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Enabling Technologies for Unified Life-cycle Engineering of Structural Components 2 DESIGN CURRENT ENVIRONMENT Design is a hierarchical process that normally extends over a period of several years, through several phases and numerous personnel changes, and is performed in a team environment. All engineered products pass through the sequential steps of conceptual design, preliminary design and final detailed design, although the time spans and value weights depend on the class of product. Similar hardware and software tools, data forms, design considerations, and design processes apply to all these products. Conceptual Design The first step in design involves broad system concepts and is often referred to as the conceptual phase (Raymer, 1989). It begins with an indication by a customer (in the examples used here, the Air Force) that a new product is needed. Interested and qualified industrial firms work with the customer to establish whether the needs can be met. These usually are defined in broad terms of vehicle type (e.g., fighter, bomber, transport, trainer) and other broad figures of merit, such as gross weight, unit cost, initial production schedule, general flight performance parameters, force-structure mix, operational threat environment, and total program costs. At conceptual design time, the desired performance parameters (e.g., weight, size, fuel use) are only loosely defined. Producibility parameters (e.g., testability, process stability) are even less well defined, and supportability parameters (reliability and repairability) are least defined. In the conceptual phase, engineers and managers perform studies showing one or more configurations that meet the customer's objectives. Operations analysis techniques are used to project mission effectiveness, vehicle cost, program costs, and other factors. In this phase, decisions are made that will have major impact on the ultimate production costs and supportability characteristics. Computer tools are invaluable in this phase. They provide methods to define external surface geometry and analytical capabilities to compute automatically important geometric parameters for flight performance analyses. Associated procedures are available for weight and balance, fuel volumes, preliminary lift and drag, and performance analyses. Unfortunately, these computer procedures are not compatible with those used later in the final detailed design stage. In
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Enabling Technologies for Unified Life-cycle Engineering of Structural Components particular, the procedures used early in the design process are insufficiently accurate to support automated numerical control hardware manufacturing methods. Preliminary Design In the next design phase the customer prepares detailed requirements that are highly refined from the earlier needs and often altered substantially, based on evaluation of the conceptual design(s) and evolving changes in enemy threats, economics, and politics. The requirements are voluminous and may stipulate firm fixed-price proposals with detailed schedules. At this stage many of the long-term production, operational, and supportability features of the vehicle are established. Final definitions are made of the specific materials and their distribution. Structural arrangements are described for all major structural members, and construction details (e.g., honeycomb, stiffened structure, corrugations, monocoque) and major manufacturing processes are defined. Many sophisticated tools and methods are utilized during the preliminary design phase. Finite element models are employed for general sizing of structural members and computerized design synthesis methods are used throughout to establish the best structural arrangements and material selections for lowest weight. However, despite the impact on life cycle costs associated with this stage, the ability to view the design from a life-cycle perspective is limited because there are not widely available tools for assessment of life-cycle costs from the preliminary design. The engineering team in this phase, although substantial in number (e.g., 200 or more during the later stages), is still just a core team that ultimately executes the final detail design. The design team is normally composed of a mix of professional preliminary design engineers (with little or no hardware experience) and experienced design and analytical personnel. Producibility and serviceability are addressed based on the knowledge of the team supplemented by the advice and critiques from and consultations with producibility and maintainability-reliability experts. Frequently, however, these attributes are included in the design based primarily on the knowledge gained by informal means by the design engineers. While computer-aided design assistance is generally available it supports only geometric definitions and finite element models (FEMs) are not interactive with these design tools. Further, force and load inputs to the FEMs for the airframe must be estimated because of the lack of dynamic analysis models. Dynamic analysis of electromechanical and hydraulic systems requires the construction of a formal mathematical representation; (e.g., a system of differential equations) followed by a tedious sequence of programming and analysis steps to obtain a solution to the equations. Bond graph techniques (Rosenberg and Karnopp, 1983) are now mature enough to allow a designer to represent a dynamic system without recourse to complex mathematical notation; however, bond graph applications software is only available from academic institutions and is not capable of handling complex industrial systems. None of the available modeling packages is capable of analyzing the design in the presence of variability due to manufacturing, use, or service; data on production or service operations is not available on computer. Three-dimensional solid modeling techniques are increasingly being used as this technology matures (Bradford, 1988). Damage threats are considered throughout the preliminary design process and materials and construction methods are selected with regard to perils from maintenance, foreign-object
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Enabling Technologies for Unified Life-cycle Engineering of Structural Components damage, and enemy actions, but not in a rigorous manner. The specific vulnerability of structures to threats is not well understood, especially with new materials. Also, the threats are not clearly defined enough to allow mathematical simulations. Detailed Design Detailed design is the third major phase in the design process. It usually begins with the "freeze" of the configuration and a general definition of the entire product. A major expansion in personnel is associated with this phase, as is a transfer of responsibilities to a fully integrated functional engineering organization. In the structural areas, teams are divided into separate groups for major components, such as empennage, wing, body (or fuselage), landing gears, and control surface. Each group may include 10 to 50 designers, with corresponding analytical groups for stress analysis, fatigue and fracture, static loads, service loads, flutter, and dynamics. The design engineering groups create the design and coordinate the execution by interaction with the associated analytical groups and other specialist functions responsible for producibility, material technology, maintainability reliability, equipment design, routing of systems, and others. The supporting methods for detailed design are similar to those of the preliminary design process but are more refined. The designer is responsible for executing the design and obtaining approvals from 10 or more analysts and specialists. The primary objectives for each part are scheduled completion, design hours, weight, and strength (fatigue, static, and dynamic). Producibility and supportability are important and are analyzed by their respective specialists, but neither specific requirements nor audits are generally required. The current generation of computer-aided design (CAD) or computer-aided engineering (CAE) tools have limited embedded data or parameters to facilitate analysis of production costs and maintenance or spares requirements. They are typically not interactive to the same extent as, for example, FEM analysis. Three-dimensional and solid modeling software is computer-intensive, and current processors are inadequate. Too much information is required to completely represent the geometric and manufacturing descriptions. Except for strength or weight, audit trails of the decisions made during the design process are unavailable. Furthermore, the designer has no CAE access to background data from other programs regarding producibility and supportability. The designer utilizes the best knowledge accessible from the lead engineers and specialists through an informal consulting process. The designer does have the option, as time and initiative permit, to perform trade-off studies to support the decisions. But, in the absence of quantifiable, computer-accessible data, these trade-off studies can not assure optimized design. The current design methodology for structural components is based on a geometric definition. The manufacturing and support features are derived from the geometry. This method contrasts sharply with the design of integrated circuits, where the functionality (e.g., logic gates) of the design is defined by the designer and the geometry is generated by a computer from the functional definition. In summary, the current design environment is not significantly different from that in the early days of the aerospace industry, with the exception of computerized assistance for geometry definition and static analysis methods. Computers have been incorporated to perform highly detailed stress analyses of total structures, thereby greatly improving the weight efficiency of resulting design. Computer graphics methods have replaced manual drawing methods and have led to major improvements in accuracy of part definitions. However, as with computer design and analysis, these methods are not integrated and interactive, and they do not include access to historical data or provide audit trails of design decisions on producibility, damage resistance,
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Enabling Technologies for Unified Life-cycle Engineering of Structural Components maintenance, repair, initial costs, or life-cycle costs. Also, the methods do not permit analysis of stochastic parameters. After completion of preliminary design, current design assessment capabilities require the manufacture and flight evaluation of one or two prototype vehicles prior to commitment for production engineering and manufacture. The process and available tools to do this job are unchanged from the normal detailed design phase. The value of this phase is not entirely clear, in that it rarely provides the time and funding to answer all the questions. However, it does establish the general flying qualities of the vehicle and does build up a confidence that the new technologies being introduced perform in the predicted manner. If ULCE can be advanced to the point where there is enough confidence in the ability to accurately predict performance of the product, then a prototype will not be necessary. FUTURE ENVIRONMENT In the future engineering design environment, the design process will include the same general components as the past and present, but many activities will operate concurrently. The three phases of design will continue to include the traditional conceptual, preliminary, and detailed design steps. Each design phase in the future will be highly augmented by powerful computer systems and extensive data bases. These will provide data necessary to support decisions and permit interaction between the designer and other disciplines (people, expert systems, information, etc.) to exchange data as required. Design assessment tools for analysis of the functionality of designs will be readily available. These tools will include capabilities for stochastic analysis of design and manufacturing parameters as well as operating parameters. They will enable designers to analyze performance across multiple energy domains so that an integrated view of the design dynamics will be possible. The committee believes that future products will be designed and represented using features rather than dimensions. Design rules will be used to create features and fixtures libraries from which designers may select those that meet the functional requirements. Features will be based on demonstrated manufacturability, reliability, and serviceability. The design definition will make automated process planning easy to implement. The preferred method for fabrication along with standard cost and reliability measures will be part of the design base. In addition, the designer will be able to establish relationships among and between components so that the overall view of a part in the system is available. It will also be easier to establish an automatic audit trail of the decisions that were made during design so that the rationale will be available later for an analysis of changes. In the early design phases, where structural arrangement and internal equipment locations are determined, solid modeling methods will be used to produce highly accurate configuration definitions. Precise external surface geometry will be defined at design initiation. Interactive applications of computerized manufacturing assembly and maintenance operations will be evaluated progressively as the arrangement of structure is defined. At the time of original proposal, and later at the design freeze, computer simulations of all major elements and surfaces will be presented. Evaluations can be made by specialists, management, and customers to verify the completeness and acceptability of all potential hardware elements. A three-dimensional computerized mock-up will be available. Computerized design optimization methods will be applied throughout the process to derive the best solutions. Using computational fluid dynamics (CFD) and FEMs, aeroelastically
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Enabling Technologies for Unified Life-cycle Engineering of Structural Components tailored lifting surfaces will be configured that utilize advanced materials. However, the major emphasis of computerized tools will be on analysis of designs for function, manufacturability, and serviceability, not on synthesis of designs. The creative talents of the designer will still provide the design synthesis. Advanced forms of organic and metal matrix composites will be extensively tailored to obtain the optimum solutions. The detailed design process will include extensive computerized automation to support manufacturing. The work performed by the originating design engineer will greatly exceed that performed in the Current environment. The major portion of the design will be accomplished with solid-or feature-based modeling software that will contain form and feature recognition and process data. Automatic tolerancing, dimensioning, and checking functions will be embedded. Solid modeling should be sufficiently mature to represent all detail part hardware so that the traditional full-scale metal vehicle mock-up (costing $20 to $50 million) will not be required. Solid models of the hardware will also be applied to maintain progressive configuration control with complete data and change records. Individual subsystem elements will be readily interrogated in wide-screen color displays to automatically identify physical interferences or establish minimum clearances. The designer will have estimates for all the components of life-cycle costs available throughout the design cycle. The cost model will be refined as the design is refined. Computerized audit trails will be required. Each individual part design data set will be accompanied by a design decision record of the basis of each decision as well as the associated trade-off data. All levels of decision can be retrieved for later review. The reasons for material selection, structural concepts, and arrangements can be traced back to the decision based on cost, weight, corrosion resistance, fabricability, damage resistance, etc. The future design environment will by necessity involve organizational and economic adjustments. Responsibilities for computer mock-ups, design data sets, and geometrically complete and precise original data will lead to more investment prior to design release than in the current environment. Also, computer use will be intensive, requiring memory and computational capacity similar to today's supercomputers. New checks and balances will be required to ensure control of the system, and many of the current downstream checks and reviews will be eliminated. Final engineering release will be unaltered and uninterpreted until part hardware is fabricated and assembled in an automated factory environment. Although engineering costs will probably increase initially because of the additional manufacturing data, within 20 years they may equal current levels of effort with higher combined engineering and manufacturing productivity. SIGNIFICANCE OF THE CHANGE Higher skill levels in using design assessment tools will be required without the current detail skills in programming and mathematical manipulation. The designer will need to be conversant with a broader knowledge domain and with more disciplines. Manual data transfer will be minimized, but there will be a much larger set of information available to be understood, evaluated, and used for decision-making. There will be automated checklists to aid the design process. It is important to recognize that these improved tools heighten the importance of the designers contribution. The skill and the creativity of the individual designer and the design team will continue to determine the success or failure (now measured for the life cycle) of a project.
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Enabling Technologies for Unified Life-cycle Engineering of Structural Components REFERENCES Bradford, David. 1988. "Through the Labyrinth of Solids Modeling," Mechanical Engineering Vo. 110, No. 3. Raymer, Daniel P. 1989. Aircraft design: A conceptual Approach, American Institute of Aeronautics and Astronautics Inc., Washington, DC. Rosenberg, R. C., and Dean Karnopp. 1983. Introduction to Physical System Dynamics. New York: McGraw-Hill.
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