Deriving the Flight Envelope and Conversion Corridor of an eVTOL Tiltrotor Aircraft Considering Battery Discharge Rate

Urban Air Mobility (UAM) operates primarily within urban environments and adopts an electric vertical takeoff and landing (eVTOL), considering the unique characteristics of urban with high buildings. UAM has various flight types, most notably Multi-copter, Lift-Cruise, and Tiltrotor types. UAM’s complexity (eVTOL, flight type, configuration) makes its design and analysis more challenging. Traditional helicopters and fixed-wing aircraft have flight modes and characteristics for each. However, UAM has both characteristics, particularly in Tiltrotor type. The tiltrotor makes it possible to fly using lift and thrust at much lower speeds than the stall speed of conventional fixed-wing aircraft. This causes a flight envelope with a unique shape and special performance metric known as the conversion corridor (tilt corridor). Electric aircraft primarily depend on batteries, which output power varies with their State of Charge (SoC). As the battery discharges during missions, its maximum output decreases and that affects the aircraft's performance. Consequently, this affects the operational performance limits defined by the flight envelope and conversion corridor. Therefore, this study aims to present three key objectives. Firstly, propose a methodology for deriving the flight envelope and conversion corridor considering tilting mechanism and battery SoC. Secondly, verify the output power reduction according to Battery SoC through modeling. Finally, derive flight envelope and conversion corridor under the various SoC conditions. The case study shows that the battery loses roughly 24 % of its maximum power output by cutoff voltage. Using this result, a power-ratio was defined, and three representative cases were examined: 1.00 (fully charged), 0.90 (the level where the aircraft lose hovering capability at sea level), and 0.76 (corresponding to the cutoff-voltage condition). Consequently, the flight envelope narrows progressively, and after the battery has been depleted beyond a certain threshold the aircraft loses its VTOL capability. These results verify that such power-degradation considerations must be incorporated in the design of eVTOL aircraft.