Accurate yet computationally efficient modelling of fixed-wing airborne wind energy systems is key in the preliminary design phase. For the simulation framework of large scale fixed-wing airborne wind energy systems an FSI model is developed that can evaluate flight states in the order of seconds and is able to account for viscosity and multi-element airfoils. MSES is combined with a lifting line for the aerodynamic calculations and the stiffness matrix from a finite element method is used to calculate the deformations. A circular flight path is used as a validation case for the developed model, for which data is available from a panel method and CFD. The developed model shows significant improvements from the panel method and evaluates the flight states in the order of seconds. Therefore, the new model can be used for further research and development of fixed-wing AWESs.