In unsteady aerodynamics, the reduced frequency has always been considered as the predominant controlling parameter because it relates to the unsteadiness. In this study, we examined the responses of several delta wings of varying aspect ratio and a two-dimensional wing in an unsteady periodic freestream. We found that in certain operating conditions, the lift of the airfoils may or may not be a function of the reduced frequency, depending on whether the leading-edge separation vortex can remain stationary on the wing or not. For delta wings with attached, stationary leading-edge vortices, the lift forces are not a function of the reduced frequency. However, if the leading-edge vortices shed and convect downstream, then the lift force depends on the reduced frequency.
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