Exploring Transition Profiles for the RAVEN Multi-Tiltrotor Aircraft

The transition between hover and forward flight is a critical consideration for eVTOL aircraft development. This transient and aeromechanically complex maneuver is often analyzed using computationally expensive methods that may not be practical for conceptual design. This paper presents a quasi-steady method capable of optimizing transition paths for multi-tiltrotor aircraft using low- to mid-fidelity models while approximating the transient nature of transition. A 3-degree-of-freedom aircraft model with surrogate models for rotor and airframe aerodynamics was developed along with a novel download model based on momentum theory. The optimization formulation minimizes a thrust-based surrogate for induced power while enforcing trim. This allows estimates for energy-optimal transition paths to be calculated even when accurate rotor power predictions are unavailable during early design phases. Important considerations for operating lift-only rotors through transition are identified and an iteratively refined speed schedule for such rotors is presented. The method presented is applied to analyze the RAVEN multi-tiltrotor aircraft. Transition paths at various fixed angles of attack are first developed to characterize operational stall limits. Optimal paths are then calculated for both accelerating and decelerating transition for RAVEN, with results showing that the aircraft behaves optimally as a multicopter at low airspeed and increasingly like a fixed-wing aircraft at higher airspeeds.