American Institute of Aeronautics and Astronautics

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The Quiet Technology Demonstrator Program: Static Test of an

Acoustically Smooth Inlet

Bryan Callender1 and Bangalore Janardan

2

GE Aviation

Cincinnati, OH

Stefan Uellenberg 3

and John Premo4

Boeing Commercial Airplane Group

Seattle, Washington

Hwa-Wan Kwan 5

and Amal Abeysinghe6

Goodrich Corporation – Aerostructures Group

Chula Vista, California

13th

AIAA/CEAS Aeroacoustics Conference

21-23 May 2007

Rome, Italy

Abstract

The QTD2 static engine test sought to provide further insight into the noise reduction technologies

demonstrated during the 2005 flight test program. One of the technologies demonstrated in the QTD2

program was the Acoustically Smooth Inlet. This technology represented a true zero splice design with the

total treated area increased relative to a typical production design. During the flight test, this inlet

demonstrated community noise reductions of over 2 EPNdB and significant reductions in forward cabin

noise. The static test was successful in identifying key design features that led to these benefits. Multiple pure

tone noise reduction was shown to be primarily attributable to increased acoustic area in the inlet.

Reductions in BPF tone related noise was shown to be mostly due to the elimination of splices of the inlet.

Finally, a detailed investigation into the effects of splices in the forward fan case region was completed.

Nomenclature

ASI = acoustically smooth inlet

BPF = Blade Passage Frequency

dB = Decibel

EPNL = Effective Perceived Noise Level

MPT = Multiple Pure Tone

Mrel = Relative Fan Tip Mach Number PNLT = Tone Corrected Perceived Noise Level

PTO = Peebles Test Operations

QTD = Quiet Technology Demonstrator

TCS = Turbulence Control Structure

1 Lead Engineer, Acoustics and Installation Aerodynamics, 1 Neuman Way, MS W26, Member AIAA.

2 Principal Engineer, Acoustics and Installation Aerodynamics, 1 Neuman Way, MS W26, Associate Fellow, AIAA.

3 Engineer, PD Noise & Emissions, P.O. Box 3707, MS 0R-MM, Member, AIAA.

4 Engineer, Aeroacoustics & Fluid Mechanics Group, P.O. Box 3707, MS 67-ML, Senior Member, AIAA.

5 Staff Engineer, Technical Support, 850 Lagoon Drive, MZ 107N.

6 Senior Engineer, Technical Support, 850 Lagoon Drive, MZ 107N.

13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference) AIAA 2007-3671

Copyright © 2007 by General Electric, The Boeing Company, and Goodrich Corporation. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

American Institute of Aeronautics and Astronautics

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I. Introduction

ecently, a consortium of aerospace industry leaders including Boeing, GE Aviation, Goodrich and

NASA Langley Research Center completed static engine testing as part of the Quiet Technology

Demonstrator 2 (QTD2) program. The QTD2 program focused on the demonstration and characterization

of several advanced technologies for mitigation of airplane and engine noise both inside the airplane and

for the community below.

Environmental concerns such as fuel efficiency, emissions, and community noise continue to drive

aircraft engine designs to higher bypass ratios. As the bypass ratio increases, so does the relative

contribution of inlet radiated fan noise to the total aircraft system noise. As such, it is becoming

increasingly important to improve the understanding of fan inlet radiated noise source and propagation

mechanisms. Such understanding allows for more effective design of inlet acoustic treatment. Boeing

Seattle and Boeing Wichita, now Spirit Aerosystems, have sustained a continuous effort to maximize

treated area and minimize splices and acoustic discontinuities in composite inlets since the mid 1980’s.

This effort has been called Area MAXimization (AMAX). Boeing Seattle supported the effort through

funding and efforts to predict the improvements while Boeing Wichita focused on manufacturing

technologies that enabled improved acoustic area and splice minimization or elimination. Although

aspects of this technology have been applied continuously to improve production engine nacelles over

time, the first true test of the technology was the AMAX inlet tested in 2001 as part of the first QTD test

program1.

In 1986, Ganz2 used early spinning mode measurements from Joppa

3 to show that forward fan case

splices were likely responsible for increased blade passage frequency (BPF) tone levels measured on a

Boeing product. The Ganz paper was the first from Boeing that gave a physical description of the

scattering source mechanism for BPF tone increases due to splices. These results led Boeing to examine

and ultimately hardwall the forward fan case lining on selected products to improve aircraft noise

certification levels. An equivalent source model was later developed at Boeing to give a quantitative

evaluation of the scattering effects of splices. This model, named the Cargill Method, is described in a

recent work by Tester4. It shows how the highly attenuated rotor-locked field at subsonic and transonic tip

speeds can be scattered into other spinning orders where the lining suppression is greatly reduced. More

recently, Tam5 put together a model capable of evaluating more general impedance variation which was

validated against the earlier work of Tester, et al. In addition, 3-D propagation codes using the Linearized

Euler Equations (LEE) have also been applied to the problem allowing refraction effects to be captured.

Together these models have provided new insights into the effects of splices on inlet noise generation and

propagation. Premo, et al6 utilize the data collected during the QTD2 Static Test to validate some of these

models.

One of the key technologies developed and demonstrated in the QTD2 program was the Acoustically

Smooth Inlet (ASI). Goodrich, in cooperation with Boeing, has continued the development of the AMAX

technologies and developed the ASI inlet specifically for the QTD2 program7. By completely eliminating

all splices and acoustic discontinuities, the ASI improves upon the AMAX inlet demonstrated in the first

QTD program and represents a true entitlement level design of inlet acoustic treatment. This is the first

full-scale, zero-splice inlet tested on a Boeing airplane. The acoustic lining design of the inlet also

differed from past designs in terms of the tuning frequency. The removal of impedance discontinuities

and increased acoustic area allowed the inlet to target the peak Noy frequencies, which provide the

dominant contribution to Perceived Noise Level (PNL), with less consideration to lower power BPF tone

noise.

The QTD2 flight test program, conducted in 2005 at Boeing’s Glasgow, MT flight test facility,

demonstrated the benefits of the ASI for both community and cabin noise8. In terms of community noise,

the flight test demonstrated over 2 EPNdB cumulative noise benefit versus a traditional inlet design with

splices. Additionally, in-cabin noise measurements showed dramatic reductions in both BPF tone levels

and buzzsaw noise during climb. The QTD2 static engine test, conducted in 2006, sought to improve the

understanding of the flight test results. Specifically, the test sought to identify the significance of the

R

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individual design features of the ASI relative to a current production inlet design. This was accomplished

by applying tape to the ASI to simulate splices and hardwalled regions. Additionally, the data was used to

validate some of the recently developed numerical models for splice prediction. This paper will focus on

the far-field effects of the various ASI design features, while reference 6 will provide details of the splice

prediction tool validation.

II. Measurement Facilities

The QTD2 static engine test was conducted at GE Aviation’s Peebles Test Operations (PTO) facility

near Peebles, Ohio. PTO Test Site 4D has been used for static engine acoustic development and

certification tests since 1987. The site features a sound field arena composed of a flat concrete surface that

is fully compliant with the recommendations of SAE Standard ARP1846 9. For this test, both far-field and

near-field microphone arrays were deployed. The far-field array is located at a constant arc distance of

150’ and is centered on a point directly beneath the engine. This array consisted of 23 microphones

positioned over an angular range of 10° to 120° in 5° increments. The angle was defined from the inlet

direction. In addition, a near-field array was deployed for measurement of inlet radiated multiple pure

tone (MPT), or buzzsaw, noise. Located on a 28’ ground sideline, the near-field array included

microphones at acoustic angles ranging from 30° to 80° in 10° increments. All far-field and near-field

microphones were mounted in ground-plane stands. Figure 1 shows an overview of the test site, while

Fig. 2 shows the microphone mounting stand and orientation.

Both the far-field and near-field arrays were composed of Bruel and Kjaer model 4134 ½” condenser

microphones with model 2639 preamplifiers. The microphones and preamplifiers were powered by Bruel

and Kjaer model 2807/2829 power supplies. Microphone head calibration was completed by the Bruel

and Kjaer calibration laboratory in Atlanta, GA prior to the start of the test. Total system calibrations for

both amplitude and frequency response were completed frequently throughout the test program to avoid

any calibration drift. The total system amplitude calibration is provided by a Bruel and Kjaer model 4228

pistonphone, while a pseudo-random pink noise signal is used for calibration of the total system

frequency response. Acoustic data was acquired and post processed by GE’s Digital Acoustic Processing

System (DAPS). The raw digitized time series data was processed using a 32-second integration time and

the results were converted to one-third octave band sound pressure levels. Adjustments and corrections

that were applied to the acoustic data include barometric correction for the pistonphone calibration,

microphone head corrections, pink noise generator corrections, amplitude calibration, total system

frequency response corrections, and a free-field adjustment of 6 dB to account for pressure doubling due

to the microphone ground plane orientation. Finally, all data was corrected to reference atmospheric

conditions of 77° F and 70% relative humidity using standard atmospheric corrections10

. In the case of

narrowband spectra, which was processed to a bandwidth of 8 Hz, only the amplitude pistonphone

calibration was applied.

Atmospheric data was collected by sensors on a Portable Environmental Data System (PEDS) cart

that was positioned beyond the far-field array at a distance of approximately 170’ from the engine center

and at an inlet angle of 85°. Measured parameters included dew point for relative humidity calculation,

temperature, wind direction, and wind velocity. These measurements were all collected at a height

corresponding to the engine centerline height. The meteorological and engine parameters were recorded

on the transient Advanced Data System (ADS) at a rate of one scan per second during all acoustic

readings.

The QTD2 test vehicle was a GE90-115B engine, which features 22 compound swept, wide-chord

fan blades with a fan diameter of 128.3”. All pretest preparation was completed in the PTO Large Engine

Prep Building. This included installation of all test instrumentation as well as a pylon. An aeroacoustic

bellmouth was attached to the inlet lip as shown in Figure 3. The bellmouth is designed to produce similar

aerodynamic conditions over the inlet barrel to those encountered during the flight test by producing

similar boundary layer properties and fan face Mach number distributions. For all test configurations, a

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Turbulence Control Structure (TCS) was attached to the engine. The TCS reduces large-scale inflow

turbulence and ground vortices that can lead to the generation of excess noise that is not present in flight.

This also provides more stable engine operation, by removing large-scale, low frequency disturbances

which can modulate the fan speed. Finally, an aft acoustic barrier wall was used to isolate the inlet

radiated fan noise from the engine exhaust noise for the configurations described in this paper. The main

portion of the barrier is 56 feet long by 34 feet high. It is designed such that the engine exhaust noise on

the acoustic measurement side of the engine is blocked from the inlet quadrant microphones. Figure 4

shows the complete static engine test setup with the TCS and the acoustic barrier wall installed.

III. Test Inlet Configurations The same fully treated ASI barrel that was flown on the 2005 flight test was utilized as the baseline

configuration for the QTD2 static engine test. Figure 5 shows a direct comparison between a current

production type inlet and the Goodrich ASI. The white dashed lines in Fig. 5(a) define the boundary of

the acoustically treated area. In addition to the two axial splices at the 3 and 9 o’clock positions of the

inlet, much of the inlet area is untreated. In comparison, the dashed lines on the ASI in Fig. 5(b) show that

the axial splices have been eliminated and the treated area has been maximized. As described in Section I,

one of the primary goals of the static test was to identify and better understand the design features

responsible for the flight test acoustic benefits. To accomplish this goal, six different test configurations

were simulated using 3M Y434 tape to represent different splices and hardwall regions. These

configurations are presented graphically in Figure 6 and described below.

Configuration 1 – Fully treated ASI.

Configuration 2 – Tape was used to simulate hardwall surfaces forward of the inlet throat and in the

forward fan case region. Two simulated axial splices were applied to the ASI inner barrel with 180°

spacing.

Configuration 3 – Tape was used to simulate hardwall surfaces forward of the inlet throat and in the

forward fan case region. The ASI inlet barrel was fully treated with no splices.

Configuration 4 – Tape was used to simulate hardwall surfaces in the forward fan case region. Tape

was used to simulate hardwall forward fan case. Acoustic lining forward of inlet throat was exposed.

The ASI inlet barrel was fully treated with no splices.

Configuration 5 –ASI was fully treated with the exception of 2 simulated splices in the forward fan

case region. Splices were located 180° apart and spanned the axial length of the forward fan case

region. These splices were approximately 33% smaller in width than those tested in Configuration 2.

Configuration 6 – ASI was fully treated with the exception of 2 simulated splices in the forward fan

case region. Splices were located 180° apart and spanned the axial length of the forward fan case

region. These splices were of equal width to those tested in Configuration 2.

Each of these test configurations is used to assess the relative impact of treated area and treatment

discontinuities compared to the fully treated ASI. Configurations 1 and 4 are used to understand the

effects of additional treated area in the forward fan case region. The benefit of adding treatment forward

of the inlet aerodynamic throat, where convective velocities are higher, is determined by comparing

configurations 3 and 4. Splice effects are investigated by configurations 1, 2, 3, 5, and 6. The effect of

splices in the inlet barrel is considered by evaluating configurations 2 and 3, while configurations 1, 5,

and 6 allow the noise impacts of forward fan case splices to be assessed. The impact of each of these

design features is discussed in detail in Section IV.

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IV. Results and Discussion The QTD2 static engine test followed the 2005 flight test, which demonstrated the ability of the inlet

to provide significant reductions in inlet radiated fan noise7, 8

. Figure 7 presents data from the flight test

which shows the impact of the ASI on the tone corrected Perceived Noise Level (PNLT) relative to the

production inlet. These data, which were measured at a cutback power setting, show that the ASI provides

reductions of 3 – 5 PNdB at forward angles between 20° and 50°. These forward arc noise reductions

lead to a reduction in the duration correction used in calculation of Effective Perceived Noise Level

(EPNL). In addition to community noise reductions, Figure 8 shows the noise reductions that were

measured in the aircraft cabin region during the flight test. Microphones mounted in the forward cabin

showed significant reductions in both BPF tone noise and buzzsaw noise for the ASI relative to the

baseline inlet. More complete details of the inlet related flight test results are provided in references 7 and

8.

As mentioned previously, the QTD2 static engine test sought to improve the understanding of the

acoustic benefits seen in the flight test. A static engine test is more appropriate to accomplish this goal

due to reduced costs, complexities, and measurement uncertainties relative to a flight test. Therefore,

static engine tests are well suited to evaluate the effects of detailed configuration changes such as those

defined in Figure 6. Based on results seen in the flight test, two primary effects are used to assess the

acoustic benefits of the ASI. These are forward arc BPF tone level reductions and buzzsaw noise

reduction. Power level metrics are defined to quantify the benefits provided in each of these areas. For

assessment of BPF tonal impacts, narrowband data from the 150’ polar array are used. The BPF tone and

harmonics are separated from the narrowband data and averaged over three repeat test conditions to

obtain averaged levels at each microphone location. The BPF tone levels are then integrated over an

angular range of 10° to 80° to obtain the forward arc BPF power level metric. Such an integrated and

averaged metric is necessary to accurately assess changes in BPF noise due to lobe patterns and subtle

shifts in directivity that are often seen in inlet radiated tonal noise repeat data.

In a similar manner, data from the 28’ sideline near-field array are used to compute an MPT power

level metric. At each microphone location, an overall MPT level is determined by extracting the MPT

content from the narrowband data and summing over a range of frequencies from zero through 2xBPF,

with the BPF and 2xBPF content excluded. After averaging for repeat test conditions, the overall MPT

levels are integrated over an angular range from 30° to 80° to obtain the MPT power level metric. This

angular range encompasses the full extent of the 28’ sideline array. This is the reason that the angular

integration range is different from that used for the BPF power level metric.

The overall effects of the ASI, relative to configuration 2, are shown by Figures 9 and 10. Recall that

configuration 1 is the fully treated ASI while configuration 2 is a simulation of an inlet containing both

splices and hardwall regions. Figure 9 shows a comparison of the BPF power level metric as a function of

relative fan tip Mach number, Mrel. The general trend of these curves is typical of inlet radiated BPF tonal

noise. Continuous increases in BPF tone levels are seen as the fan tip approaches Mrel ~ 1.0. As the fan tip

exceeds sonic speed, dramatic increases are seen as the rotor locked pressure field becomes cut-on. The

tone level continues to increase with fan speed before peaking at a Mrel of approximately 1.3. The ASI

provides reductions in BPF tone power level at all speeds between Mach 0.9 and Mach 1.4. At lower fan

speeds, this benefit ranges from 0.5 to 1.0 dB. As Mrel exceeds 1.0, the BPF benefits begin to increase.

The maximum benefit of 3.5 dB roughly corresponds to the fan speed where the BPF noise is highest.

Beyond this speed, the benefit diminishes to a level of approximately 1.0 dB.

Figure 10 shows the impact of the ASI on MPT power level as a function of relative fan tip Mach

number. First, note that these data also show trends consistent with documented buzzsaw noise behavior.

As the relative tip speed begins to exceed sonic velocity, the MPT power level increases dramatically due

to the formation of non-uniformly spaced bow shocks ahead of the blade leading edges. As these shocks

become stronger, MPT related noise continues to increase with fan speed. At Mrel values greater than

approximately 1.3, the shocks are swallowed into the blade passages, and the MPT noise radiated through

the inlet begins to diminish. Figure 10 shows that the ASI reduces the MPT related noise at all relative tip

Mach numbers between 1.0 and 1.5. The largest reductions of 4 –5 dB are seen at Mach numbers between

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1.1 and 1.25. The peak MPT power level occurs at a Mach number of 1.3, where the ASI provides a

benefit of approximately 1.8 dB. Beyond this point, the benefit remains relatively constant between 1 and

2 dB.

While comparison of configurations 1 and 2 provides a measure of the overall ASI benefits, these

benefits are the result of two distinct types of design changes. These changes are increased acoustic

treatment area and the elimination of splices and associated acoustic discontinuities. Figures 9 and 10

provide evidence of both of these effects. For example, benefits associated with increased acoustic area

are generally relatively constant versus fan speed. There are certainly fan speed regimes shown in these

figures where this is the case. However, there are also regions where the benefits vary dramatically with

fan speed. Results such as these can indicate that the source generation and propagation mechanism are

being influenced by the ASI design changes. The four additional test configurations are structured to

provide insight into the relative effects of these two types of design features.

A. Treated Area Effects

The test configurations are designed to assess the impact of increasing the acoustic area in two

different regions of the inlet. The first region to be considered is the forward fan case region, which is

immediately forward of the fan face. Secondly, the region between the inlet aerodynamic throat and

forward bulkhead is considered. This region is referred to as the forward throat region, but should not be

confused with the lip treatment that was tested in the flight test. The lip treatment was part of the flight

inlet lip. Due to the use of the aeroacoustic bellmouth lip in the static test, it is not possible to test the lip

lining.

The effect of additional acoustic area in the forward fan case region is assessed by comparison of

configurations 1 and 4. The treated forward fan case region of configuration 1 represents an increase in

acoustic area of approximately 16% relative to configuration 4. Analysis of the data shows that the largest

impacts were to MPT related noise, with relatively small impacts seen in BPF tonal noise. Figure 11

shows the impact on MPT power level as a function of relative fan tip Mach number. These results appear

very similar to the overall MPT effects seen in Figure 10, with the maximum benefits of 3 – 4 dB

occurring at fan speeds below the peak MPT speed. This benefit is slightly less than the overall ASI

benefit of 4 – 5 dB shown in Figure 10. At the peak MPT speed, the addition of forward fan case

treatment provides a reduction of slightly more than 1 dB. These results indicate that the addition of

treatment in the forward fan case region accounts for much of the overall MPT benefit that is shown in

Figure 10.

Configurations 3 and 4 are used to assess the impact of additional acoustic area in the inlet forward

throat region. Configuration 4 represents an increase in acoustic area of approximately 7% relative to

configuration 3. Figure 12 shows the impact of additional treatment on the MPT power level. The

addition of treatment in the forward throat region produces a relatively constant benefit of 0.5 – 1.0 dB

across the speed range.

Figures 10 – 12 each show that the MPT benefits are relatively independent of fan speed. This

indicates that the overall MPT benefits shown in Figure 10 are largely a result of the addition of acoustic

area as opposed to the elimination of splices. Figure 13 provides evidence to support this conclusion by

comparing the total MPT benefit, at the peak MPT fan speed of Mach = 1.3, with the benefits provided by

additional treated area in the forward fan case and forward throat region. Figure 13 shows that these

individual effects account for over 97% of the overall MPT benefits seen at the peak MPT speed.

In addition to MPT benefit, the addition of acoustic area in the forward throat region also provided

reductions in BPF tone levels. Figure 14 shows that the forward throat treatment reduces the BPF tonal

noise at relative fan tip Mach numbers between 0.9 and 1.1.

B. Splice Effects

The second design feature of the ASI that is considered in the QTD2 static test is the elimination of

splices and associated acoustic discontinuities. The effects of such design features on aircraft acoustics

have received a great deal of attention in recent years4-7, 11-13

. Analytical methods, such as those developed

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by Tester and Tam, suggest that splices can scatter tonal energy into lower order azimuthal modes where

it may propagate more efficiently, while also being less attenuated by acoustic treatment. Additionally,

Boeing and Goodrich internal studies have also shown that splices can adversely affect fan tonal noise by

influencing both source and propagation mechanisms. These studies show that the splice count, location

and width can all influence fan tone levels in the far-field. The static test configurations are designed to

investigate the acoustic impacts of simulated splices in two different regions of the inlet. First, the effect

of splices in the inlet inner barrel region is considered. This refers to the portion of the inlet between the

forward fan case and the aerodynamic throat of the inlet. Secondly, the impact of splices in the forward

fan case region is considered.

The effect of splices in the inner barrel region of the inlet is assessed by comparison of configurations

2 and 3. Note that both configurations retain the hardwall surfaces in the forward fan case and forward

throat areas. Figure 15 shows that the presence of the two splices increases the BPF power level at all fan

tip speeds greater than Mach 1.0. The impact of the splices is greatest near the peak BPF speed, where

benefits in excess of 2 dB are seen. At lower fan speeds, the benefit ranges from 1.0 – 1.5 dB, while a

smaller benefit of 0.5 – 1.0 dB is seen at higher fan speeds. As demonstrated in the flight test, these tonal

reductions can have substantial implications on community noise certification levels. The cutback

certification condition is influenced by inlet radiated fan tone noise. Therefore, the benefits shown at the

peak BPF speed, and lower fan speeds, illustrate how the elimination of inlet splices can have positive

impacts on aircraft community noise. The reduced benefits at higher speeds may be of less significance

due to the fact that community noise is more influenced by aft radiated fan and jet noise at these speeds.

While axial splices in the forward fan case were not considered as part of configuration 2, there is

interest in the impact of these splices in order to determine if there is an increased sensitivity to splices

located closer to the fan face. Configurations 5 and 6 each simulate two axial splices in the forward fan

case of the ASI. These splices are located 180° apart at the inlet 3 and 9 o’clock positions. Configuration

6 uses a splice width equal to that of the inlet inner barrel splices (configuration 2), while configuration 5

reduces the splice width by 33% in order to understand sensitivity to splice width. Test data shows that

the addition of these splices impacts both BPF tonal noise and MPT related noise. Figure 16 shows the

effect of the two splice widths on the BPF power level relative to configuration 1. Both splice widths lead

to increased BPF power levels and a clear positive correlation is seen between the tone level increase and

the splice width. The smaller width splices generate BPF increases of up to 1.5 dB, while the nominal

width splices generate a maximum increase in excess of 3.5 dB. Recall that the nominal width splices are

equal in width to those that were applied to the inner barrel region in configuration 2. Figure 15 showed

that these inner barrel splices produced increases of approximately 2 dB. However, when this same splice

width is applied to the forward fan case region, increases in excess of 3.5 dB are seen. Since the fan case

splices represent significantly less treated area loss than the longer inner barrel splices, these results

cannot be related to any treated area effects. Therefore, Figure 16 provides sufficient evidence to

conclude that BPF tonal noise is more sensitive to the effects of splices that are located closer to the fan

face. Also, these results clearly show that there is a direct correlation between splice width and BPF tonal

noise.

While Figure 16 provides data that shows the noise impact correlates with splice width and that there

is increased sensitivity to splices located closer to the fan face, it also reveals another interesting effect in

regard to the fan case splices. Note that these splices appear to affect the tone levels at a limited range of

speeds. The effects of the smaller splices are confined to speeds between Mach 1.0 and 1.2, while the

effects of the nominal width splices extend to Mach 1.3. These effects are in contrast to the inlet splices,

which affected the tone levels at all speeds between Mach 0.9 and 1.4. The inlet splices also showed the

maximum impact to occur near the peak BPF level, while the fan case splices create the largest impacts at

speeds below the peak BPF. The net effect of the increased sensitivity to the fan case splices and the

localized speed range impact is that the peak BPF power level is effectively shifted to a lower fan speed.

Such an effect could have implications for the cutback certification condition, where efforts are made to

trade aircraft altitude and power setting in a manner to minimize noise. This procedure, referred to a

delayed or optimized cutback, typically results in a cutback fan speed that occurs below the peak BPF

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speed. Therefore, increases in BPF power level to the left of the peak, as shown by Figure 16, could have

significant implications for cutback noise. Of course, cutback optimization is highly specific to the details

of the engine/aircraft system performance and noise characteristics.

In addition to the BPF impacts seen in Figure 16, the forward fan case axial splices also produced

increased MPT related noise. These results are shown in Figure 17. First, note that fan case splices were

not represented by configuration 2. Therefore, these effects were not part of the results shown in Figure

10. Figure 17 shows that there is a positive correlation between MPT noise and fan case splice width.

However, the results seen in Figure 17 are somewhat different from the BPF impacts seen in Figure 16.

For example, Figure 16 showed that the larger splice widths produced significantly larger BPF increases

than did the smaller splice widths. In contrast, Figure 17 shows that the smaller splice width account for

much of the MPT noise increases, with only a slightly greater increase seen for the larger splice widths.

While the exact cause of these trends is not clear from the far-field data, some conclusions can be drawn

in regard to the implications. Recall that Figure 11 shows that the addition of acoustic treatment to the

forward fan case region results in MPT power level reductions as high as 4 dB at certain speed ranges.

Figure 17 indicates that the addition of splices to this treatment could potentially compromise much of

this improvement. The implication is that in order to fully realize the potential MPT benefits of forward

fan case treatment, efforts should be made to eliminate or minimize splices in the treatment.

V. Summary and Conclusions

The QTD2 static engine test validated the flight test conclusions in regard to the benefits of the

acoustically smooth inlet with reductions in both BPF and MPT related tonal noise being demonstrated.

In addition, the static test provided further insight and understanding to the specific design features that

contributed to these benefits. Two distinct types of design changes were considered in the static test.

These were the effects of increased acoustic area and the effects of eliminating splices. Measurements

showed that the addition of acoustic treatment to the forward fan case region and the forward throat

region primarily impacted MPT related noise. Peak benefits of 4 dB in MPT power level were

demonstrated for the addition of 16% treated area in the forward fan case region. Reductions as high as 1

dB were seen when treatment was added to the forward throat region. Investigation of splice effects

showed that the largest impacts were seen in BPF related tone noise. The elimination of splices in the

inlet inner barrel region produced peak reductions in excess of 2 dB in the BPF power level metric.

Detailed investigation of forward fan case splices revealed several interesting effects. First, conclusive

evidence was provided to show that there is an increased sensitivity of BPF tone noise to splices that are

located closer to the fan face. This was shown by direct comparison of the same splice width. When

located in the inlet inner barrel, these splices produced increases of 2 dB in BPF power level. However,

when the same width splice was applied in the forward fan case region, increases in excess of 3.5 dB were

measured. Furthermore, it was shown that fan case splices seem to affect the BPF power level over a

more limited range of power setting than inlet splices. The result of this can be a shift of the peak BPF

power level to a lower speed range. Finally, the fan case splices were also shown to impact MPT power

levels. Each splice width was shown to produce MPT power level increases of 2 – 3 dB. This data

indicated that care needs to be taken to minimize or eliminate splices in the forward fan case region to

avoid compromising potential MPT noise benefits.

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References

1) Bartlett, P., Humphreys, N., Phillipson, P., Lan, J., Nesbitt E., and Premo, J., “The Joint Rolls-

Royce/Boeing Quiet Technology Demonstrator Programme,” AIAA-2004-2869.

2) Ganz, U. W., Strout, F. G., , “Evaluation of High Fan Tone Noise Levels in Static Testing,” Boeing Internal

Paper NOIS-B8310-C86-229RES, October 24, 1986.

3) Joppa, P. D., “An Acoustic Mode Measurement Technique,” AIAA/NASA 9th

Aeroacoustic Conference,

Williamsburg, VA, October 15, 1984.

4) B.J. Tester, N.J. Baker, A.J. Kempton and M.C. Wright, “Validation of an Analytical Model for Scattering

by Intake Liner Splices”, AIAA 2004-2906, 2004.

5) Tam, C.K.W., Webb, J.C., “Dispersion-Relation-Preserving Finite Difference Schemes for Computational

Acoustics,” J. Comp. Phys. 107, 1993, pp. 262-281.

6) Premo, Breard, Lan, Chien, Abeysinghe and Callender “Predictions of the Inlet Splice Effects from the

QTD2 Static Test,” AIAA-2007, 2007.

7) Yu, J., Nesbitt, E., Kwan, H., Uellenberg, S., Chien, E., Premo, J., “Quiet Technology Demonstrator 2

Intake Liner Design and Validation,” AIAA-2006-2458.

8) W. Herkes, R. Olsen and S. Uellenberg, “The Quiet Technology Demonstrator Program: Flight Validation of

Airplane Noise-Reduction Concepts,” AIAA-2006-2720, 12th AIAA/CEAS Aeroacoustics Conference (27th

AIAA Aeroacoustics Conference), Cambridge, Massachusetts, May 8-10, 2006.

9) SAE Standard ARP1846, “Measurement of Far Field Noise From Gas Turbine Engines During Static

Operation,” February 1990.

10) SAE Standard ARP866A, “Standard Values of Atmospheric Absorption as a Function of Temperature and

Humidity”, March 1975.

11) Gantie, F., Batard, H., Baker, N., and Schwaller, P., “Zero Splice Intake Technology and Acoustic

Benefits”, AIAA-2006-2455, Cambridge, MA, 2006.

12) Tester, B., DeMercato, L., “Far-field Directivity of Rotor-Alone Tones Radiated from Fan Intakes with

Splices Liners for Different Intake Shapes, with Flow”, AIAA-2006-2456, Cambridge, MA, 2006.

13) Tam, C., Ju, H., Chin, E., “Scattering of Acoustic Duct Modes by Axial Liner Splices”, AIAA-2006-2459,

Cambridge, MA, 2006.

American Institute of Aeronautics and Astronautics

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Figure 1. Test Setup for Inlet Noise Phase of QTD2 Static

Engine Test. Peebles Site 4D.

Figure 2. Ground Plane Microphone.

Figure 3. Test Vehicle Fitted with Aeroacoustic Bellmouth Lip.

Figure 4. Static Test Setup with TCS Dome and Acoustic Barrier

Wall Installed

TCS Acoustic Barrier

American Institute of Aeronautics and Astronautics

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Figure 5. Comparison of Typical Production Type Inlet with Goodrich Acoustically Smooth Inlet Design

c

e f

a

1

Fan Face

d

4

Fan Face

b

2Fan Face

5

Fan Face

3

Fan Face

6Fan Face

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a

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Fan Face

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Fan Face

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Fan Face

b

2Fan Face

b

2Fan Face

5

Fan Face

5

Fan Face

3

Fan Face

3

Fan Face

6Fan Face

6Fan Face

Figure 6. QTD2 Static Test Inlet Configurations

American Institute of Aeronautics and Astronautics

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Figure 7. Flight Test Data Showing Forward Arc PNLT Benefits

for Cutback Power Condition.

Frequency

Sou

nd

Pre

ssu

re L

evel, d

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BuzzsawReduction

Blade PassageTone Reduction

BaselineFully Treated

400 Hz

10 dB

Frequency

Sou

nd

Pre

ssu

re L

evel, d

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BuzzsawReduction

Blade PassageTone Reduction

BaselineFully Treated

400 Hz400 Hz

10 dB10 dB

Figure 8: Flight Test Data Showing Forward Cabin Benefits in

BPF and MPT Noise.

1.41.31.21.11.00.9

5 dB

BPF Power Level (10°-80°)

Relative Fan Tip Mach Number

Config 2

Config 1

1.41.31.21.11.00.9

5 dB

BPF Power Level (10°-80°)

Relative Fan Tip Mach Number

Config 2

Config 1

Config 2

Config 1

Figure 9. Overall Static Test Impact of ASI on Forward

Radiated BPF Power-levels

MPT Power Level (30°-80°)

1.0 1.1 1.2 1.3 1.4 1.5

Relative Fan Tip Mach Number

Config 2

Config 1

5 dB

MPT Power Level (30°-80°)

1.0 1.1 1.2 1.3 1.4 1.5

Relative Fan Tip Mach Number

Config 2

Config 1

Config 2

Config 1

5 dB

Figure 10. Overall Static Test Impact of ASI on Forward

Radiated MPT Power-levels

American Institute of Aeronautics and Astronautics

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5 dB

MPT Power Level (30°-80°)

1.0 1.1 1.2 1.3 1.4 1.5

Relative Fan Tip Mach Number

Config 4

Config 1

5 dB

MPT Power Level (30°-80°)

1.0 1.1 1.2 1.3 1.4 1.5

Relative Fan Tip Mach Number

Config 4

Config 1

Config 4

Config 1

Figure 11. Forward Fan-case Area Effect on Forward Radiated

MPT Power-levels

5 dB

Config 4

Config 3

MPT Power Level (30°-80°)

1.0 1.1 1.2 1.3 1.4 1.5

Relative Fan Tip Mach Number

5 dB

Config 4

Config 3

MPT Power Level (30°-80°)

1.0 1.1 1.2 1.3 1.4 1.5

Relative Fan Tip Mach Number Figure 12. Throat Area Effect on Forward Radiated MPT

Power-levels

0

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Fwd Fan Case (Fig. 11) Fwd Throat (Fig. 12) Overall (Fig. 10)

MP

T P

ow

er

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en

efi

t (d

B)

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0.5 dB

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Figure 13. Contribution of Forward Fan Case and Forward

Throat Treatment to Overall Peak MPT Benefit.

Mon Feb 26 2007 22:35:29

5 dB

1.41.31.21.11.00.9

BPF Power Level (10°-80°)

Relative Fan Tip Mach Number

Config 4

Config 3

Mon Feb 26 2007 22:35:29

5 dB

1.41.31.21.11.00.9

BPF Power Level (10°-80°)

Relative Fan Tip Mach Number

Config 4

Config 3

Figure 14. Throat Area Effect on Forward Radiated BPF Power-

levels

American Institute of Aeronautics and Astronautics

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5 dB

1.41.31.21.11.00.9

BPF Power Level (10°-80°)

Relative Fan Tip Mach Number

Config 3

Config 2

5 dB

1.41.31.21.11.00.9

BPF Power Level (10°-80°)

Relative Fan Tip Mach Number

Config 3

Config 2

Config 3

Config 2

Figure 15. Forward Inlet Splice Effect on Forward Radiated

BPF Power-levels

Q

Q

Q

Q

Q

QQ

Q

5 dB

Config 5

Config 1

Config 6

Q

1.41.31.21.11.00.9

BPF Power Level (10°-80°)

Relative Fan Tip Mach Number

Q

Q

Q

Q

Q

QQ

Q

Q

Q

Q

Q

Q

QQ

Q

5 dB

Config 5

Config 1

Config 6

Q

1.41.31.21.11.00.9

BPF Power Level (10°-80°)

Relative Fan Tip Mach Number Figure 16. Forward Fan-case Axial Splice Effect on Forward

Radiated BPF Power-levels

5 dB

Config 5

Config 1

Config 6MPT Power Level (30°-80°)

1.0 1.1 1.2 1.3 1.4 1.5

Relative Fan Tip Mach Number

5 dB

Config 5

Config 1

Config 6MPT Power Level (30°-80°)

1.0 1.1 1.2 1.3 1.4 1.5

Relative Fan Tip Mach Number Figure 17. Forward Fan-case Axial Splice Effect on Forward

Radiated MPT Power-levels