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2nd
level SpecializingMaster Course in Rotary Wing Technologies
Edition 2014-2015
2014
Turboshaft engine and its
installation within rotorcraftPart 4 :Turboshaft Control System
Turbomeca courseEffective slide : 28
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Turboshaft Control SystemIntroduction
From the very beginning of TURBOMECA turboshafts, control systemskills are as important as bare engine skills
ARTOUSTE Fuel Control Unit (1951)
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Turboshaft Control SystemIntroduction
Control system is a strategic component for helicopterturboshaft application
Enhances the engine performance and its operability
Directly acts on the helicopter handling qualities and on the performance ofNR 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
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Control System General presentationVocabulary
NR rotor speed
T1, P0
Combustionchamber
Gas generatorFreeturbine
CH or WF N2P3 Torque
Collectivepitch XPC
T45
N1
N2
Engine Controlsystem
MGB
N1
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Turboshaft Control SystemTURBOMECA architectures history
Hydromecanical control
All the functions are achieved by flyweights, hydraulic spool/sleeve, pneumaticbellows
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 ina protected range
Dual channel FADEC Redundancy of critical electronic and electrical functions
Auxiliary backup mode is available for single engine applications
1990s design
2000s design
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Control System ArchitecturesHydromecanical Fuel Control
Rotor
Combustionchamber
Gas generatorPowerturbine
MainGearBox
Fuelflow
N2N1P3
Collectivepitch
N1
N2
HMU
(governor)
P0
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Control System ArchitecturesHydromecanical Fuel Control
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Control System ArchitecturesHydromecanical Fuel Control
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Control System ArchitecturesHydromecanical Fuel Control
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Control System ArchitecturesFADEC control
T1, P0
Combustionchamber
Gas generator
Powerturbine
Fuel Flow N2P3
Torque
Collectifpitch data
T45
N1
N2
EECU
+Fuel system
BTP
N1
Pilot commands(Stop, Idle, Flight)
HelicopterEngine
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Control System ArchitecturesDual channel FADEC control
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Control System ArchitecturesDual channel FADEC control
C l S A hi
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Control System ArchitecturesFuel system
C t l S t A hit t
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Control System ArchitecturesMetering unit
Failure mode fail freeze thanks to stepper motor technology : enginepower remains constant in case of electronic or electric failure
C t l t A hit t
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Control system - ArchitecturesFuel system manifold control
Control S stem Architect res
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Control System ArchitecturesFADEC control
Control System General presentation
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Control System General presentationControl system functions
The control system provides the following functions:
Fuel pumping
Fuel filtering
Fuel metering to the start injectors and the main injectors
Fuel shut-off
Electrical self-sufficiency of the control system, thanks to an alternator
Automatic starting without "over-temperature"
Automatic in-flight re-start
Control System General presentation
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Control System General presentationControl system functions
Automatic N2 control in flight mode
Acceleration control (anti-surge protection systems)
Deceleration control (anti-flame-out protection systems)
Temperature limits
Torque limits
N2 overspeed protection (not systematic)
N1 overspeed protection (not systematic)
OEI detection and management of emergency ratings (for twin engines)
OEI training mode (TRAINING) (for twin engines)
Control System General presentation
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Control System General presentationControl system functions
Indications to the helicopter cockpit
Engine maintenance assistance:
engine power check
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
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Control System General presentationOverspeed protection
In case of overspeed due to system or mechanical failure, an independentsubsystem detects the overspeed condition and energizes the fuel shut-offvalve
Control system- Control laws
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Control system- Control lawsN2 and NR control during pilot manoeuvre
Torque engine Torque resistive= inertia x dN2
dt
Torque engine Torque resistive= inertia x dN2
dt
XPC
TRQr
TRQ
TRQ > TRQrN2 increases
TRQ < TRQrN2 decreases
TRQ = TRQrN2 constant
TRQ = TRQrN2 constant
N2
Pitch decrease
N2
XPC
TRQr
TRQ
TRQ = TRQrN2 constant
TRQ = TRQrN2 constant
TRQ > TRQrN2 increases
TRQ < TRQrN2 decreases
Pitch increase
helicopter inertias (rotors, MGB) + freeturbine inertia of the engine(s)
inertia of inertial flywheel + inertia of theengine free turbine on test bedResistive torque (TRQr):
on the helicopter, this is afunction of collective pitch XPC
on the engine test bed, this is a
function of the brake valveposition
Control system- Control laws
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Control system- Control lawsSpeed control loops
Control system- Control laws
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Control system Control lawsFuel control and limitations
Control system- Control laws
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Control system Control lawsStarting control
Control system- Control laws
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Control system Control lawsAcceleration limitations
Example of limits used during a pitch increase
N2
N1*
N1L*
Anti-surgeprotection
Over-torqueProtection
Maximum N1protection(thermal)
Goal : best balance between : quick response to prevent N2/NR undershoot
mandatory surge free compressor acceleration
Control system- Control laws
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N1*
N1L*
Over-torque Protection when the N2speed increases, the acceleration
"breaks off" in order to limit over-torque and yaw kicks
N2
N1
Over-torque
Control system Control lawsOvertorque limitation
Example of over-torque limitation
N1*
N1L*
N2
N1
Engine torque
Over-torque
Without over-torque protection With over-torque protectionGoal : Protect helicopter main gear box against overtorque
Prevent yaw jerk in reaction to too high dNR/dt
Engine torque Engine torque
Control system- Control laws
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Co t o syste Co t o a sDeceleration limitation
Example of limits used during a pitch decrease
N1* N1L*
Anti-flame-out
protection
Minimum N1protection
N2
Goal : best balance between : quick response to prevent N2/NR overshoot
mandatory flame-out free deceleration
Control system- Control laws
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- - - accel trajectory with poweroff-take
- - - accel trajectory without
power off-take
Working line with power off-take
Surge line
Working line without poweroff-take
N1initial
N1final
WF/P3 limit line
Air Flow
P/P
ySurge protection by WF/P3 limitation
Benefit : adaptive surge margin control vs power extraction on gas generator shaft
Control system- Control laws
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Current fuel demand
WF/P3 fuel limit
Surge
ySurge protection by WF/P3 limitation
Benefit : ability to get out from an unexpected surge event
Control system- Control laws
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yStarting control
Example of a start-up
WFstart*
Preset fuelflow
The pilot orders the start-up: the startingaccessories are
commanded (starter, startelectrovalve, on/off electro-valve, igniters).
T45
N1
Combustionchamber ignition
End of start-up:startingaccessories cut offand the engine
switches to controlmode
T45protection
T45maximum
Control System Control Laws
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yRotor speed control Torsional stability
Tail rotor
Main rotor
blades
Main rotor huband MGB
Engine 2
Engine 1
Main rotor lag mode
Return torque
Torsionstiffness
Blade
Rotor hub
Blade lag axis
Helicopter drive train very different from an inertial load
Control System Control Laws
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Rotor speed control Torsional stability
The control system and the engine can excite the helicopter modes. To avoid thisphenomenon, the engine manufacturer generally adds corrective devices in the control loop
Tail rotor
frequency
Main rotor
frequency
Inertial mode :
depends on the
rotating parts
inertias
Example of helicopter linear model
Control System Control laws
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Rotor speed control Torsional stability
The control law correction aims to damp the helicopters modes peeks below -6dB, but withcorrect phase and gain margins conservation (ARP704 norm).
The main challenge is to damp the modes with a minimal alteration the low frequenciesband (conservation of N2/NR control loop time response)