The Search for an Ideal Bearingless Main Rotor (BMR) Design

Abstract

The Main Rotor Hub is the design centerpiece for helicopters and other forms of rotorcraft. It has been a very complex mechanical system design in the past, especially for fully articulated rotor systems. Two major efforts have been made to reduce this complexity. First, was the introduction of elastomeric bearings and dampers which have freed articulated rotor hubs from liquid lubrication and extreme mechanical complexity. This has made them economically feasible for designers and manufacturers of articulated helicopters, such as Boeing and Sikorsky in the U.S; Airbus and Leonardo in Europe; and MIL in Russia. However, the major progress in main rotor hubs has been the continuous movement "and search" toward the ideal hingeless and/or bearingless main rotor hubs. Designing the "Ideal Bearingless Main Rotor (BMR)" hub has been akin to seeking the "holy grail." One outside critic of the progress made toward the "Ideal BMR" over the years has been Thomas A. Hanson, who was involved in early designs of the Lockheed hingeless and bearingless rotor hubs in the 1960s. Having tried to go on his own after Lockheed failed and abandoned their hingeless and baringless rotor hubs, e.g. the XH-51A and the AH-56A Cheyenne, Tom revisited the status of rotorcraft hub design in the 1990s. However, due to the "not invented here" syndrome no major helicopter/rotorcraft manufacturer picked up on his innovative solutions. Helicopter/rotorcraft design engineers, especially those addressing aeroelasticity and dynamics, are a very small element in industry and government engineering organizations. The author of this paper was one of these and has been involved in developing, assessing and evaluating helicopter/rotorcraft designs for almost 50 years, e.g. UTTAS, AAH, AH-1 IRB, CH-47D, MDX, OH-58D, and LHX/RAH-66, along with accident investigations. He has also been the Georgia Tech Rotorcraft Design Professor from 1984 to 2019, where he taught and evaluated student design teams. In addition, his D.Sc. research and dissertation thesis under Dr. David A. Peters in 1978 (Schrage, D.P., "Effect of Structural Parameters on the Flap-Lag Forced response of a Rotor Blade in Forward Flight") shed new light on the tradeoffs between rotor loads and stability by developing an eigenvalue and modal decomposition approach. This included the evaluation of the Boeing and Sikorsky UTTAS bearingless tail rotors. This paper will review this search for the Ideal BMR and identify the importance it will play in future BMR designs which will be Cyber Physical Vehicle Systems (CPVS) to meet and satisfy the safety and design requirements of these new complex electrical, mechanical and adaptive control systems.