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 Post subject: foil design
PostPosted: Tue Jun 17, 2014 1:34 am 
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Why not attach the Mast directly to the fuselage at the bottom, and the fuselage to the mast right above that joint. This should make the set up more rigid as I believe it is much more important to have the main fin more rigid even at the expense of the rear.

Thoughts?


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 Post subject: Re: foil design
PostPosted: Tue Jun 17, 2014 2:01 am 
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I don't know if you are talking about putting the front wing right under the mast like the Spotz Foil?


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 Post subject: Re: foil design
PostPosted: Tue Jun 17, 2014 4:04 am 
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Very similar to what I'm saying. I was thinking to actually mount the front wing directly to the mast, and directly underneath the mast. This one has it right in front of the mast, and it seems it's not mounted on directly to the mast


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 Post subject: Re: foil design
PostPosted: Tue Jun 17, 2014 4:13 pm 
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Some people have tried it and it doesn't work too. We'll. Not sure what's going on, but I think you need some separation for stability. Maybe it's affecting the flow over the wing too.


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 Post subject: Re: foil design
PostPosted: Tue Jun 17, 2014 9:18 pm 
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Do you mean like this?
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It would be nice if this worked for two main reasons.
The lift from the front wing would be translated directly up the strut (much better in english to call it this than mast, see wikipedia for definitions) removing the huge load on the fuse.
The lever force would be less on the board as you would be centered over BOTH the front wing and strut.
BUT,
It does not seem to work.
I do not know why?
Does anybody?
R H


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 Post subject: Re: foil design
PostPosted: Tue Jun 17, 2014 9:36 pm 
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Someone tried it here, when I asked how it was he said it didn't work but I didn't have time to ask what that meant. I'll try and find out.


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 Post subject: Re: foil design
PostPosted: Tue Jun 17, 2014 10:16 pm 
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I think it comes down to placing the center of downward pressure somewhere within the CG range of the wing /Fuselage / Tail configuration.

Think of it in reverse, when flying a kite, the bridle directs the pull of the main lines (CG) slightly forward of the center of lift. The steering lines make up the difference. without the steering lines the kite would fly to the top of the window and over the top then causing slack in the lines.

So if you put the strut of the hydrofoil too far forward, you are requiring the tail to work extra hard trying to compensate for the forward CG. At slow speeds you can weight shift back on the board to make it work but the problem get worse the faster you go because as speed picks up the downward pitching moment of the main wing will require even more compensation by the already overworked tail plane.

Getting a hydrofoil to be well balanced is a combination of trade-offs. High AR wings have larger pitching moments which require larger tail plane or longer fuselage. Center of Gravity must be placed slightly behind the center of lift. The slightly part is mostly a guess based on the expected pitching moment of the airfoil choice. Too far back and you get the Sword issue of needing more front foot pressure with speed.

Trying to get the center of gravity in the proper location at the board level with rider weight shifts is only effective in a small range. If the mast is connected too far forward or too far back, the weight shift correcting is really happening 3 feet away from where it is needed causing delayed responses and rider induced oscillation.

Spotz seems to have the best balance setup with a compact wing, strut, fuselage, tail combination.

Just my take on it of couse.
David


Last edited by dlprince69 on Tue Jun 17, 2014 10:18 pm, edited 1 time in total.

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 Post subject: Re: foil design
PostPosted: Tue Jun 17, 2014 10:17 pm 
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ozchrisb wrote:
Someone tried it here, when I asked how it was he said it didn't work but I didn't have time to ask what that meant. I'll try and find out.


i dont own a kite- hydrofoil AND i dont know how to ride one.

only a fool would discount the opinion of experienced riders when they say "i doesn't work on foils they have tested"

but the Moth guys dont seem to have a problem with it.
many similarities between the two 'boats'. i wonder what differences avoid the same problem
it would seem that there would be a significant structural advantage if the details of the problem could be better defined.

imo.
-bill


Last edited by zfennell on Tue Jun 17, 2014 11:14 pm, edited 1 time in total.

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 Post subject: Re: foil design
PostPosted: Tue Jun 17, 2014 10:29 pm 
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The moth benefits from the hull and rudder gantry (or whatever they call it), it has probably >1.5m longitudunally between foils, and flaps on both wings.
Most kite foils seem to have fuse lengths 50-75cm and no flaps except a few experiments here and there, so balance has to be perfect and targeted at a narrow range.


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 Post subject: Re: foil design
PostPosted: Tue Jun 17, 2014 10:44 pm 
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Many theories here :thumb:

Mine is, that experience has shown, that having the mast to far forward, will give bad dynamics and instability regarding yaw and turning.

If the mast and fuselage is okay stiff, the COE versus CG plays no role whatsoever, but of course a structural tradeoff at some point.

I believe this is the reason why the mast is a bit aft and not above the wing (dynamics and turning/yaw), and why it is not further aft (structural balance) :naughty:

8) PF


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