Gmax can be downloaded from
www.discreet.com and can make flyable planes.
However, you need the Makemdl.exe that can be found in the FS2002 SDK's [software developement kits]. The kits can be found at
http://www.microsoft.com/games/flightsi ... dk.asp#topYou also need the FSModelExp.dle file. However, this file can only be found in FS2002pro. You either have to find someone who can send you a copy of the file or someone who can export your designs for you.
I know that sending a copy of FSModelExp.dle to someone else is illegal, but no one here is willing to buy a $40 sim just for the measly file that it has.