source of basic composite info?

[Bob Chiang]

Does anyone have a good on-line source of basic information about composites? Where to use fiberglass versus carbon versus kevlar?

My specific questions have to do with a side arm launch glider I am building from my own design. It has a balsa pod with fiberglass boom, sheet tail surfaces, and built-up balsa wing. The wing center section has spruce spars and balsa shear webbing. The outer panels have a full height balsa spar with a spruce strip laminated on top.

I am getting nervous about the strength of the wing, so before I add the leading edge sheeting and cap strips, I would like to add reinforcement to the spar, especially at the dihedral joints.

I would prefer to use Kevlar instead of carbon along the spar for aesthetic reasons.

Is there anything wrong with using Kevlar instead of carbon?

I have been told that the yellow fiber inside communication cable is Kevlar. Does anyone know if that's true?

Can Kevlar be attached with aliphatic or superphatic glues instead of laminating epoxy resin, or CA glue? I did a small test and it seems to work, but I'd like confirmation before committing the wing.

Thanks in advance,
-Bob

[Ollie]

Kevlar is tough, stiff in tension, strong in tension and light weight. However, it is terrible in compression and shouldn't be used to carry compression loads. Kevlar doesn't sand well and shouldn't be used on surfaces requiring sanding.

The best material for carrying compression loads is unidirectional graphite which is pultruded or precured from prepreg. It is stiff and strong but brittle. Because of its brittleness it should be engineered with an adequate safety factor and stress risers carefully avoided. I know of no other material that has a compression strength of 275K PSI and carbon's low density. Carry-through dihedral braces for upper and lower spar caps can be laminated from a few plies of 0.007 thick stock, with epoxy, over an angled form under pressure.

[nuevo]

I suggest sticking with epoxy. Kevlar and carbon need to be completely wet out. It is the combination of the kevlar/epoxy that makes a stronger material than either alone.

Take a hard look at the well documented designs and equations of Mark Drela's SuperGee and SuperGee2. The structure is extremely well documented, and known to work in the heat of competition.

The designs take time and study to gain insights from. Well worth the effort, IMO.

See:

http://www.charlesriverrc.org/articles/supergee/SuperGeeI.htm
http://www.charlesriverrc.org/articles/supergee/SuperGeeII.htm

Also:
http://www.charlesriverrc.org/articles/kahudlg/chriskaiser_kahudlg.htm

In my opinion, the spruce spar caps to not offer the strengh / weight ratio that is so important in smaller planes like DLG's. To be strong enough, you end up with a heavier plane.

[Ollie]

Fiberous materials are weaker in compression than in tension so particular attention has to be paid to their behavior in compression. The more parallel, tightly packed, strongly stuck together and uniformly loaded the fibers are, the stronger the composite will be in compression. The best unidirectional carbon composites are about 49 times stronger in compression than sitka spruce and about 20 times higher in strength to weight ratio. This wonderful result allows for very thin, light weight structural elements. However, there is a catch. Long thin structural elements under compression loads don't fail in compression but in buckling unless they are held in column. Holding long thin carbon fiber in column requires a stiff but light weight surrounding material.

Designing a strong, light weight structure that takes advantage of carbon fiber's good properties yet avoids its bad properties is a creative technical effort that is best studied by looking at the best examples of the designer's art, just like Jon said. Study Dr. Drela's designs.