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Flysurfer back stall

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foilholio
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Re: Flysurfer back stall

Postby foilholio » Wed Nov 09, 2016 12:43 am

axel_lotta wrote:
Mossy 757 wrote:
Jzh_perth wrote:You are certain it's related to the canopy profile and not the flying lines ? Have you got line plans ? You could measure everything...
The whole bridle shrinks on these kites very quickly, so taking individual line measurements and comparing them to the line plan will probably be more confusing than anything; the line plan is almost certainly inaccurate by the time the kite ships from the manufacturer to the retailer, just knowing how quickly Dyneema can shrink. I think you can find problems in the canopy bridles by making comparisons left side versus right side trying to find irregularities. That, of course, doesn't really account for back stall problems, but it's less frustrating than measuring from a line plan and realizing that literally ZERO of your bridle lines are the same length Flysurfer says they should be.
Having gone down the path of painstakingly measuring the entire bridle, I can 100% agree with this statement. The company who makes the lines (LIROS) claims a maximum shrinkage of 1-2% but this is complete BS. The maximum shrinkage I have measured on the small white brake lines was up to 9.5% (in real terms that equates to over 50mm on a half meter line) and that is only for the very upper bridle. I had to add 60mm to just the Z main and Z main extension to bring it up to the stated line plan... That said, my kite also has backstall problems with a 100% correct (as per the line plan) bridle, and that is just apparently a feature of these kites due to the aspect ratio. I was flying it yesterday with a very experienced landkiter who always has his trim strap depowered about a third as a baseline. After all of the stuffing about that I have done getting everything perfect, it appeared the way to get my kite flying nicely was just to reduce the length of the front lines (by shortening the black rope between the trim strap and small alloy block) by about 3 inches (75mm). I also added about 3 inches to the safety line with an small extension. This was done because when fully powered up, this line was as tight as the normal front flying line beside it (this won't cause back stalling but is nice to have right anyway).

My advice is to spend the time to understand how the bridle lengths actually work in shaping the wing and what changing the lengths is actually doing as far as performance (giving the wing more or less camber etc). There is a Facebook tool floating about somewhere from Flysurfer that explains the mixer adjustment quite well. In short, flattening the wing (less camber) will cause a stall earlier in low wind, but makes for a much more stable (if less powerful) wing and will allow for better upwind ability due do the reduced drag. Increasing the camber will increase the low wind performance (to a point, too much will introduce massive drag which will cause stalling), but will lead to a very grunty and unstable kite at a higher windspeed. Also due to the increased drag, it will sit lower in the wind window and reduce you ability to go upwind.

One last point on this that not too many people are aware is that these kites actually have what is known as a 'reflex camber'. That is by their design, the shape of the airfoil is such that the about 3/4 of the profile from the nose back acts like a standard wing giving lift, whilst the last 1/4 to the trailing edge acts in the opposite direction effectively pushing the trailing edge downwards. This is why, you can sometimes get very good results by moving B towards to kite (increasing the camber and lift) whilst leaving C where it is allowing an amount of reflex camber to remain, effectively stopping the kite from wanting to tuck it's leading edge under itself.

I hope my comments have in some way helped, rather than added to the confusion..

Cheers,

Axel.
Very well said Axel.

I will add a few things 10 to 20% shrink can be found quite commonly, just look at pulley lines.

Reflex does not need to start at the 3/4 TE or even the TE, you can have it on the LE section or anywhere ! B is interesting in that it creates reflex when it is pulled tight. The wings work well when B is allowed to tension quite a bit before C or Z engage. B is much more effective at stabilizing the wing than C or Z. The main reason altering the camber has such effects on stability is they change the moment of torque or pitching moment on the wing. The wing can be neutral or wanting to flip forward or flip back. When you have high camber the wing flips forward and you will observe this as a front collapse. It can be at the center or on the tips or the whole wing. The pitching moment forward (negative) actually also reduces the AoA, so the wing is more likely to fly out of the window! It creates a funny effect where you can sheet a kite in causing more tension onto the rears but then the kite flys forward in the window and maybe even collapses!

The interesting thing about foils is they can switch from say a distorted camber with B being pulled a bit too much which is stable, to a high lift high camber curve. Avoiding the unstableness of the High camber and then the low lift and easy stall of the distorted(reflex) camber.

Also I would avoid looking at airfoils in any traditional sense as that is wrong. The angles of the surfaces and how they deflect air is whats important. Any airfoil that produces lift at say 0 AoA is just confusing with it's shape. The airfoil is not at 0 AoA, if you look carefully you will see that the LE has been altered to make it look like it is at 0 AoA, if you were to shape the LE back to symmetrical you would see the rest of the airfoil was at something like 10-20 degrees AoA! Oh and the span(shape) of the wing is important too, i.e. 2D views are very incomplete!

windrider1 wrote:ABCZ all equal does not really work sometimes instead try A and Z a little longer than B and C.
I am not trying to be rude to you windrider but this is wrong. What you suggest reduces camber ,doing this will create more stall. He will want to do the opposite and lengthen C and maybe B. Which will increase camber.

Stall is the point at which LIFT drops while AOA increases. More camber has both more LIFT and will continue to produce lift till a higher AOA than a lower camber.

Now more camber also has worse LD, so you need to make a compromise. With a mixer and things like WACs or Diablo lines you can have a bit of both.

Altering a foil to have such high camber that the lift almost doubles is quite worth trying just to feel the difference!


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