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 Post subject: Re: Building a hydrofoil
PostPosted: Tue Nov 05, 2013 5:23 pm 
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Location: Langebaan (Cape Town)
Strut it will be then. With riser as an alternative word and keel for an explanation if needed.

Now back to manufacturing them.

Here's a photo of my current foil:

Attachment:
G10-2.jpg
G10-2.jpg [ 26.57 KIB | Viewed 535 times ]


the whole hydrofoil is G10 material, but unfortunately glass is a little bit heavier than I had hoped. Total mass for the foil is 4.5kg.


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 Post subject: Re: Building a hydrofoil
PostPosted: Tue Nov 05, 2013 8:09 pm 
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very nice Chris! try it out and let us know how it works.


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 Post subject: Re: Building a hydrofoil
PostPosted: Tue Nov 05, 2013 10:34 pm 
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Location: rhode island
question for the folks that have a strut that they consider acceptable:

actual numbers for longitudinal and torsional stiffness of the structure would make the layup more predictable.

i.e
1) apply load at prescribed distance from end of strut (cantileverd) and measure displacement
2) apply moment at end of strut and measure degrees rotation.

from that, the amount of fiber to replicate results in geomety of choice is doable
or
the amount of fiber to alter stiffness to desirable levels is possible

so far, the numbers suggest that hollow alumium (NACA 12, 100 mm chord) with wall thickness of 3mm is up to the task.
principal stresss appox 15,000 psi
peak shear approx 5, 000 psi

carbon with similar net modulus (10-15 million psi) doesn't sound too extravagant.
any opinions?


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 Post subject: Re: Building a hydrofoil
PostPosted: Tue Nov 05, 2013 11:01 pm 
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ChristoffM wrote:
Strut it will be then. With riser as an alternative word and keel for an explanation if needed.

Now back to manufacturing them.
...

the whole hydrofoil is G10 material, but unfortunately glass is a little bit heavier than I had hoped. Total mass for the foil is 4.5kg.


"STRUT" it Is !!

Did YOU build That ?? :o

zfennell wrote:
...

so far, the numbers suggest that hollow alumium (NACA 12, 100 mm chord) with wall thickness of 3mm is up to the task.
principal stresss appox 15,000 psi
peak shear approx 5, 000 psi

carbon with similar net modulus (10-15 million psi) doesn't sound too extravagant.
any opinions?


I'm gluing This , (pre-made) stuff in strategic places along where the Major stress is
located ; it's good for Tensile Strength 300,000 psi (2068 mpa)

You can NOT get those numbers in a hand lay-up.

http://www.marskeaircraft.com/carbonrod_p2.html

=============================================
The CNC shop that does the hot-wiring is backed-up till
this weekend, so i got till then to decide if i
want a full 5" cord, (127mm) , right now i'm thinking
of just 120mm and can't decide ?

When i Hot-wire, i make my stencils from 1/16" , (1.5mm) thick aluminum
because everything else i've tried, makes the hot-wire stick and make
full-length grooves down my foam airfoil. I've cut a Lot of RC airplane wings
in my life. ----------------------------------------------------i still Suck at it !! :(
That's why i'd rather have a Pro do it.

Bille


Last edited by Bille on Wed Nov 06, 2013 3:04 am, edited 1 time in total.

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 Post subject: Re: Building a hydrofoil
PostPosted: Tue Nov 05, 2013 11:20 pm 
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Location: rhode island
not sure i understand.

anyplace you can find room for a solid rod/rectangle will NOT be the location of peak stress.

the spars in fabric planes do something similar, but no compressive loads are carried in the skins.
seems like a waste of carbon for your molded surface. (or a waste of carbon for your spar)

they never mention the torsional stiffness of the uni-rods. (not so much, i bet)
the fibers that do that job will need a home, too

Bille, i bet you already know better, but are too busy selling the carbon.
i'm sold.
however, i still believe in "good, fast, cheap, ....."


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 Post subject: Re: Building a hydrofoil
PostPosted: Wed Nov 06, 2013 1:23 am 
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zfennell wrote:
...

they never mention the torsional stiffness of the uni-rods. (not so much, i bet)
the fibers that do that job will need a home, too

Bille, i bet you already know better, but are too busy selling the carbon.
i'm sold.
however, i still believe in "good, fast, cheap, ....."


Making Molds, filling them with composites IS what i consider
"good, fast, cheap, ....." ------------------carbon can be had for $25 a pound
and an aircraft grade of Epoxy goes for $100 .

I do Not sell carbon, i Do sell the idea that composites are a Good
alternative for making stuff though.

** You don't get the torsional strength from the rods, unless they are also
laid up on a + - 45deg.

In the Strut :
My 12K stitched carbon on a + - 45 will carry the torsional loads. The carbon rods
will carry the bending loads of the Strut on there 0/90deg arangement; they go on
the High-point of the airfoil
and run parallel to the length of the strut. Then i place in a core, (nomex honneycomb)
or balsa, and then another inner skin to carry the shear loads of the outer skin
which makes the Local stiffness of the skin Way higher than without it.

I WILL place a shear-webb to hold the rods in space ; Much better
than the hollow aluminum your talking about, and after i make (1) the
next one will take 1/2 the time. Most of the labor is in the actual Mold.

Haven't got a mold yet, and a picture is worth a Lot of explaining, so
i need a bit of time here please !! Just ONE glance at the lay-up, and
i'm Certain you will immediately see ...

A guy COULD get this same lay-up by bending balsa around a broom-stick ! :D
No Mold , but you will need a bit of filler to finish the exterior before paint.
I'll be spraying My paint directly into my mold.

Bille


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 Post subject: Re: Building a hydrofoil
PostPosted: Wed Nov 06, 2013 5:21 am 
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Location: Langebaan (Cape Town)
Quote:
Did YOU build That ??


Yip, lots of shaping. It does help to have a slave in the garage doing a lot of cutting all day, aka a CNC mill.

Quote:
actual numbers for longitudinal and torsional stiffness of the structure would make the layup more predictable.

i.e
1) apply load at prescribed distance from end of strut (cantileverd) and measure displacement
2) apply moment at end of strut and measure degrees rotation.


The French guys are a few steps ahead of us. They started doing it over at http://kitefoil.forumactif.org/, but I cannot seem to find the topic right now (it would also help if I could read french).

Quote:
so far, the numbers suggest that hollow alumium (NACA 12, 100 mm chord) with wall thickness of 3mm is up to the task.


Did you calculate that? I think you are right, but I believe you are right. I have a solid 16mm aluminium strut which is far more than stiff enough. I think 12mm in aluminium might not be quite stiff enough, but it should work if you make a shorter strut (800mm or less). It definitely has more than enough torsional stiffness.

By the way, I would not go NACA12 profile for the strut. I would rather make the strut 2mm thicker and use a laminar flow profile, which I suspect will give the same drag numbers and make construction much easier I think?

My G10 strut is 20mm thick (yes very thick), but I do not notice it having too much drag.

The G10 foil works very nicely except for the fuselage part that I made too thin. I am busy reinforcing it with some carbon fibre.


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 Post subject: Re: Building a hydrofoil
PostPosted: Wed Nov 06, 2013 7:03 am 
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ChristoffM wrote:
Quote:
Did YOU build That ??


Yip, lots of shaping. It does help to have a slave in the garage doing a lot of cutting all day, aka a CNC mill.

...

---By the way, I would not go NACA12 profile for the strut. I would rather make the strut 2mm thicker and use a laminar flow profile, which I suspect will give the same drag numbers and make construction much easier I think?

My G10 strut is 20mm thick (yes very thick), but I do not notice it having too much drag.

The G10 foil works very nicely except for the fuselage part that I made too thin. I am busy reinforcing it with some carbon fibre.



You do GOOD work !!! :thumb: :thumb: :thumb:

Those carbon rectangle from Marske Would make your fuselage work with the shape you
have now, if you make it from a mold.

Instead of adding More skin, to the one you have now ; Mold it "FIRST" because
it's a Sweet Shape !!
Then beef it up and use it like it is , while you make a Super-Bitchen one from out of the
mold!!!
It WILL flex a bit, but the return to center would be Fast with the addition of some Spectra
and that carbon rod material.

Could you please tell me what airfoil you used on Your strut ; with 20mm thickness, i
could make that thing weigh near Nothing, with some rather exotic composites from
out of a mold. Stiffness increases by the Cube of the height, or (thickness) ; so going
from 15mm to 20mm gives a LOT of extra stiffness. That extra increase can be
traded off for weight reduction.

My foam cutter is backed up till this weekend, so i can change my order now, if
i hurry ; an airfoil name & # would be good to have ?

Bille


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 Post subject: Re: Building a hydrofoil
PostPosted: Wed Nov 06, 2013 9:53 am 
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Posts: 661
question -
why not make the width of the keel/tower much longer?
(meaning: longer chord of the airfoil that acts as a keel/tower)

in low winds, while there is no lift from the hydrofoil, it would help the board going up-wind (as the fin keel in a boat)

once the hydrofoil starts doing his magic the keel will be out of the water anyway and as such would not drag much

i´m sure there are downsides as no one is design it that way - interested in hear about it

@ Christoff
did you ride already those elliptic wings?

what´s your take on the way it stalls? is it an issue?


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 Post subject: Re: Building a hydrofoil
PostPosted: Wed Nov 06, 2013 1:59 pm 
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Location: rhode island
ChristoffM wrote:

Quote:
so far, the numbers suggest that hollow alumium (NACA 12, 100 mm chord) with wall thickness of 3mm is up to the task.


Did you calculate that? I think you are right, but I believe you are right. I have a solid 16mm aluminium strut which is far more than stiff enough. I think 12mm in aluminium might not be quite stiff enough, but it should work if you make a shorter strut (800mm or less). It definitely has more than enough torsional stiffness.

By the way, I would not go NACA12 profile for the strut. I would rather make the strut 2mm thicker and use a laminar flow profile, which I suspect will give the same drag numbers and make construction much easier I think?

My G10 strut is 20mm thick (yes very thick), but I do not notice it having too much drag.

The G10 foil works very nicely except for the fuselage part that I made too thin. I am busy reinforcing it with some carbon fibre.


Christoff.
yes, i should have explained.
I drafted a shell model of a foil section (100mm chord, 12% thickness, 1m overall lenght)
applied my own assumption for boundary conditions (rider weight , weight moments, torsion from lift on foils, ...still assumptions)
the math comes from finite element program.
it can cope with laminate properties, but i kept the material isotropic just to see how dramatic the stress was.
the program can also address buckling and resonanat modes . which were reasonable

but, any real life measurement of stiffness from working foil would support /refute any load assumption i'm playing with at the moment

i have no argument with your choice of foil profile .
this was just a structural calc to help me assess the maginitude of the loads , stress., deflections.


nice job on your foil.
building a CNC machine in the garage has all the makings of a good story.
-bill


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