American Institute of Aeronautics and Astronautics 1 Aircraft Noise Reduction Technology Roadmap Toward Achieving the NASA 2035 Noise Goal Russell H. Thomas 1 NASA Langley Research Center, Hampton, VA 23681 USA Yueping Guo 2 NEAT Consulting, Seal Beach, CA 90740 USA Jeffrey J. Berton 3 NASA Glenn Research Center, Cleveland, OH 44135 USA And Hamilton Fernandez 4 NASA Langley Research Center, Hampton, VA 23681 USA With technology level assumptions corresponding to a 2025 timeframe, a NASA- modeled 301 passenger size class hybrid wing body was previously predicted to achieve a noise level of 40.2 EPNL dB cumulative below the Stage 4 limit. A new set of technologies is selected that build on previously successful developments and, in some cases, are new early stage noise reduction approaches. These technologies are consistent with a noise reduction technology roadmap for the 2035 timeframe of the NASA Far Term goal. Aircraft system noise predictions are performed for this selected set of promising noise reduction technologies beginning with developing a predicted noise reduction for each added design feature or technology. When all eighteen predicted configurations are added to form the final aircraft concept, a total of 10.5 EPNL dB cumulative noise reduction is achieved above an updated baseline, reaching a margin to Stage 4 of 50.9 EPNL dB for this Far Term hybrid wing body concept. The center plug liner is the most promising internal nacelle liner concept with a system level noise reduction of 1.3 EPNL dB. Of the other newer, innovative and less developed noise reduction approaches, three are of particular importance and together are attributed with the large majority of the 10.5 EPNL dB of additional noise reduction. First, validated source levels for the Krueger flap as well as both cove filler and aligned bracket noise reduction approaches account for 4.4 EPNL dB at the system level. Second, an innovative pod gear approach to shorten and then integrate the main gear with the airframe reduces the system level noise by 3.3 EPNL dB. Third, accounting for 2.5 EPNL dB are various design features and technologies that increase shielding effectiveness, the added noise reduction from shielding achievable with the same airframe dimension. I. Introduction he NASA Advanced Air Transport Technology (AATT) Project is focused on developing and demonstrating technologies for aircraft systems that could meet aggressive goals for fuel burn, noise, and emissions, particularly for the Far Term period of 2035. The fuel burn goal is a reduction of 60-80% relative to a best-in-class 2005 aircraft; the noise goal is 42-52 EPNL dB (Effective Perceived Noise Level) cumulative below the Stage 4 requirement; and the emissions goal is a reduction of greater than 80% in NOx (oxides of nitrogen) levels below the CAEP 6 (Committee on Aviation Environmental Protection) standard. The target date is 2035 and beyond for key technologies to be at a technology readiness level 1 Senior Research Engineer, Aeroacoustics Branch, MS 461, AIAA Senior Member, [email protected]2 NEAT Consulting, 3830 Daisy Circle, Seal Beach, CA 90740, AIAA Associate Fellow 3 Aerospace Engineer, Propulsion Systems Analysis Branch, MS 5-11, AIAA Senior Member 4 Aircraft Noise Reduction Sub-Project Manager, Advanced Air Transport Technology Project, AIAA Senior Member T
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American Institute of Aeronautics and Astronautics
NASA Langley Research Center, Hampton, VA 23681 USA
Yueping Guo2
NEAT Consulting, Seal Beach, CA 90740 USA
Jeffrey J. Berton3
NASA Glenn Research Center, Cleveland, OH 44135 USA
And
Hamilton Fernandez4
NASA Langley Research Center, Hampton, VA 23681 USA
With technology level assumptions corresponding to a 2025 timeframe, a NASA-
modeled 301 passenger size class hybrid wing body was previously predicted to
achieve a noise level of 40.2 EPNL dB cumulative below the Stage 4 limit. A new set
of technologies is selected that build on previously successful developments and, in
some cases, are new early stage noise reduction approaches. These technologies are
consistent with a noise reduction technology roadmap for the 2035 timeframe of the
NASA Far Term goal. Aircraft system noise predictions are performed for this
selected set of promising noise reduction technologies beginning with developing a
predicted noise reduction for each added design feature or technology. When all
eighteen predicted configurations are added to form the final aircraft concept, a
total of 10.5 EPNL dB cumulative noise reduction is achieved above an updated
baseline, reaching a margin to Stage 4 of 50.9 EPNL dB for this Far Term hybrid
wing body concept. The center plug liner is the most promising internal nacelle liner
concept with a system level noise reduction of 1.3 EPNL dB. Of the other newer,
innovative and less developed noise reduction approaches, three are of particular
importance and together are attributed with the large majority of the 10.5 EPNL dB
of additional noise reduction. First, validated source levels for the Krueger flap as
well as both cove filler and aligned bracket noise reduction approaches account for
4.4 EPNL dB at the system level. Second, an innovative pod gear approach to
shorten and then integrate the main gear with the airframe reduces the system level
noise by 3.3 EPNL dB. Third, accounting for 2.5 EPNL dB are various design
features and technologies that increase shielding effectiveness, the added noise
reduction from shielding achievable with the same airframe dimension.
I. Introduction
he NASA Advanced Air Transport Technology (AATT) Project is focused on developing and
demonstrating technologies for aircraft systems that could meet aggressive goals for fuel burn, noise,
and emissions, particularly for the Far Term period of 2035. The fuel burn goal is a reduction of 60-80%
relative to a best-in-class 2005 aircraft; the noise goal is 42-52 EPNL dB (Effective Perceived Noise Level)
cumulative below the Stage 4 requirement; and the emissions goal is a reduction of greater than 80% in
NOx (oxides of nitrogen) levels below the CAEP 6 (Committee on Aviation Environmental Protection)
standard. The target date is 2035 and beyond for key technologies to be at a technology readiness level
1 Senior Research Engineer, Aeroacoustics Branch, MS 461, AIAA Senior Member, [email protected] 2 NEAT Consulting, 3830 Daisy Circle, Seal Beach, CA 90740, AIAA Associate Fellow 3 Aerospace Engineer, Propulsion Systems Analysis Branch, MS 5-11, AIAA Senior Member 4 Aircraft Noise Reduction Sub-Project Manager, Advanced Air Transport Technology Project, AIAA Senior Member
T
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(TRL) of 5-6 (system or subsystem prototype demonstrated in a relevant environment) [1]. As were earlier
Near Term and Mid Term NASA goal levels, this Far Term noise goal is intended as an aggressive
technical challenge. It has been recognized for some time that a large reduction, a step change, in aircraft
noise would require a change in the aircraft configuration [2]. The studies of Thomas et al. [3] and Czech et
al. [4] established a technology roadmap for achieving the Mid Term (previously referred to as N+2) noise
goal of 42 EPNL dB cumulative below Stage 4. A series of studies [5-13] during the Environmentally
Responsible Aviation (ERA) Project matured the low noise aspects of the hybrid wing body (HWB) aircraft
configuration including propulsion airframe integration, propulsion, and noise reduction technologies.
Another series of studies improved the processes for modeling an HWB aircraft and by which higher
fidelity noise assessments could be made with increasing confidence [14-24]. At the conclusion of the ERA
project, the modeling of the advanced hybrid wing body aircraft concept with 2025 technology assumptions
was reported by Nickol and Haller [25] as part of a portfolio of thirteen advanced aircraft concepts
including four configuration types across five passenger size classes. The noise assessment of the ERA
portfolio was also reported by Thomas et al. [26] together with an uncertainty quantification of the system
noise prediction [27]. The HWB was the quietest aircraft configuration in the ERA portfolio with a
cumulative noise level of 40.2 EPNL dB below the Stage 4 limit. In terms of reaching noise levels close to
the noise goal, the analysis clearly showed that the single largest differentiator between the HWB aircraft
and others was the favorable propulsion airframe aeroacoustic (PAA) interactions of the HWB, producing
large noise reductions from shielding. The HWB had the most favorable PAA interactions of the four
configurations studied.
The purpose of this study is to begin with the HWB aircraft model and system analysis results achieved
at the end of the ERA study and to determine a technology roadmap that could reduce the noise of the
HWB toward the NASA Far Term noise goal.
This framework of the study has several guidelines. This is an exploratory, pathfinding study focused
on noise reduction technology and design features of the aircraft and nacelle. These proposed changes
should not result in significant changes to the aircraft design or performance; however, certainly in
following studies, the aircraft design should be analyzed to include the impacts of the proposed
technologies and design features. The proposed set of technologies should include those that have a sound
basis for producing the predicted noise reduction; that is, they have theoretical basis, already have been
demonstrated with a successful proof-of-concept, or have been studied experimentally or analytically.
As an exploratory study, this is intended to be the beginning of a larger study toward the maturation of
an HWB concept that could reach the NASA Far Term noise, fuel burn and emission goals. With the noise
reduction approach established in this study, future studies are essential to mature the concept and analyze
the impact on the aircraft performance and to possibly explore reoptimization of the engine and airframe or
innovative operational procedures.
II. NASA Hybrid Wing Body Aircraft Concept
The HWB aircraft in the Large Twin Aisle (LTA) class, 301 passenger size, shall be the vehicle used
for the technology roadmap of this study. During ERA, the HWB was designed for a 7500 nautical mile
mission equivalent to a NASA model of the 777-200LR-like reference aircraft, including payload, range
and reserve mission requirements. The NASA design and predicted performance of the HWB concept
aircraft have been developed over time, based on improved design processes, prediction models and test
results obtained throughout the duration of ERA, including information from industry partners. An
overview of the HWB vehicle model results and performance used in this study are shown in Table 1 for
the NASA model of the HWB at the end of ERA. More details on the tools, modeling assumptions and
technologies included in the ERA analysis are provided by Nickol and Haller [25]. The results of the
analysis of this vehicle, HWB301-GTF (GTF for geared turbofan-like), were reported in 2016 [25-26]. In
Thomas et al. [27] and for the purposes of this study, it will be referred to as the HWB-2016.
The airframe technologies included a lighter weight structure enabled by damage arresting composites,
natural laminar flow wing and nacelle, and smaller vertical tails due to active flow control enhancements.
An advanced high lift system for the leading edge was modeled as a Krueger flap enabling a laminar flow
wing by providing protection from insect and debris accretion. The ultra-high bypass ratio geared turbofan
engine architecture included technologies such as a low pressure ratio fan with short inlet, swept and leaned
fan exit stators, a highly loaded high-pressure compressor, enabling fewer stages to achieve a desired
pressure ratio, and a low NOx combustor. In addition to the direct and indirect impact of these technologies
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on vehicle noise, a set of specific noise reduction technologies was also considered. These included a soft
vane technology, an acoustic liner integral to the fan exit stator vane, and a partial main gear fairing.
Figure 1 shows the rendering of the HWB-2016 vehicle. Table 1 identifies several of the design and
performance parameters of the vehicle, all of which have an influence on its noise performance either
directly or indirectly. It is noted that the fan diameter reported in [27] was the unscaled fan diameter
although the correct, scaled for thrust, fan diameter was used in calculations. Below the correct fan
diameter is shown, 135.0 inches. Tables 2 and 3 provide a summary of an additional number of important
features, specific noise reduction technologies, or parameters that were included and that can be significant
to understanding the noise assessment levels. However, for completeness, it is important to note that not
every parameter that can influence aircraft noise could be listed.
Table 1 Summary of ERA HWB Vehicle Model and Performance Metrics
Units HWB301-GTF 2016
Abbreviated Nomenclature HWB-2016
Entry Into Service 2025
Takeoff Gross Weight lb 535,164
Operating Empty weight lb 253,806
Payload lb 118,100
Passengers 301
Range NM 7500
Total Fuel lb 163,258
Cruise Mach 0.84
Start of Cruise L/D 23.7
Number of Engines 2
Thrust per Engine (sea level static) lb 70,124
Fan Diameter in 135.0
Fan Pressure Ratio (FPR) at Aerodynamic
Design Point (ADP)
1.35
Bypass Ratio at ADP 17.65
Start of Cruise Specific Fuel Consumption lbm/hr/lbf 0.475
Throttle: Approach % 11.4
Throttle: Sideline % 100.0
Throttle: Cutback % 61.7
Takeoff Field Length ft 8023
Approach Speed Knots 133.0
a) Front view
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b) Rear view
Figure 1. Rendering of the Mid Term technology HWB-2016 with GTF-like engines and in-board
vertical surfaces, a) front view, b) rear view.
Table 2 Summary of key engine, propulsion airframe integration, and acoustic liner technologies and
design parameters for the HWB-2016 at the end of the NASA ERA Project
HWB-2016
Engine location Core nozzle exit plane at one fan nozzle
exit diameter from aircraft trailing edge
Jet noise reduction
technology Conventional round nozzle, no chevron
Acoustic duct liner
technology
Multi-degree of freedom (MDOF) inlet
and aft duct liner, spliceless
Inlet duct liner effective
length to radius ratio 0.67
Aft duct liner effective length
to height ratio 2.54
Interstage liner effective
length to height ratio 0.5 (reported in error as 0.25 in [27])
Table 3 Summary of key airframe technologies for the HWB-2016
HWB-2016
Leading edge device type Krueger with sealed gap on
approach, takeoff, and flyover
Main landing gear type 6 wheel, 777-like
Landing gear noise
reduction technology Partial main gear fairing
Centerbody elevon
deflection for all three
certification points
Up 10 degrees
III. Noise Prediction Process
A. Previous ERA Studies
The overall noise prediction process for future low noise aircraft has developed significantly over recent
years. This progression of the noise prediction process through the ERA project together with uncertainty
quantification is detailed in Thomas et al. [27]. The noise assessment process includes utilizing the best
noise assessment practices, databases, and methods developed at NASA over the previous decades for
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predicting community noise. A key aspect of this process has been to directly predict the PAA integration
effects with a process based on an experimental database. This section will describe the cumulative noise
metric calculation and then describe how the overall noise assessment process has progressed during ERA.
An overview chart of the noise certification process is shown in Figure 2. Specifically, the noise
predictions for the 2025 aircraft models follow the certification rules found in the Code of Federal
Regulations (CFR) Title 14, Part 36 illustrated in the figure. Part 36 defines specific operational parameters
for each of the three certification points. At each of the three certification points, the EPNL dB is predicted
for the aircraft. Separate computations are performed to obtain each of the approach, lateral, and flyover
EPNL noise levels. This procedure is consistent with previous assessments performed under NASA
projects [3, 19, 23, 24, 26, 27]. The cumulative noise is the addition of the EPNL of the three points. The
cumulative noise is referenced relative to the certification level required by Part 36 which is currently Stage
4 and is a function of aircraft weight and the number of engines. The cumulative noise below Stage 4 is the
final noise metric reported.
The ERA noise assessment results were first reported [26] in January 2016 including a cumulative noise
of 40.3 EPNL dB below Stage 4 for the HWB-2016 and also included a detailed description of the process
used at that time. The assessment for the same vehicle HWB-2016 reported in [27] occurred several months
later and included several updates to both vehicle modeling as well as the noise assessment process. These
numerous final changes reported in [27] had offsetting impacts yielding the cumulative noise of 40.2 EPNL
dB reduction for the HWB-2016, slightly different from the level originally reported.
Figure 2. Noise certification flight paths and metric definitions used in the system noise assessment
process. (Definitions guided by the Code of Federal Regulations (CFR) Title 14 Part 36.)
B. Updated Noise Prediction Process and Configuration Nomenclature
For the current study, there are no vehicle (airframe and engine) modeling changes. By the framework
of the study, the vehicle modeling including flight path and engine throttle are fixed to those used in the last
reported results of [27] for the HWB-2016. It is important to note again for the reader following this series
of publications that the result of [27] is slightly different from that reported earlier [26], 40.3 EPNL dB, for
the reasons listed in [27].
In the course of beginning this study there did arise, once again, several additional acoustic prediction
updates and miscellaneous corrections that needed to be addressed. These updates are part of the ongoing
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process of pursuing the most accurate acoustic prediction results and introducing improvements as soon as
possible. In addition, this establishes a more accurate starting point for the technology roadmap that will
follow. The most notable update was to a version 2 of the noise prediction method for landing gear (version
1 of the landing gear prediction method having been reported by Guo et al. [20]). For application to the
HWB, the updated gear noise prediction method removed a gear/flap interaction effect that is not present
for an HWB. The net effect of several corrections and the updating to Guo-LG-v2 was that, as is sometimes
the case, the changes largely offset one another and the cumulative system noise changed slightly to a new
total of 40.4 EPNL dB below Stage 4. For the purposes of this study, this result establishes the new updated
result for the HWB-2016 at the end of ERA and, in this study, is designated Configuration 0 (C0).
As a result, the updated noise prediction process used in this study is shown in schematic form in Figure
3. Only those elements of the process used for an HWB prediction are shown. In this figure, ANOPP stands
for Aircraft Noise Prediction Program, FLOPS for Flight Optimization System and NPSS for Numerical
Propulsion System Simulation.
Figure 3. Overview of the HWB noise prediction process used for the HWB-2016 in the current study.
Notes: ITD 51A, 35A, and 50A were teams in the ERA Project.
As a result of several improvements to landing gear, duct lining, and Krueger flap prediction methods,
several configurations were developed and will be described next. Even though these are improvements in
the acoustic prediction process, they came after the beginning of the study and are given configuration
numbers with noise prediction results in Section V.
The design of the vehicle includes a total length of the main gear strut, and in prior results [26, 27], that
total strut length was used incorrectly as an input to the landing gear prediction. A part of the length of the
strut is inside the landing gear cavity with a lower velocity and, therefore, lower noise generation. The
more appropriate length of the gear strut to use is that length exposed to the external flow. This was
highlighted in the work reported in [28]. In this HWB-2016 application, the main gear cavity depth and the
nose gear cavity depth are estimated. For Configuration 1 (C1), the main gear strut used in the noise
prediction is reduced by 2.2 feet, and the nose gear strut length used in the noise prediction is reduced by
1.6 feet.
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For the acoustic duct attenuation prediction, the TREAT method is used as indicated in Figure 3,
modified in several ways including the incorporation of multi-degree of freedom (MDOF) liner technology
(Table 2). Configuration 2 (C2) tunes the liner to the most effective frequency for noise reduction, a
process that was not done in the prior results [26, 27] and not done automatically by TREAT. However,
tuning the liner is something that would be done in a typical engine design and is, therefore, a logical
improvement to the prediction of the HWB-2016.
As indicated in Table 2, the effective length of the interstage liner was reported [27] as a length-to-
height, or L/H = 0.25. This was an error in [27]; the length used in that prediction was 0.5. NASA subject
matter experts estimate that only 50% effectiveness should apply to the actual length of the interstage liner.
For this HWB-2016 GTF engine, the effective L/H is now set at 0.28 (50% of actual length) for
Configuration C3.
A further more realistic liner treatment issue is the application of acoustic liner to the thicker lower
bifurcator (passing through the structural attachment to the airframe on the HWB) and the thinner upper
bifurcator. The TREAT method does not include treatment on the bifurcator. In modern engines and likely
in future engines, bifurcation treatment is common and, therefore, is logically applied here. For
Configuration C4, treatment is applied to the lower bifurcator covering 75% of the available area due to the
thickness of the lower bifurcator, and liner treatment is applied to the upper bifurcator assuming a coverage
of 50% of the available area.
Similar to the update for the Guo-LG method to a version 2, ongoing work also resulted in an update to
the leading edge prediction method used for the Krueger flap, Guo-LE-v2. This new version builds on the
v1 reported in Guo et al. [21] by modeling the reflection of the bracket subcomponent from the Krueger
device itself. This change impacts the directivity of the bracket subcomponent and the Krueger component
as a whole. This improved v2 is implemented in Configuration C5 and replaces Guo-LE-v1 in the process
outlined in Figure 3.
For the ERA analysis of [25-27] and based on the available low speed wind tunnel test results, the
centerbody elevon upward deflection was set at 10 degrees for all three certification points. The acoustic
prediction did include the impact of this elevon deflection, which is to increase shielding of the aft radiating
fan and core noise sources. However, this elevon effect on shielding was modeled with a simple
suppression map and assuming a 0.5 dB reduction at all angles. Based on the analysis of the latest wind
tunnel results in 2017 by the Boeing HWB team the elevons are more correctly set at up 10 degrees at
approach, up 28 degrees at lateral (sideline), and up 18 degrees at cutback. For a more accurate acoustic
prediction, these adjusted elevon settings are used in Configuration C6. In addition, a more accurate
suppression map is developed that is based on geometric mapping of experimental data [11] to the
geometry and conditions for the HWB-2016. As a result, the new C6 elevon suppression maps are now a
function of polar () and azimuthal () angles, frequency and elevon angle.
At this point in the study, C1 through C6, can be viewed as largely improvements in the acoustic
prediction process of a more realistic HWB-2016. In addition, they also improve the prediction process to
enable better prediction of some of the more advanced configurations to follow in the roadmap.
IV. HWB Far Term Technology Roadmap
This section consists of a brief description of the remaining eleven configurations that are included on
the HWB configured with a Far Term noise reduction technology level with the final resulting concept
referred to collectively as the HWB-FT-2017.
A. Lip Liner C7
Typically, the inlet duct acoustic liner extends from the fan casing to the throat. As inlets are shortened
to reduce weight and drag, the noise reduction from the inlet duct liner is reduced. Extending the liner to
the lip of the inlet has been an attractive noise reduction approach that has been previously investigated on
the Boeing-led Quiet Technology Demonstrator 2 flight test project [28, 29]. Icing protection, aerodynamic
drag, and inlet off-design performance have continued to be challenges for this technology. Development
continues, and it is reasonable to expect this technology to be ready for service well within the NASA Far
Term timeframe. To predict the impact of the lip liner, the available TREAT method, including MDOF
liner technology, is used by extending the liner treatment length out to the full inlet length available on the
short inlet designed for the GTF-like engine on the HWB.
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B. Center Plug Liner C8
For low frequency combustor noise there has been promising development of a folding cavity liner
applied to the center plug [30]. The concept was tested on a GE CF34 engine [31]. The perforated face
sheet of the liner covers the converging section of the core nozzle plug. Figures 6-8 of Yu and Chien [30]
show the location on the engine and interior chamber design of the Center Plug Liner. For this study, based
on the test results reported, a suppression map is developed with peak attenuation of just over 8 dB at 400
Hz and rolling off quickly for higher and lower frequencies. The suppression is applied at all angles and
engine conditions.
C. Over-the-Rotor Treatment C9
The Over-the-Rotor (OTR) acoustic treatment is a technology integrated into the fan casing in the rub
strip area and has had successful proof-of-concept [32-33]. The development of this technology continues.
For this study, based on past results, the noise reduction impact of the OTR treatment is predicted by
development of a suppression map that reduces the fan noise component by 1 EPNL dB.
D. Center Elevon PAA Liner C10
For the HWB application, the Center Elevon PAA Liner would be applied only to the centerbody
control elevons of the HWB. It is a PAA liner because both the fan aft radiated noise and the core aft
radiated noise would be attenuated as these components propagate over the treated surface. The Center
Elevon PAA Liner has shown a successful proof-of-concept for an experimental configuration with a
counter rotating open rotor propulsion noise source and the effect of forward flight [34]. Aircraft system
level results have been calculated [35]. For the current study, a suppression map that is a function of polar
and azimuthal angles is implemented to more accurately reflect the experimental results and the geometry
of the HWB. Peak attenuation is 6 dB over a polar angle range from 70 to 130 degrees and an azimuthal
range of 30 degrees. The targeted frequency range covers four 1/3-octave bands.
Application to an aircraft product presents many challenges for the maturation of this technology;
however, the development of lower drag facesheets for acoustic liners is one enabling technology [36].
E. Maximized Upper Bifurcator Liner C11
For the installation of the GTF-like engine on the HWB, the upper bifurcator is thinner and therefore, in
C4 only 50% of the available area was used. In C11, the upper bifurcator is thickened intentionally in order
to increase the available treatment area to the same 75% coverage area of the lower bifurcator. The
thickness of the upper bifurcator has not been determined; however, it is expected to be only a few inches
thicker and is acceptable within the framework of this study. The impact of this technology is to increase
the attenuation of fan noise in the aft duct by adding more liner area. This is implemented simply by
increasing the effective L/H of the aft duct corresponding to the liner treatment area added. This technology
was implemented because it is possible that it could also increase shielding effectiveness of fan noise due to
its strategic location at the crown (top) of the aft duct. However, this impact was not implemented in time
for this study and is left for a future iteration.
F. PAA Chevron C12
Shielding Effectiveness (SHEF) is the general approach by which a set of noise reduction technologies
is designed as a system with the objective to change the spectral content, location, and/or directivity of a
noise source in order to produce more noise reduction from shielding as compared to that obtained without
the SHEF design objective. For propulsion airframe aeroacoustic (PAA) integration applications, the SHEF
approach has been successfully applied to the jet noise component with specifically designed chevrons and
pylon integration that changed the azimuthal and axial source distribution in the jet plume. In addition, the
chevrons designed with this PAA integration objective also produced additional low frequency jet noise
reduction. For a conventional application, engine-under-wing aircraft, this type of PAA chevron nozzle was
flight tested successfully on the Boeing-led Quiet Technology Demonstrator 2 [28, 37]. Application to
HWB experiments have demonstrated the increased cumulative noise reduction achievable for the shielded
jet noise component [4,9,10].
While developed considerably during ERA, the PAA chevrons were not applied in the ERA HWB-2016
noise assessment results [26, 27] due to time constraints and the expectation that for the ultra-high bypass
ratio engines, the jet noise component would be considerably lower than the fan and core components.
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However, after duct liner attenuation and shielding is applied, jet noise is within a few decibels of the other
engine components and, therefore, additional noise reduction can be obtained.
Based on experimental results [9], suppression maps as a function of frequency, polar, and azimuthal
angle are developed for a low power condition to better represent the application to the higher bypass ratio
of this HWB GTF-like engine.
G. Krueger Flap Bracket Alignment (C13) and Cove Filler (C14)
Because of the balanced aircraft system level design objectives, all of the thirteen aircraft in the ERA
portfolio were designed with Krueger flaps as the leading edge high lift device in order to enable laminar
flow wings and the resulting fuel burn reduction [25]. The acoustic importance of the Krueger flap, as well
as the absence of a dedicated prediction method for the Krueger, was highlighted in recent years [5, 19].
The noise of the Krueger flap represents a challenge due to the mechanical complexity of the deployment
mechanism, Figure 4. Based on experience with system noise modeling for the conventional slat and on
physics-based analytical modeling, Guo et al. [21] developed the first dedicated Krueger flap system noise
model for ANOPP. As a result of the need for improved understanding of the acoustic features of the
Krueger and for validation data, an initial 2D computational and experimental study was conducted on the
Krueger flap noise source [38, 39]. Another notable study included a semispan wing design optimized for
different leading edge devices including a Krueger flap followed by a wind tunnel test [40].
Figure 4. Krueger flap and bracket from computational grid of Ref. 41. Reproduced with permission.
Similar to a conventional slat and supported by system level noise analysis [5, 19], the most logical
noise reduction approach for the Krueger component, sealing the gap, was investigated extensively and
successfully for the HWB ITD51A configuration of ERA [6]. As a result, the ERA HWB-2016 is already
designed with the Krueger gap sealed for all three certification points and the noise predicted using the
method of GuoLE-v2. The Krueger flap prediction method of Guo et al. [21] includes the prediction of four
subcomponents of the Krueger: cove, gap, bracket, and cavity. With the gap sealed, the next most logical
Krueger subcomponent sources to be reduced are to align the brackets (C13) with the freestream flow and
to use a cove filler (C14).
In the HWB-2016 design, the brackets are aligned normal to the leading edge of the HWB wing.
Aligning the brackets with the freestream flow changes the source level and directivity of the bracket
subcomponent, a capability already included in the prediction method of Guo et al [21]. Recent studies [41,
42] have computed, at high fidelity, the complex flow field over a realistic Krueger flap design for the
ERA/Boeing HWB design [6]. These flow field results show the strong cross flow over the brackets when
the brackets deploy normal to the wing, as shown in Figure 5. This observation confirms the likely noise
reduction that would occur with alignment of the brackets to the freestream, shown in yellow in Figure 5.
Validation of the predicted noise reduction due to bracket alignment by the method of Guo et al. [21] would
be a desirable next step. Furthermore, an open question for this noise reduction approach is the mechanical
design of aligning brackets with the freestream instead of normal to the leading edge.
A cove filler for conventional slats has been a noise reduction approach that has been extensively
studied in computational and experimental research [43, 44]. Implementations of slat cove fillers have been
developed including the use of shape memory alloy approaches [45]. Recently, a slat cove filler was flight
tested in a joint EcoDemonstrator project between Boeing and Embraer [46] indicating significant maturity
in the technology and successful noise reduction of about 10 dB.
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Figure 5. Unsteady surface pressure computed on the lower surface for a full scale HWB, from Ref.
41. Reproduced with permission. Indication of cove filler and aligned brackets added for this study.
In earlier NASA system studies, the slat cove filler noise reduction was implemented as a reduction to
the slat noise component with a uniform 10 dB reduction over all angles and frequencies [47]. While this
implementation and level of noise reduction was reasonable at the time, a more accurate reduction estimate
would include a function of directivity angle and frequency.
To develop as realistic an estimate of the noise reduction of a Krueger cove filler, the following were
considered:
Prior work on the cove filler approach applied to slats,
Differences between the cove flow of the slat and the Krueger,
Differences in the deployment of the slat and the Krueger.
With these considerations, a prediction of the noise reduction of a Krueger flap cove filler was
developed as a function of frequency and polar angle with a peak reduction of 4 dB over a limited angle
and frequency range.
H. Fan Shielding Effectiveness
In addition to the Center Elevon PAA Liner (C10), additional individual technologies available for the
aft fan noise SHEF design objective include soft vane stator, aft duct liners, bifurcation liners, core cowl
liners, pylon and bifurcation shape, and fan exit nozzle shape (i.e., scarfing). These technologies can
produce the same noise attenuations as if implemented individually. However, as a system they can also be
designed to change the directivity or spectral shape of aft fan noise. The result will produce more noise
reduction from shielding of aft radiated fan noise from the same airframe surface that is aft of the engine
exit plane. The following three technology configurations (C15, C16, and C17) are all approaches to fan
SHEF.
I. Fan Shielding Effectiveness via a Duct Liner (C15) The idea of this approach is to use both aft duct and bifurcation liners to achieve the same overall liner
attenuation but with a fan noise directivity that is shifted by 5 degrees in the upstream direction, that is, a
peak noise directivity that is closer to the propagation angles shielded by the airframe aft of the fan nozzle
exit plane.
J. Fan Shielding Effectiveness via PAA (Nozzle/Pylon/Aft Section) Design (C16) With an understanding of the spectra and the directivity of propulsion noise, aft fan noise in this case,
certain design features can be introduced that increase SHEF. For this configuration, three design changes
are made, two to the fan nozzle and one to the inboard vertical tails. The latter is illustrated in Figure 6,
together with other noise reduction concepts to be discussed in the following sections.
As mentioned in reference to C11, the fan noise radiating from the crown of the nozzle is the hardest to
shield because of the propagation angles to the trailing edge, with or without elevons deflected up. An
elliptical nozzle can decrease the height of the nozzle above the airframe surface and, therefore, improve
(for shielding) the angles relative to the trailing edge. This nozzle design feature is implemented as a 12
Baseline Bracket Aligned Bracket
Flow Direction
Cove Filler
American Institute of Aeronautics and Astronautics
11
inch decrease (relative to the 135 inch diameter) in the crown of nozzle height. Area is preserved by
bulging out, increasing diameter, on both sides of the nozzle. The bottom of the nozzle in the area of the
lower bifurcation is unaltered.
A second modification of the fan nozzle is to introduce a negative scarf, longer on the bottom (side
closer to the airframe). This is effectively similar to lowering the whole engine to be flush with the surface
of the aft airframe with the effect of improving the propagation angles to increase the shadow region below
the airframe. A 10 degree negative scarf is implemented. While the elliptical shaped nozzle and the
negative scarfed nozzle are proposed here to enhance the noise shielding efficiency, it should be recognized
that practical implementation difficulties need to be overcome, especially when the nozzle is designed with
a variable area mechanism.
The third modification is to add a root extension to the vertical tail. This type of root extension has
precedent in a number of in-service aircraft. For this application, the purpose is to increase shielding to the
sideline angles because of the relative position of the vertical tail and the engine.
A prediction of the change in shielding of each of the three modifications was made for each aircraft
condition corresponding to the three elevon deflections of C6. The prediction was a function of polar and
azimuthal angles and frequency with elements of the prediction developed by geometric mapping of
experimental data and other elements of the prediction confirmed by analytical prediction [48]. The effects
of each design feature were combined into one suppression map applied to aft fan noise for each elevon
deflection angle corresponding to approach, lateral (sideline), and cutback.
Figure 6. Original wind tunnel model photo from Ref. 9, modified to show examples and placement
of C10, C12, one of three C16 features, and C17.
K. Trailing Edge Diffraction Treatment (C17) The diffraction of propulsion noise around the trailing edge of the centerbody section, in particular, can
be impacted by treatment applied on the edge. There are many edge treatments that have been studied for
jet-flap interaction noise reduction [49] and trailing edge noise reduction [50, 51] including many variations
of serrations, combs, and brushes that, in addition to their originally intended function, may also be
C17 Trailing Edge
Diffraction Treatment
Verticals
C10 Center Elevon
PAA liner C16 Vertical Tail Root
Extension (One of Three for
C16)
C12 PAA Chevron Nozzle
American Institute of Aeronautics and Astronautics
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modified to alter the diffraction of engine noise at the trailing edge of the airframe. All these concepts are
collectively illustrated in Figure 6.
This application is targeted at attenuating fan tones only in the partially insonified region that is most
influenced by the airframe edge characteristics. A predicted attenuation is developed with a peak reduction
of 6 dB over a limited frequency range of four 1/3-octave bands and only in the partially insonified region
corresponding to 110 to 140 degrees in the polar angle. Based on the geometry of the centerbody trailing
edge, the azimuthal range is also limited to 30 degrees.
L. Pod Gear (C18) The HWB-2016 result [26] showed that the main landing gear generates the highest noise level on
approach and, therefore, represents a barrier to further aircraft system level noise reduction. During ERA,
the noise reduction technology applied to the main gear was a partial main gear fairing [13, 52]. This is a
logical noise reduction approach, in particular for application to conventional aircraft. In ERA, the partial
main gear fairing was successful with a main gear component level reduction of 0.8 EPNL dB [26] when
applied to the HWB301-GTF.
An HWB unconventional aircraft with the engines mounted on top of the airframe changes the
paradigm for landing gear by allowing the gear to be shortened significantly compared to that of the
traditional engine-under-the-wing configuration. A number of in-service high wing transports have landing
gear that deploy from the fuselage. Noise reduction studies have been conducted on main gear that deploy
from a fuselage fairing for these type of high wing transports [53].
The pod gear is a main landing gear noise reduction concept that has been proposed recently [54]. The
pod gear design concept changes the main gear integration with the vehicle to more fully take advantage of
the unconventional engine-over-wing configuration. The recent exploratory study reconfigured the ERA
Mid-Fuselage Nacelle (MFN) aircraft with the pod gear concept, as illustrated in Figure 7, and then, using
the Guo-LG-v1 system level method, predicted that with the pod gear concept, the main gear component
noise was reduced by 5 EPNL dB [54] relative to the component noise of the more standard landing gear
concept on the original MFN in the ERA study.
Figure 7. Example of the difference between the a) conventional main gear and the b) Pod Gear
concept, applied to the NASA Mid-Fuselage Nacelle aircraft from Ref. 54.
For this study, the HWB-2016 vehicle design has not been reconfigured for the pod gear concept. Thus,
the predicted noise reduction for the pod design for the MFN vehicle is used here, factoring in necessary
adjustments for different strut lengths, flight path, and airframe planform (reflection effects). The predicted
noise reduction is a complex function of frequency, polar and azimuthal angles, and aircraft condition.
a) Conventional Main Gear
b) Pod Gear Concept
American Institute of Aeronautics and Astronautics
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It is important to note that with the implementation of the pod gear design concept, the predicted noise
reduction from the partial main gear fairing would not be applied to the main gear since the main gear strut
and dressing are inside the pod fairing. Specifically, for the prediction in this study, the partial main gear
fairing was included in C0 as a part of the ERA project, and it is carried through all configurations to C17.
For C18, this effect is removed from the prediction, and the predicted impact of the pod gear is then added.
Table 4 summarizes the discussion of this section by summarizing all (C0 to C18) configurations to be
predicted in the following Section V including noting the aircraft noise sources or PAA effects that are
impacted by each configuration.
Table 4 Summary of the baseline and all eighteen configurations included in HWB-FT
Configuration Description Noise Source or
PAA Applied to:
Direct Dependencies of Predicted
Noise Reduction (Output):
C0
Baseline, HWB-2016 at
the end of ERA with
corrections
Polar and azimuthal angle,
frequency, engine and aircraft
condition
C1 Gear prediction uses
exposed length
Main gear and nose
gear with reflection
Polar angle, azimuthal angle,
frequency and aircraft condition
C2 Tuned Duct Liner Fan Polar angle and frequency
C3 Interstage Liner
Effectiveness Corrected Fan Polar angle and frequency
C4 Bifurcation Treatment Fan Polar angle and frequency
C5 Upgrade to GuoLE-v2 Krueger Polar angle, azimuthal angle,