Carbon rovings/tows

[mr_wood]

So who is an expert on using carbon tows to make spars ? I am building a 2 meter glider and want to know if anyone has a good method of using tow ?

I don't have bagging equipment.

Thanks
Tim

[davidfee]

If you don't have bagging equipment, I'd say it's best to buy pre-made spar strips. They are available from several sources, including CST here in California. Sometimes kite shops have flat stock carbon these days.

If you want to use tow, you want to debulk the spar caps as much as possible to get the most carbon in with the least resin. Often vacuum is used to do this debulking (pressing). An alternative is to build a mold for your spars and press the epoxy & tows together using clamping pressure.

Simply laying the wetted-out tow on a surface will result in a lot of extra weight and only a modest improvement in strength. The resin is heavy and contributes little to the strength of the part.

-David

https://www.cstsales.com/Carbon/carbon-laminates.htm

[mr_wood]

Really ?

What if I wet them out and them use a hard roller and kitchen towls to get rid of most of the epoxy ? Then clamp them one end and pull them out on a flat surface to cure. Would they then be as strong a pre-cure ?

Thanks
Tim

[davidfee]

Hi Tim,
Yes, if you did that they'd be pretty strong. It might become a messy, difficult, job though. And if the carbon spars will be self-supporting (not glued to a wood spar) then they really need to have some thickness as well to resist buckling (compression failure). Usually, carbon spars will be bonded to a shear web... often made of vertical grain balsa or Rohacel foam.

See here:
https://www.cstsales.com/How_to_Articles/SparBuilding.htm

As you can imagine, for optimum strength and stiffness the fibers should be as straight as possible. They should also be as tightly packed as possible since they are much stronger and stiffer than the resin matrix which binds them together. The pre-made strips are much easier to deal with, IMHO.

I have made spars using tow, but I have always used either vacuum or a 4-sided pressure mold. That's not to say it can't be done any other way.

[mr_wood]

I think the way to go for me is to wet out the tows, wipe away the excess and then pressure bond them to some spruce (using a press) before using them on a shear web spar.

Thanks for the info.

By the way I live in the UK and here carbon strip is expensive !! about treble the price. I can however get 2mm carbon rods cheap (£1.60 for 1.6m) - I suppose they would work on a balsa shear web ? what do you think ?

Tim

[davidfee]

Sure, you can use carbon rods as spar caps on a balsa shear web. A 2mm rod isn't very strong for a 2M wing, but you can use several in parallel.

It may be that spruce spars are all you need. Or are you looking at carbon because spruce isn't working for you?

Ollie is the guy I turn to around here for glider spars (I do mainly racers). Maybe he'll chime in soon... ;)

good luck,
-David

[madkiwi]

Could you plat (or weave) them for added strength? Wet them out and straigthen as mentioned above? just a thought!

[davidfee]

I'm not completely sure what you mean, but wing spars are in tension and compression, so you want the fibers running as straight as possible to maximize strength. If you weave the tows you will greatly reduce the strength in this application.

-David