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Over-achiever fin fillets?

Old 06-27-2015, 05:05 PM
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Default Over-achiever fin fillets?

I see a lot of high-power rocket build threads where the builder shows off the construction of ample-to-monstrous fin fillets. So far, I have refused to do this if Iím able to get ample epoxy inside between the motor mount and airframe.

So I donít want to shed fins, either, but my meager understanding of aerodynamics says that fillets can actually have more drag than a 90-degree butt-joined airfoil on a round fuselage. Case-in-point, the F4U Corsairís bent wings eliminated the need for fillets by joining at 90 degrees (while giving good prop clearance with short landing gear). I looked at pictures of the X-1 and X-15 and they appear to me to have tiny, if any fillets, too.

So what gives? Is this tribal knowledge gone out of control or do the standard rules not apply at smaller scales?

Old 06-29-2015, 04:04 AM
Bob Jablonski
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Small fillets IMOHO help fill the small voids in the tube slots which also stiffiins up the fin to prevent flutter (a bad thing). The large fillets on some rockets would be more for the looks then the structure.
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JonnyJohnston (03-21-2022)
Old 04-26-2020, 12:17 PM
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It's been a long time since I built a high power rocket (early 1990's), but I only filleted enough to not have a sharp break between fin and body, unless scale detail dictated it. I think a lot of people over-fillet, thinking it will be a stronger bond. I found the best strength when building with thin ply for fins and a paper or fiberglass tube was to extend the fin root to the motor tube, and add a center positioning ring between the aft bulkhead and the one at the forward end of the fin roots.. I did a lot of scale rocket builds, flying from H to L motors and usually built the fins and motor tube as an assembly of its own, with a short section of outer body tube that stopped just above the tip of the wing root. Then I added a tube joiner to the outer tube and the end of the motor tube to extend them the needed length for the rest of the rocket body or in the case of the motor tube, to the base of the parachute compartment. Never stripped a fin.
Old 03-08-2021, 09:59 AM
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I have gotten to the point I don't tape my fillets anymore. Any excess I have to wipe away but it saves the PITA of running all that tape and having tape edge dams. I've tried to pull the tape early enough so it will self level the edges but that never works. If it is thick enough to hold the shape of the fillet, it is thick enough to hold the edge dam as well. I've used card edges, wood dowels, popsicle/craft sticks, PVC fittings and so on to pull fillets and it still sucks. I hate that step more than anything I think. I have yet to find the technique I like best.

I use filler in all my fillets. Be it wood glue or epoxy. I can't see how wood glue without a filler does much good but I'm no expert in that department for sure. You will just have to find your technique and get in the groove. Then decide if that small ripple or pin hole or crease is really anything to sweat over.

Head over to US Composites and pick up some of their 150 Thick Epoxy.
Then head to the Fillers page and grab a quart of Cabosil, a quart of SM Fairing Compound, and a 1 lb tub of 1/4" Chopped Strand fiberglass. That will be enough to last you a long time and ready to build (almost) any rocket.

Use the chopped fiber on internals to get good strength if you can get in there too them. I build my MMT/fin assembly outside the rocket so I can get good fillets on the MMT/fin joint.

Use the Cabosil to lay a root fillet on the externals and smooth it with your preferred smoothing tool. Doesn't need to be perfect.

Use the fairing compound to finish the fillet with a slightly larger smoothing tool. It is much easier to sand than the Cabosil. The root gives you strength so the faring compound is just for looks. You can use hobby store 5 minute here if you are quick enough. No need for fancy smancy high strength epoxies to make it aesthetically pleasing.

If you are one of the folks who inject fillets, the Cabosil will work there as well.

They also have Fasco 110 Epoxy Glue that works well too, both for gluing and making fillets. You can also pick up mixing sticks, containers, cups, squeeze bottles and even empty caulk tubes. Nearly a one stop shop for what we do. And they have pigments to color your epoxy.

And yes, if you had a good root fillet laid, then you could've just used spackle over it to fill the holes and rough spots. But that "get you high" in a tube spot and glazing putty works better (auto section at Wal-Mart); or the fairing compound I mentioned mixed with epoxy to make a paste. I use the spot putty on occasion on my fillets but usually for dings and dents and small jobs. I don't really care for it.

It should be fairly easy to use a syringe (trim the end if need be) to squeeze out a small Cabosil fillet without too much mess, then cover with the lighter, easier to sand fairing compound to make it look good if need be. Only way to get good is to practice.
Old 03-08-2021, 01:44 PM
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