Carbon Fiber and Balsa Sandwich

[RocketFeller]

I pulled out the test piece of carbon fiber and balsa sandwich that we made yesterday. I have to say that it exceeded our expectations. I don't have easy access to mold release wax or spray, so we simply covered the carbon fiber with contact paper. We used the clear kind that is designed to cover textbooks. It worked great and didn't cost anything. Now we just need to build a bigger press for the fins...

The weight worked out to almost exactly one eighth ounce per square inch. This will work out to a little over seven pounds for the four finished fins.

 

[McKailas Dad]

Mmm, drooling!

Is that going to be a solid balsa cored, or will there be ribbing of some sort?

 

[RocketFeller]

Mmm, drooling!

Is that going to be a solid balsa cored, or will there be ribbing of some sort?

We are going with a solid balsa core. It should be plenty strong and light. The balsa weighs about a fifth of the weight of plywood, so any ribbing was going to add on weight fast. The edges will be covered with an aluminum strip for the finished product.

 

[g zilla]

Think he is talking about balsa ribbimg

 

[JDcluster]

You can use wax paper. It works okay.
I've fiber glassed tubes wrapped in wax paper.

JD

 

[McKailas Dad]

Think he is talking about balsa ribbimg

Yes, I was actually. Would it have been feasible to have had balsa ribbing, with empty (Or even foam?) voids, to save even more weight?

You are working well above my 'normal' scale, but it seems it could be practical to do that in smaller projects?

 

[RocketFeller]

Yes, I was actually. Would it have been feasible to have had balsa ribbing, with empty (Or even foam?) voids, to save even more weight?

You are working well above my 'normal' scale, but it seems it could be practical to do that in smaller projects?

Got it. I was thinking more along the lines of Rharshberger's Cherokee D fins (which we had considered). We had thought about hollow voids a bit, but we need all the stiffness we can get. Using foam instead of balsa would have saved around three or four ounces per fin, but balsa is stronger and more thermally stable.

 

[RocketFeller]

You can use wax paper. It works okay.
I've fiber glassed tubes wrapped in wax paper.

JD

I had thought about wax paper as well. The nice thing about the contact paper is that the adhesive background keeps it in place.

 

[Eyesinthesky]

Now we just need to build a bigger press for the fins...

I use two granite surface plates, one on top and one on bottom, to make my composite fins. Here is a link https://www.grizzly.com/products/18-x-24-x-3-Granite-Surface-Plate-2-Ledges/G9655 but there are other sizes available. They are very flat and smooth. You could just add release to the surface for a smooth finish. I used this technique when I made some fins that had flaps in them. I did however, add carbon tubes to the edges before laying up the carbon (in a vacuum bag). That way the carbon fabric wrapped around the rounded edge of the fin. The granite slabs hold the fins very flat while curing.

 

[rharshberger]

Got it. I was thinking more along the lines of Rharshberger's Cherokee D fins (which we had considered). We had thought about hollow voids a bit, but we need all the stiffness we can get. Using foam instead of balsa would have saved around three or four ounces per fin, but balsa is stronger and more thermally stable.

And I got the idea from Tim Dixon's Cherokee D project (dixontj#####, #=numders in username).

 

[sed6]

The weight worked out to almost exactly one eighth ounce per square inch. This will work out to a little over seven pounds for the four finished fins.

Just checking your math, that works out to over 6 square feet of fins. How big is this bird?

 

[ksaves2]

Dan,

You just used simple laminating epoxy to adhere the the GF to the balsa and put it in a press? Kurt

 

[RocketFeller]

I use two granite surface plates, one on top and one on bottom, to make my composite fins. Here is a link https://www.grizzly.com/products/18-x-24-x-3-Granite-Surface-Plate-2-Ledges/G9655 but there are other sizes available. They are very flat and smooth. You could just add release to the surface for a smooth finish. I used this technique when I made some fins that had flaps in them. I did however, add carbon tubes to the edges before laying up the carbon (in a vacuum bag). That way the carbon fabric wrapped around the rounded edge of the fin. The granite slabs hold the fins very flat while curing.
View attachment 283822

Those look sweet! This is sort of a one-off project, but those would be nice to have. I would want to try to pick them up locally - shipping is as much as the slab.

And I got the idea from Tim Dixon's Cherokee D project (dixontj#####, #=numders in username).

Yours look great!

Just checking your math, that works out to over 6 square feet of fins. How big is this bird?

It is a 4.9x upscale of the Binder Design Dragonfly. It is 12.75" in diameter with a five foot fin-span. It will be almost too big to fit through the door when it is done...

Dan,

You just used simple laminating epoxy to adhere the the GF to the balsa and put it in a press? Kurt

We did. First we covered the glossy sides of the carbon fiber sheet with contact paper to protect it from the resin. Then we spread Aeropoxy laminating resin on the textured sides (the CF has a matte textured side for bonding), placed the sandwich between two scraps of melamine covered particle board backed by two scraps of 3/4" plywood, and clamped it with a c-clamp.

 

[taiwanluthiers]

I use two granite surface plates, one on top and one on bottom, to make my composite fins. Here is a link https://www.grizzly.com/products/18-x-24-x-3-Granite-Surface-Plate-2-Ledges/G9655 but there are other sizes available. They are very flat and smooth. You could just add release to the surface for a smooth finish. I used this technique when I made some fins that had flaps in them. I did however, add carbon tubes to the edges before laying up the carbon (in a vacuum bag). That way the carbon fabric wrapped around the rounded edge of the fin. The granite slabs hold the fins very flat while curing.
View attachment 283822

How do you put the other granite plate on top of that? That thing looks heavy.

Given the cost + freight it would have been cheaper to go with a vacuum pump.

 

[Winston]

Great idea.

I have tested Tyvek lamination epoxied on 1/4" plywood (Hey, USPS mailers are made of Tyvek!), but the shards of plywood during core failure sliced through the Tyvek. A soft core material like balsa would not.

Have you though of trying "blue foam" as a core? It's used a lot for custom R/C plane construction and its even a material choice within OpenRocket. Depron might also be a decent choice.

 

[RocketFeller]

Thanks!

I did look at a few foam options (Rohacell, Divinicell, SP? on both...) and definitely considered them. In the end we went with Balsa for a few reasons. Balsa is a bit stronger (at least compressive strength) and not much heavier. It is supposed to be superior at resin absorption/adhesion. Finally, it is more thermally stable.

Foam definitely works well with this kind of sandwich - lots of companies do it - but for this we needed the absolute strongest material we could come up with.

 

[Eyesinthesky]

How do you put the other granite plate on top of that? That thing looks heavy.

Given the cost + freight it would have been cheaper to go with a vacuum pump.

Good Questions. The slab I used is about 150 pounds so I built a frame around it with 8020 (total weight was close to 200 pounds) but used a winch from Harbor Freight to lift it and set it down. You are correct, if you run the numbers the pressure applied using a vacuum pump far exceeds the psi from the slab. But I don't use it for that reason. Frequently I use both, put the fins in a vacuum bag and then use the press. The press is used to keep the fins flat while the bag removes the excess epoxy. While bagging alone, you can still get warped fins.

 

[taiwanluthiers]

I realize there's the issue with warped fin, but my guess is you could use a lighter but stiff material and just use clamps to apply the necessary force (or clamp between 2 sheets of steel) and still use vacuum bagging.

I seen in those videos of people manufacturing composites that they just use a mold of some kind. They would either lay the saturated cloth into the mold then bag, or they would lay the dry cloth into the mold, bag and then use the vacuum to inject the resin into the cloth. Others use a pre impregnated cloth (mostly bike manufacturers) that would require special equipment for it to cure at all. Still others use cloth pre-impregnated with solid resin, and the whole bag would be heated to melt and set the resin...

For the mold thing, I really do not know how they join the 2 halves... They can really mold an airfoil into the mold, and then spray the mold with release, gel coat, then cloth so that the entire thing is finished as soon as it is removed from the mold.