The Blender AC3D import/export plugin that is recommended for Speed Dreams work, was written by a FlightGear user. The FG forum has a very, very long thread discussing the plugin (http://forum.flightgear.org/viewtopic.php?f=18&t=13442
). The thread dates back 2007 so the earlier posts are probably obsolete, but some of the advice may be useful to SD modellers. The thread actually mentions a second plugin which may be worth investigating.
I'm not a Blender user (I can barely use it) so this is the only help I can offer.