Abstract:
With the market introduction of both the Airbus A350XWB and the Boeing 787,
Carbon Fibre Reinforced Plastics (CFRP) has been applied to primary structure of large
commercial aircraft, as a means of enhancing overall performance. Both these aircraft
are being developed and produced in a unique way where Airbus and Boeing are acting
as System Integrators and using Risk Sharing Partners to develop the majority of the
principal components.
To support this new business and technological model it is necessary that the System
Integrator has sufficient knowledge and tools to support the development of the
components. Of particular interest are items such as the wing covers, as they are both
heavy and expensive items, thus offering large opportunities for optimisation, in
particular when the benefits of applying CFRP are considered. This creates the forum
for this thesis, i.e. to thoroughly understand all factors that influence a CFRP wing
cover, from which an optimisation methodology is developed, incorporating design
constraints, while seeking the lightest weight solution, with a resultant Life Cycle Cost
(LCC). Based on this, different solutions can be compared based on weight and LCC.
In general stringer-stiffened panels are, from a weight perspective, the optimal
configuration for wing covers, and thus are solely considered. Serendipitously, due to
their prismatic shapes, buckling calculations of stringer-stiffened panels can be solved
with reasonable accuracy and ease using the Finite Strip Method (FSM), as opposed to
more time consuming methods such as the Finite Element Method. A suitable FSM
program is available from ESDU, which when used in combination with a configured
Excel spreadsheet can take into consideration constraints established from the extensive
literature review. Once the lowest weight solution is obtained under buckling
constraints, the solution is then checked for in-plane and if desired out-of-plane
strength.
Based on the structurally optimised wing cover, the manufacturing cost is calculated
using a Process Based Cost Model (PBCM), which has been developed based on
different CFRP materials for the skin and stringer fabrication, as well as suitable
manufacturing and integration methods. In order to consider the LCC, i.e. all costs from
cradle to grave, the PBCM factors in both the cost of recycling scrap material during
manufacture and after retirement. Furthermore, when more than one solution is
compared then the Economic Value of Weight Saving, which is based on the range
equation, can be factored in to consider the financial benefit of weight saving.
The optimisation methodology and PBCM has been evaluated on diverse wing cover
examples, which has considered both uni-directional prepreg, non-crimp fabric and
braids materials in combination with autoclave and liquid composite moulding
techniques. The results demonstrated a trend which can be considered realistic, although
the cost estimation is very much dependent on the assumptions made. In conclusion, the
thesis and the optimisation methodology can be used to compare different
configurations.