*


All times are UTC + 1 hour



Post new topic Reply to topic
 [ 78 posts ]  Go to page Previous  1 ... 4, 5, 6, 7, 8
Author Message
 Post subject: Re: Twin Tip Hydrofoil
PostPosted: Sat Dec 07, 2013 6:38 am 
Offline
Rare Poster

Joined: Sat May 25, 2013 4:00 am
Posts: 11
Thanks Hawaiis, heres a bit of theory of mine.
Any shape can be used for a foil and the shape carries its performance characteristics...wider foils are more stable side to side and longer foils are more stable nose to tail, but how do you work out the best dimensions ?
Over the past decade this is what Ive tried to find out...I started with twin lateral foils much like the commercial foils you can buy but I had both foils the same size and shape. This meant that I wasnt riding off the front or back foot but had a longer sweet spot to stand over.
The twin foils were 23 or 24 inches wide, NACA profiled, with an 8in chord so there was ~400 sq in of planing area in total and those foils would plane at just a few knots. Sounds great and I thought Id cracked a great secret but as the speed increased the foils would go wild and start leaping out of the water.
The problem was too much lift, to maintain a submerged position, foils dont need constant lift but just enough for the speed and weight of the rider.
Soon enough I found a basic formula that said that Lift is multiplied by a factor of 4 when the speed is Doubled. so a foil that lifts 80 kgs at 5 knots will lift 320 kgs at 10 knots..... the problem with foils isnt lift, its control.
The 400 sq in foil setup shrank down to 100 sq in and they were well behaved at most speeds.
But the other problem with foils is that lateral foils lift, at any instant, for the distance of their chord, so no matter how wide a foil was, the measure of its chord was linked to its stability.
Now I could see why commercial foils had a larger lifting foil at the front and then a smaller regulating foil aft, much like an airplane where the bulk of the lift was in the bigger front wings.
The problem there was that there was part of the submerged setup lifting and another part not lifting so the aft section was definately playing a part but it wasnt focussed on pure lift... so why not build a foil setup that was inherently stable and totally designed for stable lift.
The foils had come down from a total of 400 sq in to a total of 100 sq in, no NACA profiling- just flat plate.
Then I started altering the distance between fore and aft foils and although the setup was more stable as the distance increased I found that the overall setup felt too wide. So as i brought the width of the foils down I increased the chord to maintain the overall area, the more the width decreased, the looser the foils were but they started to be a much smoother ride, a better feel overall, it seemed that the longer the chord, the better the foil would run through chop, the better it would handle minor drops in speed, longer chord and less width were a step forward up to the point where I was riding just one long foil that was 13 inches wide. I went down to a single foil 8 inches wide but that was too unstable.

Previously I had used twin lateral foils with an overall length of 30 inches and now I was riding a single foil 13 in wide X 23 inches long. 3mm flat plate Aluminium.
Hvave I bored everyone to death yet ??

viewtopic.php?f=107&t=2381964


Top
Profile
 Post subject: Re: Twin Tip Hydrofoil
PostPosted: Sat Dec 07, 2013 7:13 am 
Online
Very Frequent Poster
User avatar

Joined: Wed Aug 24, 2011 11:39 am
Posts: 890
Surffoils:
Wow, that is a lots of useful information accumulated in a decade.

Mahalo for sharing

Titanium Twintip Hydrofoil going upwind for the first time


Top
Profile
 Post subject: Re: Twin Tip Hydrofoil
PostPosted: Sat Dec 07, 2013 10:11 am 
Offline
Very Frequent Poster

Joined: Sun Jul 24, 2005 6:39 pm
Posts: 2922
SURFFOILS wrote:
Previously I had used twin lateral foils with an overall length of 30 inches and now I was riding a single foil 13 in wide X 23 inches long. 3mm flat plate Aluminium.
Hvave I bored everyone to death yet ??


Quite the opposite - that's a lot of useful info.

The 23" by 13" foil sounds like it would work as a bidirectional on a twintip?
Has it got a directional shape?

Also, have you come to a conclusion about angle of attack - ie. do you set the plate flat and angle it as the board angles or do you set an angle of attack from the boards under surface?


Top
Profile
 Post subject: Re: Twin Tip Hydrofoil
PostPosted: Sat Dec 07, 2013 12:03 pm 
Offline
Rare Poster

Joined: Sat May 25, 2013 4:00 am
Posts: 11
Thanks for the comments guys.
With the twin foils I did play around with AOA all the time as well as setting them at different heights so the front foil didn't create turbulence for the trailing foil, but now with a single foil there's no AOA , just parrallel to the hull, it makes it easier to know the angle of the foil, if its exactly the same as the board angle.
Some of the foils are bidirectional, some are single direction where Ive tried to work out if a tapered outline would be better.
The wood foil below, ( at 6 o'clock) is a design that gives a reduced nose area for soft turns with an increasing width and concave at the tail for more stability and grip off the back foot.

Image

The Blue foil to the left has a wider nose and then splits into a swallow tail configuration that gives stability without stiffness, I know I can push into turns and have it slide when I want.
Instead of doing a series of foils with varying outlines and then degrees of dihedral or anhedral, Ive focused on one element at a time and then worked out the best combination of elements.
With the Yellow foil in the middle Ive split the planing surfaces into 3 areas over 2 levels... there's the 2 side arms that give balance and the higher central area that gives greater area at low speeds but lifts out of the water at greater speeds leaving just the side arms submerged. This foil is great for fast acceleration in the surf and for wake boarding.
There's so many aspects to foil design but a long time ago I learnt that simplicity works best in the water, sure you could create a massively complex beast but every control surface is a drag surface so for the sake of efficienct design, simplicity is best.


Top
Profile
 Post subject: Re: Twin Tip Hydrofoil
PostPosted: Sat Dec 07, 2013 1:40 pm 
Offline
Very Frequent Poster

Joined: Sun Jul 24, 2005 6:39 pm
Posts: 2922
SURFFOILS wrote:
Thanks for the comments guys.
With the twin foils I did play around with AOA all the time as well as setting them at different heights so the front foil didn't create turbulence for the trailing foil, but now with a single foil there's no AOA , just parrallel to the hull, it makes it easier to know the angle of the foil, if its exactly the same as the board angle.
Some of the foils are bidirectional, some are single direction where Ive tried to work out if a tapered outline would be better.
The wood foil below, ( at 6 o'clock) is a design that gives a reduced nose area for soft turns with an increasing width and concave at the tail for more stability and grip off the back foot.

Image

The Blue foil to the left has a wider nose and then splits into a swallow tail configuration that gives stability without stiffness, I know I can push into turns and have it slide when I want.
Instead of doing a series of foils with varying outlines and then degrees of dihedral or anhedral, Ive focused on one element at a time and then worked out the best combination of elements.
With the Yellow foil in the middle Ive split the planing surfaces into 3 areas over 2 levels... there's the 2 side arms that give balance and the higher central area that gives greater area at low speeds but lifts out of the water at greater speeds leaving just the side arms submerged. This foil is great for fast acceleration in the surf and for wake boarding.
There's so many aspects to foil design but a long time ago I learnt that simplicity works best in the water, sure you could create a massively complex beast but every control surface is a drag surface so for the sake of efficienct design, simplicity is best.


That all sounds good for the prospect of a twintip foil that works. Is the blue plate at 11 o'clock the 23" by 13" plate?


Top
Profile
 Post subject: Re: Twin Tip Hydrofoil
PostPosted: Sat Dec 07, 2013 8:55 pm 
Offline
Rare Poster

Joined: Sat May 25, 2013 4:00 am
Posts: 11
The blue one at 11 o'clock is one of the extra small one at 8" wide X 18" long with 118 sq in.
The yellow one in the middle has a total area of 277 sq in but that includes the side arms (@58 sq in), the middle section and the angled beams that connect the middle to the side arms. Everything is measured and is a part of the design.

Image

You can read the dims in the pic, even the dihedral angle of the foil on the left is 8 degrees and the area of the dihedral is 14 sq in.

The overall shape of the yellow foil is anhedral but the main planing surfaces are the side arms, and now the shape is getting so wide and long that I decided that instead of just continuing to expand the size of the foil I could drop a lot of weight by splitting the foil into just 2 longitudinal arms and making minimalised struts to hold the arms. Using front and back struts that are 20 inches apart also split the 'yaw' pivot that comes from the standard single main vertical support of a commercial hydrofoil.
Spreading out the red struts gives better low speed stability.

Image

And having the wide set foils gives better stability when youve got the board over on an angle or when coming around at speed because theres more lifting area directly under you rather than a wingtip.

Image

I like having components that can be swapped over, the red Gullwing struts aren't suitable for kiting but the foil arms are. As you know you can change the 'board' on top to whatever you like because once its in the air it has no input on the performance unless it's too heavy or long and then 'swing- weight' becomes a problem.
So I tend to take the foils with me as I change sport and only the struts change and making new struts cost about $50 per set.


Last edited by SURFFOILS on Sun Dec 08, 2013 12:51 am, edited 1 time in total.

Top
Profile
 Post subject: Re: Twin Tip Hydrofoil
PostPosted: Sun Dec 08, 2013 12:45 am 
Online
Very Frequent Poster
User avatar

Joined: Wed Oct 16, 2002 1:00 am
Posts: 5562
Location: Denmark
Really interesting, and good R&D there :thumb:

So many variables to play around with !

8) Peter


Top
Profile
 Post subject: Re: Twin Tip Hydrofoil
PostPosted: Mon Dec 09, 2013 1:14 am 
Offline
Rare Poster

Joined: Sat May 25, 2013 4:00 am
Posts: 11
Thanks for your comments guys, its good to know I'm not talking to myself !

Here's a bit more on foil design...
Ive seen all the complex calculations to do with the foiling all things that go in the water, from the very complex NACA designs and consideration of lift Vs drag
And I compared the lift Vs speed numbers of all sorts of complex profiles to a 'flat plate' and the basic flat plate isn't that different from the very complex profiles. If you check a program like Foilsim you'll see that a minor adjustment in AOA will result in substantial lift so maybe a flat plate would be a much simpler foil to build.
The Flat Plate also has the benefits of having minimal displacement and form drag, so it will have less overall drag at all reasonable speeds, be lighter to carry and ride, and its a lot quicker to build and replicate as a DIY project.
But having a lighter, slimmer foil also brings an issue of foil flex, some of the ply and glass foils that are about 6 mm thick have had beneficial flex that can be fun to exploit, but when using flat Aluminium foils only 3mm thick there's a potential problem of keeping the foil shape stable as under pressure it could twist.
Instead of making the foil thicker I put a 90degree bend in it and used the downward bend to create a variable fin.

Image

So now Ive got the two planing surfaces (foils) out wide for stability, with the downturned sides to create grip in a twin fin fashion that gives the best control in critical positions.

Image

The downturned fins not only provide mechanical rigidity to the flat foil surface but it also creates a 25 in long fin effect.

Image

The fin is divided into two areas, with a relaxed leading edge curve that makes for a gentle introduction into a turn and a rear section with a square trailing edge for sharp release. The idea behind the two sections is that they give a feeling of longer drive and hold but when you put more pressure on your back foot it will pivot off the back section. There's still more to discover with this concept but if any of you have ridden quads then you know how you can work off the front or rear fins.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic
 [ 78 posts ]  Go to page Previous  1 ... 4, 5, 6, 7, 8

All times are UTC + 1 hour


Who is online

Users browsing this forum: Bing [Bot], BudhaNl, eloico, kjorn, oldkiter, ORSales, Peter_Frank and 36 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group