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 Post subject: why anhedral ?
PostPosted: Wed Dec 04, 2013 8:58 am 
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Most of the foils for kitesurfing has an anhedral cant on the wings. Why is that ? I know it makes the foil roll better, which we use for going upwind - but I can see that the moth foils tend to be straight and they also roll the foil while going upwind.

Has anybody tried straight or dihedral angles of the wings? What did it do ?


Last edited by lander on Wed Dec 04, 2013 9:26 pm, edited 1 time in total.

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 Post subject: Re: why anhedral ?
PostPosted: Wed Dec 04, 2013 3:12 pm 
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Kite boards usually are not ridden with the bottom of the board parallel to the surface of the water, but rather at an angle. With a flat high AR wing, this would cause one of the wing tips to surface.

http://kitehydrofoil.com/index.php/desi ... wings.html


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 Post subject: Re: why anhedral ?
PostPosted: Wed Dec 04, 2013 7:59 pm 
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The writer is making the same question as I did, but he also draw his own conclusion which makes sence.
If it was "just" about the wingtips not getting to surface, and the dihedral was more effective - you could just make the dagger/mast/keel longer. It would be more complicated finding deep water, but in a race you would prefere this and go faster. hmmmm funny that the writer has tried two different foils. One with straight wings and one with anhedral wihout feeling the big difference. Both my foils are anhedral and that made me wonder why...
Thanks for the link :thumb:


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 Post subject: Re: why anhedral ?
PostPosted: Thu Dec 05, 2013 6:24 pm 
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Location: Langebaan (Cape Town)
I have made some more foils, and am still getting my head around exactly what all the benefits of anhedral gives. I would say the main reasons I think it is nicer are:

- so that it does not come out the water when riding with the board angled. The anhedral gives a lot of effective keel length for high aspect ratio wings. More than I thought would be the case.
- roll dynamics as explained in the link, which I think is nice, but one tends to get in tune with the gear that you have and the dynamics make small differences compared to what a rider can subconsciously compensate for
- I suspect strongly that anhedral helps to stop ventilation (bubbles forming) due to the way it forces the water inwards. I think a dihedral wing will ventilate a lot more on the wingtips than a flat wing. The loads on all wings are also large, so most flat wings have a bit of curve (dihedral) when loaded I think. I think a wing that has anhedral recovers from ventilation (bubbles) faster but I have not found any solid evidence to support it. Just my tinkering around and suspicion at this stage. More prototypes needed...

Any other ideas?


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 Post subject: Re: why anhedral ?
PostPosted: Thu Dec 05, 2013 7:08 pm 
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Good points ChristoffM.
I think if you ventilate the center a little, you lose a fraction of lift, but lift is more or less still balanced across the foil, so you could usually recover.
Ventilate one tip but not the other, you may have off-center lift, leading to a crash, no?
With anhedral, tips stay immersed, ventilation would be starting from the strut in the center, anhedral could help that bubble just flow aft off the wing, rather than flowing spanwise to tip and creating unbalanced lift.
I think dihedral would enhance flow along the span and also allow the bubble to expand due to lower pressure the farther out the span it progressed.
Does that sound plausible, or is it just my fantasy?


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 Post subject: Re: why anhedral ?
PostPosted: Thu Dec 05, 2013 7:32 pm 
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BWD wrote:
Good points ChristoffM.
I think if you ventilate the center a little, you lose a fraction of lift, but lift is more or less still balanced across the foil, so you could usually recover.
Ventilate one tip but not the other, you may have off-center lift, leading to a crash, no?
With anhedral, tips stay immersed, ventilation would be starting from the strut in the center, anhedral could help that bubble just flow aft off the wing, rather than flowing spanwise to tip and creating unbalanced lift.
I think dihedral would enhance flow along the span and also allow the bubble to expand due to lower pressure the farther out the span it progressed.
Does that sound plausible, or is it just my fantasy?


Hmmm....

I think, a ventilation at the center, of a swept wing, is by far the worst of all :o

As the flow (ventilation) will continue across the whole wing very rapidly then.

I have no idea if a ventilated tip would cause a "yaw" effect leading to a crash, or not.
You may be right, or it might not mean anything - as when riding a foil, it is amazing how stable and rigid it is.

A wing with anhedral, has been used "often" in airplanes, in order to minimize tip vortices (induced drag).
But, the downsides in handling are too big, so mostly used for fighter planes because of its instability, thus very agile.

Meaning, you can not use these experiences for ANYTHING at all.

In theory, you would make a wing seem higher AR, by having anhedral.
But the downside is, the lift is not perpendicular anymore - just like a C kite is never as effective as an SLE kite or foil kite.

So not sure this is the reason...

Handling, and the ability to ride without ventilation issues, would be my best bet :naughty:

But, a very new sport, so you never know if the current most used solution is "the best" for most, or just how it is right now (and thus many follows)

8) Peter


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