Dynamic Behaviour of a Rotorcraft Main Rotor System with Variable Speed

Current Rotorcraft Developments like under the Future Vertical Lift Program in the USA (e.g. Bell V-280, see figure 1) respectively RACER (Airbus Helicopters, see figure 2) and NextGENCivil Tiltrotor (Leonardo, see figure 3) in Europe deal with High-Speed Rotorcraft or Tiltrotor-/Tiltwing Aircraft. They can expand and optimize their performance by using a variable rotor speed either to adopt the rotor speed to high forward speed or to meet the different requirements of a rotor in Hover and Aircraft Mode of a Tiltrotor-/Tiltwing Aircraft. As the required speed range can not be covered by the turbine, TU Munich (Germany), TU Wien (Vienna, Austria), ADT - Advanced Drivetrain Technologies (Austria) and Zoerkler Gears (Austria) work in the transnational project "VARI-SPEED II" on a rotor system that can change the rotor speed via change of the ratio of the transmission (variable rotor speed with constant turbine speed). The project is based on the results of "VARI-SPEED" and the direct follow-up project. VARI-SPEED showed that a rotor speed variation performed by a transmission system is possible. The efficiency and the flight envelope of the rotorcraft can be improved by this technology. Furthermore, a method for rotor blade design in a RPM range was invented. VARI-SPEED II now has built up a model of the complete dynamic system from engine to rotor of a helicopter with a variable speed rotor. This model is used for dynamic simulations of the components and the whole system. A scaled model of the module that changes the speed will be developed and pilot studies in a simulator are planned to find out the characteristics of such a system. Aim of the project is to reach TRL 3 as basis for further development of the technology with interested OEM's. This paper deals with the dynamic behaviour of the components and the whole drive train. The Bell V-280 is in the flight testing phase, RACER has recently conducted its maiden sortie and for the NextGENCtr the first flight is announced for the first half of 2024. All mentioned aircraft use transmission systems with a fixed gearbox ratio; this is feasible as a new technology like Vari-Speed normally is not added to completely new rotorcraft. However as the OEM's rate Vari-Speed positive, variable rotor speed can be integrated in a later development stage of the rotorcraft and thus development timelines of the rotorcrafts and Vari-Speed match quite well.