Relative Aeromechanical Stability Characteristics for Hingeless and Bearingless Rotors

A study has been performed to compare the aeromechanical stability characteristics for two models of conventionally designed soft-inplane main rotors. The comparison of aeromechanical stability trends for these two models is made for hover operations. One model is that of a bearingless configuration with a flexure to accommodate flatwise, edgewise and torsional (pitch control) motions. The flexure is enclosed by a torsionally stiff cuff that is cantilevered to the blade/flexure joint at its outboard end and shear-restrained to the flexure at its inboard end. The shear-restraint includes an elastomeric damper to stabilize edgewise motion. The second model is a hingeless rotor with a flexure for flatwise and edgewise bending motion. Blade pitch control inputs are made by a feathering bearing located inboard of the flexure. The bearingless rotor model data were previously available in the literature. Data for the hingeless rotor model were recently acquired. The purpose of this paper is to compare test data from these two models, identify their respective aeromechanical stability characteristics, and determine the mechanisms that most influence these characteristics. Results presented in the paper indicate that the hingeless rotor concept offers better air-resonance stability margins due to its aeroelastic characteristics. For ground-resonance, however, the bearingless rotor is better because of the larger structural damping arising from its shear-restraint damper. It is anticipated that this comparison will lead to a better understanding of bearingless rotor behavior and, perhaps, suggest means for improving its aeromechanical stability characteristics.