Lift-curve characteristics for an airfoil pitching at constant rate

Abstract

This paper reports the results of wind-tunnel studies of dynamic stall for an NACA 0015 airfoil pitching about the midchord at a constant rate. Time-varying pressure readings from 16 locations on the airfoil were collected and used to determine the lift, pressure-drag, and moment coefficients as functions of angle of attack for 100 test cases, covering 20 dynamic airspeed/pitch rate combinations. The dynamic-stall effects of the change (from steady flow) in the angle of attack at which separation occurs at the quarter chord and the change in the angle of attack at which stall occurs were extracted from these data and found to collapse onto a nondimensional pitch rate given by the chord times the pitch rate divided by two times the freestream velocity. The results showed that relatively slow pitch rates had dramatic effects on both the delay of stall and the magnitude of the maximum lift coefficient. The nondimensional rate is a measure of the speed of the leading edge divided by the speed of the freestream; it was found that nondimensional rates of less than 0.03 more than doubled the maximum coefficient of lift. The reduced data also clearly indicate that quarter-chord separation is systematically linked to dynamic stall.