Analysis of Aerodynamic Interaction Effects on eVTOL Stability and Control Using Prescribed and Free Wake Models

This paper investigates the effect of rotorwake interaction models on eVTOL stability and control characteristics. The Combined Momentum Theory and Simple Vortex Theory (CMTSVT), based on a prescribed wake model, and the Comprehensive Hierarchical Aeromechanics Rotorcraft Model (CHARM), based on a free wake model, are both used to model aerodynamic interactions between rotors and lifting surfaces in a flight simulation of a generic lift + cruise eVTOL aircraft, and the results compared to a simulation that includes no aerodynamic interactions. The prescribed wake model is less computationally intensive compared to the free wake model but may be less reliable in capturing rotor-aircraft component interactions due to lack of wake distortion effects. The stability and control derivatives and aircraft trimmed states and inputs are analyzed for a range of airspeeds in transition flight conditions to evaluate the differences between the two models and to understand the potential impacts of using the CMTSVT model. Simulation results reveal that CMTSVT has reasonable agreement with CHARM in longitudinal stability predictions but diverges in lateral predictions at low airspeeds, likely due to its inability to model wake distortion. This discrepancy impacts flight controller performance: CMTSVT-trimmed initializations yield stable, smooth controller responses at higher airspeeds, yet demand increased effort to stabilize the aircraft at lower speeds. These findings highlight the critical role of wake model selection in achieving accurate stability and control predictions.