Unsteady Airfoil with Dynamic Leading- and Trailing-Edge Flaps

T HE dynamic overshoot in lift force and the accompanying large nose-down pitching moment observed on oscillating and pitching airfoils, as a result of the formation, convection, and shedding of an energetic dynamic-stall vortex (DSV), continue to make dynamic stall and its control an important topic in unsteady aerodynamics. Moreover, once the DSV passes the airfoil trailing edge and moves into the wake, the flow progresses to a state of poststall full separation over the upper surface and an abrupt loss of lift is incurred, causing a large degree of hysteresis in the lift coefficient. Various dynamic-stall flow control methods, such as trailing-edge flaps [1–5], pulsating and synthetic jets [6–8], leadingedge slots with and without blowing and suction [9,10], and dynamically deformable and variably drooped leading edges [11,12], etc., capable of minimizing, or eliminating, the hysteresis in the dynamic-Cl loop and the peak negative pitching-moment coefficientCm;peak, have been proposed. It should, however, be noted that for rotorcraft, the dynamic-stall flow control is aimed at the prevention of the DSV occurrence and the mitigation of nose-down pitching moment on rotor blades, whereas for highly maneuverable aircraft, the purpose is to delay the DSV spillage and to enhance the dynamic lift. The trailing-edge flap (TEF) dynamic-flow control concept has been considered widely for dynamic lift enhancement and nose-down pitching-moment suppression. Trailing-edge flaps have also been used extensively as a routine practice of controlling the lift by temporarily altering airfoil camber on an airplane in steadylow-speed operations, especially during takeoff and landing, without penalizing cruise performance. Rennie and Jumper [2] investigated the effectiveness of deflecting a 27% TEF control surface in negating the unsteady lift generated during a pitchmotion of a NACA 0009 airfoil atRe 2 10. They reported that the dynamic TEF effectiveness was larger than the steady-state value and had direct ramifications on unsteady lift control. The effectiveness of a deflecting control surface was also found to be higher while the control surface was in motion and was rapidly decreased at large deflections, as a result of the thickening or separation of the boundary layer on the trailing-edge flap. Furthermore, the motion of TEF appeared to delay the occurrence of dynamic stall on the airfoil. Gerontakos and Lee [5] examined the alleviation of the nose-downpitchingmoment by using a 25%c TEF, actuated dynamically in response to the airfoil oscillation, at Re 1:65 10. The prescheduled TEF motion consisted of a brief pulse, represented by a constant ramp-upmotion, remained steady briefly, and was followed by a constant ramp-down motion. They found that the reduction in j Cm;peakj was mainly a consequence of the suction pressure introduced on the lower surface of the upward deflected TEF and that a relatively early flap actuation (initiated between the static-stall angle ss and themaximum angle of attack max during pitch-up) with a rather long duration (about half of the oscillation cycle time) and a maximum deflection max 60% max should be more effective at reducing the nose-down pitching-moment excursion, and, in the meantime, provide a good compromise between the various aerodynamic requirements. Also, the magnitude of the maximum lift coefficientCl;max was found to be rather insensitive to the flap-actuation duration td, whereas the poststall lift was decreased with decreasing td. More important, the DSV formation and detachment were not affected by the upward TEF motion. In addition, dynamic leading-edge-flap lift control has also been employed by researchers elsewhere to allow high-performance aircraft to achieve a rapid and sustained high-lift coefficient. The purpose of leading-edge devices is to increase camber and thus suppress leading-edge separation during rapid arbitrary airfoil pitching maneuvers. Rennie and Jumper [13] reported an experimental determination of a 20% leading-edge flap schedule used to maintain attached flow during arbitrary dynamic pitching motions of a NACA 0009 airfoil at Re 2 10. The leading-edge flap (LEF) schedule kept the flow attached dynamically at angles of attack that were separated during static tests. The use of the LEF avoided catastrophic dynamic-stall flow-separation events. Also, for a static leading-edge flap schedule, the attached flow could be maintained by keeping the leading-edge flap aligned with the oncoming flow. In summary, it is known that both leadingand trailing-edge flaps can serve to increase the camber and manage the boundary layer effectively and thus increase the maximum lift of an airfoil. However, despite much predictive work, published experimental data on the unsteady aerodynamic loads induced by the dynamically deflecting leadingand trailing-edge flaps are still sparse. A preliminary experimental study was conducted to examine the effects of an 18%c leadingand a 25%c trailing-edge flap, actuated dynamically and independently, on the dynamic-load loops of a sinusoidally oscillatingNACA0015 airfoil in a subsonicwind tunnel at Re 2:86 10. The dynamic-load loops were obtained by integrating the unsteady surface pressure distributions. Both upward and downward flap deflections actuated at a fixed start time (ts 0 ) were investigated. This ts value was chosen to maximize the influence of the flap motion on the transient DSV-induced effects. Special emphasis was also placed on the simultaneousmeasurements of airfoil and flap deflection histories, synchronized with the surface pressure measurements.