Before enlarging on the main subject of this paper, it will be helpful to clarify some fundamental conceptions of various rotor drive configurations.

In the past the original shaft-driven helicopter presented two major problems to the designer. These were first: the necessity to choose between either some torque balancing device or a multi-rotor layout and, second: the necessity of a large reduction gear box, the weight of which seemed to become prohibitive with increasing all-up weight of the design. Big helicopters with either single rotor/tail rotor configuration or multi-rotor configuration, are flying successfully and prove that these afore-mentioned problems, although still in existence, were mastered in an admirable way. Nevertheless, in the old days the words “ tip. drive ” had a magic sound to the harassed designer.