This paper describes the design process for the control system of an advanced geared turbofan engine. This process is applied to a simulation that is representative of a 30,000 lbf thrust class concept engine with two main spools, ultra-high bypass ratio, and a variable area fan nozzle. Control system requirements constrain the non-linear engine model as it operates throughout its flight envelope of sea level to 40,000 ft and from 0 to 0.8 Mach. The control architecture selected for this project was developed from literature and reflects a configuration that utilizes a proportional integral controller integrated with sets of limiters that enable the engine to operate safely throughout its flight envelope. Simulation results show the overall system meets performance requirements without exceeding system operational limits

Chapman, Jeffryes W.

Litt, Jonathan

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Control Design for an Advanced Geared 
Turbofan Engine
Jeffryes W. Chapman, 
Vantage Partners, LLC
Jonathan Litt, 
NASA Glenn Research Center
AIAA Joint Propulsion Conference
Atlanta, GA
July 10-12, 2017
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https://ntrs.nasa.gov/search.jsp?R=20170008458 2019-08-29T23:26:33+00:00Z
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Outline
• Background and Objective
• Simulation Description
• Engine Model
• Controller Design
• Actuator Hardware Model Design
• Mission Demonstration 
• Sample Study
• Details on Model Operation
• Summary
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• As preparation for the next generation of aircraft, advanced high-
efficiency engine concepts have been developed to demonstrate new 
technology.
Background
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The Advanced Geared Turbofan, 30,000 lbf (AGTF30) engine simulation 
was developed to investigate possible next generation engine system 
designs including:
1. Ultra-high bypass
2. Small core
3. Variable area fan nozzle (VAFN)
• Objective:
– Detail the generation of a full envelope, classical control system for the AGTF30 
engine.
– Highlight challenges associated with the control system concept of this advanced 
engine. 
• Purpose:
– To provide a dynamic platform for next generation engine system research.
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• The AGTF30 was created using the Toolbox for the Modeling and 
Analysis of Thermodynamic Systems (T-MATS)
• Modular thermodynamic modeling framework created by NASA
• Built on top of MATLAB/Simulink
• Package highlights:
– General thermodynamic simulation design framework
– Variable input system solvers
– Advanced turbo-machinery block sets
– Control system block sets
– Non-proprietary, free of export restriction and open source with +4500 
downloads, https://github.com/nasa/T-MATS/releases
Modeling Platform
4
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• Advanced Geared Turbofan features
– Variable area fan nozzle (VAFN)
– Dual spool with low pressure shaft connected to fan via a gear box
• Performance
– BPR = 24, OPR = 50, TIT = 3000, TSFC = 0.46 at cruise
– 30,000 lbf takeoff thrust 
• Control Effectors: VAFN, fuel flow (Wf), and variable bleed valve (VBV)
Engine Model Description
5
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Fuel Control Architecture
• Fuel Control methodology based on literature
– Power Management generates fan speed request based on power lever angle (PLA)
– Fan speed controller generates a fuel flow request
– Sets of limiters adjust the fuel flow request to operate the engine safely, avoiding engine 
stall, exceeding structural limits, combustor blowout, etc.
– Controllers utilize PI method, tuned to meet requirements throughout the envelope
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Acceleration limit 
for stall margin mitigation 
Structural limits
Deceleration and Pressure
limits for combustor blow 
out protection
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Fuel Control Power Management
• Fuel is the main effector to control the engine, and thrust is the dominant 
engine output.
• AGTF30 utilizes a classical control strategy
– Thrust cannot be sensed, so fan speed (Nf) was selected as the thrust surrogate
– Advantages of using fan speed:
• Generally linear relationship with thrust (given constant environment and consistent 
variable geometry positions).  
• Sensor availability and low susceptibility to noise
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Relationship between 
Thrust and Nf sea level and 
static conditions
Nf, rpm
800 1000 1200 1400 1600 1800 2000
104
0.5
1
1.5
2
2.5
3
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Fuel Control Tuning
• PI controller gains tuned to ideal values throughout envelope
– Linear models were generated throughout the envelope and at various power 
levels
– PI controller gains were tuned for each defined linear model.
– Gains were  collected into schedules that provide the optimum gain at each 
operational point.
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Operational Envelope
Speed Controller
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Setting Fuel Limiters
• Limiters designed to maintain safe engine operation
– Set to avoid engine stall, structural limits, and engine blow out.
• Structural limits based on anticipated next generation requirements.
• Stall mitigated by limiting acceleration with a maximum Wf/Ps3 limit
• Hypothetical engine blow out mitigated with minimum Wf/Ps3 and Ps3 limits
– Limiters tuned to allow acceleration from idle to 95% takeoff power within 5 
seconds
– Minimum stall margin requirement set to 8%.
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VBV Control Architecture
• Variable bleed valve opens to reduce low pressure compressor (LPC) 
pressure ratio (PR), increasing stall margin.
– Schedules constructed to maintain 10% stall margin during steady-state operation. 
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Opening VBV to 
increase LPC stall margin
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VAFN Control Architecture
• Variable area fan 
nozzle area scheduled 
to maintain optimal fan 
efficiency.
– Nozzle area increased to 
reduce fan PR
– Nozzle area decreased to 
increase fan PR
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Fan Performance
VAFN Schedule
Optimal efficiency 
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Actuator Modeling
• Fuel metering valve (FMV)
– First order actuator with a dynamic response much faster than rotor dynamics
• Variable bleed valve (VBV) 
– First order actuator with a dynamic response much faster than rotor dynamics
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• Variable area fan nozzle (VAFN)
– Research into VAFN actuation is ongoing
– Thermally activated shape memory alloy is being considered as a solution
• Advantages 
– High power-to-weight ratio
• Challenges
– Maximum area reduction may not meet ideal nozzle requirements
– Slew rate may not meet engine transient requirements.
– Low technology readiness level (TRL)
– Due to uncertainty in actuator characteristics the AGTF30 utilizes an idealized 
actuation system as default, settling time equivalent to 0.8s. This value will be 
updated as more research becomes available.
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VAFN response study
• The AGTF30 was used for a simple study to find the minimum settling 
time requirement for a hypothetical shape memory alloy actuator.
– Traces show acceleration from Idle to Full power then a deceleration back to idle
– Plots b and c show divergence from ideal actuation (tracks control request perfectly)
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Settling times greater than 9.8s, generate large losses of thrust
Thrust loss during 
acceleration
Increase in 
fan speed 
during 
acceleration
Stall margin 
positive at 
all points
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Model Validation
– Engine idling 
– Acceleration from idle 
to full power followed 
by a take off at sea 
level static conditions
– Engine climbs to cruise 
at 35,000 ft
– Deceleration and 
descent
– Aircraft lands then  
returns to idle
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• Engine Model validation
• Simulation of an abbreviated mission profile
0 1000 2000 3000 4000 5000 6000
104
0
2
4
0 1000 2000 3000 4000 5000 6000
0
0.5
1
Time, s
0 1000 2000 3000 4000 5000 6000
0
50
100
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Model Validation, full profile
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For the validation profile, all parameters remain within acceptable parameters 
and the engine performs as expected
Control regulators hit: accel, 
T45, Ps3, and Nf.
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Model Validation, takeoff and climb
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During acceleration and climb to altitude the control regulators act to maintain 
stall margin and maximum T45 limit
T45 max
Wf/Ps3 max
Approaches 
min limit
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Model Validation, approach and landing
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During approach and landing the control regulators act to maintain stall margin, 
maximum Nf limit and minimum Ps3 limit
Ps3 min
Wf/Ps3 
max
Approaches 
min limit
Nf max
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Simulation Operation
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Folder containing setup files
Link to input definitions
Engine Model
Steady-State, Dynamic, and 
Linearization models, all use 
the same Engine Model file
Excel spread sheet for 
quick input definition
Plotting scripts
Simulation setup and clean 
up scripts
AGTF30 class definition
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Setup Everything
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Setup_everything.m - Loads bus 
objects and MATLAB Workspace 
(MWS) structure containing all 
simulation  inputs
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Input File
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• Enter inputs manually 
• Or use an excel spread sheet
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Running the Model
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Data Presentation
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Data gathered in an 
output structure.
Formatted to 
make use of 
T-MATS auto 
plotting tools
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Summary
• A simulation of a next generation engine has been presented
– Advanced Geared Turbofan 30,000lbf (AGTF30)
• Ultrahigh bypass, small engine core, VAFN design
• Full envelope dynamic control system 
• Built with the Toolbox for the Modeling and Analysis of Thermodynamic 
systems (T-MATS), https://github.com/nasa/T-MATS/releases
• Simulation awaiting approval to be made publically available
• Control system design described 
– Fuel control based on classical architecture
– Variable geometries scheduled
• Sensitivity study on VAFN slew rate
– Shape memory alloy is currently being considered for use as the VAFN 
actuator, and actuator slew rate has been shown to be a limiting factor.
– Analysis of ideal and potential slew rates show significant performance 
degradation at actuator settling times greater than 9.8s
• AGTF30 simulation meets all requirements 
– Simulation provides a realistic and dynamic platform for research into 
advanced geared turbofan technologies.
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Acknowledgments
Funding for this work was provided by NASA 
Transformational Tools and Technology (TTT) project.
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Control Design for an Advanced Geared Turbofan Engine

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