Rotary-Wing Aeroelasticity: Current Status and Future Trends

Concise Perspective and Previous Surveys W HEN reviewing research in rotary-wing aeroelasticity (RWA), it is important to mention a few historical facts. The Wright brothers flew in 1903, and Sikorsky built and started flying the first operational helicopter, the R-4 or (VS-316), in 1942. The R-4 was a three-bladed helicopter with a rotor diameter of 11.6 m and was powered by a 185-hp engine. Thus, there is an initial gap of approximately four decades between fixed-wing and rotary-wing technologies. Therefore, it is not surprising that certain rotary-wing problems, particularly those pertaining to unsteady aerodynamics, are still not well understood. The situation is further compounded by the complexity of the vehicle when compared to fixed-wing aircraft. The field of RWA has been the most active area in aeroelasticity during the last three decades. This vigorous research activity has generated a considerable number of survey papers as well as several books that have been published on this topic. These review papers, when considered in chronological order, provide a historical perspective on this evolving field.1−13 One of the first significant reviews of rotary-wing dynamic and aeroelastic problems was provided by Loewy,11 where a wide range of dynamic problems was reviewed in considerable detail. A more limited survey emphasizing the role of unsteady aerodynamics and vibration problems in forward flight was presented by Dat.2 Two comprehensive reviews of rotary-wing aeroelasticity were presented by Friedmann.3,4 In Ref. 3, a detailed chronological discussion of the flap-lag and coupled flap-lag-torsion problems in hover and forward flight was presented, emphasizing the inherently nonlinear nature of the hingeless-blade aeroelastic stability problem. The nonlinearities considered were geometrical nonlinearities because of moderate blade deflections. In Ref. 4, the role of unsteady aerodynamics, including dynamic stall, was examined, together with the treatment of nonlinear aeroelastic problems in forward flight. Finite element solutions to RWA problems were also considered, together with the treatment of coupled rotor-fuselage problems. Another detailed survey by Ormiston12 discussed the aeroelasticity of hingeless and bearingless rotors, in hover, from an experimental and theoretical point of view.