Building a hydrofoil

[Tiago1973]

guess took the day for stupid questions...

my understanding is that the second wing (the smaller one) is there to stabilize the pitch given by the main wing by creating negative lift; assumption is Cg is aft the main wing

would not 2 smaller wings - 1 in the front, the other in the back - would do the same effect? with
the advantage of having 2 wings creating positive lift

off course no idea on the side effects of this setup....

[BWD]

You don't want the main foil to pitch down, and too far up will stall it, naturally.
So the tail is there to help with stability
I think the conventional design is to have the tail wing be neutral to very slightly down, and develop a small trimming moment when AOA changes, so the rider can control the main wing AOA by weight shift on a human timescale over a range of speeds, rather than reacting to every small scale variation as the foil passes through waves -that would probably be impossible at speed.
Foiling boats have less ability to trim by weight shift so tend to have trimmable flaps or trimmable foils (as in the AC boats) to address trim/stability.

[ChristoffM]

@Bill
I believe your assumptions are very good. I have tried a solid 16mm alluminium keel and I believe 12mm would be a minimum one would need.

The CNC is becoming a lot of fun as I get better at it.

@Bille
Sorry, I already added one layer of carbon. It has not altered the shape too much, although it still feels like it is not stiff enough. It now resonates at a higher frequency and might be stiff enough, but I want it more solid so that I can really manoeuvre my foilboard hard around the waves. I am going to keep adding carbon till I am happy with this foil. The wings and strut parts are very nice on this model.

@Tiago
Yip, I tested it already and I do not think the elliptical wing has any stall problems. But my fuselage was so weak that it was seriously affecting performance and causing sudden collapse syndrome.

That is a made up term for a foilboard that flexes too much and causes a sudden stall on some part of the foil and throws you off without warning. I do not believe the stall to come from the elliptical wing, but from the fuselage flexing too much and the angle on the rear wing changing just too far.

It also vibrates at a rather high frequency (I'm guessing in the 10Hz range) which I believe is a twisting vibration of the fuselage.

[stefaans]

wait wait wait, was everything just renemaed.

Does a foil still consist of a. Mast, T-bar, Wing and stabilizer?

[Bille]

...

@Bille
Sorry, I already added one layer of carbon. It has not altered the shape too much, although it still feels like it is not stiff enough. It now resonates at a higher frequency and might be stiff enough, but I want it more solid so that I can really manoeuvre my foilboard hard around the waves. I am going to keep adding carbon till I am happy with this foil. The wings and strut parts are very nice on this model.
...

What direction is your Fiber orientation , in the thinnest part of your fuce ?
What type , carbon count , and weave are you adding ?
How Long is your strut once again ?

Bille

[ChristoffM]

I put carbon braid over the rear section of the fuselage. There is only one type for the thickness of braid I used at my local supply store, I think it was 50MM/+-45/54g and looks like this (I do not know if it identical to this though): http://www.easycomposites.co.uk/carbon- ... -40mm.aspx.

I think this is the best method to add stiffness to resist twisting?

With one layer on I still think my fuselage is not quite stiff enough in the twisting direction, but the flex up and down and left and right feels like it is just about good enough. Hopefully I can test it out today.

[ChristoffM]

Sorry I missed your other question: my strut is 900mm long, which makes a big difference over my other foils which are 800mm. I ride mainly in the sea with some chop or waves.

My fuselage is almost 700mm long.

And I see I missed one of your earlier questions as well. I made my strut an eppler 836 profile as shown here http://airfoiltools.com/airfoil/details?airfoil=e836-il. Even though I scaled it to 20% thick (20mm thick strut and 100mm chord) it still feels very good. I personally prefer a stiffer strut at the expense of some extra drag. Other guys might not mind a bit of a loose/wobbly feeling (or gummy response) to get a better glide feeling of a thinner profile.

[Bille]

...

I think this is the best method to add stiffness to resist twisting?

With one layer on I still think my fuselage is not quite stiff enough in the twisting direction, but the flex up and down and left and right feels like it is just about good enough. Hopefully I can test it out today.

I "WAS" going to say No , until i read the part about it's the Twisting moment
that needs More strength !

I believe your attacking the problem correctly with that (+ - 45 braid) ; only
way to do it better would be to use a uni-carbon in the same fiber orientation ---
but that would be a Pain without a mold. WITH a mold it's Easy to use stitched.

The stitched + - 45 carbon is Stronger than your braided weave of the same weight & tow-count
because the tows are already Straight ; about almost 30% stronger in tensile and
compression.

Bille

This one ?
Eppler E836 hydrofoil
Max thickness 12.6% at 42.8% chord.

And i "LIKE" it
Hoping Ya don't mind that i just Stole it from Ya ??
I'm putting That airfoil , on Mine !!!

*** I Just re-did my order to that Super-Cool Eppler airfoil ; mine is staying
at 12.6% thick, with a cord of 127mm, (5"), and the mold length will be 111cm.
That gives me a 16mm thick profile , (should be plenty) ! Time to go do something
else, till my cut-foam comes in .

[ChristoffM]

Bille, could you take some pictures while you make your foil? It sounds like we can all learn a lot from you about the moulding. I have been messing around with it, but my grinding and CNC skills are much better.

I'm glad I could help on to get a nice profile. I just found it on the internet somewhere. I think any laminar symmetrical profile will look quite similar to that one. That one is specifically for hydrofoils if I remember correctly. Probably military boat hydrofoils if I had to guess, but it works great for kitefoils too.

[Europ2]

As you will see in the following posts, the French cousins have started a few years ago and there is a lot to learn from on the Kitefoil Forum expecially the 2 first pinned posts here in the DIY section:
http://translate.google.fr/translate?sl ... s-amateurs

Gold mine no 1: mast based on G11 core (E837 profile, 11cm chord, 13 mm thickness)):
Dups Foil V3

Gold mine no 2:
Split foil based on celeron or G11 core

Enjoy !