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Hydromechanical System Modelling

Using Simscape & SimHydraulics to

Expand Capability

Martin Kirkman and Adam Brooke

Rolls Royce Controls and Data Services

Fuel & Actuation Sub-Systems 7th October 2015

Contents

Background

• Who are Rolls Royce CDS ?

• Systems modelling in Rolls Royce CDS

• Use of Simscape/SimHydraulics

Case studies

• Case 1: Valve seal cavitation

‒ Root cause

‒ Fix

• Case 2: Servo valve damage

‒ Root cause

‒ Further understanding

‒ Potential fix through rapid modelling

The way forward

Background

Who are Rolls Royce CDSElectronic Engine Control (EEC)

i.e. software

Pump & Fuel

metering Unit

Engine Burners

Engine Variable Geometry

Design & Manufacture

of Engine Control

Systems.

The Engine

“Intelligence”

Who are Rolls Royce CDS Engine Manifold Ring

Intricate Connecting Pipework

Fuel

Control

Fuel

pump

Fuel

filter

Pipework to

engine

manifold

Hydromechanical Unit

Aircraft

supply

Significant potential for

mechanical and/or fluidic

resonance.

Systems Modelling in RR CDS

Matlab and Simulink• Dynamic models of hydromechanical units used for performance analysis and control law

design • Uses models of pressures, flows & valves to represent the system

• Models built from complex, non-linear, stiff PDEs.

• Reasonably accurate representation

• Aid investigation analysis work through application of tolerances and special cases

Application of SimHydraulics• Using SimHydraulics to increase fidelity of models through use of the physical modelling

environment.

• Applications include:• Pump ripple

• Assess overboard leakages in engine pipe breakage

• Emergency shutdown – water hammer & fluid compressibility

• In-service support issues

- Dynamic seal cavitation investigation

- Servo valve O-ring

Case Studies

𝒄 = 𝑲 𝝆

Where

c = Speed of sound (proportional

to peak frequency)

K = Bulk modulus

𝝆 = Density

Property SimHydraulics

Density (𝑘𝑔/𝑚3) 745

Bulk Modulus (P𝑎) 8.9× 108

Temperature 100oC

𝒄 = 8.9×108745 = 1093 m/s

Principles of Resonance

Res

pons

eR

espo

nse

Res

pons

e

Single source closed ended system

• Metering Valve bore

Sou

rce

Sou

rce

Sou

rce

Sou

rce

¾ standing wave – maximum oscillation

Whole standing wave – minimum oscillation

¼ standing wave – maximum oscillation

½ standing wave – minimum oscillation

Res

pons

e

Case Study 1 - Valve Dynamic Seal

Valve Dynamic Seal- Background• Cavitation damage observed on in-service units after being returned for

failed engine starts

- Only occurring on relatively new units incorporating a new design of the valve.

• Assistance asked by Customer Support Services to investigate the problem.

• Beyond the normal capability of “conventional” Simulink models- SimHydraulics used to model internal passageways leading to damaged area

- Pump ripple suspected to be causing the issues

• Significant resonance observed in the model- Used to support testing to focus accelerated damage on the test rigs

• Cavitation caused by amplification of pump ripple within the valve bore.

Definition of internal path

Cross sectional areas continually decreaseThis amplifies the pressure wave at resonant point

Lp(pump ripple)

Amplification of 1030Hz down the bore (1/4 wave)

Pre

ssu

re

7854Hz around the seal ring 2 holes

Pre

ssu

re

Valve seal - Solution Support

Modelling conducted on original entry-into-service (EIS) metering valve

to observe the differences in resonance excitation

- No damage had been observed on any of these units

- Main reasons for this;

• Larger internal volumes – prevent amplification of ripple

• 10 cross drilling removing high frequency peak.

Comparison conducted on resonant response of EIS, the revised design

and a suggested mod for the revised design which included;

- New screw cap to increase volume

- Reworked piston to increase internal volume and give additional cross

drillings

Valve Dynamic Seal Cavitation Solution

• Similarities to EIS – design pedigree

• Larger fluid volumes

• Re-introduction of 10 holes

Piston bore diameter increased to

increase internal volume

Piston with additional machining to

increase internal volume

Screwed Plug with additional machining to

increase internal volume

The number of radial holes in the Piston

increased from 2 to 10

500 600 700 800 900 1000 1100 1200 1300 1400 15000

5

10

15

20

25

Frequency of excitation (Hz)

Ord

er

of

ma

gn

ific

atio

n o

f L

p s

ou

rce

rip

ple

Response to excitation frequencies at the bal seal midpoint between two cross drillings

FMU EISFMU RevisionFMU Modified on TestFMU New Production Standard

EIS, Revision and new standard response to ripple

Idle range

Geometry of EIS, revision and

new modified standard

compared for amplitude

increase relative to frequency

of oscillation.

Modifications can be seen to

bring the peak amplitude back

to around 700Hz and the

amplification at flight

conditions below that of the

EIS standard.

These peaks can be expected

to shift ±70Hz with fuel type

and temperature.

Rest of flight regime

7854Hz around the seal ring 10 holes

Pre

ssu

re

Case study 2 – Servo Valve ‘O’ Ring

Servo Valve (SV) - Background

• In-service SV ‘O’ ring seals cracking

resulting in fuel leaks

• Approached by Customer Support after

the successes of valve dynamic seal

analysis to see if resonances are

present.

• Details of the internal drillings of the unit

provided in the Low pressure (LP) return

pathways for all servo valves.

Internal Body Drillings (fluid pathways)

VSVSV LP port

VSVSV LP port

MV - Metering Valve

VSV - Variable Stator Vane

SimHydraulics Representation

Conn1

SOVSV

HP estimate

In

Out

PATH ABCD

HP estimate

In

Out

PATH 1234

HP estimate

Conn1

Oscilatory source

A B

LP inlet

Conn1

FRTT1FRTT

Resonant peak of all three servo valves at 20oC

0 200 400 600 800 1000 1200 1400 1600 1800 20000

2

4

6

8

10

12

14

16

18

20

Frequency (Hz)

Rip

ple

mag

nific

atio

n (fa

ctor

)

MVSV

SOVSV

VSVSV

Taxi

Cru

ise

Clim

b

Take off

Temperature sensitivity at Servo Valve

0 200 400 600 800 1000 1200 1400 1600 1800 20000

5

10

15

20

25

30

Frequency (Hz)

Rip

ple

mag

nific

atio

n (fa

ctor

)

-45 DegC

0 DegC

100 DegC

165 DegC

20 DegC

SimHydraulics determines

density, compressibility and

viscosity through lookup tables

based on temperature.

Temperature Peak frequency Peak amplitude

-45 1289 16.42

0 1156 18.55

20 1109 19.44

100 964 22.86

165 880 25.54

𝒄 = 𝑲 𝝆

Taxi

Pump ripple definition

Pump ripple is made up

of multiple sinusoidal

waves being driven at

different harmonics of

the pump ripple

frequency.

For this reason we

cannot just consider

the simple frequency of

the pump.

0 1000 2000 3000 4000 5000 6000 7000 80000

10

20

30

40

50

60

70

80

90FFT of source pump ripple MTO (7950RPM)

Frequency (Hz)

FFT

Am

plitu

de

0 1000 2000 3000 4000 5000 6000 7000 80000

10

20

30

40

50

60

70

80

90FFT of source pump ripple Taxi (5000RPM)

Frequency (Hz)

FFT

Am

plitu

de

FFT of three pump signals

0 1000 2000 3000 4000 5000 6000 7000 80000

10

20

30

40

50

60

70

80

90FFT of source pump ripple Cruise (7500RPM)

Frequency (Hz)

FFT

Am

plitu

de

Taxi: 1st, 2nd, 3rd

Cruise: 1st, 4th, 3rd, 2nd

MTO:, 4th, 2nd ,1st ,5th ,3rd

Extended frequency response

Due to the amplitudes

of pressure ripple at the

higher harmonics must

consider the frequency

response plot beyond

the fundamental pump

frequency.

Horizontal blue lines

represent cruise

harmonics for

reference.

Is there a solution?

Drilling directed towards FRTT Sol V in addition to RSOV

Feeding in test data from rig

0

1

2

3

4

5

6

7

8

3000 4000 5000 6000 7000 8000 9000

Am

plif

icat

ion

Pump speed (RPM)

In-service pump amplification of ripple at the SOVSV

CurrentHMU

New HMU(easy mod)

Use select samples of

pump ripple from test

data.

Pressure ripple is complex

(not simple sinusoidal as

used for the frequency

sweeps)

Can see clear maximum in

the Taxi regime the region

of 7 times magnification.

Taxi

Dry

ru

n

Cru

ise

Clim

b

Take

off

The way forward

How to tackle the future

Return of units

Test rig data

SimHydraulicsanalysis

Solution and lessons learnt

Lessons learnt

SimHydraulicsanalysis

More robust design

Current successes Future desire

Design of dead ended pathways should

not reduce in cross section down the path.

Path length resonances should be

considered for pump ripple coincidence.

Build in greater analysis depth through

use of tool. Replace current practice with

higher fidelity models.

Investigate potential issues before they

occur on young units and ensure good

design going forward on new projects.

Any Questions?