A theoretical analysis was made of the effect of aerodynamic hysteresis on stalling flutter. The assumption was made that the absolute magnitude of the oscillatory aerodynamic forces and moments are the same at stall as at zero angle of attack hut that the vector magnitudes of these forces and moments are changed, this change being caused by the lag of aerodynamic damping and restoring forces behind the velocities and displacements at stall, thus giving rise to a hysteresis effect. The decrease of critical flutter speed at stall was thus theoretically shown. The results were applied to a given airfoil and correlation of the experimental and theoretical results was found possible by assuming that the angle of aerodynamic lag varies as the slope of the static-lift curve. The aerodynamic lag was shown to cause the effective torsional damping to decrease thereby explaining the low values of torsional aerodynamic damping obtained at stall.
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