Aircraft Noise Reduction Technology Roadmap Toward ... · Aircraft Noise Reduction Technology...

American Institute of Aeronautics and Astronautics

1

Aircraft Noise Reduction Technology Roadmap Toward

Achieving the NASA 2035 Noise Goal

Russell H. Thomas1

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


2

(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


3

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


4

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])

Additional liner

application/technology Soft stator vane acoustic liner

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


5

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


6

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.


7

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.


8

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.


9

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.


10

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


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


12

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


13

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,


C6 Updated Elevon

Deflections

Shielding of fan and

core


frequency and elevon angle

C7 Lip Liner Fan Polar angle and frequency

C8 Center Plug Liner Core Frequency

C9 Over-the-Rotor Fan Engine condition

C10 Center Elevon PAA Liner Shielding of fan and

core

Polar angle, azimuthal angle, and

frequency

C11 Increase Upper

Bifurcation Liner Fan Polar angle and frequency

C12 PAA Chevrons Jet source and

shielding of jet


frequency

C13 Krueger Flap Bracket

Alignment Krueger



C14 Krueger Flap Cove Filler Krueger Polar angle, azimuthal angle, and


C15 Fan SHEF via Duct Liner Fan Polar angle and frequency

C16

Fan SHEF via PAA

(Nozzle/Pylon/Aft

Section) Design

Fan Polar angle, azimuthal angle,

frequency, and elevon angle

C17 Trailing Edge Diffraction

Treatment Fan tones only


frequency

C18 Pod Gear Main gear source

and reflected noise


frequency, and aircraft condition


14

V. HWB Far Term Technology Roadmap Results

Based on weight and engine number, the Stage 3 certification limit for the HWB-2016 is 304.0 EPNL

dB. The Stage 4 limit (Stage 3 minus 10 dB) is 294.0 EPNL dB. As described in the Introduction, one of

the criteria for the Far Term technologies is that they are expected to have relatively small impact on total

aircraft weight and, therefore, consistent with the framework of an exploratory technology roadmap, the

weight and the certification limits for the HWB-FT are assumed to be the same. Table 5 shows the order in

which the Far Term technologies were applied to the HWB-FT, the predicted aircraft noise levels at the

three certification points, and the resulting cumulative margin to the Stage 4 limit.

The configurations in Table 5 were added in the order shown based on the logic of three general groups:

an additional group of modeling updates (C0-C6), a group that have been developed to a considerable

extent already (C7-C12) and finally, a group of newer approaches that have had much less research to date

(C13-C18).

A. System Noise of C0 to C6 The baseline HWB-2016, C0, is the last reported ERA result [27] together with updates developed

before this study began. C1 through C6 represent an additional set of updates that occurred in the beginning

of this study such that the result with C6 could be seen as yet another, newest update to the final ERA result

because these configurations are focused on a more accurate modeling of realistic effects that arguably

should be included in the HWB-2016 prediction.

C1 shows noise increasing at approach due to the change in the length of the gear used in the prediction

of gear noise. With the length being decreased by 2.2 feet for the main gear strut, the source noise is

reduced because less of the strut is exposed to the higher velocity freestream; however, the total gear noise

goes up due to the gear source moving closer to the airframe surface and the reflection directivity being

changed. This effect is shown in the plots of the reflected component only of main gear noise, Figure 8, for

C0 and C1. Note the slightly higher levels in the flyover ( = 0 degrees) plane that result in higher gear

noise and the 0.2 EPNL dB change in the cumulative margin for C1.

Figure 8. Reflected component only of main gear noise for C0 (left) and C1 (right) as a function of

polar () and azimuthal () angles.

C2 tunes the duct liner to a frequency about one-half that automatically set by the TREAT prediction

model, and the result is a reduction of fan noise at lateral and flyover points. By correcting the effective

length of the interstage liner, the result is as expected, an increase in fan noise component for C3. Some

reduction in fan noise is next produced by adding treatment area to the bifurcation for C4.


15

Table 5 Predicted aircraft system noise levels for each configuration of the HWB-FT, EPNL dB. Note:

each configuration includes impact of all prior configurations

Configuration Description Approach Lateral Flyover

Cumulative

Below

Stage 4

Change from

prior

Configuration

(positive is

more noise

reduction)

C0

Baseline, HWB-

2016 at the end of

ERA with

corrections

90.92 82.74 79.88 40.4 -

C1

Gear prediction

uses exposed strut

length

91.15 82.74 79.88 40.2 -0.2

C2 Tuned Duct Liner 91.16 82.68 79.46 40.7 0.5

C3

Interstage Liner

Effectiveness

Corrected

91.18 82.78 79.55 40.5 -0.2

C4 Bifurcation

Treatment 91.16 82.65 79.42 40.8 0.3

C5 Upgrade to

GuoLE-v2 90.73 82.4 79.33 41.5 0.7

C6 Updated Elevon

Deflections 90.72 82.3 79.22 41.7 0.2

C7 Lip Liner 90.72 82.25 79.21 41.8 0.1

C8 Center Plug Liner 90.71 81.7 78.88 42.7 0.9

C9 Over-the-Rotor 90.68 81.49 78.64 43.2 0.5

C10 Center Elevon

PAA Liner 90.65 81.47 78.34 43.5 0.3

C11 Increase Upper

Bifurcation Liner 90.65 81.45 78.32 43.6 0.1

C12 PAA Chevrons 90.65 80.9 78.17 44.3 0.7

C13

Krueger Flap

Bracket

Alignment

90.07 80.32 77.34 46.3 2.0

C14 Krueger Flap

Cove Filler 89.9 80.11 76.82 47.2 0.9

C15 Fan SHEF via

Duct Liner 89.88 79.77 76.68 47.7 0.5

C16

Fan SHEF via

PAA

(Nozzle/Pylon/Aft

Section) Design

89.85 79.67 76.48 48.0 0.3

C17

Trailing Edge

Diffraction

Treatment

89.85 79.67 76.13 48.3 0.3

C18 Pod Gear 87.27 79.67 76.13 50.9 2.6

Total Noise Reduction from C0 3.65 3.07 3.75 10.5


16

C5 changes the modeling of the Krueger flap as described in Section III.B. Because the Krueger is a

significantly high noise component at all three certification conditions, an impact in total aircraft noise is

seen. The impact of adding the reflection effect of the Krueger on the bracket subcomponent is manifested

noticeably in the directivity of the Krueger flap component. Figure 9 shows the source breakdown at the

approach condition for both C4 and C5 in order to highlight the impact of this change, a reduction of the

Krueger noise levels in the forward angles (as the aircraft approaches the observer) because the bracket

sub-component noise is reflected to more aft angles by the Krueger itself. The change at the system level

due to this more realistic model is substantial.

Figure 9. Tone corrected perceived noise level (PNLT, dB) for Approach, C4 (left) and C5 (right).

The last configuration in the prediction improvement group, C6, contains the most accurate predictions

of the ERA HWB aircraft. Its certification noise levels are shown in Figures 10, 11 and 12, for the approach,

lateral and flyover conditions, respectively. These results incorporate prediction improvements [27] since

the results reported in [26], as well as those discussed here. An important conclusion of these results is that

the main landing gear and the leading edge Krueger device are two main noise sources for this HWB

aircraft; the former dominates the approach noise, and the latter has significant contributions at all three

certification points. The engine noise, from the fan, the jet and the core, still has noticeable contributions at

lateral and flyover conditions, but its levels have been reduced significantly by shielding, so that the

Krueger noise is of equal magnitude to engine noise at these two conditions. It should be noted that the

importance of Krueger noise for the HWB is not because the Krueger device is noisy. Instead, it is because

the other components have been reduced significantly; the importance is in relative terms, and in absolute

levels, the Krueger noise is of the same order of magnitude as conventional leading edge slats [21].


17

Figure 10. Tone corrected perceived noise level (PNLT) of the final ERA HWB (C6), including all

prediction improvements. Approach certification point.


prediction improvements. Lateral certification point.


18


prediction improvements. Flyover certification point.

B. System Noise of C7 to C12 New technology that was specifically not added to the HWB-2016 for the ERA project begins with C7.

The grouping C7 through C12 are configurations for which there is considerable prior experience including

flight tests, static engine tests and wind tunnel tests. C10 has had only one proof-of-concept wind tunnel

test and, therefore, probably is the least developed of this group. C12 has been very effectively developed

for a range of bypass ratio nozzles; however, for these very high bypass ratios of the HWB-FT, an

additional development cycle would likely be needed. A possible key would be to implement the PAA

chevron with a variable geometry function, which was also flight tested on the Boeing-led Quiet

Technology Demonstrator 2 [28, 37]. In general, for eventual feasibility and effective application, all

would need further development toward maturation.

The lip liner adds 16.6% more effective length to the inlet liner; however, the impact is small largely

due to fan inlet noise shielding. C11 increases the effective aft duct L/H by just 2.6%, and therefore, the

impact at the system level is also small. As explained in Section IV.E., if the impact on fan noise SHEF

could also be predicted, then the impact of this relatively small amount of added liner could be more

substantial.

The impact of the center plug liner, C8 (Section IV.B.), is particularly notable, 0.9 EPNL dB at the

system level, and reflects the facts that core noise is an engine component on par with the other engine

components and that the impact of this liner is up to 8 dB, albeit over a narrow frequency range. In contrast,

the over-the-rotor treatment reduces fan noise uniformly by 1 EPNL dB reduction, C9 (Section IV.C.), and

yet produces an impact at the system level of only 0.5 EPNL dB cumulative. The impact of a technology at

the system level is a function of the impact of the source being reduced relative to other sources. The

impact of a technology is also a function of the suppression level and the frequency and angular range.

C10 and C12 are both SHEF technologies and, combined, reduce fan, core and jet noise sources.

C. System Noise of C13 to C18 The final group of C13 to C18 are a set of configurations that are based on sound noise reduction

principles and experience; however, they are relatively newer and would clearly need more focused and

substantial development. These technologies were also identified or created for the unique challenges of

reaching the NASA Far Term goal. The need for new approaches is highlighted by the fact that through


19

C12 the system noise has reached a margin to Stage 4 of 44.3 EPNL dB, an encouraging result but still far

short of the goal.

The approaches of C13 to C18 are focused on achieving noise reduction in three areas:

Krueger flap (C13 and C14) with a combined system level reduction of 2.9 EPNL dB cumulative,

Fan Shielding Effectiveness (C15, 16, and 17) with a reduction of 1.1 EPNL dB cumulative and,

Main Gear with the implementation of the Pod Gear approach (C18) with a reduction of 2.6 EPNL

dB cumulative.

Together C13 through C18 increase the margin to Stage 4 all the way to 50.9 EPNL dB, a final result

that includes all updates and configurations in this HWB-FT technology roadmap study. Clearly, the newer

innovative approaches to noise reduction in C13 through C18 are necessary to accomplish the majority of

the noise reduction toward approaching the level of the NASA Far Term noise goal.

With the final result, an aircraft concept with this low noise level, it is helpful to examine the source

ranking through the PNLT versus time plots. At the approach condition, Figure 13, and as expected, the

main gear and the Krueger flap remain the dominant sources over the entire aircraft flyover. At the peak,

the two are almost at the same level. Clearly, if further noise reduction were to be sought at the approach

condition, it would have to come from both the main gear and the Krueger flap components.

For the lateral condition, Figure 14, due to a low noise engine design, shielding, liner suppression and

the configurations in the roadmap, the engine source levels (with the PAA effects included) are all similar

and surprisingly close to the Krueger flap noise level. With its very different directivity influencing the

forward angles, the Krueger has a disproportionate impact on the EPNL due to the integration between the

10 dB down points. Further noise reduction at the lateral condition would need to come by reducing all

engine sources and the Krueger equally.

At the flyover (cutback) condition, Figure 15, the Krueger flap dominates the peak noise of the aircraft,

and therefore, further system noise reduction would most likely come from further reductions in the

Krueger noise.

Figure 13. Tone corrected perceived noise level (PNLT) of the final HWB-FT, including all

configurations through C18. Approach certification point.


20


configurations through C18. Lateral certification point.


configurations through C18. Flyover certification point.

In contrast to the build-up of the configurations (accumulating technologies sequentially), a preferred

approach to quantify the impact of each technology configuration on the most equivalent basis is to

perform a one-off analysis where, starting from the final configuration C18, one technology only is

removed at a time and the change in cumulative noise is quantified. A second configuration is then

removed, after the first is added back, to compute the system level change from the second configuration,


21

again with all other configurations in effect. This process is continued for all configurations, and the results

are shown in Table 6.

Table 6 One-off analysis calculating the impact at the aircraft system level of the HWB-FT with only

one configuration turned off at a time

Configuration Description

Cumulative below

Stage 4 with one

technology “off”

One-off cumulative

noise reduction due to

technology

C7 Lip Liner 50.9 0.0

C8 Center Plug Liner 49.7 1.3

C9 Over-the-Rotor Treatment 50.6 0.4

C10 Center Elevon PAA Liner 50.4 0.5

C11 Increase Upper Bifurcation Liner 50.9 0.0

C12 PAA Chevrons 50.0 0.9

C13 Krueger Flap Bracket Alignment 48.4 2.6

C14 Krueger Flap Cove Filler 49.8 1.1

C15 Fan SHEF via Duct Liner 50.5 0.4

C16 Fan SHEF via PAA Design 50.6 0.3

C17 Trailing Edge Treatment 50.5 0.4

C18 Pod Gear 47.7 3.3

Aircraft cumulative margin to

Stage 4, with all technologies 50.9

It should be noted that the results in Table 5, building up sequentially, and those of Table 6, one-off,

will not agree as to the benefit of each of technology. This is due to the impact of integrating multiple

sources to compute the aircraft system level result. The final aircraft level, 50.9 EPNL dB, will be

unchanged, but the benefit of each technology will not be the same.

VI. Ground Noise Footprint Impact

In addition to certification cumulative noise reduction, another metric of impact is to predict the

contours of noise on the ground or ground footprint for a single event landing and takeoff. This metric

shows the impact of the predicted full azimuthal directivity of the aircraft configuration and all

technologies. The resulting ground footprint is also relatable to the population that would be impacted. The

ground footprint of an early HWB concept showed an 80% reduction in the footprint area relative to a

current technology B777-like aircraft calibrated to a cumulative margin of 11.3 EPNL dB [3].

A companion paper [55] describes the current process used to predict the ground footprint along with

the predicted area for the HWB-2016 C0 configuration. A ground footprint is also computed for a B777-

like aircraft that is the current technology reference for this class [25] and that has a certification noise of

7.8 EPNL dB. The area is also computed for an aircraft in this class that is recalibrated to just meet the

Stage 4 limit [55].

As mentioned in Section V.B., the HWB-FT up to and including C6 involved prediction improvements

and, therefore, could be seen as an updated, more accurate, prediction of the HWB-2016 at the end of the

ERA project. Finally, including all configurations through C18, the HWB-FT-2017 ground footprint can

also be calculated in a similar process. Figure 16 shows the ground footprints of all four of these aircraft

predictions in order to see this visual and quantitative impact.

Table 7 shows the results of the area calculations on a relative basis. The HWB-FT-2017 is shown to

reduce the ground footprint by 94.4% relative to the current technology B777-like. The HWB-FT-2017

reduces the ground footprint by 52.2% relative to the C6 (end of ERA configuration) to demonstrate the

impact of realizing the Far Term noise reduction goal of NASA.


22

Figure 16. Ground contours of the 85 Sound Exposure Level (SEL) dB for four aircraft predictions.

Table 7 Reduction of ground contour areas of the 85 Sound Exposure Level (SEL) dB for a single

event takeoff and landing

Aircraft Prediction Area Reduction to that

of Stage 4 Aircraft

Area Reduction to that

of B777-like

Area Reduction to

that of HWB-2016 C6

Stage 4 - - -

B777-like 41.0% - -

HWB-2016 C6 93.1% 88.3% -

HWB-FT-2017 96.7% 94.4% 52.2%

The HWB-FT-2017 is rendered in Figure 17 as it would appear on approach showing the pod gear and

the Krueger flap cove filler with aligned brackets. Figure 18 shows the view from the aft of the vehicle and

shows the vertical tail root extension, PAA chevrons, in addition to the elliptical and scarfed fan exit nozzle.


23

Figure 17. Rendering of the HWB-FT-2017 on approach.

Figure 18. Rendering of the HWB-FT-2017 as seen from aft of the vehicle.

VII. Recommendations for Further Development

Several recommendations can be made for the advancement of this technology roadmap study and for

the most effective noise reduction approaches:

A full system design cycle process should be done with the HWB-FT in order to cycle the

impact of the selected noise reduction designs and technologies, including advancements in

the engine and airframe technologies for the Far Term timeframe. This will give a more

accurate assessment of the system level benefits to fuel burn, emissions, and noise.

Of the liner concepts internal to the nacelle, the center plug liner had the most significant

impact at the system level, 1.3 EPNL dB cumulative, and should be a liner concept that is

developed further.

Integrating a cove filler and aligned brackets on a Krueger that has a sealed gap is a challenge

requiring integrated aerodynamic, structural, and mechanical development. A new concept is

to design the cove filler such that it eliminates or reduces both the cove flow and the bracket

flow so that both noise components are reduced and aligning the brackets with the flow

becomes unnecessary. This is a topic under active research by the authors, and the results will

be reported in a future paper. For acoustics, high fidelity computational and experimental

aeroacoustics data for a realistic and flight-like system are necessary to validate and improve

the GuoLE-v2 system level method and improve the noise reduction performance of the

ultimate Krueger flap design.

The pod gear concept is an innovative integration of the fuselage and main landing gear that is

enabled by an over-the-wing integrated engine aircraft concept. The dramatic reduction in

main gear noise of more than 4.4 EPNL dB (on the main gear component level) should be a

strong motivator to develop unconventional high engine aircraft concepts. A detailed aircraft

reconfiguration to include the pod gear concept is needed in order to provide the detailed


24

geometry of the pod, gear, and retraction mechanisms. This design should lead to a high

fidelity acoustics experiment that would be valuable to validate and improve the system noise

predictions of the GuoLG-v2 method. From this point, development of subcomponent aspects

of the pod gear approach would then be undertaken including the pod gear door mechanisms

and interior acoustic liners.

A high fidelity PAA experiment, particularly with a realistic fan noise simulator, including

advanced design of multiple shielding effectiveness approaches would be most helpful in

advancing this noise reduction approach together with the development of prediction methods

of varying fidelity.

VIII. Conclusions

The NASA aircraft noise reduction goals are aircraft system level goals driven by the motivation that

the growth of the national air transport system can be enabled by a step change reduction in aircraft noise.

While a range of technologies inherent in advanced airframes and engines contribute, previous work in the

ERA project has shown that unconventional aircraft configurations with favorable PAA effects, such as the

hybrid wing body, are most able to reach the NASA Mid Term goal of 42 EPNL dB.

This Far Term noise technology roadmap study has now shown that there is a set of well-founded noise

reduction technologies and design approaches that have the potential to enable the hybrid wing body to

reach a noise level of 50.9 EPNL dB cumulative below Stage 4, very close to the NASA Far Term noise

goal.

This technology roadmap was developed as a true “ground up” roadmap with each configuration

selected based on realism, maturity, or potential for effectiveness on this application and without

foreknowledge of what aircraft noise level would be achieved at the conclusion. For the predicted noise

reduction developed for each configuration they:

were not based on “goal setting” for each configuration,

were based on considerable effort using improved prediction methods, experimental data, and

published information, and

included polar, azimuthal, and frequency variation to the extent it was both realistic and

possible to predict.

A total noise reduction of 10.5 EPNL dB cumulative is achieved from C0 (40.4 dB) to C18 (50.9 dB).

Notably, at 1.3 EPNL dB, the center plug liner for core noise reduction has the largest impact and is the

most promising of the liner attenuation approaches internal to the duct. However, it is the following newer

approaches to noise reduction that enable a combined 10.2 EPNL dB cumulative reduction (based on Table

6), the large majority of the total achieved. Based on the one-off results, the impact of these innovative

approaches can be grouped as:

Krueger flap, 4.4 EPNL dB cumulative (C5, C13, and C14),

Pod Gear, 3.3 EPNL dB cumulative (C18) and,

Shielding effectiveness of fan, core, and jet sources, 2.5 EPNL dB cumulative (C10, C12, C15,

C16, and C17).

The result of this roadmap on an HWB aircraft reveals a new paradigm in aircraft noise with an aircraft

that can reach this low noise level of 50.9 EPNL dB. Looking even further out, even lower noise levels

could be achieved for the HWB aircraft by concentrating further noise reduction on the Krueger flap and

the main gear components.

As this study has emphasized, advanced aircraft system noise is highly dependent on the aircraft

configuration. The conclusions of this study and, in particular, the technology configurations shown to be

promising for application to the HWB-FT-2017, may not apply directly to other unconventional, future

aircraft concepts. As a result, a series of technology roadmaps for other promising concepts is anticipated to

strengthen the NASA portfolio for the Far Term horizon.

Acknowledgments

The Aircraft Noise Reduction Sub-Project of the Advanced Air Transport Technology Project is

acknowledged for supporting this study. As integral members of our team, a special thank you to John


25

Rawls and Stuart Pope of the Aeroacoustics Branch for expert performance throughout the calculations for

this study. The Advanced Concepts Lab, Analytical Mechanics Associates, is also thanked for the

renderings of the HWB concepts. Our newest colleagues, Jason C. June and Ian A. Clark, are thanked for

assistance in the final stages of preparing this paper.

References

1. Collier, F.S. and Wahls, R.A., “NASA Aeronautics Strategic Implementation, Strategic Thrust 3A Roadmap

Overview, Ultra-Efficient Commercial Vehicles – Subsonic Transports,” NASA Aeronautics Research Mission

Directorate presentation, June 1, 2016.

2. Hill, G.A., and Thomas, R.H., “Challenges and Opportunities for Noise Reduction Through Advanced Aircraft

Propulsion Airframe Integration and Configurations,” paper presented at the 8th CEAS Workshop on Aeroacoustics

of New Aircraft and Engine Configurations, Budapest, Hungary, Nov. 11-12, 2004.

3. Thomas, R.H., Burley, C.L., and Olson, E.D., “Hybrid Wing Body Aircraft System Noise Assessment with

Propulsion Airframe Aeroacoustic Experiments,” International Journal of Aeroacoustics, Vol. 11 (3+4), pp. 369-

410, 2012.

4. Czech, M.J., Thomas, R.H., and Elkoby, R., “Propulsion Airframe Aeroacoustic Integration Effects of a Hybrid

Wing Body Aircraft Configuration,” International Journal of Aeroacoustics, Vol. 11 (3+4), pp. 335-368, 2012.

5. Bonet, J.T., Schellenger, H.G., Rawdon, B.K., Elmer, K.R., Wakayama, S.R., Brown, D. and Guo, Y.P.,

“Environmentally Responsible Aviation (ERA) Project – N+2 Advanced Vehicle Concepts Study and Conceptual

Design of Subscale Test Vehicle (STV),” NASA Contract Report 2013-216519, 2013.

6. Flamm, J.D., James, K.D., and Bonet, J.T., “Overview of ERA Integrated Technology Demonstration (ITD) 51A

Ultra-High Bypass (UHB) Integration for Hybrid Wing Body (HWB),” AIAA Paper 2016-0007.

7. Vicroy, D.D., Dickey, E., Princen, N., and Beyar, M., “Overview of Low-Speed Aerodynamic Tests on a 5.75%

Scale Blended-Wing-Body Twin Jet Configuration,” AIAA Paper 2016-0009.

8. Guo, Y.P., Brusniak, L., Czech, M.J., and Thomas, R.H., “Hybrid Wing-Body Aircraft Slat Noise,” AIAA Journal,

2013, Vol. 51, pp. 2935-2945.

9. Thomas, R.H., Czech, M.J., and Doty, M.J., “High Bypass Ratio Jet Noise Reduction and Installation Effects

Including Shielding Effectiveness,” AIAA Paper 2013-541.

10. Doty, M.J., Brooks, T.F., Burley, C.L., Bahr, C.J., and Pope, D.S., “Jet Noise Shielding Provided by a Hybrid Wing

Body Aircraft,” AIAA Paper 2014-2625.

11. Hutcheson, F.V., Brooks, T.F., Burley, C.L., Bahr, C.J., Stead, D.J., and Pope, D.S., “Shielding of Turbomachinery

Broadband Noise by a Hybrid Wing Body Aircraft Configuration,” AIAA Paper 2014-2624.

12. Van Zante, D.E., and Suder, K.L., “Environmentally Responsible Aviation: Propulsion Research to Enable Fuel

Burn, Noise and Emissions Reduction,” ISABE-2015-20209, October, 2015.

13. Fares, E., Casalino, D., and Khorrami, M.R., “Evaluation of Airframe Noise Reduction Concepts via Simulations

using a Lattice-Boltzmann Approach,” AIAA Paper 2015-2988.

14. Nickol, C.L. and McCullers, L.A., “Hybrid Wing Body Configuration System Studies,” AIAA Paper 2009-0931.

15. Gern, F.H., “Improved Aerodynamic Analysis for Hybrid Wing Body Conceptual Design Optimization,” AIAA

Paper 2012-0249.

16. Gern, F.H., “Finite Element Based HWB Centerbody Structural Optimization and Weight Prediction,” AIAA Paper

2012-1606.

17. Nickol, C.L., “Hybrid Wing Body Configuration Scaling Study,” AIAA Paper 2012-0337.

18. Lopes, L.V., and Burley, C.L., “Design of the Next Generation Aircraft Noise Prediction Program: ANOPP2,”

AIAA Paper 2011-2854.

19. Guo, Y.P., Nickol, C.L., and Thomas, R.H., “Noise and Fuel Burn Reduction Potential of an Innovative Subsonic

Transport Configuration,” AIAA Paper 2014-257.

20. Guo, Y.P., Burley, C.L., and Thomas, R.H., “Landing Gear Noise Prediction and Analysis for Tube-and-Wing and

Hybrid Wing Body Aircraft,” AIAA Paper 2016-1273.

21. Guo, Y.P., Burley, C.L., and Thomas, R.H., “Modeling and Prediction of Krueger Device Noise,” AIAA Paper

2016-2957.

22. Burley, C.L., Thomas, R.H., and Guo, Y.P., “Acoustic Scattering Prediction for Aircraft System Noise Assessment,”


23. Guo, Y.P., Burley, C.L., and Thomas, R.H., “On Noise Assessment for Blended Wing Body Aircraft,” AIAA Paper

2014-365.

24. Burley, C.L., Brooks, T.F., Hutcheson, F.V., Doty, M.J., Lopes, L.V., Nickol, C.L., Vicroy, D.D., and Pope, D.S.,

“Noise Scaling and Community Noise Metrics for the Hybrid Wing Body Aircraft,” AIAA Paper 2014-2626.

25. Nickol, C.L., and Haller, W.J., “Assessment of the Fuel Burn Reduction Potential of Advanced Subsonic Transport

Concepts for NASA’s Environmentally Responsible Aviation Project,” AIAA Paper 2016-1030.

26. Thomas, R.H., Burley, C.L., and Nickol, C.L., “Assessment of the Noise Reduction Potential of Advanced

Subsonic Transport Concepts for NASA’s Environmentally Responsible Aviation Project,” AIAA Paper 2016-0863.


26

27. Thomas, R.H., Burley, C.L., and Guo, Y., “Progress of Aircraft System Noise Assessment with Uncertainty

Quantification for the Environmentally Responsible Aviation Project,” AIAA Paper No. 2016-3040.

28. Herkes, W.H., Olsen, R.F., and Uellenberg, S., “Quiet Technology Demonstrator Program: Flight Validation of

Airplane Noise-Reduction Concepts,” AIAA Paper 2006-2720.

29. Yu, J., Kwan, H., Chien, E., Ruiz, M., Nesbitt, E., Uellenberg, S., Premo, J., and Czech, M., “Quiet Technology

Demonstrator 2 Intake Liner Design Validation,” AIAA Paper 2006-2458.

30. Yu, J. and Chien, E., “Folding Cavity Acoustic Liner for Combustion Noise Reduction,” AIAA Paper 2006-2681.

31. Martinez, M.M., “Determination of Combustor Noise From A Modern Regional Aircraft Turbofan Engine,” AIAA

Paper 2006-2676.

32. Sutliff, D.L., Jones, M.G., and Hartley, T.C., “Attenuation of FJ44 Turbofan Engine Noise with a Foam-Metal

Liner Installed Over-the-Rotor,” 15th AIAA/CEAS Aeroacoustics Conference, AIAA Paper 2009-3141.

33. Jones, M.G. and Howerton, B.M., “Evaluation of Novel Liner Concepts for Fan and Airframe Noise Reduction,”


34. Czech, M.J. and Thomas, R.H., “Open Rotor Aeroacoustic Installation Effects for Conventional and

Unconventional Airframes,” AIAA Paper 2013-2185.

35. Thomas, R.H., Burley, C.L., Lopes, L.V., Bahr, C.J., Gern, F.H., and Van Zante, D.E., “System Noise Assessment

and the Potential for a Low Noise Hybrid Wing Body Aircraft with Open Rotor Propulsion,” AIAA Paper 2014-

258.

36. Howerton, B.M. and Jones, M.G., “Acoustic Liner Drag: Measurements on Novel Facesheet Perforate Geometries,”


37. Nesbitt, E., Mengle, V., Czech, M., Callender, B. and Thomas, R., “Flight Test Results for Uniquely Tailored

Propulsion-Airframe Aeroacoustic Chevrons: Community Noise,” AIAA Paper 2007-2438.

38. Akaydin, H.D., Housman, J.A., Kiris, C.C., Bahr, C.J., and Hutcheson, F.V., “Computational Design of a Krueger

Flap Targeting Conventional Slat Aerodynamics,” AIAA Paper 2016-2958.

39. Bahr, C.J., Hutcheson, F.V., Thomas, R.H., and Housman, J.A., “A Comparison of the Noise Characteristics of a

Conventional Slat and Krueger Flap,” AIAA Paper 2016-2961.

40. Pott-Pollenske, M., Almoneit, D., and Wild, J., “On the Noise Generation of Krueger Leading Edge Devices,”


41. Kreitzman, J., Cheng, R., Moffitt, N.J., Babcock, D.A., and Bent, P., “A Qualitative Acoustic Analysis of Krueger

Device Noise Utilizing CFD/CAA Experiments,” AIAA Paper 2017-3365 to appear at the 23rd AIAA/CEAS

Aeroacoustics Conference, 5-9 June, 2017.

42. Moffitt, N.J., Babcock, D.A., Kreitzman, J., and Cheng, R., “Two Approaches to Resolving the Flow Physics of a

Krueger Flap for CFD/CAA Analysis,” AIAA Paper 2017-3366 to appear at the 23rd AIAA/CEAS Aeroacoustics

Conference, 5-9 June, 2017.

43. Imamura, T., Ura, H., Yokokawa, Y., Hirai, T., and Yamamoto, K., “Numerical and Experimental Research of

Low-Noise Slat Using Simplified High Lift Model,” AIAA Paper 2008-2918.

44. Street, C.L., Casper, J.H., Lockard, D.P., Khorrami, M.R., Stoker, R.W., Elkoby, R., Wenneman, W.F., and

Underbrink, J.R., “Aerodynamic Noise Reduction for High-Lift Devices on a Swept Wing Model,” AIAA Paper

2006-0212.

45. Scholten, W.D., Patterson, R.D., Hartl, D.J., Strganac, T.W., Chapelon, Q.H.C., and Turner, T.L., “Computational

and Experimental Fluid-Structure Interaction Assessment of a High-Lift Wing with a Slat-Cove Filler for Noise

Reduction,” AIAA Paper 2017-0732.

46. Norris, G., “Demo Duo,” Aviation Week and Space Technology, February 6-19, pp. 43-45.

47. Berton, J.J., Envia, E. and Burley, C.L., “An Analytical Assessment of NASA’s N+1 Subsonic Fixed Wing Project

Noise Goal,” AIAA Paper 2009-3144.

48. Guo, Y., Pope, D.S., Burley, C.L., and Thomas, R.H., “Aircraft System Noise Shielding Prediction with a Kirchoff

Integral Method,” AIAA Paper to appear at the 23rd AIAA/CEAS Aeroacoustics Conference, 5-9 June, 2017.

49. Mengle, V.G., Stoker, R.W., Brusniak, L., Elkoby, R., and Thomas, R.H., “Flaperon Modification Effect on Jet-

Flap Interaction Noise Reduction for Chevron Nozzles,” AIAA Paper 2007-3666.

50. Herr, M., “Design Criteria for Low-Noise Trailing-Edges,” AIAA Paper 2007-3470.

51. Clark, I.A., Alexander, W.N., Devenport, W., Glegg, S., Jaworski, J.W., Daly, C., and Peake, N., “Bio-inspired

Trailing-Edge Noise Control,” AIAA Journal 55(3), 740-754.

52. Khorrami, M.R., Humphreys, W.M., Lockard, D.P., and Ravetta, P.A., “Aeroacoustic Evaluation of Flap and

Landing Gear Noise Reduction Concepts,” AIAA Paper 2014-2478.

53. Kennedy, J., Neri, E., and Bennett, G.J., “The Reduction of Main Landing Gear Noise,” AIAA Paper 2016-2900.

54. Thomas, R.H., Nickol, C.L., Burley, C.L., and Guo, Y., “Potential for Landing Gear Noise Reduction on Advanced

Aircraft Configurations,” AIAA Paper 2016-3039.

55. Thomas, R.H. and Guo, Y., “Ground Noise Contour Prediction for a NASA Hybrid Wing Body Subsonic Transport

Aircraft,” AIAA Paper to appear at the 23rd AIAA/CEAS Aeroacoustics Conference, 5-9 June, 2017.

Aircraft Noise Reduction Technology Roadmap Toward ... · Aircraft Noise Reduction Technology...

Documents

Transcript of Aircraft Noise Reduction Technology Roadmap Toward ... · Aircraft Noise Reduction Technology...