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This paper reveals the complexities and challenges in sizing a parallel and series HybridElectric Propulsion System (HEPS) for a single-seat, light aircraft -based on a commercial airframe. Starting from a spreadsheet of key parameters, components for the two powertrain configurations are sized, based on the aircraft aerodynamics and weight. A subsequent comparison of the powertrains is then conducted in terms of component sizes (in kW), weight, control strategy and complexity to highlight their similarities and differences relative to a standard 4-stroke internal combustion engine (ICE) of similar peak power. In addition to the initial outline designs, both hybrid powertrains are optimized around their degree of hybridization to obtain the best performance. This is determined on two defined mission profiles; a short-and a long-range profile, covering a speed range from 20 to 40 m/s. Thus, the relative performance of each powertrain: series hybrid, parallel hybrid, purely electric and conventional ICE is evaluated under comparable conditions. The analysis highlights the importance of the engine design in hybrid-electric powertrains and that the parallel HEPS is favored for aviation applications, due to its inherently more efficient layout compared to the series HEPS. A parallel HEPS with a hybridization factor of 60 % provides the best performance in meeting the trade-off between efficiency and endurance. However, based on the forecast improvement in the energy density of batteries, the analysis predicts a significant electrification of powertrains in the mid-term future for general aviation (GA) applications. NomenclatureBLDC = Brushless Direct Current EM = Electric Motor GA = General Aviation GE = Generator HEPA = Hybrid-Electric Propulsion for Aircraft HEPS = Hybrid-Electric Propulsion System HEV = Hybrid-Electric Vehicle HF = Hybridization Factor ICE = Internal Combustion Engine IOL = Ideal Operating Line of the ICE J = Propeller Standard Advance Ratio L/D = Lift-to-Drag-Ratio LiPo = Lithium Polymer MAX = Maximum Operating Line of the ICE MTOW = Maximum Take-Off Weight PoFo = Power Follower -Controller Strategy RoC = Rate-of-Climb SoC = Battery State-of-Charge SOC = State-of-Charge -Controller Strategy 1 PhD Research Student, Electrical Engineering Division, cf360@cam.ac.uk, and AIAA Student Member 2 University Lecturer, Electrical Engineering Division, par10@cam.ac.uk, and AIAA Member Downloaded by UNIVERSITY OF QUEENSLAND on July 26, 2015 | http://arc.aiaa.org |

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 (HEPS) can take advantage of the synergy between two technologies by utilizing both Internal Combustion Engines (ICEs) and Electric Motors (EMs) 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 hybrid-electric 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 & 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.