active interrogation of helicopter rotor faults using trailing edge flap actuation

Active Interrogation of Helicopter Rotor Faults UsingTrailing Edge Flap Actuation

Patricia StevensDoctoral Candidate

Mechanical EngineeringPenn State University

Doctoral Dissertation DefenseApril 2, 2001

2

Outline

Background & MotivationObjectives of WorkModeling ApproachDamage IdentificationConclusions

3

Documented Rotor System Problems

CH-46 Sea Knight - Prior to Upgrade• Inspections as often as every 8

hours of flight time for some rotor components

AH-64 Apache - Early Blade Problems• Original aluminum blades pitted by

sand and disabled by hail• Composite blades suffered from

delamination

Civil• 1990-1996: 35 civil rotorcraft

accidents were caused by rotor system failures

4

What makes helicopter rotor damage detection so difficult?

CentrifugalStiffening

GyroscopicSystem

AerodynamicLoads

ComplexComponents

InaccessibleLocations

Noisy Environment

5

Previous work:Localized fault detection

• Acoustic Emission Schoess 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 impedance changes

• Limitation: – Requires a priori knowledge of

fault location

Ultrasonicsensor

crackstress waves

Acoustic Emission

PZTactuator / sensor

flawscattered 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

Measuretip displacementhub loads (vibs)

Next flightcondition

Next fault

Fault profileat eachflight

condition

TrainNeural Net

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

Composite BladeAssembly

BearinglessHub

Tab ActuatorFlap Actuator

Active Control Flap,Noise and Vibration

Trim Tab, In-Flight Tracking

HH10 Airfoil Section

BLADE CROSS-SECTION

Tab ActuatorFlap Actuator

MD 900 blade with trailing edge flap

9

Goal: Design and Evaluate the Active Interrogation Concept

Interrogation signal

Damage EvaluationAlgorithms

Blade ResponseMeasured

trailing-edgeflap

sensors

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 & MotivationObjectives of WorkModeling Approach

RotorTrailing Edge FlapDamage

Damage IdentificationConclusions

12

Rotor Model - Bearingless Main Rotor

Cantileverboundarycondition

Nel = 10

Nodal Degreesof Freedom

Flexbeam Pitch LinkStiffness '

w

w

• Finite Element Approach– Flap, torsion– 10 beam elements

• Hingeless rotor - cantilever boundary condition

• Dissimilar blades

• Aeroelastic rotor in hover

• Response via time integration

• Response measured at each node

'

w

w

13

Trailing-Edge Flap Model

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=c/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:– Cl, 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

Loss of Trim Mass

TrimMass

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

mas

sno

min

al m

ass

feather axis

x/Lel

Lel

Discrete change in mass of 0.6 lb at 95% radius

20

Crack Model - a new finite elementKrawczuk et al. (2000)

Boundary Conditions

a H

A

A

CRACK

cb=1/kb

I II

I II

w1(x)

1 (x)

w2 (x)

2 (x)

LB

L

x=0 x=LB x=LB x=L

q2

q1

q4

q3

1 2

1 1

1 2

2 3

2 4

1

2

2

1 2

1 2

0

0

" "

''

"

' '''

B B b B

B B

B B

B B

w q

q

w L q

L q

w L w L

w L w L

L L EIc w L

w L w L

From moment equilibrium

21

Crack Model - a new finite element

K

3 2 3 2

2 2 2 2

2 22

crackedbeam

3 2

2 2

2

12 6 12 6

4 6 3 6 2 6 3

12 6

4 6 3

i

L L L LL LK K L LK K

LL L K L L KEI

L LL LK Ksym

L L K

• Converges to standard beam element as K 0

• Only bending slope terms are affected

yy bK EI c

Krawczuk et al. (2000)

LB=L/2

22

0

0.5

1

1.5

2

2.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Elastic Crack Model - Relating Crack Depth to 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

22 4

72A

c F adAEB H

4

0.93 01.99 1 sintan22

cos2 2

aaHHF a a

H H

yyK EI c

Effect of depth on crack constant

1

1

0, 0, 0

1, ,

a KHa KH

K/H

a/H

23

Crack Model - Validation

0.7

0.75

0.8

0.85

0.9

0.95

1

0 0.1 0.2 0.3 0.4 0.5 0.6

Analytical modelFEM modelExperimental data

Firs

t rel

ativ

e na

tura

l fre

quen

cy

Relative crack depth, a/H

[reproduced from Krawczuk et al. (2000)]

24

Outline

Background & MotivationObjectives of WorkModeling ApproachDamage Identification

Theory & ResultsEffect of Modeling ErrorsNoise & Noise MitigationAlternate Extent Quantification ApproachMeasurability

Conclusions

25

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

(Modified from Rytter, 1993)

Can I safely completemy mission?

Can I safely completemy mission?

26



There’s a problem!

27



...in the pitch link!

28



It’s a crack!

29



It’s a small crack.

30



Safe to completethe mission!

31

Damage Detection, Location & Characterization The "DAMAGE VECTOR"EOM of damaged system:

Rearranging results in two equivalent vector expressions -- d(jw) = the Residual Force or “Damage Vector”

(1)

(2)

2 ( ) dd j M j C K X j

2 ( ) h h h dd j M j C K X j F j

d(jw) has non-zero elements only at DOFs associated with damage

d(jw) can be calculated from known parameters

2 [ ] [ ] [ ]h h h d totalM M j C C K K X j F j

0 0 0 0 00 0 0 0 00 0 00 0 00 0 0 0 0

X XMX X

Damage is perturbation matrix: , , d h d h d hM M M C C C K K K

2d d d d totalM j C K X j F j

32

Interpretation of the Damage Vector

Physical interpretation: The harmonic amplitude of nodal forces required to force the healthy system model to respond as if damage were present

3

4

5

6

00

d =

0

0

dddd

00

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

fint

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

fint

healthy

damaged

degrees of freedom:

measurements:

d3d4

d5d6

2 h h h dM j C K X j F j d j

Ojalvo & Pilon (1988)

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?

00.050.1

0.150.2

0.25

00.050.1

0.150.2

0.25

00.050.1

0.150.2

0.25

00.050.1

0.150.2

0.25

1 2 3 4 5 6 7 8 9 10

Damage Vector for Flexbeam Torsional Stiffness

measurement location

displacementw

bendingslope

w'

mid-nodetwistM

end-nodetwistA

Damage is 5% decrease in GJ of element 1

50 Hz

10 Hz

K 0K

0 K

strwwstr

str

1

0K H H' '

Tstr GJ ds

1 200

K H H'' ''cos Tstr

ww w wEI ds

Torsional stiffness damage manifests as damage vector DOFs connected to damaged element

00.020.040.060.080.1

00.020.040.060.080.1

00.020.040.060.080.1

05

1015202530

1 2 3 4 5 6 7 8 9 10

Damage Vector for Pitch Link Stiffness


Damage is 5% decrease in torsional spring representing pitch link

50 Hz

10 Hz

displacementw

bendingslope

w'

mid-nodetwistM

end-nodetwistA

Pitch link stiffness damage manifests as damage vector DOF connected to damaged element-- a single DOF

0

5

10

15

0

5

10

15

00.20.40.60.8

1

00.20.40.60.8

1

1 2 3 4 5 6 7 8 9 10

Damage Vector for Outboard Bending Stiffness


Damage is 5% decrease in EI of element 7

50 Hz

10 Hz

displacementw

bendingslope

w'

mid-nodetwistM

end-nodetwistA

Outboard bending stiffness damage manifests as damage vector w & w’ DOFs connected to damaged element

K 0K

0 K

strwwstr

str

1

0K H H' '

Tstr GJ ds

1 200

K H H'' ''cos Tstr

ww w wEI ds

00.020.040.060.080.1

00.050.1

0.150.2

0.25

00.020.040.060.080.1

00.020.040.060.080.1

1 2 3 4 5 6 7 8 9 10

Damage Vector for Outboard Bending Crack


Damage is crack of depth a/H=0.05 at midpoint of element #7

50 Hz

10 Hz

displacementw

bendingslope

w'

mid-nodetwistM

end-nodetwistA

Crack damage manifests as damage vector w' DOFs connected to damaged element

00.20.40.60.8

1

1 2 3 4 5 6 7 8 9 10

0

0.3

0.6

0.9

1.2

00.5

11.5

22.5

0

1

2

3

Damage Vector for Ballistic Damage


Damage is 5% decrease in mass of element 7

50 Hz

10 Hz

displacementw

bendingslope

w'

mid-nodetwistM

end-nodetwistA

Ballisitic damage manifests as damage vector w, w’, and DOFs connected to damaged element

ww w ww

w

M M M 0M

M M 0 M

1

0

Tww A w wdsM H H

1

0

TA ds M H H

1

w 00cos T

w A g we ds M M H H

00.20.40.60.8

1

1 2 3 4 5 6 7 8 9 10

0

0.3

0.6

0.9

1.2

00.5

11.5

22.5

0

1

2

3

Damage Vector for Ballistic Damage


Damage is 5% decrease in mass of element 7

50 Hz

10 Hz

displacementw

bendingslope

w'

mid-nodetwistM

end-nodetwistA

Why is damage vectorcontaminated?

Centrifugal Stiffening

Damage Vector for Compound Damage


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

0

10

20

30

1 2 3 4 5 6 7 8 9 10

pitch link damage

ballistic damage

0

1

2

3

ballistic damage

50 Hz

10 Hz

0

5

10

15root bending stiffness

ballistic damage

0

5

10

15

ballistic damage

root bending stiffness

displacementw

bendingslope

w'

mid-nodetwistM

end-nodetwistA

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 symmetric

matrices

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”

-1 K B B X B* *Stiffness damage:

Damping damage:

Mass damage:

-1 1C B B X B int* *

-1 2M B B X B int* *

Where [ B ] =matrix collection of damage vectors (step 1)

= [ d1, d2, …, dp ] [ jint ] = diagonal matrix of interrogation frequencies [ X ] = matrix collection of damaged system response

= [ {X(j1)}, {X(j2)}, …{X(jp)} ]

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

• 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. kK

./

elstrstr wwww

nom

EIEI

5 10 15 20 25 30 35 40

5

10

15

20

25

30

35

40 -2.5

-2

-1.5

-1

-0.5

0

0.5

1

x 104

510152025303540

5 10 15 20 25 30 35 40

Exact K

DOFD

OF

x 104

1

0

-1

-2

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

1 2

0

' 'TRCFww w wx

m d ds K H H

1 2 2 22 1 00

cos2CF Tm mm k k ds

K H H

1 200

' 'cosT

CF CFw w g w wxm e ds K K H H

ww w ww

w

M M M 0M

M M 0 M

1

0

Tww A w wdsM H H

1

0

TA ds M H H

1

w 00cos T

w A g we ds M M H H

ww w ww

w

K K K 0K

K K 0 K

47

interrogation frequencyrangeDamage

10-50 Hz 30-50 Hzroot torsionalstiffness

0.00% -0.81%

pitch link 0.00% 0.00%

outboard bendingstiffness

4.91% -1.48%

outboard cracka/H=0.05

-0.05% -0.10%

ballisticdamage

-48.14 -38.5

Damage Extent Summary

AMRPT Damage Extent Quantification Error AMRPT results show improvementusing higher interrogation frequencies

exact avg

exact

x xerror

x

where x is damagedparameter (EI, GJ, A)

Errors stem from small errors in damage vector

48

Outline



Conclusions

49

Effect of Modeling Errors

0

5

10

15

1 2 3 4 5 6 7 8 9 10

Damage VectorDisplacement

DOFs

Measurement Location

02468

10

1 2 3 4 5 6 7 8 9 10

Damage VectorRotation

DOFs


10% modelingerror

no error

Model Error: 10% stiffness error in baseline modelDamage: 5% outboard bending stiffness

Damage Detection Destroyed!!

50

Correction of Modeling Errors

0

5

10

15

1 2 3 4 5 6 7 8 9 10


DOFs


02468

10

1 2 3 4 5 6 7 8 9 10


DOFs


no correctioncorrected dno error

Model Error: 10% stiffness error in baseline modelDamage: 5% outboard bending stiffnessInterrogation: +/- 2.5 deg., = 40 Hz

Use damage vector correction: d=dd-dh

51

Effect of Modeling Errors on Damage Extent CalculationsDamage: 5% outboard bending stiffnessInterrogation: +/- 2.5 deg., = 32 & 40 Hz

Case

10% Increase inBaseline Stiffness

10% Decrease inBaseline Mass

% Error

45.4

-17.22

AMRPT Extent Quantification Error

• Extent quantification error for perfect model = 0.04%• Damage vector correction d=dd-dh is utilized

Small errors in damage vector result inlarge errors in frequency domain AMRPT

52

Outline



Conclusions

53

Noise

• How does measurement noise affect the results?

• How can noise effects be reduced?

Uniform Random Noisein Harmonic Signal

5%

2%

10%

54

Effect of Noise on Damage Vector

0

5

10

15

1 2 3 4 5 6 7 8 9 10

0

5

10

15

20

1 2 3 4 5 6 7 8 9 10Measurement Location

Noise: uniform random noiseDamage: 5% outboard bending stiffnessInterrogation: +/- 2.5 deg., = 40 Hz


DOFs


DOFs

Damage Vector Magnitude

5% noise2% noiseno error

55

Noise Mitigation Procedure

Number of Cycles in Average

0

1

2

3

4

5

6

0 20 40 60 80 100

10% uniform random noise




% RMSNoise

Cycle Averaging ofHarmonic Signal with Noise

56

Benefits of Cycle Averaging

02468

10

1 2 3 4 5 6 7 8 9 10

02468

10

1 2 3 4 5 6 7 8 9 10Measurement Location


DOFs


DOFs

Noise: 5% uniform random noiseDamage: 5% outboard bending stiffnessInterrogation: +/- 2.5 deg., = 40 Hz

Damage Vector Magnitude

Threshold = 1.8

Threshold = 2.5

5% noise40 cycle averageno error

57

Effect of Noise on Damage Extent Calculations

Damage: 5% outboard bending stiffnessInterrogation: +/- 2.5 deg., = 32 & 40 Hz

Case

2% noise20 cycles

5% noise40 cycles

% Error

114%

130%

AMRPT Extent Quantification Error

• Extent quantification error for perfect model = 0.04%

Small errors in damage vector result inlarge errors in frequency domain AMRPT

58

Outline



Conclusions

59

Damage Vector Sensitivity Approach

• Motivation– Damage vector is clean & reliable– AMRPT very susceptible small errors in

damage vector– AMRPT only applicable for null symmetric

systems– Aerodynamic damage is non-symmetric

• Does magnitude of damage vector indicate damage severity?

60

Damage Vector Sensitivity Approach

0

20

40

60

80

100{d(w

6)}

{d(w7)}

0

50

100

150

0 0.1 0.2 0.3 0.4 0.5

{d(w'6)}

{d(w'7)}

Bending Stiffness (EIyy

/EIyy

nominal)

Damage Vector Magnitude vs. Damage ExtentOutboard Bending Stiffness Fault

DamageVector

Magnitude

DamageVector

Magnitude

• Simple relationship relates damage severity to damage vector magnitude

• Nearly linear for small damage

61

Outline



Conclusions

62

Measurability

• Sensitivity– How large is the response of the healthy

system at sensor locations?

• Resolution– How does damage change the magnitude of

response?

63

Measurability: Sensitivity

Direct Measurements– Displacement < 0.25"

– Rotation < 0.25 deg.

– Twist < 1 deg.

Strain Measurements– Bending Strain < 250 -strain

– Shear Strain < 60 -strain

• Frequency averaged 10-50 Hz, 2 Hz step.

• Peak-peak harmonic response amplitudes

64

Measurability:Change in Direct Measurement Response

root bending stiffnessroot torsional stiffness

root crack a/H=0.05root crack a/H=0.2

pitch linkoutboard bending stiffness

outboard torsional stiffnessoutboard crack a/H=0.05outboard crack a/H=0.2

ballistic damagetrim mass

* Results averaged over frequency and blade length

0 0.5 1 1.5 2 2.5

displacementbending slopetwist

% change from healthy system response

65

Outline

Background & MotivationObjectives of WorkModeling ApproachDamage IdentificationConclusions

66

Recommendations & Conclusions

Summary

– A unique active-interrogation damage evaluation approach for helicopter rotor systems using trailing-edge flap actuation has been designed and implemented in a numerical rotor code.

– Residual force vector and AMRPT adapted for active interrogation approach

– Residual force vector sensitivity approach formulated as alternative extent quantification approach

– Detection & Extent demonstrated in hover using trailing edge flap actuation within the bounds set by vibration control requirements (< 50 Hz, +/- 2.5 deg. deflection)

– Effects of noise & modeling errors assessed and mitigated

– Preliminary measurability study

67


Successes – Damage detection

• very clean for mass and stiffness faults• not sensitive to interrogation frequency• faults detected & characterized in the presence of 5%

noise with cycle averaging• faults detected & characterized with 10% baseline

model errors using damage vector correction– Damage extent measurement via AMRPT

• stiffness faults within 5% error (without noise or modeling errors)

68


Limitations– Damage extent very sensitive to noise & errors– AMRPT damage extent algorithm modified to account

for CF stiffening effects BUT sensitivity to errors in damage vector is severe

– AMRPT damage extent algorithm inappropriate for aerodynamic faults

– Measurability• Typical change in response = 1%• Change in response << 1% for cracks, flexbeam

torsional stiffness fault

69


Remarks– Damage detection in helicopter main rotor using active

interrogation with trailing edge flap is promising– Damage extent using frequency domain AMRPT is difficult

due to sensitivity to errors in damage vectors

Recommendations– Extent calculations using damage vector sensitivity– Optimize sensor placement– Optimize interrogation frequency – Implementation of strain-based approach– Investigate alternate detection and extent algorithms

• non-linear time series feature extraction (Todd et al, 2001)

70

Acknowledgments

This work was funded in part by – The ONR MURI in Integrated and Predictive

Diagnostics through the Penn State Applied Research Lab

– The ARO MURI in Active Noise and Vibration Control Technologies for Jet Smooth, Quiet Rotorcraft

active interrogation of helicopter rotor faults using trailing edge flap actuation

Documents

rotor diagnostics

rotor componentsah

rotor system failureswhat

trailing edge flap actuators

trailing edge flapgoal

effective damage evaluation

presence of noise

hours of flight time