Three-Dimensional Flight Envelope for V/STOL Aircraft with Multiple Flight Modes

Vertical or short take-off and landing (V/STOL) aircraft generally have three flight modes, namely, vertical take-off and landing (VTOL), conversion, and cruise, according to the variable angle of propulsion direction to the fuselage axis. However, the traditional flight envelope or conversion corridor lacks the capability to comprehensively present the flight characteristics of these three modes. Pursuant to this, the purpose of this paper was to present a three-dimensional (3D) flight envelope that combines propulsion direction, airspeed, and altitude in one figure. The 3D envelope was constructed subject to the constraints of power rating and equilibrium conditions. To verify its effectiveness, the flight data of XV-15 was used to generate the 3D envelope, following the projection along with the cross-section, which was compared with the traditional flight envelope and conversion corridor, respectively. The maximum specific excess power (SEP) of each flight state was also promptly obtained. In the case study, the flight performance of a rotor-wing aircraft was comprehensively analyzed using the generated 3D envelope. The proposed method in this study exhibited its versatility and capability to demonstrate the performance in all flight modes intuitively, which promoted the efficiency of V/STOL aircraft flight performance analysis as well.