This paper presents a conceptual feasibility assessment of replacing the conventional mechanical tail rotor transmission of the Sikorsky S-70i Black Hawk tactical helicopter with a series-hybrid electric drive architecture. The proposed configuration comprises a quad-redundant axial-flux permanent-magnet motor (BETA V600A, 455 kW peak power, 2 900 Nm peak torque), an onboard generator providing primary electrical power during normal operation, and a buffer battery pack sized for a 6-minute emergency fail-safe envelope rather than continuous mission energy supply. Energy requirements are determined from a representative transport mission profile of 78.56 minutes duration, encompassing pre-flight preparation, take-off, cruise, landing, and shutdown phases. A detailed mass-energy balance is performed to verify whether the hybrid installation is achievable within the helicopter's operational maximum take-off weight (MTOW) of 9 072 kg. The results demonstrate that the selected motor satisfies both continuous and transient power and torque requirements with margins of 34.6% (power) and 6.9% (torque). The net mass increase of the hybrid system is approximately 175.5 kg (1.93% of MTOW), obtained after offsetting the added electrical components (350.5 kg) against the removed mechanical drivetrain (175 kg). Sensitivity analysis confirms that the feasibility conclusion is robust across ±10% variation in battery specific energy density, with the payload penalty bounded between 1.73% and 2.18% of MTOW. The net fuel saving for the analyzed mission is 20.14 kg, corresponding to a 3.43% reduction in total mission fuel consumption. The study identifies the conditions under which the proposed architecture may be feasible at the conceptual design stage and highlights that continuous motor power, battery power capability, component location, and aircraft integration constraints may be more restrictive than net mass increase alone. The findings require further validation through detailed dynamic, thermal, structural, electromagnetic compatibility, and safety analyses before any engineering or certification conclusions can be drawn.