Here's another design that might be worth playing around with.

[Senior Space Cadet]

 

[Senior Space Cadet]

I had a bit of a disaster. I needed something to make fillets for balsa fins on a plastic tailcone. I found some System 3 Melt Weld, that I'd bought and never used. Probably ten years old. The hardener is like thick honey and the resin is like toothpaste. To make a long story short, I mixed some up and put it on the rocket and it won't harden.
I've wiped off as much as I could, but there is still a sticky mess on the rocket. I'm not sure where that leaves me. I think the fins were on pretty good before I made the mess, so I might just try paining over it and see what happens. Otherwise, I'll have to start all over. Really chaps my hide, jacks my jaws, etc.

 

[XrayLizard]

when all you needed was thick CA to make a fillet... well at least you are learning from personal experience.

 

[Senior Space Cadet]

when all you needed was thick CA to make a fillet... well at least you are learning from personal experience.

And I actually made a fillet out of super glue gel before putting on the epoxy.
I think I'm going to be able to save it. I got most of the junk off and I've made a nice fillet from Titebond Quick&Thick, multi-surface glue, one of the most useful products I've come across. When that dries, a couple coats of primer and some sanding and it should be ready for paint.

 

[XrayLizard]

the super glue was enough.... the titebond on plastic is useless. Hopefully you realize Fillets are a thing about aerodynamics on larger, faster rockets. a fillet is not structural in and of itself. a blob of epoxy on a larger faster rocket would do the same... it just not pretty or the extra few percent in Cd.

 

[jqavins]

I beg to differ, a little. The fin edge to rocket body joint has very little bonding area and is quite weak. The real strength of the bond is the fillets. For that, little fillets like the gel CA is generally sufficient, and a nice smooth cove fillet is, as you said, mostly cosmetic. Yet some kind of fillet is completely necessary.

 

[Senior Space Cadet]

the super glue was enough.... the titebond on plastic is useless. Hopefully you realize Fillets are a thing about aerodynamics on larger, faster rockets. a fillet is not structural in and of itself. a blob of epoxy on a larger faster rocket would do the same... it just not pretty or the extra few percent in Cd.

This isn't Titebond wood glue, this Titebond multisurface glue. I normally use it as a last coat over a wood glue fillet, as it dries smoother. In this case, I used it to cover up the mess left by the epoxy, as well as add some strength. FYI: Titebond makes a huge assortment of glues.

 

[XrayLizard]

Yeah

u might want to read up on that glue

 

[XrayLizard]

I beg to differ, a little. The fin edge to rocket body joint has very little bonding area and is quite weak. The real strength of the bond is the fillets. For that, little fillets like the gel CA is generally sufficient, and a nice smooth cove fillet is, as you said, mostly cosmetic. Yet some kind of fillet is completely necessary.

The fillet is a word.. whereas a glue joint is another word.
that’s the point I was trying to make. We do use the word fillet as to mean a glue joint with a shape.
I think you know what I mean we are both on the same page lol
I think someone else has not even opened the book...

 

[Senior Space Cadet]

Yeah

u might want to read up on that glue

You are right, it does say it isn't for plastic, but did do a good job of covering up the mess and does actually bond somewhat to plastic.

 

[Senior Space Cadet]

The fillet is a word.. whereas a glue joint is another word.
that’s the point I was trying to make. We do use the word fillet as to mean a glue joint with a shape.
I think you know what I mean we are both on the same page lol
I think someone else has not even opened the book...

I hope you weren't referring to me about someone not even opening the book. I understand fillets quite well, and have been making very nice ones up till this rocket. I simply made a mistake in using a product I've never used before and one that was very old. I made some really impressive epoxy fillets on my stitch and glue boat and I've never had a fin come off one of my rockets. In fact, I had a fin break off above the fillet, leaving the joint intact.
I admit that I don't have experience bonding balsa to plastic and I've never heard of making a super glue fillet, since it dries to almost nothing, but I came up with the idea on my own, but wasn't impressed with it, it did dry to almost nothing, which is why I tried the epoxy paste.

 

[Senior Space Cadet]

I cleaned up the epoxy mess reasonably well and yesterday morning I put on a couple coats of primer, which was bone dry in about ten minutes.
I did a little smoothing of the primer and put on a coat of red enamel. It looked really nice and I went for a long ride on my MTB, assuming it would be dry enough to sand by the time I got back. When I got home and back into painting clothes, I grabbed the rocket and found that it was still really tacky. Now, 18 hours later, it's getting pretty dry. When it gets light, I'll give it a light sanding and give it another coat and just leave it, without touching, for about 24 hours.
I could take a photo now, but, with the finger prints all over it, I'd rather wait.

 

[Senior Space Cadet]

Fillets not only add contact area, the distribute the stress, like a gusset. They aren't just for aerodynamics. A fin with a well made fillet is far less likely to come off. The balsa will break before the joint will. A fin without a gusset, if glued well (I use the double glue method) will hold reasonably well, but a fin with a fillet is far more secure.

 

[Senior Space Cadet]

What the heck. Here's a photo of the first coat with finger prints all over it. No sanding.

 

[dr wogz]

Fillet vs. Chamfer?

Any build-up of a material to act as a support / to provide some structure is typically finished / dressed in some fashion. Fillets are also used to ensure there is no sharp corner which can lead to failure thru metal fatigue (Why airplane windows are almost round)

Welds are typically 'fillets' once grounds down to the stated radius..

https://en.wikipedia.org/wiki/Fillet_(mechanics)

 

[jqavins]

True enough, but where did chamfer come into it?

 

[XrayLizard]

All I know is my fins will break partially off from time to time hahaha

 

[dr wogz]

True enough, but where did chamfer come into it?

it's an "argument" I have sometimes with my machinist..

And sometimes, in order to meet my NSF requirements (Food safe cleanability) 2 bends are easier to clean than a tight ben d radius (sheet metal) And it easier to make two "standard" bends that try to make a nice big bend radius!

 

[XrayLizard]

Looks like you painted it with the nose cone attached ? You may find that to be interesting when you try to remove it lol.

What the heck. Here's a photo of the first coat with finger prints all over it. No sanding.View attachment 434213

 

[XrayLizard]

Fillets not only add contact area, the distribute the stress, like a gusset. They aren't just for aerodynamics. A fin with a well made fillet is far less likely to come off. The balsa will break before the joint will. A fin without a gusset, if glued well (I use the double glue method) will hold reasonably well, but a fin with a fillet is far more secure.

I could have built that without a fillet, and the result would be stronger
When your dealing with plastic to wood there are alternative methods.
I’m going to leave it up to you to discover.