Andrew,
You should probably state what your design objectives are. To just make airports or 3d environments like this:
http://www.flightsimnetwork.com/dcforum ... 48599.htmlFor just airports there is Airport For Windows, Flight Sim Scenery Creator and ScenGenX, just to name a few.
To build unique objects will require a bit more work and a program like gMax.
All of these are free. So it is just a matter of what you want to do and how much time and effort you are willing to invest to learn the processes involved.
Lou