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 inf luences cost, i.e. the tighter the tolerance, the higher is the manufacturi ng cost. Discrete surface roughness arising from manufacturing tolerance at the wetted surface may be seen as an 'aerodynamic' defect. Features such a s steps, gaps, waviness and fastener flushness (termed excrescence). seen a s defects, contribute to aircraft parasitic drag. The study is conducted on an isolated nacelle which is considered to be rep resentative of an entire aircraft. Eleven key manufacturing features at the wetted surface of a generic long duct nacelle are identified, each associa ted with surface roughness. The influence of tolerance allocation at each o f the key features is investigated to establish a relationship between airc raft aerodynamics and associated costs. The initial results offer considera ble insight to a relatively complex problem in a multi-disciplinary environ ment. Excrescence drag arising out of these 'aerodynamic' defects is assessed by using CFD and semi-empirical methods. Cost versus tolerance relationships a re established through in-house methods using industrial data. The aircraft unit price typically contributes from two to four times more t han the fuel burn to aircraft direct operating costs. A trade-off study bet ween manufacturing cost and aircraft drag indicates that, in general, there is scope for some relaxation of present-day tolerance allocation, to reduc e aircraft acquisition cost, which would in turn reduce direct operating co sts.