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problems were encountered during the work, and not all of the problems could be
resolved in the short time frame. The work therefore was followed up by a validation of
the designs via traditional hand stressing methods, and qualification of the
ribs/structure against fatigue and bird strike is still ongoing.
Figure 6: Topology, Sizing and Shape Optimised A380 Droop Nose Ribs.
3.1 Topology Optimization of A380 Leading Edge Droop Nose Ribs
The first question that arose when considering topology optimisation of the droop nose
ribs was how to best represent the attachment of the ribs to their surrounding leading
edge structure (droop nose skin, main wing box front spar and skin overhang) and also
how best to model the diffusion of air pressure loads into the droop nose ribs. In the
section on optimisation of main wing box ribs, this was done applying super element
techniques. However, for the optimisation of the A380 droop nose ribs we had not
investigated such modelling techniques and therefore had no experience on how they
would work with topology optimisation.
Some preliminary studies had been undertaken at Airbus UK, studying issues with
boundary conditions. Leading edge droop nose ribs had been topology optimised
considering the ribs in isolation and considering the ribs as part of the leading edge
droop nose structure. The global compliance formulation used in the traditional
formulation of the topology optimisation method had shown difficulties giving any
structure, when optimising ribs as an integral part of the leading edge droop nose
structure.
Copyright © Altair Engineering Ltd, 2002 11/8
This problem was put down to the global compliance objective function, which included
the total elastic energy in both the droop nose rib being designed but also in all of the
surrounding structure. Better results had been obtained optimising ribs in isolation, but
again the topology optimisation was shown to be very sensitive to stiffness of the
rib/droop nose skin attachment flange. This problem was put down to the global
compliance objective function used in the traditional topology optimisation method. The
objective function now included both the energy in the designable area of the rib but
also the energy in the rib flange that was generally considered to be non-designable.
From the very start of the new droop nose optimisation program, the decision was
taken not to attempt to model the surrounding structure, as this would result in several
detailed modelling issues and also increase the optimisation run times. Instead
simplifying assumptions were made and all attachments to the surrounding structure
were modelled using single point constraints. All lateral translations around the edge
of the ribs were for example restrained to represent the very stiff span wise support
from the main wing box front spar, sub spar and the droop nose skin. Constrained
degrees of freedom in the plane of the ribs were also used to represent the
attachments to the main wing box front spar and skin overhang.
The topology optimisation was again seen to be quite sensitive to the constrained
degrees of freedom, and several studies was performed to accurately model the load
transfer between the rib and the main wing box front spar and skin overhang. These
boundary condition modelling issues have since been resolved using super element
techniques. An example of a result of a topology optimisation is shown in Figure 7.
Figure 7: Topology Optimisation of Leading Edge Rib.
(The left picture shows the designable and non-designable areas for the rib while
the right picture shows the design suggested by topology optimisation. A total
of between 6-12 load cases were used for the topology optimisation of the
leading edge ribs)
Copyright © Altair Engineering Ltd, 2002 11/9
3.2 Sizing and Shape Optimization of A380 Leading Edge Droop Nose
Based on the topology optimisation results, which are used to determine a design with
optimal load paths, engineering solutions were created. Interpreting regions with high
density of material as structure and regions with low density of material as holes, the
topology optimised designs could be interpreted as truss-like structures.
Engineering designs incorporating a mixture of truss-design and shear-web design
were now formed in collaboration with the A380 designers. The ribs were also given
some out of plane stability by adding vertical stiffeners at the centre of the truss
members, resulting in T-sections for single-sided machined ribs and cruciform shaped
sections for double-sided machined ribs (Figure 8). The engineering designs were
initially built as finite element models (Figure 9) which served as initial designs for a
　
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