Ok....well that's just the 3d meshes, the 2d textures and if a jsbsim FDM the flight model....
Any improvements made here would need to be given back...
No, I think you're misunderstanding the GPL. The GPL doesn't require that you actively give contributions back to upstream, only that any distribution of derived works also be licensed under the GPL.
If I take some GPL work, change it, post it online under the GPL license, I've fulfilled the terms of the GPL; upstream need never be aware of it. (Also, if I take some GPL work, change it, and never distribute it publicly my changes need not even be made available). The GPL forcing people to "give back" is just a metaphor in the sense that if upstream
wants the changes they're available for use.
If people are looking for models to use FlightGear seems like a great choice specifically because of how it's licensed.