Rotor Blade Stall Flutter, Stable or Unstable? An Exploratory Investigation of Nonlinear Aeroelasticity

An investigation of rotor blade stall flutter is undertaken to explore the fundamental nature of the phenomenon. An example of UH-60 stall flutter in a full scale pull-up maneuver is presented for illustrative purposes. The Leishman-Beddoes dynamic stall model is used to investigate effective cycle pitch damping for sinusoidal oscillations of varying amplitude and frequency and to guide selection of a simple damper for a simulated stall flutter analysis. Fast Fourier Transform processing is used to relate FFT harmonic content to incipient stall flutter instability. An example of stall flutter in hover is presented using the Leishman-Beddoes dynamic stall model. The full nature of stall flutter remains elusive, but it is suggested that stall flutter is not likely to result in a divergent aeroelastic pitch instability largely because nonlinear pitch damping of dynamic stall oscillations generally increases as the amplitude of pitch oscillations increases.