Electric propulsion has the potential to induce a paradigm shift in how aircraft perform, provided the same shift occurs in their design. Improvements to conventional aircraft have reduced CO2 emissions per passenger kilometre by, on average, 3.8% per year since 1960, but these incremental improvements to conventional designs are not sufficient to meet the ambitious goals for reductions in the environmental impact of air transport. To meet these goals, radically different architectures and propulsion systems are required. These radical architectures are by definition difficult to design due to the lack of historical data to enable configuration choices, normally enabled by prior knowledge of performance trends for various configurations. Without the configuration established, optimisers cannot be used due to the small integer variable choices, such as number and position of propulsors, embedded in configuration design. This challenge is solved by developing a optimisation scheme for hybrid-electric aircraft that can deal with the small value integer variables that derail conventional schemes. This scheme is applied to conventional aircraft, for conventional missions, and replicates the expected performance and design of these aircraft. Then, the scheme is applied to missions currently infeasible with conventional aircraft. First of these missions was a STOL aircraft with 500 km range carrying 10 people. Application of this optimisation scheme produced an aircraft capable of a 241 m take-off run, cruise range of 500 km at 200 kt, burning only 287 kg of fuel. Second of these missions, a STOL survey aircraft designed to operate in the High Himalayas, an aircraft that bridges the capability gap between the remote access of helicopters and the endurance of fixed wing aircraft, coupling a 250 m balanced field length with a 1200 km range whilst burning only 217 kg of Jet-A.

This work shows how the unlocking of this integer variable problem is critical to evaluating the true potential of electric propulsion architectures and how they shift and morph conventional design space maps, and unlock performance previously unavailable to both conventional designs and conventionally configured aircraft with electric propulsion.