Since more and more modern civil aircraft are equipped with UHBR-engines
for reasons of fuel efficiency and environmental aspects, the need to tackle
specific engine related dynamic problems has occurred.
The request for UHBR-engines with high bypass ratio numbers and with their
intrinsic advantages of economic fuel consumption and lower acoustic emission
asks for enhanced vibration prediction capabilities.
Beside the energetic benefits such engines add to the aircraft design their rotating
large diameter fans can influence the dynamic behaviour of the complete
elastic aircraft fuselage in a very unfavourable manner.
Additional questions which arise with regard to structural dynamics and
aeroelastic stability are treated in this work.
Especially in the scenario when large rotating engine masses are to be combined with elastic
wing structures the possible occurrence of specific structural vibration
problems can be avoided by taking the gyroscopic effects into account.
As another important engine related aspect the modelling and the impact of the
engine thrust is highlighted by integrating the first order deformation
induced terms into the dynamical simulation model.
By introducing an increased coupling level between the degrees of freedom in the
equation of motion (through additional off-diagonal terms) both
eigenfrequencies and eigenmodes are affected. In the case of high engine
participation in the structural deformation we can observe a lowering of the
eigenfrequencies (in the test aeroplane up to 6[%]) and a loss in symmetry
properties in the now strongly asymmetric eigenmodes.
With the occurrence of flutter cases the critical speed had been experienced to
shift about an amount of similar magnitude. Although in the presented cases
the flutter speed moved to higher values, it was found indespensable to check
every individual aircraft configuration with regard to the stability margin.
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