Why Don't Skydiving Pilot Chutes Have Vented Apexes?

[GaryP 0]

I agree with Mark and have highlighted the basics covered in his & my posts to simplify.

From Mark's post:

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...at some point as you enlarge the vent(s), the amount of air going through the vent(s) exceeds the amount of air that would have spilled from underneath the skirt. Then the canopy has less drag and descends faster....

From my post:

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as long as the amount of air escaping through the vent does not exceed that which is spilling out from the bottom of the skirt there will be no loss of drag

The key here is to ensure the vent doesn't allow more air to escape from it than is escaping around the bottom skirt of the canopy/pc.

g.

+=

"Altitude is birthright to any individual who seeks it"

.

[teason 0]

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vented apexes are there to decrease ocillation... that's what they're for.

That's actually a common skydiving myth. Vented Apexs affect the critical opening velocity. As vent size increases, critical opening velocity decreases.

That's the purpose of a vent and also why you get slow fallers using vented apexs.

For the question of stability, on a round, vents on the skirt do far more than at the apex.

Another key to stability on a canopy is line length.

So, to answer the question, Base jumpers and Birdmen use vented apexs to decrease the critical opening velocity. It's a waste for the average jumper.

But ....

if you would like to reduce oscilation (as minor as it is) the PC must be constructed with the mesh in a cone shape. The way we make them now (to over simplify) is by placing the ripstop on the mesh and cuting a circle. This is the same as have a round with line = to the radius.
If you make the cone shape and sew the ripstop to it, it will be far more stable (although, you'll probably only notice it if you have a container lock)

I would rather be a superb meteor, every atom of me in magnificent glow, than a sleepy and permanent planet.

[teason 0]

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There is NO LOSS OF DRAG with a vented p/c once it is inflated, compared to a p/c of equal size & construction that is not vented at the apex.

Well ....

On a round, the vent is usually 1% of the square footage. increasing to 20% decreases drag, but only 7%.

Yes, there is less drag. No, it's not very much.

I would rather be a superb meteor, every atom of me in magnificent glow, than a sleepy and permanent planet.

[mark 102]

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That's actually a common skydiving myth. Vented apexs affect the critical opening velocity. As vent size increases, critical opening velocity decreases.

That's the purpose of a vent and also why you get slow fallers using vented apexs.

Just so I understand "critical opening velocity:" is this the minimum velocity required to inflate the canopy?

Does this effect on critical opening velocity also apply to canopies with a vent at a pulled-down apex? If so, why, since the location of the vent is not at the top of the canopy?

Mark

[mjosparky 3]

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vented apexes are there to decrease ocillation... that's what they're for.

venting in other places on a round canopy are for the purpose of steering or drive.

Right and wrong. The apex vent does help reduce oscillation but very little. It main purpose is to vent high-pressure air during inflation. Without it under the right conditions, high speed-heavy load, the canopy would blow up.

Venting at other places on a canopy can produce drive but steering is dependant on being able to manipulate the vents during descent.

For a better explanation see PPM VOL I, page 468 or refer to Decelerator Systems Technology by Helmut G. Heinrich

Sparky

My idea of a fair fight is clubbing baby seals

[base689 0]

Sorry to be picky, Bill, but your statement "adding an apex vent would..." means very little if you do not mention the area of the vent versus the area of the whole PC.
Because if we go to the other extreme (=very, very small vent) there would be a very neglectable effect on delaying the opening.
Let's consider that the vents in BASE PC's used nowadays are about 1% the area of whole PC (=a small increase in diameter would compensate the "hole" in the middle and so would yield having the same "pulling" area).
In the end, I think it is a matter of properly dimensioning the vent in order it to be the best compromise between stability and drag.
Consider also that a lot of BASE jumpers I know (myself included) are using a vented 32" ZP PC when jumping terminal walls (12"+ of freefall).

Stay safe out there
Blue Skies and Soft Walls
BASE #689 - base_689AT_NO_123_SPAMyahoo.com

[phoenixlpr 0]

I think his answer was correct. Its no need for vented PC in skydiving. It would bring more complex design without any real improvement.

[tso-d_chris 0]

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I think his answer was correct. Its no need for vented PC in skydiving. It would bring more complex design without any real improvement.

The point is, the lack of improvement is assumed, without experimental data (in sport skydiving). It would be interesting to see how experimental data compares to this assumption.

Currently, most people I have talked to consider F111 PCs to be superior to ZP PCs, bungee collapsible PCs excepted. However, if there is a benefit to a vented apex, and if that benefit exceeds the benefit of F111 PCs over ZP PCs, it would be beneficial, since ZP outlasts F111. The higher cost of production would be offset by a longer lasting PC.

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[phoenixlpr 0]

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The point is, the lack of improvement is assumed, without experimental data (in sport skydiving). It would be interesting to see how experimental data compares to this assumption.

I jump a "small" ( 86 cm in diameter) F111 kill-line PC. I had some trouble with it. I had the pin on the kill line and it was pulled back inside the bridle. From 150 jumps it was not killed 15 times. That was a problem, my bridle was spooned up and I had a "funny" landing with an anchor on my canopy. I got it fixed anyway. My pin is on the bridle finally. I'm happy with it.

Why do you wanna fix something that isn't broken?

[stevebabin 0]

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It seems to me that if round canopies oscillate less with a vented apex then the same principle would apply to pilot chutes. I cannot remember ever seeing a non BASE PC with a vented apex. Is there a reason it is not done for skydiving pilot chutes?

Yes.
There doesn't seem to be a problem caused by oscillating pilot chutes in skydiving. During deployment they're not inflated long enough for any slight oscillation to cause a real problem. Essentially it would be a solution to a problem that doesn't exist skydiving.
The same cannot be said of base. The larger pilot chutes and the slower airspeed at deployment allow an oscillating pilot chute to have a much greater effect on the opening. (specifically on-heading performance).
Just my opinion, though.YMMV.

Why do you ask? Are you still having trouble with that Heatwave spinning up on you?
Blues!
Steve

"Science, logic and reason will fly you to the moon. Religion will fly you into buildings."
"Because figuring things out is always better than making shit up."

[tso-d_chris 0]

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It seems to me that if round canopies oscillate less with a vented apex then the same principle would apply to pilot chutes. I cannot remember ever seeing a non BASE PC with a vented apex. Is there a reason it is not done for skydiving pilot chutes?

Yes.
There doesn't seem to be a problem caused by oscillating pilot chutes in skydiving. During deployment they're not inflated long enough for any slight oscillation to cause a real problem. Essentially it would be a solution to a problem that doesn't exist skydiving.
The same cannot be said of base. The larger pilot chutes and the slower airspeed at deployment allow an oscillating pilot chute to have a much greater effect on the opening. (specifically on-heading performance).
Just my opinion, though.YMMV.

Why do you ask? Are you still having trouble with that Heatwave spinning up on you?
Blues!
Steve

No. I ditched that canopy long ago.

I'm not trying to fix a non-existent problem, only wondering if there is room for improvement. If we didn't look for such things we would still be jumping T-10s, which weren't broke, either.

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[stevebabin 0]

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we would still be jumping T-10s, which weren't broke, either.

They weren't broke, but there were lots of real problems to be solved when using them for skydiving. Landing hard and landing where you want to come mind immediately. I'm glad the problems were overcome.
What skydiving problem are you trying to remedy with a vented pc?

"Science, logic and reason will fly you to the moon. Religion will fly you into buildings."
"Because figuring things out is always better than making shit up."

[tso-d_chris 0]

Refinement requires no problem, just a desire to improve things beyond the status quo. It is tough, if not impossible, to buy a poor rig today. I have no doubt, however, that the containers that will be on the market in ten years will be superior in nearly all aspects.

Like I said, if there is no potential for improvement, it would be pointless. However, "we've always done it that way" is not reason to believe a different way would not be an improvement. Refinement is a good thing.

It's unrealistic to believe every gear improvement is going to have a big effect as, say, the 3-ring release system. Even small improvements are improvements.

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[kleggo 138]

The reason I am curious is that I have talked to hard core CRW dogs in the past that swear that, all else being equal, a ZP pilot chute will contribute to off heading openings, whereas as deployments from F111 pilot chute are straighter. I have also heard this elsewhere.
__________________________________________________
2 1/2 years ago i convinced the US. 8 way crew team that we could benefit from using vented pilot chutes. Adam Filipino, then of CR, was kind enough to sponsor us and provided 32" ZP vented pc's.
When i recently asked Chris Gay what he thought of the change he said, " they've made a huge difference in on-heading performance, ( that's good).
Now Martin Tilley of Asylum Designs continues to manufacture these for the CReW community.
They are higher bulk due to the large mesh, 6 external load tapes and mesh vent, but we don't make a competition jump without them. They excel in "low speed deployments" to reduce PC oscillation, just as they were originally designed for in the Fixed Object jumping milleau.

be safe
kleggo

[bob.dino 0]

Have you tried taking them to terminal with otherwise normal sport gear?

[tso-d_chris 0]

Thanks for sharing that. There seems to be a limited amount of experimental data on the subject.

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[980 0]

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Have you tried taking them to terminal with otherwise normal sport gear?

I've got around 200 skydives (almost all terminal) with a standard sport rig and 155 canopy, using a 32" AV pilotchute by Asylum.

I replaced that pilotchute with a 28" AV by Asylum when I downsized to a 120 canopy and I have around 260 (almost all terminal and almost all wingsuit) skydives using that PC.

Why do I use an Asylum BASE PC for skydiving? - it's quite simply the best made pilotchute I have seen and the idea of the vent makes sense. Both these PCs also have the external plastic tube handle, I wouldn't jump a wingsuit with any other handle configuration.

As for AV pilotchutes inflating slower than non-vented pilotchutes, in my limited experience (293 basejumps all under 6 seconds delay and split about evenly between AV pilotchutes and non-vented pilotchutes, I can't tell the difference in inflation speed or drag comparing like sizes. But then, I retire my base pilotchutes at 50 jumps - I figure $1.50 a jump for that warm and fuzzy feeling a fresh ZP pilotchute in your BOC or hand gives you is cheap.

edited to add: I had a special bridle with a kill line made up that has a loop for PC attachment (standard BASE style), this allows me to use these BASE PCs with a kill-line to still have a collapsible PC, like you would want for a smallish canopy.

cya
sam