Rotor Blade Wake Flutter - A Comparison Of Theory And Experiment

During early whirl testing of the hingeless main rotor of the AH-56A Cheyenne helicopter, a high-frequency (7P), highly coupled, flap-torsion-inplane mode flutter occurred at rotor overspeed at a condition of near zero lift at the rotor tips. The flutter disappeared at higher and lower values of rotor lift rather than being nearly lift independent as had been predicted by quasi-steady aerodynamic theory. Wake flutter was suspected and corroborated by subsequent analysis. This discussion covers the theoretical flutter analyses and the effects on flutter of design changes made both to eliminate the flutter and to improve vehicle stability and control. A theoretical analysis employing quasi-steady aerodynamics was the basis for predicting the complicated reactionless interblade mode shape and frequency, but not the lift level of instability. The flutter stability analysis was then amended to include the effects of previous blade passage wakes and interblade mode coupling, in the four-bladed rotor, on unsteady aerodynamics and the new analysis results agreed very well with the experimental flutter. The flutter was shown to occur in the specific region of lift near zero, being stable at higher and lower levels, and at a critical overspeed rpm with stability reappearing with increase of rpm above the critical.