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Flow control to avoid or delay rotorcraft retreating blade stall can be an enabling technology for future high performance rotorcraft. Aerodynamic experiments and computations have indicated that appropriate unsteady excitation can delay boundary layer separation and stall on airfoils. Work is in progress to determine the control requirements for helicopter rotor blades at full scale Mach numbers, Reynolds numbers, and with unsteady pitching motions. Compact, powerful, and efficient flow actuation and control systems will be needed. Three actuation concepts were favorably evaluated during initial studies: electromechanical directed synthetic jets (DSJ), periodic flow modulation, and plasma actuation.Electromechanical DSJ and plasma actuators are being developed further and will be evaluated in full scale pitching blade section experiments. These experiments will determine the required control authority, validate the actuator concepts, and study open and closed loop control approaches. Computational studies are being performed of the combined external and actuator flow fields to determine preferred actuation geometries and operating points. System analyses are being used to quantify the benefits for representative aircraft configurations and missions.

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This work presents progress on a detailed aerodynamic evaluation of Miniature Trailing-Edge Effectors (MiTEs) for active rotor control. We begin with a 2D computational fluid dynamics (CFD) study focused on establishing the dependency of MiTE effectiveness and performance upon basic geometric parameters on a VR-12 airfoil. The CFD study demonstrated that a MiTE placed at 10% chord upstream of the trailing-edge and sized at approximately 1% chord is capable of delivering moment coefficient increments of approximately ±0.03, or the same moment authority as a conventional flap moving ±2.3 degrees. Wind tunnel experiments were performed on a model blade section equipped with an operational MiTE in order to validate the CFD results, and strong agreement was shown. Finally, a small set of 3D unsteady CFD simulations with prescribed blade motion of a rotor equipped with MiTEs were performed. Under high-thrust, moderate speed conditions, MiTEs deployed sinusoidally at 4/rev frequency were capable of reducing 4/rev integrated aerodynamic loads in the vertical direction by approximately 80%.