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phoenixlpr

Crossbraced tandem canopy?

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It looks like JoJo is now selling a 280 SqFt. x-braced tandem canopy, the HXP 280. I found this video on their website:

http://www.jojowing.com/...g/hxp/hoplanding.mpg



Does anyone here have any experience with these?

I am flying a Icarus 300 tandem main for work and as a light weight tandem instructor I find the smaller size to be much safer than the 330's and the 365's.

the reason for these thoughts is that a high percentage of our customers are small asian girls and our combined weights are often less than 120kg's.

Going backwards in 20knt winds is not my idea of fun.

Speed is your freind when it comes to landings.

Loading is your freind when it comes to turbulance.

If I land off at my current dropzone it would be highly likely to land in sugar cane that has Taipan snakes slithering around in there. If you get bitten by one of those you should have your last thought before you go into spasms and die!

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

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Unfortunately, speed is your enemy when it comes to turbulence. "High loadings for safety!" has been proven not to work.



For aircraft maybe. But my experience tells me that a higher loading causes a higher airspeed, which in turn causes a higher ram air pressure, there fore a more rigid wing, to better counter turbulence.

And I've got quite a bit of experience with tandems in turbulent conditions.
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You're not as good as you think you are. Seriously.

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Looks a lot smaller than 285 sq ft to me.


I attached a pic of a 285 (non-x braced) shot with hand cam. They prob just look small because of the wide angle lens.

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Unfortunately, speed is your enemy when it comes to turbulence. "High loadings for safety!" has been proven not to work.


Wait...what? Can you elaborate?


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Unfortunately, speed is your enemy when it comes to turbulence. "High loadings for safety!" has been proven not to work.



Lies!

Where this may be true for rigid aircraft parachutes and paragliders rely on internal pressure in order to maintain lift.

Internal pressure is what keeps a canopy inflated.

Higher loading = higher internal pressure = more stable in turbulance.

Are you saying you would rather be on a 7 cell canopy loaded @ 0.6 than a 9 cell crossbraced canopy loaded at 2.0 (assuming you have the skill to fly one of those) in turbulant conditions?
"When the power of love overcomes the love of power, then the world will see peace." - 'Jimi' Hendrix

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>which in turn causes a higher ram air pressure, there fore a more
>rigid wing, to better counter turbulence.

The "rigid wing that slices through turbulence" is largely a myth, akin to the "if you're going fast enough when you hit a pothole you won't feel it" myth. What stabilizes a canopy is the force of lift working against the stabilizing effect of the lines. That is several orders of magnitude greater than the stabilizing force of any pressure within the canopy.

Brian Germain once did an experiment that demonstrated this. He "opened" several airlocks on one of his canopies. If the "rigid pressurized wing" theory was correct, the canopy would have become less resistant to turbulence. However, when he tested it, it did not.

Higher speeds also have another side effect - they increase the magnitude of turbulence seen. Turbulence is not something separate from air; it's just moving chunks of air. When it changes slowly we call that wind. When it changes rapidly and erratically we call that turbulence. When it changes dramatically in a short distance we call that wind shear.

Your canopy is always moving through the air, transiting different areas of wind. The faster you move, the faster you see changing airspeeds when the air is moving erratically. If you are going fast enough, you can change severe turbulence into a canopy-collapsing wind shear event.

Why does the rigid-wing-that-slices-through-turbulence propagate? The main reason is the packing effect. The packing effect leads people to believe odd things about packing. It works something like this:

Jumper tries to pack his new Pilot 188, and can't keep the slider on the stops. He gets bad openings. He tries rolling the tail, different rubber bands, packing on damp grass, adding talcum powder, and finally standing on one foot. His openings improve! It must have been the standing on one foot. Heck, he can PROVE it - his openings have gotten better.

In reality, of course, he finally just got enough experience packing that he can reliably keep the slider against the stops during packing.

Likewise, many jumpers get to be better and better canopy pilots, and downsize at the same time. They learn more about flaring, flying in turbulence, making small corrections, turning in the flare etc etc. At some point they notice that they are dealing with turbulence better. Many people assume that this means that it's the smaller canopy that does this, instead of the normal increase in their skills.

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>Where this may be true for rigid aircraft parachutes and paragliders rely on
>internal pressure in order to maintain lift.

No, they don't. Single-surface lifting canopies have been used, and they produce lift just like ram-air canopies.

>Internal pressure is what keeps a canopy inflated.

Again, no. Single surface canopies stay inflated as well. Ram-air design gives you a lot more flexibility in shapes you can concoct while having it retain its shape, and lends itself well to more complex airfoils.

The primary stabilizing force on any canopy is the lift working against the lines. Try cutting a few holes in the bottom skin of your canopy. Chances are you will still be able to fly (and likely land) it, even though pressurization has been severely compromised. Now try cutting the two leftmost line groups of your canopy. It will immediately fold up and you'll start spinning out of control. This is because the stabilizing force of the lines is FAR greater than the stabilizing effect of pressurization.

>Are you saying you would rather be on a 7 cell canopy loaded @ 0.6 than
>a 9 cell crossbraced canopy loaded at 2.0 (assuming you have the skill to
>fly one of those) in turbulant conditions?

In windy conditions I'd rather be on the more heavily loaded canopy because I will be more likely to land on target. In very turbulent conditions I'd rather be on the 7 cell, because I would be more likely to survive the landing.

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In windy conditions I'd rather be on the more heavily loaded canopy because I will be more likely to land on target. In very turbulent conditions I'd rather be on the 7 cell, because I would be more likely to survive the landing.



So if it's *just windy* with clean air you prefer higher loadings. But if *just turbulent* you'd take a 7 cell?

But...in all reality doesn't turbulence come with high wind a majority of the time? What would you pick with very high wind AND turbulence???

Note to self:Wow turbulence is such a strange word.


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>which in turn causes a higher ram air pressure, there fore a more
>rigid wing, to better counter turbulence.

The "rigid wing that slices through turbulence" is largely a myth, akin to the "if you're going fast enough when you hit a pothole you won't feel it" myth. What stabilizes a canopy is the force of lift working against the stabilizing effect of the lines. That is several orders of magnitude greater than the stabilizing force of any pressure within the canopy.



I think you've assumed something. I did not state that the wing "slices through turbulence better" simply because it's loaded. What DOES occur is there is an improvement to how a canopy responds to the effects of turbulence at higher loadings.

Case in point. The SAME canopy at a 1.1 lading and a 1.6 loading, or to put it another way me flying a 330 square foot tandem with 350lbs or 550lbs suspended weight. With no change in experience level the canopy respons to turbulence in different ways. The collapses are larger with a lighter loading, and recover slower thus producing a more violent reaction to turbulence or wind shear. A heavier loading and thus a heavier internal pressure and ram effect help keep the effects of turbulence less.

I think your interpretation has to do with the speed of the canopy. Small loaded canopies are fast, so people assume that they "slice" through the air. Aircraft flying at higher speeds are more suspect to the destructive effects of turbulence and wind shear, but if you increase the gross weight they are less susceptible. This is a problem for aircraft in that with higher gross weights at higher speeds a pilot can increase the load factor beyond the airframe's stress points. Hence maneuvering speeds. Parachutes don't encounter this problem in flight.

A collapse is in essence a stall through a portion of the wing. So in addition to a heavier loading mitigating the effects of that stall through re-inflating faster, the increased inertial of the suspended weight helps to prevent the stall from occurring in the first place as the canopy is less likely to encounter the critical angle of attack. This is also why full flight is preferred over braked flight.
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You're not as good as you think you are. Seriously.

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Brian Germain once did an experiment that demonstrated this. He "opened" several airlocks on one of his canopies. If the "rigid pressurized wing" theory was correct, the canopy would have become less resistant to turbulence. However, when he tested it, it did not.



What was the difference in internal pressure with and without airlocks?

Derek

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>Aircraft flying at higher speeds are more suspect to the destructive
>effects of turbulence and wind shear, but if you increase the gross weight
>they are less susceptible.

That's a common belief, brought about by how turbulence _feels._

If an aircraft is flying along with a gross weight of 2000 pounds, and it experiences a sudden gust (which changes the AOA) that causes the wing to generate 6000 pounds of lift, you feel 3G's, which is pretty violent. If you load it to 3000 lbs, and you get that 6000 lb AOA-induced turbulence, then you only feel 2G's. However, the wing spar is still seeing just as much stress. You just feel less of it.

Now, if the wing spar is good only to 6000 lbs, and you see a greater gust speed than that, then the wing _may_ fail. This depends on speed. If the AOA change is sufficient to cause the wing to stall (as happens at lower speeds) then no further lift is generated and the wing survives. If the AOA change is sufficient to generate 8000 lbs, and the airspeed is high enough that the wing does NOT stall, then it may fail.

>A collapse is in essence a stall through a portion of the wing.

I disagree! A stall is the separation of the boundary layer of air on the top of the wing which reduces lift a bit and GREATLY increases drag.

Let's take a light aircraft example again. It's flying along at an angle-of-attack of 5 degrees. A gust comes along and changes that AOA to -2 degrees (i.e. nose down in relation to wind.) The wing may now generate negative lift. It is NOT stalled - indeed, it is doing just what it was designed to do. It feels like you've been slammed down, though, because you weren't expecting it.

On a parachute, there is zero resistance to downward pressure (beyond pressurization, which I contend is a very secondary effect compared to line tension.) If you get a gust that changes the AOA on your parachute to negative (i.e. a gust from _above_) then your parachute will collapse, and there's nothing you can do about it. The wing is not stalled, it's just seeing air coming from a direction that it can't handle and remain deployed.

>the increased inertial of the suspended weight helps to prevent the stall . . .

In the above case, weight doesn't matter. You will start falling at 32 fps^2 until you change the AOA back to positive and the wing can reinflate and start flying again. A heavier jumper will not fall faster than that, because you're nowhere near terminal.

Likewise, higher loadings do not mean more force on the lines. A typical canopy has 48ish lines connecting the jumper to the canopy; the weight is divided over all these lines. Going to a smaller canopy does not change that, and does not change the force on any one of those lines (barring design/trim changes, of course.)

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>So if it's *just windy* with clean air you prefer higher loadings.
>But if *just turbulent* you'd take a 7 cell?

Yes. Let's take a few more examples:

Windy and a little bumpy, tight landing area? I'd go with a higher loading over a lower loading.

Very bumpy with not much wind? A larger square. A Triathalon might be a good choice.

Incredibly turbulent over a huge landing area? A big round (30') might be the safest possible canopy to jump.

>But...in all reality doesn't turbulence come with high wind a majority of
>the time? What would you pick with very high wind AND turbulence???

There's the rub! In general I'd pick a stable canopy sized so I could get some penetration. On the Bangkok demo, which was into a small area and turned out to be quite windy/bumpy, I took a Silhouette 170, and it worked out well. It's a quite stable canopy and still gave me enough penetration that I could fly a halfway normal pattern.

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While it is true that turbulent air penetration speed in an airplane is reduced as the weight is reduced, I believe that the dynamics and aeroelasticity of fixed, rigid wings are so different to those of ram-air canopies that any comparison is quite meaningless.
...

The only sure way to survive a canopy collision is not to have one.

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One of the advantages of the tandem FX, however, is NOT the higher speed or the swoop-ability of it. IMHO, it is the almost disgusting amount of low end lift/flare these FX canopies have, by design. They can easily be brought in at 1/2 brakes or deeper, and still have plenty of flare to shut it down. We all know the stall/surge approach a lot of people use to get a decent flare on the old SET canopies. Heck even the Sigma tandem canopies (which I think have a GREAT flare to them) won't have as good a flare/shutdown-ability if put in a low speed/braked approach situation. This is a huge advantage to be able to shut them down with a minimum of forward speed (eliminating the lazy legged student to do the scary heelplant-toe-knee-shoulder roll resulting in embarrassing video footage and a broken student ankle, or worse...). Don't forget, of course, that as a first time student, they are already flipping out over the whole thing. Speeding the damned thing up toward the ground at the very end is only going to scare the hell out of them more, and keep them from performing their job properly.

Personally I'd rather have a canopy which can get back from a long spot well, and plop myself and the student down after a nice, smooth shutdown. I think that this is a great project, and while the canopies are not for everyone, they will be perfect and maybe even safer in the right hands.
Tan-I, AFF-I, S&TA, Freefall Photographer, Skydive University Coach

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Seems that history does not back up the smaller is better in turbulence philosophy. That's not to say people didn't get hurt in turbulence periodically in the past. But people weren't dying in the numbers they are now under canopy, in ANY fashion, as they are now.

I see this as one more way to 'justify' high wing loadings rather than calling it what it is.
"Any language where the unassuming word fly signifies an annoying insect, a means of travel, and a critical part of a gentleman's apparel is clearly asking to be mangled."

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I'm not pointing to any one incident. But it seems turbulence has become more of a discussion factor in them. I'm just saying that since smaller canopies have become popular a growing number of people seem to think lightly loaded canopies are just flat out dangerous. The past doesn't show this to be true. People were NOT dying of collapsed parachutes 20 years ago when an 'extreme' wing-loading was what novices seem to be jumping today.

And I'm using the term 'smaller' as a highly-loaded canopy.
"Any language where the unassuming word fly signifies an annoying insect, a means of travel, and a critical part of a gentleman's apparel is clearly asking to be mangled."

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I'm not experienced, so I'm asking wise more experienced jumpers:
why did I feel so uncomfortable on my 170 zp Cayenee in windy turbulent day, and much better on the next jump on Sa2 150?
My thought will be higher speed (WL) on similar canopy, but ... I can be wrong.
Regs
JanuszPS
Edit:
I had about 130 jumps that time.
Back to Poland... back home.

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