A gas turbine engine consists of stationary and rotating components.  The rotating components include compressor blades and turbine blades mounted on disks which are interconnected by rotors and shafts.  The stationary components are the engine case assembly static structure, and include frames and cases.  The frames and cases provide structural support for the rotors and shafts and create a passageway that constrains the flow of air and combustion gases, or flowpath, through the engine.  The engine case static structure also includes bearings which are used to connect the rotating components to the stationary frames.  Frames include an inner wall, struts, and an outer wall.  The struts cross the flowpath, enabling support loads to be transferred from interior rotor support bearings to the outer case walls.  Cases include an outer wall but do not have an inner wall or struts. 
An engine case static structure designer begins with a flowpath which is an aerodynamic engine definition provided by the flowpath designers, and performance data which is a thermodynamic engine definition.  The engine case static structure designer must then lay out an engine case static structure configuration, including bearing locations, case boundary locations, strut locations, flange locations, wall thickness, mounts, manifolds and standard parts.  This process is referred to as the conceptual design and can take approximately one month to complete.  The purpose of the conceptual design is to evaluate several candidate flowpaths and select the best overall design configuration.  Due to fixed time constraints and the one month conceptual design period for each design, an engine case designer is limited in the number of flowpaths that can be evaluated before it becomes necessary to select the “best” configuration and proceed into the preliminary and final design phases. 
A shorter design period for creating an engine case static structure would allow more time for gas turbine engine system level analysis and concept trade studies focusing on weight of the gas turbine engine.  The shorter period would also permit the gas turbine engine designers to generate additional design cycles for evaluation of alternative flowpaths and engine designs, and evaluation of the design for performing maintenance, thereby generating more efficient engine case static structures.  In addition, the engine case static structure design should incorporate vibration analysis and finite element structural analysis of blade loss design requirements and backbone bending assessment. 
The engine case static structure is the foundation of the gas turbine engine.  A shorter design cycle which includes a more thorough evaluation and analyses of the engine case static structure, including the cases and the frames, during the conceptual design phase improves the selection of the best overall configuration, thereby reducing the risk of major redesigns during the preliminary and final design.  Since the design of an engine case static structure must be an efficient integration of all components with no wasted space, any configuration changes have a domino effect, which is time consuming to implement and can have serious consequences to manufacturing schedules and cost orders for items requiring a long lead time, such as raw material, tooling, castings and forgings. 
It is known to design various products using a computer-aided design (“CAD”) system, a computer-aided manufacturing (“CAM”) system, and/or a computer-aided engineering (“CAE”) system.  For sake of convenience, each of these similar types of systems is referred to hereinafter as a CAD system.  A CAD system is a computer-based product design system implemented in software executing on a workstation.  A CAD system allows the user to develop a product design or definition through development of a corresponding product model.  The model is then typically used throughout the product development and manufacturing process.  An example is the popular Unigraphics system commercially available from Unigraphics Solutions, Inc. (hereinafter “Unigraphics”). 
In addition to CAD systems, there is another type of computer-based product design system which is known as a “Knowledge-Based Engineering” (“KBE”) system.  A KBE system is a software tool that enables an organization to develop product model software, typically object-oriented, that can automate engineering definitions of products.  The KBE system product model requires a set of engineering rules related to design and manufacturing, a thorough description of all relevant possible product configurations, and a product definition consisting of geometric and non-geometric parameters which unambiguously define a product.  An example is the popular ICAD system commercially available from Knowledge Technologies, Inc. KBE systems are a complement to, rather than a replacement for, CAD systems. 
An ICAD-developed program is object-oriented in the sense that the overall product model is decomposed into its constituent components or features whose parameters are individually defined.  The ICAD-developed programs harness the knowledge base of an organization's resident experts in the form of design and manufacturing rules and best practices relating to the product to be designed.  An ICAD product model software program facilitates rapid automated engineering product design, thereby allowing high quality products to get to market quicker. 
The ICAD system allows the software engineer to develop product model software programs that create parametric, three-dimensional, geometric models of products to be manufactured.  The software engineer utilizes a proprietary ICAD object-oriented programming language, which is based on the industry standard LISP language, to develop a product model software program that designs and manipulates desired geometric features of the product model.  The product model software program enables the capturing of the engineering expertise of each product development discipline throughout the entire product design process.  Included are not only the product geometry but also the product non-geometry, which includes product configuration, development processes, standard engineering methods and manufacturing rules.  The resulting model configuration and parameter data, which typically satisfy the model design requirements, comprise the output of the product model software program.  This output, from which the actual product may be manufactured, comprises a file containing data (e.g., dimensions) defining the various parameters and configuration features associated with each component or element of the product. 
Also, the product model software program typically performs a “what if” analysis on the model by allowing the user to change model configuration and/or physical parameter values and then assess the resulting product design.  Other analyses may be run to assess various model features with regard to such functional characteristics as performance, durability and manufacturability.  The analytical results, e.g. temperature and stress, are functional parameters that are evaluated in terms of boundaries or limits.  Limits on both physical and functional parameters have been developed over time based on knowledge accumulated through past design, manufacturing, performance, and durability experience.  Essentially, these parameter limits comprise rules against which the proposed product model design is measured.  Use of these historically developed parameters, analyses, and design procedures in this way is typically referred to as product “rule-based design” or “knowledge-based design”.  The rules determine whether the resulting product design will satisfy the component design requirements, such as weight, and whether the design is manufacturable, given various modern manufacturing processes.  The rules for a particular product design are pre-programmed into the product model software program for that specific product. 
While the ICAD system provides an excellent tool for developing software product models, it is not a replacement for an organization's primary CAD system, which maintains the product model definition throughout the entire product design and manufacturing cycle.  This is because the ICAD system is a KBE software development tool rather than a CAD system.  For example, while the ICAD system can create a geometric model, it cannot easily create drawings based on that model or support other aspects of the design process typically provided by CAD systems.  As such, for the product model created in the ICAD system to be useful throughout the entire product development process, the model must be transported into a CAD system for further manipulation. 
Another inherent problem with the commercial ICAD system is that the parametric model created by the product model software program cannot be transported as a similar parametric product model into a Unigraphics CAD system.  Instead, the parametric model in ICAD must be transported into Unigraphics as a non-parametric model. 
Since design and manufacturing technology is always evolving, the product model imported from the ICAD system into Unigraphics will usually be enhanced with new technology design or manufacturing features.  Furthermore, since it is difficult to make modifications to a non-parametric model in Unigraphics, revisions to the product model must normally be made in the ICAD system and re-imported into Unigraphics.  This causes any additional features previously added in Unigraphics to be lost. 
On the other hand, the Unigraphics CAD system has inherent problems in that not all of the parametric models created by Unigraphics are “standardized” within an organization or industry.  Also, parametric models implemented in Unigraphics do not effectively implement a KBE system (similar to the ICAD system) that requires the model configuration and order of Boolean operations to vary according to design requirements.  Also, a Unigraphics parametric model cannot be structured to provide parameter relationships that satisfy both design and manufacturing requirements. 
As a result, there are instances when a product model developed solely in either the ICAD system or the Unigraphics system will suffice, even with the aforementioned shortcomings.  However, there are other instances when it is desired to transport a parametric product model developed in the ICAD system to the Unigraphics CAD system as a corresponding parametric product model. 
An object of the present invention is to provide a computer-based method of creating a parametric, two and three-dimensional, geometric product model of the engine case static structure of a gas turbine engine. 
Another object of the present invention is to reduce the design period for creating the engine case static structure of a gas turbine engine. 
Another object of the present invention is to provide a computer-based method of creating a parametric product model in a KBE system that can be recreated as a similar parametric product model in a CAD system. 
The above and other objects and advantages of the present invention will become more readily apparent when the following description of a best mode embodiment of the present invention is read in conjunction with the accompanying drawings.