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Recent studies [1]-[4] have shown that a variation of helicopter main rotor speed allows a significant reduction of the required power. Therefore an appropriate drive train technology is necessary to enable variable rotor speed. However, such a technology entails drawbacks such as increased weight and reduced efficiency [5]. This study provides arguments and results to enable a decision process towards a promising helicopter configuration incorporating a variable rotor speed and related applications. These are mainly obtained from mission performance calculations and additional transmission weight investigations. Benchmark missions are derived and presented while two promising drive train concepts are introduced. A continuously variable gearbox stage is shown to be especially useful for utility helicopter applications while a dual-speed, clutched stage gearbox is particularly suitable for tilt-rotor concepts. The capability to vary the main rotor speed extends the flight envelope and reduces fuel consumption. This study shows that the portfolio of missions that can be carried out efficiently and the efficiency itself is enhanced by this technology.