Introduction The adverse interactions occurring between the three main disciplines : unsteady aerodynamics, aeroelasticity and servo-controls are called aeroservoelastic interactions. These interactions can be described mathematically by a system of equations in a state space form. This system requires a different representation for the unsteady aerodynamic forces from that for the classical flutter equation. The unsteady aerodynamic forces, in the case of the classical flutter equation or aeroelasticity, are calculated by the Doublet Lattice Method (DLM) in the frequency domain for a set of reduced frequencies k’s and Mach numbers M’s. The error in the DLM modelling due to omitting the effect of the boundary layer, viscosity, flow separation, etc are of higher order than the error in curve fitting frequency domain modal generalized forces by jω rational functions and replacing jω by s. Since time domain Linear Time Invariant Ordinary Differential Equations (LTI ODE) are required for using modern control theory design, several approximations for the unsteady aerodynamic forces ∗Post-Doctoral Fellow †Professor, Automated Production Department, AIAA member.