Speed 3 21m Upwind adjustment

[kitexpert]

- L/D ratio decreases rapidly when AoA increases (of course after a minimum AoA otherwise lift is zero, hence L/D poor)

Actually it does so less rapidly as one could think. L/D value 8 from the graph is at 14 degrees and under 2 degrees, peaking at 5 degrees.

the rider shall find the optimum balance between pure traction value (high AoA) but better upwind traction angle (low AoA) ; this explains why it may sometimes be beneficial to sheet-out to save some upwind angle (depending on your support : TT , hydrofoil ...).

If kiter can not take power without losing the edge, he must sheet out. If he can keep his edge, he goes faster upwind at higher AoA, even though kite's pull is not optimal directed. Forward driving component of the pull is still bigger. I have tested this a lot in the winter, when conditions are much more uniform than in the water.

For example Speed3 goes fastest upwind fully sheeted in, when adjusted for no backstalling of course. If the wind is not too strong kiter can use this force - it is hard work though. Higher wind doesn't make kite worse upwind, but kiter is just not strong enough anymore. So, he sheets out and gets his maximal pull at lower AoA.

For sure there is considerable differencies between different kites. My experiences are mainly with Speeds.

Consequence in HF foil race : for speed and upwind, it should be better to have a larger kite a bit sheeted-out rather than a smaller kite more sheeted-in.

This is true and tested observation. To go fast upwind bigger kite is better.

for the opposite tuning (flatter camber = shorten B and C) as it should increase L/D ratio

It is very hard to believe that distorting kite to lower camber could increase L/D ratio. If you test some airfoils with some virtual wind tunnel, higher camber means higher L/D. When camber is increased, lift increases more than drag - within reasonable limits.

this is why the speed system not only allows to depower the kite ..., but it also allows a better upwind ride.

Possibility to depower gives kiter a choice to control the pull he can handle without losing the upwind course.

Going fastest upwind is one of the main challenges of kiting. Kite with highest L/D is best for that purpose, like modern race foil kites. Airfoils for these kites are compromises between efficiency and stability/depowering. Used airfoils have positive camber and reflex (I'm not sure if always) and they are quite thick.

[foilholio]

I'am affraid I cannot sort out your debate , but let me bring some biscuits and opinions, sorry for the long-bothering details:

- One way I found to get an estimation of L/D is measuring its angle "alpha" to the vertical when static at the zenith : L/D ratio = arctan(90-alpha). Even if hard to measure you can use this to check the impact of sheeting-in or out or check the impact of any speed system tuning if you have a good "neck" memory. It is also a good way to compare the performance of two kites if you have two synchornized pilots to put the two kites side by side. Another way to measure the L/D is to measure the speed of the kite "s" when crossing in the middle of the wind window ; L/D ratio = s / wind-speed

- L/D ratio decreases rapidly when AoA increases (of course after a minimum AoA otherwise lift is zero, hence L/D poor), as it is the case for most "Naca" profiles ; you can see a lot of curves on Google like this . ... knowing that the maximum AoA of a kite in15-20 degree.
So when AoA increases, Lift is indeed better (good for jumps, waterstarts, plannings) however traction direction points less upwind, hence to ride upwind with maximum speed, the rider shall find the optimum balance between pure traction value (high AoA) but better upwind traction angle (low AoA) ; this explains why it may sometimes be beneficial to sheet-out to save some upwind angle (depending on your support : TT , hydrofoil ...). Consequence in HF foil race : for speed and upwind, it should be better to have a larger kite a bit sheeted-out rather than a smaller kite more sheeted-in.

- Foilholio, AoA is not the angle of the line AB against the wind, but rather the line AZ (assuming that bridle were connected between intrados ad extrados) ; therefore when you pull on Z, even if A and B and C had been unchanged, then AoA will increase;


... so yes the AoA will change as soon as you pull on the bar, even on a 4-line-handles-traction-kite without speed system and keeping ABC unchanged !

- As kitexpert said, the speed system 's primary function is to change the AoA without impacting too much the camber. It iaims at counteract the lack of rigidity of foil kites ( vs LEI rigidity that need no speed systems). If you remove it, you are on the case of a paraglider or a 4-line-handles-traction-kite, where camber increases hardly when AoA increases. If you add a WAC line or a diablo line on the speed system, it is a kind of physical limitation of the speed system impact, and you come closer to a paraglider mode. It could be an optimisation for a fine tuning, but not the overall principle of a speed system that act as a rigid frame.

- I agree with Foilholio that generally speaking, a higher camber may be in detriment of the L/D ratio (as for the AoA); this is why the speed system not only allows to depower the kite (in opposition to a 4 line traction kite which keeps ABC unchanged, therefore D will align with C when depowered, so less change of AoA , so it remains powered), but it also allows a better upwind ride. PS : that is why I have always thought that when we ( in france) say we put a "performance set-up" by extending B and C for more camber, it is a wrong expression ; we should rather say "power set-up" for more camber and use "performance set-up" for the opposite tuning (flatter camber = shorten B and C) as it should increase L/D ratio which is the definition of performance.


Foilholio, except ARC that are known to have poor L/D, I do not know which "standard" foil kite has a negative camber, so I am also interested to know which kite you tested has a negative camber with a positive impact on L/D ratio

BTW about the yellow spare lines, I maintain that extending B and C using the 2*2 inox rings on the MT of the speed 3 allows to counter act the totality of the Spare lines shrink (not all the bridles of course) until the ring touches the pulley ( then SPL have to be replaced)

I agree with you that speed is a good determiner of L/D.

I get what you mean in the picture, I think what I am getting at then is that foils don't change AoA in a rigid wing sense but mainly or only with camber changes. I think the confusion stems from where the reference line is draw. On a plane the body or fixed part of the wing could be considered the reference line and even though the flaps are down and the wings AoA has increased, the planes has not. I guess then it is the angle of incidence that changes. I wonder then if extreme camber might not just cause a negative moment but also a reduction in AoA in certain situations, interesting.Thanks.

Well originally I wasn't talking about a negative camber airfoil, but negative camber, i.e. reflex etc. But it seems possible to bulge out B and C and produce a negative camber airfoil and that infact goes upwind well.

I take you bringing up pulley lines is reference to viewtopic.php?f=197&t=2392205. The mixer adjustments of only B and C ,actually to state it correctly, can correct for profile changes caused either by the pulley lines shrinking or upper bridles shrinking. They can not correct for the pulley lines themselves shrinking, and because of that the AB depower relationship ends up not correct and the final ABCZ relation too but that doesn't matter because flysurfer gives excess. With additional adjustments you can correct the AB part. The final ABCZ part could be corrected too ,but again unnecessary.

[foilholio]

For example Speed3 goes fastest upwind fully sheeted in

Speed3 is unusual in that to perform best it needs some sheeting because of the projected area changes. This was documented when it first came out, and many complained of the loss of performance over the speed2, though it wasn't huge but getting similar performance from the speed3 was more difficult like the pyscho4 is. both requiring a fine hand on the bar, sheeting just enough to open them up but not much more.

Higher wind doesn't make kite worse upwind, but kiter is just not strong enough anymore

I disagree, higher pull from the kite equals higher pull on the board, so then the boards L/D degrades. As well the change/increase in windspeed may also now be outside the kites minimum drag speed. Attempts to decrease kite speed will only decrease board speed and now that too is out of it's minimum drag speed. The whole system board and kite then become inefficient.

To go fast upwind bigger kite is better.

Not true. It is a an optimum kite that is better, one not too small and not too big. This is very basic kiteboard stuff that nearly everyone knows. Too much time on the sewing machine and not enough riding I think.

for the opposite tuning (flatter camber = shorten B and C) as it should increase L/D ratio

It is very hard to believe that distorting kite to lower camber could increase L/D ratio. If you test some airfoils with some virtual wind tunnel, higher camber means higher L/D. When camber is increased, lift increases more than drag - within reasonable limits.

For someone who calls himself an expert you express very little understanding of mixer changes.

Used airfoils have positive camber and reflex (I'm not sure if always) and they are quite thick.

Race foils have thin airfoils compared to most foils.

[foilholio]

knowing that the maximum AoA of a kite in15-20 degree

A kite like a depower foil, with relatively full control over its tow point can in fact increase it's AoA to 90 degrees, and! even further reverse it or continue past 90 degrees , which ever way you want to look at it. And this is how it can sit down wind , fly backwards and relaunch.

[Regis-de-giens]

I mean in "normal condition of ride" of course and before backstall ...

[kitexpert]

Not true. It is a an optimum kite that is better, one not too small and not too big

Of course kite can not be so big kiter loses control. But kiter who can use biggest kite almost certainly is fastest in race, especially in upwind legs. Racers are tough athletes, (I know some) they do have a lot of strength to hold kites.

I disagree, higher pull from the kite equals higher pull on the board, so then the boards L/D degrades. As well the change/increase in windspeed may also now be outside the kites minimum drag speed. Attempts to decrease kite speed will only decrease board speed and now that too is out of it's minimum drag speed. The whole system board and kite then become inefficient.

Too messy and complicated explanation. Like I wrote, upwind ability of the kite is best tested in the winter, then it is possible to achieve repeatable and precise observations.

little understanding of mixer changes

There is nothing I don't understand in "mixer changes". It is just tweaking the line lenghts to desired direction. Much better way to test low cambered airfoils in kites is to design and make one. Then upper and lower skins are not distorted, but L/D ratio is of course worse than in kite with higher camber airfoil. Stability and depowering are better though with lower camber.

Biggest (only) need for low camber is when kite is fully depowered. Unfortunately for mixer tweaking then kite flies mainly on A-lines. This means shortened B and C line rows don't have much effect at all. If they have, some depower is lost. When sheeting in there is less camber, but higher the AoA is the less useful this lower camber is.

Race foils have thin airfoils compared to most foils.

What airfoil thickness is thick or thin in your opinion, foilholio? I just measured the thickness of the airfoil of R1 yesterday - would you like to guess how thin or thick it is? Or tell which "most foils" have thicker airfoils?

[foilholio]

I disagree, higher pull from the kite equals higher pull on the board, so then the boards L/D degrades. As well the change/increase in windspeed may also now be outside the kites minimum drag speed. Attempts to decrease kite speed will only decrease board speed and now that too is out of it's minimum drag speed. The whole system board and kite then become inefficient.

Too messy and complicated explanation. Like I wrote, upwind ability of the kite is best tested in the winter, then it is possible to achieve repeatable and precise observations.

Not at all. It is quite simple and backed up by available knowledge and experience.

There is nothing I don't understand in "mixer changes".

There is plenty from the way you talk. Never assume you know everything you will come up short every time.

It is just tweaking the line lenghts to desired direction.

Way oversimplification.

Much better way to test low cambered airfoils in kites is to design and make one.

About the most stupid thing I have heard from you yet. Having to design whole kites just to test your understanding really? I previously saw you recommend a kite could be fixed by making it again! properly from the start, sigh. You are a rotten one.

but L/D ratio is of course worse than in kite with higher camber airfoil.

Repeating an incorrect statement does not make it right. A large amount of camber has worse L/D than a low amount.

"L/D is far worse in approach configuration" https://aviation.stackexchange.com/ques ... e-decrease

"Extending the flaps also increases the drag coefficient of the aircraft." https://en.wikipedia.org/wiki/Flap_(aeronautics)

"Flaps always decrease the maximum lift to drag ratio" http://forums.jetcareers.com/threads/ho ... ax.128804/

"This ratio (L/D) is also reduced when full flaps are extended" http://www.experimentalaircraft.info/fl ... evices.php

"Flaps can decrease the plane’s lift to drag ratio" http://www.decodedscience.org/wing-flap ... raft/11831

"With the wing flaps in the full retracted position, there will be a high lift to drag ratio" http://www.pilotwings.org/use-of-wing-flaps.html

"high-speed flight would be impossible with large amounts of camber, because of the great increase in drag" naca 1938 https://www.youtube.com/watch?v=q_eMQvDoDWk

"with any stage of flap, especially full flap, with enough airspeed, drag will overcome any lift created by them" and " the flap increases drag more than it increases lift" http://www.bobtait.com.au/forum/aerodyn ... e-decrease



annnnnndd that is enough surely

Stability and depowering are better though with lower camber.

or negative camber.

This means shortened B and C line rows don't have much effect at all.

Not true

If they have, some depower is lost.

yes but not mainly because they change camber, but because they change the towpoint. and one of them them does it less than the other, Take a guess? It's B.

When sheeting in there is less camber, but higher the AoA is the less useful this lower camber is.

Not really because it will have better lift to drag, because of the lower camber. or did you miss the links above?

What airfoil thickness is thick or thin in your opinion, foilholio? I just measured the thickness of the airfoil of R1 yesterday - would you like to guess how thin or thick it is? Or tell which "most foils" have thicker airfoils?

We are talking about absolute thickness as in measured with a ruler and not reference to the cord etc where you will claim a 4 meter has the same thickness as a 22meter right? We are not in silly land? Though I am starting to feel that way now. Anyway no I have not measured exactly, but my eyes do function unlike yours, one example is Sonic 11 is about as thick as pyscho4-6, give or take a bit.

[foilholio]

I mean in "normal condition of ride" of course and before backstall ...

Backstall is normal for me If you were to exclude AoA changes because of window positions and say reference from say the front lines, what you state could be correct even in the dead downwind 90 degree position. I admit I am still learning on the fly here how exactly tow points and CoP and moments interact to cause window position changes is not 100% with me, though If I think about it I can explain it better, it is just forces finding some equilibrium. It does prove hard finding written explanations but there is some, I think kites are a bit fringe in the aerodynamic field. At most it seems a towpoint creates another point for forces to act around or from, like the CoP, AC, and CG.

[kitexpert]

We are talking about absolute thickness as in measured with a ruler and not reference to the cord

WTF? You mean your observation is that 21m kite is bigger (thicker) than say 9m kite? "Absolute thickness"

Level of your ignorance does not stop to amaze me.

Having to design whole kites just to test your understanding really?

No, but showing your erraneous thoughts. Of course there already is kites with lower and higher camber, the latter are more efficient. Your "tweaked" lowered camber kites are worst of them all, because they have shape distorsions too.

annnnnndd that is enough surely

Your examples are "tweaked" ones. Using flaps is much the same than tightening very much brakes - of course it lowers L/D, it causes huge drag. I have not suggested anything like that, but using higher camber airfoils to get more efficient kites.

Not really because it will have better lift to drag, because of the lower camber

Stop arguing against known facts, foilholio. Or have a test with some wind tunnel program: take some airfoil and change it's camber. You will see when you increase the camber airfoils efficiency - both L and L/D - will increase. I did quite a lot that kind of study ten years ago and I dare to say I know which airfoils are suitable for kites And it is not unwise to study what kind of airfoils for example FS, Ozone and PL have used and are using now. I have done that too. Of course they all use cambered airfoils.

[Regis-de-giens]

Backstall is normal for me If you were to exclude AoA changes because of window positions and say reference from say the front lines, what you state could be correct even in the dead downwind 90 degree position.

Please, let exclude backstall cases were everybody honestly do not car about the L/D ratio , right ? vonlunaty backstall is just a kind of exceptional configuration (for which standard kite bar control even do not allow) and were it may just be a way to relocate the foil elsewere int the wind window, not a "upwind ride configuration".

I admit I am still learning on the fly here how exactly tow points and CoP and moments interact to cause window position changes is not 100% with me, though If I think about it I can explain it better, it is just forces finding some equilibrium. It does prove hard finding written explanations but there is some, I think kites are a bit fringe in the aerodynamic field. At most it seems a towpoint creates another point for forces to act around or from, like the CoP, AC, and CG.

The position of the kite is more simple that that (at least as a first approximation which is enough for us). Let's seen it differently from how it is "commonly" described (it is worth a dedicated topic, but I hope you will excuse this off-topic): during a ride with constant vertical angle of the kite vs the rider (i.e. not working the kite), the foil is ALWAYS at the extreme of the wind window, and the kite does not really go deeper in the wind window (don't shout too rapidly, just give me 1 minute...):

The feeling that it goes deeper is due to the fact that the "speed" wind changes the direction of the total (apparent) wind seen by the kite, i.e. the total wind will come more from the front of the rider direction (due to speed wind parallel to rider speed). So it is still the extremity of the kite wind window, but now taking into account the wind that the kite really see coming from your direction angle, i.e. the modified direction of the wind. It also means that if you would send the kite in the opposite window ( and if you have still enough speed), then the kite will go beyond the 90 degree, say 100 or 110 degree backward you , up to the rider having zero speed and come back to 90 degree.

So if you understand that and agree with, then the position of the kite is only given by the L/D ratio of the kite (at the instant AoA imposed by the bar position) and the ratio of the rider speed vs real wind speed at ground.