When it came to attaching the fins, I went down an adhesive rabbit hole for this project.
There's lots of good glue out there. Any number of laminating epoxies with various additives, Hysol, ScotchWeld, Rocketpoxy. There's a good chance that with a nice fillet, Hysol or my favorite, ScotchWeld DP-460NS would have been fine all by themselves with no other reinforcement. Also, tip to tip makes a whole bunch of it's own problems that need to be dealt with.
But I had some other external goals with this project. I wanted it to scale up to something more extreme, (N-5800s, Robert DeHate 6" motors, etc...), I wanted to use my prepreg, and I wanted to conquer tip to tip. I also wanted some cool factor, some art, if you will.
Well, the prepreg situation complicates the glue situation. Having to cure at 350°F kills all the adhesives mentioned above. No problem, there's Cotronics 4525, right? Well, I didn't have the best luck using it for fin fillets. It's really evident that it's a high temperature potting epoxy. Very runny, very black, and just plain weird. It also really doesn't like to have anything added to it. Thickening it with the usual stuff makes it very brittle, almost crumbly. Maybe I was just doing it wrong, I dunno.
So I started looking into other high temp epoxies, but with better structural qualities than the Cotronics. I found a few, and although not available to the public, I resurrected my aviation LLC in order to get quotes and maybe buy some of these amazing structural adhesives. And they are amazing. For instance, here's the price quote for my front-runner-
View attachment 445020
I was nearly speechless. (I'm never actually speechless, gets me into trouble.)
What the...? Who the heck would...? Ok, fine... Moving on.
So that brings us to what I actually used. What adhesive has great structural qualities, while at the same time has very high heat resistance, is affordable and readily available?
My guess is you'll know it when you see it.