[Chuck Bodeen]

I'm an old timer at X-Plane, but fairly new to AC3D.
If you want the plane to fly correctly in X-Plane you don't want to start with any old existing acf file.
Plane-Maker is quite similar to AC3D, but it knows about such things as fuselage, wings, etc.
Use images of the three orth views of your AC3D model as successive backgrounds in Plane-Maker. Create the fuselage, wings, etc. on top of those backgrounds.
When you have all the parts assembled and you like the way it flies, you can overlay your plane with AC3D objects (using Plane-Maker to place them) and make many or all of your X-Plane pieces invisible.