Incipient torsional stall flutter aerodynamic experiments on three-dimensional wings

The aerodynamics of small amplitude pitching motions near stall have been studied experimentally in order to improve understanding of torsional stall flutter. A model wing was oscillated in pitch at several small amplitudes over a wide range of conditions. Unsteady surface pressures were measured and integrated to determine the aerodynamic damping at five spanwise stations. Attached flow damping was positive and, for moderate Mach numbers and frequencies, in good agreement with thin airfoil theory. Strong negative damping was found for motions centered near static stall for all studied reduced frequencies, Mach numbers, and sweep angles. The 30-deg swept-back configuration was found to become negatively damped over the entire span nearly simultaneously, while the unswept model exhibited local regions of negative damping that moved toward the wingtip as the mean angle of attack was increased. (Author)