Gap between fin and body tube for through fin. Need to file down or build up with epoxy?

[Mike Thomas]

I couldn't find this answer on another thread so please forgive me if this is duplication.

New LOC IRIS 3.1 Kit and the through body fins, when bottoming out on the motor tube, they stand off from the body tube by about 1.5mm. Do I need to file/sand/cut down the height of the root chord so it sits deeper in the body tube as to make the tip rest on the body? Fin to motor tube boding will be done by wood glue with epoxy fillets on the root attachment to the motor tube; and optionally, the mating surface of the root to the body tube where it passes through the body tube on the inside diameter (as well as outside fillets with epoxy putty). Just curious if it's worth removing the 1.5mm height of the root to mate this tiny area of the fin mating to the body on the outside of the body tube or if the external epoxy fillets make up for the gap.

 

[Ronz Rocketz]

I fill it with wood filler. Glue and epoxy tends to run. As long as the shoulder is properly secured to the MMT (I prefer TBII but most use epoxy), the gap doesn't matter. Once filled, create your fillet as usual. I prefer Quick&Thick for fillet. Be sure to remove the guide tape before the fillet dries. Good luck.

 

[Back_at_it]

I couldn't find this answer on another thread so please forgive me if this is duplication.

New LOC IRIS 3.1 Kit and the through body fins, when bottoming out on the motor tube, they stand off from the body tube by about 1.5mm. Do I need to file/sand/cut down the height of the root chord so it sits deeper in the body tube as to make the tip rest on the body? Fin to motor tube boding will be done by wood glue with epoxy fillets on the root attachment to the motor tube; and optionally, the mating surface of the root to the body tube where it passes through the body tube on the inside diameter (as well as outside fillets with epoxy putty). Just curious if it's worth removing the 1.5mm height of the root to mate this tiny area of the fin mating to the body on the outside of the body tube or if the external epoxy fillets make up for the gap.

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That gap is a bit excessive. I would sand until the fin sits flush with the body tube. If you are using epoxy, this becomes less of a concern but I wouldn't build it like that with wood glue.

 

[Titan II]

Sand the root cord.

 

[Bruiser]

I would sand the root cord as well. The glue that you use on the root cord/motor tube is going to make that gap at the body tube increase, especially if you use a thicker glue like epoxy

-Bob

 

[David Schwantz]

Also when sanding laser cut fins, sanding the char off will increase the bond of glue.

 

[K'Tesh]

I'm with the others, sand the tab down until you get a flush fit.

Pointy Side Up!
Jim

 

[loopy]

Sand down the root a little until you make contact. Personally, I'd say to use wood glue for the root edge bond, then use epoxy for fillets. In my opinion, it's easier to get a good looking fillet with epoxy than with wood glue.

 

[Lord Rory Gin]

I couldn't find this answer on another thread so please forgive me if this is duplication.

New LOC IRIS 3.1 Kit and the through body fins, when bottoming out on the motor tube, they stand off from the body tube by about 1.5mm. Do I need to file/sand/cut down the height of the root chord so it sits deeper in the body tube as to make the tip rest on the body? Fin to motor tube boding will be done by wood glue with epoxy fillets on the root attachment to the motor tube; and optionally, the mating surface of the root to the body tube where it passes through the body tube on the inside diameter (as well as outside fillets with epoxy putty). Just curious if it's worth removing the 1.5mm height of the root to mate this tiny area of the fin mating to the body on the outside of the body tube or if the external epoxy fillets make up for the gap.

View attachment 630343

View attachment 630344

I built a LOC IV fins just yesterday. Knowing that I had a power sander, it took just a few seconds to sand down the tab so that the fins sits on the tube flat. I'd far rather have the fins fit flush with the tube so that less "filler" or epoxy is needed to bridge the gap. And it just looks prettier until I can get the fillets put on.

 

[Back_at_it]

The other thing to keep in mind is that a lot of builder remove the outer layer of the motor tube so adhesives will bond better. This effectively reduces the diameter of the motor tube and increases the distance from the body tube to the motor tube.

 

[bjphoenix]

I build up pretty big fillets so I think all of the strength comes from the fillets. Depending on how you make fillets you might need a tighter fit to keep the material from running through that gap. Otherwise I wouldn't worry about the gap, after gluing the fins on I would fill the gap with a bit of wood glue then go on with the construction.

 

[Mike Thomas]

Thanks, all! I sanded the root edge to get a flush fit. 120grit removed the material pretty quickly! Now the I’m trying to decide on foam encapsulation or not. My stability cal is about 2.4 as-is so I’m thinking the extra weight from the foam in the fin can is going to help, I just don’t think the foam will be that influential considering a 3” tube with a 38mm MMT the volume is so small, only 30cu/in, so ~1.5oz if using 6# foam.

 

[OzHybrid]

Thanks, all! I sanded the roof edge to get a flush fit. 120grit room the material off pretty quickly! Now the I’m trying to decide on foam encapsulation or not. My stability cal is about 2.4 as-is so I’m thinking the extra weight from the foam in the fin can is going to help, I just don’t think the foam will be that influential considering a 3” tube with a 38mm MMT the volume is so small, only 30cu/in, so ~1.5oz if using 6# foam.

There is very little advantage to foam encapsulation if you attach the fins and centering rings properly. There is a big disadvantage of weight being added at the worst possible place, at the back... Wouldn't bother.

 

[Titan II]

No foam.

 

[David Schwantz]

Also vote no foam.

hey Titan, is that a Kangaroo in your avatar?

 

[Titan II]

hey Titan, is that a Kangaroo in your avatar?

Yes. That particular one is not mine, but my first two jets were Roos.

 

[David Schwantz]

I've had a Roo or two myself. My favorite delta is the Hotspot. Still got one

 

[DAllen]

Foam not needed. Another solution is sand until it sits flush with the body tube then for the internal gap use JB Weld, West Systems (thickend with silica) or Titebond (the white thick glue.) All three of those will be plenty strong and will not run.

 

[Back_at_it]

Simply do some nice fillets on the fins where the meet the body tube and the motor mount. Foam is unnecessary.

 

[loopy]

No foam. Decent internal and external fillets, and you're good.