The problem of modeling a complicated system such as a VTOL aircraft raises the fundamental question of how complex the model should be. If too many unknown parameters are sought for a limited amount of data, the reduced fit or equation error might delude the analyst to think the model superior to a simpler one when it is not. Although a more complex model can be justified given a greater amount of data, it is not clear what the exact relationship between model complexity and data amount should be. The temptation is especially great in the case of VTOL aircraft to exceed that model complexity that is justified by the data because of the extreme number of coefficients needed to describe the nonlinear aerodynamics and various controls. Not only is it unclear as to how complex one should allow the model to be in general but also the relative importance of individual terms is not known.