Active flow control systems in aerodynamics have been under investigation as far back as the 1930’s. 1, 2 Evaluating their benefits through wind tunnel testing offers cost/time effectiveness advantages. Coanda driven Circulation Control (CC) is an efficient active flow control approach that can also be used on fixedwing unmanned aerial vehicles (UAVs) to achieve lift augmentation during all flight envelopes using small momentum coefficients of blowing. Circulation Control Wings (CCWs) are shown to be simpler, lighter and more efficient than conventional high lift systems. 1 However, there exist several challenges when applying CC on small-scale aircraft: source of air (typically bleed or bypass air from the engine or an addition of an auxiliary power unit); weight penalties due to the internal air delivery system; additional power penalties due to the air supply unit’s power consumption, and, cruise drag penalties due to Coanda surface geometry. These challenges are used as a guide to design, develop and evaluate a CCW able to achieve high lift augmentation ratios at low blowing coefficients.

Research has shown that CC flap systems, offer significant payoffs in both performance and system complexity. CC flap systems augment aerodynamic forces by entraining and deflecting the airfoil flow field pneumatically, rather than solely by deflecting a mechanical surface. 2, 3 Englar’s dual-radius flap, 2 configurations represent specially designed internally blown flaps using the Coanda effect at their curved leading edge, therefore, they are called Coanda flaps. Golden and Marshall3 explored the design of various CC flap systems on a supercritical airfoil (NASA SC (2)-0414) using 2-D CFD analysis and concluded that the largest lift augmentation was achieved with the shorter dual radius flap. However, large negative pitching moments are associated with such lift augmentation, along with large drag penalties resulting in the lowest L/D values of all flap configurations. Jensch et al. 4, 5 conducted design sensitivity studies that led to the selection of particular flap configurations, where the most important design parameters are flap deflection angle, momentum coefficient, and blowing slot height. It is shown that flap angle and blowing momentum coefficient should increase for increased lift targets and good values for the flap length are found to be 0.25-0.30 of the airfoil chord. Recent research at NASA Langley Research Center (LaRC) 6, 7 shows that a slot height to chord ratio of 0.0022 is more beneficial than larger slot height and can have the same performance with a 30% lower momentum coefficient of blowing. The FAST-MAC6, 7 is tested at various flap deflections and low speed (M∞= 0.2) and high speed (M∞= 0.88) values to investigate the lift augmentation and the efficiency of drag reduction using CC. The complexity of the plenum design and the different methods that are used to achieve flow uniformity are also investigated. The importance of the geometrical parameters in Coanda flap design and the need to apply leading edge devices for delaying airfoil stall are well addressed in previous research. 4, 5, 8, 9 This research provides insight into the geometrical effects of the CC flap and the results of this work give a better understanding of the influence of the parameters that affect the performance of dual radius flaps. A wind tunnel CCW model (modified NACA 0015) is developed and tested with different dual flap geometries at three flap deflection angles (0o, 30o, 60o). A 2-D experimental test is conducted to investigate the flap geometry that provides the maximum L/D ratio, minimum cruise drag penalties and the maximum lift enhancement during takeoff …