Turbomeca Lecture - Part 4-Engine Control

2nd level Specializing Master Course in Rotary Wing Technologies Edition 2014-2015

Turbos Turb osha haft ft en engi gine ne an and d its its installation within rotorcraft Part 4 :Tu :Turb rbos osha haft ft Co Cont ntro roll Sys Syste tem m Turb Tu rbom omec eca a co cour urse  se  Effective slide : 28

2014

Turboshaft Control System Introduction 

From the very beginning of TURBOMECA turboshafts, control system skills are as important as bare engine skills

ARTOUSTE ARTOUSTE Fuel Control Unit (1951) 1 / 

Turboshaft Control System Introduction 

From the very beginning of TURBOMECA turboshafts, control system skills are as important as bare engine skills

ARTOUSTE ARTOUSTE Fuel Control Unit (1951) 1 / 

Turboshaft Control System Introduction 

Control system is a strategic component for helicopter turbos tur boshaf haftt app applic licati ation on

    

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Enhances the engine performance and its operability Directly acts on the helicopter handling qualities and on the performance of NR speed control Contributes to the pilot workload reduction and to the aircraft safety Embeds monitoring and diagnosis functions Counts for 15 thru 20% of engine production cost and has become a major technical and economical issue

Control System – General presentation Vocabulary NR rotor speed Gas generator

Free turbine

N1

MGB

N2

Combustion chamber

T45 P3

T1, P0

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CH or WF

Engine Control system

N1

N2 Torque Collective pitch XPC

Turboshaft Control System TURBOMECA architectures history 

Hydromecanical control 

All the functions are achieved by flyweights, hydraulic spool/sleeve, pneumatic bellows… 1990’s design

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Single channel FADEC with backup manual fuel control « protected » mode  

Single channel FADEC controls a stepper motor driving the fuel metering valve Fail freeze failure mode with auxiliary backup allowing manual fuel flow change in a protected range 2000’s design

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Dual channel FADEC  

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Redundancy of critical electronic and electrical functions Auxiliary backup mode is available for single engine applications

Control System– Architectures Hydromecanical Fuel Control Rotor Gas generator N1

Power turbine N2

Combustion chamber

N1 P3

Fuel flow

HMU (governor)

P0

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Main Gear Box

N2

Collective pitch

Control System– Architectures Hydromecanical Fuel Control

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Control System– Architectures Hydromecanical Fuel Control

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Control System– Architectures Hydromecanical Fuel Control

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Control System– Architectures FADEC control Engine

Helicopter

Gas generator Power turbine

N1

BTP N2

Combustion chamber

T45 Fuel Flow

P3

EECU + Fuel system

T1, P0

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N1

N2

Collectif pitch data

Torque

Pilot commands (Stop, Idle, Flight…)

Control System– Architectures Dual channel FADEC control

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Control System– Architectures Dual channel FADEC control

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Control System– Architectures Fuel system

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Control System– Architectures Metering unit

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Failure mode « fail freeze » thanks to stepper motor technology : engine power remains constant in case of electronic or electric failure

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Control system - Architectures Fuel system manifold control

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Control System– Architectures FADEC control

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Control System – General presentation Control system functions The control system provides the following functions:

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Fuel pumping

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Fuel filtering

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Fuel metering to the start injectors and the main injectors

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Fuel shut-off

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Electrical self-sufficiency of the control system, thanks to an alternator

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Automatic starting without "over-temperature"

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Automatic in-flight re-start

Control System – General presentation Control system functions

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Automatic N2 control in flight mode

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Acceleration control (anti-surge protection systems)

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Deceleration control (anti-flame-out protection systems)

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Temperature limits

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Torque limits

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N2 overspeed protection (not systematic)

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N1 overspeed protection (not systematic)

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OEI detection and management of emergency ratings (for twin engines)

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OEI training mode (TRAINING) (for twin engines)

Control System – General presentation Control system functions 

Indications to the helicopter cockpit

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Engine maintenance assistance:  engine power check  

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Available T45 marging to deliver the required power Available N1 marging to deliver the required power

automatic counting of N1 and N2 cycles creep counting failure detection failure recording failure context recording emergency rating usage counters

Control System – General presentation Overspeed protection 

In case of overspeed due to system or mechanical failure, an independent subsystem detects the overspeed condition and energizes the fuel shut-off valve

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Control system- Control laws N2 and NR control during pilot manoeuvre Torque

engine  –

Torque

resistive  =

inertia x dN2  dt 

Resistive torque (TRQr): • on the helicopter, this is a function of collective pitch XPC • on the engine test bed, this is a function of the brake valve position

• helicopter inertias (rotors, MGB) + free turbine inertia of the engine(s) • inertia of inertial flywheel + inertia of the engine free turbine on test bed

XPC

Pitch decrease

XPC TRQ TRQr

TRQ = TRQr TRQ = TRQr N2 constant N2 constant N2 TRQ > TRQr TRQ < TRQr N2 increases N2 decreases

TRQr Pitch increase TRQ N2 TRQ = TRQr N2 constant TRQ < TRQr N2 decreases

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TRQ > TRQr N2 increases TRQ = TRQr N2 constant

Control system- Control laws Speed control loops

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Control system- Control laws Fuel control and limitations

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Control system- Control laws Starting control

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Control system- Control laws Acceleration limitations Example of limits used during a pitch increase 

N1*

Over-torque Protection N1L*

Anti-surge protection

N2

Goal : best balance between : • quick response to prevent N2/NR undershoot • mandatory surge free compressor acceleration 24 / 

Maximum N1 protection (thermal)

Control system- Control laws Overtorque limitation Example of over-torque limitation  N1L* N1L*

N1*

N1* N1

Engine torque Engine torque

N1

Overtorque

Over-torque Engine torque

N2

N2

Without over-torque protection 

Goal : • Protect helicopter main gear box against overtorque • Prevent yaw jerk in reaction to too high dNR/dt 25 / 

Over-torque Protection when the N2 speed increases, the acceleration "breaks off" in order to limit overtorque and yaw kicks

With over-torque protection 

Control system- Control laws Deceleration limitation Example of limits used during a pitch decrease  Anti-flameout protection

N1*

N1L*

Minimum N1 protection

N2

Goal : best balance between : • quick response to prevent N2/NR overshoot • mandatory flame-out free deceleration 26 / 

Control system- Control laws Surge protection by WF/P3 limitation WF/P3 limit line

P/P

- - - accel trajectory with power off-take - - - accel trajectory without power off-take

Surge line

N1final Working line with power offtake Working line without power off-take N1initial

Air Flow

Benefit : adaptive surge margin control vs power extraction on gas generator shaft 27 / 

Control system- Control laws Surge protection by WF/P3 limitation

WF/P3 fuel limit

Surge Current fuel demand

Benefit : ability to get out from an unexpected surge event 28 / 

Control system- Control laws Starting control Example of a start-up  T45 maximum

T45

Preset fuel flow

Combustion chamber ignition T45 protection

WFstart* The pilot orders the startup: the starting accessories are commanded (starter, start electrovalve, on/off electroN1 valve, igniters).

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End of start-up: starting accessories cut off and the engine switches to control mode

Control System – Control Laws Rotor speed control – Torsional stability Blade 

Helicopter drive train very different from an inertial load

Engine 1 Return torque

Blade lag axis Torsion stiffness

Rotor hub

Main rotor lag mode 30 / 

Tail rotor Main rotor blades

Main rotor hub and MGB

Engine 2

Control System – Control Laws Rotor speed control – Torsional stability The control system and the engine can excite the helicopter modes. To avoid this phenomenon, the engine manufacturer generally adds corrective devices in the control loop

Example of helicopter linear model

Main rotor frequency

Tail rotor frequency

Inertial mode : depends on the rotating parts inertias

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