A major issue which has usually prevented aircraft manufacturers from implementing efficient and cost-effective design processes is the loose integration of engine models into iterative aircraft design workflows. This work aims at reducing this gap, by introducing a simplified modeling of the behavior of a gas turbine, allowing to rubberize a generic turbofan engine. In the present context, this methodology has been implemented for a two-spool, direct-drive, unmixed flow turbofan, but it could be easily extended to different engine configurations (e.g., geared turbofan). In order to prove the effectiveness of this approach, this rubber engine model has been included in an already existing aircraft design framework and has been used to carry out parametric analyses on engine bypass ratio, aiming at guiding the selection of this engine parameter for an advanced short-haul narrow-body aircraft.