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mbohu

Why Cross-Braced only for high performance canopies?

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This might be a naive question but:
When I first read about cross-braced canopies as well as airlocks in Brian Germain's book, I thought that these were technologies that made canopies potentially safer and better and would apply to any type of canopy (in terms of the performance range).

Later I realized that "cross-braced canopy" is essentially synonymous with "extremely high performance canopy". (And I don't think airlocks or similar technologies are available in anything but the most high-performance canopies as well) Why is that? It seems to me that both technologies make canopies more stable and don't seem to have any apparent drawbacks specific to lower performance ranges or lower wing loadings (other than cost).

Is there any technical reason why one wouldn't want to build, say a student canopy, with cross-bracing and airlocks, to make it more stable in thermals and other situations? Why are they only used in extremely high performance canopies? Is it just a matter of economics or is there a performance related downside to using these technologies on bigger, lower performance canopies?

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Cost and bulk.

It would make a typical sport canopy cost a fair amount more (no idea exactly how much).

It would make a 170 pack really huge.

And for what?

It makes the airfoil more rigid and more efficient.
But the average jumper doesn't need that.

Kinda like putting ultra-high performance tires on a Kia. Yes, it will make it perform better. But not enough to matter.
"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy

"~ya don't GET old by being weak & stupid!" - Airtwardo

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Just my impressions:

Airlocks & crossbraced together: A massive bother to try to sew both. Brian experimented with that, but not worth it.

Airlocks: The concept is basically dead in skydiving. Nothing wrong with it though, other than a pain to get air out on the ground. Various threads on it over the years. They do improve rigidity in turbulence and maintain canopy efficiency. But generally if it is so turbulent that you need airlocks to save your ass, you probably should have been on the ground anyway. If it actually did collapse one side, an inflated collapsed wingtip is more dangerous than an uninflated one. So the benefits weren't seen as worth it.

Crossbracing: Yeah, not worth the time and cost to build for less than high end canopies, at least for the way the market is now.

(As a comparison, in paragliding for a while they were popular down to a lower level of market, as they already were building fancy elliptical shaped canopies with say 44 chambers even at a low intermediate level. So it wasn't as much additional bother when the canopy was already quite complex.)

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RiggerLee

PD built the Excalibur in some larger sizes. Does any one remember the largest that they built?

Lee



230 I believe. I had a 210. It changed everything about canopies forever.

I think that was a bit different than this topic because the best we had before that were 9-cell, F-111 canopies, so it was a move forward regardless of pack volume or weight. It allowed people to jump smaller canopies because of the performance increase relative to what was available at the time. No zero-P, no elliptical canopies. PD was trying to push the limits and the Exalibur was the testing ground.

It worked.
Chuck Akers
D-10855
Houston, TX

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Quote

Airlocks: The concept is basically dead in skydiving.



To be honest, I am not sure I have ever seen airlocks and know how to recognize them, but when I was at Skydive Arizona last week I saw lots of tiny canopies land and a few of them looked completely different on their noses. They did not have cells that were simply fully open in the front, but had fabric covering some part of the nose of the cells, that had smaller round openings and when they collapsed after the landing, the wing pretty much just fell down as one piece to the ground--klonk! ...like a solid piece of wing, and only then deflated. Was that something similar to airlocks? I saw quite a few of them, but didn't really have time to ask their owners about their canopies (ok, so I probably felt a bit intimidated too)

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pchapman



Airlocks: The concept is basically dead in skydiving. Nothing wrong with it though, other than a pain to get air out on the ground. Various threads on it over the years.



Not totally dead. Brian still makes the airlocked Lotus and Samurai. I have both and you still see them around occasionally. I've no idea how many new airlocked canopies he still makes, but used ones show up on the market from time to time. And once you get used to them, they're not that difficult to deal with on the ground. But, I do catch grief for jumping these at my DZ!
My Dad used to ask me if someone jumped off a bridge would I do that too? No, but if they jumped out of an airplane, that's a different question...

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You know, the interesting thing about cross braced canopies is that they are better low speed airfoils. I know that sounds strange as they are used on the fastest canopies around but that's actually the point. What allows people to jump canopies at those high wing loading's is that they are better low speed wings.

It's about how the wing behaves as the AoA increases. The wing inflates because the inlet is near the stagnation point with the highest pressure on the wing. That dynamic pressure wants to inflate the wing. The lift produced by the wing actually wants to collapse it. That's why cells bow upwards on an I beam canopy. It's trying to flat pack it self in the air. The inflation pressure fights against that. The distortion is related to the ratio of the pressure to the lift. As your air speed reduces and the dynamic pressure lowers relative to the lift the distortion becomes more pronounced. The canopy gets smaller. Example when you are in breaks and slow the canopy. You can see this as the front of the canopy narrows on landing when you flare. Basically it's in relation to your AoA. So right at the point when you need to canopy to perform it's best it's at it's worst.

In a cross brace the canopy is now less dependent on the inflation to support the load of the cell. The diagonal ribs carry the load more efficiently and the the compresive force is reduced. The canopy is less distorted and flies more efficiently. This is going to sound strange but the best place for cross bracing is on an accuracy canopy. I can see it now... five cell cross braced accuracy canopies ruling the world meet.

Lee
Lee
lee@velocitysportswear.com
www.velocitysportswear.com

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RiggerLee


In a cross brace the canopy is now less dependent on the inflation to support the load of the cell. The diagonal ribs carry the load more efficiently and the the compresive force is reduced. The canopy is less distorted and flies more efficiently. This is going to sound strange but the best place for cross bracing is on an accuracy canopy. I can see it now... five cell cross braced accuracy canopies ruling the world meet.

Lee



Could you explain that more clearly? I’m not a rigger but I am an mechanical engineer with education on aerodynamics and I’m not quite understanding what you’re saying about loading.

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I see what Lee is saying. If a canopy were super pressurized, it would be so rigid that it would be harder for the unloaded ribs to shift upward, giving that zig zag top skin effect, where the loaded ribs with lines are held down and the unloaded ones bulge up with lift pulling up. But at low pressurization due to low speed, there's less keeping the weight vs. lift distorting the canopy.

Accuracy is one area where good airfoil performance (in brakes) actually matters more, among those using big slow canopies.

Maybe if all student canopies were crossbraced, one could go one size down for all students. A more efficient airfoil might give better handling, no worse stall speed after the size decrease, and better flare. There could be concern over higher speed from the size decrease, although that might be mitigated by different trim. The industry might not feel it worthwhile though, for the extra sewing time involved and maybe only a minimal reduction in weight and bulk. (One size down vs. crossbrace bulk - not sure how that works out for a large canopy.)

So crossbracing isn't necessarily not useful for bigger canopies, but it just isn't the way the sport has evolved in this parallel universe.

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LeeroyJenkins

***
In a cross brace the canopy is now less dependent on the inflation to support the load of the cell. The diagonal ribs carry the load more efficiently and the the compresive force is reduced. The canopy is less distorted and flies more efficiently. This is going to sound strange but the best place for cross bracing is on an accuracy canopy. I can see it now... five cell cross braced accuracy canopies ruling the world meet.

Lee



Could you explain that more clearly? I’m not a rigger but I am an mechanical engineer with education on aerodynamics and I’m not quite understanding what you’re saying about loading.

If I read and understood it correctly, he's saying that the crossbrace allows the canopy to stay more rigid at lower airspeeds because it relies less on pressurization for rigidity.

If I'm wrong on that, someone please correct me.

And the idea of crossbraced accuracy canopies is amusing.

Unlikely, because they accuracy folks don't need super-efficient wings to do what they do. They can fly a (relatively) crappy and inefficient wing to where they want to go because they know how to fly it and wringing the last little bit of performance out of it isn't needed.

Swoopers, OTOH, need every last little bit (and then some) out of their wing. When fractions of a second determine who is on the podium and who is an "also ran", it matters.
"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy

"~ya don't GET old by being weak & stupid!" - Airtwardo

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An I beam canopy is one that is built with a top skin panel, a bottom skin, and loaded bearing ribs at the ends of the cell, and a non loaded rib in between. There have been other designs but that's what you see now in the majority of canopies. So imagine the cell. You have the loaded ribs at the end and the non loaded in the center and the panels stretched horizontally in between.

Two types of distortion. Pressurize it and the top and bottom skins bulge out like a balloon between all the ribs. Ribs stay straight and vertical unless they are an end rib in which case they bulge as well. The distortion is a product of the ratio of the height of the rib to the width in between each rib. This distortion gets worse as you go back towards the tail and the rib gets thinner. Thats why you see sub ribs between the main ribs near the tail on newer canopies to try to thin out the trailing edge and reduce drag.

Second type and what we are referring to here. Again imagine the ribs are vertical supports. Remember the fabric is soft. When the wing produces lift that fabric wants to move upwards between the load bearing ribs. It wants to compress the canopy span wise by folding it back flat lifting the unloaded rib till the cell is compressed. This reduces the volume of the cell. The pressurization of the cell does not like that wanting to maximize the volume. These are the two forces seeking a medium. The amount of distortion in the wing from this is a product of the ratio of the lift across that cell and the pressure ratio in that cell.

So whats this ratio? it's basically dynamic pressure, Q, .5RoV^2. Ro=density V=velocity magnitude squared, to lift. Lift is Q*CL*S, CL= coefficient of lift of the wing, fixed number, S= surface area generally a fixed number but in this case it's actually going to shrink a bit as the canopy distorts. Cl=A*AoA, A is a slope it's how the lift changes with AoA. We're kind of simplifying all this but in the end even with high induced G loading, like coming out of a dive, you can basically say that the ratio of the internal pressure to the lift of the wing is linearly related to the AoA. And so the loss of lift as you lose span width and surface area of the wing is related to AoA of the wing.

Now think about the cross braced canopy. The diagonal ribs support much of the load of the cell. They prevent the cell from drifting upwards. Analogy. Ever seen a cable bridge like in Nepal. Its just cables with a bed and more cables for the hand lines. It's a big bow. Always will be. Now think about a suspension bridge like the golden gate. Road surface is flat no distortion because those big cables support the vertical load on the road surface. Their curve is a product of those forces. There is still horizontal force supported by the anchors at the ends but the road stays pretty much straight. Same thing in a cross brace. As long as the pressure ratio stays below the angle of the angle of the diagonal rib the cell will not distort upwards. So there is a limit to how wide you can make your cell related to how tall your rib is. Fortunately most of the lift is being produced at the front of the canopy where the cell is thickest.

Think about it a bit you'll see it. That's a little dumbed down but every one else should be able to follow it.

Lee
Lee
lee@velocitysportswear.com
www.velocitysportswear.com

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It was as I suspected. You were just using terms in a slightly different way than I, as an engineer would use them. Great explanation for people who are less informed about airfoils.

For those curious, pressure is what gives a canopy its rigidity (read as hardness) and the ribs are what give a canopy its shape. This is because fabric cannot handle a compressive force. A crossbraced canopy will hold its shape better than a non-crossbraced one. For comparison in a traditional airfoil the ribs provide rigidity and shape.

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>They do improve rigidity in turbulence and maintain canopy efficiency.

Note - a while back Brian did an experiment where he cut holes in the airlocks; worked about as well in turbulence.

I think that a lot of the value of airlocks comes not from "locking" air inside, but rather from the added rigidity it gives the nose (i.e. functions more like crossbracing than a "valve.")

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Quagmirian

Cross bracing makes a canopy more efficient in both low and high speed flight. Reducing the upwards cell distortion with the diagonal braces. Pretty sure NZ made a crw canopy called the Matrix2. It was a crossbraced 5 cell.



They also made a version of the JFX for CF. My profile pic shows us jumping them. Great canopy for what we were doing, but also highly modified for CF.

We found the extra rigidity from x-bracing to make the canopies more stable in flight. And finally a damn CF canopy that landed well!

top
Jump more, post less!

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LeeroyJenkins

......... pressure is what gives a canopy its rigidity (read as hardness) and the ribs are what give a canopy its shape. This is because fabric cannot handle a compressive force. A crossbraced canopy will hold its shape better than a non-crossbraced one. .......



------------------------------------------------------------------------

May I disagree?
It is possible to load a fabric structure in compression, buy only if internal (air) pressure exceeds external loads. Just look at s car tire. For aeronautical applications look at the experimental Goodyear Inflatoplane or more recent Swiss Stingray.

Canopy ribs (or through loops in reserve containers) allow you to tailor the inflated structure thinner than a sphere. Similarly, the latest fashion in stand-on-top paddle-boards is inflated with hundreds of drop stitches to maintain a flat profile.
In a few more years, we will be able to jump drop-stitched canopies with waaaaaaay smoother top skins than are possible with the ocaissional rib.

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Great thread!

In hopes to clarify a tiny on fabric:

Quote

It is possible to load a fabric structure in compression



From all the examples you mention, the fabric is still in tension. Remove the internal pressure and the fabric crumbles. The internal pressure puts the fabric in tension.

Same for a car-tire and inflatoplane. If the internal pressure can't create tension, then the structure buckles.

I assume you wanted to say that a fabric structure can be loaded in a compressive manner, which is correct, but the differences between the statements are huge for us engineers ;)


PS: Not sure about what a swiss stingray is.

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The Stingray prototype, inflatable airplane flew a few times in May 1998. It was built by the Swiss company Prospective Concepts AG to demonstrate their inflatable structures. It was called Stingray, because it resembled the fish when seen from above. From the front it resembled a typical ultralight with an open tubular fuselage supporting the pilot, engne(s), motor and wing.

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