Crossfire's partially enclosed nose...

[airdolphin 0]

Hi:

I heard some well-respected in the industry people expressing their opinion that Crossfire is more prone to a collapse due to its partially enclosed nose, esp. at lighter loading and more turbulent conditions.

Would be interested in people's thoughts and opinions.

Thanks,
Pavel.

[TheCaptain 2]

While I have have seen people load them light and be fine, personally if the company that make it recommends a1.4 or higher I would think it would be a good idea to stay in those guild lines.

Kirk
He's dead Jim

[rhys 0]

Quote

Hi:

I heard some well-respected in the industry people expressing their opinion that Crossfire is more prone to a collapse due to its partially enclosed nose, esp. at lighter loading and more turbulent conditions.

Would be interested in people's thoughts and opinions.

Thanks,
Pavel.

There is a minimum reccomended wingloading on a crossfire of 1.4

I have seen a girl have a collapse at about 100feet due to wake tubulance on a crossfire loaded at 1.1 or so. The side that collapsed did not reinflate and the other side kept flying resulting in a very agressive turn which broke her Tib and Fib stabbing the bone into the dirt.

I have flown a Crossfire loaded at 1.8 and have had it collapse in windsheer turbulance (up high) and it reinflated just fine as most parachues will.

I would not reccomend jumping an underloaded crossfire in any significant turbulance which includes flying close to other parachutes.

On the other hand I highly recommend an appropriately loaded crossfire as my crossfire 109 was probably the most versatile and reliable canopy I have ever owned.

The minimum recommended wingloading exists for a reason.

"When the power of love overcomes the love of power, then the world will see peace." - 'Jimi' Hendrix

[diablopilot 2]

I want know who's well respected.

----------------------------------------------
You're not as good as you think you are. Seriously.

[alberto1980 0]

Correct me if I am wrong but loaded above 1.4 or not, this is definitely a drawback of a partially open ram air canopy nose. if it collapses there's less intake area to restore its shape. On the other side there's also less intake area to let the air out. But what I really think is that the intake is designed to be at the stagnation point on the wing, and if you load it light this stagnation point is above it (flying at lower angle of attack), therefore the intake is less effective.

[rhys 0]

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Correct me if I am wrong but loaded above 1.4 or not, this is definitely a drawback of a partially open ram air canopy nose. if it collapses there's less intake area to restore its shape. On the other side there's also less intake area to let the air out. But what I really think is that the intake is designed to be at the stagnation point on the wing, and if you load it light this stagnation point is above it (flying at lower angle of attack), therefore the intake is less effective.

At 1.4 or above there will be more internal pressure and the crossports can do thier thing more effectively. The nose will also 'accept' more air with a higher wing loading.

"When the power of love overcomes the love of power, then the world will see peace." - 'Jimi' Hendrix

[swilson 0]

I almost cut away from a CF2 149 loaded 1.4 that collapsed and spun up at 1800' or so. Friend and I were flying beside each other, he *touched* one corner of the canopy with his foot and it tucked under, throwing the canopy into a violent spin. Fun times.

If it were loaded heavier, I don't think it would have been a problem.

[billvon 2,384]

>Correct me if I am wrong but loaded above 1.4 or not, this is definitely a
>drawback of a partially open ram air canopy nose. if it collapses there's less
>intake area to restore its shape.

Not so sure that's true.

Keep in mind that on aircraft engine inlets, often narrowing and shaping the entrance results in more air to the engine - this feature is often used to increase engine cooling. For example, here's a Lancair mod that decreased inlet size and increased cooling:

http://www.rv-8a.net/106-111_BuildingBasics%20Cooling%20Drag.pdf

Also keep in mind that the Nova, perhaps the most collapse-prone canopy of all time, had wide open inlets.

There's a lot to canopy stability, and I think often "what does what" gets assumed without a lot of supporting data. For example, Brian Germain once cut big holes in some of the inlet valves of an airlocked canopy and didn't see a reduction in stability. That suggests that maybe the cross-bracing effect of those airlocks might have more to do with stability than the 'air mattress' effect (which by any measure is far smaller than the stabilizing effects of lift/drag vs. normal force.)

[RiggerLee 61]

Good topic. I've been kind of interested in this for a long time. I think nose design is a really interesting and some times neglected part of canopy design. I don't think it's as well understood as it should be and I think it can be linked to a number of our fatalities.

The first guy I ever watched die, or more acceratly he died from the inguries, was a canopy collaps at Eloy. This was the early ninties. It was a Pin Tail from... chute shop? He made a ninty degree turn then front risered the canopy. It bucked, break lines short? And then the whole leading edge seemed to roll under. The canopy compressed like an acordeion, fell behind a bit, one side must have inflated first as it seemed to reopen almost 180 degres off. It almost did a split S into the ground. He impacted at about 40 deg from vertical and the body was just like a rag doll. Welcom to skydiving.

Naturaly I found this very interesting. I was an aerospace major at the time. Just the day before some one was singing me the praises of the pin tail. He was telling me about how it had a large lip on the leading edge and two of the cells, 4 and 6, were compleatly closed to make them ridged to prevent this.

I think it was a few months later when stanford started getting in some Conquest from PISA. Interesting canopies. Very big lip. The opening was almost on the bottom of the nose. It had vertical baffles that blocked even more air into the nose. It was suposed to open as soft as a feather. As it happioned stanfords opened like a bomb and some of the others would snivil into the ground. It also seemd to have problems when you front risered it. I put a number of jumps on these canopies. When you looked up at this rounded almost closed nose you could see a dimple in it. Very interesting. I was a young student at the time and we had been talking about pressure coeficents across an airfoil. Well it's highest at the nose at the leading edge where it's equal to the dynamic presure. It decreases as you move away from the leading edge and this change of preasure across the top and bottom surface is what gives you lift and your pitching moment. We'd just done a lab in the low speed wind tunnel, it was cool to watch the water in all the little tubes change as you changed the angle of attack. In any case I found that I could watch the same thing here. The dimple was the area along the leading edge where the inturnal pressure was the same or less then the free stream dynamic presure. I found that by applying breaks or front risering I could change the angle of attack and move the dimple on the nose of the canopy. You could actually see it. I thought this was just the coolest thing in the world at the time and I've enjoyed watching it ever sence. What was most interesting was what happioned when you front risered it. As the dimple moved up away from the openings onto the lip of the nose you could see it get bigger. There was a point were it would become unstable. It's like the dimple would get bigger till it caved in sucking the top skin into it changing the shape of the leading edge till more would cave in. You would then get a rippel through the top skin as it would collaps, the canopy would lose lift, angle changed and the nose would reinflate. You would feel it as a slackening in the risers, hear a slight whoumph and feel your self drop. It was just one narrow band on the front risers. You would have to hold them in just the right range. The cool thing was that you could see he whole proccess.

Later I got one of the first Extreams from Nz Aerosports. Really nice canopy. Loved it. It also had a big lip and vertical baffles but displayed none of these simptoms. Then I ordered two of his new FX's This is stll from NZ. This is before Percision and Icerous. So we get them in and Bobby puts a jump on his and comes down white as a sheet. As he turned it would roll over into a dive and then the other side would roll under. It would spin the other way. Good thing he opened high. It had a new nose cut. Much more agressive, only small dimonds for vents. Much like the nose on the Veocity. Called Gyro. He was almost frantic. It was a new canopy but he had a fair number of them out there and they had been doing fine. But this one batch was bad. There were several canopies that were just deadly fucking scary. We sent them back. I figured there most be some thing wrong with the trim. The odd thing was that they couldn't find any thing. But this one batch that had been built togather was bad and I mean way bad. In the end I think he decided that it was a cross flow issue. One side of the canopy has a greater pressure on the nose then the other and he thought the flow through the canopy was allowing the nose to deflate and collaps and roll under. The openings just were not big enough to allow enough flow into the nose to keep up. So he recut the nose. I mean he rebuilt the acual canopies. He opened the nose up a bit more and that's where you get the slightly larger nose openings on the icerous canopies today.

A couple of years later at Quincy. I used to work in the rigging trailor for Wag. A guy came in with an early Crossfire and was showing it too me. He was bragging about how it had a smooth rounded nose and made a perfect airfoil with just these tiny vents almost on the bottom that made almost no drag... Things that make you go Huummm. I think that as the year they miss cut bunch of lower control lines so they were back and forth the whole boogie resewing them. Any way, I didn't rain on the guys perade at the time but I asked him to let us know how he liked his new canopy. After he was gone I made a perdiction to wag that those canopies would be a problem. Predicktig the future is not that tough if you just pay attention to the past. I should play the stock market. So they had some problems. A QC problem or slight design tweak on the trim etc. Eventualy it evolved into the crossfire two.

So Velocites are sopposed to be safe, I mean their from PD right? They also have a very agressive ie closed nose. Why don't they have problems? Well in point of fact Mandy has a Velocity. Loves it. But it has bitten her. It collapsed bigger then shit. One whole side rolled under. She had just plained out and had killed almost all of her vertical as it hapioned. Dove to the side and bitch slapped her right into the ground bigger then shit. The vid is great but you got to hear the sound. We still give her shit about the squral she landed on and the high pitched squeaky noises it made. Did she fly through the wake of another canopy? Maybe. There is video. The posability exest. You can see other people land but I cant say that she crossed a wake. I've landed behind other people and never seen any thing like that. She was pulling out of her dive, G loaded. That was the one point where a canopy should be rock fucking solid. Help me out, wasn't there an acedent at a pond. Posable roter over the tree line? Not saying it's a bad canopy but it's not exactly busting the curve in my data eather.

Can't remember the guys name, gray and navy mirage. Busted him self into little bitty peaces out a the corner of the field. We were not at all certin that all the kings horses and all the kings men would ever be able to put him back togather again. Poor bastard bought this old VX off Slithers. He was light under it but I'm not convenced that that was the whole problem. I don't think it played a role in any thing else I've described and in some cases I can tell you that higher wing loadings make the problems worse. Part of it was the fact that a light guy can unload a high performance canopy. Part was the fact that he passed over the corner of a plowed field in summer. But I think a lot of it was the way he flew the canopy. A more agressive pilot loads the hell out of a canopy. I'm not talking about weight but how we fly it. I don't think it's really a contious act or just the product of likeing to go fast but any one else would have been driveing the thing in and swooping the shit out of it. And consiquently flying it at a much higher angle of attack. In short he gave it an opertunity and an excuse to collaps an it did. There is some debate about the mode of collaps. I've seen the tape and it looked like a classic nose tuck to me. Other saw it after it was cleaned up on a computer and think it was something else but I suspect they were missinterpeting. I think the point here is that a Saber would not have tried to kill him.

I'm not bitching about pericular caopies so much as useing them and incedets as examples. What I'm trying to talk about is nose design. I'm not even saying that I wouldn't jump any one of these canopies again. But I would try to fly them in the proper manner. In other words a maner that would not lead to them collapsing and me dieing. I don't even think that it's a bad thing that there are modes that will cause a canopy to collaps. I see knowing and understanding your canopy as part of being a pilot. And I accept that there simply are things that I shouldn't do with some canopies and conditions that I shouldn't try to fly them through. It's too much to ask of a canopy that you should be imune from the consequences of all your actions. If you want that, sit at home playing video games. What does bother me is that we don't aways seem to learn from our mistakes. Both manufacterers and jumpers and that the adverage understanding of this seems to be decreasing.


Lee

Lee
[email protected]
www.velocitysportswear.com

[Ronaldo 0]

Hello Billvon,
I believe the comparison is not exactly like this. Narrowing inlets and using diffusers have more to do with improving gas expansion. Less air is forced more efficiently through the inlet which creates a higher expansion at the chamber. Gas expansion draws heat which promotes cooling. IMHO the extremely flat trim of the Xfire plays a big role in the collapsing issue.
Safe skies

Ronaldo

Engineering Law #5: The most vital dimension on any plan drawing stands the most chance of being omitted

[pchapman 261]

Great history lesson Lee!

BillVon has a good point with:

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There's a lot to canopy stability, and I think often "what does what" gets assumed without a lot of supporting data.

I agree that the mattress effect is overrated. There is some effect in having a partially closed nose or airlocks to prevent air escaping too easily, and avoiding too much 'breathing' of the canopy. And speed does make the canopy more solid.

But in the end, it is mainly angle of attack that will determine whether the nose folds under or not.

Lee's example of front risering the Conquest really shows the effect. Depending on the nose design, one can get the area of highest pressure around the stagnation point wandering off of the inlet, bulging in some of the shaped nose (and thus equalizing pressure again in that area, between inside and outside). But if that highest pressure area goes too far, one can get too much nose distortion or the whole nose folding under.

It is hard to get enough anecdotal evidence to figure out what is really happening. Parachutes are messy to analyze because of the interaction of the canopy and the payload, the jumper down below. Speed up or slow down and the drag and weight of the jumper will affect to what degree the canopy pitches up or down, trimming to a different angle, with some angle of attack changes happening too.


Say that it is correct that if one decreases the load on a canopy, it will fly at a lower angle of attack. (As Alberto has suggested.) For an aircraft flying level at a given speed that of course is true, but as I said it gets messier to prove for a parachute.

A low angle of attack might not exist if the canopy was trimmed to match that light wing loading in the first place -- but it might if the canopy is flying at a much lighter loading than designed for, or tested & tweaked.

The effect of a lower angle of attack might then not really be noticeable unless the canopy has small nose openings.

If someone is on a lightly loaded, partially closed nose canopy, and does a sudden one sided front riser pull (at altitude), it should be easier to unload the nose and get the feeling that it about to fold, that one better not pull down as fast / far / hard.

That tests the issue in this whole thread. If a lightly loaded canopy with a partially closed nose is more susceptible to jumper induced nose collapse, then it is probably also more susceptible to turbulence. So if someone could do some front riser turn tests with a Crossfire at under 1.4 WL that would be great...

The natural angle of attack won't be the only factor though. As the canopy flies slower, any given turbulence, if hit suddenly, will change the canopy's angle of attack more if the canopy is slow, than if the canopy is fast. (That's not the effect of all turbulence though, as for some turbulence, it will be better to get into it slowly to let the canopy adjust to the conditions. So the exact nature of the turbulence also affects to what degree faster or slower is better.)

[rhys 0]

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So they had some problems. A QC problem or slight design tweak on the trim etc. Eventualy it evolved into the crossfire two.

I had a crossfire 1 for 800 jumps, it only collapsed on me in significant sheer turbulace, as did most canopies on that load.

I beleive the crossfire 1 problem was limited to a batch made by precision in the USA.

Thanks for the great post some food for thought.

I have noticed that 'dimple' too the velocity has a significant one. There are ample velocities out there, if it was a problem i'm sure we would all know about it.

"When the power of love overcomes the love of power, then the world will see peace." - 'Jimi' Hendrix

[alberto1980 0]

Great posts as always This is a really interesting thread.

PS everyone: DZ.com has a 'Check spelling' button, let's use it

[billvon 2,384]

> Narrowing inlets and using diffusers have more to do with improving gas
>expansion.

For nacelle inlet designs, agreed. The article referenced was all about how to improve cooling, and that means higher and more consisten flow inside the cowling.

However, the article talked about two effects: the shape of the inlet and the shape of the cowling inside the inlet. Both were a factor. It might seem intuitive that a 'funnel' shaped inlet would ram more air in, but in fact the opposite is true; a narrower tube that stuck further forward worked better. It also had the nice side effect of reducing overall drag.

Once you get inside the inlet, then design (expander, diffuser) was still critical to ensure that the air expanded smoothly to effectively cool engine parts.

In the case of a parachute, once it's inflated, then you don't have any flow issues at all; there's almost no flow in an inflated ZP parachute (at least a new one!) And even when deflated during turbulence, you don't care much about smooth flow; you just want to reinflate it quickly. So the internal structure doesn't seem as important.

As to the inlet size, it's tempting to say that "bigger is better" - but as the cowling design indicated, that may not always be true. (Witness the Nova's wide open nose.) In addition, as Lee pointed out, stagnation point plays a critical role; the canopy has to be able to take air from the direction it's coming from. Any canopy that successfully inflates can take air from very steep AOA's since that's where it comes from during opening. The other extreme is front risering, where you're intentionally making the angle of attack steeper. A well designed inlet will be able to keep a canopy inflated during both of those extremes.

>IMHO the extremely flat trim of the Xfire plays a big role in the
>collapsing issue.

Given the role that angle of attack plays in canopy reinflation that's very likely, I think.

[alberto1980 0]

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> Any canopy that successfully inflates can take air from very steep AOA's since that's where it comes from during opening.

Well the slider here plays a big role since it makes the canopy 'arch' a lot when it is up there close to the fabric (extreme camber during deployment, see attached pic) so this is not true and canopy collapse beyond stall angle of attack proves it. Note though, canopy stall is not related to its failure to get air through the nose. I am pretty sure that NOVA issue was in fact related to airfoil / planform shape or a combination of the two more than air intake design.

[faulknerwn 36]

Interesting thoughts - especially because of a canopy I bought recently. I ended up with a used rig with a brand new ( <20 jumps) 1994 Tomcat. A Tomcat is all ZP, fully square. The canopy collapses if you front riser it. At first I just assumed it was the brakes, so I lengthened them. Still did.. Then I videoed some jumps - even without my hands in the brakes, and the toggles not pulling on the tail, any front risers made it collapse..

Looking more carefully the end cells always collapsed first. Did a very detailed comparison with a PD 150 I have from the same era.. The line trim is extremely similar. The nose is slightly smaller on the Tomcat, but not much and both are wide open. The only major difference was the stabilizers. The PD 150 (which front risers fine) had much bigger stabilizers than the Tomcat. Which kinda makes a bit of sense since on video you can clearly see the end cells collapse when you front riser even a bit for just a second or two...

Canopy design is interesting..

[talon2 0]

NZ manufactured xfires stayed at original specs .Only issues on record related to specs were xfires made elsewhere

[RiggerLee 61]

I never said that the exestance of a dimple was a bad thing. I think it's inevitable.It's a product of the pressure being equal to Q. You got too choices, eather the nose opening is huge, way bigger then nessasary, think PD f111, falcon, raven et with the big flat cut through the nose of the air foil. Or you have some type of lip. If you have a lip of almost any kind it can not be optomised for all conditions, there will be a dimple. With out tha you get all kinds of flow in, flow out going on. For example durring flare as the canopy pitches back ever notice how the nose on a saber pops out? It infates as you increas the angle of attack, traps air in the nose, and makes a smooth surface for the upper part of the air foil to dirrect air over the top of the wing. All good things. Note it was still a big flat nose cut just with this... air dam, slat, what ever you want to call it on there. I don't know if that was the original intent but basically it's a variable geomitry inlet.

Now contrast this with some f the more "modern canopies where the idea is kind of taken to the exstream. With a more rounded nose with a more compleat airfoil trying to maximise this. As you flare the nose inflates into a much better airfoil. This is awsom. Very efficent. It's all about the quest for more speed. All fine and good. I think the problem can result from what happions the the nose of the airfoil aerodynamicly as the dimple changes. The rounder the nose is the deeper the pocket becomes and that's your "leading edge shape" that's what the wind sees. Now where is the new stagnaton point on this new air foil? and so it goes till it reaches an equallibream. There's a limit to all things. Sooner or later you reach a point were the aoa is low enough that the nose becomes a big dent, the intets on the lower half of the nose becomme less efficent and the leading edge in no longer in equallibream. And all that dent in the nose rolls back through the top skin of the airfoil breaking the lift on that part of the wing. That part of the wing drops back and down.

Thats just talking about nose design. There's another part of this which is what angals of attack a canopy can reach. I've used front risering as an example here but that's actually not a very good way to look at this. I'll go back to tha conquest as an example. As you pulled the risers all the way down it actually became more stable. Remember you're pulling the A and B lines. That means you're basically maintaining the angle of attack of the nose of the canopy. At first there is a bit more change as the rest of the canopy exerts more pressure on the B lines tilting the cascade. But for the most part you're just brakin the thing in half and makeing it less efficent. If fact as the airfoil becomes less efficent and your path steepens I think the AOA on the nose actually incrased hece the second range of stability. Compare this to a canopy with a speed bar like a paraglider. I've flown both paraglideing canopies and skydiving canopies wit speed bars and where you could pull indevidually on the A lines. Great oppertunity to study leading edge collapes. It doesn't take much at all to roll the leading edge under bigger then shit. That's an actual change in the AOA not front risers.

I think the lowest angels you'll see on a skydiving canopy are not from front risering. Note the high riser pressure you get from pulling on the front risers. No the lowest angals come from the dynamic pitching of the canopy. example: From full flight reach up and tweek the fronts, note the pressure. Now give the canopy a partial flare and then quickly reach up and pull the fronts so that you are releaseing the breaks letting the canopy surge as you pull the fronts. Try this with diffrent degrees of flare but do it up high, some canopies have nasty surprises waiting for you. As you flare the canopy dynamicaly your body swings infront of the wing changing it's AOA. If you let the canopy surge the AOA can go way down. The thing is you do this to one degree or another all the time with out even knowing it. any little tweek on the toggle ses the canopy back then as you release the imput the canopy surges forwards again with a corisponding change in the AOA. As an example small break imputs. Students and low time jumpers are notorious for this as they come in to land. It' exagerated by the size of their canopies and the length of their lines. They make a small corection, let off, the canopy surges slightly and is not ready to flare. All of this depends on line length, trim, and the camber of the airfoil. All of this factor in to how stable it is ie how much it is willing to move forwards and back above you. I think this is where the Nova ran into trouble. Going into or exiting a thermal, flying slowly, stearing with toggles, quick releases of toggle imputs, all of thease can effect your AOA and make the canopy want to sit back or surge. If you run out of lift on the canopy, if it trys to pull negative g's, your screwed. Those lines are not stiff.

So basically there can be a lot of diffrent failure modes. Things can push the nose over the edge into collaps and the canopy can over pitch and simple run out of lift under the front of the nose ie slack rises bad.

Lee

Lee
[email protected]
www.velocitysportswear.com

[RiggerLee 61]

Windy, You might want to check something. Ths will be a bit of a pain but might be interesting. Take span wise measurements at diffrent cord points across a cell for both top skin and bottom and the air foil thickness measured in a line to the three ring. And check the shrinkage on the lines.

So here's what I'm thinking. You've got a airfoil and a line trim. As you go to turn this into a canopy I'm guessing they basically rotate it around a line in space to create the panel shapes. Dependng on exactly how you do this, the angle of the line, you'll get diffrent pannel shapes. This effects the AOA at diffrent points along the span. It was one of his early canopies. I'm guessing he didn't quite have this figured out yet and has basically too much twist in the wing for lack of a better term. I think it should basicllylook like a section of a cone around the line of flight, maintaining a good AOA across the span rather then a section of a cylinder loseing AOA at the ends.

Need to come up and visit you and do some crw. How bussy are you people, say durring the week?

Lee

Lee
[email protected]
www.velocitysportswear.com