We examine the directional stability of a two-dimensional neutrally buoyant foil in an ideal fluid. To take advantage of the greatest simplicities we consider a symmetric Joukowsky airfoil and use the method of images. J.N. Newman in Marine Hydrodynamics [1] states, “For a nonlifting body with a pointed tail ... the vessel is always unstable. This situation results from the destabilizing effect of the Munk moment; in general an elongated nonlifting body will be stable only when moving broadside to the flow. Directional stability of a streamlined body depends on a tail fin, as in the case of an arrow or wind vane.” While the Munk moment on an ellipse leads to broadside stability, it does not have a Kutta condition. This Kutta condition causes many challenges to the analysis and simple computations. A symmetric Joukowsky has a “tail fin” and it is a lifting body when inclined to the flow. Instability is prevalent for many cases. We show that the stability depends on how the Kutta condition and the wake are implemented and indicate how stability may be increased by mass redistribution or manipulation of body motion. We demonstrate this with some simple experimental demonstrations and computations.

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