Hi all,
I do not want to create a controversy here but simply put your finger on a strangeness that does not seem to bother many people when it should be. GPL is access to the sources, etc. using free ... Finally, you know. It is also required for any project using all or part of GPL code to be GPL also.
And here is the contradiction. Outerra is under a proprietary license. No access to sources, free demo, but paying for full software etc. ... and Outerra uses ...... JSBSim!
What to conclude? JSBSim is not GPL or Outerra blithely violates GPL with the blessing of the authors of JSBSim.
Nevertheless two things are possible and imperative.
1 - Either JSBSim withdrew from the Outerra project to comply with the GPL
2 - Let JSBSim withdrew from the FG project to enforce the GPL
Now you can resume it in all directions. This is far more serious and important that some non-free files that I could sometimes forget in my creations and that made a fuss a few fanatics.
And if it's pretty fantastic rendering of Outerra you like, to point to ignore the GPL tell you though there are other equally powerful project (see best in my opinion) but totally GPL.
I think of the project "Proland" which in addition to allowing free access to sources provides good documentation and APIs ready:
http://proland.inrialpes.fr/
http://www-evasion.imag.fr/Membres/Eric.Bruneton/
Regards Emmanuel