Jeffrey T. Csank

Glenn Research Center, Cleveland, Ohio

Alicia M. Zinnecker

N&R Engineering, Parma Heights, Ohio

Application of the Tool for Turbine Engine

Closed-loop Transient Analysis (TTECTrA)

for Dynamic Systems Analysis

NASA/TM—2014-218449

December 2014

AIAA–2014–3975
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Jeffrey T. Csank

Glenn Research Center, Cleveland, Ohio

Alicia M. Zinnecker

N&R Engineering, Parma Heights, Ohio

Application of the Tool for Turbine Engine

Closed-loop Transient Analysis (TTECTrA)

for Dynamic Systems Analysis

NASA/TM—2014-218449

December 2014

AIAA–2014–3975

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

50th Joint Propulsion Conference

cosponsored by AIAA, ASME, SAE, and ASEE

Cleveland, Ohio, July 28–30, 2014
Acknowledgments

The authors would like to thank Dr. Sanjay Garg of the NASA Glenn Research Center for providing guidance on developing the 

TTECTrA tool and this research. The authors would also like to thank Jeffrey Chin and Jonathan Seidel, both of NASA Glenn 

Research Center, for their help in understanding the systems analysis process and providing feedback regarding the TTECTrA 

tool. Our thanks go to the NASA Fixed Wing Project for funding this work.

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2014-218449 1 

Application of the Tool for Turbine Engine Closed-loop Transient 

Analysis (TTECTrA) for Dynamic Systems Analysis 

Jeffrey T. Csank 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Alicia M. Zinnecker 

N&R Engineering 

Parma Heights, Ohio 44130 

Abstract 

The aircraft engine design process seeks to achieve the best overall system-level performance, weight, 

and cost for a given engine design. This is achieved by a complex process known as systems analysis, 

where steady-state simulations are used to identify trade-offs that should be balanced to optimize the 

system. The steady-state simulations and data on which systems analysis relies may not adequately 

capture the true performance trade-offs that exist during transient operation. Dynamic Systems Analysis 

provides the capability for assessing these trade-offs at an earlier stage of the engine design process. The 

concept of dynamic systems analysis and the type of information available from this analysis are 

presented in this paper. To provide this capability, the Tool for Turbine Engine Closed-loop Transient 

Analysis (TTECTrA) was developed. This tool aids a user in the design of a power management 

controller to regulate thrust, and a transient limiter to protect the engine model from surge at a single 

flight condition (defined by an altitude and Mach number). Results from simulation of the closed-loop 

system may be used to estimate the dynamic performance of the model. This enables evaluation of the 

trade-off between performance and operability, or safety, in the engine, which could not be done with 

steady-state data alone. A design study is presented to compare the dynamic performance of two different 

engine models integrated with the TTECTrA software. 

Nomenclature 

CMAPSS40k Commercial Modular Aero-Propulsion System Simulation 40k 

DSA Dynamic Systems Analysis 

Engine A Scaled version of CMAPSS40k 

Engine B Scaled version of CMAPSS40k with smaller acceleration limiter 

FAR Federal Aviation Regulations 

HPC High-Pressure Compressor 

LPC Low-Pressure Compressor 

NPSS Numerical Propulsion System Simulation 

PI Proportional-Integral 

Ps3 High Pressure Combustor Static Discharge Pressure (psi) 

SA (Steady-state) Systems Analysis 

SM Surge Margin (%) 

TSFC Thrust Specific Fuel Consumption 

TTECTrA Tool for Turbine Engine Closed-loop Transient Analysis 

T40 Turbine Inlet Temperature (degrees Rankine) 

Wf Fuel Flow (lb/s) 
NASA/TM—2014-218449 2 

1.0 Introduction 

Systems analysis (SA) is a complex process that uses steady-state system-level simulations to 

evaluate performance, weight, and cost of a given design. The process requires extensive analysis of 

trade-offs in order to optimize and evaluate individual technology benefits offered by the system. When 

applied to aviation propulsion systems, these analyses produce results that help guide technology 

investment, architecture, and program planning and formulation throughout the life of the program. 

There are a multitude of tools available for SA that may be used to determine the steady-state 

performance of a conceptual design, such as custom cycle decks, which are steady-state engine models 

typically developed by engine manufacturers, and the Numerical Propulsion System Simulation (NPSS), 

developed by NASA (Refs. 1 and 2). These tools can be integrated with other software to perform a 

steady-state system-level optimization. During these simulations, an engine model is driven to specific 

power reference values, typically defined by fuel flow, thrust, or fan speed, and the engine and engine 

components’ data are recorded for analysis. This analysis usually includes several specific flight 

conditions of importance, such as at takeoff and cruise. The engine components’ actual transition from 

one operating point to another is not taken into consideration by traditional systems analysis. The goal of 

dynamic systems analysis (DSA) is to incorporate the performance data during a transition early in the 

design process and in parallel with traditional SA. DSA requires that the control system can be modeled, 

and that a dynamic engine model, containing at least rotor speed states, is available. Some software tools, 

such as NPSS, already have the open-loop dynamic simulation capability. 

The dynamic performance of an engine is regulated by a closed-loop controller designed to ensure 

that the engine is capable of moving from one operating point to another while maintaining adequate 

operability margins (Refs. 3 and 4). These margins are preserved through the inclusion of limiters in the 

controller, but not all of the limiters have a large impact on the closed-loop dynamic performance. The 

limiters that protect the engine’s physical bounds, such as rotor speeds and pressures, primarily affect the 

amount of thrust produced but not the transition between operating points. To capture the relevant impact 

of the controller on the overall system performance for DSA, a primitive controller containing only the 

structure that directly impacts the transient response of the engine must be included in the simulations. 

The Tool for Turbine Engine Closed-loop Transient Analysis (TTECTrA) (Ref. 5) software package 

provides this capability. The TTECTrA software integrates with a user’s engine model, and designs a 

controller that meets user-defined performance specifications, such as bandwidth and operability margin 

limits. With this controller, the TTECTrA software provides an estimate of the transient capability of the 

conceptual engine design at a given flight condition. Since this analysis does not require the full-envelope 

nonlinear controller to be designed, the time and effort required to obtain the transient data are reduced, 

making DSA more accessible earlier in the engine design process. 

This paper is organized as follows. Section 2.0 of this paper provides a high-level overview of SA and 

describes the information regarding dynamic performance of the design that is made available through 

SA. The TTECTrA tool is discussed briefly in Section 3.0. Section 4.0 contains a high-level overview of 

the DSA concept along with a discussion of the general results anticipated from this process. To illustrate 

this concept, and the type of information gained from DSA, a design study is presented in Section 5.0, 

where a relationship that can used to evaluate the dynamic performance of a model is defined. 

Conclusions can be found in Section 6.0.

2.0 Systems Analysis and Engine Performance 

A turbine engine is designed to satisfy criteria ranging from system-level objectives (weight, thrust, 

and fuel burn rate goals) to component and sub-component-level limits (on efficiency, rotor speed, 

pressure, and temperature). These objectives are usually evaluated in steady-state. Safe operation of an 

engine requires that operating margins, such as surge margin, are not exceeded during the transition from 

one operating point to another. These margins take into consideration off-nominal operation due to engine 

degradation, atmospheric disturbances, vehicle maneuvers, angle-of-attack, etc., and attempt to account 
NASA/TM—2014-218449 3 

for dynamic changes as well. Current steady-state SA is not adequate for evaluating the ability of 

advanced technologies to meet transient performance requirements without better-defined requirements 

for the component operating margins. The dynamic performance of a design is only observed through the 

use of detailed physics-based models of the engine components along with a detailed controller. The 

development and maturation of these models and controller are typically done later in the design process, 

offering little opportunity for information regarding the dynamic capabilities of the engine to influence 

the design process. 

Designing an engine component through SA to meet both system and component-level objectives 

yields steady-state operating data representing the best performance given the design constraints. For 

example, a design constraint for a compressor is the surge margin, which is the distance the compressor 

operates from the surge line. The target operating line in a compressor, the relationship between corrected 

flow through and the pressure ratio across the compressor, is designed such that the compressor operates 

the most efficiently while still meeting the constraints; this corresponds to the lowest acceptable surge 

margin in steady-state. 

Steady-state surge margin fundamentally accounts for two types of surge margin reduction: the 

uncertainty allowance and the transient allowance. These allowances affect the transient performance, 

safety, and efficiency of the engine but cannot be analyzed individually with steady-state data. The 

uncertainty allowance represents the maximum reduction anticipated in surge margin due to mechanical 

imperfections and tolerances (engine-to-engine variation), Reynolds Number effects, inlet distortion, tip 

clearances, and engine degradation (or aging), etc. The transient allowance accounts for the reduction in 

surge margin that occurs during the transition from one operating point to another. Combining the 

uncertainty allowance and transient allowance produces the target operating line, as shown in Figure 1 for 

the generic high-pressure compressor (HPC) map (left) and low-pressure compressor (LPC) map (right). 

Also shown in the figure are the theoretical surge line, the uncertainty allowance, and the transient 

allowance for each compressor. If the allowances are defined correctly, a new engine operating in normal 

conditions (no engine damage or severe faults) at steady-state will do so along the target operating line 

and will be able to transition from one operating point to another without entering a surge condition. 

Often the defined size of the total surge margin allowance, a generic name referring to the sum of the 

uncertainty and transient allowances, is determined based on historical data and generic rules-of-thumb. 

Even though this process produces compressor designs that provide adequate steady-state performance, 

the system may be designed to operate with a transient allowance that could turn out to be too small or too 

large. Even though the surge margin allowances could be adjusted later in the design process, before 

production begins, not accounting for this in the SA phase could potentially result in a performance 

reduction. With the total surge margin allowance fixed, if the transient allowance is defined too large, 

during a large transient the engine (compressor) may operate closer to the surge line than intended, 

Figure 1.—Generic high-pressure compressor (left) and low-pressure compressor (right) 

maps illustrating the theoretical surge line (solid red), the uncertainty and transient 

allowances (green dashed and solid), and the target operating line (red dashed). 

Corrected Flow, lb/s

Pressure Ratio

Corrected Flow, lb/s

Pressure Ratio

Surge Line

Uncertainty Allowance

Transient Allowance

Target
NASA/TM—2014-218449 4 

potentially leading to compressor surge, particularly in off-nominal operation with a degraded engine. 

Conversely, a transient allowance that is defined too small could produce an overly-conservative engine 

response that may be unable to meet Federal Aviation Administration (FAA) Federal Aviation Regulation 

(FAR) Part 33, Section 33.73(b), which regulates the transient thrust response. Failure to meet this 

regulation could prevent the aircraft from attaining the dynamic performance for necessary maneuvers 

such as an aborted approach or go-around. The relevant portion of the FAA FAR Part 33, 

Section 33.73(b) reads: 

From the fixed minimum flight idle power lever position when provided, or if not provided, from not 

more than 15 percent of the rated takeoff power or thrust available to 95 percent rated takeoff power or 

thrust in not over 5 sec... 

This regulation can be used as a transient goal for DSA. 

While SA may indicate that a specific engine design operates at a high efficiency for a defined 

operating line (surge margin allowance), DSA may reveal that this increased efficiency comes at the cost 

of an unacceptable decrease in transient performance or an unacceptable surge margin reduction in order 

to meet the transient performance requirement. This delicate balance between performance and 

operability, in terms of transient response and other factors such as efficiency and safety, should be 

accounted for more accurately as may be accomplished through modeling of the closed-loop dynamic 

response of the system. 

3.0 The Tool for Turbine Engine Closed-loop Transient Analysis 

Any tool used for transient analysis must be able to model the dynamic operation of an aircraft engine, 

which is dependent on the closed-loop controller. From a high-level perspective, the engine controller can 

be considered to perform two functions: power management and engine protection (Ref. 4). The power 

management function regulates the controlled variable (typically engine pressure ratio or fan speed) based 

on the thrust commanded by the pilot via the throttle. The engine protection controller ensures that the 

engine does not violate any physical bounds, such as those on the rotor speeds and pressure, and ensures 

safe operation by avoiding compressor surge and engine flame out. The power management and engine 

protection controllers are integrated via min/max logic. The min/max logic compares the output of each 

individual controller to determine which is closest to meeting its setpoint, and then selects this as the control 

input to the engine. For DSA, it is necessary to consider the impact of both the power management function 

and the engine protection function on the transient operation of the engine. 

A tool has been developed to demonstrate and estimate the dynamic performance of the closed-loop 

system through the design of a simple controller. Known as the Tool for Turbine Engine Closed-loop 

Transient Analysis (TTECTrA), this semi-automated control design tool can be easily integrated with 

subsonic turbine engine simulations developed in the MATLAB/Simulink environment (The MathWorks, 

Inc.). At a single flight condition, defined by an altitude and Mach number, TTECTrA is capable of 

automatically designing a controller containing only the fundamental limiters that affect the transient 

performance based on the user’s specifications; this controller is a subset of the standard full-envelope 

controller designed for high-bypass turbofan engines, found in other work (Refs. 3 and 4). Simulation of 

the engine model with this controller allows for the collection of realistically-achievable dynamic 

performance data for the design. 

The general architecture of the TTECTrA controller is shown in Figure 2. The Pre-Filter and Actuator 

subsystems are implemented as unity gain first order filters with user-defined bandwidths. The Setpoint 

subsystem is an empirically derived relationship between thrust and control variable, which is model 

dependent (typically fan shaft speed or engine pressure ratio). The Proportional-Integral (PI) controller 

gains are calculated to meet user-defined bandwidth requirements. The Accel Limiter is designed to 

prevent the HPC from surging during engine acceleration by restricting the fuel flow delivered to the 

engine. This maintains the core shaft acceleration below its limit for a given core speed. The Decel 

Limiter preserves a minimum surge margin in the LPC through a limit on the relationship of fuel flow 

divided by the compressor static discharge pressure (Wf/Ps3). 
NASA/TM—2014-218449 5 

Figure 2.—The TTECTrA generic closed-loop architecture. The Setpoint, PI controller, Accel Limiter, and 

Decel Limiter subsystems are designed by the TTECTrA controller, whereas the Actuator and Pre-Filter 

subsystems are user-defined. 

The TTECTrA software package contains a Simulink block that the user can integrate in his/her 

engine model in Simulink. The Simulink block contains other functions that produce the inputs necessary 

for designing the setpoint controller. In addition to integrating this block with his/her model, the user 

must also modify a custom MATLAB script to allow the TTECTrA tool to set up and run a simulation of 

the Simulink model file by passing data to and from the model. For more information regarding the 

TTECTrA tool, the reader is referred to the TTECTrA User’s Guide (Ref. 5). 

4.0 Dynamic Systems Analysis 

The main objective of dynamic systems analysis is to incorporate dynamic performance data with the 

steady-state data used in traditional SA. These transient operating data include the pressures, temperatures, and surge margins of the engine components, some of which are traditionally unmeasured. Generic 

maps for the HPC and LPC in a high-bypass two-spool turbofan engine are shown in Figure 3 (left and 

right, respectively), where SA data (blue circles) and DSA data (cyan dots) are plotted in relation to the 

surge line (solid red line) and uncertainty allowance (dash-dotted red line). The DSA data were obtained 

by applying a burst-and-chop thrust profile at a takeoff flight condition. Assuming that the operating line 

defined by the SA data meets the designed surge margin allowance, it can be seen from Figure 3 that this 

large engine transient causes a small violation of the transient allowance in the HPC. This implies that, at 

this particular flight condition, an engine operating under worst-case conditions (used to define the 

uncertainty allowance) may operate on or over the surge line. Based on these data, there are three possible 

choices to make regarding this engine design: increase the transient allowance, accept small violations of 

the uncertainty allowance, or modify the transient limiter in the controller. Each choice has drawbacks. 

By moving the steady-state operating line farther from the surge line and increasing the transient 

allowance, the efficiency of the compressor would be reduced. The decision to allow small violations of 

the uncertainty allowance requires accepting that the uncertainty allowance is overly conservative, an 

assumption that may not be valid. If the transient limiter is modified to slow the engine response, it must 

still be able to meet the FAA 5-sec requirement. The drawbacks related with this latter choice may be 

addressed by TTECTrA, which enables investigation of the trade-off between response time and surge 

margin in evaluating an engine design. 

 
NASA/TM—2014-218449 6 

Figure 3.—Generic HPC map (left) and LPC map (right) illustrating the 

theoretical surge line (solid red), the uncertainty stack (red dashed 

dotted), steady-state data available from SA (blue circles), and 

dynamic data available from DSA (cyan dots). 

5.0 Design Case Study 

To demonstrate the concept of DSA through the use of TTECTrA, a design case study was performed 

to compare two engines related to the Commercial Modular Aero-Propulsion System Simulation 

(CMAPSS40k) (Ref. 6). CMAPSS40k is a nonlinear, physics-based, component-level dynamic engine 

model with a closed-loop controller written in the MATLAB/Simulink environment. CMAPSS40k 

models a 40,000-lb thrust class, high-bypass, dual-spool turbofan engine. The low-pressure components 

(fan, LPC, and low-pressure turbine) are connected by the fan shaft, and the high-pressure components 

(HPC and high-pressure turbine) are connected by the core shaft. The fan, compressors, and turbines are 

modeled using performance maps that relate the pressure ratio, mass flow rate, and corrected speed for 

each component. 

For this work, TTECTrA was integrated with the standard CMAPSS40k engine model. A second 

engine model was obtained by scaling the compressor and turbine maps and changing the rotor intertias in 

the CMAPSS40k engine. The second engine was scaled so that its operating characteristics were different 

enough from the standard CMAPSS40k engine to make it possible to demonstrate the potential benefits of 

the TTECTrA tool and DSA. Other aspects of the design, such as the turbomachinery sizing, are not 

within the scope of this paper. Figure 4 compares the HPC and LPC compressor maps of both engines, 

where the maps for the original CMAPSS40k engine are shown as blue solid lines (referred to as 

CMAPSS40k), and those for the scaled version of CMAPSS40k are shown as red dash-dotted lines 

(referred to as Engine A). The compressor surge lines, shown as solid black lines, are the same for each 

engine. The biggest difference between these two engine designs can be seen in the large shift in the 

speed lines of the HPC map. The thrust specific fuel consumption, TSFC, at the cruise flight condition of 

30,000 ft., 0.8 Mach is lower for Engine A than for the CMAPSS40k engine, as shown in Figure 5, 

suggesting Engine A would have a lower operating cost. 

The TTECTrA software is used to design a controller for each engine using the control design 

requirements listed in Table 1. The limits on T40 and fuel-to-air ratio are set so as not to impact the 

design of the transient limiters, allowing them to be based solely on the desired compressor surge margin 

limits. In Table 1, the Acceleration Limit is the minimum HPC surge margin and the Deceleration Limit 

is the minimum LPC surge margin. 

Once the TTECTrA controllers are designed, both closed-loop engine systems are simulated with a 

burst-and-chop thrust profile to compare their performance. The burst-and-chop profile in this study 

transitions from a near-idle throttle position to full power (burst) then back to idle (chop), where each 

transition takes 1 sec. Figure 6 compares the thrust output (top left), high pressure turbine inlet 

temperature T40 (top right), HPC surge margin (bottom left), and LPC surge margin (bottom right) for 

the two models. While the minimum surge margin for each engine satisfies the design requirement, at the 

Corrected Flow, lb/s

Pressure Ratio

Corrected Flow, lb/s

Pressure Ratio

Surge Line

Uncertainty Allowance

SA

DSA
NASA/TM—2014-218449 7 

Figure 4.—Comparison of the HPC map (left) and LPC map (right) of the 

original CMAPSS40k engine design (CMAPSS40k, blue solid lines) and the 

scaled version of CMAPSS40k (Engine A, red dash-dotted lines). The surge 

lines for both models (black solid line) are the same. 

Figure 5.—Comparison of the thrust specific fuel 

consumption TSFC of CMAPSS40k (blue solid) 

and Engine A (red dashed). 

TABLE 1.—TTECTrA TOOL CONTROL DESIGN PARAMETERS 

Parameter Value 

Thrust range 2,300 to 40,000 lbf 

Bandwidth 1.75 Hz 

Phase margin 45° 

Feedback filter bandwidth 10 Hz 

Pre-filter bandwidth 10 Hz 

Acceleration limit 11% 

T40 limit 3,500 °R 

Fuel to air ratio 0.0325 

Deceleration limit 15% 

high power steady-state operating point both the HPC and LPC surge margins are lower in Engine A. In 

addition, T40 is lower for Engine A both in steady-state and during the transient, which may improve 

engine life/degradation. The improvements in Engine A over the CMAPSS40k engine come at the cost of 

a longer time to transition from 15 percent maximum power to 95 percent maximum power (5.225 sec for 

Engine A, compared to 3.35 sec for CMAPSS40k). 

The controller for each engine was designed for a minimum HPC surge margin limit of 11 percent, 

which defines the uncertainty allowance in the HPC at this flight condition. Because the scaling of the 

compressor maps reduces the steady-state surge margin of Engine A by 4 percent compared to 

CMAPSS40k, the transient allowance for Engine A is reduced when the controller is designed using the 

 

20 40 60 80 100

0

5

10

15

20

Corrected Flow, lbf

Pressure Ratio

50 100 150 200

1

1.5

2

2.5

3

3.5

Corrected Flow, lbf

Pressure Ratio

CMAPSS40k

Engine A

1 1.5 2 2.5

x 104

7

7.5

8

x 10-5

Corrected Thrust, lbf

TSFC, lbf/(lb/s)

CMAPSS40k

Engine A
NASA/TM—2014-218449 8 

Figure 6.—Comparison of the CMAPSS40k engine (CMAPSS40k), the scaled version of 

CMAPSS40k (Engine A), and the scaled version of CMAPSS40k with a modified transient 

limiter (Engine B). The dashed black line represents 5 percent HPC surge margin. 

same uncertainty allowance as used for the CMAPSS40k engine. This reduced allowance produces the 

increased transient response time for Engine A observed in Figure 6. By considering the uncertainty 

allowance for Engine A to be overly-conservative, some of this allowance can be shifted to the transient 

allowance, preserving the steady-state operating line performance while improving the transient response 

time. A thrust response similar to the CMAPSS40k engine is obtained through reduction of the surge 

margin limit to 5 percent, as shown by the results labeled Engine B in Figure 6. The response time for 

Engine B is reduced to 3.885 sec, around 0.5 sec slower than the CMAPSS40k engine but almost 1.5 sec 

faster than Engine A. This closed-loop engine system also realizes the benefit of a lower T40 observed for 

Engine A. The bottom left plot of Figure 6 shows that, during the engine acceleration, the HPC surge 

margin reaches a lower value than for Engine A, but does not violate the limit of 5 percent, shown as a 

dashed black line, for which the Engine B controller was designed. The reduction in operability margin 

(uncertainty allowance) to achieve a performance gain (decreased response time to the 95 percent 

maximum thrust point), demonstrates the type of trade-offs that can be studied through DSA. 

The overall dynamic performance of the closed-loop system design may be evaluated by more closely 

examining this trade-off between performance and operability. For a given engine model, a controller can 

be designed with several acceleration limits (minimum surge margins) using TTECTrA, as in the previous 

example. The relationship between the response time and actual minimum HPC surge margin of each 

design can be plotted to visualize the trade-off, as shown in Figure 7 for both CMAPSS40k and Engine A. 

The baseline minimum surge margin for the CMAPSS40k design was chosen to replicate the performance 

of the full CMAPSS40k simulation. TTECTrA designs the acceleration schedule with a fuel flow ramp as 

the input to the engine model, bypassing the fuel metering valve, and the output of the engine helps shape 

the acceleration schedule. During actual use, the controller and fuel metering valve actually filter out the 

high frequency component of the fuel flow signal and often the actual minimum surge margin differs 

from the design value by a small value, typically 0.5 percent. 

10 20 30 40

0

1

2

3

4

5

x 104

Time, s

Thrust, lbf

10 20 30 40

1500

2000

2500

3000

3500

Time, s

T40, R

10 20 30 40

0

10

20

30

40

50

Time, s

HPC SM, %

10 20 30 40

0

20

40

60

80

Time, s

LPC SM, %

CMAPSS40k

Engine A

Engine B
NASA/TM—2014-218449 9 

Figure 7.—Dynamic performance evaluation plot which demonstrates the trade-off 

between performance (acceleration time) and operability (surge margin). 

The requirements imposed by the original controller (11 percent minimum surge margin and 5 sec 

response time) are indicated in Figure 7 by the box with a solid outline, labeled Original Requirement; 

designs “inside” this box satisfy both requirements. As can be seen from Figure 7, only CMAPSS40k 

satisfies these requirements for the acceleration limiters designed in this investigation. 

Assume that the minimum acceptable surge margin could be decreased to 5 percent. In Figure 7, 

designs satisfying the new requirements are located “inside” the box outlined with dashed lines, labeled 

Modified Requirement (this includes the area “inside” the Original Requirement box). With the reduced 

surge margin limit, both CMAPSS40k and Engine A meet the performance and operability requirement. 

Since both of these engines meet the dynamic design requirements, other requirements, such as efficiency, may be considered in comparison of the two designs. Engine A has a lower TSFC, as shown in 

Figure 5, suggesting that the additional surge margin in the uncertainty allowance of the CMAPSS40k 

design is traded for a better TSFC in Engine A. 

Considering only CMAPSS40k, both the acceleration time and minimum surge margin requirements 

are met by controllers and limiters designed for a large range of minimum surge margin. This indicates 

that the transient allowance assumed during the systems analysis phase may be overly conservative and a 

redesign of the compressor may perhaps move the operating line to a more efficient region. For example, 

the acceleration schedule for CMPASS40k could be changed from one designed to meet an 11 percent 

minimum SM to one meeting a 15 percent minimum SM without affecting the ability of the system to 

meet both goals, indicating an additional 4 percent surge margin in the transient allowance. This 4 percent 

surge margin could be reduced by changing the surge margin of the target operating line of the compressors from, say, 23 to 19 percent, which may allow the compressor to operate more efficiently. To fully 

evaluate the effects of such a design change requires the redesign of the compressor map and performing 

additional systems analysis to ensure other higher-level goals are met; this analysis is not within the scope 

of this paper, but may be pursued in the future. 

5 10 15 20

2

3

4

5

6

7

8

9

10

Minimum Surge Margin, %

Accel Time, s

CMAPSS40k

Engine A

Modified

Requirement

Original

Requirement
NASA/TM—2014-218449 10 

6.0 Summary 

Dynamic systems analysis (DSA) seeks to incorporate dynamic performance data with the Systems 

Analysis (SA) process to improve this process and aid in meeting future engine design goals through 

better characterization of engine bounds. These bounds are often only reached during an engine transient 

and therefore are not captured during steady-state operation or analysis. The additional information made 

available through DSA allows for the trading of overly-conservative operating margins for better engine 

efficiency, while maintaining the necessary transient performance. The dynamic performance of an 

engine design can be evaluated by defining the relationship between the response time of the closed-loop 

design and the minimum surge margin. This relationship allows the closed-loop dynamic performance, 

and tradeoffs between performance and operability, to be incorporated into the design process by 

providing information about whether a given engine design is able to meet the performance and 

operability requirements. 

To obtain the dynamic performance data used to define this relationship, the Tool for Turbine Engine 

Closed-loop Analysis (TTECTrA) was developed. The TTECTrA software is capable of producing a 

controller at a single flight condition, defined by an altitude and Mach number. This provides an estimate 

of the closed-loop performance of the engine model. TTECTrA is open source software developed in the 

MATLAB/Simulink environment that can integrate with any Simulink-compatible engine model. 

A case study was presented to demonstrate how TTECTrA may be used as part of a dynamic systems 

analysis. Two engines were studied, the standard CMAPSS40k engine and an engine constructed by 

scaling the compressor maps and adjusting the inertias of the rotors in the standard engine. A baseline 

controller was designed for each engine using TTECTrA with identical controller requirements. While the 

modified CMAPSS40k engine has better TSFC, lower surge margin, and lower T40 temperature, the 

response time of the model was unable to meet the 5-sec thrust response requirement. Modification of the 

acceleration limiter to allow for a lower HPC surge margin enabled the more efficient engine to meet the 

5-sec thrust response requirement. Modifying the acceleration limiter in TTECTrA for different HPC

surge margins also allowed the relationship between the transient performance and operability (surge 

margin) of the engine to be quantified. The information made available through this relationship provides 

a quantitative view of the trade-off between operational limits (surge margin) and performance (response 

time) that otherwise would not be available from traditional (steady-state) systems analysis. 

References 

1. Jones, S.M., “Steady-State Modeling of Gas Turbine Engines using the Numerical Propulsion System 

Simulation Code,” Proceedings of ASME Turbo Expo 2010, GT2010-22350, Glasgow, UK, 

June 14–18, 2010. 

2. Jones, S.M., “An Introduction to Thermodynamic Performance Analysis of Aircraft Gas Turbine 

Engine Cycles Using the Numerical Propulsion System Simulation Code,” NASA/TM—2007-

214690, March 2007. 

3. Jaw, L.C., and Mattingly, J.D., Aircraft Engine Controls: Design, Systems Analysis, and Health 

Monitoring, American Institute of Aeronautics and Astronautics, Inc., 2009. 

4. Csank, J., May, R.D., Litt, J.S., and Guo, T.-H., “Control Design for a Generic Commercial Aircraft 

Engine,” Proceedings of the 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, 

AIAA–2010–6629, Nashville, TN, July 25–28, 2010. 

5. Csank, J.T. and Zinnecker, A.M., Tool for Turbine Engine Closed-loop Transient Analysis 

(TTECTrA) User’s Guide, to be published as NASA TM. 

6. May, R.D., Csank, J., Lavelle, T.M., Litt, J.S., and Guo, T.-H., “A High Fidelity Simulation of a 

Generic Commercial Aircraft Engine and Controller,” Proceedings of the 46th AIAA/ASME/ 

SAE/ASEE Joint Propulsion Conference, AIAA–2010–6630, Nashville, TN, July 25–28, 2010.