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.