High-Lift Enhancement Using Fluidic Actuation

The performance of an airfoil equipped with a high-lift system based on a single-element flap configuration is enhanced by the manipulation of vorticity concentrations near the airfoil surface using fluidic actuation. The actuation, which is effected downstream of the juncture between the main airfoil body and the flap, leads to a significant extension of the attached flow domain along the flap. Consequently, a substantial increase in lift at large flap deflections is produced (at δ = 40o and α = 4o, the lift is increased by 49% relative to baseline). Actuation is effected using spanwise arrays of fluidic oscillators consisting of jets oscillating in the spanwise direction. It is shown that the momentum coefficient Cµ required for a given lift increment can be significantly reduced through appropriate design of the interface geometry between the jet and the boundary layer. I. INTRODUCTION The performance of high-lift systems has a significant impact on the design and performance of transport aircraft. Maximum takeoff weight, required runway length and stall speeds are some of the parameters influenced by high-lift system performance. Historically, high-lift systems have consisted of complex, multi-element designs with intricate positioning mechanisms to derive maximum performance and efficiency. While high-lift systems have been simplified significantly on modern transport aircraft, there remains room for improvement in terms of weight, parts, fabrication costs and cruise efficiency. Active flow control provides an avenue for such improvements and creates the potential for high-lift performance beyond what is possible for conventional systems. A study conducted in 1999 by Boeing and NASA (McLean et al. 1 ) estimates that, among other things, a simplified high-lift system using unsteady flow control can reduce the empty weight of a 737-class aircraft by 3.3%. This weight reduction, along with the aerodynamic advantages of a simplified high-lift design, such as the removal of fairings required for the external mechanisms of conventional high-lift systems, equate to a bestcase cruise drag reduction of 3.2%. The authors also conclude that a further study is required to define the aerodynamic performance of high-lift systems using unsteady flow control. Gomes et al. 2 have concluded that it is possible to design a practical AFC highlift system based on synthetic jets. However, the performance benefits of such a system still remained to be established and would determine whether or not the technology could be integrated on an aircraft. They have also suggested that the application of AFC to the trailing-edge flap would yield the greatest benefit to the airplane as far as high-lift system application was concerned. Strategies for active flow control on lifting surfaces have primarily focused on mitigation of partial or complete flow separation over stalled flaps or wing sections, where the separating shear layer is dominated by a strong coupling to the instability of the wake that