the higher you load your canopy, the better off you are. I guess I figure even if lower wing loadings are 'more prone' to collapse, you still have more fabric above you if it does. Think about it. People weren't dying under open canopies in the eighties.


fallingfaster  (B 29136)

Jan 29, 2007, 8:24 PM
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Re: [kelpdiver] A NOT so lightly loaded Sabre (2) and turbulence [In reply to] Can't Post

The context was in a discussion of different landing areas, the value judgment he was referring to was that by flying a bigger canopy, that you are flying more conservatively/safely.

Quote:
We're talking about the guy who suggested the most conservative mandatory wing loadings ever, and who stressed the importance of getting comfortable in slow flight modes in his class.

I think the quote references people who have "really thought about parachute stability" and that they are the people who have the experience and skill to jump smaller canopies. Perhaps the skydiver with low jump numbers would assume a greater risk by jumping a smaller canopy than by jumping a large one with a risk of collapse in turbulence. Conversely, the more experienced jumper mitigates the risk of jumping a smaller canopy with skill and experience and would be put at a comparatively higher risk by jumping a large canopy in turbulence.



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