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H. Amri, R. Feil, M. Hajek, M. Weigand:
"Possibilities and difficulties for rotorcraft using variable transmission drive trains";
CEAS Aeronautical Journal, 7 (2016), 2; 335 - 346.

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 CO2 production, and the flight envelope may be
extended.

Variable speed rotor; Variable transmission drivetrain; Power optimisation; Economic rotor craft; Flight envelope extension; Future technology

http://dx.doi.org/10.1007/s13272-016-0191-6