The evolution in the missions' complexity of both military aircraft and Unmanned Aerial Vehicles (UAVs) and the increase in civilian air-traffic, with limited runways, have led to the development of a new category of aircraft called Transitional Aircraft (TA). The goal of TA is to take advantages of both the characteristics of fixed-wing aircraft, like high speed, range and endurance, and of rotorcraft such as hovering, low-speed flight and Vertical Takeoff and Landing (VTOL). Despite the abundant knowledge on designing both fixed-wing aircraft and rotorcraft, designing TA has proven to be a challenging process and poorly understood. Currently there is no well accepted generic methodology that can be used for designing this type of aircraft such as tiltrotor systems due to the fact that there are numerous challenges regarding the selection of the proper TA configuration. Although somewhat effective, previously developed TA suffer from problems regarding the mutual aerodynamic interaction that occurs during the transition flight phase when the aircraft transitions between fixed-wing and rotorcraft flight modes. In order to cope with these problems, this paper presents a preliminary design methodology for TA with a set of newly developed mathematical formulations that integrate traditional fixed-wing and rotorcraft aircraft design methodologies into a comprehensive TA design methodology. Due to the potential large set of TA configurations that exist and can be proposed, this paper focuses on transitional tiltrotors which have been identified as critical importance. The proposed TA design approach is used to analyze a hypothetical TA configuration under development by the authors of this paper showing that the proposed design approach can be used to prevent some of the frequent shortcomings found on previously developed aircraft. The methodology is able to assist TA designers obtain adequate power, wing, and rotor disc loadings using the developed sizing mathematical formulations for tiltrotor aircraft based on integrating the performance requirements in the fixed-wing, rotorcraft, and transition flying modes.

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