Unsteady Boundary Layer Transition on the DSA-9A Rotor Blade Airfoil

The unsteady boundary layer transition on the pitching helicopter main rotor blade airfoil DSA-9A was experimentally investigated by the use of hot film anemometry and unsteady pressure measurements. The unsteady flow characteristics on the upper and the lower side of the airfoil were analyzed for steady test cases and dynamic cases with sinusoidal pitching motion in attached flow conditions at M = 0:30 and Re = 1:8 x 10^6. The paper discusses the unsteady transition characteristics in detail and presents the influence of the pitching frequency on the unsteady transition. The results indicate that a large transition hysteresis exists on both sides of the airfoil, and that the hysteresis is much larger than can be explained by the unsteadiness in lift. Significant transition zones exist on the airfoil with sizes of up to 55% chord. The frequency influence is seen in an increase in the hysteresis and in a reduction of the size of the transition zone with increasing frequency