## 4 Discussion
The transfer matrix method gives valuable insights into the unsteady aerodynamics of propellers, as it allows for an assessment of the change in individual hub load components with frequency and among different methods. This frequency dependency can then be transferred to frequency-domain flutter analysis, giving a proper unsteady representation of the propeller in the (whirl) flutter analysis process of, e.g., turboprop aircraft. 6. Regarding the comparison of individual aerodynamic methods, the different mid-fidelity methods showed in general good agreement in unsteady aerodynamic and stability predictions. Among the low-fidelity methods, the BEM+Wagner method was, in most cases, the closest one to the mid-fidelity methods regarding unsteady aerodynamics. Regarding stability, it could give the same trends but had larger quantitative differences from the mid-fidelity methods. Despite the comparison of different methods, operating points and structural configurations, some limitations apply to this work (see Section 2.2). Although delving into the detailed impact of the limitations on the outcomes is beyond the scope of this paper, recommendations for future research are provided to support additional investigations. The main limitation with respect to practical applications in whirl flutter analysis is the assumption of linear airfoil polars and incompressible flow in this work. Changes in the local airfoil polars (either due to steady angle of attack or compressibility) have already been shown to have a significant impact on whirl flutter predictions. The methods used in this paper allow one to account for this in different ways (nonlinear airfoil polars in the low-fidelity methods and lifting line, Prandtl-Glauert compressibility correction in the others). CFD simulations could give a good reference for compressible unsteady aerodynamic predictions. Future work could compare those to the different correction approaches to extend the findings of this work into the compressible regime. In general, even high-fidelity CFD predictions need to be validated with experimental data (either with forced-motion experiments or propeller whirl flutter tests on a reference configuration) to validate the findings. These studies (either high-fidelity numerical or experimental ones) could also lift the assumptions on aerodynamic interaction effects of the spinner, nacelle and wing. To further shed a light on the sensitivity of whirl flutter prediction with regards to input parameters, a structured uncertainty quantification approach could be applied. Regarding configurations, the main limitations are the geometry and operating points used. Although representative for a turbopro propeller, the advance ratios are still quite high; results might, therefore, not be applicable to new configurations, e.g., urban air mobility concepts. Propellers in this type of configuration operate under smaller advance ratios but at higher angles of attack and under higher loading, where different effects might play a role. The geometry of the propeller blades remained unchanged in this study. However, as mentioned in the literature, factors like blade aspect ratio and sweep are known to influence aerodynamic predictions, as well as whirl flutter stability. These parameters could be subjected to variations in future parameter studies to assess their impact and the prediction capabilities of the different methods. The results in this paper give valuable insights into the performance of different aerodynamic modeling techniques on whirl flutter stability predictions. They can be used by flutter engineers to choose a method based on requirements for conservatism and computational time. For final modeling guidelines, however, further validation studies are required. Conceptualization, C.K. and N.B. ; methodology, C.K., N.B. and H.V. ; software, C.K., N.B, H.V. and O.H. ; validation, C.K. and N.B. ; formal analysis, C.K. and N.B. ; investigation, C.K. and N.B. ; data curation, C.K. ; writing--original draft preparation, C.K., N.B.