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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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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.

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