VAF Forums

VAF Forums (https://vansairforce.net/community/index.php)
-   RV General Discussion/News (https://vansairforce.net/community/forumdisplay.php?f=2)
-   -   Which carbon fiber tape? (https://vansairforce.net/community/showthread.php?t=33764)

BJohnson 09-04-2008 11:02 PM

The basics
 
Danny7,

Balanced and symmetric laminates are rule #1 for composite materials. If it is not symmetric (about the mid ply be material type and orientation) it will warp with changes in temperature. If it is not symmetric (for every positive fiber angle there is a negative fiber angle) the laminate will twist while bending. Classical laminate theory predicts the behavior quite nicely, but if you want to experiment, laminate a ply of carbon to a ply of fiberglass and cure it flat (assuming at room temperature). Once cured, heat it up. The different coefficients of thermal expansion will warp it like a potato chip. THis is more critical with elevated cure composite materials where just cooling down from the cure cycle will warp the part. If t-here is no change in temp, then nonsymmetric layups are not as critical.

Dan,

I agree, the carbon is much stiffer but about the same strength, except as with E glass, S glass, and quartz, there are many different types of cabon fibers with dramatically different stiffness and strengths. Whole families of standard modulus, intermediate modulus, and high modulus fibers exist allowing designers to pick optimal properties for their needs. Most fabrics are typically standard modulus fibers. Adding some carbon fiber will add some stiffness without adding thickness that fiber glass would add.

When I get to that point, I am leaning towards composite skirts with cosmetic carbon fiber outer plies, if i can get them to come out smooth, and left clear coated. I Need to get more composite content on my RV because I got in trouble at work for building a metal airplane.

Danny7 09-05-2008 08:30 PM

Quote:

Originally Posted by BJohnson (Post 253629)
Danny7,

Balanced and symmetric laminates are rule #1 for composite materials. If it is not symmetric (about the mid ply be material type and orientation) it will warp with changes in temperature. If it is not symmetric (for every positive fiber angle there is a negative fiber angle) the laminate will twist while bending. Classical laminate theory predicts the behavior quite nicely, but if you want to experiment, laminate a ply of carbon to a ply of fiberglass and cure it flat (assuming at room temperature). Once cured, heat it up. The different coefficients of thermal expansion will warp it like a potato chip. THis is more critical with elevated cure composite materials where just cooling down from the cure cycle will warp the part. If t-here is no change in temp, then nonsymmetric layups are not as critical.

Dan,

I agree, the carbon is much stiffer but about the same strength, except as with E glass, S glass, and quartz, there are many different types of cabon fibers with dramatically different stiffness and strengths. Whole families of standard modulus, intermediate modulus, and high modulus fibers exist allowing designers to pick optimal properties for their needs. Most fabrics are typically standard modulus fibers. Adding some carbon fiber will add some stiffness without adding thickness that fiber glass would add.

When I get to that point, I am leaning towards composite skirts with cosmetic carbon fiber outer plies, if i can get them to come out smooth, and left clear coated. I Need to get more composite content on my RV because I got in trouble at work for building a metal airplane.


when you are talking symmetric in your response to me, would this be addressed by cutting materials on a 45 degree bias, that have the same thread for the material in the warp and weave (is that the correct terminology?)

and re your reply to dan, what is quartz?

i found this for quartz http://www.freepatentsonline.com/6979172.html, it references quartz as being particulary well suited for impact resistance. is that how it is used on airplanes?

tgmillso 04-23-2017 10:44 PM

So I happen to have some carbon uni tape on hand I was thinking about using for this canopy bow, however I was looking at coefficient of thermal expansion, and this raises a question. The coefficient of thermal expansion for fiberglass is about half that of Perspex. The coefficient of thermal expansion of carbon fiber is virtually nothing. Thus if I use carbon, in temperature extremes should I be expecting my slider to get stuck under the windscreen bow, and worse still, crack, or for some reason is this just a non issue. Anyone input from those with field experience of carbon bows would be appreciated.
Cheers.
Tom.
RV-7 slider.

6 Gun 04-24-2017 06:25 AM

canopy
 
I used 3 bids of carbon on my RV6 over base of fiberglass because everyone getting in and out grabs it. After screwing windshield to rollbar and finishing with fiberglass cloth then the carbon I added a layer of dynel over the top for smoothness and easy final filling less pin holes.This leaves a big gap between rollbar and skirt which filling with West epoxy in the caulking premixed tube really stiffens the top skirt so grabbing it for getting in and out works.Took about 1 1/2 tubes to fill.
Bob

ChiefPilot 04-24-2017 06:37 AM

I'd seen several sliders where the paint was cracked on the bow, so wanted to prevent that. I used CF on my -6A the canopy bow not for the strength but for the stiffness: passengers do grab it and I didn't want it flexing and cracking the paint. It's just a layer of CF cloth sandwitched between two layers of bidi FG cloth.

bret 04-24-2017 10:41 AM

My first experience with this FG stuff was building this plane, so, no experience here, but on the top bow of the slider WS I did, 4 FG, 1 CF, and 2 FG, the 2" CF tape had the strands going the wrong way for the strength I wanted for the bow, so I cut them into squares and laid them in, what a pain, but al in all it came out great and no paint chipping or cracking.

tgmillso 04-25-2017 10:16 PM

Thanks.
 
So the general concensis seems to be that carbon is fine and doesn't lead to Perspex cracking. I guess in my mind I still do not fully understand the mechanisms at play given that we go to all the effort to make these 1/8" spacers for the sikaflex application, but then bond a product like carbon to the outside that has virtually no coefficient of thermal expansion.
I guess this is one of those situations where I need to stop thinkin and get moving.
Tom.

ChiefPilot 04-26-2017 06:24 AM

Quote:

Originally Posted by tgmillso (Post 1168554)
So the general concensis seems to be that carbon is fine and doesn't lead to Perspex cracking. I guess in my mind I still do not fully understand the mechanisms at play given that we go to all the effort to make these 1/8" spacers for the sikaflex application, but then bond a product like carbon to the outside that has virtually no coefficient of thermal expansion.
I guess this is one of those situations where I need to stop thinkin and get moving.
Tom.

Ah, great clarifying statement!

I bonded my canopy with Sikaflex, and likewise ended up bonding the composite bits (skirts, etc.) to the plexi using Sikaflex as well after finding epoxy to be less than satisfactory.

rvbuilder2002 04-26-2017 08:27 AM

An entirely different perspective........
 
One problem that can occur with these fiberglass fairings is abuse from people kicking / banging into them while climbing in or out.
They rarely suffer damage that would be prevented by having carbon cloth in the lay-up, but it does often cause the start of a de-lamination from the surface the fairing is bonded too.
The stiffer the fairing, the more that any impact on the fairing will get focused on the bond zone and start a delamination
Adding carbon cloth to the lay up does add stiffness.

ChiefPilot 04-26-2017 11:15 AM

Quote:

Originally Posted by rvbuilder2002 (Post 1168622)
One problem that can occur with these fiberglass fairings is abuse from people kicking / banging into them while climbing in or out.
They rarely suffer damage that would be prevented by having carbon cloth in the lay-up, but it does often cause the start of a de-lamination from the surface the fairing is bonded too.
The stiffer the fairing, the more that any impact on the fairing will get focused on the bond zone and start a delamination
Adding carbon cloth to the lay up does add stiffness.

Certainly that's true. But that doesn't mean that you need to accept cracking or delamination - I chose to accept neither after seeing enough damaged fairings and talking to multiple builders who wished they had done something differently. So I learned from their experience and used CF + sikalfex. Almost 5 years and 700 hours later, no issues to date and it's been kicked (accidentally, of course) multiple times. The sika is still holding strong and no cracks on the paint.


All times are GMT -6. The time now is 09:07 AM.