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One can write exporters in Ruby for the free version of Sketchup (hint: Sketchup.active_model.entities[].mesh is your friend). So in theory it should be possible to write a SketchUp->MDL exporter via .x format. The main problem is the lack of documentation on the FSX-specific extensions to the .x format. (I notice that the FSX SP1 SDK includes more information on materials, but I haven't picked through it carefully enough to tell whether the documentation is now sufficient).If I remember correctly you need to Pro version of SketchUp, as the free version can only export to the KML format for Google Earth. With the Pro version you can export to 3DS as well, which can be imported into GMax for exporting to a Fs MDL file.
I made a model in SketchUp, exported as 3ds, load into 3DMAX8, exported with PANDA, compiled.
The problem is - the Z-axe in SketchUp became a Y-axe in FS2004.
Axis in 3Dmax set correctly i guess.