X-Plane contains subsonic and supersonic flight dynamics, allowing users to predict the flight characteristics of the slowest aircraft to the fastest. X-Plane includes thirty-five aircraft on its master disk, spanning the aviation industry (and its history), sporting aircraft from the Bell 206 JetRanger and Cessna 172 to the supersonic SR-71 and the Space Shuttle. Additionally, more than 1,400 additional aircraft models (such as those at X-Plane.org) can be downloaded from the Internet , nearly all of which are free. If those aren't enough, users can design their own airplanes and test fly them!.