In the published literature addressing boundary layer separation and its control, the effect of wing motion on the fluid dynamics is given almost no attention. In the stalled regime where separation control is of interest, some degree of unsteady wing motion is always present. The effect will become increasingly more important in the future because of the current paradigm shift towards lightweight yet super strong wing structures. This push towards modern composite construction (glass and carbon fiber, Kevlar, etc.) is driven by prospects of drastic increases in payload, a considerable reduction of energy consumption, and thus also significant increases in range and endurance (reduced drag; increased fuel capacity due to weight savings). Airplane wings will be orders of magnitude more elastic than before, so that separation and its control will be strongly impacted by the motion of the wing structure. Therefore, the consideration of structural motion constitutes the crucial and necessary next major step towards the design of aerodynamically efficient wings, and in particular the successful implementation of flow control strategies in future advanced military and civilian aircraft–both manned and unmanned. This trend was recognized by the Air Force some time ago, which led to the AFRL sponsored X-56A program with the goal of developing a flight-test vehicle to be used to explore active control of lightweight, aerodynamicallyefficient aircraft configurations. 1 The research conducted with the X-56A is critical for the successful development of future slender, lightweight, high aspect ratio wings that will be used by energy efficient transport and unmanned aircraft. The X-56A, also known as the Multi-Utility Technology Testbed (MUTT) flight demonstrator, is a product of the AFRL-led Multi-Utility Aeroelastic Demonstration (MAD) program. This program is a joint effort between AFRL's Air Vehicles Directorate and NASA Dryden. The airplane was designed and constructed by Lockheed Martin’s Skunk Works.

Because flow separation limits the useful angle of attack range, its control has been of interest since the early days of aerodynamics. Steady stall at low-Reynolds number conditions is often characterized by the sudden opening up of a short laminar separation bubble (“bubble bursting”). 2 For example, Poirel et al. 3-5 observed self-sustained lowfrequency oscillations (St= 0.007) of a NACA 0012 airfoil near stall for 4.5 x104< Re< 1.3 x105. Similarly low frequencies (St= 0.01) have been reported for rigidly mounted airfoils at angles of attack near static stall. 6-10 Bragg et al. 6, 7 argue that the flow oscillations result from a cyclic growth and bursting of a laminar separation bubble. More recently, unsteady separation control has attracted considerable attention because of its reduced energy requirements when compared to steady control. A considerable body of research has shown that separation control becomes effective and efficient (ie, minimizing the energy required for reaching a control objective) when underlying flow instabilities are exploited.