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morris

No stabilizers(JVX)=less drag???

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Hi again,
it has been said that the fact that the JVX has no stabilizers would reduce drag. Even though that seems to make sense, I dare to doubt it, I even think the opposite might be possible (but I don`t know for sure). Reason: Winglets help to reduce the induced drag! A stabilizers might be kind of a winglet. Any thoughts on that?
Morris

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Sorry Alex, I think you`r not right. Most (but not all!)winglets point upwards, that`s correct. But the reason for that is just that they want the wingtips to have as much clearance from the ground as possible... the principle is working both ways!
Morris

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Winglets and stabilizers are different things that work on a different principle. Stabilizers are there to help keep the high pressure air underneath the wing from spilling over to the low pressure air on top of the wing. This is supposed to reduce the wing tip vorticies and thus the resultant induced drag that comes with them. The problem is that stabilizers have to be pretty big to actually help, and they add parasite drag as they get bigger. By the time they are big enough to significantly affect the induced drag the parasite drag has increased enough to offset the benefit.

Winglets are actually small airfoils placed at the ends of the wings. It's hard to explain without a good diagram, which I don't have available, but basically, these small airfoils are placed at an angle of incidence such that the lift they create is vectored slightly forward and thus offsets the induced drag. That may not make sense or may sound like BS - it took me a while before I believed it. I do know that the winglets you see on modern airliners actually work pretty well.

There is a book out there called 'Illustrated Aerodynamics' by Skip Smith. It does a pretty good job of explaining things.

I believe that stabilizers on small HP canopies don't help much, so removing them is a benefit.


"Holy s*** that was f***in' cold!"

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this is just from my experience so it's not really the most technical info.

removing the stablizers definitely reduces drag just because there is less material being exposed to the air. however, i do think there is a trade off in performance by a reduction in bottom end lift. i feel i have more bottom end lift in my velo then i did in my JVX. again, this is just my perception now that i have a few jumps under my velo.

also, i don't think there is as much a performance increase, if any at all, as many people think when it comes to the JVX and other canopies out there. i say this for a couple of reasons, well actually some perceptions:

1. I've flown my old JVX 87 against/with pilots under velocity 90's at the same WLings, both with the blue HMA lines and the HMA lines that PD has put on. and flying next to each of them in full flight there was minimal to no difference when it came to forward speeds or decent speeds. i know this isn't the only indicator in performance, however, it's pretty interesting that in this flight mode they fly nearly the same.

2. i have noticed that i am going just as far and just as fast under my velo than with my JVX. HOWEVER, i am going these same distances and speeds more consistantly with the velocity, which i find pretty interesting.

in the end it just depends on what the pilot experiences. and as far as stablizers being more/less efficient/benificial, well that i can't really measure. i can just give you my opinion, which like everyone else's, doesn't really matter.
Slip Stream Air Sports
Do not go softly, do not go quietly, never back down


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My JVX 87 doesn't just fly bigger than a Velo 90, it actually is bigger if you lay it on the ground. I think you have to compare something like an 83-84 JVX to a Velo 90 in order to actually compare similar square footages. This means the wingloading actually has to be higher on the JVX (based on placarded size) in order to be making an even comparison - this is just my observation based on flying the canopies and on laying them out on the ground.

Even beyond that, I think the JVX likes even more wingloading. It really starts to come alive at higher wingloadings. Even with a smaller placarded size, my JVX 83 likes having a higher suspended weight then my Velo 90. The Velo seems to peter out around 2.5 - the JVX is still going strong at 2.8.

If you fly the same sized JVX as a Velo, you will probably find the JVX to be slower, less responsive, and harder to swoop with because of the shorter recovery arc.


"Holy s*** that was f***in' cold!"

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i know this isn't the only indicator in performance, however, it's pretty interesting that in this flight mode they fly nearly the same.



just like with cars, at 30mph (ie full flight) drag is a non issue but at 90mph it comes into play. since drag is proportional to the square of speed. I am no pro swooper but any extra speed you can get at the gates will make a difference. I am not sure about the stabilizers but I do know that when I put MEL's blue HMA on my VX it made a big difference. you can bet PDs next HP product will not have stabilizers....

rm

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My JVX 87 doesn't just fly bigger than a Velo 90, it actually is bigger if you lay it on the ground



so does that mean someone will be loading a jvx 83 the same as a velo 90 because they are measured to the same size? which would mean that all this hype about the jvx likes to be loaded "x" amount and the velo likes to be at "xx" amount isn't an accurate representation of what it really is?

also, it's impossible to get an accurate comparison when just laying 2 canopies on top of one another, they need to be inflated to see what their real sizes are. but i know you already know that ;)
Slip Stream Air Sports
Do not go softly, do not go quietly, never back down


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just like with cars, at 30mph (ie full flight) drag is a non issue but at 90mph it comes into play. since drag is proportional to the square of speed.



i wonder if that's why i'm going just as far and fast, but more consistantly? ;) (doubtful)

i'm definitely not going 665' let alone 678' though.
Slip Stream Air Sports
Do not go softly, do not go quietly, never back down


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Hi "superstu",
You`r saying "I feel I have more bottom end lift in my velo then I did in my JVX." This matches with what Jyro(!) told me at the WorldParachutingChampionships in Germany. He said that HAVING stabilizers would help/be useful at the bottom end of the swoop!
Morris

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One more for Superstu,
the induced drag is a function of the inverse(!) of the square of the airspeed - it is smallest at high speeds and increases as you slow down. This explains why stabilizers make more and more sense as you slow down, at the bottom end of the swoop, where you "feel" (good perception!) that the velo has more bottom end lift.

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The JVX is not the 1st to take off stabilizers.
The Xaos also has no stabilizers, Read about there StabilRib Technology on there web page.

I think with small HP canopys heavily loaded, the stabilizers don't do much more then flap in the wind, while with bigger bulkier parachutes you can see them doing there job helping to keep the high air pressure under the canopy.

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And this one goes to "polarbaer",
you`r right with what you`r saying about the difference between stabilizers and "modern winglets", but just kind of...
The way a modern winglet reduces induced drag with "vectored forward lift" is of course way more advanced. But that doesn´t mean they aren´t both doing the same, they do, a winglet is just more effective, but both (stabilizers and winglets) reduce the induced drag by reducing wingtip vorticies...

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it has been said that the fact that the JVX has no stabilizers would reduce drag.



Had Jim Slaton not been banned from this site then he would have explained to you by now that they did many tests with pressurised stabalisers and No sabalisers. and found the no stabaliser configuration to produce less drag and more performance. If you do a search it was all well explained on here some time ago.

stablisers are for the opening sequence. one the parachute is open they simply produce Drag.
"When the power of love overcomes the love of power, then the world will see peace." - 'Jimi' Hendrix

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they did many tests with pressurised stabalisers and No sabalisers. and found the no stabaliser configuration to produce less drag and more performance




Was this in a wind tunnel? I would think that you would need a constant like a wind tunnel to accurately test for what amounts to very slight gains.

Especially in this case, where the stabs may be producing some favorable effects. The key question is if the advantage they create is enough to offset the increase in drag.

I know that in aircraft design, advantages of 1% or 2% in efficiency or drag reduction are enough to produce a measurable gain in performance or economy, and designers will put considerable effort into gains on that scale.

This is where more sturctured test conditions would be needed to verify the advanatges or disadvanatges.

I just can't accept that test jumping is an exact enough science to verify certain claims.

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Well...I wouldn't call it hype, but it is a fact that if you are comparing a JVX to Velo, you are comparing two different canopies made by different companies and measured by different methods.

What you said is true, you have to measure the size when inflated...but I would bet that the JVX comes out bigger than the Velo. What really matters is that you load each canopy appropriately. If you compare a Velo and a JVX, of equal size, loaded with the same suspended weight, I can almost gaurantee you will like the Velo better. It will be faster, more responsive, and have a longer recovery arc, making it easier to swoop with. You HAVE to load up the JVX to a higher wingloading to see the benefit.


"Holy s*** that was f***in' cold!"

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The way I have been thinking about stabilizers on HP canopies is as follows...

The stabilizer is there to help keep high pressure air underneath the wing from bleeding over to the the low pressure air on top of the wing. That means that if it is working, the stabilizer should be 'bulged out'...since the stabilizer isn't rigid, the high pressure air underneath should push the stabilizer outwards. This should be visible.

I've spent some time watching HP canopies, both in first person and in third person, and what I see is that in most flight modes, the stabilizers aren't bulging out...they're just flapping in the breeze. This indictaes to me that stabilizers are not containing the high pressure air underneath the wing and thus are not helping.

The only time I see the stabilizers bulge out is when you are deep in the toggles, like at the end of your flare. This would reinforce that stabilizers make the biggest difference at the end of the swoop, when you are deep in toggles.


"Holy s*** that was f***in' cold!"

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This is where more sturctured test conditions would be needed to verify the advanatges or disadvanatges.



longer and faster swoops by the same person in the same conditions is enough for me!

dunno about you though?

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

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