This publication shows advantages and possible applications for variable transmission drivetrains within rotorcraft. The power requirement of a generic helicopter with constant and variable rotor speed was calculated. Various drive train technologies that support a variable transmission were described. The prospects of this technology, its influence on the dynamic behaviour of a rotor and further areas that need to be investigated extensively are presented. This technology is applicable to some rotorcraft architecture. Requests from the rotorcraft industry underline the necessity for future rotorcraft using variable rotational speeds. However, the A160 or the EC145 and Mi-8 already show the potential of this technique. Reduction of required power of the rotor should be possible and also an extension of the flight envelope towards higher flight speeds, higher altitudes, better manoeuvrability, etc. By using a variable transmission gearbox, turbine and auxiliary units can still be driven at their design point, independent of the current rotor speed. Excessive loads may occur when discrete speed transmission are used. Frictional or fluid transmissions with continuous variable ratio may fail due to overheating. Other continuous concepts are favoured. The design of a variable speed rotor focuses specifically on its dynamic behaviours and on structural and geometrical optimisation to avoid operation at rotational speed resonance frequencies. Morphing structures may support this. Some rotorcraft architectures can benefit from a variable speed rotor technology. It probably will increase efficiency, decrease noise levels, fuel consumption and CO 2 production, and the flight envelope may be extended.

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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.

The research 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 performance and cost simulation model was set up for the Sikorsky UH-60A. Three different types of drivetrains were investigated: The Standard drivetrain, a 2 Speed gearbox and a continuously variable transmission (CVT). Flight missions of 3 industry sectors- Oil and Gas, Construction and Search and Rescue with a total of 33 different missions were simulated. The simulation was performed to evaluate the effects of the variable rotor speed technologies on performance and costs in the context of missions. It could be shown that rotor speed variation in a range of 50% can lead to less fuel consumption and mission costs in every industry sector and the performance of the helicopter can be increased. The CVT seems to be the better solution for multi purpose helicopters. In single missions the results can be different, therefore it is important to have a look on more different missions in an industry sector. The mass of the transmission system has an significant impact on the results. The results of the investigation can give an idea about the boundary conditions for the usage of rotor speed variation and show the critical points of rotor speed variation. The technology increases the efficiency, decreases fuel consumption and CO2emission and reduces noise and environmental impact of rotorcraft. Furthermore it can have also economic advantages for the operator.