A Study of Bearingless Rotor-Hub System for XOH-1

A basic principle of the Bearingless Rotor-Hub System has been decided at the beginning of the study. The study consist of 3 phases. (1) Research of each elements (2) Ground Test (3) Flight Test Specially the research of elements is the most important, because many troubles occurred at this phase. At first, the torsion element which has a function of feathering and lead-lag hinge was studied. To get a good combination of the torsional stiffness and lateral stiffness, the cross section of a torsion element has been changed from original one to “X” shape and finally to the “Asterisk” shape. Also newly the “V” beam concept to fix a equivalent lead-lag hinge point was introduced to prevent the “S” shape deflection in lead-lag motion. Original pitch-housing equipped with the elastomeric damper in it was designed to get lead-lag damping by means of its deflection. But this shape showed such a poor lead-lag damping that the shape was changed. In the new shape, open spaces to install a damper on the top and bottom in pitch-housing was filled, then the elastomeric damper was installed outside of it. Consequently good lead-lag damping was obtained. Also the flapping-plate which has a function of a flapping hinge was studied. Its cross section was changed from a original one to a new one. The elastomeric damper was studied, too. During the study, basic principle was not changed, but shape of each elements were changed drastically. On the ground test, retired OH-6J installing the Bearingless Rotor-Hub System was used and vibration test was done. In the flight test, instead of the original one, the modified OH-6J installing the new Bearingless Rot-Hub System was used. About 100 hours flight test was conducted. A ground and air resonance has not occurred. Consequently, the technology of Bearingless Rotor-Hub System has been established before the start of XOH-1 development.