Experimental Investigation of Separation Control Using Upper-Surface Spoilers

T HE control of flow separation over wings and airfoils has been a subject of intense study since its observation and characterization. Separation culminating in airfoil or wing stall provides a natural limit to the lifting ability of wings, thus affecting takeoff and landing performance and maneuvering capability. Commonly used techniques to delay stall include leading-edge flaps as well as slats [1]. The mature technology of flow control has also spawned a plethora of devices to control flow, both passive and active (e.g., blowing, suction, blowing and suction, oscillatory blowing/suction, moving surfaces, compliant surfaces, resonant cavities, etc.) [2–4]. Of these devices/actuators, few have realized application in a production vehicle. The recent interest in extracting or emulating (sometimes coincidentally) biologically inspired technology has a long-standing precedent in aerodynamics. Wingtip sails as described by Spillman [5] are seen on many soaring birds, and leading-edge flaps are also seen to be deployed by birds in the form of the Alula feather. Before the Second World War, it was observed by German engineers that some birds appear to raise a feather on the upper wing surface when landing [6]. It was surmised that the action of the feather was to delay the forward progression of flow separation, thus delaying massive stall. An implementation of this idea (not entirely successfully) was evaluated on an aircraft using a strip of leather to simulate the action of the feather. Approximately 50 years later, studies conducted by Bechert et al. [6] evaluated the concept again with favorable results. In these studies, the spoilers (or flaps, as they designated them) were made from soft flexible material (plastic sheet) and were selfactuating. The investigations of Meyer et al. [7] indicated that the spoilers were effective for Reynolds numbers ranging from less than 150,000 to 1,000,000. Benefits of the spoilers are that they are passive, lightweight, do not require any form of control, and do not occupy internal wing volume.When not actuated, theywere found to show a small performance degradation, depending on their design. This was attributed to the flap behavior with attached flow: the flaps may rise slightly, causing a negative localized camber; consequently, lift drops and drag increases slightly. This behavior was ameliorated by cutting a sawtooth pattern into the rear of the flaps, a naturally occurring embodiment seen in birds. Naturally, a self-actuating flap would need to be light to raise, but sufficiently rigid so as not to crumple. As noted by Schatz et al. [8], the flap should not protrude into the flow above the wake, as this would increase its size and thus drag; their computational data suggested that the flap should just touch the shear layer. The effect of the flap was attributed to it decambering the airfoil when actuated; this alters the pressure distribution to delay the forward stall progression. A similar operating mechanism was cited by Hu et al. [9] for the effect of stall delay by flexible membrane airfoils. Optimal flap angles were experimentally determined to be between 60 to 90 deg, depending on the flap design. Schatz et al. [8] suggested that the spoiler should be located close to the trailing edge to halt the initial onset of separation. They also found that increasing the flap length for their particular configuration increased the maximum lift coefficient. Most testing was conducted for a 12%-chord spoiler. Their computational data suggested that the spoiler could stabilize the separated-flow region and decrease the size of shed vortices. A survey of the literature indicates few studies of these spoilers beyond those mentioned. However, their potential for performance enhancement on unmanned aerial vehicles and similar types of aircraft could be significant. Consequently, an experimental investigation has been undertaken to systematically evaluate the effect of several spoiler geometric parameters. Self-actuating spoilers were evaluated, with effects of location and design explored. Presented results include force balance and motion visualization.