Tip curling - luffing or arc and cell pressure

[Herman]

As I am slowly getting more into tuning foil kites it seems to me that there are at least two distinct types of tip curl. I have often seen luffing discussed and obviously if half the kite starts to luff then a tip is bound to go out of shape. However on my kites the more prevalent tip curl seems to be the mechanical failure of the canopy to support the sideways pull of the bridle. This effect is most easily seen in flat high aspect kites where the angle of bridle pull will be greatest. The tips curl inwards but still keep flying, the tip uncurls as line tension is reduced by sheeting out. The LE does not fold.

Perhaps a tighter arc would possibly reduce the sideways pull on the outer cells and help keep an old porous kite flying. Before I experiment with an old very porous Speed 12m I would be interested in any comments on the above hypothesis.

[edt]

This is exactly the stuff I want to know too

[Jyoder]

I thought my old 2016 Diablo v1 11m was going to need serious bridle adjustment as the tips were folding whenever I sheeted in, even at 12 o’clock. My many times rebuilt mixer initially seemed correct with long and short mixer tests, but I realized my C pulley line was too long after comparing to a similar kite. After shortening the C-pulley line, and of course lengthening B to compensate, it flies perfectly again.

Bottom Wingtip will fold a bit on even correctly tuned race kites if wind lulls enough when kite is at lateral edge of window, right?

[jakemoore]

Porosity and especially air leaking from seams is definitely an issue. You can see low air pressure in the leading edge when the kite flies. The ultimate answer for this internal pressure problem is probably a Loco4Viento open cell mod.

A powered up kite should have more internal pressure and lift pushing the wings out unless it is stalling backwards.

The first thing I would try based on your description is to extend the outermost Z riser a few cm with a pigtail.

[edt]

I actually went to the trouble of measuring every single line of my 12m soul, then putting everything into a spread sheet and then when I looked at it I'm like "everything is WAAAAaay off" so now I'm trying to figure out how to make it fly better by learing which adjustments do what things to my kite.

It does help tho, just measure everything then put it in a spreadsheet, that will tell you where it's off. Make sure you get the line plan before you waste a day measuring your kite lines, some kites don't have line plans.



flysurfer has great customer support all the line plans of every kite are there i'm pretty sure

[Herman]

Thanks everybody for your thoughts. It has allowed me to revise my thinking. Jakemoore’s observation that, it is lift as well as internal pressure keeping the wing tips out, gave me a bit of a eureka moment in the way I will be thinking about what I regard as the two types of tip curling. The two types of tip curl are simply luffing or stalling (backstall in kite terms). For my kites it is the more docile back stall curl that is prevelant.

Thinking about it I had a perfect example yesterday when trying out a 15m Pansh Aurora3ul for the first time in a nice steady 6-8knots in a bug. When I sheeted in hard to turn the kite on the side of the window it would turn ok but if I kept it sheeted in the bottom tip would curl; obviously the bottom tip was stalling and the loss of lift resulted in the curl. I could do this predictably, preventing or moderating the curl with sheeting. This was definitely lift dependant as the kite was very airtight when packing down. ( The stall was after the kite rotated and before I let it fly across the window, the new bottom tip would curl after the rotation if I kept the sheeting fully in. All very progressive and gentle with no loss of shape other than the curl ie no tucking of the LE.)

I am sure porosity/cell pressure is a factor in how much loss of lift can be tolerated before curl sets in. My porous 12m speed flaps like a bird and is probably well beyond a touch of Z. I also think joyriders points regarding race kites support the notion that flatter kites are more susceptible to tip curling, ( unless they have infinitely long bridles, ie attachments in line with the lift vector). Good to be on the same page as edt. Thanks again for the inputs.

[Jyoder]

Weight of the kite is a factor. At the edge of the window, the top half or even 1 third of the kite is supporting most of the kite weight and the bottom is hardly loaded. If the kite is heavy, it stalls quicker and slips sideways toward the ground. The bottom tip is lighter and unloaded, and can create a small amount of lift even when inverted/curling, exacerbating the folding.

I didn’t like the 12m Soul in light wind, even though it has very high peak power for water start, because it is heavy and if it stalled at the edge of window the bottom tip would curl as the weight of the kite slide down and kite would taco in half and collapse quicker than lighter kites I’m used to where I have more time to gently backstall the upper half back into the window and open the tip curl. The Soul doesn’t backstall easily and that actually makes saving a tip fold more difficult. My 11m and even 18m Diablo backstall very nimbly and I can recover them 100% of the time when tip folds.

My 6m Soul folds the bottom 1/3 when I slack the lines aggressively, but it is so tiny it is easy enough to backstall turn it to the other direction and open it up while riding. Even if it folds completely in half, a few tugs on center lines opens it up.

[Herman]

Oops duplicate.

[Herman]

I have, hopefully, clarified the description of the stall of the Aurora in my above post. See above. This is not occurring at the front edge of the window. The loss of lift in this case is caused by stalling of the airflow across the wing as the canopy has too much AoA at the tip which results in a loss of lift to support the tip, and hence the curl of the tip.

Conversely, the sudden inversion or tucking of a tip at the the front edge of the window is caused by too low of an AoA in the LE and the LE luffing. I say AoA of LE because the canopy can have plenty of AoA but too much camber which allows the LE to luff. Hence the two types of tip curl. All imho etc.


PS Tricky to explain in just a few words!

[edt]

I'm slowly getting rid of all my panshes and replacing them with flysurfers. On my old 19m pansh aurora it didnt fly right so I added some adjusters behind the pulleys. After flying it about 100 times I finally figured out to move the right B mixer in about 2cm. Now it flies good, but I wish I knew more about foils so I didn't have to go through so much grinding and trial and error. I've also made a 10m control bar just for tuning foils as it takes less time to make a change put the kite in the air, test and bring it down. All these guys with expert knowledge of foil kites and they keep their secrets about how to tune them so close to the vest. Share your knowledge guys!