Load analysis of open rotor aircraft engines

Abstract: The pressures on the airline industry today due to environmental issues are
high, and as the fuel prices go up the demand for more efficient engines
increase. In the 1980’s a technology with counter rotating fans was tested,
and the results were impressive in terms of fuel efficiency. Nonetheless,
due to fuel prices going down, the engines never came to production. But
the technology was proven and today the need for efficient engines has
dusted off this concept again.

Volvo Aero is part of the development of demonstrator Open Rotor engines
and there is a need for an understanding of the loads the engine is
subjected for during different flight conditions. The purpose of this
thesis has therefore been to get an understanding of the loads on the
engine and its mounts. Furthermore the loads on specific parts that
are of interest for Volvo Aero have been investigated.

To accomplish this, mechanical layouts of the engines have been drawn to
get an overview of the structure. Two different mounting configurations
have been proposed, with one of them being suggested as a suitable system
for the types of engines examined in this thesis. The fans are driven by
either a counter rotating power turbine or a planetary gearbox. In the case
of the power turbine, a performance analysis has been carried out to
investigate the loads that are present in such a configuration, especially
axial loads. The engine concept incorporates large diameter fans and
gyroscopic effects are therefore of interest.

The results show that gyroscopic effects are of small importance for the
engine and its mounts due to the counter rotating nature of the fans.
However, the gyroscopic moments are large in the structures and shafts
connected to the fans and cannot be neclected. The loads in the shaft
bearings differ according to their position and function, but they will all
have to withstand high loading during Fan Blade Off.

The turbine performance analysis was quite extensive and since the correct
design and other parameters are not known, the results should be seen as
guidelines. The axial loads in the turbines are sensitive to changes in
mass flow and pressure distributions, and the loads in the bearings are
difficult to estimate. Due to the shape of the channel used in this thesis,
these axial bearing loads are large. As a result, reverse thrust might
involve difficulties.