Sandwich Fin Layup Questions

[lr64]

Airplanes still have to take some load in the negative direction, but most airfoils will stall out at much lower lift coefficients, so they typically might be designed to take maybe a third to a half as many g's negative as positive. If a rocket had crooked fins, they could be designed the same way. ;-)

I'm sure you know for sure, but I imagine perpendicular grain might be very useful for preventing buckling in very thin skins. Does anyone ever show up with balsa cored fins made from beer cans? The really high tech free flight guys used to build that way, before carbon fiber was so available, though I think they used a higher and more consistent grade of material than beer cans. With the internet, maybe no one has time to build that way any more.

How do we get the lazier aeronautical engineers to avoid using Lehsse's circle?

 

[Rschub]

I feel like Navin Johnson. "I'm somebody now!"

 

[kjhambrick]

Fins are where it's at, the rest is just payload.

Yes, Fins ! Especially when you come up with something as totally kool as your Minnie-Magg drag mods for Level 2?

In the case of @kjhambrick, his sandwich is not the classic 2 slices of bread with peanut butter in between, but rather one slice of bread, with peanut butter coating both sides, because he likes the taste of peanut butter.

And peanut butter ( balsa or basswood ) is much easier to shape than crusty bread ( fiberglass cloth and epoxy )

-- kjh

 

[lr64]

snip

snip

Then I cut out 1/32 inch basswood 'steps' to apply to each side of the fin sandwiches. I cut the steps along the mid-chord and I beveled the underside of the mid-chord cuts so that the Kevlar mid-chord spines would not interfere with the fit of the steps and I epoxied the on the steps and squashed them between the tiles until the 30-min epoxy hardened. At that point I trimmed the kevlar spines along the root and tip chords ( more step-wise pictures next time ):
snip

Anyhow ... thanks for the input.

I will definitely try some of the suggestions here when I have larger fins to build !

-- kjh

Kevlar isn't much good in compression, so if the fins are under a heavy load, then the Kevlar is only helping on the tension side.

--------------

Herk Stokely wrote about hot wiring balsa in one of his columns in Flying Models magazine. Apparently it works, but makes a lot of smoke. I once hot wired a maple tree branch that was in an awkward place. Slow, but better than lopping my arm off with a chain saw while hanging upside down. I found it necessary to work the wire back and forth, though. Anyway, if you took your hot wire rig to a big rocket launch, maybe no one would notice the smoke. ;-)

 

[kalsow]

I'm building a set of 4 rockets that together will use 12 fins. My usual technique is to cut out plywood frames, fill the cores with foam or honeycomb or balsa or whatever, then cover with fiberglass and/or carbon. This method usually results in a bunch of wasted core material since the the core shapes don't tesselate.

My random thought for the day is to stack the frames separated by a layer of masking tape (on just the plywood), clamp them in a stack, and fill the stack with poured 2-part foam. After the foam has set, I'd use a bread knife or hot wire to slice the chunk-o-fins back into separate fins. Finally, a little sanding will flatten the rough cut faces of the foam core.

One advantage would be that the core material will fill 100% of the fin core and it'll be edge-bonded to the frames. Both good things.

Still, I'm not certain this process will save much time, money, or core material. But, it's intriguing...

Has anyone tried this before? Any obvious gotchas?

 

[lr64]

I read an article, possibly by Stan Hall, about using 2 part urethane foam. He made cores for his aircraft's ailerons out of it. (Or maybe it was the flaps?) Apparently, the foam wasn't done expanding when he covered it, so he ended up with lumps. Maybe, after you slice out the rough fins, you should put them someplace hot for a long time until they're really finished changing.

I've always heard that hot wiring urethane generated toxic gases.

Are these fins just fairly thin and flat? There are structural advantages to using some kind of airfoil shape. And, at least in subsonic flight, aerodynamic advantages.

 

[Rschub]

Hot wiring urethane foams creates cyanide gas, might be one step up from toxic.