The wings on aircraft can experience unsteady loads due to atmospheric disturbances (e.g., wind), the wakes of other aircraft, structural vibrations, or some combination of these sources. In some cases flow unsteadiness can be beneficial by augmenting the lift and reducing the drag; under different circumstances temporal variations in the flow can induce stall or cause structural failure. In this study, the response of isolated and tandem airfoils to time-periodic variations in the flow angle and velocity magnitude have been investigated. The predicted results indicate that both sources of unsteadiness can cause the airfoil(s) to produce thrust (instead of drag) during a significant portion of an unsteady oscillation cycle.
