Concepts for Improving Hingeless Rotor Stability

Techniques for improving the lead-lag damping of hingeless rotor blades in hover are investigated. Primary consideration is given to soft inplane configurations at low blade pitch angles where blade damping is small and ground resonance instability may be encountered in the coupled rotor-body system. Previous results for the isolated rigid blade flaplag problem are reviewed, and additional results are presented for elastic, torsionally flexible blades. The elastic blade results confirm the basic rigid blade results and, in addition, show the effectiveness of configuration parameters such as twist and droop for increasing the blade lead-lag damping. These parameters may produce instability, however, if excessively large values are used. Preliminary investigation is made of the ground resonance characteristics of a simple, coupled rotor-body configuration. The results indicate that hingeless rotors are much more sensitive to aerodynamic coupling than are articulated rotors and that the isolated rotorblade analysis is not always valid for evaluating the effects of aeroelastic couplings on the coupled rotor-body system.