An accurate description of individual blade oscillations is essential for aeroelastic analyses. As a result of the structural coupling of the ideally identical blades – so called ‘tuned’ rotors – double eigenfrequencies, or natural frequencies, occur predominantly, each with the same eigenform. These are rotated by a specified angle with respect to each other. For aeroelastic considerations, it is purposeful to combine these characteristic eigenform pairs into two circulating, mathematically complex ‘Travelling Wave Modes’. This combination must be consistent with the requirements for calculating the unsteady aerodynamics. To achieve reliable aeroelastic calculations, the combination of eigenforms, which depend on the finite element program used (for example Calculix, NASTRAN) or on the data provided by partners, is generally needed. The manual combination of these eigenforms is currently being replaced by a software module within the process sequence, thus increasing the reliability of aeroelastic calculations, also for structurally strongly coupled blades.
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