Effect of aircraft surface smoothness requirements on cost

Reduction of aircraft manufacturing cost benefits aircraft direct operating cost (DOC). The degree of stringency in specifying aircraft smoothness influences cost, i.e. the tighter the tolerance, the higher is the manufacturing cost. Discrete surface roughness arising from manufacturing tolerance at the wetted surface may be seen as an ‘aerodynamic’ defect. Features such as steps, gaps, waviness and fastener flushness (termed excrescence), seen as defects, contribute to aircraft parasitic drag. The study is conducted on an isolated nacelle which is considered to be representative of an entire aircraft. Eleven key manufacturing features at the wetted surface of a generic long duct nacelle are identified, each associated with surface roughness. The influence of tolerance allocation at each of the key features is investigated to establish a relationship between aircraft aerodynamics and associated costs. The initial results offer considerable insight to a relatively complex problem in a multi-disciplinary environment. Excrescence drag arising out of these ‘aerodynamic’ defects is assessed by using CFD and semi-empirical methods. Cost versus tolerance relationships are established through in-house methods using industrial data. The aircraft unit price typically contributes from two to four times more than the fuel burn to aircraft direct operating costs. A trade-off study between manufacturing cost and aircraft drag indicates that, in general, there is scope for some relaxation of present-day tolerance allocation, to reduce aircraft acquisition cost, which would in turn reduce direct operating costs.