Automation of structural cross sectional rotor blade modelling for aeromechanical rotor blade optimization

Abstract

It is often difficult to predict the behavior of helicopters, given their complex aeromechanical operating environments. Given these uncertainties, it is often the case that flight tests need to be conducted prior to a design being "frozen". To improve the whole design and validation procedure, it is essential that the design freeze (and iterations within) occurs before the expensive and timeconsuming process of building and flying. However, in order to do so, the virtual modelling needs to be more accurate and thus with less uncertainty. The DLR project Victoria (Virtual Aircraft Technology Integration Plattform) with its work package "Virtual Helicopter" aims to lay the foundation for a next generation comprehensive rotor code to overcome these challenges. Improvement on structural modelling within this code has high potential enhancing the overall development process, regarding time and accuracy. The improvement of rotor blade design is often driven by aerodynamic shape optimization, which means changes in airfoil shapes and sizes as well as their distribution and alignment, to fit different demands. Such changes will always have a major influence on the structural properties, because the inner structure geometry depends on the outer shape. Thus to still maintain an accurate aeromechanical model for the rotor simulation in the optimization process structural properties have to be adjusted. The most common approach is to calculate cross section data for various cross sections over the rotor blade span and feed the information into a beam-based rotor blade model. This is typically done by using approximations and scaling laws e.g., or by reducing the geometry complexity e.g. A high fidelity structural FEM-model will provide higher quality structural data. In general such FEM models are complex and require significant time to setup and process, starting with generating the blades inner geometry with CAD software, then meshing and performing the actual FEM analysis. This is very time consuming and hardly feasibly for an optimization with multiple loops. This paper presents the development of a tool for the automation of this process. The inner geometry is generated in CATIA V5 and can handle arbitrary cross section shapes (within reason). Additional parameters and boundary conditions are needed to obtain an inner geometry which is reasonable in terms of its structural integrity. These parameters include the center of gravity, basic spar shaping parameters and the skin thickness. This very accurate geometry model is then passed on to the FEM software (ANSYS). Here a mesh representing the geometry is created and then an analysis with the ANSYS tool SaMaRA is performed. SaMaRA calculates the structural properties of the cross section. The exchange of all data between the different disciplines (e.g. structural and aerodynamic) is performed via CPACS (Common Parametric Aircraft Configuration Scheme) to ensure data integrity and enable modularity of this structural code. The focus in developing this code was on the quick generation of highly accurate structural data for an aerodynamically driven optimization. The meshing automation in Ansys is not yet finished and in ongoing development. Follow up steps will be the extension of modelling options in terms of the inner structural setup and meshing quality.