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H. Amri, P. Paschinger, M. Weigand et al.:
"Possible Technologies For A Variable Rotor Speed Rotorcraft Drive Train";
Talk: 42nd European Rotorcraft Forum 2016, Lille, France; 09-03-2016 - 09-05-2016; in: "42nd European Rotorcraft Forum 2016", (2016), Paper ID ERF2016_108, 15 pages.

This publication shows possible technologies to enable variable rotor speed for rotorcraft. The technologies are divided into four categories: Turbine technology, gearbox technology, electric drive train technology and rotor technology. They were analysed and designed, based on a defined reference configuration. The analysis shows which technologies enable a speed variation, the expected mass increase, the change of efficiency and the possible difficulties in realisation.
Using a turboshaft engine to vary the rotor speed enables wide speed range and adds only about 5% of the turboshaft mass. But due to the possible high torque increase at lower speeds, also the gearbox weight increases. A decrease of maximal available power at lower speed has to be taken into account. The rotor speeds can´t be controlled individually and the auxiliary units are influenced by a speed change.
Using a gearbox enables a wide speed range but causes mass increase which is higher, compared to the turbine technology, but not much higher, if the thereby linked gearbox mass increase is taken into account. It is important that the part for transmission variation is not part of the main power flow. To gain most advantages it is necessary to place the gearbox close to the rotor. Then the auxiliaries are not influenced by speed variation, an independent change of the rotor speed is possible and the turbine can operate in the optimum operation point. Existing inventions would have a too high mass increase from 100% to 175% of the initial gearbox.
Variation of the rotor radius could lead as well to increased efficiency of the rotorcraft. It could be an addition to the speed variation. The "Derschmidt Rotor" rotor technology would allow a faster and more efficient forward flight, but due to the unsolved problem of vibrations it doesn´t seem to be usable.
An electric drive train is seven times too heavy to be used in the CS-29 class. Small electric engines may be used to support a drive train system in speed variation.
The knowledge of pros and cons of different technologies for rotor speed variation could be used in future rotorcraft designs to enable variable rotor speed and to help to choose the most suitable drive train system. The results are used in the project "VARI-SPEED" to find the best combination of rotorcraft configuration and gearbox design.

Variable rotor speed, variable speed drive train, variable transmission for helicopter, helicopter gearbox transmission