Scott Manleys interstellar quest was pre 1.0.0, so the atmosphere was totally different then (don't remember if he used Ferram Aerospace though). I'm not too experienced with planes myself, so the only advice I can give you is to make sure the center of mass on your ship is not too far back, and maybe add more drag to the back of your plane too (we have airbrakes now since 1.0.0 )