Aeroelastic Stability of Hingeless Rotor Blades in Descent Flight

In this paper, the aeroelastic stability of hingeless rotor blades in the descent flight by including the effects of vortex ring state (VRS) has been qualitatively investigated. The blade is modelled as a cantilevered beam using an exact formulation, while the aerodynamic loads are modelled using the quasi-steady Greenberg’s aerodynamic theory combined with a prescribed induced velocity due to the vortex ring state. An identified VRS-induced velocity, from a curve fit to flight-test data, is coupled to the aerodynamic model to complete the formulation for the descent flight. The aeroelastic model is first validated in hover against the published results for a four-bladed hingeless rotor. Quantitatively, the maximum deviation between the present predictions and the reference results is approximately 6%, confirming the accuracy of the proposed modelling approach. The model is then used to qualitatively investigate the effect of the vortex ring state on the aeroelastic stability of the rotor during descent flight. The results show that the instability region of the blade shrinks during descent compared to hover, an effect that is more pronounced at lower precone angles. Increasing the torsional and lead-lag frequency ratios, as well as the descent velocity, are both shown to further influence the extent of the instability region. However, when the identified VRS-induced velocity model is used in place of a constant-inflow assumption, the predicted instability region expands and a greater descent velocity is required before the rotor becomes fully stable, highlighting the importance of accounting for VRS aerodynamics when assessing the qualitative aeroelastic stability trends of rotor blades in descent flight.