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S.FINETTE ESA-FT 1/1

3rd MSC.Software Worldwide Aerospace Conference and Technology Showcase

Parametric finite element model of AIRBUS engine pylon withMSC.Patran.

Paper 2001-104

Sylvain FINETTE

AIRBUSM0121 / 4, ESAE

316,Route de BayonneF-31060 TOULOUSE

Tel: (33) 5 61.18.60.30Email: [email protected]

ABSTRACT:During pre-design phase, the generation and analysis of finite element models for aeronautic structure is a

complex task for two main reasons.First, aircraft designers have to be provided at any time with updated information on structural analysis inorder to validate or eliminate a new concept. The FEM analysis cycle time has a direct impact on the pre-

design process, and should be shortened as much as possible.Moreover, classic automatic mesh generators are not directly suitable to generate the 1D and 2D mesh, asmodels are mainly composed of stiffened panels. Even for experienced MSC.Patran users, the generation

of the mesh is the most time consuming part of the analysis. In the case of the engine pylon, extensive useof finite element techniques during the last ten years shown than three to four weeks were necessary topredict weight and thickness of a new pylon configuration. These studies also pointed out that this time

could be significantly reduced to less than one week with the help of automatic programs.

As a result, in 1999, AIRBUS decided to develop PCL programs to improve the structure analysis cycle. It

covers mesh generation, definition of design variables, pre-design with fully stressed design techniques(SOL101) or optimization (SOL200), and post-processing. The three main objectives of the study were:- To setup a tool for A380 project work, and reduce the engine pylon pre-design cycle time.

- To understand the risks and the benefits of such an automatic pre-design tool.- To evaluate the capacity of MSC.Patran and PCL to handle vertical application.

This project was carried out for six months and gave way to an operational program improving the pre-design process. It allows the user to perform fast analysis of engine pylon, and give easy access toknowledge about AIRBUS internal pre-design techniques. On A380, the pre-design cycle was reduced by

5, and AIRBUS increased its competitiveness by optimizing weight on the pylon structure (approximately200kg for four pylons)Also, the architecture of the program is such that the costs of development and maintenance are

minimized. Finally, encouraged by the good results, other similar PCL programs are currently developed forother AIRBUS aircraft structures.

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INTRODUCTION:

For many years, parametric finite element models have been commonly used in aerospace industry. It hasbecome a usual practice to use developments (with PCL or other tool), to generate automatically meshes,

analysis and post-processing. However, this practice was only limited to local studies, on small and genericmodels, to give indications and approximations about the behavior of the real structure.

In 1999, when AIRBUS decided, due to project work pressure, to improve the pre-design process, andreduce the analysis cycle time, these parametric finite element techniques were naturally chosen and astudy was carried out to apply them to a level 1 model, i.e. which is part of the primary structure of the A/C.

This type of model implies more constraints than local studies, and this paper presents how the parametrictechniques were adapted to fit with all requirements.

PROBLEM DEFINITION

Efficient use of analysis tools.

In project work, top quality analysis tools are required to workout the pre-design of structures. Pre and post

processing tools have been significantly improving for the last few years, and many new efficientfunctionalitys concerning mesh generation are available for finite element models. As a consequence,engineers have to be trained regularly to be fully operational and proficient. Moreover, the growing number

of engineers and subcontractors make it more difficult to have an individual level control.Therefore, it is not only essential for analysis tools to define clear rules and instructions but also, wheneverpossible, to develop and include automatic procedures. Such programs guarantee the users proficiency

and the quality of models.

Reducing pre -design cycles.

Since the beginning of AIRBUS aircraft production, engineers have gained a strong experience in the field

of pre-design of metallic A/C structures. Specialists have defined clear procedures to quickly estimate

weight and dimension of models. However, although the pre-design process is well known, the analysis isstill a long process in the overall pre-design cycle. In most cases, the work achieved is repetitive from one

model to another, and can easily be described in sequential algorithm. Moreover, engineer must produceformatted data, with validated and systematic methods, in order to be able to compare differentconfigurations together. Also, level 1 models need to be exchanged between AIRBUS partners, and

therefore require to respect standard modeling rules agreed within AIRBUS organization.

The particularity of pre-design phase is that analysis engineers must perform these tasks many times, as

the structure can be significantly modified. Also, if the weight and dimensions predictions can not be righton time with the project life, some manual and rough approximation techniques will have to be used, whichwill decrease the quality of investigations carried out for the pre-design phase.

Now it is clearly stated that there is room for improvement in the pre-design cycle, by automating some

analysis tasks, the problem consists in choosing the most strategic platform and the appropriatedevelopment methods that provide with a maximum performance gain, for a minimum cost.

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Application on Engine pylon

Parametric finite element model can not be used efficiently on all aircraft structures. Models must berelatively simple to generate, to avoid very long and costly developments. Also, the number of designvariables must large enough, to justify the need to investigate different configurations.

For this study, the engine pylon is selected as an industrial case, as it fits with these requirements. In newaircraft project like A380, this structure contains a lot of uncertain parameters. To define the optimum

configuration, engineers must work on as many models as possible, which requires fast and systematicanalysis cycle.

Other constraints due to project work.

Usually, the quality of a parametric model is conditioned by initial specifications, which must clearly define

all design variables and constraints. The more precise and thorough is the specification, the less expensiveis the maintenance of the program. This is particularly true for local and independent parametric studies.

Within the framework of aircraft project, modifications applied to the geometry of the structure can notalways be anticipated when the development is initiated. It has to be taken into account at the early stageof parametric model developments. The program, based on a modular approach, should be flexible enough

to accept changes in the shape of the structure, and in the way parameters are defined.

Fig 1 : Overview of a pre-design finiteelement model of a typical AIRBUS engine

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ANALYSIS

Generalities synopsis

The modular approach of the parametric model is shown above. First, the mesh is generated on the groundof a simple concept, consider that all ribs are vertical and equally spaced. Then, this basic mesh can bemodified by some transformation modules. Users have the possibility to modify the position and orientation

of ribs, without modifying geometry of the spars and panels connected to them. It is also possible to addkinks and holes to the lateral panels.In a third step, users define loads applied to the model, as well as design variables, when optimization is

used to work out the pre-design.In the last place, the analysis and pre-design are processed automatically by MSC.Nastran. All the resultsand new properties are stored into the database.

For each study, the initial set of parameters is fixed and can not be modified. However, any othercharacteristics (model transformation, material properties, loads and analysis) can be investigated in aiterative process, within a unique MSC.Patran database.

Digital mock up

Geometrical data :

Length, width, number of ribs...

Generation of mesh

Geometrical data :Rib pitch, orientation angle ofribs etc...

Modification of the

mesh

Generation ofMSC.Nastran model

Boundary conditions :

Position of center of gravity

Predesign of 2Delements

Load cases: Loads at COG

Predesigned MSC.Nastran model

SOL101 results for all load cases

History of parameters

Weight of model

MSC.Patran

Fig 2: Overall process of the parametric program

Iteration

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MSC.Patran environment

The parametric model is available directly from the standard MSC.Patran environment. Therefore user is

working with a unique interface, which avoids most training and licensing questions.

In this process, users can call several independent modules. At any stage, standard MSC.Patran isavailable to modify manually the model, and to import other part of the structure that have to be taken into

account during the analysis, etc

This aspect is very important, as the overall architecture of the program depends on how much integrated it

must be. Tools dedicated to vertical application like MSC.Acumen were investigated for this study, but itappears they are not suitable for this analysis. In fact, at any time, user may need access to all theMSC.Patran functionalitys, whenever the tool does not cover such parameter, or such concept. On one

hand, this will slow down the analysis process, but on the other hand, the program is more flexible, andfurther developments are not necessarily required to adapt the tool.

Geometric parameters of the model.

The geometry of the model is entered by the use of a comprehensive user interface. Users are guided

through a set of menus, which contain databoxes, default settings and figures which give explanationsabout the meaning of each parameters. Once the configuration is fixed and the model generated, theseparameters can not be changed anymore. However, user still have full read access to the menus and

figures in order to verify model data.

Fig 4 : Typical user interface to input geometrical parameters

Fig 3 : Access to parametric program in MSC.Patran environment

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Originally, engineers used to handling sets of geometric points that were imported into MSC.Patran, tomanually generate the mesh. With our parametric model, users do no longer manipulate point coordinates,or geometry references. The only necessary geometric data are physical dimensions and characteristics of

the model: length, width, number of ribs, position of spar flanges, etcParametric model can be used even when the geometry of the structure does not exist in CAD system.Analysis of new configurations can be performed, even before designers start to create CAD parts.

Mesh definition

The mesh definition on a pre-design model needs to be carefully validated. The number of elements isminimized, in order to speed up the analysis process, and to simplify the post-processing.

For many years, complete studies have been carried out to precisely define modeling rules which ensurecorrect analysis results.

The figure below shows the mesh transition required to calculate the element forces in the wing-pyloninterface. Triangles and degenerated elements are tolerated to make the transition between two meshes.Nevertheless, the bad quality elements must be far from the region of interest (i.e. where stresses and

forces are calculated)

Also, AIRBUS defined accurate manners for using analysis tools, which make the models easy to

exchange between the various sites (MSC.Nastran parameters, numbering convention, etc). As we cansee, modeling requirements can be of a different nature, and therefore owned by different specialists. Thissituation is very typical and often leads to many iterations before getting an acceptable finite element

model.

The parametric tool gives the opportunity to code all of them in a unique environment, allowing the user to

focus mainly on interpretation of results.

Position of the wing-

pylon interface

In the area of interest, meshis made only of rectangleelements.

Fig 5 : Top spar, wing-pylon interface region

Mesh transition : triangles

and low quality elements :

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Mesh modification

Some characteristics of the structure are difficult to specify, and can not be coded into an integrated

development. This is the case for the position of ribs: User needs to have the ability to change the ribdefinition on a existing model, without having to create a new model. These modification modules can beused anytime during the parametric process. They also are generic oriented, and can be used on other

similar models (wing box, center box, etc)

In the frame of our project work, the main geometry of a model can be modified, which implies importantmaintenance work to adapt the parametric model to the changes. With our developments, users have thepossibility to modify very easily their model, without changing overall parametric model.

Storage of parametric data

Parametric models enable users to generate models in a short time. But they have to deal with a lot ofinformation. In order to guarantee a maximum tracability, two types of data storage are used:

MSC.Patran database: extensive use of Client data to store all the parameters and the history of themodel inside the database. At any time, users can have access to parameters, and get a precise

understanding of the model.

Neutral file: Any operation performed with the parametric model (Parameter set, model transformation,loading of the structure) is written in a session file that can be played in an other MSC.Patran session.

In general, any work achieved at some point should be done only once and can then be automaticallyreproduced.

Example : modification of the

orientation of a rib :

Fig 6 : Illustration of a possible modification to the finite element model

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Pre-design

The pre design of the structure is aimed at determining optimum thickness which respect the materialallowable (given a set of load cases), and minimizing the weight.

Two approaches are implemented to achieve it:

Fully stressed design

This iterative approach requires FORTRAN programs which call MSC.Nastran analysis.FORTRAN runs a MSC.Nastran SOL101 on the model, and then modify the 2D element properties in order

to find out the stress equal to the material allowable. After several iterations, all MSC.Nastran results areread in MSC.Patran database which update automatically the properties.

The following hypothesis are required to obtain good results:

Initial 2D element thickness must be set to the minimum value.

The thickness can only be increased (to avoid convergence problems)

CTRIA3 elements are ignored in the stress analysis

This method is implemented to reproduce automatically what users would have to do manually to make the

structure pre-design. It gives a first estimation of the weight, easy to analyze and to understand.

Optimization

This program offers the possibility to generate automatically MSC.Nastran optimization entries, based onMSC.Patran groups. The user interface, showed below, is available for all kinds of analysis on MSC.Patrandatabases.

This user interface allow user to perform easily simple SOL200 analysis. However, it might be necessary to

add more constraints (Links or equations between design variables) in order to produce a realistic design.

Definition of design variables Definition of design constraints

MSC.Patran

groups

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This requires good understanding and experience about optimization techniques, which is usually providedby specialist engineers.For this reason, the pre-design is performed in two steps: first, fully stressed design is used to make fast

and rapid pre-design analysis, in order to select the preferable engine pylon configurations. In a secondstep, optimization takes place when a precise estimation of weight is required, which leads then to thedefinition of the optimum structure.

CONCLUSION

Parametric finite element techniques have been extensively used to pre-design of the A380 AIRBUS

engine pylon.

For A380 project, this study was very successful, and helped to improve significantly the pre-design cycle.

A complete analysis of a new configuration has been brought down from 3 weeks to 2 days, and any pre-design iteration on this configuration is now few hours long, compared to the 1 week doing the jobmanually.

With this reduction of analysis cycle and the use of optimization techniques, AIRBUS has the ability tocreate lighter engine pylon which will increase performances of the aircraft. This weight saving can beroughly approximated to 200kg (for four engine pylons) on A380.

In 2001, other similar developments have already started on different aircraft structure.

As demonstrated in this paper, the parametric model is aimed at two contradictory objectives:- The implementation of as many internal rules and knowledge as possible, to centralize and facilitate

access to technical information.- A good adaptability, as some model evolutions are not predictable.

A good compromise has been obtained, with two important principles:- Choice of an appropriate development platform: MSC.Patran has all the required functionalitys to

manually generate good quality finite element models. For many years, it is the AIRBUS standard pre-processing tool for aircraft models. Without any question, it is therefore the most strategic tool todevelop vertical application, in the structural analysis field.

-User profile: To some extend, the user of such an automated pre-design tool has a good understandingand a large experience of structural analysis techniques. This means if the parameters are not properly

used, the model might be wrong, and the program wont warn the user. With or without the tool, it is stillhis responsibility to check the data is correct.

This parametric tool is an open access to various modeling rules, and a working frame allowing the user towork quickly, but by no means it is designed to replace the finite element analyst.

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