Tip-to-Tip: Do I need fillets? Would they hurt anything?

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ActingLikeAKid

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I'm thinking I'm going to glass the fins on my L2 project - a scratch-built, Blue Tube-based rocket. It's probably overkill - Blue is Mach-proof, and the fins themselves are birch ply, which is pretty stout stuff (though I have snapped it on landing before). But I've never glassed anything and the rocket is overstable as it is, so a little weight in the bottom won't hurt.

Question:
Would it be a bad idea to do external fillets on the fins? I'm thinking that even if not for strength, it would make it easier to glass - it would make for a gentler transition from body to fin. Laminating epoxy ought to stick just fine to Rocketpoxy, yes?
 

soopirV

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I'm thinking I'm going to glass the fins on my L2 project - a scratch-built, Blue Tube-based rocket. It's probably overkill - Blue is Mach-proof, and the fins themselves are birch ply, which is pretty stout stuff (though I have snapped it on landing before). But I've never glassed anything and the rocket is overstable as it is, so a little weight in the bottom won't hurt.

Question:
Would it be a bad idea to do external fillets on the fins? I'm thinking that even if not for strength, it would make it easier to glass - it would make for a gentler transition from body to fin. Laminating epoxy ought to stick just fine to Rocketpoxy, yes?
I'm no expert, but I did T2T on one build that I wanted to be especially durable, and I think you're on the right track- the fillet made it easier to lay the cloth and wet it out. If I had to try to hold it in a narrow "v" it would've been tough, but doable, I think. This is an n=1 and I'm pretty inexperienced with glassing though, so YMMV!
 

Bat-mite

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Not my area of expertise so please ignore everything I say from this point forward.

I believe the fillets still add aerodynamic improvement, and geometrically increase the strength of the joint.

If I'm completely wrong, someone will correct me. Be gentle. :)
 

Steve Shannon

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If you don't do the external fillets the fiberglass doesn't reinforce the fin root very much. The larger the radius of the fillets the more strength the tip to tip glass provides.
 

dhbarr

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This is because ( he guessed, probably using the wrong terminology ) the fibers are good under tension but not much else?
 

Steve Shannon

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This is because ( he guessed, probably using the wrong terminology ) the fibers are good under tension but not much else?
It's been several years since I looked up the material properties of a fiberglass composite but i think it does have strength in compression also.
Fillets serve the same purpose as struts, preventing the fin from rotating about its root. The deeper the fillets, the more leverage the strut has, both in compression and tension. Having no fillet, there would be no strut.
Using tip to tip fiberglass places a layer of material that is much stronger than the fillet itself and along the outermost layer where it provides the greatest leverage.
 

Viperfixr

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Don't know about the strength aspect, but having fillets makes laying down the FG or CF much easier. The more of an angle in a short distance the harder it is to get the cloth to lay down correctly. In fact, the last tip to tip I did used slightly bulked up external fillets for just this reason (Madcow Tomach 54), and the CF laid down very nicely. Pulling a vacuum would help this, but that is more complexity too--I just used peel ply, breather fabric and some rice bags atop the wetted out CF.
 

ActingLikeAKid

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So back in the day, the generous and kind TopRamen sent me some glass to play around with. Looks like I have a home for it! Big question: 2oz or 9oz for glassing fins?
 

dhbarr

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How thick are your fins? And how fast are you planning to go?
 

ActingLikeAKid

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1/8" birch ply, and flirting with Mach.
Eventually I may try to get a Loki K in there, which sims to >1100mph
 

dhbarr

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Well, I'm still not an expert, but if the point is to get some experience would it make sense to do 9 in the middle and 2 end-to-end? I'd imagine the straight 9 would be mighty rough to fill.
 

BDB

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I agree. If you want to use the 9 oz stuff, you definitely want to put a layer of 2 oz on top of it.
 

tfish

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So back in the day, the generous and kind TopRamen sent me some glass to play around with. Looks like I have a home for it! Big question: 2oz or 9oz for glassing fins?
I'd get some 6 oz or a few layers of the 2 oz. The 9 oz will be tough to learn with.

[video=youtube;PtYYsX3VEmc]https://www.youtube.com/watch?v=PtYYsX3VEmc[/video]

[video=youtube;7Aqj3xcCWjU]https://www.youtube.com/watch?v=7Aqj3xcCWjU[/video]

Tony
 

ActingLikeAKid

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Thanks for those videos, those are perfect. I'm thinking that if I did a 1/2 coverage with the 2oz then tip-to-tip over that it ought to be overkill :)
I actually built a little fincan of some 29mm BlueTube and some basswood fins to practice on. Obviously it's a lot smaller than the actual rocket, so if I can do that, the actual rocket should be a breeze.
(and once that fincan is done I'm going to have to find something .... fun to do with it. ;)
 

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