Abstract
A technique is introduced for aeroelastic stability analysis of certain hingeless helicopter rotors termed bearingless because of their lack of a pitch-change bearing. The rotor is modeled as three or more rigid blades, each joined to the hub by means of a flexible appendage known as the flexbeam or strap. The pitch-control system twists the flexbeam to provide blade pitch change. The analysis is capable of treating effects of several different pitch-control configurations, geometic nonlinearities associated with the equilibrium deflected shape of the flexbeam, and the built-in angular offsets of the flexbeam and blade. Numerical results are presented for a variety of system parameters. The stability of the system in both hub-fixed motion and coupled rotor-body motion is considered. System parameters can be chosen to stabilize most soft in-plane configurations for the hub-fixed case. The same parameters will also, under certain conditions, stabilize the coupled rotor-body system in hovering flight (air resonance). When the rotorcraft is in ground contact, however, at zero thrust, it appears that these parameters are not as effective in stabilizing the system (ground resonance).
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