Paper # Application of MSC.Nastran for Airframe Structure Certification Sven Schmeier

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1 Paper # Application of MSC.Nastran for Airframe Structure Certification Sven Schmeier Fairchild Dornier GmbH P.O. Box 1103 Oberpfaffenhofen Airfield D Wessling Germany sven.schmeier@faidor.de Abstract This presentation is to discuss the use of MSC.Nastran and MSC.Patran, applied to the certification process of the Fairchild Dornier 328JET wing. The process of modeling the primary wing structure using this software package will be demonstrated, particularly the comparison between FEM and static test data will be closely examined (SOL 101). Extracted from the complete wing model, a more detailed Submodel of a rib-bay will serve as a means of investigating a modified wing-joint area. The aim is to satisfy JAA certification requirements without having to perform any further tests of the modified wing structure. The presentation will also illustrate the use of these models in everyday Liaison Engineering. Special attention will be drawn to Patran s possibilities in Postprocessing visualization. All the encountered findings will be critically examined and should provide helpful advice to fellow users. 1/1

1. Introduction When the German-American aircraft manufacturer Fairchild Dornier decided to turn its DO328 turboprop into a jet-engine aircraft, certification procedures required a full-scale static

2 1. Introduction When the German-American aircraft manufacturer Fairchild Dornier decided to turn its DO328 turboprop into a jet-engine aircraft, certification procedures required a full-scale static test on the entire wing box. This 328JET basic version is known as the As soon as customers demanded an increase in take-off weight by around 500kg, the wing box structure had to be modified. Thicknesses of panels and spars were enlarged, and stringer dimensions were adapted to account for higher loads. These and other changes required a whole new type certification through JAA authorities of the so-called MOD10. It was decided to have the wing modeled and analyzed under MSC.Patran and MSC.Nastran to show stresses to be below ultimate and stability criteria to be acceptable. If this approach found JAA approval, it would save time and cost inherent to a full-scale static test. Fig Demonstrator Aircraft 2. Problem Definition The basic certification philosophy for the increased take-off weight version consists of three steps which will produce three different FE models (see fig. 2.1): Creation of a global wing model, that resembles the basic properties. This model is to be validated by showing an overall good correlation of its behaviour with the fullscale static test. 2/2

3 Modification of that model to meet the new MOD10 properties. The Principle of Similarity to the baseline version is to be appplied. Creation of a very detailed outer-to-inner wing-joint sub-model, which will serve as a means for justifying the new joint design. Baseline FEM global wing Validation Baseline Static Test Change properties Principle of Similarity MOD10 FEM global wing Extract MOD10 FEM Submodell of wing joint Fig 2.1 Certification Process 2.1 FEM Requirements The global baseline wing model has to fulfill the following requirements: Has to include all parts that contribute to the wing s stiffness and behaviour. Has to be detailed enough for comparison of strain and displacement values with the actual static test. Has to be as coarse as possible to reduce modelling effort and CPU time. As the thickness modifications for the MOD10 have proceeded steadily in the same ratio as the load increase, the overall distribution of stiffness and therefore the behaviour of the wing structure is common to both versions. 3/3

differs significantly from the previous design, a detailed analysis of this highly loaded area becomes inevident.

4 Fig 2.2 Overview of complete left hand Wing FE Since the skin joint of the MOD 10 version, where outer and inner wing box meet, differs significantly from the previous design, a detailed analysis of this highly loaded area becomes inevident. This investigation will require a fine mesh which is not necessary for the global wing model. Consequently an FEM technique known as sub-modelling is to be used on a portion of the global model to analyze the joint. Fig 2.3 Rib 11 Sub-model 4/4

5 3. Analysis 3.1 Global Wing Model Due to nearly symmetric conditions only one half (left-hand side) of the wing structure has to be modelled, including all relevant structural parts and control surfaces: Wing box (upper and lower panels, spars and ribs) Landing flap with actuator and bearing arms Aileron and bearing arms Bridge (engine pylon attachment structure) Leading edge Wing-to-fuselage attachments Flight- and Groundspoilers will be left out due to their negligible effect on stiffness and their absence during the actual static testing. Where rivet or bold forces are desired for result review, coupling of the nodal Degrees of Freedom (DOFs) is introduced by means of RBE2 multi-point constraints: pin points of the flap bearing arms, fastener connections at the rib-to-panel attachments and the rivets in the bridge structure. That way forces and moments in these coupled nodes will be available during postprocessing. The bridge structure is of special interest and hence modelled with a relatively fine mesh of QUAD4 and TRIA3 shell elements, whereas for the remainder of the wing QUAD8 and TRIA6 shell elements with a slightly coarser mesh are preferred. They will provide better accuracy in areas of high curvature due to their quadratic shape function. Fig 3.1 Modelled Rib Structure Importing CATIA surfaces through CATXPRESS provides an outer loft that the shell elements can be projected onto. 5/5

6 All struts, leading edge ribs and spar stiffeners are modelled as line elements (rods and beams respectively). Fig 3.2 Rod and Beam Elements This causes the final model to be reasonably sized with about DOFs. For the wing box mid-plane nodes (y=0) symmetric boundary conditions are applied: T2=R1=R3=0 Although the actual y-strut is located slightly away from the mid-plane, the accuracy will be satisfactory with the y-strut being left out of the idealization completely. For overall positioning and analysis the global wing coordinate system, which can also be found on drawings, is used. Later into the project numerous additional coordinate systems will be introduced with respect to control surfaces and parts. 3.2 Load Introduction Supplied loads are wing section loads for relevant flight configurations applied to the elastic axis. They are acquired through 2D beam calculations, representing the entire wing, including the flap. For the 3D FEM-model these wing section loads must be applied to a virtual elastic axis in the spar box center. Hence, at each rib location RBE3 MPCs have to be introduced to distribute these forces and moments (see fig. 3.3). Only for the static test comparison, the relevant loadcase is introduced the same way as on the test rig, using only every other rib location and pulling up on lower panel contour boards (MPCs attached to all lower panel nodes at that rib location). The air pressure loads acting on the flap are reduced to four separate single forces, that will act on attachment ribs (again through RBE3 MPCs). But since these flap loads are also included in the wing cross-section loads, they have to be subtracted from the wing loads, ensuring they will not be applied twice. This is accomplished by another set of MPCs 6/6

7 connecting a negative force on a co-incident node in the flap s elastic axis with the wing box as also shown in fig. 3.3, thus taking them out again. Fig 3.3 Load Introduction into wing box and flap Certain loadcases require a flap deployment, which is easily achievable by rotating the flap around its hinge axis. For this task the utility FEM-Nodes / Enhanced Node Translation provides a fast and easy way, ensuring that all elements keep their associated properties. Furthermore engine pylon loads need to be superimposed and therefore added as external forces. Since they are also implemented in the supplied cross sectional wing loads, the same MPC method is used here as for the flap. As a result the wing model is loaded with a combination of section loads, flap loads and pylon loads, which are statically equivalent to the supplied wing section loads. 3.3 Global MOD10 Wing Model The MOD10 version of the wing FEM is created by modifying a copy of the baseline wing FE. Panel and Spar thicknesses are adapted to new properties and the new stringer heights and run-outs now found on the wing joint are modelled. Other than that the basic geometry hasn t changed. The load introduction philosophy remains untouched; only loads are changed to suit the new figures. 3.4 Rib 11 Sub-Model The global model serves well to reflect the overall stiffness even in the outer-to-inner wing joint area, but as this joint has been modified significantly, it becomes necessary to examine the new stress distribution in a special model. Six rib-bays (rib 7 to 13) are cut out of the wing model, enhanced in detail and remeshed using QUAD4 and TRIA3 elements. The DOFs could be limited to about /7

8 The boundary conditions for this sub-model (all six DOFs) are acquired from the global model s results as displacements and applied to all nodes making up the boundary ribs,. This produces a free-body balanced sub-model for the analysis of the skin joint at rib 11. All models are to be analyzed linear statically using SOL Discussion/Results Having run all relevant loadcases with MSC.Nastran 70.6, all models went through global force balance summation and checked good. Validation of the baseline model is performed in the following way: 1. Wing deflection checks good in comparison with test results (see fig 4.1). Rib 3, Z-struts Rib 11 Rib 17 Rib 23 δz d z FS test FS fem RS test RS fem Rib stations Fig 4.1 Wing deflection Front and Rear Spar: Test data vs. FEM 2. Stresses/Strains measured through strain gauges during the static test are similar to those in the FE-model (fig 4.2 works as an example). Deviations occur where local effects come in, for example a strain gauge position near a load introduction area. WSQUER is an in-house analytical tool for multi-cell calculation, providing stress distribution in these sections. FEM, test and WSQUER results show an overall good correlation. In high stress gradient areas the FEM extracted value depends pretty much on the exact positioning in the model as well as on the real wing. Plus local effects can cause some values to differ significantly, like access covers and their equivalent thickness approximation in the model. 8/8

9 Rib 3 Rib sy Refuelling Cover FEM WSQUER TEST Access Cover locations Position Bays Fig Upper Panel Center Stresses Since the highly loaded wing joint area is of special interest, all relevant parts were checked for their stress levels (fig 4.3) and found to be below the ultimate load level. The detailed modelling enables one to use the MPC connections as force transfering fasteners. MSC.Patran s postprocessing abilities allow for visualization of the magnitude and direction of resulting forces as shown in fig 4.4. Fig 4.3 Upper inner wing joint 9/9

stresses are used as a buckling analysis input and can prove the safety margin

10 Fig 4.4 Resulting MPC forces (= rivet forces) in the upper outer joint Forces and stresses are used as a buckling analysis input and can prove the safety margin for the redesigned wing joint to have increased by up to 33% in comparison to the previous setup. Fig 4.5 σ y stress distribution in inner strap and lower panel 10/10

11 5. Conclusions The FE-models have provided a reliable source for stress distribution and deflections of several loadcases and wing configurations. The Principle of similarity found JAA approval and thus saved time and cost during certification procedures of the MOD10 wing. No actual static test had to be performed. As a further benefit all models come in handy for use in everyday liaison engineering, as these days they suppply a means for: Check for stress distribution in affected areas Local modifications of the model to evaluate effects of discrepancies, that occur in production and lead to concessions Additions of other structural parts (e.g. Ground Spoilers) Source for wing deflection data that may be applied to separate models of control surfaces In addition these models have served for fatigue analyses, collecting data that will then be run through internal fatigue programs. They are also scheduled to aid in future projects, be it enhancements, modifications, optimizations or redesign studies. 6. Acknowledgements Frank Alberstadt EF5 Modification of basic 300 model to MOD10 version Wolfgang Kurz EF5 Load distribution and various enhancements Hans Grooteman EF5 Comparison of static test with model Marco Theiss ME1 FE-Modell of the landing flap 7. References [1] MSC.Nastran, Version 70.6, MSC.Software Coproration, 1999 [2] MSC.Patran, Version 8.5, MSC.Software Corporation, /11

Applications of structural optimisation to AIRBUS A380 powerplant configuration and pylon design

Applications of structural optimisation to AIRBUS A380 powerplant configuration and pylon design ABSTRACT 2001-122 Stéphane GRIHON AIRBUS 316, Route de Bayonne 31060 Toulouse CEDEX France stephane.grihon@airbus.aeromatra.com