Internal LoadsInternal Loads - Montana State · PDF fileDesign and Analysis of Aircraft ......

Internal LoadsInternal Loads

Class Objectives

• Identify Internal Loads group functions

• Understand total airplane FEM process

• Apply basic modeling concepts• Apply basic modeling concepts

Design and Analysis of Aircraft Structures 9-2

Internal Loads Group Has a Crucial Role in Airplane Design

Develops integratedCoordinates between stress

Finite Element Model (FEM)

group and external loads

groupInternal loadsInternal loads data is critical to

airplane schedule

Supports Stress group with modeling

Documents internal load results group with modeling

expertise (needed for life of airplane)


The Internal Loads Group Brings the External Loads Inside

• Produces internal loads for various stress groups• Produces internal loads for various stress groups• Generates stiffness data for external loads group• Develops and maintains major finite element p j

models used for internal loads analysis in support of an airplane certification processC di t b t t l l d d t• Coordinates between external loads and stress groups


Internal Loads

Structural analyst engineer• Sizes structure• Verifies that model represents actual structure

Finite elementmodel (FEM)Finite elementmodel (FEM)

FEM engineer• Acts as a go-between• Builds models

Understands models


• Understands models• Understands theory

Agenda

I t t d FEMIntegrated FEM

Support

Technical Background

Validation


Complete Integrated Finite Element Model (FEM)


Integrated FEM Contains Enough Detail to Accurately Describe the Structural Behavior


Integrated FEM Includes the Major Structural Elements

SkinsStringersFramesRibsFloor beamsLoad-carrying doorsLoad-carrying doorsSills BulkheadsPressure deckKeel beamPickle forksWheel wellsLongeronsLongeronsWindow beltDoor cutoutsSeat tracks*L di


*Landing gear *Nacelles/struts

*Not used for stress analysis.

The Integrated FEM Does Not Use Detailed Models For Components

• Leading and trailing edges of both wing and empennagep g

• Control Surfaces• Plug-type doors• Fairings


Wing Models Use Simple Concepts

Vent stringers

• Skins: modeled with membranes

• Stringers: modeled with bars and


gshears to create fixed and free flanges

Ribs Are Modeled With Bars and Shears


Body Models Have More Variety

• Skins: modeled with membranes

St i h b di i ti• Stringers: have bending inertia (beams)

• Window belt is included (anisotropic properties)


Frames and Floor Beams Use Bars and Shears

W bChord Web

Web

St hi (b )

Outer

Stanchion (beam)

Outerchord

InnerchordFail-safe


1chord

Bulkheads Use Beams and Shears

Center section wing box

Aft wheel wellbulkhead

Ring chordRing chord (beam

elements)

Webs(shear elements)

UpStiffeners(beam element)

BL0

UpOutboard Forward


BL0

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)M b d i i f ’ O l• Most body sizing comes from stress group’s Oracle database “APARD” (Analysis PARameter Database).

• Other body sizing comes on paper or in Excel (e.g.,Other body sizing comes on paper or in Excel (e.g., floor beams, keel beam, door sills).


The Integrated FEM IS a Collection of Several Individual Models

Vertical fin

Horizontal stabilizer

Section 48

Horizontal stabilizer

Outboard wingCenter section

Section 46Forwardcargo door

Raked tipOutboard wing

Section 45

Section 43

S ti 41

Main gear


767-400EREngine jigSection 41Nose gear

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 suddenPressure: loads due to cabin pressure and sudden decompression (13.65 psi internal cabin pressure is added to all flight cases).

Mi ll l d d t h diti ti b tMiscellaneous: loads due to such conditions as tire burst or center section fuel slosh.


Each FEM Scenario Causes the Engineer’s Workload to Multiply

• Load-carrying doors (door-in and door-out)• Main landing gear (up and down)Main landing gear (up and down)• Fail-safe conditions (limit load)• Discrete-source damage conditions

(70% of limit load)


Results From the Model Are Used in Several Ways

• Stresses and internal loads are post-processed to calculate margins of safety

• Deflections are provided to the control surface groups (e.g., flaps)

d tand 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

D fl ti ti id d• Deflections are sometimes provided to solve manufacturing problems

Structures workstation


The Integrated FEM Plays a Key Role in the Overall Design Process

Wind tunnel dataStability and control data

Pressure distribution

External loadsStiffness update Mass distribution update

Balanced forces

I t l l d d lInternal loads model

Stresses or internal loads

Post-processing

Sizing


g

Update detail drawings

Internal Load Schedule is Critical and Highly Visible

Major Milestones

1997 1998 1999 2000Qrt 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

Firm Config.

25% Drawing Release

90% Drawing Release

Start Major

Roll-Out

First Flight

Cert/ETOPSApproval

1st Delivery

Configure/Requirements

Loads

Milestones 1st DeliveryConfig. Memo

Release

Firm Structures Config. for Loads

Firm Config.Firm Structures Config.

Start Loads Prelim Internal Loads Comp.Design Internal Loads Comp.

Commitments & Compliance

Product Definition

Program Plans Complete

Start Structures W/S and Parts D-E Negotiations IPT Description of Change Comp.

IDAS Compl (Parts, Plans, Tools and CSD Negotiations

All t i EPICFirm Systems, Payloads and

Wheels and Tires SCD, Initial EAMR Release (MLG)

Product Release

Fab and Assembly (Ref)

1st Machine Print (Fixed LE)

Definition All parts in EPICPropulsion I/F to Structures

MLG ODAssembly (Ref)

Lab Test

Airplane Test

Lab Test Plans Start MLG Fatigue Test Valid of Sys. Funct/Integ in Labs Complete

Ground/Flight Test Plans First Flight Flight Test Comp.


p

Certification

Start Ground Test

Application to FAA/JAA

Preliminary Type Board w/FAA/JAA

Final Cert. Plan to FAA

Cert. Plans Approved

95% Compliance Doc. Submitted

Cert./ETOPS Approval

Cert. Basis Closed

Why Use the Integrated FEM?

ProsServes as a means of uniting

ConsStress group is somewhat• Serves as a means of uniting

disparate groups

• Consistency of idealization and analysis

• Stress group is somewhat dependent on FEM results

• Requires coordination• Idealization disagreementsand analysis

• Preserves lessons learned from previous programs

• Idealization disagreements• Culture clashes

– 767 versus 777– SAMECS versus ELFINI

• Easier to find errors (debug)

• Better interface loads

SAMECS versus ELFINI


Use of the Finite Element Method Is Diverse

• Typical applications

• Structural modeling (static, dynamic, and weight analysis)

• Preliminary-design airframe stressAi l i /b d j ti• Airplane wing/body junction

• Detailed internal loadsCrack growth and residual strength• Crack growth and residual strength

• Nonlinear geometry• Propulsion/structures integration• Propulsion/structures integration• Structure/acoustic interaction• Bird/blade impact


Bird/blade impact• Controlled airplane crash

Guidelines for Modeling

• Use simple elements.• Use simple modeling concepts.Use simple modeling concepts.• Keep the model size small.• Spend time to verify/validate/check out the model.


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,

d )and stress groups)• Subject to many demands

– Many load casesMany load cases– Many scenarios– Many groups use the results

• Critical item in the program schedule


Agenda


Support


Validation


Study Models are Built to SupportStress Groups

767-300 Validation Model767 300 Validation Model


Study Models Are Built to Support Stress Groups

767-400ER Frame Idealization Study Model


Internal Loads Group Builds Detailed Stress Models for Analysis and Verification

Design and Analysis of Aircraft Structures 9-30777 Drag Brace Fitting

737-X Overwing Escape Hatch Cutout

StStress group worried about fatigue interactions gbetween corners of the three cutouts

Total weight savings of 10 lbper airplane


737NG Rear Spar Pickle Fork


Model used to analyze bolt loads

Internal Loads Group Builds “Hybrid” Models

777-300 Overwing Door

• Goal was to find optimum contour foroptimum contour for corners of door cutout

• Optimizer was used to minimize weight

• Skins are 1-inch• Skins are 1-inch thick in this area


Internal Loads Group Loaned Engineers to Stress Group for 767-400ER Main Landing Gear Analysis

Coarse modelCoarse model with fine-meshed upper


Coarse model fine meshed upper torque link

Internal Loads Group Supports External Loads and Flutter Groups

• Creates finite element modelsP id tiff d t• Provides stiffness data

767-400ER flutter FEM 767-400ER external loads FEM


Support Summary

• Internal loads builds detailed stress models for analysis and verification.y

• Hybrid models are built to be more detailed than the regular internal loads model.S d d l i i f id li i• Study models investigate software or idealization changes.

• Internal loads engineers are sometimes loaned toInternal loads engineers are sometimes loaned to other groups to assist with modeling efforts.

• Internal loads group supports flutter and external loads with FEM data.


Agenda


Support


Validation


Textbook Definition: What Are Internal Loads?

Forces and Moments Carried byForces and Moments Carried by the Structure of the Aircraft

Examples• Axial force in a fuselage stringer• Shear flow in a bulkhead• Shear flow in a bulkhead• Hoop load in a fuselage skin panel• Segment load (skin plus stringer) in a wing


Simple, Easily Understandable Elements, and Properties Are Used

Internal Loads Model Solid Models

• Spring• Bar • 10-noded tetrahedron

Internal Loads Model Solid Models

• Beam • Shear*• Membrane*

• 8-noded brick• 20-noded brick

Membrane• Bending Plate*

* Properties for shear/membrane/bending elements

Isotropic

More common Less Common

Composite Honeycomb Sandwich


Springs

• Easy to understand• Control direction of loadF K Control direction of load• Allow for easy, quick check of load

path

F = K x

• Used to attach pieces of major structure in the FEM

• For very stiff elements set K= ∞• For very stiff elements set K= ∞• Use 3 translational and 3 rotational

stiffnesses at each node


Bars

• Easy to understand• Allow for quick check ofAllow for quick check of

load path• Used for modeling of

– Fuselage stringers– Built-up structure

(“chord-web-chord”)

Area

• The only variable is the area


Beam

• Advanced features:– Hinges (releases) at each endTorsional Hinges (releases) at each end– Variable area– Variable inertia (3 per beam)

Variable shear areaBending inertia

inertia

– Variable shear area

AreaBendinginertia


Shear, Membrane and Bending Plate

ShBending

Shear Membraneg

plate

•Carries shearforce only

•Adds axialcapability

•Adds bendingcapability


Solid Elements

10-noded tetrahedron 8-noded brick 20-noded brick


What Is a Degree of Freedom (DOF)?

• A node can have 6 structural degrees of freedom– 3 translations relative to an axis systemy– 3 rotations about an axis system


Degrees of Freedom are Determined by Element Properties

Example:BeamBeam

Without torsional inertia With torsional inertia

10 DOF 12 DOF


All elements attached to a given node can potentiallyaffect the degrees of freedom at that node

Finite Element Method

Stress/ strain relationship(spring f = Kx)

Write stiffness matrix for each element

Assemble global stiffness matrix

Each elementcontributes to theoverall stiffness matrix

Invert global stiffness matrix

of the model

Invert global stiffness matrix

Back-substitute to obtain Back-substitute to obtain


displacementsdisplacements

What Software Tools Do Internal Loads Use to Create an Integrated FEM?

CATIAPreprocessor:

Solver: CATIA-ELFINI (batch, not interactive)

Post-processor: CATIA-ELFINI (interactive)<or>Create a large ASCII text file to transfer to other codes


How Are Loads Applied to the Integrated FEM?

• ELFINI aeroelastic using detailed model (“TRLOAD”)

• ELFINI aeroelastic sing coarse model (“U CONNECT”)• ELFINI aeroelastic using coarse model (“U-CONNECT”)

• Using “point loads” (Unit loads and factors)


“ELFINI Aeroelastic” Using Detailed Model

Direct node-to-node transfer

• External loads calculated using a model that is 95% ith th i t l l d d l

Internal loads FEMExternal loads FEM

95% common with the internal loads model

• CATIA-ELFINI “TRLOAD” function used to transfer node loads


• Used on 737NG

“ELFINI Aeroelastic” Using Coarse Model

Spreading algorithm must be used

“U-CONNECT”Internal loads FEMExternal loads FEM

• External loads calculated on coarse model

Internal loads FEMExternal loads FEM

• Stiffness approximates that of the internal loads model

• CATIA-ELFINI “U-CONNECT” function used to transfer node loads


node loads

• Used on 777-200X & 777-300X

“Point Loads” Using Detailed Model

• Hundreds of unit load cases created (representcases created (represent airload, fuel, cargo, OEW, etc.)

External loads group• External loads group provides factors

• Unit cases factored and added together to create each final case on the integrated FEM

• Used on 767-400ER

• Labor intensive Internal loads FEM


Many Programs Can read ResultsFrom the FEM

MARGIN: Wing stress

FEADMS:

Moss/Duberg:

Oracle database for body structures

body skin and stringer sizing

FAMOSS:

POST-ELF:

IDTAS

body frames

Wichita

F tiIDTAS: Fatigue

Plus other IAS and Excel applications


Boeing Uses a Variety of Finite Element Codes

Internal loads, stress, flutterCATIA-ELFINI: Internal loads, stress, flutter

legacy internal loads

weights flutter landing gear dynamic

CATIA ELFINI:

SAMECS:

ATLAS: weights, flutter, landing gear, dynamic loads, and legacy internal loads

legacy internal loads stress PSD

ATLAS:

NASTRAN: legacy internal loads, stress, PSD, vibration

stress, landing gear, systems

NASTRAN:

ANSYS: stress, landing gear, systems

systems, stress

advanced nonlinear code

S S

ALGOR:

ABAQUS:


advanced nonlinear codeABAQUS:

CATIA-ELFINI Has Many Differences Compare to Other Finite Element Codes

• Uses a “history” file (cutting, copying, and pasting blocks of commands)blocks of commands)

• Uses topological meshing (10, 1, 4, 1, instead of 1001)

• Integrated into CATIA (same place as geometry)

S b t t i d l t d d• Sub-structuring and super-elements are very advanced

• Load and displacement transfer between meshes is peasy

• Limited non-linear capability


Limited non linear capability

Super-Elements Add Flexibility to the Overall Process

• Incorporate sub-structuringI di id l ti th i• Individual sections can rerun on their own

• Useful for fail-safe and discrete-source damage runsruns


Example of Super-Elements, Step 1

MeshNever changes

MeshChanges often

BA

Load

{Coincident

To solve (without super-elements):

1. Join Mesh with Mesh .A B

Coincident

2. Assemble global stiffness matrix [K].3. Invert global stiffness matrix [K].4 Back-substitute to get global displacements {U}


4. Back-substitute to get global displacements {U}.Note: Each time changes, must re-solve for .B A

Example of Super-Elements, Step 2

B

LoadSuper-elements

representing stiffness of

mesh A

Coincident {mesh A

1. Create super-element for Mesh(reduce loads and stiffness at boundary with Mesh ).

To solve (using super-elements for Mesh ):A

A B( y )

2. Join Mesh with super-element that represents Mesh .3. Assemble global stiffness matrix [K].4. Invert global stiffness matrix [K]. 5 Back substitute in Mesh and to boundary of MeshB A

A BB


5. Back-substitute in Mesh and to boundary of Mesh . B A

Technical Background Summary

• Simple, easily understood elements and properties contribute to overall stiffness of the p pmodel.

• Internal Loads group uses CATIA-ELFINI to create integrated FEMintegrated FEM.

• External loads are applied to the integrated FEM, using one of three methods.g

• Many Boeing software programs use results from the FEM

• Other finite element codes are in use throughout Boeing


Agenda


Support


Validation


777-200 Static Test ConditionMaximum Positive Wing Bending - 110% Limit Load

280

Tip deflection:Test: 205 6 inches200

240

280

Test: 205.6 inchesAnalysis: 204.7 inches

160

200

Side of b d

Nacelle80

120Vertical

deflection (in) body

0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 10

40(in)


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1— Analysis – - TestETA station

Summary

• Internal loads group develops the integrated finite element model (FEM)( )

• Internal loads group coordinates the overall modeling effortI l l d h b• Internal loads group supports other groups by building models and providing data to the flutter, stress, and external loads groups, g p

• The FEM uses simple, easily understood elements and properties

• FEM results correlate well with static test


Internal Loads

• Successes777-200 Established many current processes737-X767-400ER

y pUsed ELFINI Aeroelastic for external loadsPreliminary cycle made shorter

Two entire loads releases with composite

• Lessons Leaned

757-300properties in the frame webs

Program canceled just prior to release of internal loads

747-600xinternal loads

Software change did not go smoothly777-200X/300X


Why Work in the Internal Loads?

• Get to see entire airplaneGet to see entire airplane• Lots of variety• Lots of exposure to other groupsp g p• Trips to Wichita (future:

maybe Long Beach)• Recognition lunches with upper

management (in the cafeteria)


Can You Trace the Internal Load Paths?


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