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knuthansen
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Kite design questions

Postby knuthansen » Tue Jun 01, 2010 5:33 pm

Just reading some stuff on http://kitesurfingschool.org/kite.htm to get a clue about some kite design principles.. nevertheless, I got some questions for ya: :jump:

- What determines light wind performance? Apparently, high aspect kites have a higher Lift/Drag ratio and fly faster, thus generating more dynamic power. Sacrifice is less projected area. So is a flatter shape/higher projected area generally better for light wind, or is there an optimum value somewhere?

-Comparing a c-kite and a bridled, flatter kite with no pulleys, is the latter gonna have more depower (with less bar throw)? (depower=more change in angle of attack) If so, why?

-How do todays c-kites achieve more depower than the ones in the ol' days?

-Are low aspect kites better upwind? (Leaving angle of attack, leading edge diameter etc. constant)

-How does the size of the wingtips affect stability and turning speed?

- Does anyone have that document about LEI sphere theory, as mentioned here: http://kitesurfingschool.org/kite.htm#LEI Spherical Theory ? Links are broken, unfortunately.


That's all I can think of for now! :thumb:

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Re: Kite design questions

Postby knuthansen » Wed Jun 02, 2010 8:07 pm

Ok in terms of light wind performance, I found this:
Quote Peter Lynn:
"Myth Three.
That high aspect ratio equates to high performance.
Correct in theory but misleading for kites in practice. Aspect ratio (defined as span squared divided by area) is a strong determinant of induced drag, the dominant form of drag at low speeds for efficient airfoils- but kites are not efficient airfoils by any definition, so aspect ratio determined induced drag is not the major drag component for kites. It would be possible to make a square wing (A.R=1.0) that is more efficient than the highest aspect ratio high performance kite currently available- (but making it useable as a kite would be another matter)."

So does that mean the lower AR the better low end, or is there an optimum value?

Regarding bigger wingtips: They probably act like a spoiler, so that the kite twists less in the center, thus losing less power in turns. What about stability?

Another question: How do different trailing edge shapes affect flying characteristics?

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Re: Kite design questions

Postby Peter_Frank » Wed Jun 02, 2010 9:38 pm

knuthansen wrote:Just reading some stuff on http://kitesurfingschool.org/kite.htm to get a clue about some kite design principles.. nevertheless, I got some questions for ya: :jump:

- What determines light wind performance? Apparently, high aspect kites have a higher Lift/Drag ratio and fly faster, thus generating more dynamic power. Sacrifice is less projected area. So is a flatter shape/higher projected area generally better for light wind, or is there an optimum value somewhere?

-Comparing a c-kite and a bridled, flatter kite with no pulleys, is the latter gonna have more depower (with less bar throw)? (depower=more change in angle of attack) If so, why?

-How do todays c-kites achieve more depower than the ones in the ol' days?

-Are low aspect kites better upwind? (Leaving angle of attack, leading edge diameter etc. constant)

-How does the size of the wingtips affect stability and turning speed?

- Does anyone have that document about LEI sphere theory, as mentioned here: http://kitesurfingschool.org/kite.htm#LEI Spherical Theory ? Links are broken, unfortunately.


That's all I can think of for now! :thumb:
A lot of questions... Would take weeks to answer I think he hee.

But will take a few of them right away:

- What determines light wind performance? Apparently, high aspect kites have a higher Lift/Drag ratio and fly faster, thus generating more dynamic power. Sacrifice is less projected area. So is a flatter shape/higher projected area generally better for light wind, or is there an optimum value somewhere?

Some wrong conclusions here...
Light wind ability is usually not only about "dynamic" power - as noone want to "work" a kite - but instead have a kite that does the work just by raw pull.
And high AR kites definitely dont have less projected area (nor higher).
Projected area is depending on outline, meaning narrow or full tips - AND on C or flat, meaning flat SLE kites has very high projected area (no matter how high AR they have - two independant issues).
Generally no doubt at all - a flatter kite is much better for light wind, almost the flatter the better :thumb:
But it will suck regarding turning and if you have to work the kite, it becomes very inefficient :o
This is why C kites work quite well in really light wind, when worked 8)

But to get raw pull, you need a kite with an efficient low wind camber (airfoil/profile), and most often low AR kites with huge area in the center, generate a lot of static pull/power even at low speeds.
But they are not that good upwind because of lower L/D (although good Lift and hangtime).

Really not easy to say what works best for light wind, as it depends on usage AND many different design parameters.


-Comparing a c-kite and a bridled, flatter kite with no pulleys, is the latter gonna have more depower (with less bar throw)? (depower=more change in angle of attack) If so, why?

Yes, the latter is gonna have more depower, simply because of the triangle and frontlines.
Take a fully flat plate with four lines at the corners, and you have 100 % depower when the rear lines are slack, obvious, right ? (and yes - it WILL be able to give lift/power actually).
The more curved (C-like) it gets, the less depower you can obtain, as the frontlines is connected far away from the center of effort (force/lift "center") - drawing it will show the triangle that limits the depower.
With pulleys you get even more depower though, thats why they are used almost on all SLE kites :thumb:
Regarding less bar throw or not - you can not say anything about that, as it depends on the actual kite and design.
Early flat kites had big throw, never can have very little throw.
And it can not be compared to a C kite (more or less throw ?) as a C can only depower a fraction of a really flat kite.


-How do todays c-kites achieve more depower than the ones in the ol' days?

Because they are not C kites anymore in their original sense, but have sligthly swept tips usually - so a little delta like or slight bow like (swept a bit).


-Are low aspect kites better upwind? (Leaving angle of attack, leading edge diameter etc. constant)

No !
High AR kites are better upwind by far, because of higher L/D, everything else being equal (but NOT leading edge diameter kept constant - has to be LE diameter/chord ratio).


Because a kite is a relatively inefficient foil, there will be some early limits, where AR dont mean much, compared to other parameters - this is why things are not that simple :naughty:


Kindly, Peter Frank

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Re: Kite design questions

Postby knuthansen » Wed Jun 02, 2010 10:42 pm

I knew I can count on ya Peter :thumb:

Lightwind performance: My first kite was a super flat bow which was pulling like a truck at the side of the wind window. However it did not have any grunt and I always struggled to get through the lulls.
My current SLE is more grunty and turns faster, a bit higher aspect, but with light bar pressure and very little pull at 12. Definitely better low end!
That's why I thought there might be an optimum value somewhere..

Upwind: Somehow I thought c-kites are less good upwind, but I reckon that's more when using the wakestyle setting with a higher angle of attack, which reduces the size of the wind window. On the other hand, why are race kites fairly flat? I thought in course races upwind capability is crucial.

Depower: Kinda hard to imagine, I will try to draw it. Another thing I was wondering about: When choosing the more forward frontline setting, manufacturers usually say that gives more depower. What happens is that the kites sits deeper in the window. However the angle of attack with the highest lift is between 15-20 degrees, so if I pull in some depower in that setting, would I get even more lift?

Another thing that was not mentioned in the document I mentioned: How affect different trailing edge shapes the flying characteristics?

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Re: Kite design questions

Postby Peter_Frank » Thu Jun 03, 2010 7:01 am

knuthansen wrote:I knew I can count on ya Peter :thumb:

Lightwind performance: My first kite was a super flat bow which was pulling like a truck at the side of the wind window. However it did not have any grunt and I always struggled to get through the lulls.
My current SLE is more grunty and turns faster, a bit higher aspect, but with light bar pressure and very little pull at 12. Definitely better low end!
That's why I thought there might be an optimum value somewhere..

Upwind: Somehow I thought c-kites are less good upwind, but I reckon that's more when using the wakestyle setting with a higher angle of attack, which reduces the size of the wind window. On the other hand, why are race kites fairly flat? I thought in course races upwind capability is crucial.

Depower: Kinda hard to imagine, I will try to draw it. Another thing I was wondering about: When choosing the more forward frontline setting, manufacturers usually say that gives more depower. What happens is that the kites sits deeper in the window. However the angle of attack with the highest lift is between 15-20 degrees, so if I pull in some depower in that setting, would I get even more lift?

Another thing that was not mentioned in the document I mentioned: How affect different trailing edge shapes the flying characteristics?
Thanks !


Although I did not say anything about C-kites being better or worse upwind ?

A flat kite is more efficient, thus better upwind 8)
So more power for the same flat area, so better L/D and better upwind, in rough terms :thumb:

Essential part of racing, where turning and kitelooping is unimportant, as long as it still works good with power sweeps downwind - and added the higher windrange and gust control, that could determine who will end up in the top if the wind changes during the race.


Just to be sure - some mix up AR and C versus Flat kites :o
That is two totally independant issues.

A flat kite dont have higher nor lower AR than a C kite :naughty:

Aspect Ratio = flat span divided by middle chord.


Of course the flat kite has higher "projected" AR - but this is not a parameter one uses in daily terms to my knowledge (although it would be a good idea in fact, IMO) 8)


Regarding trailing edge - this is a huge issue and some facts and a LOT OF RELIGION too (= what you believe, but not necessarily factual supported).

I assume you mean the trailing edge outline, and not the profile - as the trailing edge "profile" (camber) is in fact one of the very important parameters regarding how much power and depower and drag, and how stable a kite is :thumb:


Trailing edge outline (bat wings, swept or not as swept etc) can influence in at least these areas:

Stability - how well it drifts and how it handles gusts without "tumbling" over.
Turning ability.
Performance in low wind with small gusts.
Physical stability - the kite frame can become more or less rigid thus maybe heavy/light, and you might get more or less "flutter" issues (trailing edge flapping so to speak).

And in other cases it might not affect much but the looks, really :naughty:


I understand why you compare your different kites to find a "meaning" about the design parameters :)

But to be honest, I would really say, that you could almost design both kitetypes you have had, to do one or the other or vice-versa.
By changing many of the more "subtle" design parameters like camber and line connections and wingtip design.

So the evident visual things one see, might not be the reason for the way a kite perform and feel :-?

Above is also why it is not a simple task being a kite designer !

:D Peter

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Re: Kite design questions

Postby knuthansen » Thu Jun 03, 2010 7:27 pm

Ok just trying to understand why C's have less depower..
In the graphic below, the kite on the left has a shorter lever for the steering line to affect the AoA. So if the laws of leverage apply (?), it should be possible to change the AoA with even less bar throw, but more power (not more overall effort, of course) is needed.
Which would not explain why C's have less depower, rather the opposite. Or are the c-kite bars just designed with much less throw, because you are not meant to change the AoA that much? If so, why? What would be the penalty of offering more depower on the bar?
depower.PNG
depower.PNG (72.13 KiB) Viewed 1653 times
Aspect ratio: "Aspect Ratio is approximately Span/Chord of the kite or more exactly Span*Span/Area. "
So the Flysurfer Speed is very "narrow", which means it has a big span, hence a high AR? I thought it was rather the other way around..

Apart from that: When a kite is very narrow (regardless of looking flat from sideways or like a letter C), does that improve upwind performance? I would expect so, because relatively more surface is at the side of the wind window, so to speak.

Wingtips: Thinking about the Windwing Batwing and kiteloop performance: On the way down, the big tip would act like a big spoiler and improve turning speed. However, the big tip has to be brought all the way up again as well.
So, does bigger tips mean smaller turning radius? (Leaving other aspects like AoA constant, without bridle)
Would it also increase turning speed throughout the loop?
Obviously when riding parked, bigger tips would mean more drag, so does that design generally shine more in powered turns?

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Re: Kite design questions

Postby sms-kite » Fri Jun 04, 2010 9:56 am

Hi,

I work as kitedesigner as many years now, and to answer to your questions I will start "by everything have to be well balanced", it is the most important things to me.

Hight aspect is good in light wind, but not too much. You have to ride as the same speed of the kite So if the kite goes speeder or slower than you it is not good. And you are loosing alot of turn ability with big AR.

Yes a flat kite with no depower have more power. Take a look on buggy kites you can ride at 50km/h in less than 4-6kts...But as I tell you you have to be able to ride at this speed with a very lowdrag performance as you can found with buggy or snowkiting. So very different than a kiteboard

Low aspect is better for stabilty and wind gust. More drag so more stable...And in high wind you don't need top performances, you need smoth performances, control...

Bigger tips size, bigger turning speed...more drag, more surface for turn less surface for power...and that's not good :D :D

There is a lot of theorie in the aerodynamic..

To make a good light wind kite it is clearely difficult. It depends of you weigth, your board, the way you ride (in power or not)

We make a kite with my friend for LW condition, you can found information here
http://forum.flysurf.com/viewtopic.php? ... iew=unread.

This kite is clearly make for him, surface, way to ride, what he wants to do with it...It will be not good for me because of the weigth difference. He can start at 6kt without any problems
To give you an other exemple we ride on snow when the foils kite don' stay in air.


Norbert

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Re: Kite design questions

Postby knuthansen » Fri Jun 04, 2010 5:20 pm

Interesting!
Questions:
- How can you go speeder or slower than the kite? Are you talking about the relationship between windspeed and riding speed?
- Can someone explain that c-kite and less depower issue that I described in the previous post?

Apparently, there are things where extreme values are good if you want to optimize a kite for just one specific feature (totally flat kite more efficient, ignoring bad turning performance etc), and others where it is about finding an ideal value in between, e.g. most lift at 15-20 degrees.
So..

-If you are only after big air (ignoring hangtime), would that mean high AR and built in AoA 15-20 deg? Or rather 0 degrees for better L/D. Obviously you cant make the AR super high, because it won't stay in the air with all that drag and little projected surface.
As Peter Lynn said, induced drag is less important since kites are not efficient foils.. so rather go for 15 deg AoA and hence max lift, combined with a deep tractor profile?

-What effects to different trailing edge profiles (not outlines) have on depower, stability etc. ?

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Re: Kite design questions

Postby sms-kite » Tue Jun 08, 2010 9:29 am

Hi,

1-For exemple if you move the kite in the windows, it is because the kite goes speeder than you.
Like this the kite fly a bigger distance than you ride with your board. It is because you are not at the optimum position. The same when you are full power on the bar. You are trying to make the kite slower than you, you increase the power but you increase the drag too. If you are full depower it is because you ask the kite goes speeder.

2-You have to think that the result of the power is align with the ligne. It is always like this in all the kites. If you have a very long tips, the AOA can not moove a lot. If you have a smaller tips the AOA can moove. So to resume if the distance between the lines and the power center is long. the kite is really stable so it can not moove to depower. If the distance is short you can moove and you can have depower.

3-Have an only whay kite is not a good way to think. If you need big air...you need good turn ability and good depower. You have to be able to ride in gust and need an explosive kite to take off.
The EGO from Fluid is a really big air kite because of the turn ability, pop and low drag kite.
If you put a big ratio on a kite, the kite can fly at an increadible speed, and you will have no power, only speed....Not a good result I think
To answer at Peterlynn. I think that tube kite or foil kite are not like a ASH25, it is clear. But the most of the plane are not too. For me this way to think is not good. The goal is to be well with your kite. If you want a kite like a glider the better way is to take a buggy race kite. Less drag top speed, big AR...you will flying like a paraglider witha 8 or 9 glide ratio. But good luck to ride this kind of kite on the water.

4-Trailling edge without the other part are not my way to design. As I tell you all the design have to be think as one part. Or all the parts are playing with the others. If you take only a caracteristic, there is no sense.

Norbert

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Re: Kite design questions

Postby knuthansen » Tue Jun 08, 2010 7:25 pm

Cheers for the info! :thumb:

I am always looking for the ultimate big air kite.. :o
I read an interview with the JN designer, where he said that a properly designed c-kite would jump bigger than a bow, because the center does not twist and maintains its ideal AoA throughout the jump, thus not loosing power when you redirect it.
However, the lacking depower would be a downside for jumping..
So would you say that a high performance hybrid has a better big air potential than a pure radical c-kite?

Questions:
-What makes a kite have pop?
-I assume with "explosive" you mean a high sled boosting effect? Do more c-shaped kites have a higher sled-boosting effect, and if so, why? Apparently they rip you off the water more aggresively.

I have not considered the Fluid EGO yet, unfortunately there are very few reviews about it, and the company's website is quite bad - no info about the company, no user forum, manuals etc.
But its on my radar now :wink:


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