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Effect of split point on bar trim

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edt
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Re: Effect of split point on bar trim

Postby edt » Tue Jan 17, 2017 5:42 pm

the difference can be pretty significant especially on larger kites so it's worth remembering to adjust your trim! On a 20 meter kite I calculate it's 2 inches of trim or 5cm you will definitely notice it. I never change the trim on my bars to fix the problems. I keep all my control bars equal line lengths that makes it much easier to tune them, and add pigtails as necessary to the kite itself.

Of course better than calculating is just go out there and feel how you want to adjust it that's better than calculations. I am providing the below diagram for fun and because Pythagoras is cool not meant to be definitive (the amount of pull the placement of the split and width of the kite all need to be measured and then plugged in and recalculated).

It's a skill any intermediate kiter should know. To be able to figure out quickly when a kite is well balanced. You should be able to put a kite into the sky and within a few seconds know exactly how many inches or cm of pigtails need to be added or subtracted to get the kite to fly how you want, but that's another topic.

Image
Last edited by edt on Tue Jan 17, 2017 10:29 pm, edited 1 time in total.

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Re: Effect of split point on bar trim

Postby kitexpert » Tue Jan 17, 2017 6:21 pm

The "problem" with different setups is there if line lenghts are measured by same method for low and high Y's.

Backlines are always straight, so these should define the nominal line lenght. In that respect I see Sandras' calculations a bit misleading, even though differencies between the lenghts of different Y's are apparently correct.

Frontlines are in practice straight if Y is very low. Then normal line check method is always valid (trimmer out, bar fully sheeted in = all lines are equal lenght). But if Y is high and same method is used, it gives different trim (when adjusted all lines equal) because now front lines are not straight when connected to kite, but go via Y route. Then frontlines "are shorter" compared to backlines, because they have that bend.

This difference is of course countered by the bridle of the kite for which high Y is originally designed for. So normal line lenght check method works still, but only for that kite and that bar combination, or similar high Y designed kite. This "design" work is just simple bridle line lenght paring to high Y bar.

So, there will be trim problems if high Y and low Y bars are cross used.

But like Peter Frank wrote it is usually quite easy to correct these normally not so big differencies when tuning a kite. There is nothing strange here, but some less experienced kiters may become puzzled why different bars - "correctly" checked - will work differently if cross used.

Pinching is very theoretical problem. LEI's have very low projected AR's, meaning wing span is small. Then line attachment points are close to each other, and there is also some stiffness in LE strut. So these kites tolerate very well high Y's, because "pinching angles" are low even then.

However low Y is universal and it has some other advantages, so there is no reason not to use it. It works of course in high Y kites, but then trim must be corrected by ....... backlines. (better leave this unanswered :wink: )

Big high AR race foil kites which can have nearly 10m wingspan and not much canopy curve will not tolerate very high Y's, or at least as low as possible Y is better.

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Re: Effect of split point on bar trim

Postby edt » Tue Jan 17, 2017 7:19 pm

pinching is a little bit uncommon on tube kites but a very real problem. I have run across at least 3 or 4 kites which did not work correctly on a high Y or vice versa on a low V, the kite couldn't correctly breath or would be too open on the wrong sort of bar. You don't see this on smaller kites 7m or less because the tubes are able to completely control the structure of the kite but on larger kites say 15m and up it is a very real problem and something to be aware of. I wish I could list all the exact models I have had this problem on but my memory isn't what it used to be. If you aren't aware of the pinching or if you only use small kites you will never run across it but if you keep an eye out for the pinching you will notice it. The effect is usually that you feel like the kite is underpowered feels choked no matter where you set the trim, or doesn't loop right, doesn't climb back up to the window properly after a loop, stuff like that. So if you haven't come across it yet just keep it in mind, you'll eventually notice it on a kite. On a 20 meter kite you will have about 4 pounds of pinching on a high Y vs about 2 pounds on a low V, so blow up a big kite, put it on its side, get a 4 pound weight and put it on the wing tip, just take a look and see if the kite deforms or not. If it does you might have a pinching problem on a high Y.

I haven't run a Y on any of my high aspect foils but it sounds reasonable that a high Y could be a problem there.

The high Y is sort of dying out, as bridled kites are all moving to single line flagging so it's left to tensioned 5th line kites like the rebel or torch and race bars seem to be using a slack 5th with a low V.

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Re: Effect of split point on bar trim

Postby Peter_Frank » Tue Jan 17, 2017 9:04 pm

edt wrote:Snip...
It's a skill any intermediate kiter should know. To be able to figure out quickly when a kite is well balanced. You should be able to put a kite into the sky and within a few seconds know exactly how many inches or cm of pigtails need to be added or subtracted to get the kite to fly how you want, but that's another topic.
Precisely, a basic skill that comes pretty fast, just like bodydragging upwind :thumb:

You can not "measure" the perfect trim - it has to be "felt" :D

8) Peter

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Re: Effect of split point on bar trim

Postby Sandras » Tue Jan 17, 2017 9:10 pm

merl wrote:
Sandras wrote:
merl wrote: EDIT: Your delta calculation is wrong. The shortening delta is not relative to La + Lb, but SQRT((La+Lb)^2 - b^2). I.e. Just use the same formula with La = 0.
I'm afraid I have to ask you to explain it more... I do not understand.
Sorry, I realise that you are calculating something else with the delta (what is it good for?). I was thinking what is the difference in front line length relative to back lines between high Y and lowest possible Y (a V). It translates to the pigtail length you need to add to the back lines to get back to the factory trim* if you move from Y to V. Your delta is something else.

* Accepting that factory trim is uninteresting if you prefer to always add your own pigtails and adjust by personal feel accordingly.
The length of the front lines is (La+Lb)
If you do the manufacturers trim. This should be equal to the length of the back lines as well.
What I calculate is how much trim you would need in order to compensate the different Lt you get by different split point heights

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Re: Effect of split point on bar trim

Postby grigorib » Tue Jan 17, 2017 11:05 pm

why are you guys discussing those? Just buy a proper bar where all lines are of equal length. It's safer too.

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Re: Effect of split point on bar trim

Postby Dan-at-Duotone » Tue Jan 17, 2017 11:45 pm

I feel like this has been hashed over many times. The OP's math looks correct to me, however I don't understand his conclusion that this somehow reflects badly on North kites...

A. We've been aware of this affect of the V height on the relative lengths of front/back lines since we started using different V heights and we've designed our kites and bars with it in mind. When you use a higher V, such as on our 5th Element, the front lines are relatively shorter vs. the backlines than if you have a lower V, assuming all lines are the same length before pulled sideways to attach to the pigtails. The kites where we were worried about this effect such as the Fuse (back in 2012) up through the new Neo have two separate knots for attaching the center lines, one marked for 5th Element, one marked for Quad.

B. This effect is not very big. According to the OP chart it can look big if you're looking at a V that splits at 21m vs one that splits at 1m, but that's just dumb. No bar splits at 21m. Our quad vs our 5th are approx 2m apart in the V height, resulting in about a 1cm change in tuning if you want to be precise (which is approx how far apart those knot choices are on the front pigtail of the Neo/Fuse/etc. for exactly this reason). This change is much less than most people's bars are out of tune after half a season.

C. As many other people have noticed, besides this obvious small trigonometric change, there are some performance differences you may notice when changing V height. This is less easily quantified, especially as different models seem to react differently to that change in V height. I have asked each of our kite designers for specific thoughts (you can probably find some of that info in previous posts of mine on this forum), but it seems that there is no simple "higher V means the kite flies more like this". As I said, it's different for each model. And since many of the changes are subjective, it's hard to get overly specific.

D. All North Kites are designed around the V height for the model's normal bar. For our 4-line friendly kites, the Evo, Dice, Neo, Mono, and Juice, the kite is designed around the V height of our Quad control 24m bar. For all of our loaded 5th line kites, Rebel and Vegas, they are designed around the 5th element V height (the testing team also uses the wakestyle bar for testing, but the extremely low V of that bar just seems to accentuate the pop/slack/steering delay that the Vegas is known for. That bar, for anyone interested, comes with the front lines shorter than the other three specifically to tune out the trigonometric issues the OP brought up.) However, I recommend you experiment with different V heights if you're curious to see what happens. Please fill us in on what you find.

E. The Click bar offers all of our different height Vs, including the super-low V of our wakestyle bar. It should make testing the different setups simple, and should allow you to match the V pretty close to any other bar on the market, in case you prefer the feeling of your kite on a different bar.

If anyone has any questions on this or anything else feel free to message me directly anytime.

Hope this helps.

-Dan

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Re: Effect of split point on bar trim

Postby kitexpert » Wed Jan 18, 2017 12:40 pm

edt wrote:On a 20 meter kite you will have about 4 pounds of pinching on a high Y vs about 2 pounds on a low V, so blow up a big kite, put it on its side, get a 4 pound weight and put it on the wing tip, just take a look and see if the kite deforms or not. If it does you might have a pinching problem on a high Y.
It is true that "problem" of pinching is bigger for big kites, because they have more tow point separation. This means big kites always have more pinch, but because it isn't real problem it is not worth calculating. Small changes on how much there is canopy curve are not so critical any way.

If Y is very high problem of course becomes real at some point, but I don't think there is that kind of bars there.

I've sometimes used 14m LEI with 10m lines, equaling quite high Y. I've also used small foil kite with 3m lines. In latter case there perhaps was some distortion (added canopy curve), but not anything very noticeable and kite was flying ok. LEI tolerated short lines well. These setups are useful for extremely high winds or teaching work, then kite control is harder to lose and if it is lost nothing very serious happen. These experiments were relatively easy with FS, which flying lines had 3m, 6m and 12m sections.

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Re: Effect of split point on bar trim

Postby evan » Wed Jan 18, 2017 1:33 pm

Interesting topic, especially for racers on big foilkites and shorter lines since the trim-differences become more noticable on the big foilkites and short lines.

Made my own spreadsheat to get a better idea and expanded it to a full bar-setup instead of only split-heigth.
But first the split heigth effect on a bigger kite:
18m foilkite split vs trim.JPG

Then the expanded calculations wich take in account also the angle of the steering lines and bar-width and display the trim-difference on a fixed-height split when you shorten or lengthen your lines:
18m foilkite line length vs trim low-V.JPG
And a higher split:
18m foilkite line length vs trim high-V.JPG

Note that these calculations are a big approximation since the length between the pigtails gets pinched at higher angles/shorter V thus counteracting the trim difference. The effect of this pinching depends a lot on the kite and load you put on the lines, also called yellyfishing as older c-kites deformed a lot when a gust hits for example. Although newer kites are a lot stiffer, the effect is still there.
Hard to take this into account in a spreadsheat unless you go almost into full simulation mode.


To get an idea of this pinching effect: if I guess the tow-points of the kite gets pinched from 4800mm to 4000mm when you go from 24m to 16m lines your trim difference of 19mm gets counteracted by 11mm so you are only left with 9mm trim difference.

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Re: Effect of split point on bar trim

Postby edt » Wed Jan 18, 2017 4:57 pm

kitexpert wrote: I've sometimes used 14m LEI with 10m lines, equaling quite high Y. I've also used small foil kite with 3m lines.
I've used a 19m fuel on 10m lines and it was perfectly fine. However I have used other kites on 24m lines with a high Y and they had the problems I listed above. I wish I could remember which they were. Also I have used kites meant for a high Y with a low V and they also had problems. But other kites are fine! Just like you said, you can have a dozen kites all work perfectly fine with high Y and low V but there are kites out there that just die if you put them on the wrong bar. For a few years I would grab everyone's kite and beg to demo it so I rode a lot of kites with a lot of control bars, now I have settled down a bit and don't really experiment as much but if you go out and try every kite you can get your hands on eventually I think you'll see the effect. It's definitely real even if you haven't come across it yet. It's the same with the center line settings on some kites, you know like how the RPM has settings for "wakestyle" and "free ride" where you can move the center lines up and down. On some kites (like the RPM) you can move those settings however you like and the kite will just adjust and still ride fine, but on other kites (notable some wainman models) if you move the center line settings in the wrong position the kite will fly terribly. The same kind of thing happens on a high Y most kites it doesn't bother (actually most kites prefer the low V because it helps them "breath") but a few kites will perform terrible if you pinch them. You can't talk to kite manufacturers about this because they never "cross dress" that is you will never see a manufacturer demo a naish kite used with a liquid force control bar they just don't do that, they only try control bars they build with kites they build, so if you want to see this you can't talk to kite manufacturers (who I admit are the experts at this kite thing, no question they know what they are doing) in this case because we are the only ones cross dressing, the amateurs, the pros never cross dress, so we have to go out in the field and try it ourselves. This is a fun thread, lots of good math in it, and I do love me some good Pythagoras.


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