Parallel hybrid turboprop engines propose a means to reduce fuel consumption of regional aircraft due to lower flight velocities. They feature an electric drive, typically on the free power turbine, and require a design trade-off between the gas turbine and electric power sub-system characteristics. Degrees of freedom include the nozzle expansion, the propeller power loading, the gear ratio, and the selection of shaft speeds. The latter for instance requires a trade-off between propeller and free power turbine efficiency. For a parallel hybrid, the electric machine efficiency becomes a third factor to consider. The objective of this paper is to expose some key aspects of these trade-offs in terms of efficiency and weight. The paper applies sophisticated methodology in both the gas turbine and electrical power domains. For the gas turbine, multi-point design is used. Here, an extension of established synthesis matching schemes is used, which covers the design and operation rules also for the electric components of the hybrid. For the electrical machine, fully analytical sizing is used, which also captures the impact of cooling. For all main gas turbine components and the electric machine, the geometry is estimated based on the sizing methodology, and used as input for the weight estimation. Results are presented for parallel hybrid electric 2.5-spool geared turboprop architectures fulfilling requirements of a notional 19 passenger regional aircraft. Uninstalled fuel consumption can be lower for the hybrid than the conventional baseline, and the key relations to typical cycle parameters such as overall pressure ratio and shaft speed selection are exposed. Overall, the benefit of hybridization is low however with the concept of operations inspired by hybrid turbofans. This is related to differences in contradicting cycle design requirements.

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