The desktop version of x-plane is a very detailed/indepth flight simulator... detailed enough to model your own aircraft including sizing control surfaces, estimating drag and lift coefficients, predicting stability, ect. We used an older version to model and simulate a UAV we were building as part of the AIA Design-Build-Fly competition while I was a sophomore in AE at Georgia Tech. It did a fairly good job predicting stability, thought there were some issues surrounding the software not being written for small-scale (RC) aircraft. The plane flew, but our pilot sucked and we crashed on takeoff during the competition