A Modeling Study of the Use of Burner Exit Temperature in
the Control of a Geared Turbofan Engine with NPSS
by
David P. Shore
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING IN MECHANICAL ENGINEERING
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, CT
December, 2010
Abstract
This project presents the results of a modeling study designed to develop new
ratings using burner exit temperature (T4) for the control of a geared turbofan engine.
The model was developed from an existing simulation using the Numerical Propulsion
System Simulation (NPSS) software, which utilizes turbomachinery theory to determine
various engine parameters based on a specific set of inputs. A matrix of burner temperature targets across the entire operating envelope of a Pratt & Whitney customer’s aircraft
was developed for use in simulated thrust output control. The design objective was to
fulfill thrust requirements specified by the customer. Requirements included ratings for
23k, 21k, and 18.9k pounds of thrust for a number of different customer bleed off take
combinations at maximum takeoff (MTO), continuous (MCT), and climb (MCL) power
requirements. The process of rating creation is demonstrated. The final result was a table
of ratings that meet customer demands which is currently used in the customer’s aircraft
design process.
ii
Table of Contents
Abstract
ii
List of Tables
iv
List of Figures
v
Nomenclature
vi
Glossary
viii
1. Introduction
9
1.1
Geared Turbofan Basics
10
1.2
Numerical Propulsion System Simulation
11
2. Methodology
14
2.1
Review of Gas Generator Turbomachinery
14
2.2
Engine Rating Characteristics
15
2.3
Aircraft Flight Envelope, Power Settings, and Derates
17
2.4
Customer Requirements and Thrust Logic
20
2.5
Method Used to Develop Ratings Tables
21
3. Results and Discussion
23
3.1
Ratings File and Selection Logic Result
23
3.2
Technical Requirements Document Thrust Points
23
3.3
Derates and Meeting Customer Requirements
24
3.4
Behavior Checks
27
4. Conclusions
29
5. References
30
6. Appendices
31
6.1
Appendix A – Maximum Takeoff Selection Logic Example
31
6.2
Appendix B – Scaled T4 Table Example
33
iii
LIST OF TABLES
Table 1 – Burner Exit Temperature Schedule Summary
20
Table 2 – Scaled TRD Thrust Requirements
23
iv
LIST OF FIGURES
Figure 1 - Two-Spool Turbofan Cross Section with Station Designations
10
Figure 2 - Simple Conceptual NPSS Model
12
Figure 3 - Aircraft Requirements & Engine Limitations
15
Figure 4 - Typical Rating Characteristic
16
Figure 5 - Modified Operating Envelopes
18
Figure 6 - Low rotor speed limitation
24
Figure 7 - 23k to 21k Derate – N1 limiting Issue
25
Figure 8 - 21k to 18.9k Derate Contour Plot
25
Figure 9 - 23k to 21k MCT Bleeds OFF Contour Plot
26
Figure 10 - MCL to MCT Bleeds OFF Contour Plot
27
Figure 11 - MCL to MCT Bleeds OFF Contour Plot
27
v
LIST OF SYMBOLS
BPR = fan bypass ratio
Cp = constant pressure specific heat
Cv = constant volume specific heat
Fn = Raw Thrust (lbs)
FNTST = Stream tube corrected thrust (lbs)
h = enthalpy (BTU/lbm)
ISA = International Standard Atmosphere
LHV = Lower Heating Value of fuel (BTU/lb)
m a = mass flow rate (lb/sec)
MN = Mach number
N1 = Low rotor speed (RPM)
N1F = Fan speed (RPM)
N2 = High rotor speed (RPM)
P0, Pamb = Outside free-stream air temperature (psia)
P2 = Pressure at the face of the fan, core stream (psia)
P12 = Pressure at the face of the fan, fan stream (psia)
P2.5 = Pressure in between the LPC and HPC (psia)
P3 = Pressure at the exit of the HPC (psia)
P4 = Pressure at the exit of the burner (psia)
P4.5 = Pressure in between the HPT and LPT (psia)
P4.9 = Pressure at exit of the LPT (psia)
R = gas constant (BTU/°F/lbm)
T0, Tamb = Outside free-stream air temperature (°F)
T2 = Temperature at the face of the fan, core stream (°F)
T12 = Temperature at the face of the fan, fan stream (°F)
T2.5 = Temperature in between the LPC and HPC (°F)
T3 = Temperature at the exit of the HPC (°F)
T4 = Temperature at the exit of the burner (°F)
T4.5 = Temperature in between the HPT and LPT (°F)
T4.9 = Temperature at exit of the LPT (°F)
vi
TSFC = Thrust Specific Fuel Consumption
Wf = Fuel flow (lbm/sec)
Wa = Airflow (lbm/sec)
ρ = Density (lbm/ft3)
η = efficiency
γ = Ratio of constant pressure specific heat to constant volume specific heat (Cp/Cv)
vii
viii
Glossary
Derate
Derate is defined as an engine thrust rating that is at a lower value than the
maximum thrust rating. For the subject engine, the derates are thrust targets of
21000 and 18900 lbs.
GTF
Geared Turbofan. A term used to describe the new family of engines being
developed by Pratt and Whitney.
MTO
Maximum Take-Off thrust. Typically the maximum thrust output of the engine
available to the pilot on demand. Used predominately in takeoff situations and
situations in which maximum thrust is necessary.
MCT
Maximum Continuous thrust. Typically the maximum thrust used by the pilot
during cruise operation of the aircraft.
MCL
Maximum Climb thrust. The maximum thrust used by the pilot during climb
operation of the aircraft.
ix
1. Introduction
The latest Pratt & Whitney engine offering is a high-bypass geared turbofan engine
currently selected as the exclusive engine for the customer’s aircraft. Due to the sensitive
nature of this material, the name of the customer is being withheld to alleviate any
potential legal issues that could arise. At the time of this project creation, the first engine
is currently being tested in Pratt & Whitney’s outdoor testing facilities in Florida. With
the target aircraft estimated entry in to service date of 2013 rapidly approaching, there is
a substantial amount of work that needs to be completed. One such task is defining a
rating that will be used to determine performance (thrust targets) of the engine. This
rating will eventually be defined by low turbine rotor speed (N1), but as an intermediate
step, will be defined by burner exit temperature (T4).
The final goal of this project will be a number of T4 matrices organized into a
comprehensive ratings file. This file will be packaged in a customer simulation and
shipped to the customer for use in their aircraft design process. The customer simulation
is essential for accurate expectations on thrust targets, station temperatures and pressures, bleed pressures and temperatures, fuel burn, etc. Additional to the design process,
it will be used by the Customer’s engine performance group to quote aircraft thrust
capabilities to potential aircraft buyers.
9
1.1 Geared Turbofan Basics
The geared turbofan engine is a typical two spooled engine design with a twist.
Normal two spool engine designs are driven by two rotors, called the high spool and low
spool. The low spool is composed of the fan, low pressure compressor (LPC), and low
pressure turbine (LPT). The high spool, often called the core when combined with the
burner, is composed of the high pressure compressor (HPC) and high pressure turbine
(HPT). In the typical two spool design, all components on the low spool spin at the same
rate, referred to as the low rotor speed (N1). All components on the high spool also spin
at the same rate, referred to as the high rotor speed (N2). The figure below shows a
simplified cartoon of a two-spool engine. Additionally, the figure has numeric station
designations that will be used in the remainder of this project.
Figure 1 – Two-Spool Turbofan Cross Section with Station Designations
Modified from [4]
In the geared turbofan design, the low spool components are separated by a reduction
gearbox. In this design, the LPC and LPT will spin at the same rate, but the reduction
10
gearbox will create a slower spinning fan. The reason behind this design is fan bypass
ratio (BPR). For subsonic commercial aircraft, the goal in engine design is multipronged,
but all aircraft suppliers would agree that the engine needs to be as lightweight as
possible with low thrust specific fuel consumption (TSFC), while still being able to meet
thrust requirements. A high bypass ratio gives a lower exhaust speed, which helps reduce
TSFC. Unfortunately for the two spool engine design, a high bypass ratio requires a slow
spinning fan. Since the fan is driven by the LPT, to slow the fan down to an optimal
speed would require more stages on the LPT, which increases size and, more importantly, weight of the engine. By adding the reduction gearbox, the fan can spin at an optimal
speed (N1F) without needing to increase the size of the LPT.
1.2 Numerical Propulsion System Simulation
The Numerical Propulsion System Simulation, hereby referred to as NPSS, is a
detailed aerothermomechanical computer simulation developed by the NASA Glenn
Research Center with support from the US aeropropulsion industry. It is used in the
aircraft engine design process to realistically model the physical interactions that take
place inside an engine, which can not only reduce the need for expensive and complicated testing, but to also verify results of such testing as it is required [1].
NPSS is a component-based, object-oriented simulation. It is designed to perform
cycle design, steady state and transient off-design performance prediction, test data
matching, and day to day engineering tasks that require simulated engine performance
[1]. For the purpose of this project, only steady state performance predictions will be
used, since transient simulations are not necessary for rating table definitions. NPSS is
composed of a series of individual, interconnected components, linked together through
mechanical, fluid and fuel ports. The overall assembly of components is controlled by a
solution algorithm by way of a solver object that will impose fundamental conservation
laws (continuity, energy, momentum) as well as any other user defined constraints. To
support engine simulation analysis, several gas property packages are supplied with the
simulation. For this project, I will be using the Pratt and Whitney internal version of
NPSS, so the gas property package that will be used in “GasTbl”. Additionally, NPSS
11
incorporates a full-featured user interface via a built-in programming language modeled
after C++.
All NPSS simulations are created from a collection of six (6) basic object types
(classes) or building blocks. These classes (building blocks) represent the engine components (hardware) and how those components are linked together, as well as the more
abstract features that a simulation comprises. The six basic object types are elements,
subelements, sockets, flow stations, ports and tables. Elements are typically the biggest
blocks in the model, typically representing the major components of the engine, such as
turbines, compressors, and the fan. Subelements are specific to elements, and usually
contain more detailed information on the element, one example being a turbine map.
Sockets are a means to connect the subelement to it’s specific element. Ports are a means
to connect each element together, and tables are specific information found in subelements. A conceptual figure illustrating how these concepts are used to create an engine
simulation is included below in Figure 2. This particular figure is simplified for a single
shaft turbofan. The highlighted area represents the portion of the simulation that would
be doubled for the typical dual shaft engine design. Objects represent both engine
Simplenozzle,
Conceptual
components (compressor, turbine, shaft,
etc.) as well as the more abstract feaEngine Model
tures
such as “FlightConditions”, “Solver”, “DataViewers”, etc.
Model
FlightConditions
Element
s
FuelStart
Element
Fluid Port
Fuel Port
Assembly
F
P
Inlet
Element
F
P
Fuel Port
F
F
P Compressor P
Element
F
P
F
F
P Burner P
Element
Shaft Port
Bleed
F
P
Turbine
Element
F
P
F
P
F
Duct P
Element
F
P CDNozzle
Element
F
P
Fluid Port
Shaft Port
Fluid Port
S
P
Shaft
Element
S
P
FlowEnd
Element
CompressorMap
Subelement
F
P
Socket
Compressor
Element
Shaft Port
F
P
F
P
Compressor, Shaft, and Turbine will have both LP and HP
objects for a 2-spool engine
Solver
Data Viewer(s)
Figure 2 – Simple Conceptual NPSS Model, from Figure 1, p. 1-3 in [1]
12
For the purposes of this project, a baseline NPSS simulation for the customer’s
engine has already been developed by various groups within Pratt & Whitney, including
modeling, performance, aerodynamics, etc. Various “building blocks” from these
departments and others within Pratt & Whitney were supplied to create this baseline
simulation. These building blocks include input files for component maps, component
setup, subsystems (bleeds, drag), engine control, ratings, and installation effects (customer bleeds, port losses, flight envelope, etc.). This baseline simulation is what will be
sued to develop the target thrusts and combustor exit temperatures needed to hit customer thrust requirements. The ratings input file of the baseline simulations is what will be
modified to meet the customer requirements for the new simulation.
13
2. Methodology
2.1 Review of Gas Generator Turbomachinery
Methods of engine control in past engine designs at Pratt & Whitney have varied
quite a bit over the years. Legacy engine models have used engine pressure ratio (EPR),
or PT7/PT2 as a method of controlling the engine powersetting. Newer engine models,
such as the Engine Alliance GP7000 and PW6000 have used low rotor speed (N1) as the
means for setting engine power. The Customer’s engine will also eventually use an N1
rating to define powersetting, but in advance of this, combustor exit temperature will be
used for preliminary customer simulations. T4 can be directly linked to engine thrust
with basic thermodynamics of turbofan engines. For a turbofan engine, we can approximate specific thrust with the following equation [3]:
Fn
 (1  f )  ue  BPR  uef  (1  BPR )  u
m a
where
Fn
= specific thrust
m a
f = fuel air ratio
ue = core engine exit velocity
uef =
fan nozzle exit velocity
u = free stream velocity
In the above equation, it is assumed that the core engine flow and the bypass flow
expand separately to the ambient conditions behind the engine. Since we are concerned
with burner exit temperatures for control, we want to focus on the core engine flow.
Allowing for the work done by the engine core on the fan stream, assuming the rate of
mass flow in both the turbine and compressor, as well as the average value of Cp to be
equal, the turbine power balance becomes [3]:
T4.9  T4  (T3  T2 )  BPR (T18  T0 )
where
T18 = Fan bypass air temperature
14
We can also model the core engine exit velocity as such [3]:
1
 

 Pamb   



ue  2 n
R  T4.9 1  

 1
P4.9  



where
 n = Fan nozzle efficiency
Therefore, the LPT exit temperature is dependent on the upstream burner exit
temperature, the core engine exit velocity depends on the square root of the burner exit
temperature, just as is the case for the specific thrust. Thus, we can conclude engine
thrust for a given flight condition (MN, ALT, ambient pressures and temperatures) is
directly proportional to T4 as well as ambient conditions. This is further discussed in
Section 2.2.
2.2 Engine Rating Characteristics
When considering the development of an engine rating, a number of different limitations need to be considered. Two key limitations are aircraft requirements, and
physical material limits of the engine hardware. While these are not the only limitations
that need to be considered (others include physical engine limits, derates, etc.), the rating
characteristic curves can be developed by focusing on these two specifically. Figure 3
shows schematically the aircraft requirements and material limits with respect to thrust,
T4, and ambient temperature.
Figure 3 – Aircraft Requirements & Engine Limitations
15
Figure 3 above shows the aircraft requirements in a plot of thrust versus ambient
temperature. Considerations in this particular figure include take-off gross weight
(TOGW) requirements and aircraft structural concerns. The engine limitations section
takes the maximum combustor exit temperature into account, as well as engine life and
deterioration concerns. Higher temperatures can result in failure of the engine hardware,
or over an extended period of time, less time between overhauls for the customer. Figure
3 also shows combustor exit temperatures vs. thrust at various ambient conditions. It is
fairly obvious that hot day conditions will strain the engine much more than cold day
conditions. Putting these two requirements together will give us a typical engine ratings
characteristic [Fig. 4].
Figure 4 – Typical Rating Characteristic
Based on aircraft requirements, the net thrust on the cold day is constant, up until the
flat rated temperature, or breakpoint. At the breakpoint, we have hit an ambient temperature in which the maximum T4 has been reached. At all hotter conditions than the
breakpoint temperature, the rating logic will hold T4 constant so as not to violate the
physical limits of the engine. This will cause net thrust to drop on the hot day. This is
typically call this the “hot-day line”. The rating files I created model this type of behavior. Furthermore, subtracting flow out of the compressor for engine bleeds will have a
noticeable effect on thrust. By reducing core flow, if all other conditions are held constant, the overall thrust value will decrease, and in most cases, the breakpoint value will
move to a lower DTAMB [2]. This will be evident in the results section when constant
T4 schedules are applied to different bleed configurations.
16
2.3 Aircraft Flight Envelope, Power Settings, and Derates
The initial requirement for the GTF engine was a maximum take-off (MTO) thrust
capability of 23,000 pounds of thrust. Further discussions with the customer have led to
the requirements for thrust derates on the engine for the purpose of prolonging engine
life on wing. Derates have been defined at 21k and 18.9k pounds of thrust respectively.
Specific requirements for these derates have been supplied by the customer and have to
be met for this particular engine simulation.
Power settings required for the simulation are maximum take-off (MTO), maximum
continuous (MCT), and maximum climb (MCL), ordered in descending levels of thrust.
The baseline simulation used as a starting point in this project has 23k and 21k levels of
MTO, MCT, and MCL thrust defined for only a small number of engine bleed configurations. The new simulation has MTO, MCT, and MCL power settings defined for 23k,
21k, and 18.9k levels of thrust at a larger number of bleed conditions.
There are three different engine bleeds that will be taken into account for the rating
definition. These bleeds are the environmental control system (ECS), wing anti-ice
(WAI), and cowl anti-ice (CAI). ECS bleeds are responsible for feeding bleed off-take
air into a number of different systems that are required for a commercial flight with
passengers, including breathable air, cabin pressure regulation, heat and air conditioning
systems, etc. WAI and CAI bleeds are fairly self-explanatory, in that their specific
purpose is to combat the effects of wing and cowl icing in conditions in which icing can
become a potential problem. Engine bleeds can be utilized in both single and dual
(nominal) engine operation. Single operation occurs if there is only one engine on the
aircraft producing bleed flow for these systems. This will incur a higher bleed flow for a
particular engine, and hence the performance of that engine will be more restricted than
an engine flowing less bleed for dual operation. There are a large number of different
bleed configuration permutations, but only eleven configurations were outlined in the
requirements between P&W and the customer. The eleven different bleed configurations
that were taken into account for rating design are as follows:

Bleeds OFF

ECS OFF / CAI ON

ECS OFF / CAI ON WAI ON DUAL
17

ECS OFF / CAI ON WAI ON SINGLE

ECS ON DUAL

ECS ON DUAL / CAI ON

ECS ON DUAL / CAI ON WAI ON DUAL

ECS ON SINGLE, ECS ON SINGLE / CAI ON

ECS ON SINGLE / CAI ON WAI ON DUAL

ECS ON SINGLE / CAI ON WAI ON SINGLE.
All schedules had to be defined for the entire operating envelope of the Customer’s
aircraft. The operating envelope of the aircraft is defined as a combination of Mach
number and pressure altitude values in which the aircraft is designed to operate. Since
the operating envelope is an internally controlled and proprietary document, for the
purposes of this project, it has been modified to eliminate any exposure. Figure 5 describes the modified operator envelopes. Figure 5 outlines two separate envelopes. The
take-off envelope is defined for all points in which the aircraft could be expected to take
off from a runway. The combination of the takeoff envelope and all other points within
the boundaries of Figure 5 is considered the entire operating envelope.
CSeries Operating Envelopes
40000.00
39,000 ft
35000.00
0.77M
30000.00
Pressure Altitude (ft)
25000.00
20000.00
15000.00
14,500 ft
12,000 ft
10000.00
Take-Off
Envelope
5000.00
0.00
-2,000 ft
-5000.00
0
0.1
0.2
0.3
0.4
0.5
Mach Number
18
0.6
0.7
0.8
0.9
Figure 5 – Modified Operating Envelopes
It is only required to hit MTO target thrusts within the take-off envelope, which accounts for realistic airports around the globe. For MCT and MCL target thrusts, all
points within the operating envelope are required. To hit specific thrusts for every point
in the envelope would create ratings tables too large for the simulation, and fidelity on a
level which is not required. As a result, grids of nodes were agreed upon with the
customer to meet requirements. Typically, this results in a node at every 0.05 Mach
number and every 2000 feet.
Creating individual schedules for all possible permutations of derate, power setting,
and bleed configuration would require a total of 99 different schedules. If we include all
other bleed configurations outside the 11 defined previously, the number of schedules
required gets out of control in a hurry. Combine this with the fact that there are nodes
every 0.05 Mach and every 2000 feet across the entire envelope, and the size of the
ratings file balloons very quickly. As a result, schedules were targeted for specific bleed
conditions which were most important. For bleed conditions in which a specific schedule
was not targeted, the bleed configuration with the next highest bleed flow was chosen to
represent this case. For example, for the 21k MTO rating, bleed configurations that were
specifically targeted were Bleeds OFF, ECS ON SINGLE, and ECS ON SINGLE / CAI
ON WAI ON SINGLE. Obviously, bleed flow for the Bleeds OFF configuration is zero.
Total bleed flow for ECS ON SINGLE will be less than ECS ON SINGLE / CAI ON
WAI ON SINGLE, as the later is flowing bleed for the anti-ice system. Logically, it
would also follow that ECS ON SINGLE / CAI ON would be flowing greater bleed than
ECS ON SINGLE. As such, the next targeted bleed configuration with the highest flow
would be ECS ON SINGLE / CAI ON WAI ON SINGLE, so that schedule would be
used. This will result in the same T4 schedule being used for both the targeted and nontargeted condition. Since the non-targeted condition is operating with less bleed flowing,
but the T4 remains the same, the corresponding output thrust will be greater.
Table 1 below illustrates the T4 schedule summary. The numbers below the bleed
configurations correspond to the bleed flow ranking, with (1) being the least amount of
19
bleed flow, and (11) being the max case. Schedules were also given an identifying
number between (1) and (14) for organization purposes.
Table 1 – Burner Exit Temperature Schedule Summary
2.4 Customer Requirements and Thrust Logic
Customer requirements are taken into account in every aspect of this project. Initial
customer requirements that we work off of as a base are defined in the Technical Requirements (TRD) document. For our purposes, we are really only concerned about the
thrust requirements outlined in the TRD. Specifically, the TRD outlines the 23, 21, and
18.9k thrust targets for takeoff, cruise, and maximum continuous operation at a few
specific locations within the envelope. These targets are absolute, and the final ratings
file will hit these targets precisely as required.
Additional to the TRD requirements, and at the core the reason for this project, there
are specific derate and bleed flow requirements requested by the customer. These
requirements can be summed up into the following list:

For all ratings, derates, and specific bleed configurations - MTO > MCT > MCL

18.9k derate shall be 90% of 21k derate for all specific bleed configurations

21k MTO = 0.9 * 23k MTO for all specific bleed configurations
20

21k MCT = 0.94 * 23k MCT within the takeoff envelope. Outside the takeoff
envelope, 21k MCT = 23k MCT. In other words, 21k MCT thrust uses the same
values of T4 and thrust as 23k MCT outside the takeoff envelope.

21k MCL shall be the lower thrust value of 21k MCT and 23k MCL within the
takeoff envelope. Outside the enveloped, 21k MCL shall be equal to 23k MCL
Meeting these requirements based on single bleed condition would not be an overwhelming feat. However, by adding different bleed configurations into the process, the
specific thrust targets of 90% or 94% in some areas will be missed. The reason for this is
schedules have only been developed for 14 specific conditions (See Table 1). For the
conditions in which a schedule has not bee specifically developed, we will miss the
derate target by a lower value. For example, the MCT condition for ECS off, CAI on,
does not have a target schedule for the 23k thrust level. The 21k requirement at this
condition is 94%. Unfortunately it is not a targeted bleed condition. Therefore, the derate
will be less than 6% because the schedule we are using for this bleed condition is
targeted to a bleed configuration with more bleed off-take. The key takeaway from this
is that we will never be giving the customer less thrust because of a missed target.
To keep track of various bleed configurations, schedules, and ratings in the actual
ratings file, I have developed some NPSS code that includes selection logic. This will
ensure the model is using the correct schedule based on the given bleed configuration
and thrust setting input by the user. Closer examination of Table 1 shows that there are
no schedules targeted for any of the 18.9k ratings. This is because the selection logic for
the 18.9k includes a simple few lines of code that will target exactly 90% of whatever
the 21k thrust value is. This will not work on 21k derates from 23k thrusts due to other
limitations that will be discussed in the results section.
2.5 Method Used to Develop Ratings Tables
Developing the T4 ratings table is a model intensive process, but fairly straightforward. As discussed previously, the base model used to develop the new T4 tables has the
capability to run 23k and 21k MTO, MCT, and MCL conditions. However, the output
21
thrust values for the base model is not at the target thrusts that are required by the TRD
and the customer.
To develop a schedule for a specific target, the following process was used. The
model was run at all nodes throughout the envelope at the 23k rating for MTO, MCT,
and MCL at the targeted bleed conditions. Output values of T4 were saved and organized into a comprehensive table for each condition. The 21k rating was then run for
MTO, MCT, and MCL at the targeted bleed conditions. Once again, output values of T4
were saved and organized into tables for the 21k. Thrust values for each condition were
then checked between the 23k and 21k results. If the T4 had to be adjusted due to the
derate being off target, the T4 values were modified and the process was repeated until I
got the correct answer. Additionally, breakpoint temperatures had to be modified for a
number of schedules to meet requirements. This process was repeated over a substantial
period of time until I was comfortable with the results.
22
3. Results and Discussion
3.1 Ratings File and Selection Logic Result
The result of the process is a ratings file that went from about 5000 lines of code
for the base simulation to a ratings file that is just under 10000 lines of code. Additional
to the new T4 schedules in the ratings file, selection logic in the form of nested if/then
statements was added to ensure the correct schedule is being applied for the given thrust
level and bleed configuration. Examples of the ratings code T4 schedules and the
selection logic can be found in the Appendix.
3.2 Technical Requirements Document Thrust Points
The thrust targets included in the Technical Requirements Document are an essential part of this project. It is essential that the output thrust of the new customer
simulation meet these targets exactly. Special consideration was taken into account in
the T4 ratings file to ensure there was no deviation from target. Table 2 below shows the
scaled targets and the results of the new ratings file. The values have been scaled down
by a constant factor as the actual values are considered proprietary, and a contract
agreement between the customer and Pratt & Whitney.
Table 2 – Scaled TRD Thrust Requirements
Point Altitude
1
2
3
4
5
6
7
8
Speed
Amb.
Rating
Temp.
(ft) M / KCAS (ISA + C)
00M
15 MTO
0 0.2 M
15 MTO
3000 0.25 M
30 MTO
15000 0.45 M
0 MCT
3000 250 KCAS
10 MCL
18000 290 KCAS
10 MCL
30000 0.78 M
10 MCL
34000 0.78 M
10 MCL
Fan Interstage
Bleed
Bleed
(lb/s)
(lb/s)
0 OFF
0 OFF
0 OFF
0 OFF
NOM NOM
NOM NOM
NOM NOM
NOM NOM
VFG
Power Thrust Thrust Delta Delta
Bleed Extraction TRD Actual
(lb/s)
(HP)
(lb)
(lb)
lbs
%
0
60 23300 23300
0
0.00
0
60 16000 15998
-2 -0.01
0
60 11900 11898
-2 -0.02
0
60
9600
9598
-2 -0.02
NOM
35 10500 10500
0
0.00
NOM
35
6800
6799
-1 -0.01
NOM
35
4700
4699
-1 -0.03
NOM
35
3995
3995
0 -0.01
With the new T4 ratings table and the schedule selection logic in place, the
simulation hits the TRD required thrust points within two hundredths of a percent, which
is within the tolerance of the simulation itself.
23
3.3 Derates and Meeting Customer Requirements
The portion of the project that dealt with meeting customer requirements outside
the TRD was the most time intensive and the most difficult. For the majority of the
customer requirements, plot packages including large numbers of contour plots were
created to ensure the simulation was behaving as intended.
One major issue that I ran into while creating the derates was the inability to hit
90% of the 23k level of thrust for the 21k MTO rating. While running through the
process, the rating characteristic curves were showing a double breakpoint temperature
near standard day temperatures. The reason for this double breakpoint was another
engine limit was getting in the way of the target T4. In this particular case, the maximum
low rotor speed limit was forcing the thrust and T4 to be cut, creating this condition.
Figure 6 below illustrates how another engine limit can impact the rating structure.
FNTST
Original Breakpoint
N1 Limit
DTAMB
Figure 6 – Low rotor speed limitation
Figure 6 illustrates the idea behind a “double breakpoint”. Because the engine
simulation is running on the low rotor speed limit, the thrust is clipped in the area of the
original breakpoint. This results in a complication with targeting 90% of 23k thrust for
the 21k rating. By targeting 90% of the output 23k MTO thrust (clipped), the resulting
thrust for the 21k rating was less than desired by the customer. The customer further
requested that the 21k level of thrust be based off the 23k level of thrust had the N1 limit
not interfered with thrust output. The process had to be re-run with the N1 limit moved
out of the way, creating a final value for 21k MTO that was less than a 10% derate from
23k. This can be seen in the following contour plot.
24
Altitude
Mach Number
Figure 7 – 23k to 21k Derate – N1 limiting Issue
Figure 7 above shows the numerical values of percent delta thrust, or derate, between the 23k and 21k MTO rating is 10% throughout a good portion of the take off
envelope. The requirement for the 21k MTO rating was to be 90% of the unconstrained
(no N1 limit) 23k MTO thrust target. The points that are less than 10% derate from 23k
MTO in the upper right hand corner of the envelope are clipped because of the N1 limit.
Contour plots similar to Figure 7 above were created for a number of ambient conditions, power settings, and bleed configurations to ensure all customer requirements we’re
met. Below is an example of the 21k to 18.9k check. For all conditions, the below
contour plot applies. There were no cases in which the 21k to 18.9k plots were different
than a 10% derate.
25
Figure 8 – 21k to 18.9k Derate Contour Plot
Checks were also performed to ensure the special logic requested by the customer for the MCT and MCL ratings worked correctly. From section 2.4, one such
requirement was to ensure 21k MCT was equal to 23k MCT everywhere outside the
takeoff envelope, and equal to 94% of the 23k MCT inside the envelope. Figure 9 below
illustrates this well. The green domain in Figure 9 is the area outside the takeoff envelope, and it represents the 0% delta between the 23k and 21k MCT rating, which is the
customer requirement for this flight regime.
26
Figure 9 – 23k to 21k MCT Bleeds OFF Contour Plot
Additionally, Figure 9 shows that the new ratings file is meeting the customer
requirements with some small deviation in the upper left hand portion of the takeoff
envelope (in red). This is mostly due to the grid differences between the ratings files and
the contour plots. At points in which the ratings file does not contain a node, there is
usually a deviation from the target by a small amount. In this particular case, the upper
left hand corner of the contour is being influenced by a point outside the flight envelope
all together. Also notice the transition points between the takeoff envelope and the
remainder of the flight envelope. These points rapidly go from a 0% difference to a 6%
difference linearly, which is by design.
The last set of contour plots was created to ensure MTO>MCT>MCL at a given
thrust setting and bleed configuration. This was a relatively simply check. Contour plots
needed to show that the simulation resulted in a higher thrust for MTO than MCT, and a
higher thrust for MCT over MCL. An example showing the simulation is working
correctly is shown below in Figure 10.
27
Figure 10 – MCL to MCT Bleeds OFF Contour Plot
Figure 10 actually illustrates the requirement that for all points, MCT >MCL.
There is not a single positive delta on the plot, indicating that at all conditions, 21k MCT
is always equal to or greater than 21k MCL.
3.4 Behavior Checks
The last checks to be performed were to ensure the simulation was behaving as
expected with regards to thrust lapses, using fir tree plots. One such fir tree plot can be
seen below in Figure 11.
Figure 11 – MCL to MCT Bleeds OFF Contour Plot
28
The above figure shows the 23k MTO thrust lapse for all the different bleed configurations we are concerned with. There a couple key takeaways from this plot. As
bleed flow increases, the resulting thrust on the hot day line will decrease. While on the
cold day “flat”, bleed flow has no impact on the net corrected thrust. The shape of the
rating characteristic matches expectations as well. For increasing levels of bleed flow,
the break point temperature moves to a colder condition. This is due to the reduced
performance of the engine with higher levels of bleed flow. By taking more flow off the
engine, the T4 levels will increase and we will once again be limited by our maximum
T4 temperatures. Plots similar to this were created for all target thrust levels and conditions to ensure compliance.
29
4. Conclusions
After creating an additional 5000 line matrix of T4 schedules, adding schedule
selection logic to the simulation, and checking about every possible flight condition, I
was comfortable enough to present the results to my department managers and executives, and finally distribute the new ratings files to the Pratt & Whitney modeling group.
The modeling group took the input and created a new NPSS simulation which was
readied for distribution. The simulation has since been distributed to the customer and
has been used extensively by their engine performance group. As a result, the customer
now has a better idea of the thrust output of the geared turbofan engine, and those of us
in the P&W performance group have a set of thrust targets in which to meet with future
simulations based on an N1 rating.
30
5. References
1. NPSS User Guide, Software Release: NPSS_2.3.0, Rev. 2, July 2, 2010
2. Lewis, John H., Fundamental Engineering Principles as Applied to Gas Turbine
Performance Analysis, 2nd Edition, September 2003
3. Hill, Philip and Peterson, Carl, Mechanics and Thermodynamics of Propulsion,
2nd Edition, Addison-Wesley Publishing Company, 1992
4. Aainsqatsi, K. “Turbofan operation.svg”,
http://en.wikipedia.org/wiki/File:Turbofan_operation.svg
31
Appendix A – MTO Selection Logic Example
real TB_sched2targ (real alt, real MN) {
if (Pset.switchTOrating == "23KTO") {
return TB_sched2targ23(alt, MN, Amb.dTs);
}
if (Pset.switchTOrating == "21KTO" ) {
if (DblPtRat.SecondPointActive == 0.0)
return TB_sched2targ21mtobld0(alt, MN, Amb.dTs);
}
else {
if ((CbldECS.switchW == "OFF" || (CbldECS.switchW == "INPUT" && CbldECS.W_in == 0.0)
|| (CbldECS.switchW == "FRAC" && CbldECS.WqWref_in == 0.0)) && ((CbldWAI.switchW
== "OFF" || (CbldWAI.switchW == "INPUT" && CbldWAI.W_in == 0.0) || (CbldWAI.switchW
== "FRAC" && CbldWAI.WqWref_in == 0.0)) && (CbldCAI.switchW == "OFF" || (CbldCAI.switchW == "INPUT" && CbldCAI.W_in == 0.0) || (CbldCAI.switchW == "FRAC" &&
CbldCAI.WqWref_in == 0.0)))){
return TB_sched2targ21mtobld0(alt, MN, Amb.dTs);
}
else { // ECS OFF, WAI OFF, CAI ON
if ((CbldECS.switchW == "OFF" || (CbldECS.switchW == "INPUT" && CbldECS.W_in == 0.0)
|| (CbldECS.switchW == "FRAC" && CbldECS.WqWref_in == 0.0)) && ((CbldWAI.switchW
== "OFF" || (CbldWAI.switchW == "INPUT" && CbldWAI.W_in == 0.0) || (CbldWAI.switchW
== "FRAC" && CbldWAI.WqWref_in == 0.0)) && (CbldCAI.switchW == "CALCULATE" ||
(CbldCAI.switchW == "INPUT" && CbldCAI.W_in > 0.0) || (CbldCAI.switchW == "FRAC"
&& CbldCAI.WqWref_in > 0.0)))){
return TB_sched2targ21mtobldOEI(alt, MN, Amb.dTs);
}
else { // ECS ON (Single or Dual), WCAI OFF
if ((CbldECS.switchW == "CALCULATE" || (CbldECS.switchW == "INPUT" &&
CbldECS.W_in > 0.0) || (CbldECS.switchW == "FRAC" && CbldECS.WqWref_in >
0.0)) && ((CbldWAI.switchW == "OFF" || (CbldWAI.switchW == "INPUT" &&
CbldWAI.W_in == 0.0) || (CbldWAI.switchW == "FRAC" && CbldWAI.WqWref_in
== 0.0)) && (CbldCAI.switchW == "OFF" || (CbldCAI.switchW == "INPUT" &&
CbldCAI.W_in == 0.0) || (CbldCAI.switchW == "FRAC" && CbldCAI.WqWref_in ==
0.0)))){
return TB_sched2targ21mtobldOEI(alt, MN, Amb.dTs);
}
32
else { // All other bleed states
return TB_sched2targ21mtobldOEIAI(alt, MN, Amb.dTs);
}
}
}
}
}
33
Appendix B – Scaled T4 Table Example
34