Class Objectives. Identify Internal Loads group functions. Understand total airplane FEM process. Design and Analysis of Aircraft Structures 9-2

Transcription

1 Internal Loads

2 Class Objectives Identify Internal Loads group functions Understand total airplane FEM process Apply basic modeling concepts Design and Analysis of Aircraft Structures 9-2

3 Internal Loads Group Has a Crucial Role in Airplane Design Develops integrated Finite Element Model (FEM) Supports Stress group with modeling expertise Internal loads data is critical to airplane schedule Coordinates between stress group and external loads group Documents internal load results (needed for life of airplane) Design and Analysis of Aircraft Structures 9-3

4 The Internal Loads Group Brings the External Loads Inside Produces internal loads for various stress groups Generates stiffness data for external loads group Develops and maintains major finite element models used for internal loads analysis in support of an airplane certification process Coordinates between external loads and stress groups Design and Analysis of Aircraft Structures 9-4

5 Internal Loads Structural analyst engineer Sizes structure Verifies that model represents actual structure Finite element model (FEM) FEM engineer Acts as a go-between Builds models Understands models Understands theory Design and Analysis of Aircraft Structures 9-5

6 Agenda Integrated t FEM Support Technical Background Validation Design and Analysis of Aircraft Structures 9-6

7 Complete Integrated Finite Element Model (FEM) Design and Analysis of Aircraft Structures 9-7

8 Integrated FEM Contains Enough Detail to Accurately Describe the Structural Behavior Design and Analysis of Aircraft Structures 9-8

9 Integrated FEM Includes the Major Structural Elements Skins Stringers Frames Ribs Floor beams Load-carrying doors Sills Bulkheads Pressure deck Keel beam Pickle forks Wheel wells Longerons Window belt Door cutouts Seat tracks *Landing gear *Nacelles/struts *Not used Design for stress and analysis. Analysis of Aircraft Structures 9-9

10 The Integrated FEM Does Not Use Detailed Models For Components Leading and trailing edges of both wing and empennage Control Surfaces Plug-type doors Fairings Design and Analysis of Aircraft Structures 9-10

11 Wing Models Use Simple Concepts Vent stringers Skins: modeled with membranes Stringers: modeled with bars and shears to create fixed and free flanges Design and Analysis of Aircraft Structures 9-11

12 Ribs Are Modeled With Bars and Shears Design and Analysis of Aircraft Structures 9-12

13 Body Models Have More Variety Skins: modeled with membranes Stringers: have bending inertia (beams) Window belt is included (anisotropic properties) Design and Analysis of Aircraft Structures 9-13

14 Frames and Floor Beams Use Bars and Shears Web Chord Web Stanchion (beam) Outer chord Fail-safe chord Inner chord 1 Design and Analysis of Aircraft Structures 9-14

15 Bulkheads Use Beams and Shears Aft wheel well bulkhead Center section wing box Ring chord (beam elements) Webs (shear elements) Stiffeners (beam element) Up Outboard Forward BL0 Design and Analysis of Aircraft Structures 9-15

16 FEM Sizing Comes From a Variety of Sources Wing and section 41 groups provide complete sized models. Some models are converted from other codes (e.g., landing gear: ATLAS) Most body sizing i comes from stress group s Oracle database APARD (Analysis PARameter Database). Other body sizing comes on paper or in Excel (e.g., floor beams, keel beam, door sills). Design and Analysis of Aircraft Structures 9-16

17 The Integrated FEM IS a Collection of Several Individual Models Vertical fin Horizontal stabilizer Section 48 Forward cargo door Section 46 Section 45 Center section Outboard wing Raked tip Section 43 Main gear Nose gear Section ER Engine jig Design and Analysis of Aircraft Structures 9-17

18 The Model Is Subject to Many Types of Load Conditions Ultimate and fatigue: Loads due to flight maneuver, gusts, ground maneuver, and landing Floor/frame: loads due to such items as seats, lavs, galleys, and cargo. Pressure: loads due to cabin pressure and sudden decompression (13.65 psi internal cabin pressure is added to all flight cases). Miscellaneous: loads due to such conditions as tire burst or center section fuel slosh. Design and Analysis of Aircraft Structures 9-18

19 Each FEM Scenario Causes the Engineer s Workload to Multiply Load-carrying doors (door-in and door-out) Main landing gear (up and down) Fail-safe conditions (limit load) Discrete-source damage conditions (70% of limit load) Design and Analysis of Aircraft Structures 9-19

20 Results From the Model Are Used in Several Ways Structures workstation Stresses and internal loads are postprocessed to calculate margins of safety Deflections are provided to the control surface groups (e.g., flaps) and systems groups Stiffnesses (EI/GJ) provided to loads and flutter groups Super-elements (reduced stiffness and loads) given to stress groups to iterate the component FEM s Deflections are sometimes provided d to solve manufacturing problems Design and Analysis of Aircraft Structures 9-20

21 The Integrated FEM Plays a Key Role in the Overall Design Process Wind tunnel data Pressure distribution Stability and control data Stiffness update External loads Mass distribution update Balanced forces Internal loads model Stresses or internal loads Post-processing Sizing Update detail drawings Design and Analysis of Aircraft Structures 9-21

22 Internal Load Schedule is Critical and Highly Visible Major Milestones Configure/ Requirements Loads Commitments & Compliance Product Definition Product Release Fab and Assembly (Ref) Lab Test Qrt 1 Qrt 2 Qrt 3 Qrt 4 Qrt 1 Qrt 2 Qrt 3 Qrt 4 Qrt 1 Qrt 2 Qrt 3 Qrt 4 Qrt 1Qrt 2 Qrt 3 Qrt 4 Qrt 1 Manag. Config. Plan Complete Config. Memo Release Start Loads Firm Config. Firm Structures Config. 25% Drawing Release Firm Config. 90% Drawing Release Firm Structures Config. for Loads Prelim Internal Loads Comp. Design Internal Loads Comp. Start Roll Major -Out Program Plans Complete IDAS Compl (Parts, Plans, Tools and CSD Negotiations Start Structures W/S and Parts D-E Negotiations IPT Description of Change Comp. Firm Systems, Payloads and Propulsion I/F to Structures Wheels and Tires SCD, Initial EAMR Release (MLG) 1st Machine Print (Fixed LE) All parts in EPIC First Flight MLG OD Lab Test Plans Start MLG Fatigue Test Valid of Sys. Funct/Integ in Labs Complete Cert/ETOPS Approval 1st Delivery Airplane Test Certification Application to FAA/JAA Ground/Flight Test Plans First Flight Flight Test Comp. Preliminary Type Board Final Cert. Plan w/faa/jaa to FAA Cert. Basis Closed Cert. Plans Approved Start Ground Test 95% Compliance Doc. Submitted Cert./ETOPS Approval Design and Analysis of Aircraft Structures 9-22

23 Why Use the Integrated FEM? Pros Serves as a means of uniting disparate groups Consistency of idealization and analysis Preserves lessons learned from previous programs Easier to find errors (debug) Better interface loads Cons Stress group is somewhat dependent on FEM results Requires coordination Idealization disagreements Culture clashes 767 versus 777 SAMECS versus ELFINI Design and Analysis of Aircraft Structures 9-23

24 Use of the Finite Element Method Is Diverse Typical applications Structural modeling (static, dynamic, and weight analysis) Preliminary-design airframe stress Airplane wing/body junction Detailed internal loads Crack growth and residual strength Nonlinear geometry Propulsion/structures integration Structure/acoustic interaction Bird/blade impact Controlled airplane crash Design and Analysis of Aircraft Structures 9-24

25 Guidelines for Modeling Use simple elements. Use simple modeling concepts. Keep the model size small. Spend time to verify/validate/check out the model. Design and Analysis of Aircraft Structures 9-25

26 Integrated FEM Summary A collection of separate models (similar in detail and idealization) Contains the major structural details A cooperative effort (internal loads, external loads, and stress groups) Subject to many demands Many load cases Many scenarios Many groups use the results Critical item in the program schedule Design and Analysis of Aircraft Structures 9-26

27 Agenda Integrated t FEM Support Technical Background Validation Design and Analysis of Aircraft Structures 9-27

28 Study Models are Built to Support Stress Groups Validation Model Design and Analysis of Aircraft Structures 9-28

29 Study Models Are Built to Support Stress Groups ER Frame Idealization Study Model Design and Analysis of Aircraft Structures 9-29

30 Internal Loads Group Builds Detailed Stress Models for Analysis and Verification Design 777 and Drag Analysis Brace of Aircraft Fitting Structures 9-30

31 737-X Overwing Escape Hatch Cutout Stress group worried about fatigue interactions between corners of the three cutouts Total weight savings of 10 lb per airplane Design and Analysis of Aircraft Structures 9-31

32 737NG Rear Spar Pickle Fork Model used to analyze bolt loads Design and Analysis of Aircraft Structures 9-32

33 Internal Loads Group Builds Hybrid Models Overwing Door Goal was to find optimum contour for corners of door cutout Optimizer was used to minimize weight Skins are 1-inch thick in this area Design and Analysis of Aircraft Structures 9-33

34 Internal Loads Group Loaned Engineers to Stress Group for ER Main Landing Gear Analysis Coarse model Coarse model with fine-meshed upper torque link Design and Analysis of Aircraft Structures 9-34

35 Internal Loads Group Supports External Loads and Flutter Groups Creates finite element models Provides stiffness data ER flutter FEM ER external loads FEM Design and Analysis of Aircraft Structures 9-35

36 Support Summary Internal loads builds detailed stress models for analysis and verification. Hybrid models are built to be more detailed than the regular internal loads model. Study models investigate i software or idealization i changes. Internal loads engineers are sometimes loaned to other groups to assist with modeling efforts. Internal loads group supports flutter and external loads with FEM data. Design and Analysis of Aircraft Structures 9-36

37 Agenda Integrated t FEM Support Technical Background Validation Design and Analysis of Aircraft Structures 9-37

38 Textbook Definition: What Are Internal Loads? Forces and Moments Carried by the Structure of the Aircraft Examples Axial force in a fuselage stringer Shear flow in a bulkhead Hoop load in a fuselage skin panel Segment load (skin plus stringer) in a wing Design and Analysis of Aircraft Structures 9-38

39 Simple, Easily Understandable Elements, and Properties Are Used Internal Loads Model Spring Bar Beam Shear* Solid Models 10-noded tetrahedron 8-noded brick 20-noded brick Membrane* Bending Plate* * Properties for shear/membrane/bending elements Isotropic More common Composite Honeycomb Sandwich Less Common Design and Analysis of Aircraft Structures 9-39

40 Springs Easy to understand F = K x Control direction of load Allow for easy, quick check of load path Used to attach pieces of major structure in the FEM For very stiff elements set K= Use 3 translational and 3 rotational stiffnesses at each node Design and Analysis of Aircraft Structures 9-40

41 Bars Area Easy to understand Allow for quick check of load path Used for modeling of Fuselage stringers Built-up structure ( chord-web-chord ) The only variable is the area Design and Analysis of Aircraft Structures 9-41

42 Beam Torsional inertia Bending inertia Bending inertia Area Advanced features: Hinges (releases) at each end Variable area Variable inertia (3 per beam) Variable shear area Design and Analysis of Aircraft Structures 9-42

43 Shear, Membrane and Bending Plate Shear Membrane Bending plate Carries shear force only Adds axial capability Adds bending capability Design and Analysis of Aircraft Structures 9-43

44 Solid Elements 10-noded tetrahedron 8-noded brick 20-noded brick Design and Analysis of Aircraft Structures 9-44

45 What Is a Degree of Freedom (DOF)? A node can have 6 structural degrees of freedom 3 translations relative to an axis system 3 rotations about an axis system Design and Analysis of Aircraft Structures 9-45

46 Degrees of Freedom are Determined by Element Properties Example: Beam Without torsional inertia With torsional inertia 10 DOF 12 DOF All elements attached to a given node can potentially affect the degrees of freedom at that node Design and Analysis of Aircraft Structures 9-46

47 Finite Element Method Stress/ strain relationship (spring f = Kx) Each element contributes to the overall stiffness of the model Write stiffness matrix for each element Assemble global stiffness matrix Invert global stiffness matrix Back-substitute to obtain displacements Design and Analysis of Aircraft Structures 9-47

48 What Software Tools Do Internal Loads Use to Create an Integrated FEM? Preprocessor: CATIA Solver: CATIA-ELFINI (batch, not interactive) Post-processor: CATIA-ELFINI (interactive) <or> Create a large ASCII text file to transfer to other codes Design and Analysis of Aircraft Structures 9-48

49 How Are Loads Applied to the Integrated FEM? ELFINI aeroelastic using detailed model ( TRLOAD ) ELFINI aeroelastic using coarse model ( U-CONNECT ) Using point loads (Unit loads and factors) Design and Analysis of Aircraft Structures 9-49

50 ELFINI Aeroelastic Using Detailed Model Direct node-tonode transfer External loads FEM Internal loads FEM External loads calculated using a model that is 95% common with the internal loads model CATIA-ELFINI TRLOAD function used to transfer node loads Used on 737NG Design and Analysis of Aircraft Structures 9-50

51 ELFINI Aeroelastic Using Coarse Model External loads FEM Spreading algorithm must be used U-CONNECT Internal loads FEM External loads calculated on coarse model Stiffness approximates that of the internal loads model CATIA-ELFINI U-CONNECT function used to transfer node loads Used on X & X Design and Analysis of Aircraft Structures 9-51

52 Point Loads Using Detailed Model Hundreds of unit load cases created (represent airload, fuel, cargo, OEW, etc.) External loads group provides factors Unit cases factored and added together to create each final case on the integrated FEM Used on ER Labor intensive Internal loads FEM Design and Analysis of Aircraft Structures 9-52

53 Many Programs Can read Results From the FEM MARGIN: Wing stress FEADMS: Moss/Duberg: FAMOSS: POST-ELF: Oracle database for body structures body skin and stringer sizing body frames Wichita IDTAS: Fatigue Plus other IAS and Excel applications Design and Analysis of Aircraft Structures 9-53

54 Boeing Uses a Variety of Finite Element Codes CATIA-ELFINI: Internal loads, stress, flutter SAMECS: legacy internal loads ATLAS: weights, flutter, landing gear, dynamic loads, and legacy internal loads NASTRAN: legacy internal loads, stress, PSD, vibration ANSYS: S stress, landing gear, systems ALGOR: systems, stress ABAQUS: advanced nonlinear code Design and Analysis of Aircraft Structures 9-54

55 CATIA-ELFINI Has Many Differences Compare to Other Finite Element Codes Uses a history file (cutting, copying, and pasting blocks of commands) Uses topological meshing (10, 1, 4, 1, instead of 1001) Integrated into CATIA (same place as geometry) Sub-structuring t and super-elements are very advanced d Load and displacement transfer between meshes is easy Limited non-linear capability Design and Analysis of Aircraft Structures 9-55

56 Super-Elements Add Flexibility to the Overall Process Incorporate sub-structuring Individual id sections can rerun on their own Useful for fail-safe and discrete-source damage runs Design and Analysis of Aircraft Structures 9-56

57 Example of Super-Elements, Step 1 A Mesh Never changes Mesh B Changes often Load { Coincident To solve (without super-elements): 1. Join Mesh A with Mesh B. 2. Assemble global stiffness matrix [K]. 3. Invert global stiffness matrix [K]. 4. Back-substitute to get global displacements {U}. Note: Each time B changes, must re-solve for A. Design and Analysis of Aircraft Structures 9-57

58 Example of Super-Elements, Step 2 B Super-elements representing stiffness of mesh A Load Coincident{ To solve (using super-elements for Mesh A ): 1. Create super-element for Mesh A (reduce loads and stiffness at boundary with Mesh B ). 2. Join Mesh B with super-element that represents Mesh A. 3. Assemble global stiffness matrix [K]. 4. Invert global stiffness matrix [K]. 5. Back-substitute in Mesh B and to boundary of Mesh A. Design and Analysis of Aircraft Structures 9-58

59 Technical Background Summary Simple, easily understood elements and properties p contribute to overall stiffness of the model. Internal Loads group uses CATIA-ELFINI to create integrated FEM. External loads are applied to the integrated FEM, using one of three methods. Many Boeing software programs use results from the FEM Other finite element codes are in use throughout Boeing Design and Analysis of Aircraft Structures 9-59

60 Agenda Integrated t FEM Support Technical Background Validation Design and Analysis of Aircraft Structures 9-60

61 Static Test Condition Maximum Positive Wing Bending - 110% Limit Load Tip deflection: Test: Analysis: inches inches Vertical deflection (in) Side of body Nacelle ETA station Analysis -Test Design and Analysis of Aircraft Structures 9-61

62 Summary Internal loads group develops the integrated finite element model (FEM) Internal loads group coordinates the overall modeling effort Internal loads group supports other groups by building models and providing data to the flutter, stress, and external loads groups The FEM uses simple, easily understood elements and properties FEM results correlate well with static test Design and Analysis of Aircraft Structures 9-62

63 Internal Loads Successes Established many current processes 737-X Used ELFINI Aeroelastic for external loads ER Preliminary cycle made shorter Lessons Leaned x X/300X Two entire loads releases with composite properties in the frame webs Program canceled just prior to release of internal loads Software change did not go smoothly Design and Analysis of Aircraft Structures 9-63

64 Why Work in the Internal Loads? Get to see entire airplane Lots of variety Lots of exposure to other groups Trips to Wichita (future: maybe Long Beach) Recognition lunches with upper management (in the cafeteria) Design and Analysis of Aircraft Structures 9-64

65 Can You Trace the Internal Load Paths? Design and Analysis of Aircraft Structures 9-65