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 Post subject: Foil wing basics
PostPosted: Thu Apr 03, 2014 11:19 am 
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Hi

I was wondering if anyone could have any use of some basic wing experience and theory.

I am really experienced in aerodynamics.
Hydrodynamics are different in some parts though, as the water is more dense (reynolds numbers), and regarding cavitation which is an important issue at high speeds.

The cavitation and the profiles to avoid this (to avoid rapid pressure changes) is a science of its own, and I wont get into that.


The basics regarding the wing, as I see it, when comparing wings with the SAME surface area :

A low AR wing has more drag, because of induced drag (tip vortices from the pressure differences top/bottom = turbulence = drag)

A high AR wing has more lift than a low AR (some think the opposite, I know), because the low AR wing will lose lift at the tips, because of the vortices.
The high AR wing also has less drag of course.

So why use low AR wings ???

Because, the very same feature, the tip vortices (which is generally the pressure difference top to bottom that will try to equalize around the wing tip, thus lose a lot of lift and add drag), will make such a wing MUCH easier to ride, because it wont stall abrubtly, like a high AR wing.
A high AR wing will typically be able to lose its lift at the same time over most part (or all), when pressed over the limit, or when sucking air down from the surface from a rider error (or big chop), and it will stall (stall = too high AOA for the given speed = separation on the top of the wing = lose its lift thus you crash).
Whereas a low AR wing will be much more foregiving, and make a very soft stall, actually more often recover from the stall again. The reattachment hysteresis is also "better", meaning easier to recover.
It will also be less prone to a stall from air from the surface, as the AOA is different over the wing, because the tip vortice changes AOA.

Apart from that, one can also diminish AOA at the tips, if having a swept wing - and this way obtain a much more stable wing, easier to ride without "bopping" up and down, and with very gentle stall characteristics (and/or use a smaller tail wing as not as needed).

Gentle stall is really important for beginners, and when riding on the lower speed limit.

The physical strength and stability is also much better with low AR wings.


A fast wing is a high AR wing, right ?

No, not directly connected this way.
A fast wing is a wing with a low drag profile and lower max lift, which means a more flat profile maybe not even concave at the bottom.
A wing like that rides with less lift and less drag, so faster (higher top speed).
If you increase AR, you will get even less drag indeed, but that is only part of it.

Increased AR has much more influence at high lifting profiles, so for going max upwind a cambered high AR wing is the best actually - whereas for pure speed, a low cambered but only medium AR wing will work fine :thumb:

Above is not what we typically see from the brands, but this is, IMO, merely because most have chosen to carry two types:
One lower AR lots of camber for beginners and easy to do tricks/waveride.
One higher AR fast(er) camber for race.
And not a foil for pure speed...


If you go solely for speed (not race), the trick would IMO be to go with a low drag (low camber) profile, quite high AR, and a smaller wing (less area = less drag).
But everything is connected - as if the wing gets too small, it needs more lift, and will maybe operate at too high AOA for having the lowest drag, both because of the profile polar and more induced drag.
So all these parameters, polar curves and induced drag (which is higher with stable swept wings) and size drag has to be calculated across, to find the optimum size and type.


I might be wrong if something does NOT match in hydrodynamics, like it goes for aerodynamics.
Please correct me if wrong...
But I think it is the very same (apart from cavitation issues).


Hope above can be useful as knowledge for those interested ?


8) Peter


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 Post subject: Re: Foil wing basics
PostPosted: Thu Apr 03, 2014 11:45 am 
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Interesting Peter. I had to look up exactly what camber was.


I have no thoughts of ever foiling myself, but it is very interesting as a subject.

There seems to be a compromise looming with regard to getting going and not developing too much lift at speed.

With regard to pumping a foil onto the plane, do you think that would be dependent much on the camber or on the area? I'm thinking that if the area is what is key to the pumping action, you could have a big area with low camber which you get planing by pumping your legs, so you need less kite power? Then when you get the apparent wind you are not going to be getting too much lift from the foil?


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 Post subject: Re: Foil wing basics
PostPosted: Thu Apr 03, 2014 3:09 pm 
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Well explained Peter. The only part that I would disagree with is that beginner wings should be low aspect ratio. I am starting to find that the increased stability of a high aspect ratio wing far outweights the benefits of having a lower aspect ratio wing. The lower aspect ratio wing is simply easier to manufacture (it needs less stiffness/strength).

I find that my high aspect ratio wings do not stall badly at all, and if they do stall I simply recover by tilting the nose of the board down. It recovers long before my board touches the water surface. And it hardly ever happens. The high aspect ratio wings can ride quite slowly if they are big enough, and the foilboard becomes mushy and difficult to control before the wings stall, so you get a warning of an imminent stall. Other foils might be different, but that is the case for me on my high aspect G10 wings and Spotz foil.


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 Post subject: Re: Foil wing basics
PostPosted: Thu Apr 03, 2014 3:39 pm 
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Could you address the issue of wing incident between the front foil and rear wing?
Would they be set even,~0~,
Would this vary for high or low AR?
How would you know when it is right when on the water?


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 Post subject: Re: Foil wing basics
PostPosted: Thu Apr 03, 2014 4:56 pm 
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Location: Vancouver, Canada. rides Naish,Ozone,North, Spotz,Aguera
Thanks Peter for a very good explanation. It matches my experience.

I have two wings for my Spotz foil, one with low AR and one with a high AR. The low AR one works well and is forgiving and it starts at low board speeds. The high AR one does not feel too much different at first but then I noticed that it stops foiling at very low speeds. It is also allows me to reach higher maximum board speeds. When it stops foiling it is very abrupt. All this is consistent with your explanation.

Everyone I talk to who has a Spotz foil seem to use the high AR wing mostly. As you gain more experience you learn to maintain higher board speeds and you stall less often. I expect that as the typical foil rider gets more skilled we will see the higher AR wings more commonly. I remember the very early days of kiteboarding when very low aspect kites were common because everyone was a beginner, but now higher aspect kites are the norm.

:wink: CG


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 Post subject: Re: Foil wing basics
PostPosted: Thu Apr 03, 2014 5:28 pm 
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Great post,
book-marked for my inevitable foil build.


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 Post subject: Re: Foil wing basics
PostPosted: Thu Apr 03, 2014 5:34 pm 
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cglazier wrote:
Thanks Peter for a very good explanation. It matches my experience.

I have two wings for my Spotz foil, one with low AR and one with a high AR. The low AR one works well and is forgiving and it starts at low board speeds. The high AR one does not feel too much different at first but then I noticed that it stops foiling at very low speeds. It is also allows me to reach higher maximum board speeds. When it stops foiling it is very abrupt. All this is consistent with your explanation.

Everyone I talk to who has a Spotz foil seem to use the high AR wing mostly. As you gain more experience you learn to maintain higher board speeds and you stall less often. I expect that as the typical foil rider gets more skilled we will see the higher AR wings more commonly. I remember the very early days of kiteboarding when very low aspect kites were common because everyone was a beginner, but now higher aspect kites are the norm.

:wink: CG


I have the same two wings.

The low AR is the Race wing: it starts to foil at lower speeds so it's useful for very low end conditions and for the upwind legs of race courses. It's also much more easy to start with for beginners since less power is needed to foil and the sudden jump in the apparent wind is lower when it starts to foil. It's also easier to start to learn jibes if you are afraid of high-speed crashes.

The high AR is the Speed wing, this is useful in the French Speed-crossing type races. Some say that it's easier to learn to jibe with it.

The surface of the Race wing is higher so the comparaison of Peter (who assumed comparable surfaces) does not fit exactly to compare the two Spotz wings.

Both Maxime Nocher and Nicolas Parlier used relatively high-surface low AR wings in La Ventana, because of the important upwind leg. These wings are then quite difficult to keep in the water on faster downwind legs.


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 Post subject: Re: Foil wing basics
PostPosted: Thu Apr 03, 2014 6:01 pm 
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Quote:
A fast wing is a high AR wing, right ?

The amount of lift from the foil depends on the rider weight and line tension (force equilibrium), which should not be variables of AR. Therefore drag is inversely proportional to L/D ratio and maximum L/D ratio is therefore desired for high speed, which (as you know) means a high AR foil. Naturally, the wing itself should be smaller for high speeds, but that is another discussion unrelated to AR.

I guess, the limititing factor for AR is structural, i.e. if the wing is too flexible, it becomes impossible to control.


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 Post subject: Re: Foil wing basics
PostPosted: Thu Apr 03, 2014 7:44 pm 
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A low AR wing is like a F22, a high AR wing is like a Glider.


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 Post subject: Re: Foil wing basics
PostPosted: Thu Apr 03, 2014 8:23 pm 
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ChristoffM wrote:
Well explained Peter. The only part that I would disagree with is that beginner wings should be low aspect ratio. I am starting to find that the increased stability of a high aspect ratio wing far outweights the benefits of having a lower aspect ratio wing. The lower aspect ratio wing is simply easier to manufacture (it needs less stiffness/strength).

interesting, as in theory a high AR wing is more ´lift sensible´ to AOA changes than a low AR wing, i.e., (simplifying) a small change in AOA leads to a big change on lift for a high AR wing, not so much for a low AR

guess is as the saying goes - the slaying of a beautiful hypothesis by an ugly fact...

can relate of what you saying for lateral stability

ChristoffM wrote:
I find that my high aspect ratio wings do not stall badly at all, and if they do stall I simply recover by tilting the nose of the board down.

was expecting things to be set-up in a way that under a stall there was an "automatic" pitch
moment that would put the board nose down and allow the front foil to recover from that stall (without the rider input)

is this the way things work?

Peter?


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