Boeing Vertol Bearingless Main Rotor Structural Design Approach Using Advanced Composites

The Bearingless Main Rotor (BMR) being developed by the Boeing Vertol Company under contract to the Eustis Directorate of the U.S. Army Air Mobility Research and Development Laboratory. will be the tirst hingeless main rotor system to eliminate pitch bearings as well as the flap and chord hinges. The design. fabrication, and testing of this concept will lead to a flight-test program on the BO-105 helicopter. The chief objective of the BMR program is to demcnstrate that the bearingless main rotor system is aeroelastically stabie through an extensive research program culminatii. g in the flight-test program scheduled tor 1978. Because of the dependency ot' the BMR on structural tailoring to achieve the required stiffness. dynamic. and strength characteristics, the program has strongly emphasized advanced structures and materials technology. By using composite materials and their high strength-to-weight ratio and the variety of tension modulus/shear modulus ratios that are available through fiber orientation, it is possible to match the dynamic characteristics ot the BO-105 and still maintain a high level of structural integrity. The careful selection of a graphite composite material for the torque tube. used for pitch control input. allows for a rigid torsion and soft bending member with high strength.