In parallel with the automotive industry, hybrid-electric propulsion is becoming a viable alternative propulsion technology for the aviation sector and reveals potential advantages including fuel savings, lower pollution, and reduced noise emission. Hybrid-electric propulsion systems can take advantage of the synergy between two technologies by utilizing both internal combustion engines and electric motors together, each operating at their respective optimum conditions. However, there can also be disadvantages to hybrid propulsion. We are conducting an analysis of hybrid-electric propulsion for aircraft, which is looking at modelling systems over a range of aircraft scale, from small UAVs to inter-city airliners. To support the theoretical models, a mid-scale hybridelectric propulsion system for a single-seat manned aircraft is being designed, built, and tested to generate data for validation and development of the simulation models. This paper draws parallels between the synergy of hybrid-electric propulsion for automotive and aviation applications, and presents an innovative theoretical approach integrating several desktop PC software packages to analyse and optimize hybrid-electric technology for aircraft. Our findings to date indicate that hybrid-electric propulsion can have a significant impact in the small- and mid-scale sectors, but only a minor impact in the large-scale sector assuming battery energy densities predicted for the next decade. Fuel savings of up to 50 and 10 % have been calculated for a microlight aircraft and inter-city airliner, respectively, over the mission profiles considered.