We consider the identification of a nonlinear energy sink (NES) designed to limit the vibration of an aircraft wing by attracting and dissipating energy before a transient response can build into a limit-cycle oscillation (LCO). The device studied herein is the prototype of an NES intended to be mounted at the tip of a scale-model wing, housed in a winglet, and capable of interacting dynamically with the wing over a broad frequency range. Because the stiffness of the NES is essentially nonlinear (i.e., its force-displacement relation is nonlinearizable), it cannot be regarded as a perturbation of a linear system. Furthermore, the action of the NES requires the presence of some amount of damping, here assumed to be viscous. Both the nonlinear stiffness and the linear viscous damping have been evaluated using the restoring force surface method (RSFM), and found to be repeatable across trials and across builds of the system. These findings are summarized and used in simulations of the NES attached to the wing. The simulations are then compared to experiments (ground vibration tests), revealing good agreement of transient responses and of frequency-energy dependence, the latter revealed by wavelet transforms of the computed and measured time series.