Why dont reserve PCs have limiter tapes?

[Westerly 61]

I looked at my reserve PC today and noticed to my surprise there are no limiter tapes in it like the main PC has. I am curious why. I know the spring helps kind of act like a limiter, but I can easily stretch the spring to full length without that much effort (far less force than a fully inflated reserve PC at 120 MPH would produce).

I noticed that when I collapse the PC to the point that the mesh is the only thing holding it's shape, the PC diameter in essence goes from 30" or so to about 15". Thus I would imagine when the reserve PC inflates, as it extracts the bag and there is a load on the PC, the PC in essence kind of collapses partly because there are no limiter tapes which would reduce its diameter and the amount of force it can produce.

[pchapman 279]

Jeez, you're nosy, it's like you want actual scientific design, testing, and application of life saving gear.

Reserve pilot chutes aren't made that way. If you have a Vector PC, you have the same PC for a 375 reserve or a 99*, despite maybe a 3.7 weight ratio (Ref: PD manual for OP-99 vs TR-375). If you have a Javelin, you are using an MA-1 spring dating back to the '50s or something. And major companies produce reserve pilot chutes with maybe 10%, 25%, or 50% mesh/open space. Lots of variation...

Pressures on the PC 'canopy' won't be directly in line with the spring, but also outward around the whole surface of the canopy. That will keep some shape to the canopy, making things less dire than if one just pulls on the PC spring on the ground until it stretches to the full length of the fabric seams.

I don't recall who has actually done wind tunnel (or in air) tests at full terminal speeds.

Jump Shack posted a graph of pilot chute drag vs. speed for various pilot chutes but the lines are perfectly straight so it is likely extrapolations based lower speed tunnel tests to establish drag coefficients -- where there would be less distortion. You'll also want to look at PIA-TR-401 Low Reserve Opening Investigation Report. Jump Shack's likely extrapolated data shows (expected) 75-175 lbs forces at some terminal speed, while the PIA report shows sustained (rather than peak) forces at terminal of only 34 to 57 lb on average. This seems however to be including the effect of being in the wake of the jumper in actual freefall - despite presumably being at the end of a full length reserve bridle when tested. It wasn't a wind tunnel test of PCs alone.

Some of those values are rather low when one hears about how much drag there is supposed to be on main pilot chutes, e.g,

"A 36" pilot chute, at 120 mph, pulls at about 120 lbs.
A 27" pilot chute, at 120 mph, pulls only at about 56 lbs., if it is properly made, and in still in "trim". "
[Bill Booth, DZ.com, 2004]

Even the worst pilot chute (brands not show), at 34 lb sustained, would accelerate a canopy away from a rig fairly fast -- say 3.8+ g acceleration for a PD-253. (Data: PD-253 is 8.3 lbs in the manual, let's be conservative and say 9 lb inc. bag, bridle, etc)

I'm not sure that I have any conclusion, other than that a wide variety of pilot chutes vs. reserve canopies have been used, without a lot of fine tuning of drag requirements.



(* They have the small cap Vector PC's for small rigs now, but that's the only variation.)

[skydiverek 63]

Wind tunnel Reserve PC tests here:

https://www.youtube.com/watch?v=Zva4gHVX_zI


.

[accumack 14]

If you had a center line on a spring loaded pilot chute it could limit the jump/ launch of the pilot chute.

[billbooth 10]

I put the center line tapes, what you called "limiter tapes", in the original hand deployed pilot chute for two reasons:
1. To make the pilot chute open faster. (Without the initial "holding open effect" of a spring, the shapeless hand deployed pilot chute tended to streamer for a second or so.)
2. A pilot chute (or any round) with a pulled down apex creates 11% more drag, and could therefore be make a bit smaller and less bulky. But, opening speed was the main reason.

[RiggerLee 61]

Historically the well designed spring loaded pilot chutes did have a limiter. They were much better designs with better drag. The designs got compromised to make them easier to manufacture. Look at the MA-1. That was actually a well thought out design. The spring is enclosed in a tube preventing any thing from intangibility. A simple limiter tape would tend to get tangled around the coils when it was compressed.

The MA-1 is in much better proportion then modern pilot chutes. A round canopy wants to inflate into a hemisphere. The out wards force has to balance the inward force of the lines. It really needs lines at least one diameter in length. So if a PC is 36 inches across it wants to have 36 inch long lines from base to edge. Longed is actually better but 1 D is about the point of diminishing returns. If you look at the MA-1 it has a long point way below the limited length of the spring. The spring does not actually define the shape of the PC in any way. If you want a semi modern example look at some of the older strong PC's... the Little grabber? As I recall it was basically the same design only with modern mesh. Could use a better spring but that's another story.

Now look at most of the abortions that we build today. Every thing from hand deploys to most of the reserve pc. A lot of them are just two disks, one of fabric, one of mesh. So you can hot cut them together and just sew around the edge. This makes the "lines" about half the length they should be. Pulling down the apex helps but they still suck. The bottom line is that we didn't actually want some thing good. We wanted some thing easy. Some thing we could build. Some thing that would fold flat and roll up or pack small. And the truth is that in most cases we didn't need that much drag so any thing would do.

Some of the earlier PC's had nice long springs like the 357 or strong tandem but then there seemed to be a movement towards smaller shorter springs and letting them act as a limiter. I've seen these stretched and mangled on high speed deployments. For example we tried using Power Racer PC once on a recovery system and blew them up. We wound up basically building mini MA-1 around that spring and they worked great.

So the answer to your question is that good PC's do in fact have a limiter but it's usually in the form of a tube. Modern PC's don't have one because we are lazy and cheap.

Lee

Lee
lee@velocitysportswear.com
www.velocitysportswear.com

[richoH 6]

RiggerLee

Historically the well designed spring loaded pilot chutes did have a limiter. They were much better designs with better drag. The designs got compromised to make them easier to manufacture. Look at the MA-1. That was actually a well thought out design. The spring is enclosed in a tube preventing any thing from intangibility. A simple limiter tape would tend to get tangled around the coils when it was compressed.

The MA-1 is in much better proportion then modern pilot chutes. A round canopy wants to inflate into a hemisphere. The out wards force has to balance the inward force of the lines. It really needs lines at least one diameter in length. So if a PC is 36 inches across it wants to have 36 inch long lines from base to edge. Longed is actually better but 1 D is about the point of diminishing returns. If you look at the MA-1 it has a long point way below the limited length of the spring. The spring does not actually define the shape of the PC in any way. If you want a semi modern example look at some of the older strong PC's... the Little grabber? As I recall it was basically the same design only with modern mesh. Could use a better spring but that's another story.

Now look at most of the abortions that we build today. Every thing from hand deploys to most of the reserve pc. A lot of them are just two disks, one of fabric, one of mesh. So you can hot cut them together and just sew around the edge. This makes the "lines" about half the length they should be. Pulling down the apex helps but they still suck. The bottom line is that we didn't actually want some thing good. We wanted some thing easy. Some thing we could build. Some thing that would fold flat and roll up or pack small. And the truth is that in most cases we didn't need that much drag so any thing would do.

Some of the earlier PC's had nice long springs like the 357 or strong tandem but then there seemed to be a movement towards smaller shorter springs and letting them act as a limiter. I've seen these stretched and mangled on high speed deployments. For example we tried using Power Racer PC once on a recovery system and blew them up. We wound up basically building mini MA-1 around that spring and they worked great.

So the answer to your question is that good PC's do in fact have a limiter but it's usually in the form of a tube. Modern PC's don't have one because we are lazy and cheap.

Lee

What's your opinion on the Snatch (And SkySnatch) ?

https://squirrel.ws/equipment/skydiving/skysnatch

[LeeroyJenkins 3]

richoH

What's your opinion on the Snatch (And SkySnatch) ?

https://squirrel.ws/equipment/skydiving/skysnatch

I hope reserve PC move towards the shape of a snatch. /they work so well.

[RiggerLee 61]

You could argue about some of the dimensions but the snatch is the first commercial effort to build a better PC. It's head and shoulders above the two flat plates that you get with most PC.

Lee

Lee
lee@velocitysportswear.com
www.velocitysportswear.com

[riggerrob 643]

How wide is the hard red cap on the pilot-chute for 300 series Vector 3/Micron?
Are red-cap pilotchutes compatible with Sigmas?

[pchapman 279]

riggerrob

How wide is the hard red cap on the pilot-chute for 300 series Vector 3/Micron?
Are red-cap pilotchutes compatible with Sigmas?

Check out
http://www.chutingstar.com/upt-reserve-pilot-chutes

That has all the details on the red cap / small cap pilot chutes:
Can use the small cap 5" (vs. regular 6") on any Vector 3 or Sigma.
Same spring, etc.
Not for Vector II and earlier -- not tested.
V300 size rigs are the only ones that MUST use it.