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Tom, are you 100% that vented PC of same size and construction has lower drag at same airspeed?

Not at all.

A properly constructed vented PC ought to have exactly the same drag as an identical unvented PC. It will just spill the air through the vent, rather than the around the skirt.

However, I have yet to conclude that any vented PC is contructed to an ideal design.

Also, and most importantly the vent may increase the chance of hesitation. This is well documented in round parachute openings, and is the reason that BASE jumpers tend to put a rubber band over the apex vent of a round used for BASE.

The vent ought to have virtually no effect on drag, once the PC is open. But the thing that gets you at low altitude isn't low drag, it's hesitation, and the vent is definitely an issue there.

im whith Tom,i just got myself an unvented 46´zp pc for that reasson.That said,i only once had a problem on a vented (46),at a sub 200ft freefall,but thats the reasson i want a unvented,to sub 200ft freefalls.

Also, and most importantly the vent may increase the chance of hesitation.

Simply NOT TRUE.
The PC has central line attacment which completly changing the dynamic of the opening.
Physics of opening / in theory and practise / is completly diferend with or with out cental line.

robi

Robi,

Do you have any research on this?

I've seen vented PC's hesitate on go and throw deployments far more often than unvented PC's. I'm wondering if there is some other cause?

I do know that the US military did a bunch of research in which they concluded that apex vents created noticeable increase in opening times. I don't know much about the aerodynamics of the various parachutes used (i.e. which were PDA, and which were not) but I do know that they tested both PDA and standard (non PDA) rounds. Let me see if I can dig up the reference material...

Also, do you know if this is true regardless of the location of the vent?

In other words, will an apex vent have the same effect (or lack of effect) on opening as a vent located somewhere else (say, in a higher pressure area) on the PC?

sounds like it's time to bust out the Knacke (which is at home).... Not sure I'm buying it.

gardner

I'm not buying anything from a redneck like you.

Seriously, though. Robert probably knows about a hundred times more than I do about parachute inflation. And I'm really curious about vent locations. I wonder if the vent locations matter at all?

I've split this off the "Non ZP PC" discussion, since it looks like a separate topic (vents on PC's) and hopefully will involve me learning some things.

Tom,

''I've seen vented PC's hesitate on go and throw deployments far more often than unvented PC's. I'm wondering if there is some other cause?''
Your statement does not prove anything.

I have seen cases were both were slow in pressurizing.

Are you sure that both PC ( with and with out vent ) were from same material?!
Same size?!
Same weight?!
Were launch same way?!
Weather condition were the same ?! Wind?!
Did you time the opening?!

As you see, this are just few factors which have huge effect on ''testing'' the PC or any parachuting component.

Opening process:

First of all we should know that all round canopy openings are following the same principle. The ''dynamic pressurization.
Most people were witnessed the big round canopies (military or sport ) opening. Starting with ''mushroom'' form first and after, bottom opening follows.
(When the round is stretched the pressure around the canopy is lower than inside of the canopy which create the ''mushroom'' form.)
Well, same happening with small or big rounds, same happening with vented apex or non vented apex.
However, closing the apex by rubber band does not changing the basic principal of opening in round canopies. Yes, it opens slightly faster when the rubber band is placed on the top but also because there is no diaper on the bottom, which is primary reason, because the bottom of the canopy starting to open before the line stretch, ( we have similar issue with the square canopies it called ''line dump'').. Once we have full pressure under the canopy apex vent starting to have real purpose, not before. However, if this set up would be placed back to skydiving we would be for sure witness many line over and inverts.
Same principle of openings is happening with PCs for BASE jumping or skydiving. Problem is that we can't really see that cause our eye is to inert to spot and recognize that process.
( In order to speed up the opening process on round canopies , best result so far were made by having the round made in Cross shape or made from many small parts with load of vents. Both you can see at combat aircraft brake chutes)
Vents Apex:

Vent on the apex job is to stabilize the PC once when is fully inflated ( pressurized) Vent has no real or noticeable effect on opening time nor effect during the process till the full pressurization.
Best placement of the vent of the PC would be the top of the arc circle ( 1/4 diameter ). This can be made, but the cost would be much higher, while the PC would not be so much better to justify the price anyway.

Centre line attachment:

Purpose of having centre line attachment on round canopies is:
a) to increase the surface of the canopy
b) to increase the lift !!!!! ( interesting, This is the main idea behind the centre line on the round canopies, first time made on sport round canopies )

Well, with small PCs we only have to worry about the ''b''.
PC for base has this to increase the surface. As the side effect centre line also increasing the opening time too, because once the pressure inside and outside of the canopy or PC is equal the exposed surface area is bigger and the drag is higher , resulting in faster final opening.

IMHO most of the PCs with the vent on the apex has the center line to short. This void to oscillating again.
Optimizing the length of the centre line is the main issue on nowadays PC with vent.

Regards Robi

Of course it doesn't. It's just an anecdotal observation, not any kind of actual scientific observation.

Thanks for explaining. As I said earlier, you know a lot more about the inflation mechanics than I do.

<tongue-in-cheek mode>Well if that's the case, then why don't I see any terminal wall jumpers using high aspect ratio annular PC's? (apart from the obvious reason that I'm not hanging around bigwalls alot)</tongue-in-cheek mode>

The annular ring parachute achieves the same drag with far less oscillation angle than today's PC's (vented or not). I guess they don't open as well. Why?

Still not sure that I'm buying the, "vent has no real or noticeable effect on opening time" notion...

Maybe not noticeable, but I bet it can be measured.

Gardner

Tom, your reference to PDA, is that pull down apex, or pull down vent line?
Sparky

Pulled Down Apex. But I'm not a rigger, so I may not have a very good understanding of the proper terminology.

What's the difference between a pulled down apex and a pulled down vent line? Or, more accurately, what's a vent line?

gardner,

there is many ways to go in order to make good more stable PC. The problem is that PC will not be so much better to justify the production price.
Also, whatever way you go, PC will be more complex, more sensitive on production error, heaver...

Benefit would be very hard to notice and as the bottom line the ''keep it simple '' politics is by far best for BASE jumping

To prove that you can have the Vented PC for example.
PC vented or non vented or Z/P or 1-11..
All works good as long the user is mature enough to understand the sizing, how to properly attach the PC on bridle how to pack and throw...

In short ..
All PCs are good enough, all works and IMHO jumpers like to accuse PC for all kind of shit openings... Most of those openings PC was the last one to blame.. Especially on low slider off jumps.
:)
Robi

Yes Robi,

And I completely agree with you regarding the production costs/benefits/keep it simple politics.

I must admit that I have a tendancy to get lost in the theory of it all rather than the application of theory to production. But ultimately I do see the very real necessity of keeping things simple and keeping costs down. To that end, that is probably why I don't see any high aspect ratio annular PC's around (not to mention they wouldn't lend too much confidence at the exit point).

You should see something that I've dreamed up; a real "rube goldberg" of a PC. Heh heh... show it to you sometime.

Gardner

P.S. in case you don't know rube goldberg...

http://www.rube-goldberg.com/

A pull down vent line is just that, a line that pulls the canopy vent down like you see on a Para Commander. Pulling the vent down during a low speed situation reduces fill time and the canopy opens faster. By rigging it with a weak link it does not affect high speed openings. Sometimes used in ejection seat canopies.
Sparky