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 Post subject: Re: Carbon fiber lay up
PostPosted: Wed Apr 09, 2014 6:56 pm 
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Joined: Sun Feb 24, 2008 6:15 pm
Posts: 295
Location: France
Good point tpink.
Maybe measure from point above strut. ceiling, or another horizontal member mounted above and paralle to strut?
I agree weight would be in the way it seems trying to measure from below.
Or place a bubble level on the strut that would extend beond weight to get good measure?
As for twist it still seems to me that placing the weight a 30 cm behind strut center gets rid of many additional parts?
But,
I agree that you need a strut stiffner attached to measure twist so as not to have strut flex come into play.
We look forward to your vid. :thumb:
R H


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 Post subject: Re: Carbon fiber lay up
PostPosted: Fri Apr 11, 2014 2:36 pm 
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Posts: 84
So I set up my measurment jig and all looked good until i actually measured my strut flex!
What I thought was a really stiff strut wasn't so stiff... with the 5kg weight (bucket of water confirmed weight a number of times) hung at the 85cm mark, i got just shy of 9cm's (not mm's) of flex!

I was shocked. Also astonished the the Franky foil only flexed 2mm's! With that much weight hanging out 85cm's i could bend a 2x4 two mm's. I don't quite get how he got his foil so stiff, or if that measurement was accurate.

So back to the drawing board... i know i need to add carbon to the strut, which isn't that big a deal, but i'm now wondering how to get the best bang for my buck while keeping the foil thickness as small as possible. I'm thinking one layer on either side of the highest weight unidirectional i can find. i found online some uni that is 8.9oz but didn't see anything higher. They were selling at $10/ft by 24" wide.

Any thoughts?

Thanks,


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 Post subject: Re: Carbon fiber lay up
PostPosted: Fri Apr 11, 2014 3:09 pm 
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Location: France
TPINK,
Better you tested the flex now than it breaks on the water and you don`t find it! :thumb:
I thought at that weight it was not strong enough. :idea:
F K uses high mod carbon and epoxy and because he has built MANY foils he knows what carbon to put where. And his core is tiny.
I also think the one with so little flex was a very speical factory carbon plate like what I am lucky enough to have in 2 of my foils.
Sorry, but I would think your core is just to big to start with.
You can add a thicker uni and for sure it will be stiffer but of course thicker as well.
Have you done the twist test? :?:
This will also open your eyes to the need for bi bias "X" on a strut. :o
R H


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 Post subject: Re: Carbon fiber lay up
PostPosted: Fri Apr 11, 2014 3:18 pm 
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out of curiousity, how much do franky's foils weigh all up? I re-weighed mine and it is all up 1.3kg.

my strut is currently just over 11mm thick, and i wouldn't have a problem going to 15mm, which i think will get me a bunch of stiffness if i use the right stuff.

What would you think is an acceptable amount of flex for a recreational foil?
i'm not planning on racing the thing...


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 Post subject: Re: Carbon fiber lay up
PostPosted: Fri Apr 11, 2014 6:03 pm 
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TPink wrote:
...

So back to the drawing board... i know i need to add carbon to the strut, which isn't that big a deal, but i'm now wondering how to get the best bang for my buck while keeping the foil thickness as small as possible.
...

Any thoughts?

Thanks,


I said this before, but someone Ruded me off there thread for saying it :

Basically there IS NO better way to add stiffness to a home built project, than
by adding Carbon fiber , "High Modulus rods" (.)

Aircraft home builders have bin using this material since it's onset, in a
variety of ways. They do it because the rods come out of this machine, with
the Perfect amount of tension and epoxy, (something you can NOT do at home).

Scroll down a bit here :
http://www.cstsales.com/hm_carbon_shapes.html

Second up is to get RID of ALL the woven material in your project, Ya loose
up to 30% on Any fiber, (glass, Kevlar, carbon) the moment it's woven.

And last, if your fibers weigh 100g, then after there wet-out with epoxy, they
should NOT weigh more than 190g , (185g is My target weight).

Bille


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 Post subject: Re: Carbon fiber lay up
PostPosted: Fri Apr 11, 2014 6:59 pm 
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Joined: Tue Nov 09, 2010 10:00 pm
Posts: 84
Bille wrote:
TPink wrote:
...

So back to the drawing board... i know i need to add carbon to the strut, which isn't that big a deal, but i'm now wondering how to get the best bang for my buck while keeping the foil thickness as small as possible.
...

Any thoughts?

Thanks,


I said this before, but someone Ruded me off there thread for saying it :

Basically there IS NO better way to add stiffness to a home built project, than
by adding Carbon fiber , "High Modulus rods" (.)

Aircraft home builders have bin using this material since it's onset, in a
variety of ways. They do it because the rods come out of this machine, with
the Perfect amount of tension and epoxy, (something you can NOT do at home).

Scroll down a bit here :
http://www.cstsales.com/hm_carbon_shapes.html

Second up is to get RID of ALL the woven material in your project, Ya loose
up to 30% on Any fiber, (glass, Kevlar, carbon) the moment it's woven.

And last, if your fibers weigh 100g, then after there wet-out with epoxy, they
should NOT weigh more than 190g , (185g is My target weight).

Bille


Bille, I would NEVER rude you!

I should have paid more attn to you though. The rods look sweet and less expensive than i thought.

I'm not gonna strip anything off my foil, cause that would be more work than building a new one. But would be interested in adding some vertical rods down the strut.

So how many? do you suggest anchoring the ends? What's best to use to fair them into the foil shape?

Thanks!!


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 Post subject: Re: Carbon fiber lay up
PostPosted: Fri Apr 11, 2014 7:45 pm 
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Joined: Sun Feb 24, 2008 6:15 pm
Posts: 295
Location: France
TPINK,
My best flying friend has a fk t bar and we made the wings but they are simalar to fks.
I will weigh it and let you know.
Add a layer of thicker uni and a thicker layer of bi bias vac sacked if possible and respect temperature even if a normal grade epoxy.
Make sure to rough up well your already done work as you have pased the chemical bonding time window, now it will be mechanical , so also clean well with acetone.
15 mm is just fine if you have a good profile. better thicker=stronger vs thinner=weaker for sure!
BILLIE, if you happen to see this may I ask what thread you are talking about so I can see?
And I agree that using quality factory made carbon tubes and or rods would be a good choice but needed to be used from the beginning of the build I think.
R H


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 Post subject: Re: Carbon fiber lay up
PostPosted: Fri Apr 11, 2014 8:24 pm 
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Joined: Wed Nov 20, 2002 1:00 am
Posts: 933
Location: rhode island
Bille wrote:
TPink wrote:
...

So back to the drawing board... i know i need to add carbon to the strut, which isn't that big a deal, but i'm now wondering how to get the best bang for my buck while keeping the foil thickness as small as possible.
...

Any thoughts?

Thanks,


I said this before, but someone Ruded me off there thread for saying it :

Basically there IS NO better way to add stiffness to a home built project, than
by adding Carbon fiber , "High Modulus rods" (.)

Aircraft home builders have bin using this material since it's onset, in a
variety of ways. They do it because the rods come out of this machine, with
the Perfect amount of tension and epoxy, (something you can NOT do at home).

Scroll down a bit here :
http://www.cstsales.com/hm_carbon_shapes.html

Second up is to get RID of ALL the woven material in your project, Ya loose
up to 30% on Any fiber, (glass, Kevlar, carbon) the moment it's woven.

And last, if your fibers weigh 100g, then after there wet-out with epoxy, they
should NOT weigh more than 190g , (185g is My target weight).

Bille



That was probably me and i should appologise if i was rude.
i agree that commercial high modulus rods are good quality compared to garage built.
but the location is still a poor choice.
There are 2 factors to maximise the stiffness of any cross-section.
Area Moment of Inertia (I)
and
Fiber Modulus (E)...

moment of inertia is increasesd as fibers are located as far the neutral axis as possible.
(increase the thickness outward, instead of filling the core)

I do agree with everything else.
fibers only work in one direction. if you need added stiffness in any particlular direction, use only stiff fibers aligned in the same orientation.

if you know some of the details of your layup, its possible to compute section modulus from geometry. most folks selling similar shapes will provide I for their shapes. any cadd software will do it for you. you can also do it by hand w/o too much work
The % carbon that is aligned with the main axis of the strut will allow you to estimate E

from simple beam deflection formula
(http://www.advancepipeliner.com/Resourc ... rmulae.pdf)
stress= Mc/I
defl= PL^3/3EI


the above will let you can guess what should have happened.
and since you already know the answer, it will let you guess how much to add to get what you want.

-bill


Last edited by zfennell on Sat Apr 12, 2014 4:41 pm, edited 1 time in total.

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 Post subject: Re: Carbon fiber lay up
PostPosted: Fri Apr 11, 2014 11:27 pm 
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Joined: Sun Feb 12, 2006 8:16 pm
Posts: 407
Location: Kettle Point Ontario Canada
bottom line is if you want to use a wood core it needs to be hard wood not something that you can crave for decoys . I have been looking at doing something for a while now.. and some of you know were I am on this project.

Frankie is a god father in the sport and has built a shit load of foils to get were he is now.. It is great to see people doing DIY foils and I wish all the best too them...

When you start buying the high end carbon and epoxy you start getting the price up in material alone.
The lay up has to be 3-4 different kinds of carbon of different types. it takes a lot of research to figure out some of it and the other part is a shot in the dark to get it right .. The epoxy I use has to have heat to cure and not 100 degrees .....

Not saying I can pull this off but I am going to give it 110% shot to do it...

here are a few shots of were I stand now.
Image
Image

tomorrow hope to have all the molds cnc with any luck...

We have been building boards for a lot of years now , so we do have some know how with carbon and high end epoxy .
We have a snowboard/ twintip press ( not vacuum baging) with silicon heat blankets in it which I will but using , Will press them around 60ton with 180 degrees of heat to cure the resin.

Just drawing the molds has taken me 20-30hrs alone.

I will keep everyone posted how it goes. I will start a new thread as I go


Terrie
www.jellyfishboards.com


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 Post subject: Re: Carbon fiber lay up
PostPosted: Sat Apr 12, 2014 3:22 am 
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Joined: Thu Dec 04, 2008 1:50 am
Posts: 333
Location: No fixed address
To build on what zfennell and billie wrote and to expand on resin choice
I want to invite "the rule of mixtures" to the party
I care not WHAT epoxy I use (at least for stiffness) however I care obsessively HOW MUCH epoxy is in my part.
I suggest others should do the same.

The stiffness of the part is dictated by the modulus of the composite Ec.
The modulus of carbon fiber (Ef) is somewhere around 33,000,000 psi or up to 50,000,000 psi for TRUE high modulus carbon. (some scumbags will try to sell basic 33,000,000 psi carbon as "high modulus"...... know what you're paying for)
The modulus of Epoxy is somewhere around 400,000 psi for an average resin
Now when you put these two together. You get the following stiffness.

Ec= fiberpart+ resinpart
Ec= Vf x Ef + (1-Vf) x Em

where Vf = the volume fraction of fiber...

As I've built parts I've hit Vf's from 30% to 55%
So let's work through the numbers so this makes sense
@ 30% Vf
Ec = 0.3*33,000,000 + 0.7 * 400,000
= 9,900,000 + 280,000
= 10,180,000

The first thing you should notice here is the resin contributed almost nothing to the overall modulus.....less than 2%....i.e. resin doesn't matter much for stiffness, use whatever resin can give you a better Vf, doesn't poison you and is easy to work with.

So what about build quality
@55% Vf
Ec = 0.55*33,000,000 + 0.45*400,000
= 18,150,000 + 180,000
= 18,330,000

Almost 2 x the modulus based on build quality ALONE

What this means ISN'T that a part which has 1 layer of carbon at 60% vf is 2x stiffer than a part with 1 layer of carbon at 30% Vf.

they will have similar stiffnesses, in fact the 30% Vf will be (mildly) stiffer, cuz it's thicker and has more goo. but it's way heavier and takes up a lot of space
Modulus compares materials of the same thickness so a given thickness of a 60% Vf part will be ~ 2x stiffer than the same thickness of a 30% Vf part.

In other words I can throw in 2 layers of the same carbon at 60% Vf, in the same space as a single 30% Vf part.

So when you want to make a Thin part that is as stiff as possible, (like a hydrofoil mast) you better know
1) What is your Vf
2) how can you make it better.

In general
worry more about
Vf, Moment of inertia, fiber direction, actual fiber modulus

worry less about
weave type, epoxy type, core type

Tkettlepoint: 60tons sounds like a proper force, but please consider how to press the part or design some means to get the high tech resin OUT of your mold. I think you may have done this with the side channels but I'm not sure how you're pressing it to know if this will work or not.
It appears you have the tools to make the most amazing parts possible and you're well on your way to making them....keep at it.


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