16.4.4 Cost Formulas and Results

Table 16.7 shows that both nacelles have six stages (m = 6). Each stage has four classes of parts: structure, minor parts, AGS, and EBU. The EBU is separated from the other classes, which are classified as one (n = 1).

Although geometrically similar, in the DFM/A study, Nacelle B has fewer parts to reduce the assembly time. Although there are few parts for Nacelle B, the parts fabrication needs about the same amount of time for both nacelles. Therefore, the various factors are as follows:

size factor Ksize = 1.133 from Equation 16.1

geometry factor F1 = 1 (geometrically similar)

complexity factor, F2 = 1 (functionality issues, same for both nacelles)

parts-manufacturing philosophy factor (methods factor), F3 = 1.0

learning-curve factor = 1.022 (slightly higher)

Engineering process sheets provide all the information for Nacelle A to compute cost; for nondimensionalizing, all the factors use 1 as the baseline value.

Therefore, using Equation 16.7, the parts-manufacturing cost of

Nacelle B = (1.133)0.5 × 1.022 = 1.0878 × Nacelle A

3. Nose-Cowl Assembly Cost, CAsm. The manhours required for assembly at each stage are listed in Table 16.10 in nondimensional form. The nose-cowl assembly is expressed as:

assembly cost (C Fab) = rates × manhours × learning-curve factor × size factor

The rate and factor for Nacelle A are 1; Table 16.2 lists them for Nacelle B:

i i

(16.11) In this case, m = 6 stages and n = consists of the following cost-driver factors:

F1 = 0.735 for the tooling concept (assembly methods – Nacelle B takes less time)

F2 = 1.0, the complexity factor (functionality – same for both nacelles)

F3 = 1.0, aerodynamic smoothness requirements (surface tolerance is the same)

Then, Equation 16.7 reduces to:

To simplify, all stages are combined to obtain the Nacelle B cost. Using 1 as the baseline Nacelle A factors and indices, the Nacelle B assembly cost is expressed as follows:

assembly cost of Nacelle B = (1.133)0.25 × 0.735 = 0.759 × Nacelle A

It is interesting that considerable assembly costs can be reduced at the expense of a slightly increased parts-fabrication cost and, of course, with some increase in

the tooling cost (i.e., NRC). Establishing these factors is the main purpose of the DFM/A trade-off studies. Basically, they summarize the manhours required compared to the baseline manhours.

4. Nose-Cowl Support Cost, C Sup. Separate support costs are taken as a percentage of material, fabrication, and assembly costs. Support costs arise from reworking and concessions when the build quality is not met (e.g., tolerances). In this book, the support cost is flat-rated as follows:

C Sup = 0.05 ×(material cost + parts-fabrication cost + assembly cost) (16.12)

5. Nose-Cowl Amortization Cost, C Amr. Cost is amortized for 400 finished products. It is a variant design and therefore has low amortization costs; it can be included in the manhour rates at all stages or separately at the end. In this book, the cost is accounted for in the manhour rates and is not computed separately. Typically, because it is produced in twice more in number, the amortization cost is taken as 2% of material costs plus parts-fabrication costs plus assembly cost:

where N = 400, or

CAmr = 0.02 × (material costs + parts-fabrication costs + assembly costs

6. Nose-Cowl Miscellaneous Costs, CMisc. Miscellaneous costs are unavoidable, as follows (taken as 3%):

CMiscp = 0.03 × (material costs + parts-fabrication costs + assembly costs) (16.14)

Total Costs of the Nacelle B Nose Cowl. The final costs of the Nacelle B nose cowl can now be computed, in nondimensional form – that is, relative to the Nacelle A costs. Using Equation 16.4, the following is estimated (i.e., costs of amortization embedded in manhour rates and costs of support and miscellaneous cost estimated as 10% of other costs):

5

CNoseCowl = Ci = CMat + CFab + CAsm + CSup + CMisc

Nacelle B nose-cowl cost:

CNoseCowl B = 0.8302CMat NacA + 1.0878 CFab NacA + 0.759 CAsm NacA

+ 0.1 × (0.8302 CMat NacA + 1.0878 CFab NacA + 0.759 CAsm NacA)

=0.9132CMat NacA + 1.1966CFab NacA + 0.8349CAsm NacA

From company records, the Nacelle A cost fractions are as follows:

CMat NacA/CNoseCowl A = 0.408

CFab NacA/CNoseCowl A = 0.349

CAsm NacA/CNoseCowl A = 0.149