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Designed foil behaviour?

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fluidity
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Designed foil behaviour?

Postby fluidity » Wed Nov 07, 2018 10:16 am

Hi all, I've not ridden foil before.
I usually design and build my own stuff (I'm currently riding my own 4'th design twintip which I resin infusion moulded.)

So two questions, the answer to the first may influence the feasibility of the second:

1. On an average foil setup, what's the difference in angle from the horizontal of the main wing and the rear stabiliser wing?
i.e. is there a natural tendency for the board to carve upwards or downwards?

2. At the moment I have no driving desire to cut over to a surfboard. I understand with more buoyancy a smaller kite is needed and the focus shifts from the kite to surfing but I'm interested in a more radical change, I've already windsurfed a lot years ago. So the question is, how effective do you think a bidirectional foil arrangement could be made? (to mount under a twintip)
Flip in/out front and rear stabilisers are one option with direction controlling the in/out flip.
Obviously you can't use a perfect hydrofoil wing shape with curved leading edge on the main foil and thin trailing edge taper but from reading builder's threads this wouldn't be much of an issue?

tomtom
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Re: Designed foil behaviour?

Postby tomtom » Wed Nov 07, 2018 10:58 am

It will be very very inefective. To the point no one making it.

Further whole nature of surfing is directional. So why anyone want bidirectional foil? Jibing is 80% of fun on foil

windmaker
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Re: Designed foil behaviour?

Postby windmaker » Wed Nov 07, 2018 11:54 am

It has been done numerous times. It s a big compromise (just like with a normal TT ) to the extend that you loose any advantage gained with the foils. Think of an airplane flying backwards :)

Mossy 757
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Re: Designed foil behaviour?

Postby Mossy 757 » Wed Nov 07, 2018 8:42 pm

fluidity wrote:
Wed Nov 07, 2018 10:16 am
1. On an average foil setup, what's the difference in angle from the horizontal of the main wing and the rear stabiliser wing?
i.e. is there a natural tendency for the board to carve upwards or downwards?
Some are set in the 2-3.5 degree range, tending towards a "nose-up" attitude. I've also seen setups that were at 0 degrees, but I don't think that's particularly effective for most setups.
2. At the moment I have no driving desire to cut over to a surfboard. I understand with more buoyancy a smaller kite is needed and the focus shifts from the kite to surfing but I'm interested in a more radical change, I've already windsurfed a lot years ago. So the question is, how effective do you think a bidirectional foil arrangement could be made? (to mount under a twintip)
Flip in/out front and rear stabilisers are one option with direction controlling the in/out flip.
Obviously you can't use a perfect hydrofoil wing shape with curved leading edge on the main foil and thin trailing edge taper but from reading builder's threads this wouldn't be much of an issue?

I think this is one of the stupidest ideas I've seen since I became aware of hydrofoiling. This is what I think of whenever I see a bidirectional foil:

Image

Conceptually...close, but nowhere near what actually works in the real world.

fluidity
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Local Beach: Ngati Toa, Plimmerton, Titahi Bay, Waikanae, Petone, Seatoun, Lyall Bay, Eastbourne, Lake Wairarapa
Style: Wave, jump
Gear: Old Flexfoil 9, Switch Element V4 11m, Ocean Rodeo Prodigy 7m 2014, Nitro14m v6, Nitro9m v6 12mSwitch Legacy2
DIY CAD ultra-concave wave twintips 1500 and 130mm with my own fin designs. Easy upwind, awesome carving.

Re: Designed foil behaviour?

Postby fluidity » Fri Nov 09, 2018 9:50 am

Interesting attitudes and comments to my query about a deviation from what's still a pretty new sport, even for kiteboarding which is also so brand new that it's barely a generation old!

I'm a bit disappointed though that I had to work out for myself the most basic issue (and only technically hard to overcome one) which is that a twintip transition can include sliding of the board in the toe-side or heelside directions but with a foil board this is impossible assuming the mast is extended and it's profile parallel with the board. This one is no doubt obvious for those who have already foiled and explains the emphasis on a gybing transition otherwise you would have to either restart your foil or switch heelside to toeside and stay toe-side for the other tack (which would reduce the usefulness of bi-directionality)

Still, challenges are for overcoming!

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Re: Designed foil behaviour?

Postby evan » Fri Nov 09, 2018 1:11 pm

There are tt-hydrofoils available, even ones with sliding systems.

The problem is that there doesn't exist a bi-directional profile with a good enough L/D ratio to give any low-end benefits compared to big twintips, skimboards and directionals. Besides that the tt-hydrofoils are inherently unstable, combined with the immense drag it is no wonder every attempt to make one gets frowned upon by directional foilers.
It is just a dead-end design-wise, a wish from twintip kiters to make foiling more accessable by removing the need to gybe. But in the process making foiling a lot harder by trying to foil on a unstable draggy platform.

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Re: Designed foil behaviour?

Postby plummet » Fri Nov 09, 2018 6:52 pm

I am a designer and builder of my own boards. I also love TT/Mutant boards and have designed and built my own foil. I too considered a TT hydrofoil. But the compromises to make it work are far to great. You will end up with a foil that real draggy and slow, has to be stopped to change direction if you want to stay heal side. All for what? because you dont want to learn to jybe?

Far better is to build a directional hydrofoil that you can bolt to a TT style of board. Put in the hard yards, learn to jybe and the need to have a Bi-directional foil is eliminated.

Further to foil design. The rear stabilizer is typically flipped and provides negative lift. This is to alight the center of lift and the center of gravity together. With 2 lifting wings, the C of G moves rearward of the center of lift and you get instability. Also rear stabaliser is typically 1/3 the size of the front wing. A bigger rear wing will increase drag significantly. A Bi Directional foil starts of life with inherently unstable and draggy design brief.

My suggestion is this.

Buy a shitty cheap foil. Learn to foil first. Then design and build your own foil after you have been through the learning curve. Then after you have learnt and build your own traditional foil, if you are still interested in a bi directional foil go about designing one then.

PositiveNegativeLift_Zeeko.jpg

tomtom
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Re: Designed foil behaviour?

Postby tomtom » Fri Nov 09, 2018 8:58 pm

The plane that can fly backward is most exact definition of what you want to achieve. Our comments wasnt rude or anything it was just reality check. There are lot of tinklers here you are not alone making own gear. So it is possible to make bidi foil - which will fly - it just will be very draggy with no benefits to normal TT at all. Also as former windsurfer too i always find directional nature of sport appealing.

edit - i wrote this before plummet - and little redundant to his detailed explanation

fluidity
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Posts: 136
Joined: Sat Mar 19, 2016 11:20 pm
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Local Beach: Ngati Toa, Plimmerton, Titahi Bay, Waikanae, Petone, Seatoun, Lyall Bay, Eastbourne, Lake Wairarapa
Style: Wave, jump
Gear: Old Flexfoil 9, Switch Element V4 11m, Ocean Rodeo Prodigy 7m 2014, Nitro14m v6, Nitro9m v6 12mSwitch Legacy2
DIY CAD ultra-concave wave twintips 1500 and 130mm with my own fin designs. Easy upwind, awesome carving.

Re: Designed foil behaviour?

Postby fluidity » Fri Nov 09, 2018 11:17 pm

Thanks evan, plummet and tomtom.

plummet, I'm trying to understand your bottom right diagram. I think critical is that the feet combined with the foil and stabiliser control a pivot point. Without the stabiliser the board would be super sensitive and likely near impossible to gybe.
With acceleration from kite transferred through the rider's feet the tendency would be for the board to rotate forward over the foil and cause a dive.
With deceleration from kite transferred through the rider's feet the tendency would be for the board to rotate backwards and stall or breach the foil.
So at a neutral setting and with neither + or - lift the stabiliser would dampen the above tendencies but only by speed vs acceleration forces so the reaction is limited by time for acceleration to occur..
With the negative lift setting for a rear stabiliser the stabiliser together with the main foil causes the nose of the board to lift with more force as speed increases. (why we see lots of breaches in videos of learning kite foilers)
Has anyone experimented with a low tech arrangement to make the rotation from acceleration dampen from force rather than speed increase? (live)?

plummet
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Re: Designed foil behaviour?

Postby plummet » Sat Nov 10, 2018 10:37 pm

George at delta hydrofoil explains stab importance and angles very well.

phpBB [video]


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