Emission reduction is the most important challenge that aviation has ever faced. This work aims to define a design procedure that is simultaneously applicable to electric and non-electric aircraft. The starting point is the creation of a general model that can describe the propulsion system. Then, the effect of this system on the aircraft performance is calculated according to methods revised from the literature or completely new. Aero-propulsion interactions, aircraft weights, reliability and flight performance are thus evaluated. The work is concluded by two applications on a regional transport aircraft and a commuter aircraft.