Magnetic Suspension with Minimum Coupling Effects for Wind Tunnel Models

In a multidegree-of-freedom suspension system coupling effects are undesirable because they make the calibration difficult and they can cause deterioration of the dynamic stability of the system. This paper presents an analysis of the coupling problem leading to the design of a particular system configuration. First, the support of spherical models by means of a three-degree-of-freedom magnetic suspension system is analyzed in terms of the forces acting on a magnetized point. It is shown that coupling effects due to displacements are eliminated only if the magnetization, the resultant force to be balanced and the axis of the field providing the balancing force, are aligned with each other. In the case of axisymmetrical models this condition can always be met, except for small disturbances, in a vertical wind tunnel leading through the coil which balances the resultant of the drag and the weight. The same coil provides the magnetization of the model. Orthogonality of the force axis system is achieved by using a pair of coils, operating in push-pull, for each of the force axes perpendicular to the axis of the wind tunnel. This arrangement eliminates first-order coupling effects between the force axes. It is shown that long axisymmetrical bodies can also be supported by the same system. This is made possible by decoupling the angular and the translational degrees of freedom. Restoring moments are produced by the field of the magnetizing coil.