Modeling lift hysteresis on pitching airfoils with a modified goman-khrabrov model

Wind-tunnel experiments were conducted on two symmetric airfoils with different thickness ratios as a test of the ability of Goman-Khrabrov-type models to predict the lift coefficient history during pitching maneuvers. The primary difference between the two airfoils was that the thin airfoil had no hysteresis in its lift curve during quasisteady maneuvers, but static hysteresis was observed with the thick airfoil over a range of 16 < α < 22 deg. Both airfoils exhibited dynamic hysteresis in the lift coefficient when the wing was pitching. The existence of static hysteresis had a strong effect on the lift response during periodic pitching and must be accounted for in the model. A modified version of the Goman-Khrabrov model was introduced that captured the static hysteresis behavior and the largescale features of the dynamic hysteresis for the thick airfoil, even when the thick airfoil was in a deep-stall condition. Some deviations between model and experiment were observed at the highest angles of attack, which were attributed to the convection of the dynamic stall vortex and its backwash effect as it leaves the airfoil.