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Foil Kite folding in half when going upwind!?

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Nash556
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Foil Kite folding in half when going upwind!?

Postby Nash556 » Fri Jul 06, 2018 6:53 pm

Hello.

Please help me.

My 12m Liquidforce Elite has a very nasty habit of folding in half and falling out the sky when I'm pointing upwind in powered up conditions. It totally collapses in the middle and folds in on itself. It happened today resulting in me being rescued. I've checked the knots where the bridle joins the mixer. They are inline in accordance to the manual. It only seems to happen in 18knts plus with medium depower. Lines are 18m.

Are there any experts out there who can help diagnose what might being causing this?

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Re: Foil Kite folding in half when going upwind!?

Postby Horst Sergio » Fri Jul 06, 2018 8:05 pm

Hi,
who is your reseller? Does he has any experience with triming foil kites? If yes, this normaly is your first contact.

Next easiest way make a clean photo of the whole mixer and pully system and somebody here may tell you the most basic way to trimm it towards better stability.

For more precise help, would be good to see a video of what happens, or even better an really experienced foilkiter to try it.

You're not the first with questions about Elite. Thats a bit the problem with the label bunnys, as liquid force, if something goes wrong, nobody feels responsable and can help.

(seen you're not in North America, so too far to send it to designer Ben from ConceptAir)

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Re: Foil Kite folding in half when going upwind!?

Postby foilholio » Sat Jul 07, 2018 4:18 am

Extend Z ~10cm.

Kite is front stalling and collapsing. You can also use the bar to prevent it. Just about any foil kite can do it given the right conditions, gusty direction changing etc. By extending Z you are altering the kite to be less likely to do it. It's no free lunch you'll loose power with Z longer. Play with the Z length until you are happy. You can often vary Z +-10cm with different conditions.

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Re: Foil Kite folding in half when going upwind!?

Postby Nash556 » Sat Jul 07, 2018 11:01 am

Hey guys thank you so much for your replies. It sounds like the front stall and collapse. See attached image, a,b,c & d knots are all aligned. Do you recommend putting a 4cm spacer between the mixer knot (black) and the D bridle (blue)
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153095762179821703744.jpg

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Re: Foil Kite folding in half when going upwind!?

Postby foilholio » Sat Jul 07, 2018 3:39 pm

Yep D=Z

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Re: Foil Kite folding in half when going upwind!?

Postby Nash556 » Sun Jul 08, 2018 7:46 pm

Awesome thanks for that....

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Re: Foil Kite folding in half when going upwind!?

Postby kitexpert » Sun Jul 08, 2018 8:41 pm

foilholio wrote:
Sat Jul 07, 2018 4:18 am
Extend Z ~10cm.
Kite can very well collapse no matter how loose Z is. Because main reason for front collapsing is too low AoA for the wing specifications, AoA or wing (airfoil) shape should be adjusted. Extending Z is a weird advice, there is a bit sense in it but not very much.
foilholio wrote:
Sat Jul 07, 2018 4:18 am
Just about any foil kite can do it given the right conditions, gusty direction changing etc. By extending Z you are altering the kite to be less likely to do it. It's no free lunch you'll loose power with Z longer. Play with the Z length until you are happy. You can often vary Z +-10cm with different conditions.
Not all foil kites collapse, some of them ever not even once 8) I know how to do kite like that and it has nothing to do with loose Z.

For OP I suggest to stretch all A bridle lines, and of course before that to check flying line lenghts and the mixer.

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Re: Foil Kite folding in half when going upwind!?

Postby surfer » Sun Jul 08, 2018 10:16 pm

I am with kitexpert on fix
Had the same problem with FS Speed 12. Kite has not been used off close to one year, bridle shrank.
Stretch A or shortened B only. Then fly kite for few sessions, kite should be stable right away and no more frontal colapse. Then try to rectify mixer again after few strong wind sessions

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Re: Foil Kite folding in half when going upwind!?

Postby foilholio » Mon Jul 09, 2018 1:04 am

Well then you both don't know what you are talking about.

It's 100% likely dealing with an old kite that B too needs to be lengthened. You do not fix something correctly that has shrunk by further shrinking (shortening) it. The fix you experienced by shortening B, is because it will partially correct the profile or mainly B engagement time, but the side effect is you have now reduced the AB limit and with that you have lost range the component parts being depower and lowend.

The primary bridle that experiences the most shrink of any bridle is Z. If you were to restretch one bridle it should be Z. You can restretch all the bridles and the kite will be close to level. Often though adjustments at the mixer or the kite bridles will last longer, or be easier to implement.

You can make a kite more stable by reducing B, but the bar(&trimmer) should be the way you do this. The kite designer has spent much effort finding the correct limit for B which is usually slack on modern kites, i.e. kites fly off A and not AandB at full depower. Reducing it will just impose arbitrary limits that are not needed.

How can you reduce B without reducing it? I.e. change the bridles so B engages more, but the AB limit stays the same or even increase the AB limit which is often needed but then reduce B in the profile by lengthening it?? Well simple you lengthen Z and C. Often though Z is only needed but C can be a help. Remember Z is 4x the length of B in the standard 421 mixer. So if you altered B 2cm and it fixed things, you would need to lengthen Z 8cm ( and C 4cm being 2x B) while keeping B the same. B does not stay the same when you alter a flysurfer mixer, which actually makes their mixers a bit annoying to adjust when fine tuning.
kitexpert wrote: Kite can very well collapse no matter how loose Z is. Because main reason for front collapsing is too low AoA for the wing specifications, AoA or wing (airfoil) shape should be adjusted. Extending Z is a weird advice, there is a bit sense in it but not very much.
Yes AoA is what causes collapsing. Camber affects AoA. Z affects camber. Situation in foil kites exist where bar is pulled in but AoA is reduced.
kitexpert wrote: Not all foil kites collapse, some of them ever not even once 8) I know how to do kite like that and it has nothing to do with loose Z.
All foil kites collapse. Wind can shift 90, even 180 degrees, kite can very easily find it's AoA temporarily negative, or itself way out of the window.
kitexpert wrote:For OP I suggest to stretch all A bridle lines
This is silly advice as A is rarely shrunk much. Z is the main problem he should stretch that, or just lengthen it. Ask any paraglider which bridle they always stretch, they will say the brakes (Z).

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Re: Foil Kite folding in half when going upwind!?

Postby evan » Mon Jul 09, 2018 7:04 am

This time I am with foilholio, old kite with too much and long bridles with high AR are sensitive to bridle shrinkage. As C and Z shrink the most you get a kite with increased camber that has lost its reflex profile to automatically levels the kite when flying maximum depowered.
With too much camber you allow the kite to fly on a lower angle of attack than what it can handle and collapsing the leading edge when depowering.

I admit that it feels counter intuitive to lengthen or stretch the back of the kite when experiencing front stall. It is the same as tip instability, that also happens by a too high camber in the the tips whith a too tight Z.


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