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Foil design details

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Peter_Frank
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Re: Foil design details

Postby Peter_Frank » Sat Oct 05, 2013 10:34 pm

Looks really good Hawaiis !

8) Peter

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ChristoffM
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Local Beach: Langebaan and Cape Town.
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Gear: Cape Town Foilboard and Spotz hydrofoils.
Any kite that can keep up with the hydrofoils.
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Re: Foil design details

Postby ChristoffM » Sun Oct 06, 2013 7:24 am

Yes, that titanium wing does look nice!

Interesting that you say the bent wing makes it more stable. I think it confirms my theory as to why the front wings have anhedral. I do not think it is there for any of the reasons posted about induced drag or stalling. Induced drag is a very minor in the dense water and the types of loading angles I experience on the foil. Drag is mostly from parasitic (flow) drag (i.e. non smooth finishing) and vibrations.

A sudden stall is also of no concern, even on the elliptical flat wings that I have used.

I think the anhedral has to do with the roll and yaw interaction when riding. Providing a nice controllable feel to the foil, making it easier to ride, and would help to keep it stable as a whole.

I don't know if any of you saw my post on design issues here: http://kitehydrofoil.com/index.php/desi ... wings.html

Please give me some feedback what you guys think/experience.

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Peter_Frank
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Re: Foil design details

Postby Peter_Frank » Sun Oct 06, 2013 11:05 am

I agree fully with you Christoff, that roll and dynamics could be the key factor why "bent" wings are used :thumb:

Also agree that parasitic drag is a major contributor to drag and lower performance here, yes.

But whether the water is dense or not, does NOT change the induced drag principles :naughty:

Of course, at high speeds and low AOA, induced drag is of minor concern - but it is still present !
And when going upwind, and the foil operates at a bit higher AOA (less speed), it becomes really evident.

This is the reason why high AR foils perform so much better upwind especially, besides from being faster.
Less induced drag :D


The wing, will typically perform better with higher AR, because of less induced drag.
Lots of disadvantages then, as structural weakness, and especially a MUCH HARDER stall :o

Low AR wings will stall very gentle, because the AOA changes towards the tips, so instead of a full stall of the whole wing, you will get a gentle stall, that can be recovered much easier.

A wing with some wash-out can be tuned to work okay at a certain high AOA, true.
The problem comes, when you try to go fast - as with low AOA, you can only have extremely low or no wash-out, otherwise it will only be drag :(

As with kites, you can not get it all, but have to decide and make many compromises.


All of above of course only "IMO" and knowledge/experience (R&D regarding glider design and competitions), not from tests nor facts regarding hydrofoils :-?

8) Peter


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