A differential equation describing the behavior of an oscillatory vane for flow‐speed measurements is obtained. It differs from the classical equation for an ordinary flow‐direction vane in that it contains a positive feedback term representing the flipovers of the fin. No analytical solution to the equation exists, but an oscillatory vane’s response may be determined to a good approximation by simple calculation of second‐order parameters and by use of a relevant coefficient whose value has been obtained from a numerical solution and is provided here in a table for a variety of vanes. In agreement with experiments, the vane’s oscillation frequency is shown to be directly proportional to flow speed, and furthermore, to have a square‐root dependence on the density of the flowing medium, the physical dimensions of the vane, and the reciprocal of its moment of inertia.