The Flexhinge Rotor

This paper reviews the efforts of Bell Helicopter Textron (BHT) to develop hingeless rotor systems over the past nineteen years. The origin of the Model 609 hingeless rotor as a stiff-inplane design, and its evolution to a soft-inplane system is described. Flight test results which describe Model 609 rotor response and inplane stability are evaluated. The design and wind tunnel test of a 1/5 Froude-scale Model 652 soft-inplane rotor, and the manner in which the test results support findings of the Model 609 program and advance understanding of inplane stability, is discussed. Results obtained from a recently developed BHT analysis are used to explain the effects of kinematic rotor couplings on blade inplane stability for a six-degree-of-freedom fuselage/pylon/rotor system. The experimental and analytical research described in this paper forms the basis for development of the Flexhinge Rotor, presently being predesigned at BHT. The principal design features of the Flexhinge Rotor are described.