Expert System to Support Operational Safety of the TS-11

“Iskra” Aircraft and Overhauls of the SO-3 Engines

Mirosław WITOS, Michal WACHLACZENKO

Air Force Institute of Technology

Warsaw, Poland

7th International Symposium on NDT in Aerospace 16 – 18 November 2015, Bremen, Germany

Mo4-A7

Outline: Introduction Motivation Fatigue problem of the SO-3 engines Tip Timing Method Application More then 20 years of experience Conclusion

1F mode1F mode

How to detect level of fatigue of critical elements before the accident?

Introduction Some fatigue problems of SO-3 turbo jet engine

What prevention strategy (i.a. NDT and SHM methods) should be used?

Uncontroled blade fatigue:

- is a threat for service safety

- limits turbomachinery life time

- increases maintenance costs

Motivation

Compressor blades’ fatigue and crack

Reliability = Blades properties & Operating conditions

Motivation

Fatigue problems of compressor blades = {LCF, HCF, VHCF}

Tp > 30 godzTp > 30 godzTp > 30 hours

„Fish eye”

Tp < 30 minTp < 30 min

During TBO 1F mode: N = 3 – 6 Giga cycles;

1T mode: N = 10 – 25 Giga cycles

Motivation

Compressor blade - the effect of closing the crack’s gap

Disadvantages of classic NDT methods: - High man-hour consumption - Probability of detection (POD) for blades is dependent on the time after stopping the engine.

VT – crack or scratch?

15 mm

UT method: After engine stopped: 15 mm 16 hour after stoped: 5 mm 76 hour after stopped: no failure mode!

the resolution: < 0.05 mm

Structural errors of blades SO-3 engine, 1st stage of compressor

0

200

400

600

800

1000

7000 8000 9000 10000 11000 12000 13000 14000 15000 16000n [rpm]

sn [MPa]

Normal level

High level (bird)

III harm.

II harm.

Elastic

limit

Yield point

Tp < 30 min

High stress level Tp > 30 hours

Normal

stress level

nucleation

1. Is the blade cracked? Yes: engine repair, preventive actions No:

2. Is the blade overloaded? Yes: Special NDT procedure No: Normal NDT procedure

Overhaul problems

1. The time from engine stop to NDT of blades: a few weeks/months.

Required additional procedure (annealing) to open cracks before NDT.

3. Quality Guarantee made repair: typically 1 year.

The required procedure for documenting the results of NDT and minimizing

the human factors.

2. Heterogeneity of material fatigue: the blades of different production batches,

at different times of work and the history of material effort.

Required additional procedure to detect material overload.

Engine user

Mission

Loads

Material effort

& residual stress

Modal property

(C, K, M)

Compressor blades

s(w, t)

e(w, t) dC(t)

dK(t)

dC(t)

Modal property of

dynamic phenomena

Operating

range

Engine

Fatigue observer (sensitive NDT&SHM)

Fatigue control (diagnose & recommendation)

Fuel system adjustment

Correct of operating range

Active control of blade fatigue – Don’t wait for crack!

Environment

condition

T(n,H,V)

r(H,V)

The methodology has been implemented in aviation Polish armed forces since 1993.

Tip Timing

OBSERVER

ENGINE HEALTH (fuel system, bearings, rotor resonance)

Vibration level of all

blades in the same time

(Campbell, 1924)

Disadvantageous dynamic

phenomena (stall, surge, flutter, resonance)

Blades health (early phase of cracking)

Disk health (early phase of cracking)

Blade stress (TOA & NSMS systems)

INPUT

UNIT

w

TRANSDUCER

MEASUREMENT

UNIT

ERRORS

CORRECTION

&

SIGNAL

DECOMPOSITION

ADVANCED

SIGNAL

ANALYSIS

&

MACHINE

DIAGNOSIS

Code(k)

TOA(k)

Blades

Disk

Rotor

Fuel unit

Shaft

Bearings

Flow dynamics

Tip timing method

)()()()( tIttt PA www

BN

2min

min

0

)()(

ww dk RR

clock

avg

B

clock

B

tkCodekTOA

t

ktTrunckCode

w

w

)()(

1

1)()( min

TTM – method of getting complex data (aperiodic, periodic and noise parts) from rotational speed signal that gives an ability of decreasing number of sensors when designing complex data processing and acquisition algorithms.

fclock=10-350 MHz A(k) P(k) I(k)

S= A + P+ I

TTM measuring time of arrival (TOA) of flexible and vibrating blades

– irregular signal digitizing (sampling)

Tip timing method

)()(1

)(1)(

k

L

k

kconstkTOA

avg

TB

w

w

Measurement data gives THREE UNKNOWNS:

rotational speed of ideal stiffness rotor wavg

blades jitter (vibration spectrum) B

rotor jitter (rotational speed spectrum of real rotor) w

kkkkkk

kkkkk

SOWSWPUP

DPKBiPB

w

Blade jitter components: P – pitch errors;

Bi – blade vibration;

C – case vibration;

DP – dynamic phenomena of TTM sensor

Rotor jitter components: UP – influence of control unit;

WP – transverse vibration of rotor;

WS – torsional rotor vibration of rotor;

O – alignment error (eccentricity)

S – alignment error (misalignment)

dn/dt > 0

dn/dt < 0

Decreasing risk level of blades HCF & LCF

is possible by correct overhaul and operation

errors (i.e. shape of operating area changes)

Operating area

‘Actively’ control of blade HCF & LCF

Flow dynamics

• Flow clocking (interaction with stator blades)

• Stall

• Surge

• Flutter

• Combustion instability

• Foreign object in inlet

Kinematic loads: - Centrifugal force

- Compressor non-axial

- Rotor unbalance

- Compressor speed fluctuation

- Structure resonances

SO-3 engines: Since 1991 the statistical mean time between fatigue-offs

of blades has been increased: 15x for calendar-based data,

12x on the hour-referred data.

x

P(n,x,z) Tq(n,x,z) y

Pc(n)

Pu(n,re)

Tip timing method

The signal from the TTM

The signal from the strain gauge

SUITABLE is OBJECTTS2dt

iPar2d

,dt

idPar

,i

ParCVm1,2,...,i

SNDŁ-1b/SPŁ-2b diagnostic system Since 1993

Blade Excessive Vibration Device

SNDŁ-1b

(on-board unit)

Blade Crack Signaling Device

SPŁ-2b

(ground equipment)

SOFTWARE comprises: - INFO database

- MEASURING database

- MODELS database

- DIAGNOSTIC RULES database

- Log

Monitoring blade vibration spectra every:

- 50 hours – Low LCF risk

- 25 hours – Medium LCF risk

- 12 hours – High LCF & HCF risk

In 1997 a real time module on the basis of the Keithley's CTM-PER counter card has been added to the already existing diagnostic system and is now used in the repair works.

VR sensor

SjBjj eee

Numerical signal analysis SPŁ-2b software

Base equations

EXPERT OF

FUEL SYSTEM

COMPRESSOR

BLADE

VIBRATION

ANALYSIS

BEARING

SYSTEM

TECHNICAL

CONDITION

ANALYSIS

j

jjjCODE

constnn e

Time consumption of one engine test < 0.5 man-hour (PC 486DX2)

The expert system for ‘active control’ of fatigue and …

ENGINE

GROUND PERSONNEL

PILOT OVERHAUL

AFIT

Blades vibration Blades vibration

Main data base New failure procedures

New symptoms of operation errors

New symptoms of overhaul errors

(since 1997)

SNDŁ-1b

SPŁ-2b

CTM-PER/SPŁ-2b

AFIT assistance to users of SO-3 engine

Point spectrum of blade vibration SO-3 engine

PRIMARY STANDARD

28 blades made of the 18H2N4WA steel

Mode frequencies:

350 Hz (flexible vibration), Q 400

1380 Hz (flexible vibration), Q 30

1890 Hz (torsional vibration), Q > 1000

Sampling frequency ws = 200 - 1650 rad/s

SYNCHRONOUS RESONANCE ASYNCHRONOUS RESONANCE

Tip timing observer Stall & Surge

Blades response depends on both rotor and support modal properties

Deep surge

Rotor vibration Blade 1F: 2.5xfR

AFTER SURGE

Shallow surge

Stall BEFORE SURGE

Inlet blocked surge Surge limit identification (p3 signal disconnected from FCU )

Bench tests of blade cracking

The years 1989 - 1993

3x

2x

II STAGE

PHASE

MAPPING (linear SDOF)

fR

fB I STAGE

CAMPBELL DIAGRAM

22 )()0()( RBB fnBfnf

2x

3x No opened crack non-linear increase?

Prognosis: 50 engine work hour (over 9x107 HCF & 100 LCF cycles, 1/8 TBO)

2x

3x

LPF

5 minute before

blade break off

Thanks to the active control of the user over the material

fatigue process the following have been eliminated:

Cracking of compressor blades in operation despite the

existence of uncorrected design flaws and high LCF risk

in the take-off range.

The statistical time between blade cracks has been prolonged for over 1500% (despite the existing structural error)

Other fatigue problems occurring in the SO-3 engines

have also been minimised.

Safety results More then 20 years of experience

Maintenance problems of fuel system

REPEATABILITY (INDIVIDUAL CONTROL)

HIDDEN DEFECT

ERROR CONTROL

(SURGE)

I

TTTconst

dt

dn

WAWFfnnbdt

dnna

dt

nd

FST

,)()(2

2

F-34 fuel „Apply jelly” phenomena

n TANGENTIAL VIBRATION OF ENGINE

Complex analysis – bearing damage

BEARING DAMAGE

PRIMARY STANDARD

Ex post facto analysis

Crash, 11.10.2005

0

0.05

0.1

0.15

0.2

0.25

9300 9500 9700 9900 10100 10300

n [rpm]

Adec

[mm]

Flutter is danger not only for blades!

Conclusion

1. Over the 20-year operation of the SNDŁ-1b/SPŁ-2b diagnostic system

proved that the Tip Timing Method is very effective as:

contactless method of non-destructive testing and health

monitoring of rotating blades,

objective method for monitoring the operation quality and repairs of

the SO-3 engines.

2. The described method is of great importance to flight safety and

may be recommended as a completion of the existing systems of

diagnosing engines and power turbines, in particular, to identify the

LCF, HCF, VHCF and TMF of critical components.

Expert System to Support Operational Safety of The TS-11

“Iskra” Aircraft and Overhauls of the SO-3 Engines

Mirosław WITOS, Michal WACHLACZENKO

Air Force Institute of Technology

Warsaw, Poland

7th International Symposium on NDT in Aerospace 16 – 18 November 2015, Bremen, Germany

Mo4-A7

Thank you for your attention

Any questions?

More info: http://www.researchgate.net/profile/Miroslaw_Witos http://www.researchgate.net/profile/Michal_Wachlaczenko