The "Free Flight Simulator" is the more accepted tagline (and how I would rather put it as a native-ish* speaker).
Sorry Michat, but "freedom flight simulator" sounds a little awkward to me...
Though if you really wanted to use the word "freedom", you could do something like this:
...and my personal branding idea for the 15th anniversary of FlightGear:
* I learned English and Japanese around the same time as a child, so I'm not really sure which would count as my "first" language...
Look for "B-BIRD" "N127KY" or "AVA0004" -that's me.
Despite having over 1700 posts here, I am not even close to being the most skilled guy here... I'm just words and bad drawing, not experience. :P