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

A three-dimensional (3D) solid finite element analysis 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 steady-level flight regimes are considered: a low-speed transition (μ = 0.1) and a high-speed cruise regime (μ = 0.35). Two typical lift-offsets are considered: zero (LO = 0) and 10% of rotor radius (LO = 0.1). Two blade models are considered: a high flap frequency (ν_β = 1.44/rev) and a low flap frequency model (ν_β = 1.23/rev). The predicted airloads are verified qualitatively with recently published industry predictions. The predicted stresses are unique to this paper. Their 3D nature is revealed by this analysis. Predictions indicate that lowering the frequency of the first flap mode while tailoring the shape of the second flap mode might help relieve stresses significantly (50%) while also avoiding the danger of blade strike at high speed. Even though specific conclusions are premature without the exact properties of an actual aircraft and a higher fidelity aerodynamic model, it appears clear that blade flexibility may be a key factor even for these stiff rotors to relieve blade and hub stresses.