Coupled Aeroelastic Hub Loads Reduction

The conventional approach to reducing helicopter vibration is first to design the rotor with adequate frequency separation from the harmonic aerodynamic loads, and then to incorporate vibration isolation or attenuation systems. The drawbacks of this conventional approach are excessive weight, cost, and complexity. Furthermore, the results are often unsatisfactory. In this paper, theoretical approaches are presented which show that the interaction of rotor structural properties with airloads distribution is very important in minimizing hub forcing functions and, consequently, in providing for low vibration. The rotor structural properties considered include beam, chord, and torsional stiffnesses, as well as span and chord mass distributions. The airloads distributions considered include span higher harmonic lift and aerodynamic pitching moment. These are the primary excitation sources for helicopter N/rev vibration. The theory is presented by simplified rotor dynamic equations. Analytical results are provided using a fully coupled aeroelastic analysis. Correlations of limited flight test and wind tunnel data with theory are also discussed.