## 1 Introduction  
Whirl flutter, as an aeroelastic instability, affects the design of aircraft configurations with elastically supported propellers or rotors [1, 2]. Turboprop aircraft represent one category where the prevention of whirl flutter can require trade-offs in the design of, e.g., engine suspension. Reducing cabin vibrations demands low stiffness in the engine support for better dynamic isolation, while aeroelastic stability requires high support stiffness [3]. To enable the design and certification of the next generation of propeller aircraft, accurate simulation-based whirl flutter predictions integrated into the frequency-domain flutter analysis workflows for propeller-driven aircraft are necessary. For example, a classical, simplified rigid propeller-python system is shown in Figure 1a, where the propeller is located at distance \(a\) from the pylon (pivoting point). The propeller is flexibly mounted on the pylon to allow for rotations of the propeller about the pitch (\(\theta\)) and yaw (\(\psi\)) axes. Two corresponding rotary springs with stiffness values, \(K_{\theta}\) and \(K_{\psi}\), are used to model the elastic properties of the pylon, along with the respective inertia, which also represents the inertia of the engine. Linearized around the zero angle of attack, the physical  
inclusion of such derivatives (based on the Houbolt/Reed method) into flutter analysis using MSC Nastran, paving the way for its inclusion into the flutter assessment of full propeller aircraft configurations. Since then, the method has found application in certification [4], design and optimization [10; 11], as well as parameter studies [12; 13; 14], due to its computational efficiency. Despite being fast and robust, the method by [PERSON] and [PERSON] makes some assumptions: It neglects blade elasticity, as well as aerodynamic interaction with the wing, and simplifies the unsteady aerodynamics of the propeller. The stabilizing influence of blade elasticity was recently discussed by one of the authors [15] and is not be detailed in this work. Aerodynamic interaction between the propeller and the wing is often neglected in the literature, although [PERSON] and [PERSON] [9] suggested an approach to couple the Houbolt/Reed method with wing aerodynamics modeled with the doublet lattice method (DLM). The effect of aerodynamic interaction was shown to be slightly destabilizing in some cases [10], depending on the modes and frequencies involved. The last assumption concerns the aerodynamic modeling of the propeller. The Houbolt/Reed method is based on a quasi-steady, linearized strip theory approach, using Theodorsen's function for unsteady correction [5]. The steady state (and thus the thrust and torque) is neglected, and no wake model is considered for the induced velocities. While this work mainly focuses on propeller whirl flutter, there are parallels with respect to the so-called 1P hub loads in terms of aerodynamic phenomena and solution methods. These are the steady in-plane loads a propeller experiences under non-axial inflow. The thesis by [PERSON] [16] summarizes some low-fidelity methods to predict them, while [PERSON] et al. [17] compare mid- and high-fidelity approaches to the prediction of propeller aerodynamics under (even high) angles of attack. CFD-predictions of 1P hub loads have also been compared to wind-tunnel experiments with high-speed propellers [18; 19], finding, e.g., significant contributions of the spinner to propeller in-plane forces [18]. The authors also compare different numerical modeling approaches like unsteady time simulation and actuator disc predictions [19]. The literature concerning 1P hub load prediction only covers steady loads, while whirl flutter prediction requires unsteady aerodynamic loads. Although the Houbolt/Reed method is the most common method used for propeller whirl flutter prediction, other methods can be found in the literature.