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Active Interrogation of Helicopter Rotor Faults Using Trailing Edge Flap Actuation

Active Interrogation of Helicopter Rotor Faults Using Trailing Edge Flap Actuation. Patricia Stevens Doctoral Candidate Mechanical Engineering Penn State University Doctoral Dissertation Defense April 2, 2001. Outline. Background & Motivation Objectives of Work Modeling Approach

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Active Interrogation of Helicopter Rotor Faults Using Trailing Edge Flap Actuation

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  1. Active Interrogation of Helicopter Rotor Faults UsingTrailing Edge Flap Actuation Patricia Stevens Doctoral Candidate Mechanical Engineering Penn State University Doctoral Dissertation Defense April 2, 2001

  2. Outline Background & Motivation Objectives of Work Modeling Approach Damage Identification Conclusions

  3. Documented Rotor System Problems • Civil • 1990-1996: 35 civil rotorcraft accidents were caused by rotor system failures • AH-64 Apache - Early Blade Problems • Original aluminum blades pitted by sand and disabled by hail • Composite blades suffered from delamination • CH-46 Sea Knight - Prior to Upgrade • Inspections as often as every 8 hours of flight time for some rotor components

  4. What makes helicopter rotor damage detection so difficult? AerodynamicLoads Complex Components InaccessibleLocations GyroscopicSystem CentrifugalStiffening Noisy Environment

  5. Previous work:Localized fault detection Ultrasonicsensor • Acoustic EmissionSchoess et al. (1997) • Passive Approach • Acoustic Emission sensor “listens” for crack propagation • Wave Mechanics Lakshamanan & Pines (1997) & Purekar et al. (1998) • Active approach • Scattering of structural waves due to impedancechanges • Limitation: • Requires a priori knowledge of fault location crack stress waves Acoustic Emission PZTactuator / sensor flaw scattered waves Wave Mechanics

  6. Previous work:Rotor Diagnostics using Fuselage Measurements • Azzam & Andrew (1992, 1995) • Ganguli, Chopra & Haas (1995-98) • Passive generation of fixed frame loads • Measurements • relative blade position • fuselage vibration • Measurements in hover and forward flight • Limitations: • Limited detectability of small faults • Neural net required to classify faults • Forward flight condition measurements required Dissimilarblade model Seed fault Simulateresponse Fault profileat eachflightcondition Measuretip displacementhub loads (vibs) Next flightcondition TrainNeural Net Next fault

  7. Previous work:Application of Structural Damage Detection Kiddy & Pines (1997 - 1999) • Applied Modal Based SDD Technique to rotor blade environment • Modified Eigenstructure Assignment Technique to accommodate • Centrifugal Stiffening • Aerodynamic Damping • Limitations • Sensitive to noise • Limited fault coverage • Measurability & actuation not assessed Will an active interrogation structural damagedetection approach yield improved results?

  8. Next Generation Rotorcraft… Active Trailing Edge Flaps • Installed for vibration and noise control • Potential actuator for damage interrogation MD 900 blade with trailing edge flap Flap Actuator Tab Actuator Active Control Flap, Noise and Vibration Composite Blade Assembly Trim Tab, In-Flight Tracking HH10 Airfoil Section Flap Actuator Bearingless Hub Tab Actuator BLADE CROSS-SECTION

  9. Interrogation signal sensors trailing-edge flap Blade Response Measured Damage Evaluation Algorithms Goal: Design and Evaluate the Active Interrogation Concept

  10. Objectives • Determine if active interrogation of rotor faults using trailing edge flap actuators is a viable concept. • Develop active interrogation techniques appropriate for the rotor blade environment. • Demonstrate effective damage evaluation in hover. • Demonstrate damage evaluation in the presence of noise and modeling errors • Evaluate limitations of the approach.

  11. Outline Background & Motivation Objectives of Work Modeling Approach Rotor Trailing Edge Flap Damage Damage Identification Conclusions

  12. Rotor Model - Bearingless Main Rotor • Finite Element Approach • Flap, torsion • 10 beam elements • Hingeless rotor - cantilever boundary condition Pitch LinkStiffness Flexbeam • Dissimilar blades • Aeroelastic rotor in hover • Response via time integration • Response measured at each node Nel = 10 f Nodal Degreesof Freedom W Cantileverboundarycondition

  13. Trailing-Edge Flap Model d • Physical Description • Size 10% of rotor radius • Location 80-90% rotor radius • Frequency 0 - 50 Hz. • Amplitude up to +/- 5 deg • (using +/- 2.5 deg) • Lift 120 lb/deg at 0 Hz • 70 lb/deg at 50 Hz • Moment 25 ft-lb/deg • Aerodynamic Environment • Mach No. 0.45 - 0.6 in hover • Reduced Frequency up to 0.5(k=wc/2V) • Requires subsonic compressible flow unsteady aerodynamic model(Leishman, et al)

  14. Damage Models • Flexbeam Degradation • Bending Stiffness • Torsional Stiffness • Control System Stiffness • Flexbeam Crack • Outboard Stiffness Defect • Bending • Torsional • Outboard Crack • Ballistic Damage • Trim Mass

  15. Flexbeam Degradation • Distributed stiffness fault • Change in EI or GJ over flexbeam element • 5% reduction in EI or GJ for 0.0-0.1R (flexbeam element)

  16. Control System Stiffness • Crack in pitch rod or fatiguefailure in connecting hardware • 5% reduction in axial stiffness of pitch rod • 5% effective reduction in torsional spring at end of flexbeam

  17. Outboard Stiffness Defect • Adopted from Ganguli, Chopra and Haas (1995-98) • Manufacturing Defect • Delamination • 5% reduction in EI or GJ for 0.6-0.7R

  18. Ballistic Damage • Experimental study of effects ofballistic damage (Robinson & Leishman, 97-98) • Ballistic damage affects: • Cla, Clmax, Cd • aerodynamic center location • mass • “In some cases significant damage produced surprisingly mild effect on the aerodynamics” • “Mild decreases in lift, but major increases in drag” • Ballistic Damage = 5% decrease in mass from 0.6-0.7R

  19. mass nominal mass Loss of Trim Mass • Discrete change in mass of 0.6 lb at 95% radius x/Lel feather axis TrimMass Lel

  20. Crack Model - a new finite elementKrawczuk et al. (2000) A CRACK Boundary Conditions I II H a LB A L w1(x) w2 (x) q1 q3 cb=1/kb q2 q4 f1 (x) f2 (x) I II x=0 x=LB x=LB x=L From moment equilibrium

  21. Crack Model - a new finite element • Converges to standard beam element as K 0 • Only bending slope terms are affected LB=L/2 Krawczuk et al. (2000)

  22. Elastic Crack Model - Relating Crack Depth to Crack Constant Effect of depth on crack constant • Correction function, F(a/H), takes into account crack and body geometry (from stress intensity factor): • Correction function governs flexibility(elastic crack) • Flexibility determines constant, K K/H a/H

  23. Crack Model - Validation [reproduced from Krawczuk et al. (2000)]

  24. Outline Background & Motivation Objectives of Work Modeling Approach Damage Identification Theory & Results Effect of Modeling Errors Noise & Noise Mitigation Alternate Extent Quantification Approach Measurability Conclusions

  25. Structural Damage Detection Background (Modified from Rytter, 1993) Four Levels of Damage Identification • Level 1: Detection • Level 2a: Level 1 + Location • Level 2b: Level 1 + Characterization • Level 3: Level 2 + Quantification of Severity • Level 4: Level 3 + Prediction of Remaining Life Can I safely completemy mission? Can I safely completemy mission?

  26. Structural Damage Detection Background Four Levels of Damage Identification • Level 1: Detection • Level 2a: Level 1 + Location • Level 2b: Level 1 + Characterization • Level 3: Level 2 + Quantification of Severity • Level 4: Level 3 + Prediction of Remaining Life There’s a problem!

  27. Structural Damage Detection Background Four Levels of Damage Identification • Level 1: Detection • Level 2a: Level 1 + Location • Level 2b: Level 1 + Characterization • Level 3: Level 2 + Quantification of Severity • Level 4: Level 3 + Prediction of Remaining Life ...in the pitch link!

  28. Structural Damage Detection Background Four Levels of Damage Identification • Level 1: Detection • Level 2a: Level 1 + Location • Level 2b: Level 1 + Characterization • Level 3: Level 2 + Quantification of Severity • Level 4: Level 3 + Prediction of Remaining Life It’s a crack!

  29. Structural Damage Detection Background Four Levels of Damage Identification • Level 1: Detection • Level 2a: Level 1 + Location • Level 2b: Level 1 + Characterization • Level 3: Level 2 + Quantification of Severity • Level 4: Level 3 + Prediction of Remaining Life It’s a small crack.

  30. Structural Damage Detection Background Four Levels of Damage Identification • Level 1: Detection • Level 2a: Level 1 + Location • Level 2b: Level 1 + Characterization • Level 3: Level 2 + Quantification of Severity • Level 4: Level 3 + Prediction of Remaining Life Safe to complete the mission!

  31. Damage Detection, Location & Characterization The "DAMAGE VECTOR" EOM of damaged system: Damage is perturbation matrix: Rearranging results in two equivalent vector expressions --d(jw) = the Residual Force or “Damage Vector” (1) (2) d(jw) has non-zero elements only at DOFs associated with damage d(jw) can be calculated from known parameters

  32. Interpretation of the Damage Vector Ojalvo & Pilon (1988) Physical interpretation: The harmonic amplitude of nodal forces required to force the healthy system model to respond as if damage were present degrees of freedom: 1,2 3,4 5,6 7,8 9,10 healthy fint d3 d5 d4 d6 measurements: 1,2 3,4 5,6 7,8 9,10 damaged fint

  33. Results for ... • Flexbeam Degradation • Torsional Stiffness • Control System Stiffness • Outboard Stiffness Defect • Bending Stiffness • Outboard Crack • Ballistic Damage • Need to • detect & locate • differentiate between similar faults • Does interrogation frequency affect the results?

  34. 50 Hz 10 Hz Damage Vector for Flexbeam Torsional Stiffness displacement w Damage is 5% decrease in GJ of element 1 bendingslope w' mid-nodetwist fM Torsional stiffness damage manifests as damage vector f DOFs connected to damaged element end-nodetwist fA measurement location

  35. 50 Hz 10 Hz Damage Vector for Pitch Link Stiffness displacement w Damage is 5% decrease in torsional spring representing pitch link bendingslope w' Pitch link stiffness damage manifests as damage vector f DOF connected to damaged element -- a single DOF mid-nodetwist fM end-nodetwist fA measurement location

  36. 50 Hz 10 Hz Damage Vector for Outboard Bending Stiffness displacement w Damage is 5% decrease in EI of element 7 bendingslope w' mid-nodetwist fM Outboard bending stiffness damage manifests as damage vector w & w’ DOFs connected to damaged element end-nodetwist fA measurement location

  37. 50 Hz 10 Hz Damage Vector for Outboard Bending Crack displacement w Damage is crack of depth a/H=0.05 at midpoint of element #7 bendingslope w' mid-nodetwist fM Crack damage manifests as damage vector w' DOFs connected to damaged element end-nodetwist fA measurement location

  38. 50 Hz 10 Hz Damage Vector for Ballistic Damage displacement w Damage is 5% decrease in mass of element 7 bendingslope w' mid-nodetwist fM Ballisitic damage manifests as damage vector w, w’, and f DOFs connected to damaged element end-nodetwist fA measurement location

  39. 50 Hz 10 Hz Damage Vector for Ballistic Damage displacement w Damage is 5% decrease in mass of element 7 bendingslope w' mid-nodetwist fM Why is damage vectorcontaminated? end-nodetwist fA Centrifugal Stiffening measurement location

  40. 50 Hz 10 Hz Damage Vector for Compound Damage displacement w Damage is • Root bending stiffness • Pitch link stiffness • Ballistic damage Results show • Each damage type is identified • Combined damage vector is equal to sum of individual damage vectors bendingslope w' mid-nodetwist fM end-nodetwist fA measurement location

  41. Damage Detection, Location & Characterization Summary • Residual force vector (a.k.a. damage vector) requires • refined model of healthy system • measured response of damaged system • model or measurement of external force • All fault types studied were detected and located viaresidual force vector • Similar faults are distinguishable • Compound fault damage vector = sum of individual damage vectors • No clear frequency recommendation • Requires a single interrogation frequency

  42. Why are rotor system damage extentcalculations difficult? • Aerodynamic Loads • Non-symmetric aerodynamic matrices • Damping • Centrifugal Stiffening • large CF stiffening • mass / stiffness coupling • Coriolis Forces • Skew symmetricmatrices

  43. Damage Extent for Gyroscopic Systems • Yap and Zimmerman (1999) solved the gyroscopic problem via the “Asymmetric Minimum Rank Perturbation Theory” • Modal based model update • Find the perturbation matrix of minimum rank subject to constraint of null symmetry • This modal analysis based approach was extended to a FRF based approach as part of the current work

  44. Damage Extent (step 2)FRF -"Asymmetric Minimum Rank Perturbation Theory” Stiffness damage: Damping damage: Mass damage: Where [ B ] =matrix collection of damage vectors (step 1) = [ d1, d2, …, dp ] [ jWint ] = diagonal matrix of interrogation frequencies [ X ] = matrix collection of damaged system response = [ {X(jw1)}, {X(jw2)}, …{X(jwp)} ] The number of independent columns of [ B ] and [ X ] is equal to the rank of the perturbation matrix (e.g. flap only: mass=4, stiffness=2) BUT! Must know nature (mass, damping, stiffness) a priori.

  45. Calculation of Parameter Change Exact DK x 104 1 0 -1 -2 • AMRPT results in perturbation matrix of full dimension • Non-zero terms describe change in elemental matrix • For damage located in a single element, change in physical parameter is calculated using structure of elemental matrix • e.g. 5 10 15 20 25 30 35 40 DOF 5 10 15 20 25 30 35 40 DOF

  46. Mass Damage in Rotating Structure • Observations: • Off diagonal terms in mass and CF stiffness matrices depend on c.g. offset - typically small • CF affects inboard elements in flapwise motion only • Neglecting off-diagonal terms, problem is now (3 x 3) in twist • Solve problem using twist DOFs only - still coupled in mass & stiffness • Solution: • Iterate on coupled twist problem

  47. where x is damagedparameter (EI, GJ, rA) Damage Extent Summary AMRPT results show improvementusing higher interrogation frequencies AMRPT Damage Extent Quantification Error Errors stem from small errors in damage vector

  48. Outline Background & Motivation Objectives of Work Modeling Approach Damage Identification Theory & Results Effect of Modeling Errors Noise & Noise Mitigation Alternate Extent Quantification Approach Measurability Conclusions

  49. 10% modeling error no error Effect of Modeling Errors Model Error: 10% stiffness error in baseline model Damage: 5% outboard bending stiffness Damage Detection Destroyed!!

  50. no correction corrected d no error Correction of Modeling Errors Model Error: 10% stiffness error in baseline model Damage: 5% outboard bending stiffness Interrogation: +/- 2.5 deg., w = 40 Hz Use damage vector correction: d=dd-dh

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