Three-Dimensional Blade and Hub Stresses of Coaxial Rotors in High-Speed Forward Flight

Abstract

A three-dimensional (3-D) solid finite element analysis (FEA) model is developed to predict blade and hub stresses of a lift-offset coaxial rotor in forward flight. The model is open source, with a generic internal structure, but with rotor radius, planform, number of blades, hub type, and rotor frequencies loosely resembling a modern lift-offset coaxial rotor. Comprehensive analysis is carried out with a lifting-line aerodynamic model with free-wake. Two traditional high vibration regimes are considered: a low-speed transition flight ($\mu=0.1$) and a high-speed cruise flight ($\mu=0.35$). Two typical lift-offsets are considered: zero ($LO=0$) and 10\% of rotor radius ($LO=0.1$). The predicted airloads are verified by comparing them qualitatively with recently published industry predictions. The predicted stresses are unique to this paper. The highest stresses were found to occur on the advancing side and toward the front of the disk. At low-speed, the stresses and deformations were mostly as expected. At high-speed, they were more interesting, with the second elastic bending mode playing a key role in the trade-off between blade strike and blade stress. Some of the behavior were found to be counter-intuitive; for example, at low-speed, increase in lift-offset produced a 38\% increase in stresses, whereas at high-speed it reversed, so that an increase in lift-offset produced a 20\% decrease in stresses. Even though specific conclusions are premature without exact properties of an actual aircraft and a higher-fidelity aerodynamic model, it is clear already that blade flexibility may be a key factor even for these stiff rotors, and that the blade and hub stress patterns may not always be intuitive. These and other interesting phenomena are the subject of this paper.