## 4 Discussion  
In this work, a comparison of different low- and mid-fidelity aerodynamic methods with respect to unsteady propeller aerodynamics and whirl flutter stability using the transfer matrix method has been presented. Further, 1P hub loads and transfer functions from hub motion to hub loads have been examined. Based on the identified frequency-domain transfer functions, the whirl flutter stability of simplified pylon whirl systems has been examined for varying thrust coefficients, airspeed values and pylon lengths. Results have been compared with the Houbolt/Reed method commonly used for propeller whirl flutter analysis. The major findings of this study can be summarized as follows: The classical Houbolt/Reed method does not guarantee conservative results compared with methods with higher fidelity. It is, therefore, suggested to choose the most detailed feasible method for a given whirl flutter problem. Additionally, strong sensitivity of whirl flutter stability with respect to relative changes in the individual in-plane loads has been found. More detailed key findings and their implications are as follows:  
1. Clear trends are visible in the unsteady aerodynamic results when comparing different methods. The stability predictions that are based on these results, however, do not necessarily reflect those clear trends. This suggests that stability predictions are not driven by the absolute differences in unsteady aerodynamics but rather by relative deviations in the individual load components. Even slight differences in the prediction of one load component can drastically impact stability predictions. Great care, therefore, needs to be exercised in the modeling of real aircraft propellers and as few assumptions as possible should be made regarding method, geometry and aerodynamic characteristics. 2. For all configurations investigated, the Houbolt/Reed method yields the more stable (and thus less conservative) whirl flutter predictions for zero thrust than the mid-fidelity methods. If the Houbolt/Reed method is applied in its quasi-steady form (without Theodorsen correction), it results in the most unstable system, but unsteady aerodynamic predictions are far away from the other methods investigated in this comparison. Despite yielding the least conservative stability predictions, the Houbolt/Reed method was used successfully for the design and certification of several propeller aircraft in the past. This fact could be explained by conservatism in the designs or other stabilizing effects (such as blade elasticity) offsetting the destabilizing effect found. Further research is necessary to compare to experimental data or high-fidelity numerical test cases without the assumptions used in this study to identify the correct modeling approach. 3. The stability predictions for systems with thrust are all more stable than the zero-thrust operating points, which confirms previous studies in the literature. The stabilizing effect can mainly be associated with the tilting torque vector. Although not checked in this study, this effect probably extends into the region of negative thrust/torque, which is believed to have a destabilizing effect and could thus be important for concepts that use propellers in conjunction with recuperation, as negative torque might destabilize the system. 4. To analyze the reasons for a variation in whirl flutter predictions when changing parameters, it is not sufficient to look at individual hub load components (e.g., only into the destabilizing term, \(M_{2\theta}\)). Changes in several components may cancel each other out, as some load components (or derivatives in the linearized approach) have a stabilizing and others a destabilizing effect. For the same reason, as results of the Wagner method demonstrated, general over- or under-prediction of unsteady aerodynamic loads does not necessarily result in large changes in whirl flutter stability. 5.