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GoneCodFishing

Bi-cell crossbraced?

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Been doing a load of research into canopy design and construction and the absence of bi-cell cross braces in the design evolution is notable, and was wondering if anyone knows what happened with that? While the benefits of a tri-cell are obviously much higher i can't help thinking that a bi-cell would also run some pretty good benefits and afford middle ground of canopy rigidity vs pack volume/manufacturing complexity, and i'd have expected some bi-cells to have existed in the evolution line towards modern tri-cells, yet the Excalibur came out straight as the later. Was there a bi-cell excalibur proto before which wasn;t satisfactory?

Going a bit more in-depth i found the original patent from Bill Coe listed bi-cell cross braces as the 'idea' and added tri-cell designs merely as an alternative https://patents.google.com/patent/US4930727A/en

So what happened with them? Did they not offer the expected benefits? Was it deemed not worth it and preferred to go the full hog to tri-cells? Any thoughts appreciated

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An early, somewhat successful version of crossbracing was called "airlocks."  The chamber-spanning fabric of the airlock valves had the effect of crossbracing the most critical part of the airfoil.

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Uh, good call. Never thought of airlocks functioning as a crossbrace as well. IIRC they were sewn on the bias which would then work better to support the diagonal loads.

 

What benefits would be atibutable to each (the 'crossbracing' vs the airlocking) would be hard to tell, so back to square one. Ha, ha

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First to be clear. I think what you mean by bi cell is that there are two sections to each cell instead of three. One vertical unloaded rib inbetween the loaded ribs. Like a normal canopy. That would put both diagonal ribs running to the same seam. Well that's kind of over kill to support that seam. This would of course work but it's not very efficient in terms of the bulk of the canopy. Turns out that a 7 cell 3 section is more efficient bulk wise then a 2 section 9 cell.

 

If on the other hand you ment a 5 section cell with 4 diagonal ribs per cell then I can tell you that there is an issue with the vertical rib between the diagonal cells. They build nice long bridges like that all the time but that vertical member has to be able to withstand compression to really work properly.

 

There is kind of an inbetween. There is at least one design out there by... Airborne systems? They make the unloaded rib shorter. So the bow, the support, is all in the bottom skin. The top is smooth, all the top skin seams at the same hiath. The individual skins still inflate and how but no distortion as the unloaded rib shifts upwards. Basically the same thing you get from a crossbarace with out the bulk. PD played with it as well but felt that it didn't give them enough rigidity.  But that is some thing that is out there in a "bi" cell.

 

Lee

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(edited)
56 minutes ago, RiggerLee said:

First to be clear. I think what you mean by bi cell is that there are two sections to each cell instead of three. One vertical unloaded rib inbetween the loaded ribs. Like a normal canopy. That would put both diagonal ribs running to the same seam. Well that's kind of over kill to support that seam. This would of course work but it's not very efficient in terms of the bulk of the canopy.

 Yes, i meant a 'normal' cell (2 loaded ribs with an unloaded rib in the middle and support braces from line attachement points to unloaded rib). Such as

Capture.thumb.JPG.b62754a75d60b59ab3f1b0accf704684.JPG

 

Proably overkill as you say, but also, in that configuration, necessary for symmetry? Although possibly other than the centre rib it'd work, (braces running only on one side) that is being done in cells 3 and 7 in the Gangster.

56 minutes ago, RiggerLee said:

There is kind of an inbetween. There is at least one design out there by... Airborne systems? They make the unloaded rib shorter. So the bow, the support, is all in the bottom skin. The top is smooth, all the top skin seams at the same hiath. The individual skins still inflate and how but no distortion as the unloaded rib shifts upwards. Basically the same thing you get from a crossbarace with out the bulk. PD played with it as well but felt that it didn't give them enough rigidity.  But that is some thing that is out there in a "bi" cell.

Yes. That was patented by Atair some time back, but wasn't aware it had made it into production https://patents.google.com/patent/US20030209634A1/en

 It'd not be as efficient as a crossbrace as the path "line attachement>bottom skin>rib>top skin" is still longer than a crossbrace (line attachement>Xbrace>top skin) though still a shorter path than a normal height rib would produce.

 I suspect the effectiveness of the crossbrace might be relative to the angle of the crossbrace, so where a tri-cell would give a sharper angle trying to stretch the brace to the unloaded rib on a bicell might have too big of an angle which wouldn't do a hell of a lot to load the top skin with most of the 'shaping' being done from the expansion force from the canopies pressurization. But that's just academic, hence i'm curious why this 'step' was tried and discarded or bypassed altogether

 

Edited by GoneCodFishing
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19 minutes ago, GoneCodFishing said:

I suspect the effectiveness of the crossbrace might be relative to the angle of the crossbrace, . . .

 

Yes, for rigidity the optimum angle for the cross-brace is about 45 degrees.  By contrast, think about the lack of effectiveness of cross-bracing at the tail.  Some canopies cross-brace just the forward part of the cell (Icarus NEOS, for example), and some canopies add false ribs at the tail to keep the as-flown shape closer to the as-designed shape (PD Valkyrie, for example).

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(edited)

What Skylark is doing looks nice. The crossbrace design reminds me of the Atair Onyx of around 2002 - one over-hyped canopy even if some aspects were inventive. Two sections on the bottom surface become 4 sections on the top surface.

Atair onyx.jpg

Edited by pchapman
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23 hours ago, mark said:

Yes, for rigidity the optimum angle for the cross-brace is about 45 degrees.

Makes sense that.

 

23 hours ago, mark said:

By contrast, think about the lack of effectiveness of cross-bracing at the tail.  Some canopies cross-brace just the forward part of the cell (Icarus NEOS, for example), and some canopies add false ribs at the tail to keep the as-flown shape closer to the as-designed shape (PD Valkyrie, for example).

The Neos is the one canopy that i thought was closest to that 'missing link', in that by bracing the centre sells and A to C lines it tried to get the benefits with less complexity and cons so to speak. I assumed the lack of bracing on D's was due to it being purely unnecessary due to the lack of height of the rib on a narrow profile, but now that you mention it's very possible a bracing there wouldn't do anything or not much at all.

 

With regards mini-ribs at the trailing edge i found interesting that of the ones i could inspect they are not equal height/shape to the corresponding section in the loaded ribs, but usually taller. I guess so as to reduce the thickness of the cell without actually pinching it as it will invariably balloon more compared to the loaded ones

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(edited)

Skylarks approach is a really elegant solution to provide shaping and rigidity to a canopy, Atair's however i always thought it to be a pretty impressive feat given they probably didn't have much in the way of 3D software to get the patterns dialled. The amount of trial and error involved when sewing those must have been huge. (Edit: Thinking more about it, they probably designed it with ribs on and then just removed them?)

 

The rib design in those Phantom paragliders are really interesting. A bi-cell bracing nested inside a tri-cell to give a quad cell. (Edit: I'm making the names up. Don't know if there's an actual name for that) Clever the way they run the fabric threaded through the holes! Interestingly the bi-cell type seems to run at 45 degrees tying with what Mark mentions above regards angles

Edited by GoneCodFishing
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The rib design in those Phantom paragliders are really interesting.

It makes sense as part of the evolution of ribs. Starting out as solid ribs, with a few round holes knotknifed into them. The holes then become more rectangular, or a set of thin vertical slots to allow the rib to transfer loads along the weave of the fabric. With diagonal loads being taken by tapes sewn on from the line attachment points.  As computer aided design expands, more and more excess material in the rib is removed.  One can't go too far with a plain rib as soon one would have a thin strip of material on a bias, on the 45 to the main threads, that will stretch when loaded or ravel even with hotknifing. So one would need strips of material built to work along the diagonals. (Not sure how the Nova Phantom paragliders' diagonal ribs work.) ("crossbracing" in skydiving = "diagonal ribs" in paragliding)

If the production methods could be made to work without adding ridiculous cost, for great structural efficiency, each rib or diagonal reinforcing rib could end up as a bunch of bifurcating, cascaded threads or mini-lines. 

A 2D version is sketched below just to convey the idea, but obviously these could split spanwise as well to act as crossbracing, whether they could attach to the upper surface at any point or have to join into some sort of reinforced seam. Then it becomes more like organic structures from nature, thin structural members going everywhere instead of a few heavy structural beams of simplistic design.

In skydiving, having ribs that aren't just a bunch of holes might be useful for controlling inflation though.

Some more thoughts on some diagonal variations plus spanwise load bearing tapes, from a while back I think, can be seen at http://www.laboratoridenvol.com/paragliderdesign/V-H-ribs.html

 

image.png.af37a49c5290297e8f8a4e2f86bc293e.png

 

 

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(edited)

Ribs (as in parachute ribs, not pg ribs aka crossbracing. Getting complicated. Ha, ha)are already getting pretty threadbare in cases and don't particularly affect negatively, although it may have life expectancy issues in the long run. It's is more notable in BASE canopies which are not expected to last housands of jumps.

For example

1-DSC_0487.JPG

 

or

all-t.jpg

 

 When getting the geometry right they work just like an upside down suspension bridge, where loads can be transferred effectively and thus only a certain amount of 'threads' are needed to support the expected loads and they seemingly cope well.

 Similar thinking is used with regards support tapes in PG's and some sky canopies, having them form parabolas instead of the classic triangles. That supports the load more evenly accoss the to skin and avoids the pinchpoints usually asociated with traditional supprot taping. NZA's kraken does have parabolic tapes and interestingly in their marketig they have photos of what could have been an earlier pre-production model without them (or mini-ribs) and the difference seems noticeable. Could also just be judicious photography work of the later model making it look smoother though ;-)

 Other more leftfield 'solutions' do exist, for example PdF techno reserve's direct attachements to the top skin via spectra line, rendering the rib itself more of  a 'helper' in order to shpe the skin rather than supporting the load. Similarly single skin PG's have direct line attachements with no rib, ribs used as a bit of a fender to keep air in only, or parabolic tapes/lines with fabric only above between the parabolas and top skin to again, help shape the top skin and spread the load without pinchpoints. The amount of 'ideas' that have been put to use past and present outside the 'normal' stuff is pretty inspiring, so the 'lines only rib' thing does not look that far fetched in context, though it looks like a lot of faff and likely no possible benefit over other 'systems'?

  On a 'normal' RAM air canopy a rib of sorts is still needed at least at the unloaded bit even if crossbraced to shape the bottom skin and preventing it from ballooning downwards from the internal pressure.

P.S. I found that website a while ago and have been trucking through it. It is excellent in a nerdy sort of way.

Thanks

 

Edit: This is one example of a single skin pg rib made wth parabolic tapes + fabric. It'd be the best compromise as opposed to using just lines i think, so as to spread the load evenly acroos the seam

b07acb274d6f.jpg

Edited by GoneCodFishing
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On 3/9/2020 at 1:56 PM, mark said:

Yes, for rigidity the optimum angle for the cross-brace is about 45 degrees.  By contrast, think about the lack of effectiveness of cross-bracing at the tail.  Some canopies cross-brace just the forward part of the cell (Icarus NEOS, for example), and some canopies add false ribs at the tail to keep the as-flown shape closer to the as-designed shape (PD Valkyrie, for example).

Those extra tail ribs "sharpen" the trailing edge to fine tune down-wash flowing off the trailing edge.

The most efficient trailing edges are sharp or squared. The least efficient trailing edges are rounded.

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Only end ribs need to be made of ZP fabric. With internal ribs, the more porous the better.

I am so old that I have jumped canopies sewn before cross-porting was fashionable. cross-ports primarily vent air sideways to help end cells inflate. XPs also help end cells re-inflate in turbulence.

If you only care about structural loads, you could make internal ribs from mesh.

A few years back, University of Alberta teamed up with a fabric specialty company (was it Altair) to custom weave structural ribs so that individual threads aligned (fan-shaped) to distribute loads perfectly from line-attachment points to the top skin. High-priced racing yachts have been flying custom-woven, 3D curved sails for more than a decade. As computer-driven weaving looms become more wide-spread, we will see more custom-woven ribs.

The other new technology is through-stitched stand-on-top paddle boards. Currently they are only made constant thickness, but we can expect to see variable thickness SOTPBs in the next two or three years. Once TS technology is perfected, we will start seeing through-stitched mass-production canopies and a few years later diagonally woven, variable thickness canopies. Internal threads will be so thin and along so many different paths, that it will be impossible to stick your head inside to inspect the interior.

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Another option is the V-ribs patented by Carl Yarbenet decades ago. They had no bottom skin, but a complete top skin. Each suspension line was attached to a pair of ribs that branched off (roughly 45 degrees)  up to the top skin. Pairs of ribs were sewn together along the bottom. From the front, it looked like a top skin, with triangular ribs spreading loads to half as many suspension lines.

 

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