Rotor blade tip sweep effect on lead-lag damping in hover and forward flight

Abstract

Rotor blade geometry (tip sweep, planform taper and anhedral) significantly influences the aerodynamic efficiency, vibratory loads and aeroacoustic behavior of the helicopter rotor. In this paper, the focus is on analyzing the effect of blade tip sweep on the structural dynamic, aeroelastic loads and blade lead-lag damping in hover and forward flight for an isolated rotor. For carrying out this analysis, a comprehensive analytical model is developed. In the structural model blade undergoes coupled flap, lag, torsion and axial deformations. Tip sweep, pretwist, precone, predroop, torque offset and root offset are included in the model. Aerodynamic model includes 3-state Peters-He dynamic wake theory for inflow and the modified ONERA dynamic stall theory for airloads calculations. The comprehensive model is first validated by comparing the predicted results with experimental data. The predicted results of blade root loads and lag damping for a straight blade in hover, over the collective angle settings of -2 to 6 degrees, are in a very good agreement with the measured whirl tower test data. In hover and forward speeds, the lag damping values of swept tip blade are less compared to the straight blade for various collective angle settings. For a straight blade, lag damping values show slight increase and decrease with the increase in advance ratio, whereas for the swept tip blade, lag damping shows a marginal increase at lower advance ratio up to 0.1 and then a decrease in the range of advance ratios 0.1 to 0.2 and thereafter it shows an increasing trend. This kind of variation in damping trends for straight and swept blades with forward speed qualitatively agrees with experimental observations available in the literature.