Comparison of Variator Technologies for Variable Rotor Speed Drivetrains for Rotorcraft

The investigation presented in this paper is part of the international research project VARI-SPEED, which aims to invent a mass optimized speed variable drivetrain comprising a main gearbox with variable transmission ratio and a rotor suitable for rotor speed variation. A mass estimation model for the Sikorsky UH-60A drivetrain including a compound split was set up. Hydraulic, electric and mechanic variator technologies were investigated regarding their mass, torque and RPM properties. An optimization model was set up to find: 1.) The best variator technology for the mass optimized drivetrain. 2.) The best drivetrain for the mass optimized variator technology and 3.) The mass minimum if variator and drivetrain are considered both in the optimization loop. The research is performed to find out the influence of the variator technology on the mass and the design of the speed variable drivetrain. Furthermore, it is investigated, whether there is a preferable variator technology for rotor speed variation in rotorcraft. The investigation showed that the variator module has a significant influence on the optimal solution and that an individual optimization strategy is necessary for different drivetrain architectures as well as for different variator technologies. The electric variator technology seems to have the highest potential to enable an efficient variable rotor speed technology. At the moment the design is too heavy to gain benefits in efficiency. The mass estimation and optimization of the whole drivetrain including the variator is the basis to enable an assessment of usability of the variable rotor speed technology. Variation of the rotor speed within the drivetrain enables the turboshaft engine, the rotor and the auxiliary units each to operate at their optimal speeds. Rotor speed variation can overcome the divergent requirements between hover and fast forward flight, which is important for future rotorcraft like inventions in Future Vertical Lift (FVL) in the USA and CleanSky in Europe. Furthermore, it increases the efficiency, decreases fuel consumption andCO2- emission and reduces noise and environmental impact of rotorcraft.