Tube arrays in industrial components such as heat exchangers and steam generators are susceptible to damage due to fluidelastic instability (FEI). This study focuses on the modelling of fluidelastic instability in tube arrays subjected to cross-flow. There is agreement on the idea of introducing a time delay in FEI models. Although it is one of the key parameters for FEI models, the phenomenon behind this time delay is still subject to many possible explanations, and the underlying physics is still not well understood.In the present work a new time delay formulation for the class of the quasi-steady FEI models is derived in the frequency domain in the form of an Equivalent Theodorsen Function.Unsteady and quasi-static fluid forces were measured to develop the time delay formulation.The effect of Reynolds number on the static force coefficients was also investigated. The time delay function was found to be also dependent on the Reynolds number which implies that the fluidelastic instability is also Reynolds number dependent. Comparison to other time delay functions proposed in the quasi-steady and quasi-unsteady models is made. An initial overshoot in the lift force was observed for the present model.