CADac: A New Geometry Construction Tool for

Aerospace Vehicle Pre-Design and Conceptual Design

A. Berard∗ and A.Rizzi †

Dept. of Aeronautical & Vehicle Engineering, Royal Institute of Technology (KTH),

Teknikringen 8, SE 100-44 Stockholm, Sweden

A.T. Isikveren‡

Dept. of Aerospace Engineering, University of Bristol,

Queen’s Building, University Walk Bristol BS8 1TR, UK

In view of a continuous increase of computer performance, it is nowadays feasable touse CFD (Computational Fluid Dynamics) analysis very early in the conceptual designstage, if not the pre-concept phase, of aircraft. This requires the generation of a CAD(Computer Aided Design) model suitable for CFD computations, which is a tedious andtime consuming process because a disconnect exists between the geometrical definitionrequired for a CAD model and the limited number of geometry related design parametersdefined by the designer. An additional complication to this is the requirement of producinga closed and consistent CAD model suitable for problem setup of CFD computations.The CADac (CAD-aircraft) tool fills this gap by automating the generation of closed andconsistent CAD models via the implementation of a parameterized approach to conceptualdesign. CADac enables therefore to use CFD earlier and to use tools with inter-lacedfidelity at the conceptual design phase.

I. Introduction

This paper presents an advanced geometry construction tool whose aim is to enlarge the scope of aerody-namic analyses that can be performed at either the pre-design or the conceptual design stage, in particularby enabling the use of CFD (Computational Fluid Dynamics) earlier. The second goal is to facilitate opti-mization analysis using interlaced (somethimes refered to as adaptive) fidelity methods based on a uniqueparametric geometry description. It is founded on the studies done by Isikveren1 for the development of theQCARD software.Although three-dimensional parametric solid geometry construction pervades many aspects of modern-dayaerospace vehicle product development, the level of integration of such techniques is still not close to beingcomplete, particularly when it concerns defining the configuration layout in pre-design and early-conceptualdesign phases. It is nowadays common practice to employ a CAD (Computer Aided Design) tool relativelyearly in the conceptual design phase not only to permit internal/external topology visualization (space pro-vision and basic assembly), but to also begin predicting properties (areas, slenderness and mass) requiredfor assessment of performance and flight handling. However, geometry construction does not typically ex-ceed sophistication beyond 2D visualization, e.g. the general arrangement or three-view. Some dedicatedaircraft conceptual design packages with Computer-Aided Engineering (CAE) such as RDS,2 Piano,3 AAA,4

QCARD or ACSYNT5 typically construct the 3D (3-Dimensional) surfaces by geometrical lofting techniques.Another stand-alone geometry construction tool is RAGE.6 The major drawback of these tools is that theuser is compelled to use a software package that can neither support increasing sophistication in geometricdefinition with growing design maturity nor are they compatible with any industrial-grade CAD softwarepackage, i.e. you need to start from a ”clean sheet” to create a CAD model of the configuration.

∗Ph.D. Candidate, AIAA Member, adrien@kth.se.†Professor, AIAA Associate Fellow, rizzi@kth.se.‡Director of Engineering Design, Senior Lecturer, AIAA Member, Askin.T.Isikveren@bristol.ac.uk.

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II. Problem Statement

The traditional way of proceeding during conceptual design is sequential: the designer creates a layoutin which low-fidelity analyses are performed and then a CAD expert is charged to produce a solid modelas similar as possible to the original intend of the designer. Specifically, whereas a three-view drawingis usually sufficient for a designer to quickly predict minimum goals for the performance of a design, thearray of design parameters taken into consideration is not detailed or explicit enough to support a usablecomputational mesh for higher fidelity analysis. CAD-based modeling along with grid generation and high-fidelity analysis usually enters at a later stage in the design process when the configuration has developedsubstantially. In addition, the CAD-based geometry models are usually built from spline surfaces whoseparameters are numerous and poorly related to the much fewer and familiar parameters (such as referencewing aspect ratio, quarter chord sweep, area, etc.) used by the designer to describe the aircraft morphology.This transformation from intended shape to the CAD expert’s implemented model slows down the designprocess and creates difficulties for more ambitious optimization analysis. The CADac tool was designedto shortcut this transformation by creating a sufficiently detailed, closed and consistent CAD model fromdesign parameters that are intuitive and informative for the designer.For the designer, the analysis of a new aircraft candidate basically covers (geometry construction related)the interaction of weights and balance, structures and loads, aerodynamics, propulsion, stability and control,operational performance, noise and emissions, and economics. This complex multi-disciplinary problem callsfor a geometrical description protocol that is flexible in the sense that it can suitably accommodate all theseparate technical subspaces yet must not be so detailed as to produce an inconsistency in fidelity betweenproblem formulation and problem assessment. Each technical subspace needs a particular description of theaircraft but all of them should refer to the same unique geometry (see Fig.1).

Figure 1. Geometry centric conceptual design, example shown Piaggio P-180.

In addition, to maximize freedom for the designer, the geometry construction method needs to be suf-ficiently general and flexible to cover not only all expected classical aircraft morphologies, but also thoseregarded to be atypical or unconventional. Finally, the geometry description should be suitable for opti-mization procedures and simple enough to allow the designer to have a good, intuitive understanding of thepossible layout modifications and their expected effects according to previous experience and knowledge.These three conditions are fulfilled by an appropriate parametrization of the morphology.

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III. The Approach and Tool: CADac

A. Master Model and Datafile

The master model concept is the heart of the geometry construction module of the CEASIOM (ComputerizedEnvironment for Aircraft Synthesis and Integrated Optimization Methods) environment under developmentin the SimSAC project. SimSAC stands for Simulating Stability and Control and its task is to createthe multidisciplinary simulation environment named CEASIOM,9,10 oriented toward stability and controland specially suited for aircraft conceptual design sizing and optimization. The aim is to produce a largeparametrized library of aircraft components, from which the aircraft model can be created through a processof assembly and sizing that matches as closely as is practical the way in which an engineer designs an aircraft.The key is the particular parametrization that is chosen because it ensures geometric consistency throughoutthe whole design process. The same parameters drive all the geometric descriptions of the aircraft, whetherit is a three-view layout, quasi-analytical lofting, structural beam representation, panel surfacing for vortexlattice methods or 3D CAD solid model for CFD analysis (see Fig. 1).The geometry of each component of the aircraft, such as wing, fuselage or empennage, is fully described bya set of parameters chosen a priori. This ensures that by varying only a few parameters, the designer hascontrol over the aircraft morphology variations, even large ones, which lays the basis for sensitivity studyanalyses and optimization. The parameters describing each component (fuselage, wing, etc.) are organizedand stored together in a unique XML file format describing the feature tree of the desired aircraft concept.XML stands for Extensible Markup Language and is a free, open standard that is very widely used. Thisformat has been chosen for data storage because it allows the users to define their own elements in a struc-tured way and it facilitates the sharing of structured data across different information systems.

Fig.2 shows a sample of some geometries that can be modeled and illustrates the versatility of the geome-try module. The four aircraft on the left are quasi-analytical representations, whereas the four aircraft on theright are solid models. All these aircrafts are based on a unique format of input XML file but with differentinput variables. Of course, each of the XML files used to create the solid models can be used to produce aquasi-analytical representation of the same aircraft. Conversely, the XML file used in the quasi-analyticallofting can also be used to create a solid model of the same aircraft in a straightforward manner using CADac.

(a) Falcon 7X withoutwinglets.

(b) Piaggio 180. (c) Boeing 747-100. (d) Wind tunnel model withstruts.

(e) Learjet 45. (f) Bae-146. (g) Oblique wing regional jet. (h) Transonic cruiser.

Figure 2. Sample of possible parametric morphologies.

B. Master Model Component Libraries and CAPRI

This approach introduces the CAD generation at the start of the conceptual design stage and automates itsexecution to produce the identical morphology that the traditional lofting techniques create, thus matching

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the expectations of the designer. Indeed, the CADac tool mimics the usual design process and, in doingso, allows the use of analysis tools with inter-laced fidelity,for example vortex-lattice methods with Eulercomputations. Compared to low fidelity methods that are commonly used at the conceptual design stage,CFD is quite demanding because it needs a closed and consistent model featuring all the external geometricalcharacteristics of the aircraft. This model is generated using CAD in order to produce a three dimensionalrepresentation of the aircraft that is then used by a grid generator software to produce meshes suitable forCFD computations. A recurrent problem arises from the the CAD model being traditionally produced at alevel that usually needs extensive post-CAD geometry repair in order to be suitable for CFD use. Therefore,CADac works with solid models from the outset and assembles them by doing a Boolean union, i.e. mergingall the volume as a unique volume, in order to ensure that, for any automatically generated aircraft solidmodel, no geometry repair is required to produce a quality mesh.Another issue to address in order to introduce CFD at the conceptual design stage is that the representa-tion used for low fidelity analysis is not detailed or refined enough to be directly used to describe a CADrepresentation. Therefore, additional parameters describing the lofting of the different parts of the aircrafthave been introduced. For example, the lofting between the nose cone and fuselage is very important forCFD computations in order to predict correctly the aerodynamic data. Fig. 3 illustrates the result of Eulercomputations for a parameterized solid model of the same aircraft with and without nose cone lofting.

(a) Pressure distribution for a inappropriatelylofted nose cone junction.

(b) Pressure distribution for a properly lofted nosecone junction.

Figure 3. Illustration of the importance of lofting for CFD computations.

Different CAD systems vary quite a lot with respect to details, yet are sufficiently similar to enablethe definition of a common user interface powerful enough to support the generation of appropriate CADmodels as a basis to create computational meshes for CFD. The Application Programming Interface (API)CAPRI11 (www.cadnexus.com) offers this functionality. The use of a master model concept in conjunctionwith CAPRI results in the following benefits:

• Unique and consistent model definition of the geometry. Any change of parameter or of morphology iscascaded through all the analyses modules, in particular in the CAD model.

• Unified database which ensures that the same version of the aircraft is used for all analysis modules.

• Feature management for design evolution. Features in the master model can be easily added or sup-pressed depending on the complexity level required.

• Feature management for fidelity matching. Suppressing branches of the feature tree can also be usedto match the fidelity of the geometry to the analysis being performed.

• Parameter studies and design optimization. Because a reduced set of intuitive parameters has beenchosen, it is possible to easily modify some or all of the parameters in order to analyze their effect oroptimize the whole geometry.

In addition, CAPRI is a vendor-neutral API, which means that no expertise of computational geometry orof specific CAD-vendor API programming environment is required. The CAPRI Gateway uses a geometrycentric approach, which makes the actual geometry (not the computational mesh) accessible to all phases ofthe CAE process. The connection to the geometry is made through a middleware and not a file system. Thisisolates the top-level applications (grid generators, solvers, optimizers, and visualization components) fromthe geometry engine. It also allows the replacement of one geometry kernel with another, without affecting

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the top-level applications as illustrated in Fig.4.

Figure 4. Simulation-based design/analysis suite using CAPRI.11

C. Procedure

The generation of a 3D solid model is done by means of CAPRI and the in-house developed code CADac.As explained earlier, a component based approach has been adopted and by reading the XML data filestructure, CADac understands the designer’s desired aircraft components, and associated parameters, and,assembles these components according to the defined set of geometrical parameters. For all the differentparts (wings, fuselage, empennage, etc.) that can possibly be constituents of a new aircraft morphology, amaster-model has been created in a parametrized way. They are the so-called ”rubber” components and areavailable in a component library where CADac selects the relevant ones for the design at hand, then sizesand assembles them, thus producing a CAD model that emulates the quasi-analytical lofting and is suitablefor CFD computations (see Fig. 5).

Figure 5. Emulation of quasi analytical lofting in CAD.

Such component libraries have been created in four major CAD systems: SolidWorksTM, UnigraphicsTM,Pro EngineerTMand CATIATM, allowing designers to choose and thus to work in the environment they aremost acquainted with.

The CADac code has been implemented and coupled to CAPRI in order to perform the following oper-ations automatically:

1. Load and parse the XML file describing the geometry of the aircraft.

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2. Read the XML file and identify the different constituants (e.g. fuselage, wing, etc.) and for each ofthese parts, perform the following actions:

(a) Load the part from the component library that has been created for the desired CAD software.

(b) Update the CAD part parameters according to the data contained in the XML file.

(c) Regenerate the CAD part to obtain the aircraft component dimensioned according to the originalintend of the designer.

(d) Save the re-dimensioned CAD part individually so that analysis can be performed on each of theaircraft components separately if desired.

(e) Position the part with respect to the other parts of the aircraft.

3. Do a Boolean union of all the re-dimensioned and positioned parts in order to create a meshable, closedand consistent solid model of the aircraft that can be used in CFD computations.

4. Do a Boolean subtraction of the above assembled solid model of the aircraft and of a solid sphere. Bydoing so, it creates a mold of the aircraft included in the solid sphere that represents the far field.

The main steps of this procedure are illustrated in Fig.6.

Figure 6. Selection and sizing of the ”rubber” aircraft components.

By simply modifying the parameter value in the XML file, the quasi analytical lofting representation offersa quick visualization of the effect of each parameter modification. Re-execution of CADac enables one toobtain a new solid model of the design concept within minutes. To match the requirements of CEASIOM, thefollowing ”rubber” components populate the component libraries for each of the four chosen CAD kernels:

• wing featuring two kinks and therefore three fully definable subsections per semi-span. The wing canbe symmetric or asymmetric or even be a semi-wing in case the user wants only the port/starboardwing. There is facility for multiple wing definition as will be discussed in the following paragraph.

• fuselage featuring the nose and tail cones as well as two central sections of variable vertical andhorizontal diameters.

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• horizontal and vertical tail which are both similar to the wing but with only one kink.

• fairing for the wing-fuselage attachment, bullet fairing in the case of a T-tail and sponson (to modelthe landing gear housing box in case of a high wing aircraft for example).

• ventral fins.

• engines which can be wing-mounted, blended into the wing or aft-mounted.

For the control surfaces definition, two options were experimented. The first method involves control surfacesdefinition as separate solids, in the component library, that are then mated to the lifting surfaces. Thisimplied complex operations of Boolean subtraction and subsequent Boolean union of the lifting surfaces andthe control surfaces. Indeed, the part of the lifting surface corresponding to the control surface needs to befirst erased, then the control surface needs to be deflected and finally mated to the lifting surface. The secondmethod, which has been chosen, represents the control surfaces as separated faces of the lifting surfaces (seeFig. 7). Therefore, the control surface definition is embedded in the lifting surface definition but because itis a different face, it is possible to use surface transpiration methods12 during the CFD analysis to modelsmall deflections (approximately +/- 5 degrees) of the control surfaces.

Figure 7. Control surface modelization by splitting the lifting surface face.

Any combination of the components can be done, thus enabling to model most of the classical aircraftconfigurations, but also atypical ones. These components are solid models and CADac assembles themthrough Boolean union, thus ensuring that the produced assembly is closed and consistent and can bemeshed without any need for time-consuming geometry repair.In addition, it is highlighted that the CADac module has been coded to be as flexible as possible andto adapt easily with regard to further enhancements of the geometry description, whether it is additionalparameters to add details to the existing parts, or new parts (wing struts for example). Furthermore, onehas the ability to duplicate the same component with different parameters. For example in the case of adesign with multiple wings or fuselages. This can be easily done since XML format has been chosen for datastorage, and the fact that the CADac module does not search for specific names (such as ”wing”, ”fuselage”,etc.). CADac checks for a correlation between the name of the component described in the XML file and thenames of the parts found in the component libraries. Once the appropriate part has been found and loaded,CADac proceeds in the same way for the parameter recognition and modification. It is therefore very easyto enhance the geometric model in order to automatically generate new or modified parts in the CAD solidmodel. Attention should be paid to the fact that, in such a case, the consistency of the representation withthe other analysis modules of CEASIOM is reduced or possibly lost in case major changes are made.The possibility to create a mold of the aircraft stems from the idea of streamlining, if not automating, themesh generation for CFD analysis. Indeed, once the Boolean operation of all the parts has been performed,a ”watertight” solid model is obtained but the domain around the aircraft model still has to be manuallygenerated in order to create a volume mesh for CFD computation. In many cases, and at least for theanalysis that are most relevant at the conceptual design stage, the airplane concept is studied in cruiseconfiguration, and therefore, the far field is often represented as a sphere, or sometimes a box, of about tento fifteen times the largest airplane dimension. Such simple volumes can be easily generated in an automated

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manner. Instead of assembling the different parts and performing a Boolean union, a solid sphere is createdand then the different parts are subtracted from this sphere. This produces a mold of the aircraft includedinto the far field. The diameter of the sphere can be user defined. Having the geometry described as a moldincluded in the far field sphere enables to create the volume mesh in a very straightforward way with nearlyno user intervention. This has been tested and has proved to be very convenient and is a promising steptoward automated mesh generation. Indeed, the open source grid generator TetGen (www.tetgen.berlios.de)is coupled to CAPRI to give the possibility to produce automatically a tetrahedral mesh suitable for CFDcomputations. For that purpose, the above described mold of the aircraft can be used since it is possible tomesh directly the solid volume around the mold of the aircraft.Fig.8 shows the final assembly of an example airplane and its mold, both obtained directly by runningCADac, for an example geometry XML file, without user intervention.

(a) Solid model of the aircraft. (b) Zoom on the aircraft mold.

Figure 8. The final boolean assembly of the airplane and the automatically generated far field.

IV. Proof-of-Concept: Effortless Grid Generation and CFD Computation

From the CAD solid model that is automatically generated using CADac, a mesh can easily be createdusing ICEM (www.ansys.com) for example. Indeed, because the different ”rubber” components have beencreated as solids and because particular attention is given to the sizing, assembling and Boolean union ofthem, the generated CAD model is ”watertight”, i.e. closed and consistent. This ensures that both surfaceand volume mesh can be generated with minimal user intervention and without the need for post-CADgeometry repair, which is usually a very time-consuming process when generating a mesh.To illustrate the handiness and utility of the CADac tool, the example case of a 70 PAX regional open rotorconcept named Horizon 110013 has been run. First, the XML datafile has been populated by assigning theappropriate values to each of the parameters for the different components, namely the fuselage, the wing,the wing-fuselage fairing, the vertical tail, the horizontal tail and the engines.Then, this XML file is givenas an input to CADac, thereafter, following the process described earlier, it generates automatically a solidmodel of each of the re-dimensioned components, as well as a solid model of the Horizon 1100 and a mold ofit. Then a surface mesh of 69500 triangles and a volume mesh of 1.05 million tetrahedra have been createdminimal effort in less that a dozen mouse-clicks and in about 10 minutes using ICEM. Indeed, as highlightedbefore, no post-CAD geometry repair was required. The surface mesh can be observed on Fig.9.CFD computations have been performed using this mesh in order to highlight the quality of the CAD solidmodel created by CADac and the fact that it is meshable and suitable for high fidelity computations. TheEdge solver (www.foi.se/edge) was run in Euler mode; a fully converged result was obtained in 800 MultiGridcycles (with 4 levels of MG) which were run on a modern laptop in 15 minutes (see Fig.9).

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Figure 9. CFD computations based on the automatically generated CAD solid model of the Horizon 1100.

So, to summarize, it took less than one and a half hours, starting from the XML file describing theparametrized geometry, to obtaining a CFD solution with minimal user intervention, without the need forpost-CAD geometry repair and without requiring any knowledge of the CAD environment.If the geometric parameters of the aircraft are changed, in the case of an optimization for example, the inputgeometry XML file can be easily modified, both manually or in an automated manner, and a new meshableCAD model can be obtained automatically by simply re-running CADac.

A. Application to Low and High Fidelity Aerodynamics

To further illustrate the value and utility of the CADac tool, an example case has been run for the Boeing747-100 without pylons and engines. Two aerodynamic analysis have been performed based on exactly thesame geometry XML input file; the first one using the vortex lattice method based software Tornado;8 thesecond one using the CAD model automatically generated by CADac and running Edge (www.foi.se/edge)CFD solver in Euler mode. The results for the pressure distribution are shown in Fig 10. It is highlightedthat the whole process, from XML file to CFD solution and visualization, took less than one and a half hourswith minimal user intervention. The Tornado solution was obtained within two minutes.

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(a) Tornado ∆Cp distribution.

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(b) Edge ∆Cp distribution. (c) Edge Cp visualization.

Figure 10. Aerodynamics study of the B747-100 in cruise condition.

Fig.10(a) shows the Tornado computation of the distribution of the pressure difference between upperand lower surfaces for the wing and horizontal tail. For comparison purposes, the pressure difference hasbeen extracted from the Edge pressure distribution results (see Fig. 10(c)) and projected on the same surfacemesh as the one used for Tornado computations; the result is shown in Fig. 10(b). The comparison of Fig.

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10(a) and Fig. 10(b) clearly highlights the advantages of being able to run both low fidelity methods as wellas CFD simulations early in the design process. Although Tornado predicts correctly the total lift coefficientand averages well the ∆Cp distribution over the wing and the pressure drop at the leading edge, it cannotmodel shock waves and separation for example and therefore fails to describe accurately the sudden drop of∆Cp in the midboard region as well as some finer details of the distribution. Indeed, Tornado is a vortexlattice method modeling only the camber of the lifting surfaces and not their thickness distribution. It istherefore useful to be able to run high fidelity CFD computations in order to have a better understandingof the aerodynamics.

V. Conclusion

The CADac tool brings two major improvements for the conceptual design phase of an aircraft.First, it enables to use CFD analysis earlier in the design process and to streamline the transition frommanual-design analysis into automated-design analysis. This allows a faster and more efficient processingand treatment of the data and results. It also ensures that all the results concern a unique configurationof the aircraft, without discrepancies or inconsistencies between all possible representations of the aircraft,from three-view layout to three-dimansional solid model.Second, by giving the user the possibility to utilize a wide array of methods with interlaced fidelity andenabling automated parametric Computer Aided Design (CAD) solid-modeling and dynamic mesh genera-tion, this tool facilitates multi-disciplinary design optimization. For example, using CADac, it is possible tocouple non-linear aerodynamics with advanced linear structural analysis, even during the pre-design phase.And since CADac automates the path to the use of Computational Fluid Dynamics (CFD), it is possibleto use it very early in the conceptual design phase, either to fully analyze the new aircraft concept or tobenchmark results obtained with lower fidelity methods.

Acknowledgments

CADac (CAD-aircraft) has been developed as part of the 6th European framework project SimSAC(www.simsacdesign.eu).

References

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