Experimental evaluation of an active controlled L-shaped tab for dynamic stall alleviation

Abstract

The present paper describes the experimental activity carried out to investigate the effectiveness of an active trailing edge L-shaped tab for deep dynamic stall control. Wind tunnel tests were performed on a NACA 23012 pitching airfoil in deep dynamic stall conditions. The L-shaped tab was designed to behave as a Gurney flap when deployed, as its end prong protrudes at the airfoil trailing edge, while in retracted position the tab behaves as a trailing edge flap. The active control of the deployment and retraction of the tab along the oscillating cycle was based on the use of micro pneumatic actuators guided by miniaturized servovalves. The results of unsteady pressure measurements carried out on the airfoil model midspan contour showed that important benefits for blade aerodynamic performance and structural integrity prevention could be achieved deploying the tab during the upstroke motion and retracting the tab during the downstroke. The main goal obtained by the active control system was a conspicuous increase of the net positive aerodynamic damping associated to the pitching moment, thus ensuring to avoid the risk of stall flutter occurrence. Moreover, the tab deployment in upstroke produces a conspicuous increase of lift corresponding to a higher level of available thrust very useful on the retrating side of a helicopter rotor. The retraction of the tab before the stall onset enables also to reduce the pitching moment peak with respect to the clean airfoil configuration. The present tests results illustrates that the tested L-shaped tab can be considered a very attractive device to be employed on helicopter rotr blades for dynamic stall control due to its easier integration at the trailing edge with respect to an active deployable Gurney flap.