Hi niko, my first quick attempt with fillet was successful, I've attached the 3DM file which looks like this:




So note that you have 2 open surfaces here so that means that you'll generally want to use the Trim command to slice them with each other instead of booleans.

To do the cutting, select both the fuselage and the wing, and then run Edit > Trim.

At the prompt to select cutting objects, just right-click or press Done because you want them to cut each other.

Then at the prompt to select pieces to discard, switch to "Keep" mode instead, because it's a bit easier to see the pieces you want to keep in this case. Select the part of the wing and fuselage that you want to keep and then right-click and the interior parts will be trimmed away.

Use Edit > Join to join those pieces together into one connected piece.

Then I selected those joined edges at the juncture and ran Fillet, with a radius of 15 units, which produced the attached result.

The fillet will be a bit wiggly because it is adapting to the bumps and wiggly nature of your fuselage - you probably want to have a cleaner and less bumpy fuselage to get a better quality overall result.

Hope this helps!

- Michael