Stregthening Balsa Fins

[Dogsbd]

I read about a technique for for removing warp from plywood fin material by epoxing two layers together. This got me to thinking about using this with Balsa, not to remove warp but to build a stronger fin. Take two sheets of balsa half the thickness desired and epoxy them together and traping a layer of fiberglass cloth between the two.

How would the strength to weight ratio for this compare to plain balsa and plain plywood? I'm thinking it would be much stronger than balsa alone and maybe as strong as a comparable thickness of plywood, but weighing less (maybe).

Ideas?

 

[Austin]

Dogs,

The laminated cloth would add a good ammount of strength, IMHO. Also, if you cut the other piece of balsa and install it cross grained ( grain perpendicular to the first piece's grain) you will add considerable strength and ridgidity.

Carl

 

[Dogsbd]

Originally posted by CTulanko
Dogs,

The laminated cloth would add a good ammount of strength, IMHO. Also, if you cut the other piece of balsa and install it cross grained ( grain perpendicular to the first piece's grain) you will add considerable strength and ridgidity.

Carl

The cross grain idea is good, thanks. One could also do the standard paper and CA routine to the out side of the balsa/epoxy sandwich.


It would be interesting to build samples of different construction techniques and test them to destruction.

 

[rstaff3]

I too think this would add a bunch of strength. I have sandwiched verry thin (I forget how thin) G10 between balsa on one of my 'disposable' machbuster-style rockets, and can attest to it's strength. I added the balsa beacuse it made it easy to airfoil the resulting fin. The end result was a sub-optimal performer that looked cool and could survive a fall without a chute.

If you aren't aware, this dude is testing various HPR materials. Maybe he'd do the same for some more low-power type materials if someone sent them to him???

 

[lamart72]

I decided to try a new technique for my Deuces Wild fins. I coated both sides with a light coat of Pacer Z-poxy Finishing Resin and
placed the fins between the plastic sides of a folded and cut gallon size Ziploc bag. I didn't have any wax paper! I then used an aluminum T-bar sander over the plastic to squeegee away any excess epoxy. I placed a hard cover book directly over the fins and then about 5 pounds of magazines on that. The fins did not stick at all and have a smooth, flat surface! There is dried excess
epoxy on the edges, but it sands off easily. Previously, I would coat a fin with epoxy and then lay it on a sheet of wax paper until dry. This would not leave a perfectly smooth surface though as the epoxy would tend to gather in areas and leave a wavy appearance. Squeezing away the excess definitely helps! It seems to have barely added any weight at all. The ziploc bag is ready to be used again since dried epoxy will not stick to it.

 

[swimmer]

Check out this website: www.rocketmaterials.org

The tests on various construction materials and glue/epoxys is very interesting. I spoke with the site owner over e-mail and he told me he built a mach-buster that did mach 1.136 and pulled 24g's. He used a plain cardboard tube, albeit a high quality LOC tube. He works in a testing lab.

Swimmer

 

[kgholloway]

Hey, that trick with the plastic baggie is cool!

What I've been doing for both plywood and balsa fins is to layup a light, 1.43 oz cloth on balsa and 2 to 6 oz on plywood, fiberglass layer on each side of the fin using finishing epoxy. I then place the fins between two pieces of waxed paper, sandwich the fins and paper between two sheets of 1/4" plate glass or marble tile, and then put a bucket containing 50#'s of lead shot on top of the stack. The excess epoxy gets squeezed out and the wax paper is both non-stick and smoths the surface.

When the epoxy has setup the fins are absolutely flat and take only light sanding to be ready for installation.

Ken