Motion-dependent fluid forces are measured in a water channel. From the measured fluid forces, fluid-stiffness and fluid-damping coefficients are calculated as a function of reduced flow velocity, oscillation amplitude, and Reynolds number. Once these coefficients are known, the mathematical model based on the unsteady-flow theory can be applied to predict structural instability due to fluid damping. The cases we considered are a single tube, twin tubes, tube row, triangular array, and square arrays. The results show instability regions that are based on the fluid-damping coefficients and provide answers to a series of questions on fluidelastic instability of tube arrays in crossflow.