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 Post subject: Re: Hydro Foil almost done
PostPosted: Sun Oct 28, 2012 7:18 pm 
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Joined: Mon Sep 02, 2002 1:00 am
Posts: 477
Location: Teahupo'o, Tahiti
Thanks for that link, I've "Liked" it. I'm cautiously optimistic. I've over engineered mine somewhat never had anything to base it on other than pictures. Was worried about mast not being stiff enough at first, I hear thats very critical. But without anything to base it on it's very difficult. My friend got his MHL Lift few months ago and I checked his out and I'm not far off in terms of stiffness. I also went 80mm on the mast not very wide. Figured I'd try something different everyone seems to be making them from 90-100mm wide. Tried to come in at around 12mm thick but it ended up being 15mm at it's thickest point.

Suppose the proof is in the pudding. Should be done in the new week. Hopefully I'll get some wind.


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 Post subject: Re: Hydro Foil almost done
PostPosted: Sun Oct 28, 2012 7:41 pm 
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Joined: Tue Jul 19, 2011 5:26 pm
Posts: 194
Location: Langebaan (Cape Town)
Finishing this week! Awesome. I want to check it out.

I had some nice time on my MHL custom at the coast. I think there is a noticeable difference between riding a foil in sea water (salty more dense) and in fresh water at 1500m above sea level. In the fresh water the foil wants to "stall" I think. It wants to drop to the side every now and then when a chop hits the mast. This only happens on a deep upward tack. If I keep the board flat (more parallel to the water surface) it does not happen. I can't figure out what it is so I think it is a stall. At the coast it felt like I could angle further upwind without it stalling. Could be simply me improving, but I am sure it is either the more dense salty water or probably the different type of chop. Sterkfontein dam has some unique chop being so deep.

Oh yes, the latest Magma foil looks super cool. I like the square design look.


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 Post subject: Re: Hydro Foil almost done
PostPosted: Sun Oct 28, 2012 8:08 pm 
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Joined: Wed Sep 12, 2007 10:19 am
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Location: Fuerteventura
Hi,

Yes the chord of the mast on ours is 100mm. It is 18mm thick. We wanted strong as I was snapping the other when jumping it.

We made a less wide Mast, and that works fine too. Just make sure that it has no torsion, that is a killer and cause a lot of spin outs.

Here are some videos to give you inspiration for your first session on yours:





--
Gunnar


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 Post subject: Re: Hydro Foil almost done
PostPosted: Sun Oct 28, 2012 11:07 pm 
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Location: Fuerteventura
Christoff.

I am not clear on what your foil is doing.

What do you mean with it "wants to drop to the side". Does the Foil loose grip, or does it rapidly sink down or is it maybe sucking air (the foil will loose traction almost instantly and you will crash).

Check and see how stiff the main Mast is. We had problems with the Mast sucking air when it was too flexible and had torsion.

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Gunnar


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 Post subject: Re: Hydro Foil almost done
PostPosted: Mon Oct 29, 2012 5:51 am 
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Location: Langebaan (Cape Town)
Hi Gunnar

Its hard to explain what happens but I'll try: When I point far upwind, and the board is almost at 40 degrees to horizontal (feels like 40 degrees, but probably less), then I would go upwind very nicely until suddenly the foils wants to roll to the windward side, almost like it is suddenly losing lift on the upwind side of the foil. Normally a quick correction of pointing the board's nose more upwind to stop the roll can stop a fall, but sometimes I'm not fast enough to stop it. It seems to happen at any speed, and normally when I lean hard against the pull of the kite. If I depower the kite more and keep the foil more horizontal then it never happens.

I think you might be right about torsion. The MHL is super stiff in all ways except that in torsion it feels very loose (I don't know how other foils are but it just feels like it rotates too easily). It would make sense that it might be sucking air on the windward side, causing the board to roll suddenly to windward.


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 Post subject: Re: Hydro Foil almost done
PostPosted: Mon Oct 29, 2012 7:10 am 
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Location: Mauritius, waterman since 1960
Hi Gunnar.

A practical question: have you tried any substantial extension on the fuselage to get a better stability at higher speeds (1mt-1,5mts). ???
Seems to me the potential of high speed is there, but gets to unstable when speed increases at the existing lengths.

Nico


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 Post subject: Re: Hydro Foil almost done
PostPosted: Mon Oct 29, 2012 10:12 am 
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Nico wrote:
Hi Gunnar.

A practical question: have you tried any substantial extension on the fuselage to get a better stability at higher speeds (1mt-1,5mts). ???
Seems to me the potential of high speed is there, but gets to unstable when speed increases at the existing lengths.

Nico


Hi Nico,

We have not tried such long fuselages. I can see having them that long will have the following 2 problems:

1) The longer the fuselage, the more stable and less agile the foil gets. I already noticed a big difference in agility when we increased the length by 10cm. at 1m or even 1.5m your will probably not be able to control the foil easily anymore as it will not want to turn.

2) Flexibility will be a problem. A fuselage that long will start to become flexible. We already have to put a lot of reinforcement in the fuselage to keep it stiff, so to keep the rigidity we need at 1-1.5m the thing will weigh too much. We would need to make the mast out of some very expensive materials.

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Gunnar


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 Post subject: Re: Hydro Foil almost done
PostPosted: Tue Oct 30, 2012 5:03 am 
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Location: Mauritius, waterman since 1960
Thanks Gunnar.
Yes I had envisaged such side effects, but think these could be circumvented.
The reason I wanted such stability was to be able to concentrate more on getting big power to obtain very high speeds wih absolute control.
I see record potential along this route without the need for absolutely flat water and specialized tracks.
regards
Nico


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 Post subject: Re: Hydro Foil almost done
PostPosted: Tue Oct 30, 2012 12:04 pm 
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Location: Fuerteventura
Hi Nico,

For maximum speed you will need smaller wings with more rake. This will make them plane/fly later, but will give you more speed.

With big wings like ours you need less power to get up, but they are not made for anything more than 50km/h straight line speed. Above that the lift just gets way to much to handle.

The stability will come from finding the perfect balance between wing size and Fuselage length. Smaller wings will not need such a long tail section of the fuselage.

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Gunnar


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 Post subject: Re: Hydro Foil almost done
PostPosted: Tue Oct 30, 2012 5:58 pm 
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I've built then flown D-tubes for rigid-wing hang gliders in the not so
distant past. Typically the Uni carbon along the spar-caps will control
stiffness of the wing span-wise,(the bending moment) ; "ALL" the torsional moment,
or ((resistance of the wing-tip changing angle of attack)), is controlled by placing woven carbon
at a +- 45deg around the D- part of the tube. The +-45 fiber orientation basically slowed down the tip from changing AOA.

Do you use +-45 woven in your mast ?

On the same note : " replacing the +- 45 woven carbon, with Stitched carbon of the same weight,
will add about 25% More torsional rigidity". Anytime you weave a fiber, it looses
about 20-25% of it's strength ; it's woven to make it easier to place in molds or On stuff.

Bille


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