Basejumper.com - archive

I have read a lot about the fact that PC's should have the tapes sewn on the bias of the mesh to prevent distortion. It makes sense when I read it, but when I look at all of my PC's (APEX an BadSeed), I can't tell that they are or are not. They are all big mesh and have 3 (or six depending on how you count them) support tapes on the ZP and Mesh. The mesh tapes are placed symetrically apart...thus putting some bisecting the mesh in one direct and others in another direction. How can they ALL be on the Bias?

Can someone post a picture of a tape on the Bias and off the Bias (and I guess then on the Block) of big mesh. Thanks for the help and clarity on this.

Also, what do you do to test the overall symmetry of your PC?

And what do you do to ensure you attach it symetricaly to the bridle?

Not sure what a bias is but I just check the tape line length on each attach and repeat

I shouldn't have to tell you this if you're already into base, but bias and block has to do with the zp/f111 material
It's diagonal vs. parallel with the ripstop weave.

It has nothing to do with the mesh.
Stretch some f111 or zp diagonally and then stretch it horizontally. It doesn't stretch parallel to the weave, but it does diagonally. So you want tapes going diagonally to prevent the pc from stretching and distorting causing pc asymmetry...

Who said I was into base already I'm just a troll

Touche

LOL not you
Newb2base up there. The OP

Heath just hides under the potato bridge waiting for cute girls and poses as a base jumper. He also hides in the outback and talks about Base but has only jumped out of a plane twice.

The (Marquisette) small, square mesh, positions the skirt. So it is of greater significance than the sail material in the elongation scenario of Bias vs Block construction techniques.
I think I once calculated a possible 30% drag difference between a Bias vs Block PC of the same mat. and diameter.

The attached drawing is for square weave mesh reinforcement.
Big mesh is normally a hex weave which would mean that 3 tapes on the mesh could be better.
Note: The red and green arrows represent elongation directions, not reinforcement tapes.
Take care,
space

Just the ZP/F111? Are YOU sure? The relative stretching of the ZP is very minimal. I think it is all (or mostly) about the mesh.

Is BASE283 wrong then (above)?

Is Bill Booth wrong when he states that it is related to the mesh? I'm sure you know better.

http://www.dropzone.com/...h=25;guest=105645730

Thanks 283. The visual and effect is easy to see on square (small) mesh with 4 tapes. On big mesh and 6 tapes...it seems some will be on the bias and some block. At least that is how they look to me.

There's a bias to most mesh material also.

I've seen some really cool ZP material where the block grid is hexagonal, which really reduces the issue. I'd love to see some BASE PC's made from that sort of thing.

It's really not. I have some unreinforced ZP 42" PC's that I can stretch to about 50" along the bias direction. 20% stretch isn't what I'd call minimal.

Bias (and stretch) applies to both the mesh and the ZP.

That is interesting. Thanks for that info.

Is the stretching of the ZP (sail) really as critical to ossicliatoin as the stretching of the mesh. The mesh acts like the lines on a round. Under the load of the relative wind, the mesh stretching asymetrically changes the shape of the round parachute (the PC) allowing more drag where it doesn't stretch and less where it does (because it allows the sail to be higher and thus spill out air). This can cause oscilation.

The ZP is facing an upward force from the inside and thus stretches in that direction which has less of a distortive effect on the shape of the edge of the Sail/Mesh juncture.

This is why skyjumping PC's only have reinforcement on the bias and on the mesh....right?

A quote from Bill Booth (who knows a little bit about PC design):

Think of the mesh on your pilot chute as the suspension lines of a round canopy. If the suspension lines are all different lengths, then the parachute is not going to work very well is it? We sew the support tapes on the bias (the direction the mesh stretches the most) to stabilize the mesh so that the whole skirt of the inflated pilot chute, especially that part which is between the support tapes, is roughly the same distance from the base. If the support tape is not sewn on the bias, and the apex is pulled to the junction of the support tapes and the skirt, when the pilot chute inflates, the mesh halfway between the support tapes (on the bias) stretches a lot, allowing that portion of the skirt to get way above that apex, spilling a lot of air, and lowering the pilot chute's drag. How much drag is lost depends on how much the apex was pulled in the first place. Why would a manufacturer not sew the support tape on the bias? Well...1. They either they don't know any better, or 2. it's really hard to sew tape on the bias, 'cause the damn mesh keeps stretching.

In most cases, I think it's more critical.

I've seen PC's with serious differential in the mesh at different points of the skirt (in fact, with large hole mesh this is very often the case, and most subterminal BASE PC's are like this) that still function pretty well, with minimal oscillation. The unreinforced ZP (or with the reinforcement tapes in the wrong direction) PC's that I've seen almost all oscillate dramatically.




Stretching of the sail can also cause quite a lot of oscillation. Actual rounds (and toxic style PCs) are built with each gore oriented along the block to minimize stretch.



I have no idea. If I wanted to know why skydiving PC's were built a certain way, I'd ask skydivers. The Gear and Rigging forum on Dropzone.com would be a good place to start with that.

Just the difference in stretch between fine (marquisette) mesh on most skydiving PCs and large hole mesh on most BASE PC's is enough that I'd consider them to be very different animals.

Another good quote on this topic to think of it differently:

Let' try to re-state this.
If you only use a piece of mesh - on the bottom side of your pilot chute - it is like only having four suspension lines, because the fabric will not stretch along the threads.
If you sew reinforcing tapes along the weave, it is still like only having four suspension lines.
However, if you sew tapes along the bias (45 degrees to the weft and weave and fill), then the pilot chute flies as if it has eight suspension lines = more drag, less spin, etc.

What do you mean by this exactly?

Thanks for the conversation on this. I appreciate the experience you bring to the table!

If you hold the PC upside down by the bridle attachment point and stretch the skirt down toward the floor, you can see that the skirt is at a very different distance (several inches of differential) from the apex at different points along the skirt. This is because of the differential stretch in the mesh in various directions. This difference is much larger with large hole mesh than with small hole mesh (marquisette) bottom skin.

For what it's worth, I have never seen a BASE PC that was "bad" in the way Bill Booth describes in the linked DZ.com post (the skirt falls past the apex of the PDA round).

+1 This is what I mean to say. Large hole mesh really doesn't show a dramatic effect in the performance of the pc when it stretches. I'm sure it's a much greater difference with small hole mesh based on the drag and whatnot we've seen depicted in the image above.
I simply believe the zp is the more critical element.

Hexagonal is even worse allegedly .... Heard a swooper bitch about canopy deformation at high loadings. Was flying a sensei with said material. Makes sense I guess...every side of the weave is like a joint now. Never tested, just hearsay and mental masturbation

zpX is still a block and bias weave with the rip-stop thread weaved in a honeycomb shape into it. A hex weave and honeycomb are two completely different things.

The Honeycomb of the zpX means you can puncture it with a nail and the hole barely shows after which is awesome, but it seems there are some trade offs when it comes to durability in other areas