Parallel hybrid-electric propulsion systems would be beneficial for small unmanned aerialrnvehicles (UAVs) used for military and homeland security missions. The benefits, due to the hybridrnand electric-only modes, include increased endurance time, greater range, and stealth modes. Thisrnpaper describes the conceptual design of a parallel hybrid-electric propulsion system for a smallrnUAV designed for intelligence, surveillance, and reconnaissance (ISR) missions, considers therntrade-offs which must be considered to take advantage of the hybrid-electric system, and presentsrnthe results of the conceptual design. The hybrid-electric propulsion system is a two point designrnthat includes an engine primarily sized for cruise and an electric motor and battery pack that arernprimarily sized for endurance. The electric motor provides additional power for take-off, climbing,rnand acceleration and also serves as a generator during charge-sustaining operation. Electric-onlyrnoperation provides stealth operation not available with gasoline-powered UAVs by reducing thernacoustic, smoke, and thermal signatures. The battery pack/generator that usually only providesrnpower for the sensors and avionics also provides propulsion power. A parallel hybrid-electricrnpropulsion system for a small UAV would potentially, depending on the mission, satisfy the "30%rnincrease in time-on-station (TOS) requirement with the same fuel load" and "UAV inaudible from arn500-1000 ft slant range" capability metrics described in the OSD's UAV Roadmap [1].
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