Assessment of Nonlinear Beam Models for Rotorcraft Aeromechanics Analysis with Fluid-Structure Interface

Helicopter blades are often modeled as one-dimensional (1D) beams and considered to undergo medium-to-large deformations. The degree of nonlinearity that a beam theory can handle greatly affects prediction accuracy. In this work, quantitative evaluations are made for static and dynamic behavior of beams and blades using the classic moderately-large deformation beam (MLB) model as reference to a geometrically exact beam (GEB) model. A rotorcraft aeromechanics analysis framework is constructed to incorporate both beam models. The framework contains various solution procedures such as trim, blade response, loads, and vibrations while allowing external interface to high-fidelity computational fluid dynamics (CFD) analysis. A validation study is performed to examine the extent of the accuracy in the large deformation behavior of benchmark beam problems in static and dynamic conditions. Next, the HART (Higher-harmonic Aeroacoustic Rotor Test) II rotor is applied to evaluate the relative prediction accuracy of the beam models. Numerical simulations indicate that the MLB model shows poor correlation with the test data or fails to reach stable solutions as the level of loads and geometric twist angles become increased substantially. In case of HART II rotor, however, the MLB-based predictions show reasonable correlations with the measured data while indicating good agreements with the GEB results. The centrifugal stiffening by the rotor rotation causes limitations in the blade deformation which essentially lead to comparable solutions between the two beam results.