hi again, peter
there is no exactly answer how to fix your problem, but I can write here some tips, what works for me.
BASICS
1) gMax should have good set the units - 1 unit = 1 meter (this way you can fix problem with small objects size in 3dmax)
2) remember, the gmax exported X file use BMP textures (to fix lot of issues, should be 1024x1024, 512x512, 256x256, 128x128, 64x64, or didived by 2, so 1024x512... etc.), egal, if your source material was JPG, TGA, PNG or any another format. but you can replace easy it with text editor the textures in the X file.
SPECIAL
1) for MSFS still need export the aircarfat to one mdl (or X-file) with animation, tags etc, but for 3Drad you should export it to few primary parts X-files (fuselage, wings), which are not animated.
2) if you can see nonanimated parts with 3Drad, then you won little war, and you can process try export some animated parts. if you can not see nonanimated parts with 3Drad you must delete all animation gMax features and try export it again to X.
3) if is animated x-file invisible in 3Drad, you can try fix it with fragMotion. open (import) file - export file. this can be done easy, but it is not yet sure, it is enough for 3Drad. easy way, how to check it, is open this new frag X file again in fragMotion, if it crash, you have another problem.
4) another problem, if fragMotion wont open his file, I have had yesterday. I find out, that the source x-file (little propeller for Komet) must be closed face one mesh, I will put here pictures, to clear little bit, what I mean.
3d_rad_frag_tip_01.jpg
