Experimental Investigation of Unsteady Transition on a Pitching Rotor Blade Airfoil

The unsteady flow around a pitching helicopter main rotor blade airfoil, the EDI-M109, was experimentally investigated at conditions similar to those existing on a retreating rotor blade in forward flight. High-speed pressure measurements and hot film anemometry were used to investigate the unsteady transition characteristics of the airfoil. Results are presented for dynamic test points with attached flow, light dynamic stall, and deep dynamic stall at M = 0.3, Re = 1.8 x 10(6), and f = 6.6Hz. The results include the discussion of the periodicity of the hot film signals for different flow states. The transition process of the pitching airfoil is analyzed, and the significance of the intermittent region is described. A time delay between the transition and the model motion is discussed, and a linear relationship between the transition position and time is observed. The influences of the pitching amplitude on the transition characteristics are discussed, and the flow separation initiating dynamic stall is analyzed.