American Institute of Aeronautics and Astronautics
AIP = aerodynamic interface plane
ARP1420 = Aerospace Recommended Practice 142013
BWB = blended wing body
BLI = boundary layer ingesting
BPF = blade-passing frequency
BPR = bypass ratio
BWB = blended wing body
CFD = computational fluid dynamics
EE = embedded engines
EGV = exit guide vane
ExTE = extended trailing edge
TSFC = thrust specific fuel consumption
UHB = ultra high bypass
I. Introduction
nited technologies Research Center completed the first part of the second phase of a two-phase research
program funded by NASA Fundamental Aeronautics Subsonic Fixed Wing Program to advance the design
capabilities for embedded engines in Blended Wing Body (BWB) aircraft. The performance objectives for NASA’s
Generation-After Next (N+2) BWB aircraft research require dramatic reductions in noise, emissions, and fuel burn
relative to conventional aircraft.1 These objectives require a close coupling of the airframe and propulsion systems
in ways that challenge current designs and design systems. One such concept which holds promise for meeting
these objectives is boundary layer ingestion into the propulsion system. The challenge is then shifted from the
airframe to the propulsion system where the high inlet flow distortion drives performance, aeromechanical,
stability/operability and acoustic issues within the compression system. The inlet duct and fan function as a system,
the large flow distortions lead to strong coupling between the fan and upstream flow fields. The impact of the
compromises in engine performance required to overcome these issues is a key question that must be addressed.
In recent years several system-level studies of advanced aircraft with BLI propulsion have been carried out and a
short summary is presented next Section II while a more thorough review is done by Hardin, et al.2 Some of the
benefits of BLI propulsion are reviewed by Plas, et al.3, who describe maximum propulsive efficiency benefits of up
to 28%. Actual achievable benefits are a function of several elements, such as the amount of airframe boundary
layer that can be ingested; wake properties; the engine cycle; composition of the overall airframe drag; and losses
incurred in the inlet and turbomachinery due to boundary layer ingestion and engine operation in the presence of
distortion. The study3 estimates that a net fuel burn reduction of between 3 and 4% should be achievable with a
modest level of boundary layer ingestion (on the order of 16%) after properly accounting for reductions in engine
performance.
A thorough system study of BLI propulsion benefits for a Blended Wing Body (BWB) carried out by Daggett et
al.4 predicted a net 5.5% reduction in aircraft fuel burn. The benefit of taking lower-velocity air into the engine
inlets was accounted for through a reduction in the propulsion system ram drag. Other elements included in the
study were the weight and drag reductions associated with the elimination of the pylon and reductions in nacelle
weight associated with embedding the propulsion system. The effect of reduced pressure recovery on engine
performance was included in the study. Active flow control was included in the embedded engine implementation
in order to enable a shorter offset inlet duct, however, the weight and power consumption of the flow control
subsystem was not accounted for. Following this initial study, Kawai, et al.5 carried out a more detailed follow-on
study using Computational Fluid Dynamics (CFD) simulations of the entire aircraft. Consistent with the previous
study4, for a fixed engine cycle the performance benefit provided by the lower ram drag associated with the reduced
inlet velocity for a BLI system was found to be largely offset by the cycle penalty due to reduced inlet pressure
recovery. It was concluded that with additional work to better optimize the integration and the use of active inlet
flow control to reduce or eliminate the distortion-related cycle penalties, a 10% vehicle fuel burn reduction could be
achieved.
Nickol and McCullers incorporated the use of an aircraft sizing tool into the system study process for BWB
aircraft configurations.6 This approach allows for the effects of multiple technologies, and their non-linear
interactions, to be included in system-level vehicle assessments. In their study BLI propulsion was included as a
contributing technology, as was active inlet flow control. The effect of power extraction for the flow control
subsystem was included on engine thrust-specific fuel consumption (TSFC). The BLI propulsion technology itself
was found to provide a 7% fuel burn reduction relative to an advanced technology baseline aircraft. When
U
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interactions between BLI and other technologies that would be present on an advanced BWB aircraft were
accounted for in the sizing model, the BLI benefit was found to be 5.2%.
System studies to date have reported aircraft fuel burn benefits of between 4 and 10%, with some variation in the
number and treatment of parameters that have an impact on the system-level result. All of these studies have shown
that there are relatively large benefits to pursue, but that significant performance debits to the propulsion system that
can result from BLI must be avoided or minimized in order to maximize the net vehicle-level benefit. The range of
system study benefits identified by previous investigators, along with UTRC own results2, is illustrated in Figure 1.
15%
10%
5%
Air
cra
ft F
ue
l
Bu
rn R
ed
uctio
n
Boeing(Daggett, et al., 2003)
MIT(Plas, et al., 2007)
NASA( Nickol, et al., 2009)
Boeing(Kawai, et al., 2006)
UTRC( Hardin, et al., 2012)
Figure 1. Summary of some previous and current BLI system study results.
A recent study published by Rodriguez7 highlights some of the key challenges in realizing net vehicle-level
benefits with BLI propulsion. An N+2 class BWB aircraft was used for this study with BPR 12 propulsion systems.
Using automated optimization-based design, the BLI propulsion was found not to provide a fuel burn benefit relative
to pylon-mounted baseline engines. The optimizer used a fuel-burn-based cost function to evaluate design changes
both in terms of reduced inlet total pressure distortion, as well as inlet total pressure losses subject to multiple
constraints. For a BWB/BLI-EE architecture, an accurate fuel-burn cost function requires a detailed understanding
of both the vehicle and the propulsion system. In addition, such a high level cost function may be very sensitive to
particular aspects of the propulsion system integration while oversimplifying specific aspects of the flow
requirements. Use of a single cost function employed in this manner can drive automated optimizers to regions of
solution space that satisfy system constraints yet do not achieve the proper balanced design result. The BLI
propulsion system result, for example, reduced the circumferential distortion index but did so at the cost of a 1.3%
increase in inlet total pressure loss. For a BPR 12 propulsion system, this effect alone can be expected to increase
TSFC by several percent, increasing aircraft fuel burn and largely negating any benefit provided by boundary layer
ingestion. In addition, constraints on the location of the engines on the BWB aircraft for this study resulted in strong
interference drag when the propulsion systems were integrated into the airframe. Boeing6 has identified this as well,
and it would be expected that integrated BLI vehicle / propulsion system designs will be required in order to
mitigate this issue.
II. Summary of the Vehicle-Level Preliminary System Study2
The overall goal of the BLI-EE research was to develop a distortion-tolerant propulsion system design that
simultaneously targets less than 2% reduction in fan efficiency and less than 2% reduction in stall margin relative to
a clean-inflow baseline. In order to identify BLI propulsion concepts with the most promise for providing system-
level benefits, a high-level, trade-factor-based study was conducted2, and a summary is presented here. The study
used boundary layer profiles and the external flow field associated with the Boeing N2A-ExTE blended wing body
aircraft. The aircraft configuration is derived from previous Boeing BWB design work based upon the original
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SAX-40 aircraft used by MIT in recent studies.3-5
This reference aircraft is an advanced, BWB design for podded or
BLI propulsion, with an extended trailing edge aimed at providing additional aft acoustic shielding of jet noise. The
vehicle is designed for a 7,500 nautical mile mission and is within the large commercial transport class. The design
cruise Mach number is 0.8 at an altitude of 35,000 feet. The isentropic Mach number contours on the suction side at
cruise conditions are shown in Figure 2. The percentage of the total drag that can be ingested by propulsion systems
embedded on the suction side of the reference vehicle aft of the wings is limited to around 12% or less. The
reference vehicle is configured with podded propulsion systems that do not ingest any boundary layer air. The
reference propulsion system for the current study was chosen as an ultra-high bypass (UHB) turbofan, with a
reference cycle chosen to have a bypass ratio of 16 and a fan pressure ratio of 1.35. This type of engine could enter
service in the 2020 – 2025 time frame, and would employ a suite of advanced technologies in the engine core,
bypass stream, and nacelle. The BLI-EE system study attempted to capture the vehicle impacts associated with
changes in propulsion system drag, and to a lesser extent weight, where these changes can be estimated. Propulsion
system performance changes due to BLI operation were to be included as well. Use of the same airframe and engine
cycle for both podded and BLI configurations is intended to give a more accurate evaluation of the benefits of the
BLI concept exclusive of any benefits due to airframe or engine features other than BLI.
BLI benefit limited by viscous drag accessible on N+2 vehicle upper surface (~12% orless depending on inlet location)
1Figure & CFD solution from the Boeing Company, Kawai, R. T., Friedman, D. L., and Serrano, L.,Blended Wing Body (BWB) Boundary Layer Ingestion (BLI) Inlet Configuration and System Studies,NASA CR-2006-213534, December 2006.
0.80
0.75
0.70
0.65
1.00
0.95
0.90
0.85
1.05
X/C = 0.8011% BLI
X/C = 1.0012.4% BLI
Figure 2. Isentropic Mach number contours on suction side. BLI benefit limited by viscous drag accessible on
N+2 vehicle upper surface (~12% or less depending on inlet location). Solution from Boeing, Ref. 5.
The system study used the design point computational fluid dynamics (CFD) solution of the vehicle5 to
determine how much of the viscous drag was available for boundary layer ingestion. The percentage of the total drag
that can be ingested by propulsion systems embedded on the suction side of the vehicle aft of the wings is around
11% and cannot exceed 12.4%. The maximum theoretical BLI benefit was estimated using the energy-based
propulsive efficiency in the presence of wake ingestion developed by Smith.7 Propulsive efficiency is shown to be a
function of the jet velocity ratio, as is the case for non-BLI propulsion, and in addition is also shown to be a function
of the ingested airframe drag fraction, a boundary layer pseudo-energy factor, and a wake recovery parameter. A
comparison of several of the architectures investigated to the theoretical maximum possible benefit as computed by
Smith’s method7 is shown in Figure 3.
2 The system study indicated that low-loss inlets and high-performance,
distortion-tolerant turbomachines are key technologies required to achieve a 3-5% BLI fuel burn benefit for the N+2
aircraft relative to a baseline high-performance, pylon-mounted, propulsion system. The study further identified the
key sensitive parameters and overall objectives and their desired bounds. Potential attractive configurations have
inlets with an aspect ratio of around 2.0, total pressure losses below 0.5% and close integration between the airframe
and the fan.
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Propulsion / airframe integration configuration can determine ingested boundary layer drag fraction & resulting maximum achievable upper benefit.
8
6
4
2
0
Pro
pu
lsiv
e E
ffic
ien
cy o
r T
SFC
Be
ne
fit
(%)
Max Benefit for 12.4% BLI (aircraft uppercenter area, LE to TE) (Smith)
• Inlet excess pressure loss reduced ~3x relative to Phase 1 optimized inlet & ~4 – 5x relative to original Inlet A starting point
• Dominant distortion harmonic amplitudes reduced ~30 – 50% relative to original Inlet A starting point
NASA Inlet A
1
2
0.5
1
Inle
t ex
cess
to
tal p
ress
ure
loss
(Δ
PT
/ P
T1),
%
Red
uct
ion
of
dis
tort
ion
har
mo
nic
am
plit
ud
e (n
orm
aliz
ed t
o N
ASA
Inle
t A
)
UTRC A
UTRC Parametric A
UTRC P1 UTRC P2
NASA A
UTRC A
UTRC P1
UTRC P2
0 0
RDEES Phase 1
Largest reduction of the first three harmonic amplitudes
UTRC P4
UTRC P4UTRC P3
UTRC P3
UTRC Parametric A
RDEES Phase 2
Figure 8. Performance evolution of the inlet design.
Next, the inlet lip and upstream BWB wall shaping led to the “UTRC P3” design with inlet total pressure losses
below 0.4%. At this stage, preliminary calculations to evaluate the inlet impact on the fan are carried out.
Specifically, UTRC performed high fidelity coupled inlet / fan fluid flow steady (mixing plane) and unsteady
calculations to evaluate fan efficiency, aerodynamic and stress blade loading. Some of the aerodynamic results are
summarized in the next Section IV and will be detailed in a future paper. In addition, as part of the NASA/UTRC
Robust Design of Embedded Engine Systems team, Bakhle et al.16
performed preliminary fan aeromechanics
calculations that show flutter-free behavior at the inlet design conditions analyzed with distorted in-flows. Next, it
was decided to further reduce the total pressure inlet harmonic content upstream of the fan by performing another
design iteration. That led to the final “UTRC P4” design with the inlet total pressure losses maintained below 0.4%.
The inlet design progression shows the reduction in inlet total pressure loss as well as the largest reduction achieved
in the first three distortion harmonics relative to the NASA’s “Inlet A” level. This is an aggregate metric showing
the largest of the improvements in the first three harmonics. Separately, the first harmonic amplitude was reduced
by approximately 33% relative to the NASA’s “Inlet A” baseline, the second harmonic by approximately 45%, and
the third harmonic by 20%.
The design progression is significant, given the starting point of the NASA Inlet A configuration. Also of note is
that while part of the improvement achieved in the Phase 1 program was associated with the implementation of inlet
flow control, the improvements in the current Phase 2 effort are solely the result of parametric / optimized design of
the inlet shape using both global and local parameters. All four designs “UTRC P1, P2, P3 and P4” were achieved
without the use of flow control. A three-dimensional view of the UTRC P4 inlet along with the wall pressure
distribution (normalized by the total pressure upstream of the inlet lip) and Mach number at the AIP at nominal inlet
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conditions are shown in Figure 9 a). For comparison, total pressure fields at AIP are also presented for UTRC P4
and NASA’s “Inlet A” in Figure 9 b).
NASA Inlet A
UTRC P4
a) Three-dimensional view of the pressure distribution
over inlet wall and Mach number at AIP.
b) Total pressure field at AIP.
UTRC P4
Figure 9. Flow field details for UTRC P4 inlet at nominal conditions (L/D=0.6). NASA’s “Inlet A” results
10
shown here for comparison.
Note that the initial UTRC system study2 assumed a particular correlation between the inlet aspect ratio and the
inlet length. The present CFD inlet design led to shorter inlets resulting in a 4-5% BLI fuel burn benefit for the N+2
aircraft relative to a baseline high-performance, pylon-mounted, propulsion system.
The preliminary impact of the BLI on the fan performance is discussed in the next Section IV. However, for a
given fan geometry, the inlet mass flow at nominal conditions is determined by the interaction between BLI inlet
and the fan. The ingested boundary layer develops into a distortion that leads to mass flow imbalance at the AIP,
with higher mass flow in the top half. At the same time, the top half of the clean inlet mass flow ingested into the fan
is limited by the fan requirements for higher efficiency. The overall mass flow deficit is about 5% and defines the
new nominal mass flow for the coupled BLI inlet fan system. The inlet performance defined here by the variation of
total pressure losses and total pressure harmonics with respect to the mass flow is shown in Figure 10. All quantities
are normalized by their corresponding nominal values of the BLI inlet fan system.
Change in Mass Flow Change in Mass Flow
Ch
ange
in P
TLo
ss
Ch
ange
in P
TH
arm
on
ics
Figure 10. Inlet performance: changes in total pressure loss and first two harmonics with respect to inlet mass
flow variation for UTRC P4 inlet (L/D=0.6).
IV. Preliminary Impact on Fan Performance
At the inlet throat section, the incoming flow is defined by the amount of ingested boundary layer as percentage
of total incoming flow. As the flow further develops through the inlet into the fan, the flow at the AIP is
characterized by its two main components: total pressure distortion resulting from momentum losses in the
incoming boundary layer, and three-dimensional swirl distortion. Swirl-inducing three dimensional flows can be
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created by the internal duct shape, offset, or other effects, including the static pressure distortion upstream of the fan.
Ultimately, both total pressure and swirl distortion lead to large spatial variations of blade-relative incidence.
Together, the excursions in blade relative incidence and low mass flow/total pressure in the bottom half of the fan,
have a first-order impact on the fan performance, operability, and aeromechanic response.
The excursions in blade incidence relative to its mean values are presented in Figure 11. This was done by
calculating the instantaneous relative inflow angle distribution (β), and subtracting the circumferential mean from
these values. The circumferential mean was calculated by first computing the time-mean incidence angles, and then
averaging them circumferentially. For a fan operating in clean inflow, this process will yield a spatially uniform
swirl distribution in the circumferential direction, and except for potential field effects close to the fan leading edge,
the excursions in blade-relative incidence will be zero at all locations. In the presence of distorted inflow,
excursions from the mean values will be expected. The left plot shown in Figure 11 corresponds to NASA inlet A
while the plot on the right corresponds to UTRC P4 inlet. Both results are computed at a plane section near the
leading edge upstream of the fan, in both cases the inlet is operating in a loosely-coupled manner with the fan.
Lower excursions in relative incidence from the clean flow lead to less reduction in fan stability margin and provide
a clear benefit of UTRC P4 inlet when compared with NASA’s “Inlet A”.
Δβ (deg.)
NASA Inlet A UTRC P4 Inlet
Figure 11. Excursions in fan blade leading edge relative incidence from clean inflow. UTRC P4 inlet design
significantly improves fan interaction with incoming distortion.
The harmonic content of the distorted flowfield upstream of the fan is the starting point of the aeromechanical
analysis16
. At AIP, each flowfield harmonic is treated as an “engine order” excitation for which the unsteady blade
loading and corresponding modal blade vibration responses are computed and flutter boundaries determined. The
dynamic stresses associated with the blade loading can be used to evaluate the blade integrity at different flow
conditions. Shown in Figure 12 are the total pressure contours for the first five harmonics at the AIP at nominal
conditions. Note that the total pressure field is dominated by the first three harmonics. For the fourth harmonic and
higher distortion appears only near outer diameter wall.
AIP Harmonics: Multiple Inlet Configuration
Figure 12. Total pressure distortion and the first five harmonics at the AIP at nominal conditions for UTRC
P4 inlet (L/D=0.6).
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For the final P4 inlet, UTRC performed high fidelity steady and unsteady fluid flow calculations starting with the
fan in clean and distorted (loosely coupled) in-flow and ending with the fully coupled inlet/fan stage with in-flow
distortion prescribed at the inlet throat. The configurations defined by the fan only, fan stage and coupled inlet / fan
/EGV are shown in Figure 13. Fan efficiency and aerodynamic blade loading were evaluated. Some of the
aerodynamic results are summarized in Figure 14 and will be investigated in detail in a future paper. For the results
shown here, the exit guide vanes (EGV) impact is neglected. The fan maximum efficiency with clean in-flow is
around 96.5%. The coupled UTRC P4 inlet / fan unsteady calculations show about 0.5% or less drop in maximum
fan efficiency and about 7% drop in corresponding mass flow when compared to clean in-flow. However, to
increase the fan stability margin, we selected the nominal mass flow for the coupled inlet / fan at 95% of the
nominal clean flow with a drop in efficiency of 1.5%. The 95% nominal mass flow for the coupled inlet / fan system
is consistent with all the inlet calculations performed in Section III.
Figure 13. UHB turbofan, with bypass ratio of 16 and a fan pressure ratio of 1.35.
Max eff. with “NASA
Inlet A” distortion
-6%
-0.5%
Mass Flow / Mass Flow (nominal clean)
Ad
iab
ati
c E
ffic
ien
cy
-1.5%
Figure 14. RANS calculations for steady fan alone (Clean, STD, F) with clean flow and unsteady P4 inlet/full
wheel fan (DIST, UNS, IF).
V. Conclusions and Recommendations
A high-level, trade-factor-based system study has demonstrated that significant system-level benefits can be
achieved with BLI propulsion. The study used a fixed, ultra high bypass ratio advanced engine cycle and Boeing
N+2 BWB aircraft; engine weight & propulsion / airframe integration effects were not considered.
The system study has identified a low-loss inlet and high-performance, distortion-tolerant turbomachinery as key
technologies consistent with achieving net system level benefits. Optimization-based parametric design has yielded
a significantly improved inlet that meets the requirements identified in the system study. In summary, inlet excess
pressure losses have been reduced by around three to five times, while simultaneously reducing the dominant
distortion harmonic energy amplitudes by 30 to 50%. The resulting inlet length was reduced by over 50% relative to
the initial NASA’s “Inlet A” starting geometry. Through optimized inlet design, the impact of distorted inflow on
fan efficiency was reduced from a 6% loss associated with embedding the fan into the highly-offset NASA Inlet A,
to about 0.5 to 1.5% with the current UTRC P4 inlet design. Note that the initial UTRC system study2 assumed a
particular correlation between the inlet aspect ratio and the inlet length. The present CFD inlet design led to shorter
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inlets resulting in a 3-5% BLI fuel burn benefit for the N+2 aircraft relative to a baseline high-performance, pylon-
mounted, propulsion system.
Preliminary inlet / fan calculations show that coupling effects (airframe / inlet / fan / fan exit guide vane) will
play a key role into the integration of the inlet into the propulsion design and ensure that no more than 2% reduction
in stall margin relative to a clean-inflow baseline is achieved. In addition, changes in fan mechanical design may be
required to ensure fan blade integrity.
Acknowledgments
This work was funded under NASA Contract NNC07CB59C, Robust Design for Embedded Engine Systems
(RDEES), Phase 2. The authors wish to acknowledge the technical guidance of NASA Glenn Research Center
Technical Monitor Mr. David Arend, and Principle Investigator Dr. Gregory Tillman of United Technologies
Research Center. The authors also want to thank Drs. Scott Ochs, Gorazd Medic and Paul Van Slooten of United
Technologies Research Center for their support with grid generation and numerical flow simulations. The authors
are grateful to Drs. Bill Cousins and Om Sharma for their insight into the fan stability and operability.
References 1Amendment No. 4 to the NASA Research Announcement (NRA) Entitled “Research Opportunities in Aeronautics– 2006
(ROA-2006),” NNH06ZEA001N, Release May 23, 2006. 2Hardin, L. H., Tillman, T. G., Sharma, O. P., Berton, J., and Arend, D. J., ” Aircraft System Study of Boundary Layer
Ingesting Propulsion,” AIAA 48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Jul 30- Aug 1, 2012. 3Plas. A.E., Sargeant, M.A., Madani, V., Crichton, D., Greitzer, E.M., Hynes, T. P., and Hall C.A. “Performance of a
Boundary Layer Ingesting (BLI) Propulsion System,” 45th AIAA Aerospace Sciences Meeting, AIAA 2007-450, January 2007. 4Daggett, D. L., Kawai, R., and Friedman, D., Blended Wing Body Systems Studies: Boundary Layer Ingestion Inlets with
Active Flow Control. NASA CR-2003-212670, December 2003. 5Kawai, R. T., Friedman, D. L., and Serrano, L., Blended Wing Body (BWB) Boundary Layer Ingestion (BLI) Inlet
Configuration and System Studies. NASA CR-2006-214534, December 2006. 6Nickol, C. L., and McCullers, L. A., Hybrid Wing Body Configuration System Studies. AIAA paper AIAA 2009-931, AIAA
47th Aerospace Sciences Meeting, January 2009. 7Smith, L. H., Jr., Wake Ingestion Propulsion Benefit. AIAA Journal of Propulsion and Power, Vol. 9, No. 1, pp. 74 – 82,
January – February 1993. 8Rodriguez, D. L., Multidisciplinary Optimization Method for Designing Boundary-Layer-Ingesting Inlets. Journal of
Aircraft, Vol. 46, No. 3, May – June 2009.
9Cousins, W.T., “History, Philosophy, Physics, and Future Directions of Aircraft Propulsion System/Inlet Integration,” GT
2004-54210, ASME Turbo Expo 2004. 10Florea, R. V., Reba, R., Van Slooten, P. R. , Sharma, O., Stucky, M., O’Brien, W. and Arend, D. J., ” Preliminary Design
for Embedded Engine Systems,” AIAA-2009-1131, 43rd AIAA Aerospace Sciences Meeting, January 2005, Orlando, FL. 11Sculptor User Manual, Version 2.3.0, http://gosculptor.com/products/sculptor/. 12CFL3D Version 6.6 Home Page. http://cfl3d.larc.nasa.gov/. 13Aerospace Recommended Practice ARP 1420, 1978, “Gas Turbine Engine Inlet Flow Distortion Guidelines,” Society of
Automotive Engineers. 14Davis, R. L., Ni, R. H., and Carter, J. E., Cascade Viscous Flow Analysis Using the Navier-Stokes Equations. AIAA
Journal of Propulsion and Power, Vol. 3, No. 5, pp. 406-414, 1987. 15Berrier, Bobby L., Carter, Melissa B., and Allan, Brian G., “High Reynolds Number Investigation of a Flush-Mounted, S-
Duct Inlet with Large Amounts of Boundary Layer Ingestion,” NASA/TP-2005-213766. 16Bakhle, M. A., Reddy, T. S. R., Herrick, G. P., Shabbir, A., and Florea, R. V. “Aeromechanics Analysis of a Boundary
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