Multidisciplinary optimisation of a CFRP wing cover (2009)

With the market introduction of both the Airbus A350XWB and the Boeing 787,\nCarbon Fibre Reinforced Plastics (CFRP) has been applied to primary structure of large\ncommercial aircraft, as a means of enhancing overall performance. Both these aircraft\nare being developed and produced in a unique way where Airbus and Boeing are acting\nas System Integrators and using Risk Sharing Partners to develop the majority of the\nprincipal components.\nTo support this new business and technological model it is necessary that the System\nIntegrator has sufficient knowledge and tools to support the development of the\ncomponents. Of particular interest are items such as the wing covers, as they are both\nheavy and expensive items, thus offering large opportunities for optimisation, in\nparticular when the benefits of applying CFRP are considered. This creates the forum\nfor this thesis, i.e. to thoroughly understand all factors that influence a CFRP wing\ncover, from which an optimisation methodology is developed, incorporating design\nconstraints, while seeking the lightest weight solution, with a resultant Life Cycle Cost\n(LCC). Based on this, different solutions can be compared based on weight and LCC.\nIn general stringer-stiffened panels are, from a weight perspective, the optimal\nconfiguration for wing covers, and thus are solely considered. Serendipitously, due to\ntheir prismatic shapes, buckling calculations of stringer-stiffened panels can be solved\nwith reasonable accuracy and ease using the Finite Strip Method (FSM), as opposed to\nmore time consuming methods such as the Finite Element Method. A suitable FSM\nprogram is available from ESDU, which when used in combination with a configured\nExcel spreadsheet can take into consideration constraints established from the extensive\nliterature review. Once the lowest weight solution is obtained under buckling\nconstraints, the solution is then checked for in-plane and if desired out-of-plane\nstrength.\nBased on the structurally optimised wing cover, the manufacturing cost is calculated\nusing a Process Based Cost Model (PBCM), which has been developed based on\ndifferent CFRP materials for the skin and stringer fabrication, as well as suitable\nmanufacturing and integration methods. In order to consider the LCC, i.e. all costs from\ncradle to grave, the PBCM factors in both the cost of recycling scrap material during\nmanufacture and after retirement. Furthermore, when more than one solution is\ncompared then the Economic Value of Weight Saving, which is based on the range\nequation, can be factored in to consider the financial benefit of weight saving.\nThe optimisation methodology and PBCM has been evaluated on diverse wing cover\nexamples, which has considered both uni-directional prepreg, non-crimp fabric and\nbraids materials in combination with autoclave and liquid composite moulding\ntechniques. The results demonstrated a trend which can be considered realistic, although\nthe cost estimation is very much dependent on the assumptions made. In conclusion, the\nthesis and the optimisation methodology can be used to compare different\nconfigurations.