PC, manufacture size vs real size

[JaapSuter 0]

Hi,

I search didn't give me much, but correct me if I'm wrong.

I have a Paratech 42 inch PC here that when measured is on the lower side of 40 inch (ZP fully stretched). Is this considered an acceptible margin of error?

What kind of differences have you noticed between the manufacturer claim and the measured size? Any standard deviation gurus out there?
Cheers,

Jaap Suter

[base283 0]

It is standard in the industry to state the "cut size". You were measuring the "finished size".´Tis not an error.
The difference you noted sounds right.
take care,
space

[sum1sneaky 0]

I one time measured up all of my pc's 2 inches shy as well, the manufacturers measure the top skin stretched tight, which somehow - amazingly - adds two inches. Try that and see what you get.

[base283 0]

No way, The manu can only cut the material in the specified diameter. Seam allowances and differing shinkage rates due to sewing (sewing a circle on square weave) would make it difficult to calculate an accurate finished diameter before you started sewing. Stability and Drag are more dependent on construction rather than diameter, and also more important.
hope this helps.
take care,
space

[TomAiello 25]

I think space is right on, here. The important thing is the drag that a PC makes, rather than it's measured diameter. The manufacturer ought to be working from that sort of parameter, so "Asylum 42" or whatever, is more a model number (based on the cut size of the material) than a statement of the finished diameter of the PC.

-- Tom Aiello

[email protected]
SnakeRiverBASE.com

[Faber 0]

Morpheus is the only compagny(i know off) that size their pc´s in the correct size=sewn/ready to jump..

Stay safe
Stefan Faber

[RayLosli 11]

It not that big a Deal.... You just cut your pattern 2 inches larger than your desired size.

The 2 inches of / Shrinkage due to thread tension when taping seems to be the Norm.
I think all manufactures should Cut so finished size is the stated finish size.
A.. 42 or 32.. should be that size when you measure the tape from end to end on the Top Skin.

[flite 0]

to measure- stretch load tapes on the diameter (and hold ) - then measure end to end. I have Morpheus and BR PC's and they all measure their stated size.

[SafetyNate 0]

A couple years ago a BridgeDay, I had My 48 on my rig and took it through gear inspection. I was denied because it measured 40".
That was a surprise. I had just free fell 235' with a 40" PC! It seemed to work fine though.

[Lonnie 0]

I've probably had more PC's from Paratech than anyone, at least 30, and everyone has measured to the correct size.

NEVER GIVE UP!

[base283 0]

Once again, PC diameter has very little to do with drag. Construction and stability are THE major parameters. Though I do think diameter is more important than the color. In some of the incidents and accidents I investigated including one Fatality, 3 were caused IMHO by the type of construction of the PC. The diameters were appropiate for the delay if one went by the standards we go by. I am very knowledgeable on this subject. Ask around.
Take care,
space

[edge01 1]

Ok what's IMHO?

[CanuckInUSA 0]

Quote

Ok what's IMHO?

"In My Humble Opinion"

Try not to worry about the things you have no control over

[JaapSuter 0]

Quote

Construction and stability are THE major parameters.

Can you elaborate on this?

With stability I'm assuming you mean body position, e.g. velcro in headdown position is harder to come off than in a stable belly to earth position, even if with a shrivel flap. Or are you talking about PC stability and oscillation?

What about construction parameters? Are you talking about ZP vs F111, vented vs non-vented, number of spokes, etc.? What are the 'right' parameters according to you?

I can imagine some people could write a book about this, but if you could write some thoughts on the general idea of what a good 46er would look like according to you, that would be great. Links to other material would be cool too.

Thanks,

Jaap

[nicknitro71 0]

Quote

some of the incidents and accidents I investigated including one Fatality, 3 were caused IMHO by the type of construction of the PC. The diameters were appropiate for the delay if one went by the standards we go by. I am very knowledgeable on this subject.

Could you give us more details on your findings. You can PM if you don't want it post it publically.

Memento Audere Semper

903

[TomAiello 25]

I'm guessing that block v. bias construction was a major factor. If you search the forums for those terms, it'll give you some good food for thought.

-- Tom Aiello

[email protected]
SnakeRiverBASE.com

[base283 0]

Hi, I will write more in the next days as my net dropped out. post ya soon,
take care,
space

[JaapSuter 0]

Here is an excellent post by Bill Booth about pilot chute construction.

It turns out I was measuring wrong, and after doing it properly my PT pilotchute was 42 inches.

I was also contacted by Paratech rigging after posting this and Colin gave me some very useful information. Kudos to them for taking the time to call me.

Cheers,

Jaap

[valentin 0]

Well, of course the drag capacity of a Pilot chute is more important than the diameter.
Ideally, we should refer the PC with their drag capacity, which is not easy, but if not, to use the surface area would be better, because using diameter gives the wrong idea about size proportions. For instance, a 40-inch PC look like 10 % bigger than a 36 (+ 4 inches) but if you calculate the surface, it’s 20% bigger!

The drag can vary with the material and the design of the PC, for instance, if you increase the length of the apex line, you increase the stability but you decrease the drag.

So how do we measure this drag capacity? Quiet simply, if you consider that the average effort to pull the pin and the parachute is around 10 pound, then you just have to throw from 100 feet or what ever height you have available a PC with a 10 pound weight attached to it and measure the time it takes to touch down. It should reach it's terminal speed after one second. So you can calculate the sink rate (or vertical speed) quite easily and then you can compare your pilot chutes.

If you want determine your extraction force, you take your estimate speed at deployment time (3 seconds = 50 miles per hour, etc), then subtract the speed of your PC falling with 10 pound, and the difference will tell you how fast/ strong your chute going to be pull from the container.
-If for instance your PC falls at 20 miles per hour and you pull at 3 seconds (at 50 miles per hours) you have a good difference. You wouldn't want to pull at 3 second with a PC falling at 50 miles per hour (with 10 pound under it), because it would just stay behind you in perfect balance.
- To pull fast the chute out of the container limits the risk of twist of the canopy (like in skydiving, if the D bag comes out slowly, it has more time and chance to turn).
-To have a very big PC (big speed difference) is not so good because it can violently shake your canopy at opening, maybe damaging it. It also increases the opening shock.
BECAREFUL, these principles concern only short delays of maximum 3 seconds. Also don’t use my math numbers as reference; they are theoretical, and many factors can alter the result. Always err on the safe side.

-Our PC has two jobs to do in a base jump.
1-Clear the turbulence /depression behind us, also called burble (Name that I don’t like, because it’s not accurate and it give a quiet wrong idea of the phenomenon)
2- Pull the pins or Velcro and the chute.
What’s interesting is that these two jobs work in totally different ways.
-With less than 4 second delay (around 50 miles per hour speed) you don’t have to worry much about turbulence behind you, because the speed is too low, but you have to worry about the PC size, if it’s too small or old, or if you threw it too early you may end up with a PC in tow (I seen 42 PC staying 2 second before to pull the pin on a 2 second delay and that was not because of the burble)
-Now when you go over 5 second delay (100 miles per hour and more) nearly any pilot chute will pull the stuff for you (in skydiving some guys use 24 inch f111 pilot chutes at terminal, when the smallest one sold by Basic research is a 36 ZP) but the problem is now about the turbulence behind you which increases at the square of the speed…

We seen before how to secure the size of your PC in order to pull your pin and chute correctly at delay below 4 second, now we will see how to avoid having the PC stuck in the turbulence behind your body.

This turbulence works the same way as in a river. If you look at a place where the water is flowing behind a big stone in a river, you will see similar turbulence. If you threw a ping pong ball in the water there, it would stay for a while before being swept away, If you throw a larger ball made of the same material, it will clear the turbulence faster. If it’s big enough it won’t be held in the turbulence at all. Same things with a PC if there is enough volume material (doesn’t matter if it’s F111, Zero P or mesh) it will clear the turbulence behind your body. That’s why with over 10 second (terminal speed), we use a much bigger PC then in Skydiving: we don’t have time to let the PC play in the turbulence behind us before it clears the turbulence.

I am working on a 36 zero P Pilot chute, on which I will put four vents of 1 inch each, looking to have the volume of a 36 inch with the drag power of a 32 to reduce the opening shock at terminal speed.

By the way I d like to mention that I am very new in base jump but I been working in design of paragliding and hang gliding equipment for long time. I have a bunch of others ideas, anyone is welcome to contact me for some suggestion.

For instance, if you separate on a canopy the back risers in 2 risers (Cs and Ds) you will have a canopy much easier to fly with the back risers (only the Ds now) easier to fly back word and much safer to land if you lose the toggles. And I can’t see a negative!

Jump safe
Christophe
[email protected]

[robibird 2]

...most important is that the PC size match the size which producer clame it is!!!

Space, all is important. I strongly agree w what you said but, were is the problem to make the 42 and write on top that is 42?!

To have perfectly made PC on top of the bridge of 250 ft in student hands, were the diameter is 46 written and you find that is 42 or 43 will change a lot of things in your and student head.

You'll be confident that the 42 or 43 will work good, student will be convinced that equipment is super fine, cause you said to him so....mut the ??? how the jump will go....

To me, it is very stupid to make PC and put the size number that does not shows real size of PC.

Regards

Robert Pecnik
[email protected]
www.phoenix-fly.com

[RayLosli 11]

For instance, if you separate on a canopy the back risers in 2 risers (Cs and Ds) you will have a canopy much easier to fly with the back risers (only the Ds now) easier to fly back word and much safer to land if you lose the toggles.
And I can’t see a negative!
..........

I like new ideas but what about:
More to grab at in a panic situation / Line Twists / 6 Grommet Slider / Toggle placement & positioning
.

[pBASEtobe 0]

Quote

...if you separate on a canopy the back risers in 2 risers (Cs and Ds)...

More to grab at in a panic situation / Line Twists / 6 Grommet Slider / Toggle placement & positioning
.

Why couldn't you separate the rear risers into two risers (total of 3 risers) but still have the rear two risers go through the same slider grommet so you'd still have 4 grommets? Has anyone ever tried doing something like this?

Quick flash over to paragliding...we have four risers A,B,C,D. Also when we hit the speed bar (equivalent to front riserin' it) the A riser is pulled down and then the B. They aren't pulled down at the same rate therefore you don't distort the shape of the wing as much. You actually change the trim better than pulling down both A's and B's the same amount.

[RayLosli 11]

Having the C & D's separate for... Rear Riser, Stall & heading corrections
would be an ideal set-up.
No more extreme distortion when you Yard Down on the rear risers.
If you have ever seen side video of a Canopy when doing this ? The canopy is
buckling in the center span-wise / like a V-shape.

I however can not see the two Risers being split below a downward moving
Slider Grommet.
** Big thing is.... Would the Slider let that much Riser Span-Out Freely...?
Think about it. A-B-C-D / 8 sets of line groups let free to span ?
also:
If the front A-B's one Riser & the C-D's which are split on two Risers,
are giving a un-balanced or not Welcoming at some point on its trip/slide,
down to the Riser Bumpers. / On a 4 Grommet Slider.

For No-slider jumps I do not see a problem in splitting the C-D's for deployment or
canopy inflation./ just maybe research in Reaction to line-twists and other things.
Just keep those hands/fingers away from those 3rd Riser Hits on deployment..Ouch !
..........................
The 5th break control Line helped give this extra power for Rear-Riser stall and
turns. While breaks are stowed. With less Buckling between the AB - CD's
span-wise on the canopy. That is a big plus that I feel jumpers should not
abandon totally.
The idea is a sound one but will take a while to work out a nice balance.
.
.

[TomAiello 25]

Perhaps this is a step toward a "no slider only" canopy?

-- Tom Aiello

[email protected]
SnakeRiverBASE.com

[pBASEtobe 0]

Quote

I however can not see the two Risers being split below a downward moving
Slider Grommet.

Wouldn't having a split below the grommets actually force the slider down faster? As the canopy inflates the lines would be able to spread out more than if there was only one rear riser. I would think that this wider spread would force it down faster.

edit to add: Thought about what I wrote some more. I change my mind. I don't think the lines would spread out any more above the slider with or without split rear risers. Therefore what I wrote above it crap but I'll leave the post anyway.