Dynamic Loads Reduction of Lifting Propellers through Geometric Modification

Innovative, low-emission electrical vertical take-off and landing (eVTOL) concepts require the use of highly efficient lifting propellers due to the high power demand during hover. In these eVTOL systems, the operating conditions vary between hover and forward flight. The propellers are used to generate lift during forward flight, introducing dynamic loads caused by cross-flow and the resulting unsteady inflow conditions across the advancing and retreating blades. The conventional structural design processes for general aviation propellers such severe dynamic loads are not considered. Applying these conventional methods would lead to excessively heavy propellers, as the dynamic loads-causing severe vibrations and bearing moments-would need to be fully tolerated. Within the NERO (Novel Emission Reduction through Optimisation) project, the influence of geometrically modified propellers on the resulting dynamic loads under oblique inflow conditions is investigated. This paper presents a transient aerodynamic-structural coupled method to calculate the dynamic loads on propellers subjected to oblique inflow using a time-marching approach. A Blade Element Theory (BET) coupled with a linear inflow model is used to compute the unsteady aerodynamic forces, while structural loads and deformations are determined using a one-dimensional beam theory. In the chosen parameter space, the most effective reduction in dynamic loads and amplitude is achieved using an s-shaped forward-swept propeller. The results show a dependency on the operating conditions. Additionally, the application of a pre-cone angle counteracts aerodynamic forces and leads to a significant reduction in the time-averaged bending moments. The presented method enables the evaluation and detailed analysis of the complex coupled behavior of propellers under oblique inflow conditions. Furthermore, a case study on the eMagic One is conducted to assess the potential for structural load relief under various flight conditions, taking into account the parameters of a recently developed eVTOL aircraft.