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Hi !

I'm wondering if the vented pilot chute is usefull or a new gadget for 5- terminal delay ( incuding WS base) ?

 

I would think a vented pilot chute would be better for those shorter delay jumps where separation from the object is less and on-heading is that much more important. There is obviously a limit to this. I doubt one would use a vented pilot chute on sub 200' freefalls.

Is there a different bridle length involved too ?

I did a search and noticed that WS (skydiving) is 100" & 108" (9ft) for BASE bridles,
does that extra 8" make a big difference ? or is one safer that the other ?

And is there a recommended bridle length in WS BASE ? (I looked in the wsbase.pdf for it by the way).

It's definately not a new gadget. A well designed vented pilot chute works well at all delays. Under 200 feet, I have no opinion since I don't freefall that altitude.

Longer delays: If the pilot chute orbits quickly, it will pull the pack job out asymetrically often causing either off heading or worse, line twists.

Shorter Delays: Same thing. Except the cargo is closer to the object.

If the pilot chute is too small and it orbits, the possible problems are compounded due to the slower extraction.

I've witnessed more than one pack job start out of the container square and be turned around by the orbiting pilot chute.

I use Asylum AV series pilot chutes sized 34 (yes 34), 38, 42 and 46. All of them have 6 load tapes which helps the pilot chute keep it's shape rather well.

Hey there! I am a new BASE jumper and I just purchased gear from Apex BASE and this also was a big question I had while looking at what to order. Todd and Karen at Basic Research explained as follows: They told me that of your 4 pilot chutes, lets say you have sizes 36, 38, 42, 46, you would want your 38 and 42 to be vented, 36 and 46 non-vented. They explained to me that a vented pilot chute prevents osculation that would pull the canopy into an off heading opening in the mid range freefall speeds. They said you could vent your 36 but it isn’t really necessary because a jump at terminal would allow adequate time to get away from the object where an off heading openning would not cause an object strike. They said it was also not necessary to have your 46 vented because PC osculation would not occur with the short delay (0-1.5 delay).

Hopes this helps, and again I not an expert in base. Just being helpful and sharing what I was told.

David

Just to be clear, I'm not raggin' on ya for using the wrong word here. But this post reads totally differently if you know what "osculation" is...

http://dictionary.reference.com/search?q=osculation

Michael

Dmcoco84,
If I read your post correctly, you stated that BR told you that its not necessary to have a vented 36" PC because your far enough from the wall if you have an offheading. That makes no sense to me. I personally never want an offheading on any jump. What if I decide to do 3 backflips before I start to track and don't open as far away from a terminal wall. I'm still at terminal and I still don't want an off-heading. I am a fan of the vented pc's and from what I have been told by Marty, the pull force is the same vented or not. So why not go vented all the time.

I agree with Bry. I want an on-heading opening no matter how long/short the delay and how long/short the seperation from the object.

I know we have a physicist around here () who might be more qualified to weigh in, but I personally think a vented PC is going to be useful at pretty much any delay. I've seen video of PCs orbiting violently with both short and long delays.

I've also seen enough video of vented PCs in action to believe that that vents do what they are supposed to do. My personal take is that while the effects may not be as severe with super-short (i.e. throw & go) delays, so long as the PC is catching air & taking on a load, the forces which cause orbiting will be at play.

I have a full set of Marty's vented PCs (32,38,42,46) and I swear by them.

I have heard several experienced jumpers say this, but I am at a loss for the physics of this. The argument I have heard is that the pilotchute is going to spill air regardless, it's just that with an unvented PC the air spills out from underneath (encouraging oscillation).

Taking that argument to the extreme, wouldn't this mean that if I were to completely open up the apex (have a big hole where normally the cap is), while making sure the total surface area of my pilotchute is still the same (by increasing the diameter), the pull force would remain consistent?

I find this hard to believe.

That doesn't mean you can't use vented PCs where you use unvented ones (except maybe in ultra-low freefall), but it does mean I believe there is a neglectible difference. Don't hesitate to prove me wrong.

 

Ok, so, like, everybody talks about the positive effects of the vent on an inflated pc...

Certainly not. Take a square one meter on a side. It has four meters of edge over which it can spill air. Cut it into centimeter-sized pieces, and spread them out a bunch, and you've got 400 meters of edge. If you keep them close together, though, the air won't "see" the spaces between them as well.

The key idea here (I think) is the Reynold's number (http://scienceworld.wolfram.com/.../ReynoldsNumber.html). For small holes, viscosity is very important. The optimal apex vent will let through enough air to stabilize the PC without letting through so much air that it significantly reduces drag.

Somebody better-versed in aerodynamics will hopefully drop by to correct me on all of this.

Just throwing this in to the hat..

Would having a PC spinning like a catherine-wheel with two opposed vents make it more stable ?

If you had a link like a spinner or a bearing of some sort ?

Probably. There's a bunch of US government tests that bear this out reprinted in the Knacke book. Unfortunately, they found that only very small parachutes could be made to do this without insurmountable deployment issues (by small, they were talking around 6 feet, so a PC might be doable). Even the smaller ones had some problems, though, and they didn't really put in much more effort to work it out.

As a side note, they actually found that the spinning parachutes were the most stable and had the most drag per size.

Dude, the Reynolds Number only applies after nondimensionalizing the Navier-Stokes equations. Given an average Ro of < 30, we find an experimental Poiseuille (laminar) flow that isn't turbulent enough to exhibit viscosity.


Okay, taking it to the extreme was a bad example. How about this scenario. Take a 42 inch non-vented PC. Measure the drag at a certain speed. Not cut a hole the size of a penny in the top. Measure the drag again at the same speed.

I'm pretty sure it'll have less drag. Probably neglectible, but measurable with sufficiently precise instruments.

Now enlarge the pilotchute all around (at the skirt) by the same area as what you cut out from the top. Then measure the drag again. Will it be less or the same as what it was before it was vented?

And what scenario do PC manufacturers use? Do they take a 42 inch PC and cut something out, or do they offset the area they cut out for vents by increasing the diameter by a miniscule amount.

I'm probably splitting hairs here, like I did in an older thread about breakcord strength. The bottomline is that it works in practice and that vented PCs are interchangable with non-vented PCs.

Blah...

I hated my fluid dynamics classes. Both of them!!

Wait a minute....aren't you just a video game geek!?!

hey now thats not nice!

He dosen't have to tell us everything about himself...

Some people like to be underestimated. They have the upperhand or an advantage in a sense.

Are you assuming the PC scales precisely? I think that's a bad assumption. You're essentially moving the spill points around to various different points of trim, which is going to have fairly complex effects, in my opinion.

Ro? Rotation number? For a 35-inch pilot chute at 120 mph I get Re=3E6. Could it be we're not talking about the same thing?

Anywho, theory is notoriously bad (beyond the sort of handwaving arguments I already gave) for turbulent flow, and it doesn't get much more turbulent than a pilot chute being dragged at terminal. The experiment would be interesting, but -- setting aside the fact that I'm too lazy to build dozens of test-PC's -- would, I expect, be complicated by the following:

(1) Particularly for small apex vents, the pull force of a pilot chute probably oscillates by an amount which I expect would swamp the effect of small changes in vent size; and

(2) Just the idea of trying to build a pilot chute with an apex vent more than maybe 20% of its outer diameter which still resembles a pilot chute when inflated makes my skin crawl.

Still, um... If you want to build them, I'll shoot video.

That's precisely my point, but expressed much more succinct. I have heard several people state that the drag created by a vented pilotchute is the same as the drag from a nonvented pilotchute. Their argument was that the same amount of air would spill either though the vents or from underneath the skirt. I believe the statement is correct (drag is the same for all practical purposes) but the argumentation is flawed.

If one were to compare the airflow between the two types of pilotchuts we would see very complicated differences resulting in definite measurable differences in drag. Sufficiently neglectible to argue the two types are interchangable, but real enough to warrant research.

It's too bad that manufacturers can't devote the resources to such R&D. Especially during the inflation stage (in which everybody agrees there are noticable differences) there's plenty of interesting stuff to be found.

But again, it seems another one of those cases where the science is too complicated and we just have to leave it up to practical experience to prove our hypothesis.

The latter. The difference being that you're trying to make sense and I am full of shit. I just visited that Wolfram website and couldn't help but laugh after reading about the nondimensionalization of the Navier-Stokes equations. So I took a few complicated words and slapped them together in a sentence. The dimensionalized Navier-Stokes are tricky enough for me already, and sometimes with math I just have to step back, respect the complexity of it all, and admire that somewhere in the world there are people that actually have a clue about this kind of thing.

I think I just lost the last little piece of credibility I had on these forums. Sweet!

You bugger. I spent a half an hour trying to figure out what you thought you were talking about...

Ok, that is incredibly funny. Way to go Jaap!

 
hmm you do
i have freefalled 180ft whith a 46´vetnted pc serval times... i only once had a problem but that could also happen on a unvented.
make a note that most vented pc´s are made bigger to still have the same surface as theyre named to...
I also know of serval other guys jumping this way.

I only used a 48´once(unvented) once that were a backup on a SL jump i choose not to freefall anyway...

***Justs thinking off the top of my head. I haven't thought all this out but...***

Although you have to remember that the air spilling out the bottom of a non-vented PC and the same amount of air spilling out the top through a vent isn't necessarily the same. It takes energy to turn the air around 180 and spill it out the bottom whereas that energy isn't required when it goes out the top through the vent. I'm wondering if the time the air stays in the pilot chute longer (needing to turn around and spill out the bottom) or just the energy required to turn it around (opposite reaction being the topskin pushing on the air) creates more drag than a vented pilot chute.

Ok I'm tired and I'm going to bed. Did any of that make sense?

No worryes i think us in this part of the world might use smaller pc´s than you guys in the states..

I mean most arround here use the vented 42 as low as 230ft(what i have seen)on freefalls,and 46´even lower.
However a 48´unvented is to preffere below 200ft but the vented can do the job aswell... but thats personal... Below 200ft you should do the chiken dance if it makes you feel better(qoute from Tom A if i rember correct)..

BASE pilot chutes operate in a very turbulent environment.

venting a pc correctly, this turbulence may be reduced.

generally, the smoother the airflow, the easier it is to get air to actually perform usable work. i.e. the device becomes more efficient.

so a vented pc might be able to generate the same forces as an unvented pc. much will depend on the design.

Yea...Good point. I could have sworn I was told that venting my 36 and 46 wasn't necessary. I tried to refer back to the email but I can't find it. I guess I'll just keep my mouth shut in the future.