Hello everyone. I've been reading alot in here. A world of info on the hobby. I'm building an Avistar trainer ARF for my son and I've got a problem with the vertical fin being loose after it has been epoxied in place. What can I do to make it stable?

It needs to be stable before flying. I add some triangular balsa to the stab to fin and fuselage to stab joints. It makes for a much stronger tail and will last a lot longer. I just finished a Sig LT-40 with these and many other little improvements that I think make it a better plane. Not that it isn't out of the box. Just the way I do it after more than a few years in the hobby.

EXCAP232

.I'm a bit confused here.
If you used epoxy and it took a set the vertical fin is still loose?
Can you move it and see where the movment is coming from?
Is the back of the fuse flexing when you move the fin?