Structural modelling and design strategies for lifting propellers

RWTH Publications (RWTH Aachen)

Abstract

Lifting propellers enable vertical take-off and landing for air taxis and large cargo drones. They are a hybrid between conventional propellers and helicopter rotors, presenting unique requirements for simulation and design strategies. While significant focus has been placed on the aerodynamic and aeroacoustic design of lifting propellers, limited information is available on structural simulation and design strategies. This work aims to develop an enhanced beam finite element model capable of capturing the key structural-mechanical and aeroelastic effects that influence the operation of lifting propellers. A novel frequency-domain approach is introduced to enable rapid dynamic load estimation during forward flight. Additionally, the thesis explores how conventional propeller design processes can be extended to address the unique challenges posed by lifting propellers. Analytical and numerical analyses investigate various structural-mechanical effects. The trapeze effect and centrifugal twisting moment influence the torsional stiffness, loads, and deformations of lifting propellers. A novel cross-sectional analysis is combined with an existing beam finiteelement method to capture these effects. Comparisons between the model, high-fidelity simulation approaches, and experimental results validate the method’s accuracy. By combining a linear induced inflow model with quasi-steady aerodynamic theory, the aerodynamic stiffness and damping loads are described in the frequency domain. The results provide physical insights into the loads and dynamic magnification of lifting propellers. A generic lifting propeller is sized using the conventional jig-shape approach. However, due to pronounced aeroelastic couplings and limited inherent blade stiffness, the sequential approach shows deviations at off-design conditions. Enhancing the jig-shape approach by integrating material and geometric modifications to mitigate aeroelastic coupling effects allows the sequential scheme to still be applied to lifting propellers. The identification of relevant effects and their implementation in novel simulation approaches reduces deviations between expected and measured performance data, increasing both the reliability and efficiency of the design process. The computational cost of the novel dynamic load estimation procedure is extremely low, enabling their determination already in preliminary design for the first time. Finally, the results provide a solid foundation for extending the sequential design process to multidisciplinary optimisation approaches.

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