brain

Some kinds of aero modeling can be extremely challenging to achieve in software. Another factor is that it's not always clear which of several priorities should be driving in a given case. In this case, the intention was simply that the ventral fin extending downward from the boom tube should blend from the tube to the fin without creating turbulence. These fins were added mostly to prevent the aircraft from tipping over backward too far and to prevent prop strike.

Here a structurally complete layup gets a fillet of polyurethane spray foam in the offending corner. This black kind is my favorite; it's called landscaping foam. Now it can get cut and shaved and filled with micro, then overlaid with glass and finished to a beautiful blended form.

Foams like this cure with moisture. It helps to mist or dampen the deepest areas, and it's important to allow uncured internal areas to finish curing and expanding after they are cut open. The surface shown might be hard, but the inside can swell and can create a lot of pressure until the cure is complete. I usually mist the cut surface prior to further shaping, and wait typically overnight for it to be fully cured internally.