The Role of Finite Element Analysis in Light Aircraft Design and Certification

Transcription

1 The Role of Finite Element Analysis in Light Aircraft Design and Certification

2 Nigel Bamber Wey Valley Aeronautics Ltd Engineering Consultancy Civil and Military Aerospace and Motorsport Design Structural and Dynamics Analysis Structural Testing Flight Testing, Track Testing and Race Engineering Design consultants to LAA

3 Light Aircraft Association CAA Approved Design Organisation Review design of amateur built/designed aircraft. Make recommendations to CAA for granting of UK Permit to Fly

4 Todays Lecture Overview of capabilities of FEA Guidance on acquiring FEA Capability or Subcontracting work Demonstrate applications of FEA in Design and Certification

5 What is Finite Element Analysis? Finite element method. (2009, September 25). In Wikipedia, The Free Encyclopedia. Retrieved 18:40, September 25, 2009, from method&oldid= The finite element method (FEM) (sometimes referred to as finite element analysis) is a numerical technique for finding approximate solutions of partial differential equations (PDE) as well as of integral equations. The solution approach is based either on eliminating the differential equation completely (steady state problems), or rendering the PDE into an approximating system of ordinary differential equations, which are then numerically integrated using standard techniques such as Euler's method, Runge-Kutta, etc.

6 What is Finite Element Analysis? Take a complex structure that we don t have an analytical solution or equations for. Break it down into a collection of simple structural elements that we do have equations for. (Idealisation) Solve a very large number of simultaneous equations to understand the complex structure.

7 Solid Elements Only need to define the material properties Gives full 3D stress field E.g Will capture thru-thickness stress concentrations in an I-beam Large Files, Slow Run-times

8 Shell Elements Need to define thickness, as well as material. Thickness can be defined as plies of different composite material Can show shear lag in an I-beam Smaller files and shorter run times

9 Beam Elements Need to define the cross-section as well as thicknesses and material properties. Can show bending and direct stresses in an I-beam. Can be used to transfer loads around the model

10 Mixed Element Models

11 Linear Static Analysis Types Applied forces, Displacements, Thermal expansion Non-Linear Geometric, Material, Contact Buckling Linear Euler, Non-linear Post-Buckling Modal Analysis Resonant Frequencies Dynamic Analysis Time Response, Frequency Response

12 Boundary Conditions Forces Point Loads Distributed or Pressure Loads Inertial Loads Constraints Fix points to ground, 6 DOF Tie movement of 1 point to another. (Rigid Links, Sliding Interfaces)

13 Boundary Condition Errors

14 Load Path Errors Not Just Modelling Geometric Shape Modelling Structural Behaviour

15 Load Path Errors 1

16 Load Path Errors 2

17 The F.E.A. Marketplace (CAD) CAD Add-on Modules Often embedded version of known FEA package Advantage good geometry interface Check Level of Support from FE Specialists Extent of functionality available through CAD package.

18 The F.E.A Marketplace (Specialist) Range of Prices and Capabilities Lower Price Systems e.g. STRAND7, NISA Good Analytical Functionality Less Sophisticated Model Pre/Post-Processing May need to purchase 3D CAD to provide complex geometry. Aero Industry Standards e.g. NASTRAN or ANSYS Excellent Pre/Post-Processing Wide user base Check Compatibility with CAD System Try real geometry imports as a benchmark.

19 Additional Selection Methods Benchmark Model Small benchmarks by vendor Large benchmarks using consultant Go on training course prior to purchasing

20 Gaining Experience Further Vendor Training Courses Try Modelling Closed-Form Solutions From Roark, Bruhn, Niu or ESDU Hand-holding Projects with Consultants NAFEMS an independent not-for-profit body with the sole aim of promoting the effective use of engineering simulation methods such as finite element analysis, multibody system dynamics and computational fluid dynamics. FE Primers and How-to guides

21 Analysis in Certification CS-VLA 307 Proof of structure (a) Compliance with the strength and deformation requirements of CS-VLA 305 must be shown for each critical load condition. Structural analysis may be used only if the structure conforms to those for which experience has shown this method to be reliable. In other cases, substantiating load tests must be made. Use Analysis and Test to Support Each Other

22 FEA Methodologies Finding Critical Load Case For Structural Test Wing Ultimate Case (Points A, C or D or Aileron Torsion) Finding Load Paths in Redundant Structures To Enable Sub-Assembly or Component Tests To Provide Input for Manual Calculations E.g. Fastener Loads Modelling Cases That Cannot Easily Be Tested E.g Structural Inertial Loads Calibrate Model Against a Set of Actual Test Results Then Extrapolate to Untestable Conditions Effect of Damage on Fail-Safe Structures

23 Model Quality and Accuracy 1 Check Model Against Known Theoretical Solutions or Test Quick Simple Manual Sanity Check Appropriate Idealisations and Constraints

24 Model Quality and Accuracy 2 Model Convergence Mesh Density, Element Shape 54 Elements Max Stress MPa Poor Contour Symmetry

25 210 Elements Max Stress MPa Approx 10% increase. Smoother Mesh Symmetric top to bottom. Not Left to Right.why?

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27 Max Stres 689 Elements Max Stress MPa Stress Convergence No Of Elements

28 Model Quality and Accuracy 3 Data Input Unit System Material Properties Thicknesses Beam Section Orientations Shown in pre-processor Composite Ply Orientations Force Magnitudes and Directions Sum reaction forces and moments at constraints Do they match anticipated input loads

29 Documentation and Checking Internal Company Checking Quality System Liability Limitation Ethical Assessing Sub-Contracted Analysis Information Fit For Purpose Were requirements of contract met Legislative Checking of Results Type Approval (LAA, CAA) Continued Airworthiness (Mods, Accidents) (AAIB, NTSB, Board of Enquiry, Relatives Solicitors)

30 How Do You Check FEA Models? Interrogation on Computer Pre/post processor checks of input/output data. Can be done remotely with screen dump request. But what about afterwards No access to program No access to analyst Especially if consultant, or in future

31 Review Reports Report Requirements FEA Reports List all assumptions and idealisations. Identify where these may affect future use of report. Input plots Enough to be able to verify input data Model Quality Info (Convergence, Element Shape) Results Plots Supply supporting data as report Appendix or a separate referenced report

32 Report Guidelines NAFEMS LAA Technical Information Leaflets On website, Engineering Section, TL 1.21 Also applicable to manual calculations and spreadsheets.

33 Idealisation Issues Are all possible failure modes covered? Not just static overall strength Will it buckle? Is it flexible enough that a geometric non-linear analysis is required? What about fasteners? Bearing strength, shear-out, are hand calcs required? Is the mesh or idealisation refined enough to show stress concentrations for fatigue? Will components make contact or separate? What about out-of-plane shear in a shell model?

34 Future Proofing FEA Data Insist on Comprehensive Reports Make Soft Copy of Model a Deliverable Even if you don t have program that generated it. In the Future, That Program May No Longer Exist Or the operating systems and hardware to run them Text Format Input Data Files