X-plane has been discussed herein numerous times. Basically its capabilities should not be considered any more accurate or usable for design than a good video game. It might be OK for trying out ideas or just playing around with concepts, but as you transition from playing around to actual design, then you need to go elsewhere. One of the biggest problems of the package is that there is no numerical or otherwise quantatative output. Furthermore, since it is designed for virtually anyone to use, its inherent "simplicity" forces it to be less than useful or accurate beyond very limited application.