DSO Case Studies

Optimal Sequencing of Design Analysis

Background This individual assignment concerns the structuring and scheduling of design activities and the decision making sequence in a design project. The design of a complex electro-mechanical artefact such as an aerospace product requires the definition and management of many thousands of pieces of information (often called design attributes).

Optimised Design Design managers need to ensure that design decisions are made quickly and will lead to high quality products. The design process is a race against time as immense competitive pressures force organisations into reducing design timescales. Examples of design attributes include wing-span, range, mass, component dimensions, fatigue-life and tolerances. These design attributes can be classified as explicit or implicit. Explicit design attributes can be easily measured directly. Hence dimensional attributes are explicit as they can be determined directly by use of a measuring device. Implicit design attributes are more difficult to assess directly and can only be determined by analysis, test or even operation. For a gas turbine, for example, a critical design attribute is the time between major overhauls (TBO). This is impossible to measure directly and is very difficult to predict from analysis alone. TBO prediction typically requires a mix of historical operational data, test data and analysis.

Dependencies Design attributes generally tend to fit into a logical dependency structure. This dependency structure can be used to determine the sequence in which attributes need to be defined/ estimated. This leads to a logical decision making process within a design project. This optimal sequence can save an organisation a considerable amount of design lead time. A simple example of such dependencies concerns wing area. Wing area is dependent on other design attributes such as wing span, chord, sweep angle etc. Unfortunately, many design projects have complex dependencies which cannot be topologically sorted into a linear sequence. In other words circular dependencies exist reflecting a design attribute feedback loop. Figure 1 gives a simple illustration of such a dependency loop. This shows, for example, that an Aerodynamicist needs a mass estimate before determining the wing geometry. In turn the Mass Properties Engineer needs to have an outline design of the wing structure (number and rough dimensions of spars, stringers, ribs, skins and material type). The Structural Designer is unable to generate

JP Scanlan DSO Case Study an outline design without knowing the load cases. Finally, the Loads Engineer needs the overall dimensions of the wing in order to estimate the structural loads and bending moments.

Mass of wing structure, Determine Span, Chord, Systems, fuel aerodynamic shape Section Aerodynamicist Mass properties engineer

Calculate Calculate structural mass structural loads Loads Engineer

Load cases Wing box design, Design Load Material volume, density bearing structure Structural Designer

Figure 1 Circular dependency structure for aircraft wing design attributes.

This appears to lead to a logical impasse where no activities are able to initiate. In reality organisations will use past experience and crude estimates to break this cycle and stimulate the initial design iteration. For a large project it is useful to map the dependency structure so that cyclic and non- cyclic activities can be identified and an overall decision making sequence established. The research community is very active in this area and a number of modelling tools nave been developed. One example is the Design Structure Matrix tool that can be downloaded as a spreadsheet from http://www.dsmweb.org/macros.htm . This allows a dependency structure to be modelled as a matrix as illustrated in Figure 2. t i l o e i p t t g S a s n

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i a i n t t f t f t f f f e t t t t e a e e e e e e e e s p e s s d D D E S E I D D D D E S M D D 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Determine Aircraft/ Engine Range 1 O 1 Define By- Pass Ratio 2 O 1 1 Estimate Development Cost 3 1 O Specify limit Drag 4 1 1 O EstablishDurability/ Life 5 O Identify Emissions criteria 6 O 1 Determine Engine Core Flow 7 1 O 1 1 Define High Rotor Speed (N2) 8 O 1 Define Low Rotor Speed (N1) 9 1 1 O 1 Define High- Low Spool Work Split 10 1 1 O 1 Estimate Jet Velocity 11 O Set target for Maintenance Cost 12 1 1 O 1 1 Material Selection 13 1 1 1 O 1 Define Noise Target 14 1 O Determine Overall Pressure Ratio 15 1 O Estimate Production Cost 16 1 1 1 1 1

Figure 2 Part of a Design Structure Matrix for a gas turbine (Source Pratt and Whitney).

JP Scanlan DSO Case Study Dependencies are signified by putting a “1” in the relevant intersecting cell. Hence for example the matrix in Figure 2 suggests that activity 7 (Determine Engine Core Flow) is dependent on Activities 2 (Define By-Pass Ratio), 9 (Define Low Rotor Speed (N1)) and 10 (Define High- Low Spool Work Split). Notice that the diagonal of zeros prevents self dependencies to be defined. The MIT spreadsheet has a number of macros which allow this matrix to be manipulated. The most important macro attempts to sort the activities into dependency order (macro name; “Partition DSM”). The MIT spreadsheet tools have developed the dependency relationships slightly further by allowing “fuzzy” or partial dependencies to be modelled (by entering a “2” into the matrix). Instructions on how to use the DSM tool can be found in appendix A.

Assignment Appendix B outlines a typical design project concerning an electronic design which incorporates some mechanical design considerations. I want you to assume that an organisation will be producing these generic designs for the foreseeable future and wants to optimise its design process. Your task is to  Use the DSM tool to identify the major dependencies within this class of design and sequence them into an optimal order.  Identify the key design feedback loops and partition them into distinct “blocks”.

Suggestions  Start by simply listing all the design variables.  A large matrix can get very unwieldy so try and reduce the number of design variables/attributes by filtering out the less important variables/attributes.  Identify dependencies by going through each of the design variables/attributes and listing all the things that influence decisions concerning that particular design variables/attributes.

JP Scanlan DSO Case Study Appendix A

User guide for DSM tools

User_Guide.pdf

JP Scanlan DSO Case Study Appendix B

Design Case Study

Electro_mechanical_design.pdf

JP Scanlan DSO Case Study