Simulation Based Operational Modal Analysis for Helicopter Rotor Design

In the field of helicopter rotor design, having a comprehensive understanding of the aeroelastic stability characteristics of the rotor is essential. The modal parameters of the rotor blades in operating conditions provide invaluable insights into the vibrational characteristics and overall structural stability. Nevertheless, it is typically impractical to conduct comprehensive helicopter experiments at the early stages of the design process. In this study, a digital twin modelled in CAMRAD II (CII) is employed to conduct stability analyses using time-domain simulation data. An operational modal analysis (OMA) method, namely the eigensystem realization algorithm (ERA), is employed to extract natural frequencies, damping ratios, and mode shapes solely from output data, thereby eliminating the need to evaluate system inputs. The developed identification framework is verified using CII, and accurate simulation data is generated using mid-fidelity aerodynamics modelling, thereby enabling precise estimation of operational stability characteristics. The aforementioned analysis framework provides invaluable insights into rotorcraft stability during the initial stages of design. The method is applicable to a wide range of mid- or high-fidelity aeromechanics frameworks, including scenarios involving software coupling with CFD or particle methods for aerodynamics modelling. Furthermore, the study introduces a time response concatenation approach, which enhances the precision of the mode estimations. In order to eliminate the spurious modes that are inherent to the ERA output, an automated three-step clustering algorithm is employed as the final step in the process of determining the physical modes.