Achieving sustained flight of micro air vehicles (MAVs) bring significant challenges due to their small dimensions and low flight speeds. 1 For so-called mini air vehicles, that operate in the 100,000< Re< 300,000 range, efficient systems can be designed by managing boundary layer transition via tripping at multiple locations. 2 However, at Reynolds numbers routinely experienced by MAVs (Re< 100,000), conventional low-Reynolds-number airfoils perform poorly, or even generate no useful lift. Some of the best performing airfoils in this Re range are cambered flat plates and airfoils with a thickness to chord ratio (t/c) of approximately 5%. 1 MAVs are usually designed with surveillance, sensing or detection in mind. Hence, a typical MAV mission should include a “high speed dash”(V~ 65km/h, 18m/s) to or from a desired location with significant head or tail winds and low-speed loiter (V~ 30km/h, 8.3 m/s) while maneuvering, descending and climbing. 3 Mueller defines two MAV sizes, which we can call “large”(b= 15cm, M= 90g) and “small”(b= 8cm, M= 30g). 1 As the Reynolds number decreases below about 100,000, the changes in airfoil performance are significant and boundary layer tripping becomes continuously more difficult, and at Re< 50,000 the separated laminar shear layer does not transition within the dimensions of the airfoil. 1 Consequently, unconventional approaches have been pursued, such as ornithopters that are inspired by bird and insect flight. Active control methods are also pursued. For example, Greenblatt & Wygnanski investigated perturbing an airfoil leading-edge boundary layer via twodimensional periodic excitation at Re≈ 50,000 and 30,000. 8 Near-sinusoidal perturbations at F+≈ 1 resulted in the restoration of conventional low-Reynolds-number lift and aerodynamic efficiency, while excitation-induced lift oscillations were small and hysteresis associated with stall was eliminated. However, with decreasing Re larger periodic perturbations (expresses as〈 Cμ〉) were required to generate useful lift. A similarity between the timescales associated with excitation and those characterizing dynamic stall in small flying creatures provided some insight into these observations. They observed that typical MAV dimensions are suited to actuation by means of microelectromechanical systems (MEMS)-based devices. It was also noted that the effectiveness and efficiency of actuators required to supply the prescribed excitation will ultimately determine the success and limitations of the method.

Plasma-based actuators have recently demonstrated application to drag reduction and separation control. 9-11, 13-15, 20 Separation control on airfoils at typical MAV Reynolds numbers (10,000< Re< 140,000) were first demonstrated by plasma actuation using high voltage (10–20 kV) charged corona discharge wires in 1999. 9, 10 Göksel demonstrated significant improvement to an Eppler E338 airfoil performance [eg Cl, max,(l/d) max], particularly for 10,000< Re< 70,000. 9 For a given power input (in this case~ 8.5 Watts), Cl, max was shown to increase with decreasing Reynolds number up to 2.9 at Re= 10,000. The reason for this is that the relative power input to the actuators, and presumed large relative momentum input, increased with decreasing Re. The relatively large power required to generate meaningful changes to aerodynamic indicators serves as a potential stumbling block in the way of plasmabased separation control. However, several comparisons of separation control by periodic excitation versus steady blowing have indicated that similar performance benefits (eg ΔCl) can be achieved where〈 Cμ〉 is up to two orders of magnitude …