Nose Cowl Parts and Subassemblies

The build-work breakdown of the two nacelles from start to finish is grouped in six stages, as shown in Table 16.7; however, the cost of their parts is different. Nacelle A is an existing design and its cost is known. Nacelle B is a later design with a lower parts count and assembly time, achieved by superior structural and manufacturing considerations through DFM/A studies. The nose cowl consists of pure structures (STRs), minor parts (MPs) (e. g., brackets and splices), engine-built units (EBUs) (e. g., anti-icing units and valves), and aircraft general supply (AGS) (e. g., fasteners, rivets, nuts, and bolts). EBU costs are studied separately and not herein.

The expensive components are the STR and the installation of EBU parts. Clearly, these costs are reduced to almost half, thereby saving the cost of the Nacelle B nose cowl even with a larger engine. Assembly hours are also reduced to nearly half. AGS is not expensive but there are numerous rivets, nuts, bolts, and so forth.

16.4.1 Methodology (Nose Cowl Only)

The author points out that this seemingly simple algebraic procedure with elemen­tary mathematics becomes a complex workout. Newly initiated readers may find it difficult to follow. It will require the instructor’s help and industrial data to under­stand the coursework for their project.

The methodology generates the factors and indices from existing Nacelle A, the cost data for which are known. Based on the similar geometry, these factors and indices are then adjusted using the DFM/A considerations and applied to Nacelle B. The conceptual design phase outlines the basis of the manufacturing philosophy under the DFM/A, relying heavily on the Nacelle A experience. Table 16.8 lists the necessary factors and indices for the eight cost drivers (i. e., the data from the industry). The table is followed by expanding the eight cost drivers.

Table 16.8. Normalized indices for the eight cost drivers in Group 1 Nacelle A Cost drivers (Baseline: known) Nacelle B (to predict) Remarks 1. Ksize 1.0 1.133 2. Material (weight ratio) 1.0 1.135 (a) Raw material 1.0 (See Tables 16.9 and 16.10) Nacelle B better (b) Finished material (subcontracted) 1.0 (See Table 16.9) 3. Geometry 1.0 1 Similar 4. Technical specifications 1.0 1 Similar 5. Structural design* 1.0 1.1 NRC 6. Manufacturing philosophy* (a) Nonrecurring (tool and jig design) 1.0 1.2 NRC (b) Recurring (manufacture and assembly) 1.0 0.95 Nacelle B better 7. Functionality 1.0 1 Similar 8. Manhour rates 1.0 1 Same Note: * Primary cost driver.

The shop-floor learning characteristics are an important factor in cost consider­ation. Initially, parts fabrication and their assembly take longer (actual manhours) than when it is a routine task with a stabilized time frame of standard manhours, which initially is the target time. If actual manhours do not reach standard man­hours, the investigation is required to change the standard manhours. The faster people learn, the greater is the savings for taking fewer manhours to manufacture. The number of attempts required to reach the standard manhours varies, and the DFM/A study must consider this aspect. In this case, Nacelle B has a faster learning – curve factor, with fewer parts.

1. Ksize: Geometric details of the nacelles and engine parameters are listed in Table 16.5 to estimate Ksize.

2. Material Cost: Material is classified in two categories: (1) raw materials (e. g., sheet metal, bar stock, and forging), and (2) finished materials (e. g., lipskin, engine ring, and some welded and cast parts acquired as subcontracted items). The weight fractions of both nacelles are listed in Table 16.9. The unit cost for each type varies, depending on the procurement policy (see notes in the table). The next part of the table lists details of the raw-material weight fractions. The last column provides various material costs per unit weight, normalized relative to the aluminum sheet-metal cost. The AGS consists of various types of fasten­ers including blind rivets (more expensive) and solid rivets; they are classified as raw materials because it is impractical to cost each type separately.

3. Cost of Manufacture: The core of the manufacturing cost buildup considers the cost drivers of geometry, technical specifications, manufacturing philos­ophy, functionality, and manhour rates. For this study, only the evaluation of the manufacturing philosophy is required, as discussed in the next two subsections.

Table 16.9. Material weight fraction

Nacelle A	Nacelle B	Cost of material per unit weight
weight (Wa/Wat )	weight weight (Wb/Wat ) (Wb/Wbt )	Nacelle A Nacelle B

Material weight fraction
All material	1.0	1.135	1.0
Raw material	0.7136	0.8744	0.77	see below
Finished material	0.2864	0.2607	0.23	1.0	0.92
Raw material weight fraction (finished material not included)
Total weight fraction	1.0000	1.2253	1.0
Aluminum alloy sheet	0.2288	0.4778	0.39	1.0	1.0
Aluminum alloy forging	0.1213	0	0	4.19	4.19
Aluminum alloy honeycomb	0.3104	0.38687	0.3157	2.25	2.25
Titanium alloy	0.2752	0.34254	0.2795	3.50	3.5
Composite	0.0175	0	0	3.62	2.9*
Mechanical fasteners (e. g., nuts)	0.0366	0.0050	0.0041	18.44	18.44
Solid rivets	0.0101	0.0139	0.0113	0.63	0.63

Notes:

* There is no composite in the nose cowl of Nacelle B, but it is used in the core cowls of both nacelles. The subscript “T” stands for total weight of nose cowl;A and B stand for each nacelle.