American Institute of Aeronautics and Astronautics

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Parametric Modeling of Aircraft Families for Load Calculation Support

C. Cerulli*, P. B. Meijer† and M. J. L. van Tooren‡ Delft University of Technology, Delft, The Netherlands

and

J. W. Hofstee§ Airbus, Hamburg, Germany

In the present work, a knowledge-based parametric Multi Model Generator (MMG) for reproducing a conventional aircraft family is presented. The intent is to introduce the MMG into a dedicated Design and Engineering Engine (DEE) for performing load calculation in the preliminary design phase. For this purpose the MMG has to be capable to supply different models of the same product, i.e. structure, mass and aerodynamic models, to feed a set of analysis tools. The generated models are extracted from a Knowledge Based Engineering (KBE) product tree, which is capable to hold the knowledge of the complete aircraft product. The definition of the aircraft is fully parametric, so that consistent models for the different disciplines can be generated for a large variety of aircraft configurations by variation of a single input file. The present work is mainly focused on the description of the structural model extracted from the MMG. The aircraft is modeled as an assembly of components, which in turn are built up as an assembly of so-called High-Level Primitives. The wing trunk primitive, presented in previous works, is used for reproducing all the lifting components; the fuselage trunk is presented as a new primitive used to reproduce the fuselage and engine nacelles. A flow diagram for the complete DEE is presented to show the position of the MMG in the load calculation process.

I. Introduction HE design process of an aircraft can be divided into different phases. The two first phases are the conceptual design phase and the preliminary design phase. In the conceptual design phase, the designer has to think of

possible concepts that fulfill the requirements of the customer. In the preliminary design phase, the various concepts are elaborated, analyzed and evaluated, and trade offs between solutions are made. A schematic overview of the diverging/converging design process is given in Fig. 1.

The conceptual design phase is difficult to automate because here the creativity and the experience of the designer play an important role. The preliminary design phase is usually more suitable for automation because the analyses of the various design concepts are of a repetitive nature. Nevertheless, recent developments were done on the possibility of supporting also the conceptual design phase with Knowledge Based Engineering (KBE). In this case automation is applied at concept generation level and not at process level. In preliminary design, the automation is * PhD student, Aerospace Engineering/Mechanical Engineering, Kluyverweg 1, 2629 HS Delft, The Netherlands. † Master student, Aerospace Engineering, Kluyverweg 1, 2629 HS Delft, The Netherlands. ‡ Professor, Aerospace Engineering, Kluyverweg 1, 2629HS Delft, The Netherlands. § Structural dynamics specialist, Loads & Aeroelastics, Kreetslag 10, 21129 Hamburg, Germany.

T Figure 1: Diverging/converging design process

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- Parametric multi-modeling

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45th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials Conference19 - 22 April 2004, Palm Springs, California

AIAA 2004-2019

Copyright © 2004 by Airbus. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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enabled by the creation of a computer Design & Engineering Engine (DEE). A DEE consists of a set of properly interconnected toolboxes in which the process is represented. A schematic overview of the DEE for aircraft design is given in Fig. 2.

An important block in the DEE is the Multi Model Generator (MMG). The MMG has to be capable to handle all the possible design concepts the designer comes up with. Moreover the MMG has to provide the models needed to feed the different disciplines silos. In this work the ICAD environment has been used for the development of the MMG. ICAD is an advanced tool to support Knowledge Based Engineering (KBE). In fact, the MMG is not only a CAD model, but it also captures the design strategy and the engineering rules required to reproduce a particular product. The main innovation in the MMG presented in this work is its ability to represent a complete family of products just by changing few input values.

Recently, different DEEs have been developed for preliminary design of new-configuration aircraft1, 2 and for investigating acoustics in preliminary design of fuselage panels3, 4. The present work is the starting point for the integration of the MMG into a dedicated DEE for loads calculation purposes in the preliminary design phase.

In general, performing loads analysis in preliminary design involves some problems. First, load envelope prediction requires evaluation of a large number of flight conditions, mass configurations, type of excitations (gust, maneuver, ground loads, etc.), and results in design envelopes for a certain number of quantities (loads, accelerations) monitored in a large number of points. The corresponding required computational effort is enormous, which prohibits extensive parametric studies and quick answers as required in preliminary design. Second, a high-fidelity aeroelastic model is required for accurate loads analysis. In preliminary design, such a model is either not available, or cannot be built up in a sufficiently short period of time.

The first issue is the subject of ongoing joint research at the German Aerospace Center (DLR), TU Delft and Airbus. A flexible software environment (VarLoads) is being adapted for preliminary design purposes5. In the present work, the focus is especially on the second issue, i.e. fast generation of high-fidelity aeroelastic models.

Preliminary design for an aircraft company does not automatically imply exploration of completely novel configurations. Most of the time, derivatives of existing aircraft are under consideration. Taking as starting point an existing aircraft, attention is focused on how relatively small design changes can influence aircraft behavior, in this case the dynamic response. Thus, the main difference with parametric studies for a completely new configuration is that a validated baseline model is available. By implementing this validated baseline into the MMG, it is expected that high-fidelity derivative models can be built up automatically by simply changing the values of the input

Figure 2: The DEE paradigm

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parameters, leading to an extreme reduction in engineering and computational effort required for dynamic aircraft modeling.

As a first step in this direction, the intention is to integrate the MMG into a DEE dedicated to loads calculation in the preliminary design phase. The MMG will be able to provide the loads analysis tool (VarLoads, see section II) with the models needed to perform dynamic loads analyses, i.e. structural, mass and aerodynamic models at the required level of fidelity. In the present work attention is focused on the description of the structural model extracted from the MMG. The aircraft structural model is built up in an object-oriented way by making use of two so-called High-Level Primitives (HLP) (see section III.A); the wing trunk primitive, already presented in previous works1, 2, used for modeling all the lifting components, and the fuselage trunk primitive, used for fuselage and engine nacelles. As mentioned before, defining HLP in a KBE way and using them like basic blocks to build up any aircraft configuration helps automation in the conceptual design phase, i.e. at concept generation level.

The paper is built up as follows. In section II an overview on VarLoads is given. In section III an overview is given on the structural model coming from the MMG representing a commercial aircraft family. In section IV, the link between the MMG and the different analysis tools required to build up the aeroelastic model is discussed. In section V, conclusions and further developments are treated.

II. Features of the loads analysis tool: VarLoads The Variable Loads environment (VarLoads) 5 can be described as a flexible loads analysis tool intended for

special investigations, with applications ranging from preliminary design to investigation of in-service incidents. To this effect, major characteristics of the tool are:

- Capability to handle aircraft models from variable sources and with variable level of refinement (e.g., 6 DoF model from handbook methods or fully flexible model from FEM).

- An integrated aircraft model and unified simulation environment for maneuver and dynamic response, extendable to flight dynamic and aeroelastic analyses.

- Modular, library based software structure. - Data structures and I/O interfaces consistent with certification loop software.

The above characteristics allow for easy linking with the action boxes into a DEE. Specifically, the structural dynamic aircraft model needs to be constructed from the MMG only once, and can afterwards be used for all steady

Figure 3: VarLoads scheme

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maneuver, dynamic response, and aeroelastic calculations. Furthermore, the consistency with certification loop software allows for easy validation of the parametric model against existing configurations.

VarLoads realizes the classical aeroservoelastic simulation loop in a Matlab/Simulink environment (see Fig. 3). The aeroelastic properties are collected in the aircraft block. Starting from a trimmed initial condition, typically the aircraft is either excited by a change in its environment (e.g., an atmospheric disturbance defined in FAR25), or by some given input (e.g., a checked maneuver defined in FAR25). In addition, outer feedback loops transport the aircraft states (rigid body motion, elastic vibrations) back to the sensor sub-system, where the relevant signals for the flight controls sub-system are selected and transformed. For certain aircraft states additional input from the pilot can come into action as well. Control surface deflections and engine settings are the typical output from the flight control system. These signals are fed through the actuators sub-system, which reflect the dynamic behavior of the actuation systems, back to the aircraft sub-systems, where they are translated into incremental values for the aerodynamic and propulsion forces. Finally, the post-processing sub-system computes the desired dynamic loads, e.g. shear, bending and torsion at pre-defined monitoring stations. The pilot, systems, and controls models are not treated in detail in the present work. It is assumed that sufficiently detailed representations of these sub-systems will generally be available in a parametric manner in preliminary design. The relevant parameters will however be dictated by the aircraft response rather than aircraft geometry and they are therefore not necessarily part of the MMG, assuming that pilot, sensors and control are not coming from the MMG. Nevertheless they may be included in the future.

The basis for the integrated aircraft model that is currently implemented in VarLoads is the equations of motion (EoM) for an aeroelastic flight vehicle as proposed by Waszak and Schmidt6. These equations consist of the non-linear Newton-Euler EoM of the rigid aircraft (as used in flight mechanics), and the linear elastic EoM in modal form (known from structural dynamics). As an additional feature, the number of flexible and rigid DoF of the model can be varied without having to adapt the simulation environment. This implies, for example, that a 3DoF symmetrical manoeuvre simulation and a full flexible dynamic gust response simulation can be carried out using the same underlying aircraft model and software.

The aeroelastic aircraft model is built up from a set of sub-models reflecting the individual disciplines, i.e. structure, mass and aerodynamics (see Fig. 4), needed to carry on an aeroelastic calculation. The structural and mass sub-models are combined in an elastic&inertia model, obtained by the condensation of the full FE structural model. The generation of this FE model from the MMG will be treated in the following. The sub-model reflecting the aerodynamic properties will be derived from the MMG as well. The general approach for preliminary design will be to construct a parametric unsteady panel model, which is corrected for some discrete steady points (angle of attack, sideslip, control surface deflection…) by a higher order CFD method. The aerodynamic modeling will be treated in more detail in future work. For propulsion models, it will be assumed that detailed models are either available or can be scaled from existing engines.

III. The Airbus Conventional Passenger Aircraft parametric Model

The Airbus Conventional Passenger Aircraft Model (ACPAM) presented in this section is the part of the MMG creating the input model needed to feed the structural analysis tool. This model is basically made of geometrical entities that will be translated into structure once the link with the structural analysis tool is set. This is a parametric model based Figure 5: A318, A340 and A380 are shown.

Aerodynamic model

Structural model

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Aerodynamic modelAerodynamic modelAerodynamic model

Structural modelStructural model

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Figure 4: Sub-models combined to have the aeroelastic aircraft model

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on the Knowledge Based Engineering (KBE) design approach. The ACPAM has the capability of modeling the Airbus families of conventional passenger aircraft, i.e. the A318/A319/A320/A321 single aisle family, the A300/A310 wide-body family, the A330/A340 long-range family and the A380 ultra-high capacity family (see Fig. 5), as well as possible future configurations with similar design principles.

The parameters used inside the code to generate the aircraft model are combined in engineering rules that are specific for the mentioned aircraft families. The numerical values needed to build up a specific aircraft model are univocally assigned in an input file, i.e., no parameter values are part of the MMG itself. The input file is a text file where all the parameters are grouped in a user-friendly way, according to the component they belong to (wing, fuselage, etc.) and on the kind of part they refer to (external surface, internal structure, etc.). The input data can either be interactively defined by the user or automatically modified as part of an optimizer inside the DEE.

For building up the ACPAM, an object-oriented approach has been used. As a first step, two so-called High-Level Primitives (HLP) are defined; (i) the wing trunk, and (ii) the fuselage trunk. The HLP are pieces of code that generates the models of the two main blocks needed by the user to represent every aircraft component. The choice of using HLP was taken in order to improve the flexibility and the efficiency of the code. When a library of HLP exists, the user has the capability of choosing and combining them in a preferable way in order to create different configurations. Moreover, this improves the interpretability of the model itself and allows for a high level of consistency within the model.

Every component is built up by making use of the HLP. The wing trunk is the primitive used for all the lifting components; fuselage and engine nacelles are an assembly of fuselage trunks. In order to improve the modularity of the ACPAM, components (wing, fuselage, horizontal tail, etc.) are generated independently. The modules representing the connection between the components are kept in separate blocks even if dependent on the definition of the components themselves (see Fig. 6). One of the advantages of the modularity of the ACPAM is the possibility to include design changes on a singular component, without having to update the complete aircraft model.

In the next sub-sections the HLP, the assembly of HLP into components and the assembly of components into a complete aircraft are described.

A. High-Level Primitives (HLP) 1. Wing trunk primitive

A detailed description of the wing trunk primitive has been presented in Ref.1, 2. The wing trunk is the HLP used for generating all the lifting components. The wing trunk is generated from the set of input parameters given in Fig. 7. Firstly, the root and tip airfoils are positioned. Afterwards, a surface is lofted between these two profiles.

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Figure 6: Aircraft model modularity

Figure 7: The wing trunk primitive

wing trunk

Wing Trunk parameters set- Type of airfoil (from a library)- Amount of airfoils- Positioning of airfoils- Thickness of airfoils- Reference axis- Chord length- Span- Dihedral angle- Sweep angle- Twist angle

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Optionally, intermediate airfoils can be defined in the input file to influence the shape of the trunk. The structure within the wing trunk is characterized by spars and ribs. The spars are created by positioning a spar

line between two points defined as percentage of the chord on the root and tip airfoil. By projecting the spar line upward and downward on the wing trunk surface the spar web is created. Ribs are then generated by positioning a rib reference point on one of the spar lines. The ribs can be positioned in any direction by specifying a rib orientation angle with respect to the flight direction. The rib webs are created from the intersection lines determined from vertical planes positioned at the rib reference points and oriented with the rib orientation angles with the outer surface of the wing trunk.

2. Fuselage trunk primitive

The fuselage trunk has been developed to model all the rotationally closed surfaces like the fuselage and engine nacelles. As for the wing trunk, the fuselage trunk primitive consists of an external surface and an inner structure that is generated inside the surface.

The trunk surface is generated from two sets of curves that are defined along the trunk length (longitudinal curves) and curves representing the cross sections of the trunk (circumferential curves) (see Fig. 8).

The longitudinal curves give a direct control over trunk areas with a possibility to include high gradients, such as the cockpit area. A maximum of four independent longitudinal curves are available; a crown curve, a belly curve and two side curves. These curves are defined by X-, Y- and Z-coordinates of a specified number of points (see Fig. 9). The density of these points is arbitrary in order to minimize computational effort.

The circumferential curves allow for an exact definition of the cross-sectional shape of the trunk surface. In order to assure the matching between the two sets of curves, points from the longitudinal curves are used as a basis for creating the circumferential curves. First, the longitudinal curves are cut with vertical plains perpendicular to the longitudinal axis (X-axis). Then, the four points obtained from these intersections are used as reference points for

building the cross sectional curves as shown in Fig. 10 (step b). The circumferential curves can be defined separately for the lower and the upper part. First, for each part a profile is selected from a library (see Fig. 10 step a). Second, the selected profiles are stretched in order to fit with the four points previously defined (see Fig. 10 step b). At the end, the upper and lower profiles are smoothly connected according to the dimensions that are specified by the user in the input file (see Fig. 10 step c).

Based on the definition of the curves, a three-dimensional convex hull surface is automatically created as a B-Spline surface. Input-parameters are available to

morph the resulting surface. The structural concept of the fuselage trunk consists of frames, stringers, floor panels and skin panels. For the

current use, a medium fidelity model is needed, so the ACPAM only uses the segmented skin as output report. Nevertheless, the three-dimensional shape of frames and stringers are already defined in case a more detailed model is needed.

Frame curves are defined at user specified points on the longitudinal X-axis by intersecting a plane perpendicular to the X-axis with the fuselage-trunk surface (see Fig. 11a).

The user determines the number of stringers that is generated inside the trunk. On the first frame curve of the trunk, points are distributed at equal distances to form the starting points of the stringer curves. The possibility to

Figure 8: Fuselage trunk

FuselageTrunk parameters set

- Shape of cross-section(from a library, upper and lower)

- Amount of cross-sections

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- Shape longitudinal curves(maximum 4 curves)

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Figure 9: Generation of the crown curve

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define a different density distribution of these points is under development. Connecting these points with curves lying on the trunk surface, stringer curves are created (see Fig. 11b).

Regarding the floors, the user can define the amount and the position of them inside the trunk (e.g. double deck configuration). Moreover, the floors are defined independent of other structural elements. However, the frame curves are used to determine the positions where the floors are connected to the fuselage. The intersection of the floor with the trunk surface provides a curve needed for the segmentation of the skin panels. The floor panels are created in between longitudinal and lateral bars. The user has the possibility to determine the number of these bars. The floor curves, the frame curves and the stringer curves form the blueprint of the skin panels on the fuselage trunk surface (see Fig. 11c). The standard skin panels have a quadrangular shape. A special situation can occur where the floor curves intersect the stringer curves. At both sides of the intersection point a triangular panel has to be generated. The ACPAM automatically recognizes this situation and implements triangular panels at the right position.

B. Aircraft components The ACPAM reproduces an aircraft configuration defined by fuselage, two main wings, a horizontal tail, a

vertical tail, two or four engine nacelles and optionally two winglets. All the main components are built up by making use of the HLP previously described (see sec. II.A).

As mentioned before, all the lifting components are obtained by assembling multiple wing trunks. The user has the possibility of defining the number of wing trunks used for each component and the input values for each wing trunk (position, orientation, etc.). The wing trunks are automatically connected to each other in order to obtain a smooth connection surface. The wing trunk structure including spars and ribs is then continued inside the connection parts as well. The user has the capability to specify whether to end the wing with an end-cap or by attaching a winglet. Since both wing and winglet are built making use of the

a) b) c) Figure 11: Fuselage structure: a) frame curves; b) stringer curves; c) skin panels

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Figure 10: Construction of circumferential curves

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wing trunk primitive, it is possible to connect them by using the same routine used for assembling two consecutive wing trunks. The vertical tail is still based on the wing trunk primitive. Comparing with the wing and the horizontal tail, the main difference is in the orientation in the global coordinate system. Because the model is assumed to be symmetric in the x-z plane, some input like shift in y-direction, twist and dihedral angles, are all set to zero by default. Due to the absence of change in the dihedral angle, no connection elements have to be defined between the trunks.

The fuselage is generated using the fuselage trunk primitive. No connection elements are generated in between the fuselage trunks so the user has to make sure that the surface definition of each trunk ensures a proper transition between two trunks. In the front a close surface representing the nose is created. In the rear part the bulkhead is added as well.

For generating the engines the fuselage trunk is used. The options to create the surface and structure are the same as for the fuselage trunk, apart that only the frame definition is possible. Regarding the inner structure, only frames are created because they are needed to connect the engine to the rest of the structure. The user can choose whether to have two or four engines and where to position them.

C. Connections between components Once the main components are available, it is possible to assemble the model of the complete aircraft. In this

section the modules needed to connect the main component between each other are described. As for the other modules first the external surface is built up, and then the inner structure is included.

The geometry of the connection module between fuselage and horizontal tail is depicted in Fig. 12a. First, the connection surface is defined by extending the external surface of the tail to intersect the external surface of the fuselage. Then, the inner structure is created. The inner structure consists of two parts; the center box and the

connection of it with the horizontal tail. The center box is built up like the continuation of the tail structure making use of the wing trunk structure definition; it includes spars, ribs and skin panels, but obviously no leading and trailing edges are included. The connection between the center box and the horizontal tail is created using the structural module used for connecting the wing trunks. This is possible because of the creation of a virtual airfoil close to the fuselage. It includes spars, ribs and upper and lower skin panels; obviously no leading or trailing edges structures are included.

The connection module between the vertical tail and the fuselage is obtained in a similar way. The vertical tail surface is extended to intersect the fuselage surface. The intersection curve and the root airfoil of the vertical tail are used to build up the external surface of the connection. No inner structure exists because the vertical tail is connected to the fuselage by six connection bolts. Additionally a rib is included to provide sufficient stiffness (see Fig. 12b).

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Figure 12: a) Connection fuselage-horizontal tail; b) Connection fuselage-vertical tail; c) Connection wing-engine

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The connection between the engines and the wings consists of a set of boxes that are connected to the engine structure on the one side and the wing structure on the other side. The box at the engine side has a user-defined width and height while its length is equal to the distance between the first and last frame of the engine structure. The box at the wing side has the length of the wing rib in between two spars. The user directly determines the height and the width. The two boxes (at the wing and the engine side) are connected with a third box that accounts for changes in size of both boxes as well as to fill the gap in X-direction (see Fig. 12c).

IV. Linking the MMG with analysis tools

A. Linking with structural analysis tools The ACPAM is built up in order to provide sub-models needed to feed the different discipline analysis tools. In this section the link with the structural analysis tools is described. It has to be mentioned that no attempt was made to code any FEM package directly inside the model. This enables the discipline expert to choose the tool he feels more comfortable with.

As first step, the geometric model of the aircraft (including external surfaces and internal structure) needs to be exported from the ACPAM. This model is completely represented with shell elements. This is done using routines available in ICAD that are able to write output reports in Initial Graphics Exchange Standard (IGES) format. This format is usually used for storing information about drawings, three-dimensional models and surface models in a computer readable form. The IGES format is designed to be independent of all CAD systems and it is readable by the most used analysis tools. Prior to meshing, the structural surfaces (spars, ribs and skins) need to be split along their intersections. This is a typical operation that the Patran operator has to perform and it is usually very time consuming. An automatic procedure has been programmed in the ACPAM in order to perform that. In this case the MMG interprets the structural items configuration of the aircraft model and automatically fragment skins and webs in lots of mesh-able surface patches. Spars are split along the intersections with the ribs; skin panels are split in patches along the intersections both with ribs and spars. When a surface patch is generated in ICAD, a special identification tag is applied. A special MMG module performs the scanning of the product tree, collects all the surface patches and distributes them in a predefined set of IGES files, ready to be imported in the FE environment. In order to fully describe finite element models non-geometrical information, i.e. material and elements properties, are also needed. A specific ICAD application is incorporated inside the MMG in order to transfer this additional information about the finite elements, i.e. the FEM-table system. A set of look-up tables is automatically output for each geometric patch with all the useful information generated by the MMG and transcribed in a properly formatted text file (the FEM-table). This information includes thickness, material and design variable area specification, non-structural masses connectivity, plus other generic element features such as ‘mesh-ability’, number of edges, co-ordinates of the corner nodes, etc. One FEM-table is generated for each IGES file. A proper routine has been developed to map the contents of the FEM-tables, directly into the Patran database. The co-ordinates of the nodes of each surface patch are the bi-univocal link between each surface coded in the IGES

Figure 13: Link with the structural and aerodynamic analysis tools

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file and its representation in the Patran environment. A Patran session file has been programmed in Patran Command Language (PCL) to read automatically the set of IGES files, generate mesh, apply constrains and properties and run the FEM analysis and/or the optimization.

B. Linking with aerodynamic analysis tools In this sub-section the link with the aerodynamic analysis tools is described. As for the structural link, no attempt was made to include any aerodynamic calculation package directly into the ACPAM. Depending on the kind of aerodynamic calculation the user intends to perform, models with a different fidelity level can be exported from the ACPAM. One possibility is to extract information about 2D flat panels representing the plan form of the aircraft components (see Fig. 13). These flat panels are already included in the ACPAM model. In this case a routine is available to transform the geometric information about these panels into standard Nastran input format (CAERO cards) for performing aeroelastic analysis. This means performing an unsteady Doublet Lattice calculation in the frequency domain based on acceleration potential. Consequently a rational function approximation will be carried out to get a time domain representation in order feed VarLoads (developed in time-domain) with the convenient input. Making use of the same 2D panels another routine has been developed in Matlab environment in order to read the Nastran CAERO cards and to perform a Vortex Lattice calculation for the steady case. Another possibility is to export from the ACPAM more refined 3D models, as for example clouds of points representing the wet surface of the 3D structure. These points are ordered with a certain criteria in groups of four for representing the four vertices of the aerodynamic patches. The number of patches (i.e. the amount of points) can be controlled as input into the ACPAM. Patches generated in this way are used as input for Computational Fluid Dynamic (CFD) tools. Moreover they can also be linked with a Matlab tool performing steady Doublet Lattice calculation based on velocity potential7.

The link with the aerodynamic analysis tools will be treated in more detail in future work.

V. Conclusion and outlook

In the present work, a knowledge-based parametric Multi-Model Generator (MMG) denoted Airbus Conventional Passenger Aircraft Model (ACPAM) was presented. The ACPAM has the capability to represent conventional passenger aircraft families, and to provide models at different fidelity level needed to feed the disciplines tools required for aeroelastic calculations (i.e. structural, mass and aerodynamic models). The ICAD environment has been selected for the development of the ACPAM. An object-oriented philosophy was used. This is proved by the fact that two so-called High-Level Primitives (HLP), i.e. the wing trunk and the fuselage trunk, are the basic elements to build up every component of the aircraft. This choice was made in order to improve the interpretability of the code and the consistency of the generated models. Moreover, in order to improve the modularity of the model, components are generated independent of each other and are stored in separate modules. This is computationally efficient in case of investigations on aircraft derivatives in which design changes involve one singular component. The present work was focused on the description of the structural sub-model and the interface with a selected structural analysis tool (Patran/Nastran) was described.

The model built up from the selected HLP is a shell model of medium fidelity level, i.e. not every single structural detail is reflected. In ongoing work, it will be investigated whether the adopted fidelity level is sufficient for loads calculation in preliminary design. Future developments will involve including the ACPAM into the Design and Engineering Engine (DEE) for performing loads calculation in the preliminary design phase. To this effect, the MMG will be linked with a load and aeroelastics analysis module5, a parametric mass estimation module, and a sizing tool. The overall process will be validated on existing baseline aircraft at first. It is expected that the parametric model can afterwards be used to analyze the effects on loads and aeroelastics of derivative configurations that are characterized by the same engineering design rules.

A flow diagram is presented in Fig.13 in order to show the position and the utilization of the ACPAM into the dedicated DEE. In this diagram, attention is only focused on the interface between the structural and mass sub-models of the MMG on the one hand, and the loads calculation process on the other hand. The loads and aeroelastics tool is based on an integrated aeroelastic and flight mechanics formulation6, and requires as input, among others, a condensed model of the structural and mass properties. The model condensation is required in order to reduce the computation time required for the dynamic simulation.

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The design problem (e.g., a fuselage extension, an enlarged winglet or a V-tail) prescribes the input parameter values needed by the ACPAM. The parametric model is adapted for the components under consideration and, for reproducing the structural model, the ACPAM produces two kinds of output. Firstly, the files containing model surfaces description in IGES format are exported. Secondly, the ACPAM produces the FEM tables including the element properties required to transform the geometrical shells into physical elements. The combination of surface definition and FEM tables is the input for the structural analysis tool.

In addition to the structural model, the mass model is needed in order to perform dynamic analysis. The mass model of an aircraft includes two kinds of masses, i.e. structural and non-structural masses. An estimation of the structural masses can be made using the structural model coming from the ACPAM by including density information in the material properties contained in the FEM tables. Under investigation at the TU Delft is whether this estimation is correct. Non-structural masses depend on many things, e.g. fuselage set, fuel consumption, systems positioning. Future development will be to include them directly into the ACPAM, using a parametric representation. As mentioned, in loads calculation condensed models are used. This is necessary in order to save computational time, because of the large number of cases that has to be investigated. The last step is therefore to automatically condense the complete shell FEM. This will be the subject of further research. In addition to a static condensation within NASTRAN, a dynamic condensation (modal truncation) will be carried out.

As mentioned before (see sec. II), additional models are also needed to feed VarLoads. Regarding the pilot and systems sub-models, it is assumed that sufficiently detailed representations are already available. Nevertheless, provided that such models can be adequately parameterized, the future goal will be to include them in the ACPAM. Regarding the aerodynamic model, a rough description of the undergoing research and developments was given in sec. IV.E. Further details will be presented in future work.

Figure 14: Creation and use of the elastic&inertia sub-model

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571 7van Staveren, W.H.J.J., “Analyses of Aircraft Responses to Atmospheric Turbulence,” Ph.D. Dissertation, Aerospace Dept.,

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