## 1 Introduction
[PERSON] and [PERSON] [20; 21] used nonlinear airfoil polars coupled with dynamic inflow models under trimmed conditions to study the whirl flutter stability of NASA's X-57 aircraft. They used time-domain methods embedded in rotorcraft comprehensive codes or multi-body simulation to include these more complex aerodynamic methods. Time-domain stability analysis is also commonly applied for tiltrotor aircraft, where the whirl flutter stability of the large, flexible rotors is of major importance [2]. Different studies [22; 23] demonstrated the application of mid-fidelity aerodynamic methods in tiltrotor whirl flutter analysis. For example, [PERSON] et al. [23] compared uniform inflow, dynamic inflow and vortex particle method (VPM) results, finding good agreement for the tiltrotor case studied for isolated whirl flutter results. Still, the results cannot easily be transferred to propeller whirl flutter analysis due to the differences in rotor design, operational ranges and the analysis methods used. As an alternative to time-domain stability analysis, [PERSON] and [PERSON] [24] presented an identification procedure to derive rigid propeller aerodynamic derivates from unsteady vortex lattice method (UVLM) simulations using quasi-steady perturbation of the propeller disc pitch. This approach is compatible with frequency-domain flutter analysis and yields derivatives with fewer assumptions, although it still follows the quasi-steady assumption by [PERSON] and [PERSON]. [PERSON] and [PERSON] [25] showed an unsteady, finite-state reduced-order model (ROM) identification method for unsteady aerodynamics of proprotors and applied it for a comparison of low- and mid-fidelity aerodynamic methods. They studied the influence of blade aspect ratio and advance ratio and compared quasi-steady and unsteady airfoil theory with results from a 3D boundary element solver. Their findings indicate that for high advance ratios and blade aspect ratios, the airfoil-based approach yields an under-prediction of the instability ranges. However, their ROM method is not easily transferable to other aerodynamic codes due to its level of integration into the solver. To the authors' knowledge, no studies exist in the literature comparing low- and mid-fidelity aerodynamic solution methods with the legacy Houbolt/Reed method for whirl flutter [5] in a framework fully compatible with frequency-domain flutter analysis. [PERSON] [26] suggested a method to include arbitrary time-domain models of isolated rotors into frequency-domain flutter analysis by identifying frequency-domain transfer matrices about the propeller hub using time-domain perturbations. This method (called transfer matrix or TM-method) allows for the inclusion of different aeroelastic rotor models into whirl flutter analysis, enabling the use of the wide range of tools and methods available for time-domain aeroelastic rotor modeling. It was developed based on an approach to construct helicopter rotor ROMs [27]. The TM-method is applied in this work to study the effect of aerodynamic modeling on the prediction of propeller whirl flutter, giving insights into the underlying mechanism found in unsteady propeller aerodynamics and the reasons for the differences in stability predictions observed. In the first part of this paper, the aerodynamic methods, including the legacy Houbolt/Reed method, are described and compared. Afterwards, the TM-method, and the workflow and models used are outlined. The results of the studies are presented at the end, finishing with a discussion and outlook.