Multibody Rotor Blade Modeling Based on Component Mode Synthesis

Helicopter rotor blades with several different parts, multiple load paths and/or springs and dampers can be modeled as a multibody system, into which finite element descriptions of flexible bodies can be integrated. When doing so, model order reductions can be necessary for robustness and/or performance reasons. A known drawback of such reductions is that the isolated modes of the particular bodies may not adequately describe their actual deformations in the multibody system. To alleviate this problem, the paper proposes a Craig-Bampton reduction for the flexible bodies. Compared to a standard modal reduction, the additional consideration of static interface modes in the Craig-Bampton approach significantly improves the prediction of eigenfrequencies and mode shapes, as demonstrated for a segmented steel beam with a single load path. Using the same approach, a bearingless rotor blade with multiple load paths is modeled by two beam segments. The model is assessed by code-to-code comparison with a CAMRAD II reference model. The consideration of the tension interface degree of freedom of the inner segment (flexbeam) allows for the accurate prediction of eigenfrequencies at varying rotor speeds, including the effect of centrifugal stiffening – which is not appropriately covered with a standard modal reduction. With the Craig-Bampton method, the mode shapes are plausible and in overall good agreement with the reference model. Despite these benefits, deficiencies are identified that still need to be improved. Discrepancies in the blade’s torsion eigenfrequency prediction are observed, as well as anomalies in the mode shapes that are potentially related to the flexbeam, that is – despite the consideration of interface modes – too kinematically restricted.