Nacelle Cost Drivers

Given herein are the eleven specific parameters, in two groups, identified as the design – and manufacture-sensitive cost drivers for generic nacelles. These cost drivers are applicable to all four nacelle subassembly components shown in Fig­ure 16.3. Group 1 consists of eight cost drivers, which relate to in-house data within an organization. Group 2 cost drivers are not concerned with in-house capability issues; therefore, they are not within the scope of this discussion. Indices and coeffi­cients obtained during the DFM/A study are used.

Group 1

1. Size: Nacelle size is the main parameter in establishing the base cost. Size and weight are correlated. The nacelle cowl size depends on the engine size – that is, primarily the fan diameter (DF) of the engine – which in turn depends on the thrust (TSLS) ratings as a function of BPR and the thermodynamic cycle. The relationship between the TSLS and the Dfan can be expressed as follows:

(Tsls) = (KDjan) (16.1)

where K = constant of proportionality.

The variants in the family of turbofans are the result of tweaking the base­line design, keeping the core gas generator nearly unchanged. This improves cost effectiveness by maintaining component commonality. Hence, the variant fan diameter is marginally affected, with the growth variant having a better

Figure 16.4. Cost versus tolerance

thrust-to-dry-weight ratio (T/W) and vice versa. As a consequence, the nacelle maximum diameter (Dmax) and length (L) change minimally. The size factor for the nacelle, Ksize, that affects cost is given in semi-empirical form, as follows:

The effect of size on parts-fabrication and assembly costs is less pronounced than material cost unless a large size calls for drastically different fabrication and assembly philosophies.

2. Materials: Parts weight data provide a more accurate material cost than apply­ing the size factor; Ksize may be used when weight details are not available. Two types of material are considered based on industrial terminology: raw and finished; the latter consists of the subcontracted items.

3. Geometry: The double curvature at the nacelle surface requires stretch-formed sheet metal or a complex mould for composites in shaping the mould lines. Both nacelles are symmetrical to the vertical plane. The nacelle-lip cross-section is necessarily of the aerofoil section with the crown cut, thinner than the keel cut, where engine accessories are housed (Figure 16.4). This does not make the outer and inner surfaces concentric. Straight longitudinal and circumferential joints facilitate the auto-riveting. In brief, there are four “Cs” associated with geometric cost drivers: circularity, concentricity, cylindricity, and commonality. Nacelles A and B are geometrically similar and therefore do not show any dif­ference made by the four C considerations. A geometric cost-driver index of 1 is used for both nacelles as a result of their similarity.

4. Technical Specifications: These standards form the finishing and maintainability of the nacelle including the surface-smoothness requirements (i. e., manufactur­ing tolerance at the surface), safety issues (e. g., fire detection), interchangeabil­ity criteria, and pollution standards. Figure 16.4 shows the cost-versus-tolerance relationship from [2].

At the wetted surface, Zone 1 (Figure 16.5) is in an adverse pressure gradi­ent that requires tighter tolerances compared to Zone 2 in a favorable pressure

Figure 16.5. Typical nacelle section

gradient. The tighter the tolerance at the wetted surface, the higher is the cost of production due to the increased reworking and concessions involved. Because the technical specifications are similar, both have an index of 1.

5. Structural Design Concept: Component-design concepts contribute to the cost drivers and is a NRC amortized over the production run (typically, four hun­dred units). The aim is to have a structure with a low parts count involving low production manhours. Compared to the baseline design of Nacelle A, an index factor is associated with the derivative new design. Nacelle B has a more involved design with an index greater than 1. Manufacturing considerations are integral to structural design as a part of the DFM/A requirements.

6. Manufacturing Philosophy: This is closely linked to the structural-design con­cept, as described previously. There are two components of the cost drivers: (1) the NRC of the tool and jig design, and (2) the recurring cost during pro­duction (i. e., parts manufacture and assembly). An expensive tool setup for the rapid-learning process and a faster assembly time with lower rejection rates (i. e., concessions and reworking) results in a front-loaded budgetary provision, but considerable savings can be realized. Nacelle B has a NRC index >1 and a RC index <1. Nacelle B is an improvement compared to Nacelle A.

7. Functionality: This is concerned with the enhancements required compared to the baseline nacelle design, including anti-icing, thrust reversing, treatment of environmental pollution (e. g., noise and emissions), position of engine acces­sories, and bypass-duct type. A “complexity factor” is used to describe the level of sophistication incorporated in the functionality. Being in the same family, the nose cowl of both nacelles has the same functionality – hence, a factor of 1 – otherwise, it must be revised. Other nacelle components could differ in functionality.

8. Manhour Rates and Overhead: Manhour rates and overhead are constant for both nacelles; therefore, the scope of applicability is redundant in this study.

Group 2 (These do not relate to in-house issues; therefore, it is not considered in this

book.)

9. Role: Basically, this describes the difference between military and civil aircraft design.

10. Scope and Condition of Supply: This is concerned with the packaging quality of a nacelle supplied to a customer; it is not a design or manufacturing issue.

11. Program Schedule: This is an external cost driver that is not discussed herein.

Table 16.7. Nose cowl build-work breakdown Nacelle A Nacelle A STR MP EBU AGS STR MP EBU AGS Forward-bulkhead assembly 4 3 33 482 4 4 1 0 Aft-bulkhead assembly 3 0 33 395 3 1 25 644 Primary assembly 1 6 0 393 1 1 10 970 First-stage assembly 11 0 105 939 6 0 19 708 Second-stage assembly 8 2 78 873 2 2 82 1,617 Third-stage assembly 0 5 7 1,480 0 0 15 95 Total 28 16 256 4,562 16 8 152 4,034

In summary, only four cost drivers in Group 1 – size, material, structural-design concept, and manufacturing philosophy – are required to establish the cost of com­ponent manufacture and assembly. The other four cost drivers in Group 1 can be evaluated similarly for nacelles that differ in geometry, technical specifications, functionality, and manhour rates.