Investigation of Rotor Blade Structural Dynamics and Modeling Based on Measured Airloads

The work presented herein treats measured airloads, from the UH-60A Airloads Program, as prescribed external loads, to calculate the resulting structural loads and motions of a rotor blade. Without the need to perform any aerodynamic computations, the coupled aeroelastic response problem is reduced to one involving only structural dynamics. Results, computed by RCAS and CAMRAD II, are compared against measured results and against each other for three representative test points. Results from the two codes mostly validate each other. Seven more test points, with response computed by RCAS, to form thrust and airspeed sweeps are evaluated to better understand key issues. One such issue is an inability to consistently predict pushrod loads and torsion moments well, and this was found to be amplified at the two test points with the highest thrust coefficient. For these two test points, harmonic analysis revealed that the issue is due to excessive amounts of 5/rev response that stem from high levels of 5/rev pitching moment excitation. Another issue that concerns all test points is that the phase of the 1/rev blade flapping motion is not predicted well and suggests that this is not a mere random error. A modeling conclusion drawn is that the behavior of the lead-lag damper force should not be modeled by a linear damper for accurate chordwise bending moment predictions. Overall, the investigation here, conducted with numerous test points, further confirms the methodology of prescribing measured airloads for assessing the structural dynamics capability of a computational tool.