An Analysis of the Stall Flutter Instability of Helicopter Rotor Blades

Abstract

An analytical study 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 by the United Aircraft Research Laboratories for the Sikorsky Aircraft Division on 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. It was found that under certain conditions of forward speed and high disk loading, a helicopter rotor blade could encounter regions of negative aerodynamic damping while operating as a retreating blade. The extent of the negative damping regime was sensitive to changes in incidence angle distribution, and for the cases considered herein could produce a maximum of two cycles of unstable torsional motion per revolution. However, under most operating conditions the rotor blade was found to be quite stable. Generally good agreement was found to exist between the results of the present study and those of other investigators at all stages in the development; e.g., the two‐dimensional damping data were in good agreement with the results of Halfman, et al and the final three‐dimensional damping variation with azimuth over the rotor disk was in qualitatively good agreement with the recent work of Ham and Young.