PREDICTION OF ROTOR INSTABILITY AT HIGH FORWARD SPEEDS. VOLUME 3. STALL FLUTTER

Abstract : The analytical study in this volume was carried out to determine the susceptibility of helicopter rotor blades to a stall flutter instability. This analysis was based on the use of unsteady aerodynamic data previously obtained for an NACA 0012 airfoil oscillating in pitch about its quarter-chord over a wide range of values of incidence angle, oscillatory frequency, amplitude of motion, and free-stream velocity. These data were originally available in the form of moment coefficient-incidence angle loops, and a twofold task was performed in carrying out this study. First, it was necessary to convert the moment coefficient data to an aerodynamic damping parameter form. This was accomplished by integrating the moment over one cycle of motion to yield the aerodynamic work per cycle, and this in turn was multiplied by appropriate conversion factors to produce the desired two-dimensional aerodynamic damping. Second, it was necessary to apply these two-dimensional results to a helicopter rotor to evaluate the weighted three-dimensional damping at each azimuth station, and to interpret the implications of any predicted region of instability. The stall flutter analysis was used in conjunction with the blade motion solution of Volume I to provide flight condition boundaries for stall flutter intensity.