Aerodynamics of a Jumbo Jet and a .40-powered R/C plane is quite different. The same numerical model, which describes high-Re flying pretty accyrately, may be inadequate for low-Re flying planes. Besides, R/C planes are usually way over-powered, which is never the case with real planes.

My impression of X-Plane was that it's flight model was not suitable for R/C. (I only tried it once, though.)