Publication - rotor speed variation
Comparison of Variator Technologies for Variable Rotor Speed Drivetrains for Rotorcraf
Author: Hanns Amri, Zoerkler Gears GmbH & Co KG
Rotor speed variation - the challenge for future rotorcraft aviation?
Today the single main and tail rotor rotorcraft configuration is the generally used rotorcraft configuration in the world. These rotorcraft are driven a constant rotor speed and the controls are performed with cyclic and collective rotor blade pitch control. The increasing requirements on rotorcraft especially on the hover qualities and the fast forward flight leads to a new way of thinking. The commonly used single main rotor and tail rotor configuration is not the only configuration of interest any more. Compound Rotorcraft and Tilt Rotor Rotorcraft are under development. Those configurations have different requirements on the rotor RPM in hover and in fast forward flight. A rotor speed variation seems to be necessary to enable an efficient rotorcraft design, as different studies, for example the "Heavy Lift Rotorcraft System Investigation" from NASA, showed. Furthermore the efficiency and the noise of a rotorcraft can be reduced if the rotor speed can be adjusted to the current flight state.
Knowledge of innovative engineering for variable rotor speed
There are two essential innovations necessary to enable rotor speed variation: First a device is needed to vary the rotor speed and second a rotor is needed which can be driven in the required speed range. Zoerkler together with our partners The Munich University of Technology and the Vienna University of Technology have targeted to investigate the possibilities of rotor speed variation. We are working together since 2014 in the project "VARI-SPEED" supported by the Austrian Research Promotion Agency FFG in the Programm TAKE-OFF and by the German Ministry of economics in the Program LuFo.
The RPM variation can be performed by the turboshaft engine or by the drivetrain. The turboshaft engine has the advantage that RPM variation is possible with only a little weight increase but it has an influence on the whole drivetrain and even on the auxiliary units. Furthermore, the efficiency of the turboshaft engine depends on the RPM of the turboshaft engine.
Using a technology inside the drivetrain has the advantage that rotor and turboshaft engine can be operated at their optimum RPM and the auxiliary units are not influenced by rotor speed variation. The disadvantage of this technology is the additional mass of the system.
R&D results for rotorcrafts with variable rotor speed
Research results of the consortium showed that continuous variable transmission systems are more suitable for rotor speed variation. A special configuration, the so called "compound split" transmission system is a good choice because of the high efficient mechanic main power flow part and the power efficient speed variation path.
The results were published international, for example in Springers CEAS Aeronautical Journal or at conferences like the "European Rotorcraft Forum", the "Deutscher Luft- und Raumfahrt Kongress" or at the Forum of the Vertical Flight Society in the US.
The latest results were published this autumn at the Forum 76 - The future of vertical flight.