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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 12:28 am 
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zfennell wrote:
Personally the best technical advice i have to offer is:

GOOD, FAST, CHEAP.
-Pick any 2


carry on kids.


Excellent advice!



Bille wrote:
And just for a reference here :
If a Metal foil was Better, then the AC-72 would have use metal instead of carbon.
Each team had a 100-million dollar budget, and could afford Anything they wanted.
There was a Reason they went with Carbon, it's Better.
How fast does the AC-72 go ? 45kts + ; and they weigh How much ?

Bille


Most of us cannot afford a carbon foil let alone what you are writing about here.
To make a foil that is significantly lower cost than current carbon foils, I bet metal would be a good solution.


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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 1:33 am 
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don pitcher wrote:
zfennell wrote:
Personally the best technical advice i have to offer is:

GOOD, FAST, CHEAP.
-Pick any 2


carry on kids.


Excellent advice!



Bille wrote:
And just for a reference here :
If a Metal foil was Better, then the AC-72 would have use metal instead of carbon.
Each team had a 100-million dollar budget, and could afford Anything they wanted.
There was a Reason they went with Carbon, it's Better.
How fast does the AC-72 go ? 45kts + ; and they weigh How much ?

Bille


Most of us cannot afford a carbon foil let alone what you are writing about here.
To make a foil that is significantly lower cost than current carbon foils, I bet metal would be a good solution.


OUCH --don !
i'll re-word that :

Here is a 12K stitched carbon ; its 50" wide, and $17.50 a running yard .
It's Thickness .035".
http://compositeenvisions.com/carbon-fi ... m-795.html

The 50K uni i use, $25 a POUND ; comes on a 1Lb roll.

I Don't consider that extravagantly expensive, (for what Ya get) ?

Bille


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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 2:59 am 
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Your distortion of the whole picture is amazing.

If it was so easy and inexpensive as you say, then why do people pay so much for these problems?

http://carafinouk.com

How many metal hydrofoils have these problems?


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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 3:06 am 
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zfennell wrote:
Bille wrote:
................There was a Reason they went with Carbon, it's Better.
How fast does the AC-72 go ? 45kts + ; and they weigh How much ?
Bille


I believe they said the AC-72 weighs close to 30,000lbs
pretty impressive!

Bille,
overall, I tend to agree with your choice of materials.

But i am having trouble, following the 'analysis'
Hawaiis has followed his intuition and hard work to develop a product that suits him.
You have also followed you intuition and considerable experience with composites to develop an approach that suits you.

However, despite the fact that i support your choice, i find much of the logic and math questionable.

The fiber strength of T-700 is actually quoted as high as 700 ksi (in tension)
In compession the strength drops to 350 ksi. (every tensile load in this beam example will be balanced in compression)
All these fibers need resin to tie the matrix together. A 50% ratio of 10 ksi epoxy (nominal bag process) is going to dilute the 350 ksi to 180 psi net in that particular fiber direction.
Now that you chosen to orient the fibers in an additional 5 directions the max compressive and tensile strength will be reduced even further. < 100ksi ( all because you've attempted to make this stuff isotropic?)
And none of the above addresss the shear strengh between laminates which is effectively limited by the epoxy again at 10ksi.

The benefit of the fibers is you can put them where you need them. They do very little if pointing in the wrong direction. I bet those smart guys on Team Oracle do something like that.
Iso-tropic makes the math easier, but i doubt thats a good enough reason to take a fancy composite layup and turn it into a lump of coal.

Hawaiis has covered a lot of ground on his own and so have you.

Personally the best technical advice i have to offer is:

GOOD, FAST, CHEAP.
-Pick any 2


carry on kids.



My advice is to stop discussing strength, start talking STIFFNESS and how to increase it. Start quoting Modulus of elasticity (E) for materials instead of tensile strength which in this case is not terribly relevant,

Making a foil strong enough should not be a challenge go back to grade 12 physics and draw a free body diagram, the loads are laughable. Unless thin high aspect foils are being used.
His piece of wood with a single layer of glass was STRONG enough, it just wasn't STIFF enough

Until the difference between these 2 properties are understood this conversation is not going to be terribly relevant.

Strength, toughness, and stiffness. They are very different properties and not necessarily dependent on each other. through shape, material choice, and fiber orientations, they can all be altered (almost) independently. If you ever want to build something REALLY GOOD out of composites, take the time to understand all three.

Strength - How much can you yank on it until it breaks (Ultimate tensile and compressive strength)
Stiffness - How much does it flex when you yank on it with a given force (Modulus of Elasticity)
toughness - How easily does it break when you jump on it or smash it into something. (drop testing or impact testing ) oftentimes related to the area under the stress strain curve.


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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 4:38 am 
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don pitcher wrote:
Your distortion of the whole picture is amazing.

If it was so easy and inexpensive as you say, then why do people pay so much for these problems?

http://carafinouk.com

How many metal hydrofoils have these problems?


The original post on this topic, it indicated i was quite Good with
composites ; Yes i would prefer to Stay with what i know :

Bille wrote:
Building the hydrofoil, where do i start ?
Lots of questions like aria, airfoil, length of pod, and AOA
of each wing.

There is so much info on board builders, about building boards
that it would be Easy for a guy to get the info for his first TT ; appears
to be more difficult to find that info about Hydrofoils.

I have most all the materials needed, including the composite expertise ; all
that's left is a set of workable plans or ideas. With a decent guide, i would prefer
to shape a blank & mold it because i could build a carbon wing WAY stronger
than a shaped foam or ply. Also i could bend the mold to get anhedral ; would be quite
easy for my first attempt.

Bille
...


Now if you wanna make a metal wing, then go & do it . I could see a hybrid
metal-wings & carbon body & pylon.

On that carafinouk link , looks like some mistakes were made , short-cuts
taken that didn't work out so well.

They want onwards to $2,000 for one of those things ; my cost analysis
tells me i can do one for $250. I'll make it modular, so i can guarantee
myself a decent set of odds i'll get it to work with the info i got off this
and other threads .

Bille


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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 4:58 am 
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Quote:
why do people pay so much for these problems?

because they fancy themselves athletes rather than fabricators!

Thanks to all who write about what they know and are willing to share!


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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 5:25 am 
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Johnny Rotten wrote:
...


My advice is to stop discussing strength, start talking STIFFNESS and how to increase it.
...



...
His piece of wood with a single layer of glass was STRONG enough, it just wasn't STIFF enough


...


NOW were Talking !! :D :thumb: :thumb:

If he had made a groove with a guided router-bit down both sides of his airfoil at
the thickest station, make
it 1/8" deep, and 1/4" wide , then back-fill it with 50K carbon uni ; he wouldn't
have Needed the glass. I would have placed a 1.7oz Kevlar in it at a + - 45 over
the entire wing. Twisting would-a disappeared completely . Kevlar hates UV
so paint is now needed.

Bille


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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 10:58 am 
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For the AC 72 foil design they have extensive knowledge of: CFD, FEA, hydroelasticity, process capability, post production testing...

The AC 72 foils are made in an autoclave with no expenses spared with a fiber content probably approaching 0.8 volume fraction plus they are probably getting it with custom resin and fibers with special surface modifications.

When you do hand lay-up the volume fraction of fibers is 0.5 at best and with a high porosity fraction as well. This means that the performance in any direction the cross section is half fiber half resin. Since the resins structural performance is nowhere near the fibers you can as a rule of thumb assume that half of the strength is lost right there. Then add effects of porosities and contamination.

And it is still not isotropic and thus capable of producing some very odd buckling and vibration characteristics.

You can do a good foil in carbon fiber reinforced composites it is just not easy and because it is carbon does not mean it performs like an AC 72 foil or anywhere near that.

A very high AR thin profile foil must be the best so a CFRP foil will be the best if the design is up to it.


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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 5:48 pm 
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Hawaiis,
a few page back, you made this comment.
Quote:
On my high AR wood front fin the span was 25 inches long, it was flexing too much, so I fiberglassed it ....


latter you offered this, as clarification.

Hawaiis wrote:
I lay the fiberglass 90 degrees. The vibration creates a loud low frequency noise. Any vibration will create more drag than a rigid wing. In the French forum, they are talking about using prepreg high modulus carbon fiber to make the foil more rigid.


and after some prodding from Bille, you offered this.
Quote:
It is not the twisting moment. It is the lengthwise flex that was the problem.


may i assume that your observations of low frequency vibration is the only indication you have of span-wise flex?

was the vibration noticable at all speeds or more distinct at certain speeds?

have you compared span-wise stiffness of your different foils with any static measurements?

For Johnny Rotten,
Is this the priincipal reason you believe stiffness is the most relevant design criteria?

thanks.
Bill


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 Post subject: Re: Building a hydrofoil
PostPosted: Fri Nov 01, 2013 7:45 pm 
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"may i assume that your observations of low frequency vibration is the only indication you have of span-wise flex?

was the vibration noticable at all speeds or more distinct at certain speeds?

have you compared span-wise stiffness of your different foils with any static measurements?"
Bill:
You are right, the low frequency vibration at a certain speed is the only indication of flex.
I did not do a static test.
I assumed the long wood foil was flexing because my low AR wood foil did not have that problem.
I tried to bend the wood foil and with 16mm thickness, it was definitely more stiff than my 2.5mm titanium high AR foil.


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