How thick should my carbon fibre be?

Hi guys,I am just trying to model up the skins for my wing design in Carbon fibre. Does anyone know roughly how thick the carbon fibre would need to be for a 200x50x1000mm wing? I am proposing to use wet layup so you would expect it to perhaps be a little bit thicker then prepreg. Once i get the cad model finished I am going to do some FEA using cosmos but I just wondered if anyone had a tried and tested rule of thumb for this? I suppose what I am looking for is a realistically achievable thickness for a wet layup part?RegardsOliver

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  • This seems to have resurfaced but as I have researched this myself I can give at least something that will help.
    Carbon Mods does DIY kits for CF and they recommend varying amounts for building methods rather than application. 2 layers for lay and smack type build but 4 layers for vacuum formed stuff.
    I am hoping to use this stuff when i get the "balls" to self build and vacuum form each part. If anyone has done wings with CF i would love to know more as I'm scared to hell about doing that bit.
  • That's a pretty short wing so I'd go with 2 layers on each side of the wing (top/bottom), 80-100gsm (gram per meter) plain or twilt weave lay down in one 45/45, and one layer 90/90

    R
  • This is a very old thread but just to give you an idea of how strong carbon fibre can be if laid up correctly, used with a good quality toughened epoxy, and processed/cured correctly, here's a real-world comparison to chew on:

    BTS aftermarket carbon fibre Ducati motorcycle rims - check out the total wall thickness of the carbon fibre spokes on these wheels - around 500microns - 800microns!!!!

    Nope, the spokes do not contain Rohacell or any other kind of stiffening core material - they are hollow!

    C/F (and Kevlar) can offer way out characteristics when matrixed and processed correctly - pity its not the easiest stuff to cut and shape.

  • Hi

    What sort of spar structure are you using? Are you using a foam core wing - if so, what type of foam and what density?
    If you are using a foam core, safest bet is to use some sort of a spar structure - something simple, like +-45 degree 200g or so CF laid up on a granite block or sheet of glass and stripped into full depth shear webs, with a high density foam fill and unidirectional (can make this out of tows or laid-up sheet, like the webs). This handles most of your flight loads - the skin is then there for torsional loads and surface hardness - you do need to size it adequately for handling. Don't want fingerpints and dents in your wing skin.

    Wild-ass-guess for a wing like that would be: Aforementioned spar structure, high density (>25kg/m3) extruded foam, 1 layer of 100g or so plain-weave CF, 1 layer of 60g glass (for finish, if you don't want a load of pinholes). Or, 2 layers of 80-100g satin-weave glass, no CF.
    Oh - the CF or glass should be at +- 45 deg.
  • Two layers should be sufficient with nearly any type of woven fabric, and even one would probably suffice for a lightly loaded wing. Prepreg will definitely give a better result but if you go with a wet-layup then I strongly suggest vacuum bagging it. Otherwise you will end up doing a lot of filling and sanding to get a smooth contour and you will also likely end up with voids. Also, by aligning the direction of the fibers of two or more layers of the same fabric and vacuuming bagging, you can achieve "nesting" which gives you a more rigid and thinner product.

    Back to the thickness question: all the strength of composites is in the tensile direction, so you really need to have an adequate spar and number of ribs (or equivalent internal supporting structure) and the thickness is not as critical. Like Scott said, it depends a lot on the specifics. If you are new to working with composites, I can give more specific advice about doing the layup, as I do composite work on full-scale aircraft. You may need to invest in some equipment however.
  • The real answer to this depends on what kind of CF you are using - if you are laying down very thin unidirectional, I would go with 4 layers (2 in each direction for stiffness in both directions) - If you are using thicker woven stuff (also pretty strong, but difficult to get properly soaked without some excess resin) you could go with as little as two layers...so in all the thickness is going to depend on the material you select, and the curing method.

    I know I just gave you more questions than answers - but CF is one of those materials that doesn't lend itself well to "rules of thumb" - it also depends greatly on what you are trying to accomplish with your wing...does it need to withstand high loading, or is this a fast and light thing? Anyway - if at all possible I would consider looking around for a pro in your are that has the tools (ovens etc.) to work with prepreg - you can generally achieve a better result with less finish work on the back end and sometimes even use fewer layers.
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