EXPERIMENTAL-STUDY OF THE AERODYNAMICS OF INCIPIENT TORSIONAL STALL FLUTTER

An experimental study has been made of the aerodynamics of small-amplitude airfoil torsional oscillations near stall to examine the mechanism that leads to stall flutter on propeller blades. An airfoil was oscillated in pitch at amplitudes of 0. 5, 2. 0, and 4. 0 deg. Time histories of the unsteady surface pressures show that stall initiation and reattachment at low amplitudes are determined not only by kinematic conditions but also by the presence or absence of random effects such as gusts or vibrations. In contrast to the results at higher amplitude, hysteresis loops at low amplitude near stall are dominated by a central destabilizing subloop, so that the aerodynamic pitch damping per unit amplitude has a maximum negative value near an amplitude of 2 deg. This implies that small-amplitude, oscillations near stall may be extremely unstable.