A significant amount of research activity has been carried out in the last years, in the United States and in other developed countries toward the development of an air-vehicle featuring hybrid operational characteristics, in the sense of having the ability to combine hovering and high-payload vertical lift capabilities of the helicopter, and the high-speed capability of an aircraft. Due to its enormous potential advantages, such a vehicle is expected to be adopted in both military and civil operational services. In this lecture some issues related with the theoretical foundation of a dynamic model of tilt-rotor aircraft wing-rotor system are presented, and some results and conclusions regarding the implications of some non-classical effects are outlined.