February 2008 

NASA/TM-2008-215112 

Silent Aircraft Initiative Concept Risk 

Assessment 

Craig L. Nickol 

Langley Research Center, Hampton, Virginia 
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February 2008 

NASA/TM-2008-215112 

Silent Aircraft Initiative Concept Risk 

Assessment 

Craig L. Nickol 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information (CASI) National Technical Information Service (NTIS) 

7115 Standard Drive 5285 Port Royal Road 

Hanover, MD 21076-1320 Springfield, VA 22161-2171 

(301) 621-0390 (703) 605-6000 

Acknowledgments 

This risk assessment is a compilation of information from numerous subject-matter 

experts. Therefore, the author would like to thank the following researchers and engineers 

for their valuable contributions to this assessment: 

Richard Wahls, Richard Campbell, Mark Guynn: Cruise aerodynamics 

Greg Gatlin, Craig Streett, Andy Hahn: High-lift system 

Karen Taminger, Zoran Martinovic, Vivek Mukhopadhyay: Structures 

Bill Haller, Cecile Burg: Human factors, cost 

Michael Hicks: Internal arrangement 

Jim Heidmann, Mary Jo Long-Davis, Hyun Dae Kim, Karl Geiselhart, Laurence Leavitt, 

Jeffrey Flam: Propulsion/airframe integration 

Doug Perkins, Mike Tong: Engine architecture and transmission system 

Dennis Culley, Ken Fisher: Engine operations 

Joseph Veres: Compressor, turbine, and fan design 

Ed Envia, Craig Streett, Mike Marcolini, Erik Olson: Trailing-edge brushes, low-noise 

undercarriage 

Florence Hutcheson, Andy Hahn: Quiet drag 

Mike Jones, Jeff Berton: Exhaust-duct acoustic liners 

Joe Posey: Airframe noise shielding 

Jeremy Smith: Approach operations 

Charlotte Whitfield, Russell Thomas: Noise 

Dan Bulzan: Emissions 

Neal Watkins: Experimental 

In addition, the authors appreciate the support provided by Fay Collier (NASA Langley 

Research Center), Dennis Huff (NASA Glenn Research Center), James Hileman 

(Massachusetts Institute of Technology), and Zoltan Spakovszky (Massachusetts Institute 

of Technology). Their support enabled the execution of this risk assessment. 

Trade names and trademarks are used in this report for identification only. Their usage does not 

constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space 

Administration. 
 1

Contents 

Abstract.........................................................................................................................................................3 

1.0 Introduction.............................................................................................................................................3 

2.0 Nomenclature..........................................................................................................................................4 

Abbreviations ............................................................................................................................................4 

3.0 Background.............................................................................................................................................6 

4.0 Risk Assessment Process ........................................................................................................................7 

5.0 Results...................................................................................................................................................11 

5.1 High-risk items.................................................................................................................................11 

5.1.1 Risk #8, Variable-area exhaust thrust-vectoring nozzle .............................................................11 

5.1.2 Risk #22, Displaced threshold and CDA with an increased approach angle..............................13 

5.1.3 Risk #4, BWB configuration: structures, weight.......................................................................15 

5.1.4 Risk #7, BLI and inlet design .....................................................................................................17 

5.1.5 Risk #23, Cost.............................................................................................................................18 

5.1.6 Risk #24, Overall noise estimates...............................................................................................19 

5.1.7 Risk #5, Human factors ..............................................................................................................20 

5.2 Medium-risk items ...........................................................................................................................22 

5.2.1 Risk #1, Cruise aerodynamic performance.................................................................................22 

5.2.2 Risk #2, Deployable, drooped LE and continuous mold-line (CML) elevons ...........................24 

5.2.3 Risk #9, Transmission system ....................................................................................................26 

5.2.4 Risk #10, Single-core/multiple-fan concept ...............................................................................27 

5.2.5 Risk #12, Low flight idle thrust..................................................................................................29 

5.2.6 Risk #16, TE brushes..................................................................................................................30 

5.2.7 Risk #21, Thrust-managed takeoff..............................................................................................31 

5.2.8 Risk #26, Emissions....................................................................................................................32 

5.2.9 Risk #27, Experimental ..............................................................................................................32 

5.3 Low-risk items...................................................................................................................................33 

5.3.1 Risk # 6, BWB configuration, internal layout ............................................................................33 

5.3.2 Risk #11, Propulsion/airframe integration: buried engines .......................................................35 

5.3.3 Risk #13, Low-speed fan with forward swept blades................................................................37 

5.3.4 Risk #14, High-pressure compressor (HPC) design ...................................................................38 

5.3.5 Risk #15, Low-noise, LPT design ..............................................................................................40 

5.3.6 Risk #17, Low noise undercarriage ............................................................................................41 

5.3.7 Risk #18, Quiet drag...................................................................................................................42 

5.3.8 Risk #19, Long ducts with acoustic liners ..................................................................................43 

5.3.9 Risk #20, Airframe shielding......................................................................................................45 

6.0 ATB Comparison ..................................................................................................................................45 

7.0 Conclusions and Recommendations .....................................................................................................48 

References...................................................................................................................................................51 

Appendices 

A – SAI Press Coverage References.................................................................................54 

B – BWB Literature Survery...........................................................................................56 

C – SAI Requirements................................................................................................ .67 

D – Risk Item List......................................................................................................69 

E – Additional Data Required to Support Further Analysis.......................................................72 
 2

List of Tables 

Table 1. Definitions of Consequence Metrics...............................................................................................8 

Table 2. ATB Comparisons ........................................................................................................................48 

List of Figures 

Figure 1. Three generations of the SAI conceptual design. ..........................................................................6 

Figure 2. Five-by-five risk matrix.................................................................................................................8 

Figure 3. Populated SAX-40 risk matrix. ...................................................................................................11 

Figure 4. High bending stress associated with a noncylindrical pressure vessel. .......................................16 

Figure 5. Sample 335-passenger layout for volumetric sanity check. ........................................................34 

Figure 6. Landing gear retraction geometry................................................................................................35 
 3

Abstract 

A risk assessment of the Silent Aircraft Initiative’s SAX-40 concept 

design for extremely low noise has been performed. A NASA team 

composed of subject-matter experts and systems analysts developed a list 

of 27 risk items, and evaluated the level of risk for each item in terms of 

the likelihood that the risk would occur and the consequences of the 

occurrence. The following risk items were identified as “high risk,” 

meaning that the combination of likelihood and consequence put them 

into the top one-fourth of the risk matrix: structures and weight 

prediction; boundary-layer ingestion (BLI) and inlet design; variablearea exhaust and thrust vectoring; displaced-threshold and continuous 

descent approach (CDA) operational concepts; cost; human factors; and 

overall noise performance. Risk management strategies of either 

mitigation, avoidance, assumption, or transfer were recommended for 

each risk. In addition, several advanced-technology baseline concepts 

were created to serve as a basis for comparison to the SAX-40 concept. 

These comparisons indicate that the SAX-40 would have significantly 

greater research, development, test, and engineering (RDT&E) and 

production costs than a conventional aircraft with similar technology 

levels. Therefore, the cost of obtaining the extremely low noise capability 

that has been estimated for the SAX-40 is significant. The 

recommendation from this assessment is that the next iteration for this 

design should strive to achieve an appropriate balance among a variety 

of metrics, such as maintenance costs, fuel burn, emissions, and noise. 

The SAX-40 concept design proved successful in focusing attention 

toward low noise technologies and in raising public awareness of the 

issue. 

1.0 Introduction 

On November 6, 2006, a group called the Silent Aircraft Initiative (SAI) gave a public 

presentation to the Royal Aeronautical Society on research that was conducted over the previous 

three years. The research focused on producing a conceptual design for a fuel-efficient mid-range 

commercial transport that is inaudible outside of a typical airport boundary. The conclusion of the 

presentation stated that the group had achieved its goal of producing a credible conceptual design 

that meets the objectives of a functionally silent and fuel-efficient aircraft. Dubbed the SAX-40, 

this 215-passenger commercial transport concept has garnered a high level of attention, evidenced 

by both the large amount of press coverage associated with this particular briefing and the SAI 

effort in total. A Web survey yielded over 40 references in a few minutes (see Appendix A), 

including full-length articles in Aerospace Engineering and Aviation Week and Space 

Technology. The SAI team also presented their research at the 2007 Aerospace Sciences 

Conference in January 2007 to a standing-room-only crowd. Clearly, this research has resonated 

not only within the relatively small aircraft design community but has also caught the attention of 

a much broader public segment. 

Within the NASA Fundamental Aeronautics Program, the Subsonic Fixed Wing (SFW) 

project has declared goals for reducing noise, emissions, and fuel burn relative to the levels found 

in today’s commercial transports. Near- and far-term goals have been established, with the 
 4

assumption that the near-term solution will resemble a conventional “tube with wings” approach, 

whereas the far-term solution will resemble a hybrid wing/body approach, widely known as the 

blended wing/body (BWB) concept. The SAX-40 is an example of this hybrid wing/body concept 

and is, therefore, aligned quite well with NASA’s far-term goals. Many of the technologies that 

have been utilized for the SAX-40 are highly relevant to NASA’s technology research portfolio. 

Given the high relevance and large amount of public interest generated by the SAX-40 

concept, an assessment was performed to gain additional insight into the feasibility of the concept 

and to identify the key enabling technologies to facilitate the development of NASA’s technology 

research portfolio. Therefore, this qualitative risk assessment was performed. A more detailed, 

quantitative technical assessment is a potential follow-on activity that may be performed if it is 

deemed worthwhile. 

The next section provides background information on SAI and the SAX-40 concept. 

Then, the qualitative risk assessment process is presented. Then, the results of the risk assessment 

are summarized. One shortcoming of the SAI effort that was identified during this assessment 

was the lack of an advanced-technology baseline (ATB) design to provide a consistent basis for 

comparison. SAI was aware of this shortcoming and had considered including just such a baseline 

case, however, resource constraints prevented its inclusion. A NASA ATB was developed, and 

this effort is presented in the section following the risk assessment results. The final section 

contains conclusions and recommendations. 

2.0 Nomenclature 

M – Mach Number 

M(L/D) – Mach times lift to drag ratio 

σ – stress 

T3 – combustor inlet temperature 

x/c – location as a fraction of wing chord length 

Abbreviations 

ADS-B – Automatic Dependent Surveillance - Broadcast 

AGMA – American Gear Manufacturers Association 

AR – Aspect Ratio 

AST – Advanced Subsonic Technology 

ATB – Advanced Technology Baseline 

ATC – Air Traffic Control 

BLD – Boundary-Layer Diverter 

BLI – Boundary-Layer Ingesting 

BWB – Blended Wing/Body 

CDA – Continuous Descent Approach 

CESTOL – Cruise Efficient Short Takoff and Landing 

CFD – Computational Fluid Dynamics 

CML – Continuous Mold Line 
 5

CMT – Continuous Mold-Line Technology 

DARPA – Defense Advanced Research Projects Agency 

EIS – Entry into Service 

EPNL – Effective Perceived Noise Level 

FAA – Federal Aviation Administration 

FEM – Finite-Element Model 

GRC – Glenn Research Center 

HIDEC- Highly Integrated Digital Electronic Control 

HPC – High-Pressure Compressor 

HPT – High-Pressure Turbine 

IGES – Initial Graphics Exchange Specification 

IGV – Inlet Guide Vane 

LE – Leading Edge 

LPT – Low-Pressure Turbine 

MDO – Multidisciplinary Design Optimization 

NAS – National Airspace System 

N-S – Navier-Stokes 

NTF – National Transonic Facility 

OEW – Operating Empty Weight 

OGV – Outlet Guide Vane 

OML – Outer Mold Line 

P & W – Pratt & Whitney 

QAT – Quiet Aircraft Technology 

R & D – Research and Development 

RDT&E – Research Development Test & Evaluation 

SAI – Silent Aircraft Initiative 

SME – Subject-Matter Expert 

SFW – Subsonic Fixed Wing 

TE – Trailing Edge 

TOGW – Takeoff Gross Weight 

TRL – Technology Readiness Level 

TSFC – Thrust Specific Fuel Consumption 

UHB – Ultra-High Bypass 

2D – Two-Dimensional 

3D – Three-Dimensional 
 6

3.0 Background 

The SAI project was launched by the Cambridge-MIT Institute in 2003 with an initial 

grant of £2.3M and was co-led by Professor Ann Dowling of the Engineering Department at 

Cambridge University and Professor Ed Greitzer of Aeronautics and Astrophysics at MIT (ref. 

1). The objective of the project was to discover concepts to dramatically reduce aircraft noise to 

the point at which the noise would be imperceptible outside of the airport perimeter. The team 

was organized to emphasize a collaborative, multidisciplinary approach involving academia, 

industry, and government. The team proceeded through three major design evolutions, producing 

the SAX-10, -20, and, finally, -40. See Figure 1 for depictions of the three designs. 

The first generation design, referred to as the SAX-10 (or SAX-12) was developed using 

an evolved version of a Boeing design tool called WingMOD (ref. 2). This concept was 

optimized by seeking minimum takeoff gross weight (TOGW) and utilized four “Granta-252” 

concept engines, which featured a geared low-pressure turbine (LPT) and boundary-layer 

diverters. This concept underwent a first round of industry nonadvocate reviews. The second 

generation, referred to as the SAX-20 (or SAX-29), benefited from a quasi-three-dimensional 

(3D) airframe design methodology and was optimized for low stall speed to reduce noise. The 

propulsion concept utilized three “Granta-3201” engine clusters with boundary-layer ingestion; 

each cluster included a single core that drives three fans through a gear and transmission design. 

This concept underwent a design review with Boeing and benefited from Boeing-led 3D viscous 

aerodynamic analysis. The final concept, the SAX-40, features an optimized outer wing that was 

optimized with 3D design methodology and a refined engine design referred to as “Granta-3401.” 

The refined design retained the SAX-20 concept of three engine clusters, each comprising a 

single core that drives three fans, and focused on the design of the transmission system. The use 

of boundary-layer ingestion was retained as well. This concept underwent another round of 

industry nonadvocate reviews and is the concept that was presented to the public in the fall of 

2006 and early 2007. References 3-8 provide a comprehensive description of the SAX-40 

concept. 

The SAX-40 utilizes a broad set of enabling technologies to meet its design objectives. 

The BWB planform provides engine noise shielding in the forward sector, increased low-speed 

capability, and efficient cruise performance. The high-lift system is designed to minimize noise 

and utilizes a deployable drooped leading edge (LE) and an advanced airfoil trailing-edge (TE) 

treatment. The large wing area and high angle of attack on approach eliminate the need for flaps. 

Takeoff and approach noise are greatly reduced through the combined use of faired 

undercarriage, quiet drag generation via increased induced drag, optimized takeoff thrust 

management, and low noise approach procedures. In addition, advanced propulsion technology is 

assumed in the form of the unconventional distributed propulsion concept of a single core that 

drives three fans. Three of these engine clusters are embedded in the aft section of the airframe 

and are fitted with variable-area, thrust-vectoring nozzles. The embedded engines are designed to 

ingest the boundary layer, thus increasing propulsive efficiency. 

Figure 1. Three generations of the SAI conceptual design.
 7

Clearly, a large array of advanced technologies have been assumed in the development of 

this concept. The risk assessment process is described in the next section, including the 

methodology that is utilized to characterize the level of risk that is present in this concept and to 

identify the highest risk areas to inform technology investment decisions. 

4.0 Risk Assessment Process 

Because many of the technologies that are mentioned above have been linked in the past 

to the BWB concept (e.g., noise shielding, embedded BLI engines, advanced high-lift systems), 

the risk assessment began with a literature search that was focused on the BWB concept. A large 

volume of material is available in this area, and the earliest references date to the mid-1980’s. A 

BWB chronology was created (Appendix B) that summarizes approximately 60 BWB references 

that focus on system-level studies. Numerous other references were found that address individual 

discipline areas; however, to keep the effort manageable not all of these were included in the 

chronology. The results of this literature search indicate that the BWB configuration arose 

primarily as a result of aerodynamic considerations that were associated with the reduction of 

drag by minimizing wetted area. Consequent challenges in the areas of structures, propulsion, and 

stability and control have been the subject of a large research effort over the years. The BWB 

configuration has been considered for a wide variety of applications, beginning with a large (800-

passenger) commercial transport, then a smaller (450-passenger) transport, and then, briefly, as a 

potential sonic cruiser solution. Shortly thereafter, the BWB was studied for its suitability as a 

large commercial freighter; as the basis for a family of various-sized commercial transports with 

high commonality; and as a military tanker, bomber, and cargo aircraft. For its application, SAI 

utilized the BWB configuration for a 215-passenger, low noise commercial transport, and 

indications are that the BWB is being considered for the role of a relatively small, high-value 

commercial package carrier. The BWB configuration appears frequently as a sample application 

problem for multidisciplinary design optimization (MDO) methodology development. Because of 

the lack of full-scale test data, validation is a major challenge, and relatively few system-level 

studies have been performed. Furthermore, few of these studies utilize advanced-technology 

baselines for consistent comparisons. 

With the literature search providing context for the current effort, the next step was to 

define the risk assessment focus, that is, identify the risk to be assessed. The decision was made 

to assess the risk of the SAX-40 concept in meeting the SAI requirements. A sample risk 

statement would then be, “If the actual airframe shielding benefits are less than predicted, then 

attainment of the noise requirements will be jeopardized.” A risk assessment includes an 

estimation of the likelihood of the risk statement being true and a characterization of the 

consequence if the risk statement is true. To perform this assessment, the SAI requirements were 

necessary to provide the team with a consistent basis for evaluation. A brief summary of the SAI 

requirements was developed in conjunction with the SAI team (see Appendix C). The noise 

requirement was identified as “the aircraft must be inaudible outside of a typical airport 

boundary.” A goal value of 60 dBA was selected, and a typical airport boundary was defined in 

terms of distances from the sides and ends of the runway. A 2025 Technology Readiness Level 

(TRL) of 6 (i.e., a system or subsystem prototype is demonstrated in a relevant environment) was 

assumed for the SAX-40. As such, the known technology investments that are planned for the 

timeframe between now and 2025 may be used to reduce risk; however, if no known current or 

planned research efforts exist, then the 2025 technology level is assumed to be similar to today. 

The mission requirements called for carrying 215 passengers in a three-class configuration on a 

5000-nm-range mission with a cruise speed of M = 0.8 and a reserve fuel that was adequate for a 

200-nm divert and 45 minutes of loiter. Operational requirements included an approach speed of 
 8

60.8 m/s (118 knots), a continuous descent approach on a 3.9 degree glide slope, and a 1.2 km 

(3937 ft) displaced threshold on the runway. No explicit requirements were stated for emissions 

or costs; however, the general theme was that they should be no worse than competitor aircraft. 

The next step was to identify a risk matrix and develop the appropriate definitions for the 

likelihood and consequence metrics. Figure 2 shows the risk matrix that was selected: a 

conventional five-by-five layout with low-, medium- and high-risk areas that are defined by 

green, yellow, and red, respectively. 

The likelihood definitions ranged from not likely (rating = 1) to near certainty (rating = 

5); the incremental steps between were low likelihood (rating = 2), likely (rating = 3), and highly 

likely (rating = 4). The consequence definitions were tied to the ability to meet the requirements. 

The consequences for each risk area ranged from minimal or no impact in meeting the 

requirement(s) (rating = 1) to unacceptable shortfall where less than 60 percent of the 

requirement(s) can be met (rating = 5). Incremental steps within this range are shown in Table 1. 

Next, based on a review of the SAI-related literature and several working sessions, a list 

of 27 risk items was developed. The full matrix of risk items is provided in Appendix D. These 

risk items are summarized by group. 

Figure 2. Five-by-five risk matrix.

Table 1. Definitions of Consequence Metrics

Key

G

Y

R

= low risk

= medium risk

= high risk

Key

G

Y

R

= low risk

= medium risk

= high risk

1

2

3

4

5

Significant shortfall, ~60 - 75% of requirement(s) can be met

Unacceptable shortfall, < 60% of requirement(s) can be met

Consequence

Minimal or no impact in meeting requirement(s)

Minor shortfall, ~90 - 95% of requirement(s) can be met

Moderate shortfall, ~75 - 90% of requirement(s) can be met
 9

Propulsion: 

Noise: 

Aerodynamics: 

Layout/Human Factors: 

Risk ID Risk title

5 BWB configuration: human factors

6 BWB configuration: internal layout

Risk ID Risk title

7 Boundary-layer ingestion/inlet design

8 Variable-area exhaust thrust-vectoring nozzle

9 Transmission system

10 Single-core/multiple-fan concept

11 Propulsion-airframe integration/buried engines

12 Low flight idle thrust

13 Low-speed fan with forward swept blades

14 High-pressure compressor design

15 Low noise low-pressure turbine design

Risk ID Risk title

16 Trailing-edge brushes

17 Low noise undercarriage

18 Quiet drag

19 Long ducts with acoustic liners

20 Airframe shielding

24 Overall noise estimates

Risk ID Risk title

1 Cruise aero performance

2 Deployable drooped leading-edge and continuous 

Mold-line elevons

3 Stability and control
10

Operations: 

Structures: 

Cost, Emissions and Experimental: 

Each of these 27 risk items was assigned to a subject-matter expert (SME). The SME was 

asked to review the relevant literature and provide an evaluation of the risk statement in terms of 

the likelihood and consequence metrics that are defined above. In addition, the SME’s assembled 

a list of questions to be provided to SAI that would help to clarify assumptions and provide 

support for the evaluation (see Appendix E for the list of SME questions). Unfortunately, because 

of resource constraints, SAI was unable to provide any further clarification beyond the literature 

that has already been published. The inputs that were collected from the SME’s contain 

descriptions and background information on the risk, an evaluation of the metrics and the 

rationale for the scoring, a recommendation on how to address the risk (either by avoiding, 

transferring, assuming or mitigating the risk), and references. The results were compiled and are 

presented in the next section. 

Risk ID Risk title

21 Thrust-managed takeoff

22 Displaced threshold and CDA

with increased approach angle

Risk ID Risk title

4 BWB configuration: structures, weight

25 Aeroelasticity

Risk ID Risk title

23 Cost

26 Emissions

27 Experimental
11

5.0 Results 

Figure 3 presents the populated risk matrix. The figure shows the number of risks that fall in each 

section of the matrix. As of this report, 25 of the 27 risk items have been evaluated (#3 Stability 

and Control and #25 Aeroelasticity were not evaluated because of lack of SME availability). 

Seven risks fell into the high-risk (red) area of the matrix, and another nine fell into the mediumrisk (yellow) area. Before proceeding into a full-scale development effort, a prudent program 

manager would require that all high-risk and most medium-risk items be avoided, transferred, or 

mitigated into the low-risk area. 

A brief discussion of each risk item follows. 

5.1 High-risk items 

 5.1.1 Risk #8, Variable-area exhaust thrust-vectoring nozzle 

 Likelihood = 5, Consequence = 5 

Figure 3. Populated SAX-40 risk matrix.

Consequence

Likelihood

1 2345

1

2

3

4

5

1 1

1

3

1

4

3 2

1

4

1 1

1

Variable-area exhaust and

thrust-vectoring nozzle

Human factors

Displaced threshold and

CDA with increased

approach angle

BWB configuration:

structures, weight

BLI/inlet design

Cost

1 Overall noise performance

Consequence

Likelihood

1 2345

1

2

3

4

5

1 2345

1

2

3

4

5

1 1

1

3

1

4

3 2

1

4

1 1

1

Variable-area exhaust and

thrust-vectoring nozzle

Human factors

Displaced threshold and

CDA with increased

approach angle

BWB configuration:

structures, weight

BLI/inlet design

Cost

1 Overall noise performance
12

Risk statement: If the variable-area/thrust-vectoring nozzle performance is less than 

predicted, then attainment of the mission and noise requirements will be jeopardized. 

Variable-area nozzle: At takeoff conditions, the aircraft requires a 45-percent increase in 

fan nozzle area (ref. 6). The movement of nozzle flaps for the purposes of changing both 

throat and exit area is technology that is routinely employed on high-performance 

military aircraft. Actuator sizes, as well as the upper and lower flap size and weight for an 

aircraft of this size (relative to high-performance military aircraft) may be significant, but 

the mechanical technology is off the shelf. State-of-the-art mechanical nozzle-area 

variation is approximately 20 percent (as compared with the 45 percent that is required by 

the SAX-40 concept). The integration aspects will be the issues that make or break such a 

concept because the use of these systems for high-bypass-ratio subsonic engines is 

uncommon. The items noted above, coupled with design issues such as actuator location, 

size, and weight, elevate the risk. 

Thrust-vectoring nozzle: The same technology that is required to make area changes on 

the nozzle flaps can also provide the ability to vector the exhaust flow. The F-22 (± 20 

degrees of vectoring) employs movable two-dimensional (2D) divergent flaps for both 

nozzle-area-ratio control and thrust vectoring. The differences here are the sizes and 

possibly the loads on very large flap systems and whether those actuation systems can be 

mechanically integrated into either the vehicle structure or the engine structure. This is 

mitigated somewhat by the lower operating temperatures and pressures of the highbypass-ratio engines. Regardless, this is not a straightforward integration. 

Additional risk is associated with the aeropropulsive interactions of trimming when 

vectoring nozzles occupy a large amount of the TE of the center wing. A vortex lattice 

method was used to determine the lift, drag, and moment characteristics; however, the 

vectored engines behave as jet flaps, and the literature is not clear on whether this effect 

was taken into account. This effect could be estimated by modifying the SAX-40 

planform with extended TE flaps, but the literature contained no indication that this was 

done. Even at low idle, the effect on the lift, drag, and moment of the configuration could 

be dramatically changed, thus invalidating the force and moment balance. 

Variable-area nozzle and thrust vectoring are critical aspects of the SAI strategy. The SAI 

team acknowledged that no mechanical design details exist for either the variable-area 

nozzle or the thrust-vectoring mechanisms. The SAX-40 aircraft will not meet 

performance and noise goals without the technology. Therefore, the risk associated with 

integrating a variable-area thrust-vectoring nozzle with the airframe must be mitigated. 

Recommendation: Mitigate the risk. 

Mitigation strategy: The variable-area nozzle and thrust-vectoring technologies are 

mechanically feasible. The risk arises from the integration of the nozzle and the actuators 

into the aircraft. To properly evaluate the integration of the nozzles into the airframe, 

detailed prototype designs must be evaluated. No design work has been performed on the 
13

nozzle system to date. The first step in mitigating this problem will be to initiate a 

contract with an engine manufacturer to propose and evaluate several candidate 

mechanical nozzle designs that incorporate a variable-area thrust-vectoring system. 

 5.1.2 Risk #22, Displaced threshold and CDA with an increased approach angle 

 Likelihood = 5, Consequence = 4 

Risk statement: If the use of a displaced threshold and CDA with an increased approach 

angle have penalties, then attainment of the noise and operations requirements will be 

jeopardized. 

A number of operational issues exist with the use of CDA approaches and a displaced 

threshold. An additional factor is the slow approach speed compared with that of 

conventional commercial jet traffic. The main issues are summarized here: 

An approach angle of 3.9 degrees is not compatible with conventional traffic that uses 3.0 

degree approaches. Current-day conventional air traffic control (ATC) procedures cannot 

mix like traffic (i.e., traffic in the same category, such as commercial jet transport) that 

are arriving at different approach angles. 

An approach speed of 118 knots is as much as 28 percent slower than current commercial 

jet aircraft of similar size (ref. 4). Large differences in approach speeds make optimizing 

approach spacing difficult, and slower approach speeds reduce runway arrival rates. 

Future flight-deck-based technology designed to provide precise spacing capability could 

make this less of an issue, but runway arrival rates will still be reduced by slow 

approaches. The effect on runway arrival rates is estimated in reference 4: 

"The loss of capacity due to an approach speed of 118 knots, the approach speed of the 

Silent Aircraft, could be between 5 and 20% depending on the traffic mix and buffer 

size." 

Displaced landing thresholds reduce the safety margin for pilot errors and mechanical 

failures (e.g., brakes, reverse thrusters), and may not be compatible with existing taxiway 

exits, runway markings, lights, ramp accessibility, and so on. However, displaced landing 

thresholds have been successfully used at Frankfurt airport (for enhanced wake 

avoidance). The shorter landing distance of the SAX-40 mitigates these issues to some 

extent. Displaced landing thresholds cannot be used with the shorter runways at many 

smaller airports. 

Instrument approach procedures would need to be developed for the unique approach 

path. This would require time for procedure development and certification, in addition to 

costs that might have to be partially absorbed by the sponsoring airline or airport. 
14

The likelihood is low that SAX-40 operations, with a combination of significantly slower 

approach speeds, steep CDA descents, and displaced thresholds, could be successfully 

integrated with conventional traffic during peak hours at busy airports in current-day 

operations. A loss of 5 percent or more in capacity would not be acceptable during these 

periods, and this loss is one of the big obstacles to the acceptability of CDA’s (even 

conventional ones that use standard glide slopes) in higher density facilities. 

No issues are anticipated outside of peak operating hours, however. If the SAX-40 

aircraft could make use of night operations (as a result of its quieter flight operations), 

use lower-density airports, or utilize a dedicated runway (which would require significant 

SAX-40 traffic to justify), many of the concerns expressed here could be alleviated. 

Future technologies can potentially mitigate the current-day problem with mixing 

conventional and nonconventional approaches. Greater use of CDA’s for conventional 

aircraft, perhaps with steeper glide slopes, is also possible in the future, but capacity loss 

as a result of the low-speed approach of the SAX-40 could still be a problem. 

Assessing the risk that is associated with SAX-40 operation in future air-traffic 

environments requires extrapolation of the stage of deployment of technologies that are 

currently in the research and development phase. National Airspace System (NAS)-wide 

deployment of some of the enabling technologies, such as ADS-B (Automatic Dependent 

Surveillance-Broadcast) is imminent. However, some of the tools that would enable 

SAX-40 operations (such as airborne precision spacing in conjunction with custom CDA 

approaches) require several years of further research. In the far term, the unique 

operational profile of the SAX-40 aircraft potentially could be more easily 

accommodated. 

Recommendation: Avoid the risk. 

The aircraft should be designed to operate in the range of conventional jet-transportcategory aircraft. If possible, retaining the slow-speed approach capability in addition to 

the conventional speed, for use at nonpeak times and less busy airports would be 

worthwhile if the additional cost is justifiable. 

The aircraft should be designed so that using a non-CDA approach without displaced 

thresholds still provides a worthwhile noise reduction benefit. Thus, the aircraft can fly 

with conventional approaches into large airports at busy times with a mix of conventional 

jet traffic. At off-peak times or at less busy airports, the SAX-40 can use a slow-speed 

approach, CDA, and displaced threshold where runway length permits. The SAX-40 

potentially could be used at night at some airports that have curfews or quotas on night 

operations. 
15

 5.1.3 Risk #4, BWB configuration: structures, weight 

 Likelihood = 4, Consequence = 4 

Risk statement: If the SAX-40 structural and subsystem weights are greater than 

predicted, then attainment of the mission requirements will be jeopardized. 

The SAX-40 center-body design is a noncircular pressure vessel, most likely with 

inefficient packaging of useful volume (i.e., cabin, cargo) within the vehicle outer mold 

line (OML). Accurate structural weight estimation of the primary structure (i.e., pressure 

vessel, wing) and the secondary structure (e.g., the structure between the pressure vessel 

and the OML) is a challenging problem. A number of the enabling technologies (ref. 5) 

that are necessary to achieve a viable design also pose some risk for structural weight 

prediction. The deployable drooped LE of the wing is an unknown contributor to both the 

structural and subsystem weight. Flap elimination will probably reduce the weight in 

comparison with traditional wings, but the large wing area may add more weight. 

Undercarriage fairings for noise reduction will add weight. Propulsion system integration 

with the airframe and distributed propulsion may reduce the weight. Analysis of the 

effects of the integration of engine acoustic liners within the airframe, structural analysis 

of the noncircular pressure vessel, and mechanical design of subsystems (i.e., thrust 

vectoring) were not performed. Fabrication and manufacturability (ref. 5) of noncircular 

pressure vessels and their impact on preliminary weight estimates are unknowns. 

Empirical correlations have been used extensively to predict the SAX-40 weight (“...the 

structural weight calculation was based on empirical formulae, which yields a serious 

challenge...,” ref. 9). A great deal of empirical data exists for the calculation of the 

weights of wings, tails, landing gear, and every other component on a conventional 

commercial aircraft. However, the structure of the SAX-40 has many differences from 

that of conventional commercial aircraft. The BWB design incorporates a noncylindrical 

pressurized cabin into the center wing, highly tapered outer wings, and large winglets 

with control surfaces at the wingtips. These structures are not modeled well by existing 

empirical fuselage and wing weight estimation tools. For this reason, several WingMOD 

(ref. 2) designs were used to produce a least-squares quadratic response surface model. 

This surface was then used to predict the structural weight of the SAI aircraft. 

In the BWB (refs. 10 and 11) noncircular pressurized fuselage, the stress levels may be 

one order of magnitude higher than those for a conventional design (ref. 12). The high 

stress and weight problem that is associated with the BWB pressurized cabin can be 

explained by examining Figure 4. 
16

Figure 4 illustrates both a cylindrical and a square-box fuselage under internal pressure p. 

In a cylindrical pressure vessel of radius R and skin thickness t, the pressure is resisted by 

uniform stretching; the resulting membrane stress is equal to p(R/t). In the BWB box-like 

fuselage, the nearly flat upper cabin wall resists the pressure by bending deformation. Let 

us model the flat upper cabin wall as a simply supported beam or plate of length l, with 

thickness t; then, the maximum bending stress is equal to 0.75p(l/t)2. If we assume that R

is of the same order as l, then the bending stress is one order of magnitude higher than the 

membrane stress. The problem is aggravated by the nonlinear effect of the compressive 

load as it acts on the deflected beam or plate; hence, significant effort must be made to 

design an efficient structure with a minimal weight penalty that results from the 

noncylindrical fuselage. 

Recommendation: Mitigate the risk. 

Mitigation strategy: To obtain an efficient structure, one must increase the bending 

stiffness without increasing the weight. Several alternative concepts must be developed 

and compared to determine the best approach. The options include a deep sandwich shell 

with lightweight, high-strength composite skin or a stitched composite frame and 

stiffener construction. Durability, fracture toughness, and manufacturing issues play a 

major role. One manufacturing process that appears highly promising is the use of 

stitched composite technology. Additional options include the use of a multi-bubble or 

multi-lobe concept, as shown in the inset sketch in Figure 4. With proper design, the 

resultant adjacent bubble membrane stress could be balanced by tension in the intra-cabin 

wall. However, because of manufacturing concerns, the multi-bubble and deep sandwich 

constructions may be high risk. Significant research and development investment is 

required to develop and test these alternatives. 

Figure 4. High bending stress associated with a noncylindrical pressure vessel. 

Noncylindrical pressure vessels

Pressure p

R

t

membrane σ bending = O(pR/t) σ = O[p(R/t)2]

typical: R/t = 1000 (R/t)2 = 106

2R t

Pressure p

Containing pressure in a shoebox is difficult!
17

 5.1.4 Risk #7, BLI and inlet design 

 Likelihood = 4, Consequence = 4 

Risk statement: If the SAX-40 inlet design and use of BLI does not result in the 

predicted performance, then attainment of the mission and noise requirements will be 

jeopardized. 

Several issues and concerns are associated with this risk item: 

1. The SAX-40 literature has provided no indication that the SAX-40 employs 

inlet flow control to reduce distortion levels down to acceptable values. 

Numerous studies have suggested that for this class of subsonic inlets, where a 

thick boundary layer is being ingested and where the inlets are offset vertically 

and transitioning in geometry from a D shape to a circular shape, some type of 

flow control within the inlet duct is required to reduce distortion (ref. 13). The 

use of passive control devices (e.g., vortex generators), active flow control (e.g., 

jets), or a combination of the two have been used to achieve significant 

reductions in inlet flow distortion for this class of inlets. Without such devices, 

those studies have indicated unacceptably high levels of inlet distortion. 

2. To complicate things further, the current propulsion system configuration has 

three main inlets with D-shaped apertures, each of which feeds three fans that are 

powered by a single core engine. The distortion characteristics of such a 

configuration are cause for concern, as referred to in item 1, as is the “sharing” of 

the inlet airflow amongst the three fans within each inlet duct. This sharing is of 

particular concern during crosswind and engine-out-climb operations and when 

one or two of the three fans are not operating. 

3. The concerns expressed in items 1 and 2 relate to both dynamic distortion and 

steady-state distortion and, as a consequence, lead to additional concerns about 

high-cycle fatigue on the three-fan/duct system. 

4. Finally, the required inlet flow-control devices will likely need to be integrated 

with the acoustic liners on the walls of the inlet duct. The aerodynamic 

performance of the flow-control devices and the acoustic performance of the 

liners may be affected by this integration and will need evaluation, as will the 

affect of the distortion on noise. 

All previous studies point to the absolute need for some type of inlet flow control 

for boundary-layer ingesting, shape-transitioning centerline-curving subsonic 

inlets to mitigate steady-state and dynamic inlet distortion. With the added 

complication of each of the three inlets feeding three fans, the need for such flow 

control is probably even more necessary. By not including an inlet flow-control 
18

strategy in the current concept, the likelihood of unsatisfactory inlet performance 

is high. Other factors that add to the current high risk are the lack of 

consideration of high-cycle fatigue on the fan system and the effects of the 

interaction between flow control and the acoustic liners. 

BLI is assumed to be of significant benefit to achieving improved engine 

propulsive and aerodynamic efficiency and, thus, reduced fuel burn. The current 

approach of seemingly ignoring the need for inlet flow control to properly 

manage the incoming boundary layer and, hence, provide acceptable levels of 

flow distortion to the fan system significantly increases the likelihood that the 

concept of BLI will not be successful and the benefits not achieved. 

Recommendation: Mitigate the risk. 

Mitigation strategy: 

1. Consider appropriate inlet flow control devices and then perform 

computational fluid dynamics (CFD) studies to further develop the concept 

details for this highly complex inlet design. 

2. Perform experimental verification of the resulting concept(s) in both lowspeed (takeoff and approach) and high-speed (cruise) wind tunnels. 

3. Demonstrate acceptable inlet performance (recovery, steady state, and 

dynamic distortion) to ensure the success of the concept. 

 5.1.5 Risk #23, Cost 

 Likelihood = 4, Consequence = 4 

Risk statement: If the costs of developing, producing, and operating the SAX-40 

outweigh the benefits, then attainment of the mission requirements will be jeopardized. 

The BWB is not the typical “tube with wings” concept, so the cost of RDT&E will be 

relatively significant. Aircraft costs are historically highly correlated with weight. As 

shown in the next section regarding the development of an ATB design, the SAX-40 

would likely outweigh the ATB by a significant margin (and, assuming SAX-40 

feasibility, the SAX-40 would also be significantly quieter). Therefore, the RDT&E, as 

well as the production costs, will likely be relatively high for a significant portion of the 

life cycle of the vehicle. Some of these increased costs may be offset if airports assess 

noise fees or if night operations are increased. 

Additionally, the engine placements above the wing, the fact that the engines are 

embedded in the airframe, and the fact that multiple engines and fans are used, will 

significantly impact the maintenance costs. Maintenance cost is a function of required 
19

labor hours and the amount of down time and turnaround time when the aircraft is on the 

ground and not serving customers. Several smaller engines and fans are more difficult to 

maintain than one larger engine because the many parts are smaller and harder to inspect. 

Because the engines are embedded in the airframe, the small, multiple parts are harder to 

reach, inspect, and replace when faulty. Finally, the SAX-40 engines are completely new 

technology and have a degree of complexity inherent to them, which makes the 

maintenance process more specialized and, therefore, more costly, at least for the first set 

of aircraft units. 

Recommendation: Mitigate the risk. 

If lower operating costs (because of lower noise and increased fuel efficiency) offset the 

increase in procurement and maintenance costs, then the SAX-40 concept may prove to 

be economically viable. Tradeoffs between performance and cost, as well as cash flow 

and life-cycle cost analyses, should be performed. Activity-based cost estimations should 

be developed for this type of unconventional vehicle to quantify the costs of new 

technologies, new maintenance procedures, and new material production. 

 5.1.6 Risk #24, Overall noise estimates 

 Likelihood = 4, Consequence = 4 

Risk statement: If the overall noise performance is less than predicted, then attainment of 

the noise requirements will be jeopardized. 

Although the SAI requirements were not stated in terms of certification noise, the 

implication is that the design would be certified to operate in the commercial airspace 

with existing and future aircraft. Therefore, several concerns are associated with this risk, 

primarily the following: 

1. The noise calculations require changes to the operational rules in the terminal 

area (e.g., displaced threshold, use of a variable-area nozzle on takeoff) and also 

assume that “credit” can be obtained for those techniques during certification. 

Permission would have to be granted before credit for such procedures would be 

allowed; additionally, the likelihood that permission for a displaced threshold 

touchdown would be granted in typical air traffic scenarios is questionable at 

best. (Reference risk #22.) 

2. The landing gear may not be “buildable.” (Reference risk #17.) No hydraulic 

lines are visible, and the fairing goes over the top of the wheels (like a car 

fender). Many design, fabrication, and operational issues are associated with this 

concept, including cooling, brakes, and so on. 

3. No effective perceived noise level (EPNL) calculation details are available. 

If the likelihood of obtaining credit for the landing procedure alone is 
20

questionable, then the likelihood of obtaining approval for the variable-area 

nozzle is even less. With the additional reduced benefits from a realistic quiet 

gear design, this risk is significant. If certification “credit” for the SAX-40 for 

either the displaced threshold landing or the variable-area nozzle on takeoff 

cannot be obtained and if the low-noise gear benefits are reduced, then a 

significant shortfall will exist with respect to the projected noise benefit. 

Recommendation: Assume the risk. 

Even if only two-thirds of the predicted noise benefit is achieved, the aircraft will still be 

relatively quiet. Additionally, even if credit for the variable-area nozzle is not given for 

certification, the capability potentially may still be allowed operationally, resulting in 

noise-reduction benefits to communities that are not reflected in the aircraft’s 

certification numbers. 

 5.1.7 Risk #5, Human factors 

 Likelihood = 3, Consequence = 5 

Risk statement: If the SAX-40 human-factors characteristics negatively impact the 

concept feasibility, then attainment of the requirements will be jeopardized. 

Human factors comprises many elements, some that are interrelated and others that are 

independent. Specifically, human factors includes passenger acceptability, which can be 

defined as passenger comfort (e.g., windows, number of seats in a row between aisles), 

ride quality (e.g., wing loading, seat displacement from roll axis), internal noise, and 

vibration. Human factors also includes emergency egress and airport compatibility 

requirements. 

 

The passengers are the ultimate customers; if they are not satisfied, they will not fly. 

Passenger comfort is a subjective metric and may be difficult to assess with confidence. 

Because the SAX-40 will have fewer window seats than a conventional “tube with 

wings” and may contain rows of seats with more than three adjacent seats between the 

aisles, human factor issues must be studied. 

Ride quality is also subjective, because the SAX-40 has not been flight tested. One way 

to measure passenger acceptability is to conduct a public survey; however, if such data do 

not exist, a Web site forum can provide a feel for public reaction on a topic or concept. A 

current Flight International Web site discussion post indicates a distinct concern 

regarding ride quality in the public opinion (ref. 14). Some challenges in this area have 

also been identified in previous BWB studies. The general geometry of the BWB lends 

itself to an unsteady ride. The vehicle’s inherent vulnerability to turbulence results from 

its low wing loading and the placement of all of the control power along the same 

surface. However, according to Liebeck (ref. 15) the ride quality of a 450-passenger 

BWB concept is not much worse than that of a conventional design. The analysis 
21

presented in reference 15 was performed using NASA Langley Research Center’s 

motion-based flight simulator, but not much information was provided on the details of 

the test or the assumptions involved. 

 

No analysis was performed on cabin noise or vibration levels. However, with the engines 

placed directly behind the passenger cabin and embedded in the airframe, these issues 

will have to be considered during design. Significant acoustic liner technologies will have 

to be implemented to reduce the reverberation of forward fan noise within the vehicle’s 

structure. 

 

Although the remaining two concerns relate to the passengers, they are subject to Federal 

Aviation Administration (FAA) regulations rather than passenger opinion. The first 

concern is the emergency egress regulation, which requires that all passengers be able to 

exit the plane within 90 s and that an emergency exit be located no further than 60 ft 

away from each passenger seat (ref. 16). The cabin size of the SAX-40 is less than 100 ft 

long and approximately 40 ft wide; thus, even if the emergency exits are located only in 

the front and back of the aircraft, the distance from each seat to an exit will be less than 

60 ft (ref. 4). We can safely assume that, after the detailed layout of the cabin is known, 

simulations and algorithms can be used to optimize the layout to meet the egress 

requirements (ref. 17). 

Airport compatibility requirements can be satisfied by the SAX-40 concept in terms of 

runway and taxiway separations. However, the airport design and gate operations will 

most likely require modifications to obtain optimal passenger boarding and aircraft 

servicing. Boeing proposed an x-shaped terminal for the large 800-passenger BWB (ref. 

18). A similar or star-shaped terminal may also be appropriate for the SAX-40. 

Recommendation: Mitigate the Risk. 

Mitigation strategy: 

1. Perform flight tests or high-fidelity flight simulations to ensure that the ride 

quality is acceptable. 

2. Conduct a survey and simulations with test subjects to assess the cabin layout 

comfort level. 

3. Execute an acoustic/vibration test to simulate the effects of embedding the 

engines and the effects of noise that is propagated through the aircraft structure 

both with and without acoustic liners. Ensure that the acoustic liner that is chosen 

for SAX-40 is sufficient. 

4. Determine the level of airport modifications or adaptations that are required and 

the likelihood that the changes will be made. 
22

5.2 Medium-risk items 

 5.2.1 Risk #1, Cruise aerodynamic performance 

 Likelihood = 4, Consequence = 3 

Risk statement: If the SAX-40 cruise aerodynamic performance is less than predicted, 

then attainment of the mission requirements will be jeopardized. 

The information used in this assessment was obtained primarily from two AIAA papers 

(refs. 5 and 19) that were prepared by the SAX-40 designers and from two Boeing 

proprietary internal presentations that summarize the CFD evaluations of the SAX-30 

configuration and compare it with their BWB-450-1L vehicle (refs. 20 and 21). 

Supplemental information was obtained from interviews with the Boeing principal 

investigator on the project, as well as from a researcher from NASA Langley Research 

Center who was involved in the CFD design and testing of a BWB concept with a 

different propulsion installation. 

 

The following items were identified as contributing to the uncertainty in the published 

cruise performance predictions, based on the methodologies used by the SAI team: 

1) Limitations of the quasi-3D, linear, and inviscid aerodynamic analysis and 

 design methods. 

2) Lack of propulsion system in the aerodynamic predictions. 

3) Lack of realism in wing load capability. 

4) Propulsion-airframe integration and thrust/drag bookkeeping (ref. 3). 

These concerns were in some cases addressed in a partial manner by the SAI team or by 

the Boeing studies but in general did not allow for a full quantitative assessment of the 

impact on the final vehicle performance. These concerns are discussed in more detail in 

the following paragraphs. 

Based on the quasi-3D design methodology used by the SAI team, the SAX-40 vehicle 

has M(L/D) values of 20.1 and 18.8 at the beginning and end of cruise, respectively. Both 

of these values are higher than the M(L/D) requirement of 17.5 to meet the desired range. 

The SAI team recognized the limitations of the methodology and requested that Boeing 

perform Navier-Stokes (N-S) computations on the SAX-29 configuration. (The Boeing 

principal investigator thought that the configuration was that of the SAX-30, so this 

designation is used here for consistency with the Boeing references.) For this 

configuration, the M(L/D) value that was predicted by the CFD was 13 percent lower 

than the value that was calculated by the SAI team. Applying this same factor to the 

SAX-40 values reduced them to 17.5 and 16.4 for beginning and end of cruise, 

respectively, or an average of 16.9 or about 3.4 percent below the required value. 

In the above calculations, both the quasi-3D and 3-D N-S computational models were for 
23

wing/body configurations with no propulsion system. For the Boeing 450-1L 

configuration, similar N-S computations were performed for the basic wing/body 

configuration with both pylon-mounted conventional nacelles and surface-mounted BLI 

nacelles. These computations (and subsequent high-Reynolds-number tests in the 

National Transonic Facility (NTF)) indicated that adding the BLI nacelles resulted in 

about a 4 percent drop in the M(L/D) relative to the baseline wing/body. Given the 

technical challenges that are associated with BLI nacelles, pylon-mounted nacelles may 

need to be used instead. This propulsion system integration approach is preferred by 

Boeing, and for their 450-1L vehicle the NTF tests indicated approximately a 7 percent 

penalty in M(L/D) relative to the BLI nacelles. 

Note that the SAX-30 has TE closure angles that are at least twice as large as those for 

the 450-1L over the inner 40 percent of the span, even though the airfoil thicknesses are 

comparable in this region. Some of the original BWB configurations at Boeing (Douglas 

at that time) had large closure angles that led to flow separation when the nacelles were 

added. This effect contributed to the requirement for extending the central body region 

further aft to reduce the angles, even though it added wetted area. This extension appears 

to be one of the planform differences between the SAX-40 and 450-1L configurations 

that may need to be added into the SAX-40, regardless of which propulsion approach is 

used. The additional wetted area should increase the drag, but a quantitative estimate of 

the M(L/D) penalty was not attempted. 

The final area of concern is the feasibility of using an elliptical spanload, especially 

because the outboard airfoils are approximately 10 percent thinner than those on the 450-

1L. Several modern transports have a more triangular loading that increases the induced 

drag at cruise but is based on consideration of wing bending moment (and the resulting 

structural weight), as well as approach and high-speed buffet aerodynamic characteristics. 

The 450-1L design included a fairly detailed look at these areas and has the more typical 

triangular loading. Some of this difference is attributable to the undercut nose of the 

SAX-30 body, which provides a moment balance for greater aft and/or tip loading, but 

does not account for these other concerns. Note that the outer wing airfoils on the SAX30 are strongly supercritical, with considerable aft loading. Achieving this loading 

requires a lower surface cove that results in reduced thickness near the aft end of the 

airfoil. For example, the thickness of the SAX-30 airfoil at 80 percent of the semispan 

and at an x/c = 0.85 is less than half of the thickness of the corresponding 450-1L airfoil 

at the same chordwise location. The high aft loading and thin airfoil section might prove 

unrealistic from a structural and control surface loading requirement. Although the 

Boeing studies did not individually address these items, some of the results inferred that 

shifting the spanload toward a triangular shape and using thicker airfoils with less aftloading would reduce the M(L/D) by 5 to 10 percent. These results were obtained for the 

SAX-30 configuration, which appears to have the same basic airfoil and loading 

characteristics as the SAX-40. 
24

The cumulative impact of these concerns suggests that a more reasonable estimate of the 

performance of the SAX-40 with the propulsion system installed, a more triangular 

spanload, and thicker outboard airfoils might fall 10 to 20 percent short of the desired 

M(L/D) of 17.5. Much of this deficit could potentially be recovered through the 

application of fully 3D, viscous design methods. Such methods could also be used to 

address the need to extend the body airfoils to reduce the aft closure angles to alleviate 

any flow separation that is related to propulsion/airframe integration. 

Recommendation: Mitigate the risk. 

Mitigation strategy: 

1) Redesign the SAX-40 wing/body. 

a) Obtain the geometry for the current SAX-40 configuration and any 

design constraints, such as the cabin region and wing volume. 

b) Redesign the configuration using 3D CFD design methods. 

2) Assess and redesign for aerodynamic/structural issues. 

a) Evaluate the off-design performance (e.g., buffet, approach) for the 

SAX-40 wing/body. 

b) Evaluate the impact of parametric variation in spanload, outboard 

airfoil thickness, and reduced airfoil aft loading on the performance of 

the SAX-40 wing/body. 

c) Perform detailed structural analysis to assess the adequacy of the 

current wing to handle the existing spanwise and chordwise loadings and 

make recommendations for loading and thickness changes. 

d) Redesign the configuration to meet these new constraints. 

3) Assess and redesign for propulsion/airframe integration effects. 

a) Obtain the geometry for the SAX-40 with BLI and pylon-mounted (if 

available) nacelles installed. Integration of the pylon-mounted nacelles 

with 2D thrust-vectoring nozzles may be problematic. 

b) Assess the initial impact of the propulsion/airframe integration using 

3D N-S analysis. 

c) Redesign the configuration by using 3D CFD design methods and the 

constraints from 2D body-extension airfoils, if necessary, 

4) Perform wind-tunnel verification of the design. 

a) Fabricate the model from 3(c) for testing in the NTF. 

b) Conduct tests to evaluate cruise and off-design performance. 

 5.2.2 Risk #2, Deployable, drooped LE and continuous mold-line (CML) elevons 

 Likelihood = 2, Consequence = 4 
25

Risk statement: If the deployable drooped LE and CML elevon performance is less than 

predicted, then attainment of the noise and operational performance requirements will be 

jeopardized. 

The issues that are associated with the deployable drooped LE and CML elevons are: 

1. Can these technologies provide the required high lift for takeoff and 

 approach operations? 

2. When deployed, will these technologies meet the noise requirements? 

3. Will the skins, mechanics, and actuators required result in an effective and 

 robust system? 

4. Will the systems be able to meet weight constraints? 

5. Can CML elevons, split elevons, and TE brushes be effectively integrated? 

The “all lifting body” concept eliminates the need for TE flaps (refs. 4 and 8), which are 

a significant noise source, and results in a high approach angle of attack (refs. 11 and 22). 

A slow, steep aircraft approach profile is desirable for meeting noise-reduction goals 

(refs. 4, 19, and 23). A drooped LE is intended to enhance high-angle-of-attack 

performance (ref. 5) while providing reduced airframe noise characteristics. Although 

some efforts to fill a traditional slat cove have not resulted in the desired noise reductions 

(ref. 24), others have succeeded (ref. 25). Researchers are optimistic that further slat 

noise reductions are possible and that a drooped LE may not be required. Increased lift 

from the application of circulation control techniques can further decrease takeoff and 

touchdown speeds and increase departure and approach angles (ref. 9). Continuous moldline technology (CMT) has been researched and developed for some time (ref. 26) and 

has been shown to effectively reduce high-lift system noise without compromising 

performance (ref. 25). Continuous mold-line link, a mechanically efficient, elastic 

structure that connects a deflected flap edge to the adjoining nondeflected wing, has been 

shown in wind tunnel testing to be a very effective technique for flap or slat edge noise 

reduction (ref. 25). Initial studies on TE brushes (ref. 27) have shown the potential for 

significant source noise reduction. 

The ability to combine multiple quiet technologies and quiet operations provides an 

optimistic outlook for the success of attaining the quiet, high-lift operational goals. 

Based on inputs from the noise research community, the indication is that the high-lift 

system noise goals should be able to be met for low-speed, high-lift takeoff and approach 

operations. Recent research on the noise that is generated from LE and TE high-lift 

systems has been promising. Drooped LE technology has already been developed and is 

currently being used on the Airbus A380. CMT and TE brushes are expected to satisfy 

the noise requirements. More of a challenge may exist in the development of the 

materials, structures, and mechanisms that are required for the CML high-lift systems. 

However, a CMT flight demonstration program has been completed for the Air Force 

(ref. 28), which has resulted in the maturation of a continuous control surface concept to 
26

a TRL of 5. The company Continuum Dynamics, Inc. (ref. 29), has demonstrated a 

continuously deformable shape-memory-alloy controlled airfoil section. Flexsys, Inc. 

(ref. 30), has successfully demonstrated flexible wing LEs and TEs in the wind tunnel 

environment. Some risk is associated with the ability to develop a complex split flap 

system that incorporates a CML capability and TE brushes. Several airframe 

manufacturers are working to further develop CMT, and meaningful progress is expected 

to continue to be made in these proprietary programs. The associated drag reduction that 

results from CML systems is another incentive for the continued development of this 

technology. The U.S. Air Force and the Defense Advanced Research Projects Agency 

(DARPA) are funding research on flexible material systems over the next few years; thus, 

further advancements are anticipated. Furthermore, stealth technology often has low 

noise attributes. Because of the military’s ongoing interest in furthering the development 

of stealth technology, the evolution of additional acoustical benefits is likely to follow. 

Recommendation: Two options deserve further consideration. 

1. Transfer the risk: Because of the substantial ongoing efforts in the 

development of CMT in other research organizations, outside research programs 

may assume the risk in the development of a CML elevon. Similarly, efforts by 

military research programs to pursue improved stealth capabilities could lead to 

reduced risk in the achievement of the low noise requirements. 

2. Mitigate the risk: A research plan that incorporates the coordinated assessment 

of aerodynamic high-lift performance, as well as the resulting acoustical 

performance, would be required. This assessment would involve the use of CFD 

to guide experimental wind tunnel verification testing. Iterations on the high-lift 

design would result, based on aerodynamic and acoustical performance results. 

Adequate trimmed lift also would be necessary for takeoff and approach. The 

design, fabrication, and demonstration of a split flap system that employs CML 

capabilities and TE brushes also would be necessary. 

 5.2.3 Risk #9, Transmission system 

 Likelihood = 3, Consequence = 3 

Risk statement: If the engine transmission system performance is less than predicted, 

then attainment of the mission requirements will be jeopardized. 

In general, the inclusion of a gearbox system on any aircraft engine increases the risk of 

system failure and overall engine maintenance and should be avoided whenever possible. 

An enlarged engine oil lubrication system and oil cooler are also necessary and will 

increase engine system weight. On the other hand, inclusion of a gear system can 

decrease overall system weight and improve performance and operability because of the 

improved speed matching of components. In some cases, the gear system can be used to 

reduce noise by reducing propulsor tip speeds. A number of aerospace vehicles, such as 
27

helicopters and turboprops, require the use of gearboxes. These systems have been in 

operation for many years, so the technologies involved are well understood. 

The SAI transmission system is described in reference 6. While the design philosophy is 

outlined and the physical characteristics of the system are given, the actual results for 

efficiency are omitted, and the system weight is incomplete. Thus, evaluation of the 

design is difficult. Previous internal studies conducted at Glenn Research Center (GRC) 

(ref. 31) have shown that power loss can be considerable through multiple right-angle 

gearboxes. Subsequent discussions with Pratt & Whitney regarding gearbox efficiency 

calculations have shown that significantly different values can be obtained using different 

methods, creating uncertainty in the final results. Public references do not adequately 

demonstrate whether the SAI design is conservative or optimistic. However, a cursory 

analysis by Dr. Robert Handschuh indicates that some of the gear meshes in the selected 

SAI design may have been under-designed by as much as a factor of 2. Other poor design 

practices are also evident in the SAI report; however, with proper design this concept 

should be workable. 

Recommendation: Mitigate the risk. 

Mitigation strategy: Further design and analysis is needed to validate the proposed 

design options. Other design options are also available. The appropriateness of the design 

choices are not yet clear, as the analysis is incomplete. The following tasks should be 

undertaken: 

1. Replicate the SAI designs using available NASA and American Gear 

Manufacturers Association (AGMA) design tools and determine their actual 

expected efficiency and weight. 

2. Examine alternate engine layouts and power transmission options. 

Because the overall design of the engine is highly dependent on the power transmission 

design, a feasible design must be used. The design trades made by the SAI team may be 

suspect as a result of this shortfall, leaving room for much more conceptual design work 

for ultra-high bypass ratio (UHB) configurations. 

 5.2.4 Risk #10, Single-core/multiple-fan concept 

 Likelihood = 3, Consequence = 3 

Risk statement: If the single-core/three-fan concept performance is less than predicted, 

then attainment of the mission requirements will be jeopardized. 

The SAI activity was a high-level conceptual study, that is, a point design look at what 

might be done to significantly reduce jet exit velocity. As in any conceptual design study, 

many design factors are reduced to simple correlations or models to assess the trend of 

the overall system. Because the configuration(s) under study are not well understood and 
28

differ considerably from a conventional architecture, these correlations/models may 

contain considerable unquantified error for this application. Additionally, some physical 

attributes of the system have been ignored for simplicity of analysis that may turn out to 

be significant in an eventual design. These are necessary risks that are associated with 

conceptual design. 

Obviously, if the engine performance turns out to be significantly worse than predicted, 

the overall vehicle design could be compromised. A number of vehicle programs have 

been abandoned over the years when the propulsion system design did not meet the 

requirements. 

Not all loss mechanisms have been accounted for in the SAI study. For example, 

nacelle/nacelle interactions, inlet corner flows, fan performance degradation due to 

distortion, and overall performance degradation due to the long noncircular inlet and 

nozzle duct. The sources and rationale for the engine weight analysis are unclear. Scaling 

effects are unknown. The capability of the primary performance degradation mitigation, 

which is the BLI installation, still needs to be tested in a practical design. Design changes 

will likely be required to meet operability requirements. Sufficient conservatism does not 

appear to have been used in the design process to account for all of the unknowns. 

Previous internal studies performed at GRC of similar configurations have shown 

relatively high performance penalties for such factors as inlet performance and gearbox 

efficiency. 

The basic truth is that to reduce jet velocity a large volume of air will have to be moved, 

and a large amount of drag-inducing structure and weight will be required to move that 

large volume of air. The SAI is one of only a handful of studies that has looked at design 

options for UHB systems. Many more studies will be required to map the design space 

for these low noise aircraft. The SAI result represents only one of many possible design 

options. Research needs to be conducted to better understand whether the assumptions 

that have been made in this type of study are reasonable. 

Recommendation: Mitigate the risk. 

Mitigation Strategy: Key areas for further study include: 

1. Distortion-tolerant turbomachinery, particularly the fans. 

2. Increased pressure recovery S-duct inlets with distortion mitigation. 

3. Power transmissions with improved performance and reliability. 

4. Off-design component performance models. 

5. Overall BLI performance. 

6. Coupled vehicle/engine performance (needed for BLI). 

All of these factors should also include scaling effects to better indicate where a given 

design characteristic fits in terms of the vehicle class.
29

 5.2.5 Risk #12, Low flight idle thrust 

 Likelihood = 3, Consequence = 3 

Risk statement: If the assumed low flight idle thrust is not feasible, then attainment of 

the noise and operational requirements will be jeopardized. 

The source of engine noise during landing primarily results from the fan and the LPT. 

The use of a low idle thrust setting reduces the loading on the fan blades, thereby 

minimizing broadband and tonal noise. The airframe center body provides shielding of 

the forward-radiated engine noise; this noise is addressed through shielding and duct 

length. In addition, acoustic liners are used to attenuate rearward-propagating noise. 

Engine design also reduces jet noise through two primary mechanisms. The first is to 

fully mix the core exhaust with a large amount of bypass flow from the primary fan, 

thereby lowering the jet temperature. The second mechanism is to minimize jet velocity 

through the engine thrust settings. 

To produce the required thrust while keeping jet velocity low, the mass flow rate of each 

engine is increased by augmenting the flow with two additional fans. These auxiliary fans 

are driven by the LPT through a complex transmission with a gear ratio of 1. To maintain 

the operability of the fans over the entire mission profile, a continuously variable nozzle 

area is used. This variable-area nozzle allows the mass flow rate through the fan to 

change while keeping the fan pressure ratio within reasonable conditions. 

In addition, the role of the engine is key during the landing approach; the engine works in 

conjunction with the airframe to provide stability and control. The nozzle provides thrust 

vectoring and is used to change the aerodynamic moment of the vehicle, which enables 

the aircraft trim settings that are described above. 

During the entire landing profile, the engine speed is reduced to 45 percent to both lower 

noise and facilitate drag trim. This power setting is substantially lower than the 

conventional reduction to 60 to 70 percent of maximum engine speed for approach. An 

issue with the low spool speed occurs when a go-around maneuver is required or 

additional thrust is needed to compensate for weather. At 45 percent engine speed a 

significant lag occurs when engine power is increased, which could induce pilot error. A 

similar issue occurred during the initial years of the 727 aircraft (United flight 227), 

which was eventually alleviated by pilot training. 

Several engine design issues are related to low flight idle thrust. First, the fan is the most 

critical component because it provides the majority of the thrust and affects other 

important engine performance parameters. In this configuration, with a variable nozzle in 

control of the fan conditions, the blading must be designed for off-design conditions. At 
30

low fan speed, this design may produce negative incidence on the outlet guide vane 

(OGV). Second, even though LPT blading is optimized to minimize noise, this source of 

noise is difficult to quantify and must be further evaluated. Third, the design of a 

transmission and cooling system for the multiple-fan configuration is challenging and 

must be assessed for its reliability. 

The low spool speed is likely to be an issue for weather-related engine power needs and 

for go-around conditions. Training can alleviate the pilot error that can be associated with 

a late go-around call, but weather is more unpredictable. If weather causes pilots to 

manipulate engine power, the noise benefit will be reduced with no obvious substitutes. 

Weather is also a transitory event that may cause the noise target to be exceeded on 

relatively rare occasions. 

Recommendation: Assume the risk. 

Weather-related issues may cause problems, but these types of issues may not occur 

frequently enough to require mitigation. 

 5.2.6 Risk #16, TE brushes 

 Likelihood = 3, Consequence = 3 

Risk statement: If the TE brush technology performance is less than predicted, then 

attainment of the noise requirements will be jeopardized. 

Several concerns associated with TE brushes: 

If the tips of the brushes are too thick, although the turbulent boundary layer (TBL) and 

TE noise would be reduced, TE bluntness noise could increase perhaps negating the 

benefit of the brushes. 

TE brushes have been evaluated only in a laboratory environment. A number of concerns 

are associated with the TE brush buildability, maintainability, reliability, durability, and 

so on. At this point, TE brush technology will require significant maturation before 

implementation becomes feasible. Often during such a maturation process, benefits are 

lost to gain viability. TE brushes are currently at such a low state of maturity that it is 

unlikely that all of the benefit that has been demonstrated in the laboratory can be 

achieved on a real vehicle. 

TE brushes only ameliorate one of the SAI noise sources, albeit an important one. The 

SAI assessments show that airfoil noise only dominates the overall aircraft noise levels 

well before the aircraft passes overhead; as such, the airfoil noise does not contribute to 

the highest EPNL levels that dominate computation of the certification noise level (ref. 

4). Thus, much of the SAI noise benefits can be achieved even if the TE brushes do not 

perform as predicted as the technology matures. 
31

Recommendation: Assume the risk. 

Even if only some of the noise benefit of the TE brushes is achieved, the aircraft will still 

be a very quiet aircraft. 

 5.2.7 Risk #21, Thrust-managed takeoff 

 Likelihood = 2, Consequence = 5 

Risk statement: If the thrust-managed takeoff is not viable, then attainment of the noise 

requirements will be jeopardized. 

The departure profile is segmented into three parts—acceleration, roll, and climb—each 

with different conditions for engine control. During acceleration, thrust is maximized by 

controlling fan speed and nozzle area. The roll phase commences when the required 

velocity is achieved. At this point, engine control is based on maintaining constant 

velocity, which is a function of both thrust and drag. The third phase, climb, begins when 

sufficient lift is generated to exceed the takeoff weight. In this phase, control is based on 

constant velocity and maximum climb angle. 

To meet the noise target, engine control alone is insufficient. Achieving the noise target 

will require coordinated control of both the airframe and the engine. Traditionally, engine 

control and airframe control are two separate entities that are governed by separate 

organizations. While this segregated condition is arbitrary, it is nevertheless a reality in 

present engine/airframe integration practices. 

Although additional control law processing, sensing, and actuation will be required to 

implement thrust-managed takeoff, this implementation is not deemed to be beyond the 

capability of present control-system technology. However, a more holistic approach to 

control system architecture (e.g., the open systems approach that is advocated by 

distributed control methodologies) would be beneficial for both the airframe and engine 

systems. 

For noise mitigation during vehicle takeoff, the entire approach to control is different 

than traditional practice. Noise performance requires integrated airframe and engine 

control but is not beyond the bounds of current technology. 

Recommendation: Mitigate the risk. 

Mitigation strategy: The recommendation for thrust-managed takeoff is to mitigate the 

risk by conducting the necessary additional research into integrated vehicle control. This 

additional research entails the development of new control architectures that reduce the 

impact of integrating the engine and airframe by creating functional elements with 

standardized interfaces for both hardware and software. Work that was performed for the 
32

NASA F-15 Highly Integrated Digital Electronic Control (HIDEC) experiment should be 

reviewed for applicability to this risk. 

 5.2.8 Risk #26, Emissions 

 Likelihood = 2, Consequence = 4 

Risk statement: If the emissions characteristics of the SAX-40 are undesirable, then 

attainment of the mission requirements will be jeopardized. 

Based on the SAX-40 literature, the estimated emissions reductions for the silent aircraft 

concept were expected to result from decreased fuel burn. To improve efficiency and 

reduce fuel consumption, the overall pressure ratio of the SAX-40 engines is 48.8. This 

value is higher than that of current engines and will cause an increase in the combustor 

inlet temperature T3, which will increase the NOx emissions index (i.e., grams of NOx

emitted per kg of fuel consumed). The combustion system was not addressed in the SAX40 literature. 

The assumption is made that the emissions were calculated using empirical correlations; 

however, no information on the calculations was provided. The lack of information 

makes it difficult to judge whether the calculations are reasonable and whether the 

increase in NOx emissions that results from the increase in T3 is mitigated by the 

decrease in fuel burn. 

The apparent use of a standard combustion system reduces the risk because an advanced 

low-emissions combustor does not need to be developed. However, we cannot assume 

that all of the emissions benefits can be realized through fuel savings alone. If the 

increased pressure ratio causes a substantial increase in the NOx emissions index, then 

this effect may offset the reduced fuel consumption benefit. 

Recommendation: Assume the risk. 

 5.2.9 Risk #27, Experimental 

 Likelihood = 3, Consequence = 4 

Risk statement: If the attributes (e.g., assumptions, approximations, scaling) of any of the 

experimental investigations that were conducted in support of the SAX-40 concept are 

questionable, then attainment of the mission requirements will be jeopardized. 

The phased microphone array work that was presented by the SAI team is reviewed in 

this section. The phased microphone array design is detailed in reference 32, which 

describes the design process and the initial testing. The microphone array design consists 

of prepolarized condenser microphones for which the protective grid over the diaphragm 

has been removed. These microphones are sensitive for a range of 10 Hz to 40 kHz with 
33

correction. Electronically, the array was designed to measure two frequency ranges (650-

6500 Hz and 5-50 kHz) and was flush mounted to the tunnel floor. The array was 

designed specifically for the Markham Wind Tunnel at Cambridge University, which is a 

5.5-ft by 4-ft closed-circuit subsonic tunnel with a maximum free-stream velocity of 60 

m/s. Initial testing of the microphone array was conducted using various techniques (e.g., 

loudspeaker and thin cylinder); the vortex-shedding tonal noise that was generated by a 

NACA 0012 airfoil was measured as well. All of the measurements and beam-forming 

results agreed well with the predicted values. In addition, the data analysis methods (i.e., 

the CLEAN algorithm (ref. 33)) were developed to quantify the noise levels and 

eliminate the side lobe contributions. 

This phased microphone array design was used to test various individual components of 

the SAX-type aircraft. Major tests included investigating the airfoil LE high-lift 

geometries and support (ref. 34), quantifying the noise components that are associated 

with landing gear (ref. 35), and determining the acoustic differences between rough and 

smooth surfaces (ref. 36). These results were used for a variety of design modifications. 

For example, the support brackets were the major source of noise for the LE geometries, 

and the slat was a significant source of noise overall. Experiments on the landing gear 

demonstrated a 12-dB reduction in noise when a simple landing gear that contained only 

main struts was used. 

The design seems to be well-suited for the Markham tunnel and has been validated by 

using conventional measurement techniques. However, this design was customized for 

the Markham tunnel; thus, issues may arise if experiments are performed in another 

facility. Several of the numbers that were quoted by the SAI team appear to have come 

from these experiments using this phased microphone array, and without this array (or a 

comparable system), noise measurements would be suspect at best. 

Recommendation: Assume the risk. 

5.3 Low-risk items 

 5.3.1 Risk # 6, BWB configuration, internal layout 

 Likelihood = 1, Consequence = 3 

Risk statement: If the SAX-40 configuration layout does not result in a feasible design, 

then attainment of the requirements will be jeopardized. 

The internal layout of the SAX-40 concept is presented on page 10 of reference 5. The 

cabin, cargo bay, fuel tanks, and landing gear are placed within the SAX-40 OML in a 

2D representation. The cabin density is 0.9 passengers/m2

, compared with 1.4 

passengers/m2

 for the Boeing 767; thus, the available internal volume for the SAX-40 is 

more than adequate. In fact, 335 passengers could be carried by the SAX-40 if the density 
34

factor of the 767 was utilized (ref. 5). In addition, BWB configurations tend to have 

abundant volume; thus, internal layout and packaging requirements are not typically 

design drivers as they can be for a supersonic transport configuration, for example. 

However, to provide a first-order assessment of this risk item, a 3D layout was developed 

utilizing Pro-Engineer.TM Starting with the initial graphics exchange specification (IGES) 

file provided by SAI, the cabin, cockpit, galleys, restrooms, fuel tanks, cargo bay, 

retracted landing gear, and major bulkheads and frames in these areas were sized and 

placed within the OML. A high-density seating arrangement of 335 passengers was used, 

which corresponds to the current seating density of the 767. Figure 5 shows that the 

SAX-40 can easily accommodate the design load of 215 passengers. 

The large cubes that are situated in the rear of the passenger cabin represent the volume 

that is required for the galleys and restrooms. The rectangular volumes that are located 

outboard from the seating area show the required fuel tank size. A large volume clearly 

exists in the wings for additional fuel, as pointed out in reference 5. However, any 

additional fuel would increase both the weight empty and the gross weight of the vehicle, 

which would have a negative impact on the noise performance. Figure 6 shows the 

geometry of the landing gear retraction. Again, adequate volume is available. 

Figure 5. Sample 335-passenger layout for volumetric sanity check. 
35

Based on the 3D modeling, issues with the internal packaging of this design are not 

likely. The BWB configuration in general, and the SAX-40 concept in particular, have 

relatively large amounts of internal volume. Should a packaging issue arise, however, the 

consequences would not be minor. Major redesign of the components or OML changes 

would be required. These changes could easily compromise the attainment of the mission 

requirements by 10 percent or more. 

Recommendation: Assume the risk. 

 5.3.2 Risk #11, Propulsion/airframe integration: buried engines 

 Likelihood = 2, Consequence = 3 

Risk statement: If the buried-engine concept presents propulsion/airframe integration 

issues that cannot be resolved, then attainment of the mission requirements will be 

jeopardized. 

This risk item is closely coupled with risk items #7, BLI /inlet design, and #10, Singlecore/multiple-fan concept. Because these two risk assessments are dealt with separately, 

the current assessment of risk item #11 is addressed only from the propulsion/airframe 

integration point of view as it relates to burying or embedding engines within the 

airframe. Aside from the well-known problem of inlet distortion, which is covered in risk 

Figure 6. Landing gear retraction geometry. 
36

item #7, the following issues or concerns are identified: 

1. Engine-out impact on overall vehicle aerodynamics and controllability: 

Critical engine failure effects were recognized in terms of thrust loss and 

increased drag (refs. 3 and 8). Because the three-fan/single-core propulsion 

system is closely coupled with external aerodynamics, an outboard or even a 

center engine failure may cause a reduction in airframe lift, especially near the 

vehicle aft area, and may interfere with the vehicle stability. In addition, the 

effect of flow disturbance from the failed engine on the other engines must be 

studied to minimize impact to overall vehicle performance. For the case of 

uncontained engine-blade failure, the likelihood of adjacent engine failure as a 

result of failed blades must be considered because of the proximity of these 

engines to one another. 

2. Engine maintenance: The installation and removal of a three-fan/single-core 

engine may be difficult and time consuming because of the interconnected fan 

gearing system; the long and shape-changing (axisymmetric to 2D) nozzle duct; 

and the relatively high location of the engines with respect to the ground (ref. 

37). For example, on the Lockheed L-1011, the center-engine installation and 

removal from the rear fuselage was known to be problematic. 

3. Possible interruption of rear structural spars as a result of the embedded 

engines: Embedding engines within an airframe has not been a major obstacle, 

based on historical vehicle configurations such as the Northrop YB-49, B-2, and 

small fighters. However, applying the concept in commercial vehicles may 

introduce additional difficulties in terms of vehicle operations and support. 

4. Engine noise propagation to the passenger compartment: As a result of the 

engine casing contact with the airframe structure, noise propagation to the 

payload area, as well as acoustically induced structural fatigue, are expected. The 

engine-fan gearing system will further increase the likelihood of both noise 

propagation and structural fatigue. The problem can be mitigated through the use 

of both sound insulating material and a heavier structure. 

5. Variable nozzle strike at takeoff: For conventional vehicles, a number of tail 

strikes result from pilot over-rotation. In operational situations such as vehicle 

payload loading, incidents of tail strikes have resulted from imbalanced payload 

loading. In the case of the SAX-40, variable nozzles are extended beyond the 

vehicle tail area, making damage to flight-critical and expensive variable-nozzle 

hardware a possibility. 

6. Integration of active/passive/hybrid flow-control devices with inlet duct: As a 

result of the extended acoustic liners along the long inlet duct, the integration of 

active or hybrid flow-control devices may not be trivial. 
37

7. Passenger egress with embedded engines: This issue is not seen as a major 

problem; the structure can be designed to accommodate passenger egress 

between and under the engines. 

8. Insufficient control surfaces as a result of the unusually wide propulsion 

system: If thrust vectoring is employed to augment control surfaces, this 

problem may not be critical. 

9. Thrust reverser: The three-fan/single-core combination thrust reverser may be 

mechanically complex and may exhibit unusual pitching moment if the thrust is 

directed upward. 

10. Long, acoustically treated inlet and nozzle ducts: These components will 

reduce the overall propulsion system performance. 

11. Water, snow, and ice ingestion. 

12. Pre-flight inspection requirements. 

The major issues and concerns are related primarily to vehicle operation rather than the 

technical aspects of the aircraft. From the technical point of view, no major problems 

prevent development of such a propulsion/airframe integration concept. Maintenance and 

operational issues are somewhat more complex than for conventional aircraft, but these 

could be considered as the “price to pay” for an extremely low noise aircraft. In regard to 

the vehicle safety issues that are associated with an uncontained engine blade burst, this 

risk could be minimized with newly developed technology and the use of redundant 

structure (e.g., extra containment material near the engine core). 

Recommendation: Mitigate the risk with typical developmental activities that are 

performed in support of the detailed design. 

5.3.3 Risk #13, Low-speed fan with forward swept blades 

 Likelihood = 3, Consequence = 2 

Risk statement: If the fan performance is less than predicted, then attainment of the noise 

and mission requirements will be jeopardized. 

The embedded-engine configuration places the fan into a highly distorted inlet flow 

condition during the entire operating envelope. The highly distorted flow in the 

circumferential direction at the fan face can have a detrimental effect on fan efficiency 

and surge margin (i.e., operability). If the fan performance predictions and tolerance to 

distortions are not as predicted by the analyses, the engine thrust and specific fuel 

consumption may be detrimentally affected. 
38

We already know that an embedded fan experiences circumferential flow distortion at the 

fan face. We also know that this distortion reduces the aerodynamic efficiency of the fan 

and also can have a negative impact on its stall margin. We need to determine the 

magnitude of reduction in efficiency and operability that results from inlet distortion and 

the effect that this has on the overall engine cycle. High incidence on the OGV’s is an 

additional factor that can reduce overall fan performance at several engine operating 

conditions. 

Because most of the engine thrust is dependent on the performance level of the fan, fan 

performance has a significant effect on meeting the overall engine requirements. The 

expected outcome of the inlet distortion could possibly result in reduced overall engine 

performance between 1 and 10 percent. However, this reduction in performance can be 

minimized by the development of inlet-flow-distortion mitigation technology. The 

incidence on the OGV may be mitigated with the use of flow control with plasma or flow 

injection techniques that would need development. 

Recommendation: Mitigate the risk. 

Mitigation strategy: The inlet distortion issue can be mitigated by executing a technology 

development plan that includes the following necessary events: 

1. Conduct analysis of specific engine configurations with thermodynamic 

engine cycle studies. 

2. Create a conceptual design of the embedded engine, including the inlet and 

fan design. This task should include input from engine manufacturers. 

3. Perform CFD analyses of the inlet and fan at several operating points to 

determine quantitatively the effects of distortion on fan efficiency and 

operability, as well as incidence effects on the OGV. 

4. Determine analytically (CFD) whether flow distortion mitigation techniques, 

such as flow control, could be effective at reducing the fan face distortion 

with minimal negative impact on the engine cycle. The same techniques need 

to be applied to the OGV to reduce separation as a result of high levels of 

incidence. 

5. Validate the analytical results by rig testing the embedded fan and OGV at 

several engine operating conditions. 

 5.3.4 Risk #14, High-pressure compressor (HPC) design 

 Likelihood = 3, Consequence = 2 

Risk Statement: If the HPC performance is less than predicted, then attainment of the 

noise and mission requirements will be jeopardized. 

The circumferential flow distortion that is experienced by the embedded fan does not in 

most cases completely mix out and; thus, this distortion propagates through the fan and 
39

results in some flow distortion into the core compressor. In addition, the static pressure at 

the fan exit is not uniform, which may cause additional unknown effects on compressor 

performance. The magnitude of the distortion at the compressor inlet is not constant and 

can change significantly depending on aircraft maneuvers during the flight envelope, 

crosswinds, and engine-out conditions. Varying levels of boundary-layer thickness at the 

fan face can result in large swings in the corrected mass flow rate into the core 

compressor and can cause a compressor surge. The reduced-pressure-ratio fan places an 

additional burden on the core compressor to provide an even higher pressure ratio to 

compensate for the reduced-pressure fan. A highly loaded high-pressure compressor is 

likely to be more sensitive to small levels of inlet distortion and will have reduced 

operability compared with one that has an undistorted inlet flow. Compressor inlet flow 

distortion and increased pressure-ratio requirements will reduce compressor operability, 

while anticipated swings in the corrected flow rate will require increased levels of 

compressor operability. The compressor will likely experience stability issues. 

The expected outcome of the inlet distortion at the compressor face could result in 

reduced overall engine performance and operability. Even a low level of distortion into 

the high-pressure multistage compressor can negatively impact the aerodynamic 

efficiency and stall margin, or the operability. This places an additional requirement on 

the compressor to have even higher margins of operability than those in traditional 

engines. 

Recommendation: Mitigate the risk. 

Mitigation strategy: The compressor inlet distortion issue can be mitigated by a 

technology development plan that includes the following necessary events: 

1. Conduct analysis of the specific SAX-40 engine configuration with 

thermodynamic engine cycle studies. 

2. Create a conceptual design of the embedded engine, including fan and 

multistage compressor. This task should include input from engine 

manufacturers. 

3. Perform CFD analyses of the multistage compressor at several vehicle/engine 

operating points throughout the flight envelope. The varying magnitude of 

the boundary layer into the engine/fan and the resulting swings in corrected 

mass flow rate at the core compressor inlet must be determined. 

4. Determine analytically (CFD) whether flow distortion mitigation techniques, 

such as flow control and non-axisymmetric design of the compressor inlet, 

could be effective at reducing compressor face distortion with minimal 

negative impact on the engine cycle. 

5. Determine analytically whether variable-geometry compressor inlet guide 

vanes and stators can be effective at mitigating the swings in the compressor 

inlet corrected flow that are caused by the boundary-layer thickness 

variations encountered in the flight envelope. 

6. Identify a test facility with an adequate flow rate capacity and drive power 
40

that can be used to verify core multistage compressor aerodynamic 

performance. If the drive power is excessive, then the multistage axial and 

the centrifugal compressors can be tested separately. 

 5.3.5 Risk #15, Low-noise, LPT design 

 Likelihood = 3, Consequence = 2 

Risk statement: If the LPT performance is less than predicted, then attainment of the 

noise and mission requirements will be jeopardized. 

The LPT is one of the major contributors to engine noise in the SAX-40. One way to 

minimize tonal noise from the LPT is to design a large number of rotor blades. The 

interaction between the LPT blades and vanes can generate noise in the audible frequency 

range. During the design process, the number of LPT rotor blades and vanes can be 

increased to generate higher frequencies that are less audible. The higher frequencies are 

more easily attenuated by the atmosphere. However, the maximum number of blades is 

limited by structural, mechanical, and manufacturing considerations, as well as by the 

aerodynamic performance consideration of minimum Reynolds number. As the blade 

number is increased the chord is reduced, thereby reducing the Reynolds number, 

especially at cruise operating conditions. The reduced Reynolds number has a high risk of 

increasing the LPT boundary-layer profile losses. If the boundary-layer separation bubble 

on the blade surface becomes excessively large, it may not collapse back onto the blade 

surface before the TE, which can result in even higher losses and underturning in the last 

few blade rows of the LPT, causing a reduction in power generated by the LPT. 

Recommendation: Mitigate the risk 

Mitigation strategy: The LPT boundary-layer loss profile can be mitigated by a 

technology development plan that includes the following necessary events: 

1. Perform analysis of the specific SAX-40 engine configuration with 

thermodynamic engine cycle studies. 

2. Create a conceptual design of an embedded engine including the LPT. This task 

should include input from engine manufacturers. 

3. Perform CFD analyses of the LPT at several vehicle/engine operating points 

throughout the flight envelope. Determine the varying magnitude of the boundary 

layer in the blade rows of the LPT and assess the profile losses. 

4. Identify technologies that could reduce the boundary-layer growth that results 

from low Reynolds number. Test candidate technologies such as flow control and 

plasma control in a turbine cascade. 

5. Identify a rotating test facility that can be used to verify LPT aerodynamic 

performance at the low-Reynolds-number operating conditions. The facility 

should also enable measurement of LPT acoustic performance. 

6. Validate LPT performance and acoustics by rig testing. 
41

 5.3.6 Risk #17, Low noise undercarriage 

 Likelihood = 2, Consequence = 3 

Risk statement: If the low noise undercarriage performance is less than predicted, then 

attainment of the noise requirements will be jeopardized. 

Landing-gear systems are complex mechanical devices, which require robust but efficient 

design, frequent inspection and maintenance, and high reliability with respect to 

mechanical actuation and motion. Additionally, landing gear operate in severe 

environments of heat, dust, foreign-object impact, and forceful water/snow/ice spray, and 

mishaps that involve tire failures are not uncommon. These conditions would likely make 

the operational life of fairing structures very short or else require the fairings to be quite 

robust, thereby adding both weight and complexity in maintenance. In addition, a 

maximum-energy aborted-takeoff incident usually results in a fire that involves the tires, 

wheels, and hydraulics, so unhindered fire-suppression access to these systems is 

paramount. In normal service, heat buildup in the brakes during taxi to the takeoff 

location must dissipate quickly so that the brakes are ready for a potential aborted-takeoff 

operation, and the heat buildup during landing and taxi to the gate must also dissipate 

quickly for ground safety. The fairing system that is proposed for noise reduction must 

not significantly impact these issues because they are safety related. Also, landing gear 

are geometrically complex with many adjacent components, so the sources of noise are 

surmised to be large-scale interacting turbulent wakes and their impingement on various 

surfaces. Thus, the selection of which components to fair and the manner in which to do 

so is not obvious because the flowfield around the undercarriage can change dramatically 

with aircraft attitude in the various stages of approach. 

While both U.S. and European wind-tunnel tests appear to have demonstrated landinggear noise reduction through the use of fairings and shielding, flight tests have yielded 

disappointing results at best. Depending on the longitudinal location of the undercarriage, 

the noise created by the wake interaction with the TE flaps may be an unaddressed noise 

source; on the Boeing 777, phased-array tests suggest that this component can contribute 

as much as 25 percent of the total undercarriage-related noise. Noise that is generated 

through interactions with the SAX-40 TE brushes (instead of flaps) is unknown. 

Additionally, none of the demonstrated fairings are even remotely acceptable in regard to 

integration issues, based on extensive discussions with Boeing and Goodrich 

undercarriage designers during certain NASA programs (e.g., the Advanced Subsonic 

Technology (AST) and Quiet Aircraft Technology (QAT) programs). 

Undercarriage-related noise is frequently the largest component of airframe noise during 

approach and often accounts for more than 50 percent of the total approach noise. A 

mitigating factor related to the BWB configuration is the relatively short undercarriage 

length (i.e., compared with a configuration with a high-bypass-ratio engine mounted in an 

underwing nacelle). 
42

Recommendation: Avoid the risk. Concentrate on the development of systems to enable 

the delay of undercarriage deployment until late in the approach timeline. The focus 

would need to include increased deployment reliability although these systems are 

already considered quite reliable, quiet air-brake devices, and automated fast-response 

go-around controls. 

 5.3.7 Risk #18, Quiet drag 

 Likelihood = 2, Consequence = 2 

Risk statement: If sufficient quiet drag cannot be generated, then attainment of the noise 

requirements will be jeopardized. 

An adequate amount of drag must be generated. The necessary drag is produced by 

inducing additional drag via an inefficient lift distribution over the all-lifting airframe 

during approach. This inefficient lift distribution is created through a combination of 

upward-deflected elevons and vectored thrust. For the purposes of this assessment, we 

assume that this is feasible. The risk that is inherent in the vectored-thrust system and the 

high-lift system has been addressed separately. 

Assuming that the amount of drag is adequate, we must determine whether the drag is 

generated in an adequately quiet manner. 

In the SAX-40 design (ref. 4), the required drag for a low approach speed would be 

achieved through the use of a large wing area at a high angle of attack and through the 

use of elevons and thrust vectoring. Aside from the engine noise (thrust vectoring), the 

drag noise is inherent airfoil noise (i.e., the noise generated by the air flowing along the 

surface of the wing and elevon). 

The likelihood is high that the drag noise (inherent airfoil noise) for the SAX-40 in an 

approach configuration would be much less than that generated from an aircraft with 

traditional slat and flap high-lift systems. This assumption is true because the dominant 

noise sources that are typically located in the slat and flap cove/gap regions and along the 

flap side edges are eliminated in the SAX-40 wing configuration at approach. The 

remaining noise is the noise that radiates from the TE of the elevons and wing tips; this 

noise can be controlled via the use of TE brushes. See risk #16 regarding the performance 

of the TE brushes. 

Prediction methods (well-established empirical and semi-empirical models) were used to 

estimate the noise that is generated by the SAX-40 wing in an approach configuration. 

However, accurate measurements of the overall resulting noise level (which would need 

to be performed in a facility that is designed for aeroacoustic testing) of the complete 

SAX-40 wing airfoil profile (including drooped LE, deflected CML elevons, approach 

speed, and angle of attack) were not obtained. Noise measurements were only performed 

to evaluate the benefit of using a drooped LE versus a slat (ref. 34). These measurements 
43

were acquired at a lower speed (40 m/s) than the target approach speed (60.8 m/s) and in 

a facility that has not been designed for aeroacoustic testing (i.e., an untreated, closed 

wind tunnel with relatively higher background noise). Nevertheless, these acoustic 

measurements did indicate that the drooped LE would be significantly quieter than a 

traditional slat, but quantitative measurements of the actual noise level and spectra were 

hindered by the high background noise, the proximity of the microphone array to the test 

model, and so on. 

No prediction or experimental measurements of the noise that is generated in the region 

where the side edges of the deflected elevon are linked to the main wing have been 

performed. The CML technology has previously been tested as a means to reduce flap 

side edge noise (ref. 26). The flow in the side edge region of the elevon would differ from 

that in the side edge region of a deployed flap. Nevertheless, because of the absence of 

the side edge, we can reasonably assume that no significant noise source would form in 

that region, but the TE may need to be treated differently along the CML link than along 

the rest of the elevon TE. 

In summary, the noise that is generated from the selected airfoil configuration (e.g., 

drooped leading edge, CML elevons, main-element high angle of attack) should be 

mostly TE noise. The TE does not generate as much noise as the other noise sources that 

were eliminated. The geometry of the TE plays an important role in the level of noise that 

radiates from that edge. 

For a single-airfoil-wing configuration, where the LE airfoil (slat), main airfoil, and TE 

airfoil (elevon or flap) are molded into a single airfoil, the TE noise is the only noise 

source that is left to control. This noise source is not typically important (compared with 

slat noise or flap side edge noise); however, the level of noise that radiates from the TE 

will greatly depend on the TE geometry. 

In summary, if sufficient drag can be generated without having to depart from this singleairfoil configuration, then the noise requirements could likely be met. If the necessary 

drag cannot be generates without having to revert back to the use of slats and flaps, then 

the noise requirements would most likely not be met (but the flap side edge noise could at 

least be mitigated via the use of a CML to eliminate the flap edges). 

Recommendation: Assume the risk. 

 5.3.8 Risk #19, Long ducts with acoustic liners 

 Likelihood = 2, Consequence = 3 

Risk statement: If the integration of long ducts with acoustic liners results in acoustic 

penalties, then attainment of the noise and mission requirements will be jeopardized. 
44

Based on the SAX-40 literature, the integration of long ducts with liners involves the 

following assumptions: 

1. The length-to-height ratio of the acoustically lined section of the exhaust fan duct 

will be greater than the ratio that can be achieved with conventional engines. The 

amount of attenuation in the duct will increase as the length-to-height ratio is 

increased; thus, this effect is clearly positive. 

2. Reduced fan diameters with higher rotational speeds will result in increased 

attenuation (ref. 6). As the blade passage frequency is increased, the effectiveness 

of the conventional acoustic liners increases. This effect is clearly positive. 

3. The complex propagation path will require an increase in the number or the 

width of the splices (ref. 6). The interaction of the rotor-stator noise with the 

splices causes the sound field to be scattered into higher order modes, in 

particular when the splices are close to the fan (refs. 38 and 39). This effect is 

enhanced as the width of the splices is increased. The amount of attenuation that 

is achieved by the acoustic liner depends on whether the liner is properly 

designed to match the resultant scattered modes. One way to mitigate this 

concern is to develop methods for the fabrication of liners that are acoustically 

smooth (i.e., the effects of the splices are virtually eliminated). Recent tests 

conducted by industry (ref. 40) demonstrated that an acoustically smooth inlet on 

a Boeing 777 aircraft with GE90-115B engines could be used to achieve 

excellent attenuation. Thus, although these splice effects can be important, they 

are reasonably well understood. 

4. The complex propagation path will result in a reduced “line of sight” from the 

fan to the exhaust plane (ref. 6). For this condition, the sound field will interact 

with the acoustically treated wall at oblique angles of incidence; thus, the 

absorptive capability of the liner will be enhanced. Although this shape is 

expected to have a negative impact on the performance of the engine, it will 

result in increased noise attenuation. The NASA Langley Curved Duct Test Rig 

has recently been developed to study the effects of S-shaped wall curvature on 

the sound field (ref. 41). 

5. Multilayer and or multisegment liners will be installed in these long ducts (ref. 

42). As these liner types are designed to achieve broadband attenuation, the noise 

spectrum of concern is assumed to be predominately broadband. The inclusion of 

multilayer liners will increase the frequency range over which significant sound 

absorption can be achieved but will do so at the cost of reduced peak tonal 

attenuation (ref. 39). However, the added length of the duct provides multiple 

options for overcoming this reduction in tonal attenuation. As indicated in 

reference 42, multisegment liners may result in preferential scattering into higher 

order modes. However, the work of Zlavog et al. (ref. 43) indicates that the mode 

scattering that occurs at the interfaces between each liner segment can cause the 

sound absorption to either increase or decrease (relative to uniform liners), 

depending on the relative phases of the multiple modes that interact with these 

liner segments. Thus, the modal content of the fan noise must be known to 

achieve the benefit of segmented liners. 
45

Recommendation: Assume the risk. 

 5.3.9 Risk #20, Airframe shielding 

 Likelihood = 1, Consequence = 2 

Risk statement: If the airframe shielding benefits are less than predicted, then attainment 

of the noise requirements will be jeopardized. 

The SAX-40 configuration provides shielding for the propulsive noise that is radiated 

from the engine inlets, including broadband and tone noise from the fan and the 

compressor. The amount of tone noise may be greater than it would be for podded 

engines because of nonuniform inflow, and broadband noise may be greater as a result of 

ingestion of boundary-layer turbulence. Therefore, shielding may be important to 

compensate for the increase in the noise that is radiated from the inlet. Nevertheless, the 

SAI team demonstrated (ref. 44) that even conservative estimates of the effects of 

shielding predict a reduction in noise in the direction of primary inlet radiation to the 

ground (i.e., 60 to 100 degrees with respect to the inlet axis) of approximately 30 dBA. 

These predictions are consistent with previous studies of similar configurations (ref. 45). 

Estimates were based on wave calculations at low frequency and were projected to high 

frequency. Ray calculations at a high frequency should predict even greater effects of 

shielding in the shadow zone than are predicted from the low-frequency wave 

calculations. Inlet noise will need to be controlled by making the inflow as uniform as 

possible and by maximizing noise absorption by the duct liners because of the proximity 

to the passenger compartment. Therefore, even only moderately shielded inlet noise is 

unlikely to be a major contributor to total aircraft noise. 

Recommendation: Assume the risk. 

6.0 ATB Comparison

Throughout the various SAI-related publications, comparisons have been made between the 

SAX-40 and various existing or near-term aircraft, such as the Boeing 767-300, 777, and 787-3. 

A more enlightening comparison would involve an ATB (i.e., a conventional “tube with wings”) 

concept that has been sized for the SAX-40 design mission and that utilizes similar levels of 

technology. Such a comparison would address the overall SAX-40 feasibility and the risk level 

relative to the conventional baseline, thereby highlighting the cost of optimization solely for low 

noise. 

Standard analysis practice begins with the identification of a starting point from which to create 

the baseline design. The starting point should be an existing aircraft with a range and payload 

performance that is consistent with the SAX-40 design mission and for which adequate data are 
46

available to calibrate the analytical model. Once calibrated, the analytical model can then be 

adjusted with technology and mission performance assumptions that match the SAX-40 design to 

result in a resized ATB design. 

The SAX-40 design mission is to carry 215 passengers for 5000 nm. This passenger load is 

slightly larger than the largest existing single-aisle transport (i.e., the Boeing 757-300) yet not 

quite in the range of most wide-body, twin-aisle transports. Therefore, two starting points were 

used: a single-aisle Boeing 757-300 and a twin-aisle Boeing 767-200ER. The 757 was modeled, 

calibrated, and then resized to result in a design that is termed the “Single-Aisle–AdvancedTechnology Baseline” (SA-ATB); likewise, the 767 was modeled, calibrated, and resized to result 

in a design that is termed the “Twin-Aisle–Advanced-Technology Baseline” (TA-ATB). 

The primary aircraft level sizing and analysis tool that was used was the Flight Optimization 

System (FLOPS) computer code. A baseline model of a 757-200 (186-passenger, mixed-class 

configuration) was developed using a combination of publicly available data for geometry, 

weight, and performance characteristics (ref. 46) and a PW2040-like engine model that was 

developed at NASA. In addition, a Boeing-generated 757-200 group weight statement was 

utilized. The FLOPS weight predictions were calibrated by setting the maximum ramp weight and 

landing weight to the values that were reported by Boeing (221,000 lb and 198,000 lb, 

respectively) and comparing the FLOPS-predicted operating empty weight (OEW) with the 

Boeing data. To match the OEW from the Boeing group weight statement, calibration 

adjustments were made to the individual component weights in the FLOPS model. Next, the 

model was modified by applying the 757-300 geometric and design mission parameters, and the 

OEW was recalibrated by adjusting the predicted weights for two operating items (passenger 

service and cargo containers). The maximum ramp weight for the 757-300 is 271,000 lb, and the 

OEW is 141,800 lb. The calibrated FLOPS model predicted an OEW of 141,807 lb. 

The FLOPS-predicted mission performance was then calibrated to a specific point on the 757-300 

payload-range diagram that is provided in reference 46. Assuming a payload of 58,200 lb (243 

passengers at ~240 lb per passenger), the OEW plus payload is 200,000 lb. At a fuel load of 

60,000 lb, the ramp weight for the calibrated mission is 260,000 lb. According to the payloadrange diagram, at this operating point the range is 2500 nm. This mission performance was used 

as a calibration point for the FLOPS model. The FLOPS-predicted fuel capacity (which is based 

on wing geometry) was calibrated to match the published capacity for the 757-300. Assuming 

that the weight characteristics are accurate based on the previously described weight calibration 

process, range performance is impacted most directly by the mission profile, engine-thrustspecific fuel consumption (TSFC), and the aircraft lift-to-drag ratio (L/D). Although a detailed 

mission profile is not provided in reference 46, some parameters are specified in the payloadrange diagram, such as cruise Mach number, typical mission reserves, and a 200-nm alternate 

airport. The step cruise that is specified (31,000-35,000-39,000 ft) was approximated in FLOPS 

by cruising at optimum altitude (i.e., “cruise climb”). The internally computed FLOPS 

aerodynamic performance was adjusted to achieve a range of 2500 nm. The impacts of engine 

specific fuel consumption and aircraft L/D cannot be separated when matching range 

performance. The accuracy of the PW2040-like engine model thus impacts the accuracy of the 

calibrated FLOPS aerodynamic performance. For example, if the actual engine TSFC is higher 
47

than that in the model, then the range-calibrated aerodynamic efficiency will be lower than the 

actual efficiency. 

The final step in the development of a baseline model that was representative of the Boeng 757-

300 aircraft was to ensure that the FLOPS sizing was consistent with that of the actual aircraft. 

For basic FLOPS sizing, the varied parameters are engine thrust and wing area, with an objective 

to minimize the gross weight that is required to meet the mission. One of the difficulties in sizing 

for this case is that the design mission that is described above is at a gross weight that is below 

the maximum gross weight. Performance constraints, such as initial cruise altitude capability and 

takeoff field length, need to be met when the mission start weight is at the maximum weight. A 

more significant issue is that the wing area and the thrust of the actual aircraft can be sized by 

additional considerations that are outside the scope of FLOPS. For example, the wing or engine 

may be oversized for a future “growth version” of the aircraft. Without any adjustments to the 

weight and the aerodynamic calibrations that are described above, the FLOPS-sized vehicle for 

the selected mission resulted in a gross weight of 262,157 lb (261,000 lb actual from Boeing 

data), an OEW of 142,857 lb (141,800 lb actual), and wing area of 1963 ft2

 (1994 ft2 actual). 

A similar procedure was utilized to model the 767-200ER, minus the use of an actual group 

weight statement. The calibration design point selected from the Boeing data (ref. 47) was for a 

388,000-lb maximum design ramp weight, a 181,610-lb OEW, and a range of 6600 nm with 216 

passengers in mixed-class seating. The FLOPS-sized vehicle for the selected mission, which 

utilizes a NASA developed PW4056-like engine model, resulted in a gross weight of 381,309 lb 

(388,000 lb actual), OEW of 178,290 lb (181,610 lb actual), and wing area of 3117 ft2

 (3050 ft2

actual). 

The SA-ATB and TA-ATB airframe models were then created as derivatives of the baseline 

models that are described above. The primary technological advance that was assumed for the 

airframe was the extensive use of composite materials for the airframe structure. For the Boeing 

787 that is currently in development, as much as 50 percent of the primary structure is made of 

composite materials (ref. 48). This composite construction was assumed to result in a 15 percent 

reduction in the weight of the wing, fuselage, and empennage compared with that of traditional 

metal construction. Several other minor technological improvements were assumed, based on the 

787 development, including an increased hydraulic pressure of 5000 psi and a 1 percent reduction 

in drag. Changes were also made to the mission and payload parameters that are described above 

to match the SAX-40 design mission (i.e., 215 passengers in a mixed-class configuration and a 

5000-nm range). 

The results are presented in Table 2. The 757-300 and 767-200ER data are included for 

completeness. The SA-ATB and TA-ATB gross weights are within approximately 1 percent of 

each other, with the twin-aisle option having a slightly better efficiency than the single-aisle 

design (7000 lb or approximately 7 percent less fuel burn). 
48

Table 2. ATB Comparisons 

Using the TA-ATB as a basis for comparison, the SAX-40 is significantly heavier: 46 percent 

heavier in OEW and 18 percent heavier in TOGW. The fuel burn for the SAX-40 is 16 percent 

less than the TA-ATB design. These comparisons indicate that either the SAI team’s weight 

estimates were extremely conservative or that the SAX-40 would be significantly more costly to 

design and produce (however, the fuel burn savings would result in a lower fuel costs). SAI 

selected the BWB configuration because of its inherent airframe noise shielding benefit; 

therefore, the increased design and acquisition costs can be roughly viewed as the cost of 

obtaining the extremely low noise benefit offered by the SAX-40 concept. The technology 

assumptions that are applied to the ATB designs were based on advertised Boeing 787 levels; 

therefore, the in-service date was given as approximately 2010. The assumed in-service date for 

the SAX-40 is approximately 20 years later. This 20-year period makes the ATB estimates 

conservative; that is, the actual weights and fuel burn of an ATB design with comparable 

technologies to the SAX-40 will be lower than those shown in Table 2. 

7.0 Conclusions and Recommendations

The SAX-40 risk assessment identified high-risk items in three primary areas: BWB 

configuration (structures and weights, human factors), propulsion (BLI/inlet and nozzle/thrustvectoring design), and operations (displaced threshold, CDA). In addition, the overall noise 

estimates and the cost of the concept are high-risk items. The following is a summary of the 

challenges that are associated with the high-risk items and the recommended risk management 

strategy: 

1. The challenge of building a relatively lightweight, noncircular pressure vessel must be 

767-200ER 757-300 SA-ATB TA-ATB SAX-40

In-service date 1984 1998 ~2010 ~2010 ~2030

Engine architecture

Twin 

podded

Twin 

podded

Twin 

podded

Twin 

podded

Embedded, 

three cores 

driving nine fans

Range, nm 6600 2500 5000 5000 5000

No. of passengers 216 243 215 215 215

TOGW, lb 387,000 260,000 284,650 281,110 332,560

OEW, lb 181,610 141,800 138,700 142,140 207,660

Fuel, lb 159,000 60,000 94,350 87,370 73,310

Payload, lb 45,100 58,200 51,600 51,600 51,600

Span, ft 156.0 124.8 134.1 136.9 221.6

Wing area, ft2 3050 1994 2264 2343 -

Length, ft 159.0 177.4 177.4 159.0 144.3
49

overcome prior to committing to the full-scale development of a BWB concept. Focused 

research efforts should be undertaken to design, build, and test several structural 

concepts, culminating in the construction and testing of a full-scale structural article that 

represents the preferred approach. In addition to the verification of the preferred 

structural concept, this effort should result in an empirical database for noncircular 

pressure vessels and the associated tools and methods for incorporating this data into the 

design process for BWB configurations. 

Recommended risk management strategy: Mitigate. 

2. The challenge of designing and integrating the BLI inlet design and the variable-area 

nozzle with thrust vectoring is not trivial. Both the inlet and nozzle represent highly 

complex mechanical and fluidic design challenges, which if not met can prevent the 

attainment of the SAX-40 requirements. Efforts to design, build, and test both 

components would be necessary to reduce the risk prior to full-scale development. 

Recommended risk management strategy: Mitigate. 

3. The challenge of integrating the SAX-40 into the airspace represents a serious issue. 

The significantly slower approach speed relative to the other traffic in the pattern will 

necessitate increased spacing, thus reducing airport capacity. Most projections forecast a 

large increase in demand over the next 20 years, and increasing capacity is a high 

priority; therefore, introducing a system that has the opposite effect will not be 

acceptable. Likewise, utilizing a displaced threshold may not be acceptable in all 

situations. A detailed study of airport operations and infrastructure would be required 

before a determination of the feasibility of low speed CDA’s with displaced thresholds 

could be made. One option would be to operate the SAX-40 in a conventional mode 

during peak hours (sacrificing some noise performance) and utilize the slow CDA and 

displaced threshold during off-peak hours to attain the full noise benefit. 

Recommended risk management strategy: Avoid. 

4. The challenge of producing an economically viable concept must be met for any new 

design and for the SAX-40 in particular because it represents such a large departure from 

convention. The ATB comparison indicated that the SAX-40 would be significantly 

heavier than a conventional design (comparable in all categories except noise and fuel 

burn); therefore, RDT&E and production costs would be greater. A detailed life-cycle 

cost estimate should be performed to quantify these costs, including the costs of 

operations and support. The benefit of extremely low noise must be worth any additional 

costs that are incurred, otherwise, the concept will not be viable. 

Recommended risk management strategy: Further study is needed prior to selecting a 

strategy. 

5. The primary motivation for the SAX-40 is low noise. Therefore, attainment of the 
50

overall noise requirement is a crucial requirement. Concerns that have been raised during 

this risk assessment include a reliance on operational techniques that may not be 

acceptable in all places at all times. In addition, many high-risk items, for example, 

landing gear, are designed for low noise but will require new, unconventional, and 

complex approaches. 

Recommended risk management strategy: Assume the risk. (Even if only two-thirds of 

the predicted noise benefits are feasible, the SAX-40 still represents a very quiet aircraft). 

6. The challenge of correctly assessing the public’s reaction to an unconventional 

configuration such as the BWB is critical. This issue cannot be understated given the fact 

that public perception (even if the perception is incorrect) can be a showstopper for 

commercial viability. The BWB represents a radical departure from the traditional tubewith-wings design, and the human-factors issues must be addressed. Simply identifying 

the issues is a challenge. For example, the BWB may be perceived as less safe than the 

tube-with-wings configuration because it looks different. Public assessment through 

surveys, interaction with full-scale mock-ups, and flight simulators must be performed to 

address this risk. 

Recommended risk management strategy: Mitigate. 

In addition to these high-risk items, the ATB comparisons should be considered as well. The 

results show that a significant weight penalty is associated with the SAX-40 concept when 

compared with the baseline. This weight penalty will translate into higher RDT&E and 

production costs and can be roughly viewed as the cost of achieving the extremely low noise 

performance promised by the SAX-40. Although the SAX-40 serves as an example of what a 

commercial transport concept might look like when optimized solely for low noise, the 

commercial viability of such a design is questionable. The types of commercial transports that are 

viable are those that maximize operator profits. These aircraft are specifically targeted at reducing 

acquisition and operations costs and increasing revenue. Therefore, the following design drivers 

should be utilized: 

1. Minimize labor costs (e.g., maintenance, crew, servicing) 

2. Minimize fuel burn 

3. Maximize availability 

4. Minimize emissions 

5. Minimize noise 

6. Minimize structural weight 

The challenge is to develop a design that achieves the proper balance among these cost drivers. 

Optimizing only one driver, such as noise, will not achieve this balance. Optimizing singularly 

for low noise will, however, highlight technology areas and new concepts for noise reduction; 

thus, given that perspective the SAX-40 design was successful. 
51

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 Requirements Research Evacuation Study, September 2003. 
52

 

18. Rawdon, B. K.: “BWB - Cargo Handling and Airport Compatibility,” Boeing Presentation, Oct. 2, 1997. 

19. Hileman, J. I.; Spakovsky, Z. S.; Drela, M.; and Sargeant, M. A.: “Aerodynamic and Aeroacoustic Three- 

 Dimensional Design for a ‘Silent’ Aircraft,” AIAA-2006-0241, January 2006. 

20. Roman, D.: “MIT Silent Aircraft Configuration SAX-30 Aerodynamic Evaluation,” Boeing proprietary 

 internal presentation, May 2006. 

21. Roman, D.: “BWB4501L with MIT Silent Aircraft Configuration SAX-30 Airfoil Technology,” 

 Boeing proprietary internal presentation, May 2006. 

22. Roman, D.: Alien, J. B., and Liebeck, R. H., “Aerodynamic Design Challenges of the Blended-Wing- 

 Body Subsonic Transport,” AIAA- 2000-4335, August 2000. 

23. Lilley, G. M.: “A Quest for Quiet Commercial Passenger Transport Aircraft for Take-Off and Landing,” 

 AIAA-2004-2922, May 2004. 

24. Andreou, C.; Graham, W.; and Shin, H.-C.: “Aeroacoustic Study of Airfoil Leading Edge High-Lift 

 Devices,” AIAA-2006-2515, May 2006. 

25. Streett, 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-2006-0212, January 2006. 

26. Storms, B. L.; Hayes, J. A.; Jaeger, S. M.; and Soderman, P. T.: “Aeroacoustic Study of Flap-Tip Noise 

 Reduction Using Continuous Moldline Technology,” AIAA-2000-1976, June 2000. 

27. Herr, M. and Dobrzynski, W.: “Experimental Investigations in Low Noise Trailing Edge Design,” 

 AIAA-2004-2804, May 2004. 

28. Johnson, M. J. and Billman, G. M.: “Structural Technology and Analysis Program (STAP) Delivery 

 Order 0022: Continuous Moldline Technology (CMT) Flight Demonstration Program,” AFRL-VA-WP- 

 TR-2001-3027, 2001. 

29. Continuum Dynamics, Inc., URL: www.continuum-dynamics.com Accessed November 29, 2007. 

30. FlexSys, Inc., URL: www.flxsys.com Accessed November 29, 2007. 

31. Perkins, H. D.; Frate, F.; and Handschuh, R.: “Dual-Fan/Blended Wing Body (BWB) Conceptual Design 

 Study,” Internal Glenn Research Center Design Study Report, July 2004. 

32. Shin, H-C.; Graham, W.; Sijtsma, P.; Andreou, C.; and Faszer, A. C.: “Design and Implementation of a 

 Phased Microphone Array in a Closed-Section Wind Tunnel,” AIAA-2006-2651, May 2006. 

33. Hogbom, J. A.: "Aperture Synthesis with a Non-Regular Distribution of Interferometer Baselines,” 

 Astronomy and Astrophysics Supplements, Vol. 15 1974, pp. 417-426. 

34. Andreou, C.; Graham, W.; and Shin, H-C.: “Aeroacoustic Comparison of Airfoil Leading Edge High-Lift 

 Geometries and Supports,” AIAA-2007-0230, January 2007. 

35. Quayle, A. R., Dowling, A. P., Babinsky, H., Graham, W. R., Shin, H-C., and Sijtma, P., “Landing Gear 

 for a ‘Silent’ Aircraft,” AIAA-2007-0231, January 2007. 

36. Liu, Y.; Dowling, A. P.; and Shin, H-C.: “Effects of Surface Roughness on Airframe Noise,” AIAA-2006-2510, 

 May 2006. 
53

 

37. Hall, C. A. and Crichton D.: “Engine Design Studies for a Silent Aircraft,” GT2006-90559, ASME Turbo 

 Expo 2006, Barcelona, Spain, May 8-11, 2006. 

38. Rademaker, E. R.; Sarin, S. L.; and Parente, C. A.: “Experimental Investigation on the Influence of Liner Non- 

 Uniformities on Prevailing Modes (of Acoustic Propagation in Turbofan Engines),” AIAA 96-1682, May 1996. 

39. Bielak, G. W.; Premo, J. W.; and Hersh, A. S.: “Advanced Turbofan Duct Liner Concepts,” NASA CR-1999- 

 209002, February 1999. 

40. Yu, J.; Kwan, H.; Chien, E.; Nesbitt, E.; Uellenberg, S.; and Premo, J.: “Quiet Technology Demonstrator 2 

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

41. Gerhold, C. H.; Cabell, R. H.; and Brown, M. C.: “Development of an Experimental Rig for Investigation 

 of Higher Order Modes in Ducts,” AIAA-2006-2637, May 2006. 

42. Law, T. R. and Dowling, A. P.: “Optimization of Traditional and Blown Liners for a Silent Aircraft,” AIAA- 

 2006-2525, May 2006. 

43. Zlavog, G. and Eversman, W.: “A Statistical Approach To Optimum Design Of Inlet Acoustic 

 Treatment,” AIAA-2006-2523, May 2006. 

44. Agarwal, A. and Dowling, A. P.: “The Calculation of Acoustic Shielding of Engine Noise by the Silent 

 Aircraft Airframe, AIAA 2005-2996, May 2005. 

45. Clark, L. R. and Gerhold, C. H.: “Inlet Noise Reduction by Shielding for the Blended Wing-Body Airplane,” 

 AIAA-99-1937, May 1999. 

46. “757-200/300 Airplane Characteristics for Airport Planning,” D6-58327, Boeing Commercial Airplanes, 

 August 2002. URL: http://www.boeing.com/commercial/airports/757.htm Accessed November 29, 2007. 

47. “767 Airplane Characteristics for Airport Planning,” D6-58328, Boeing Commercial Airplanes, September 

 2005. URL: www.boeing.com/commercial/airports/767.htm Accessed November 29, 2007. 

48. “787 Dreamliner Program Fact Sheet,” URL: www.boeing.com/commercial/787family/programfacts.html Accessed November 29, 2007. 
54

 

APPENDIX A: SAI Press Coverage References 

(all links accessed December 6, 2007) 

1. “The Silent Aircraft Initiative,” Aero Gizmo, March 28, 2005. 

 URL: http://www.gizmag.com/go/3884/

2. Wikipedia entry, “Silent Aircraft Initiative,” 

 URL: http://en.wikipedia.org/wiki/Silent_Aircraft_Initiative

3. “Silent Aircraft: How it Works,” BBC News, November 6, 2006. 

URL: http://news.bbc.co.uk/1/hi/technology/6120132.stm

4. Bowler, T.: “The Birth of a Quieter, Greener Plane,” BBC News, November 6, 2006. 

URL: http://news.bbc.co.uk/2/hi/business/6113418.stm

5. “Silent Aircraft Design Launched,” BBC News, August 17, 2005. 

URL: http://news.bbc.co.uk/2/hi/uk_news/england/beds/bucks/herts/4158802.stm

6. “Airport Joining Silent Jet Team,” BBC News, January 18, 2005. 

URL: http://news.bbc.co.uk/2/hi/uk_news/england/beds/bucks/herts/4184513.stm

7. Twist, J.: “Flying Into a Silent Sky Future,” BBC News, May 27, 2004. 

URL: http://news.bbc.co.uk/2/hi/science/nature/3749301.stm

8. “Joint Quest for Silent Air Travel,” BBC News, May 20, 2004. 

URL: http://news.bbc.co.uk/2/hi/uk_news/england/cambridgeshire/3732721.stm

9. Layton, J.: “What’s the Silent Aircraft Initiative?” Howstuffworks. 

URL: http://travel.howstuffworks.com/sai.htm

10. Mulligan, C.: “Boffins Show Off ‘Silent’ Aircraft,” The Inquirer, November 6, 2006. 

URL: http://www.theinquirer.net/default.aspx?article=35552

11. “Silent Passenger Aircraft No Longer a Flight of Fancy,” Environment News Service, November 9, 2006. 

URL: http://www.ens-newswire.com/ens/nov2006/2006-11-09-01.asp

12. “Silent Aircraft Readies for Take-Off,” ABC News in Science, November 7, 2006. 

URL: http://www.abc.net.au/science/news/stories/2006/1783044.htm

13. “Silent Aircraft Creates a Buzz,” Cosmos Magazine, November 7, 2006. 

URL: http://www.cosmosmagazine.com/node/827

14. Jewell, M.: “Silent Aircraft Design Complete,” Discovery News, November 6, 2006. 

URL: 

http://dsc.discovery.com/news/2006/11/06/silentplane_tec.html?category=technology&guid=20061106100

030

15. “UK Engineers Open Flight Path to Quieter Aircraft,” Science Daily, May 21, 2004. URL: 

http://www.sciencedaily.com/releases/2004/05/040521072028.htm

16. “Researchers Unveil ‘Silent Aircraft,” CNET News, November 6, 2006. 

URL: http://news.com.com/2300-11397_3-6132739-1.html

17. Osborne, H.: “Scientists Unveil Plans for Eco-Friendly Plane,” Guardian Unlimited, November 6, 2006. 

URL: http://environment.guardian.co.uk/travel/story/0,,1940936,00.html

18. Haines, L.: “UK Scientists Seek Silent Aircraft,” The Register, May 27, 2004. 

URL: http://www.theregister.co.uk/2004/05/27/silent_aircraft_project/

19. “Working Towards The ‘Silent’ Aircraft Engine,” Innovations Report, July 22, 2004. 

URL: http://www.innovations-report.de/html/berichte/umwelt_naturschutz/bericht-31558.html

 20. Jewell, M.: “Design for Silent Aircraft to be Released,” LiveScience, November 6, 2006. 

URL: http://www.livescience.com/technology/061106_ap_silent_aircraft.html

21. “Engineers Unveil Plans for Super-Silent Plane,” National Public Radio, November 10, 2006. 

URL: http://www.npr.org/templates/story/story.php?storyId=6469064
55

 

22. Jha, A.: “On a Wing and a Whisper: The Airliner to End Runway Noise,” Guardian Unlimited, November 

7, 2006. URL: http://www.guardian.co.uk/science/story/0,,1941173,00.html

23. “Researchers Reveal Design for Silent, More Efficient Airplane,” What’s Next in S&T, November 7, 2006. 

URL: 

http://www.whatsnextnetwork.com/technology/index.php/2006/11/07/researchers_reveal_design_for_silent

_mor

24. “Silent Aircraft Readies for Take-off,” IOL Technology, November 7, 2006. 

URL: http://www.ioltechnology.co.za/article_page.php?iSectionId=2890&iArticleId=3524984

25. Maino, R.: “Listen Carefully – Here Comes the ‘Silent’ Plane,” British Embassy, Muscat, October 4, 2005. 

URL: 

http://www.britishembassy.gov.uk/servlet/Front?pagename=OpenMarket/Xcelerate/ShowPage&c=Page&ci

d=1061396962117&a=KArticle&aid=1128331988280

26. “Silent Aircraft Unveiled in England,” Gate2Board, November 13, 2006. 

URL: http://gate2board.typepad.com/gate2board/2006/11/silent_aircraft.html

27. Winters, J.: “Fly Silent, Fly Cheap,” ASME Mechanical Engineering, February 2007. 

URL: http://www.memagazine.org/feb07/features/flycheap/flycheap.html

28. Birch, S.: “Beyond 2030: the Silent Aircraft Initiative,” SAE Aerospace Engineering, 

April 2007. 

URL: http://www.sae.org/aeromag/techupdate/04-2007/2-27-3-6.pdf

29. Ott, J.: “Quiet as an Owl,” Aviation Week and Space Technology, February 12, 2007, p. 41. 

30. Warwick, G.: “Video: Silent Aircraft Institute Aims for 2025 Launch of Barely-Audible, Energy-Efficient 

Blended Wing-Body SAX-40 Airliner,” Flight, November 6, 2006. 

URL: http://www.flightglobal.com/articles/2006/11/06/210471/video-silent-aircraft-institute-aims-for2025-launch-of-barely-audible-energy-efficient-blended.html

31. “The Next Airliner Sounds Like a Stealth,” International Herald Tribune, 

November 6, 2006. 

URL: http://www.iht.com/articles/2006/11/06/business/jets.php

32. “Silent Aircraft,” BBC Radio, January 13, 2005. 

URL: http://www.bbc.co.uk/radio4/science/thematerialworld_20050113.shtml

33. “US-British Research Team Working on the ‘Silent Aircraft,” Taipei Times, November 7, 2006. 

URL: http://www.taipeitimes.com/News/worldbiz/archives/2006/11/07/2003335248

34. “Scientists Design a ‘Silent’ Airplane,” United Press International, November 7, 2006. 

URL: http://www.upi.com/NewsTrack/Science/2006/11/07/scientists_design_a_silent_airplane/

35. “Researchers Reveal Concept for Quiet Passenger Airplane,” USA Today, November 7, 2006. 

URL: http://www.usatoday.com/tech/news/techinnovations/2006-11-06-silent-aircraft_x.htm

36. Sherriff, L.: “Cambridge Engineers Moot ‘Silent’ Plane,” The Register, August 19, 2005. 

URL: http://www.theregister.co.uk/2005/08/19/silent_plane_cambridge/

37. “Researchers Develop a Nearly Silent Airplane,” Earth & Sky Radio Series. 

URL: http://www.earthsky.org/radioshows/50737/researchers-develop-a-nearly-silent-airplane

38. Mikulik, A.: “Silent Planes...The Way of the Future,” AboutMyPlanet, November 6, 2006. 

URL: http://www.aboutmyplanet.com/environment/silent-planesthe-way-of-the-future

39. “Blended Wing Concept 3000 Times Quieter, 35% More Efficient,” Treehugger, November 11, 2006. 

URL: http://www.treehugger.com/files/2006/11/blended_wing_co.php

40. Howe, P .J.: “MIT Helping to Build a Jet that Could be Quiet as a Car,” Boston Globe, November 6, 2006. 

URL: 

http://www.boston.com/business/technology/articles/2006/11/06/mit_helping_to_build_a_jet_that_could_b

e_quiet_as_a_car/
56

 

APPENDIX B: BWB Literature Survey 

• May 1984 

Douglas Aircraft input to the NRC Vehicle Applications Panel Group E.

Desirable features of future commercial aircraft are low cost, noise, and emissions. BWB mentioned in 

context of long-haul military airlifters only, not commercial transports. No data. 

• October 1990 

Callaghan, J. T. and Liebeck, R. H.: “Some Thoughts on the Design of Subsonic Transport Aircraft for the 

21st Century,” SAE Paper No. 901987, October 1990.

 This paper was a response to Dennis Bushnell’s challenge to industry to develop innovative concepts 

and is cited in NASA-CR-4624 below. Contains a nice introduction showing technology and cost 

trades with return on investment crossover based on fuel price. This paper compares the 

derivative/evolutionary path for improvements with a revolutionary path (BWB). Interesting 

progression from DC-10 to MD-11 to MD-11-Advanced to a year 2000 synergistic technology 

transport. Then a MD-BWB is compared with the synergistic technology transport; the BWB offered a 

50 percent increase in L/D and a 25 percent reduction in fuel burn. 

• September 1994 

Liebeck, R. H.; Page, M.A.; and Rawdon, B. K.: “Concepts for Advanced Subsonic Transports,” NASA 

CR-4624, September 1994. 

The relatively crude initial study in SAE Paper 901987 above provided the incentive for this study. 

Purpose was to develop and compare a conventional tube with wings (baseline), a BWB, and a pure 

spanloader, all utilizing the same technology levels. Requirements envisioned for a 2020 entry into 

service (EIS) were 800 passengers, 7000-nm range, 11,000-ft takeoff length, 155 knots Vap, 0.85 cruise 

Mach, and 35,000-ft initial cruise altitude. The structural problem of pressurizing a noncircular 

passenger cabin was assumed to be solved efficiently. Given this, the BWB fuselage weight was 6,000 

lb higher than the conventional baseline. BWB cruise L/D of 27.2 resulted in 105,000 lb fuel load 

savings (27 percent less) versus conventional (cruise L/D = 20.6). The TOGW was 14 percent less than 

the conventional baseline, OEW was 10 percent less. NASA analysis showed a much better 

conventional baseline, reducing the BWB benefits to 6 percent on TOGW and 2 percent on OEW, and 

17 percent on fuel. 

• January 1996 

Liebeck, R. H.; Page, M.A.; Rawdon, B. K.; Potsdam, M. A.; Wakayama, S.; and Girvin, R.: “BWB 

Configuration Control Document, CCD-2,” NAS1-20275 Deliverable, January 1996.

 Update of CCD-1, published 12-9-94. Preliminary design of BWB-1-1 continues to offer benefits over 

conventional baseline: 15 percent lower TOGW, 28 percent lower fuel burn. Update has yielded a 

preferred engine arrangement (three engines with BLI inlets), a pressure vessel concept (5.5-in thick 

composite sandwich with aluminum honeycomb core), a new cabin arrangement, and slats on the outer 

wing. 800 passenger BWB with 7000-nm range. 2020 EIS with 2015 technology; however, 2012 EIS 

is possible. 

• October 1996 

Mukhopadhyay, V.: “Structural Concepts Study of Non-circular Fuselage Configurations,” AIAA/SAE

WAC-67, October 1996.

 Analysis of several noncircular fuselage concepts, including flat and vaulted shell, deep honeycomb 

sandwich shell, and ribbed double-wall shell. Flat sandwich shell found to be preferable to the vaulted 
57

 

shell because of its superior buckling stiffness. Vaulted double-skin ribbed shell configurations were 

superior because of weight savings, load diffusion, and fail-safe features. 

• June 1997 

Potsdam, M. A.; Page, M.A.; and Liebeck, R. H.: “BWB Analysis and Design,” AIAA-97-2317, June 1997.

 Focus on CFD methodologies for BWB wing aerodynamic design. No validation data available. 

Configuration was 800 passenger, three-engine design with t/c of approximately 18 percent. Claim 28 

percent fuel-burn reduction versus conventional, but no baseline given, no backup to support 

performance claims. 

• October 1997 

Liebeck, R. H.; Page, M.A.; Rawdon, B. K.; Potsdam, M. A.; Wakayama, S.; and Girvin, R.: “BWB 

Configuration Control Document 3, CCD-3,” NAS1-20275, October 1997.

 This update to CCD-2 focuses on the test phase results from the contract, including Langley’s 14-ft by 

22-ft low-speed test of a 4 percent model, global finite-element method (FEM) analysis, Stanford 6-

percent-scale flying model, BWB-17, and University of Southern California’s inlet testing with vortex 

generators. 

• January 1998 

Liebeck, R. H.; Page, M.A.; and Rawdon, B. K.: “Blended-Wing-Body Subsonic Commercial Transport,” 

AIAA-98-0438, January 1998.

 Summary of three-year NASA-sponsored feasibility study. 800 passenger 7000-nm-range BWB 

compared with conventional design, both with 2020 EIS. Reduction of 27 percent in fuel burn for 

BWB. Engine installation trade study, mid-bifurcated BLI was lightest but impractical, so upper Sduct BLI was utilized. Five inch thick structural shell concept for fuselage. Tests: 

 Low-speed powered 4 percent model, Langley’s 14-ft by 22-ft tunnel 

 NTF test in spring 1997 

 Six-percent flight-test model (BWB-17, Stanford) 

• January 1998 

Anabtawi, A. J.; Blackwelder, R.; Liebeck, R. H.; and Lissaman, P.: “Experimental Investigation of 

Boundary Layer Ingesting Diffusers of a Semi-Circular Cross Section,” AIAA-98-0945, January 1998.

Baseline (no vortex generators) total pressure loss coefficient = 19 percent, and distortion coefficient = 

63 percent. With vortex generators, these numbers were 17 to 21 percent and 24 to 54 percent. 

Boundary-layer diversion yielded 9.6 percent and 10.5 percent for these coefficients. 

• September 1998 

Wakayama, S. and Kroo, I.: “The Challenge and Promise of BWB Optimization,” AIAA-98-4736, 

September 1998.

 Used NASA’s Advanced Subsonic Technology (AST) Task 18 BWB concept as baseline (855 

passengers, 7500-nm range); applied an updated WingMOD tailored for BWB. Used 134 design 

variables and 705 constraints (90 active) to show design improvement due to MDO. 

• September 1998 

Wakayama, S.: “Multidisciplinary Design Optimization of the Blended-Wing-Body,” AIAA-98-4938, 

September 1998.

 Write-up of same work from previous citation, 4736, with more information on actual updates made to 

WingMOD to handle BWBs. Aerodynamics updates to handle 3D effects calibrated with CFD results. 
58

 

• August 1998 

Pelkman, R.A.: “Key Findings and Conclusions from an NTF Wind Tunnel Test of an Initial BlendedWing-Body Concept,” Boeing Report No. CRAD-9402-TR-3985, August 1998.

 Wing/body model for high-Reynolds-number (Re) testing for cruise Mach ranging from 0.5 to 0.88, 

and low speed at Mach = 0.25. Re = 3.5 million (to assess viability of transonic test), 12.3 million (to 

assess one-fourth scale model) and 25 million (max for NTF safety, close to cruise Re of full-scale 

vehicle) and low-speed (Re = 20.5 million) points were tested. Main purpose was to verify CFD 

predictions. Good matches at 0.85, poorer at 0.5. Predicted L/D at 0.85 cruise was 20.5 (with no 

engines, but Re will be higher at full scale). 

• January 1999 

Anabtawi, A. J.; Blackwelder, R. F.; Lissaman, P.; and Liebeck, R. H.: “An Experimental Investigation of 

Boundary Layer Iingestion in Diffusing S-Duct With and Without Passive Flow Control,” AIAA-99-0739,

January 1999.

 Follow-up investigation from AIAA-98-0945, compared straight duct diffusers with S-duct diffusers 

both with and without vortex generators. Ducts had thinner entrance lips and were shorter. Pressure 

recovery was 93 percent for straight duct, 90 percent for S-duct. Distortion in S-duct was 14 percent, 

vortex generators decreased this to 3.4 to 6 percent. The distortion numbers have been corrected from 

the low Mach number test condition to M = 0.85. This correction was not done in the previous work. 

• May 1999 

Clark, L. R. and Gerhold, C. H.: “Inlet Noise Reduction by Shielding for the Blended Wing Body 

Airplane,” AIAA-99-1937, May 1999.

 BWB provided significant inlet noise shielding. Noise radiating downward into the forward sector 

below the model was reduced by 20–25 dB due the presence of the BWB model. Tests: 

 4-percent scale fiberglass three-engine nacelle BWB 

• June 1999 

Anabtawi, A. J.; Blackwelder, R. F.; Lissaman, P.; and Liebeck, R. H.: “An Experimental Study of Vortex 

Generators in Boundary Layer Ingesting Diffusers with Centerline Offest,” AIAA-99-2110, June 1999.

 The S-duct total pressure recovery was 90 percent with and without vortex generators; total pressure 

distortion was 77 percent without vortex generators (normalized to dynamic pressure) and 11 percent 

in some cases with vortex generators that were the same height as the boundary-layer thickness (30–40 

percent of inlet height as calculated by CFD). Description of Mach number effects and corrections 

appears to be inconsistent with January paper. “Generally speaking, a typically acceptable distortion 

level (normalized to q) would be no more than 20 percent.” 

• July 1999 

Anabtawi, A. J.; Blackwelder, R. F.; Lissaman, P.; and Liebeck, R. H.: “An Experimental Study of the 

Effect of Offset on Thick Boundary Layers Flowing Inside Diffusing Ducts,” AIAA-99-3590, July 1999.

 Boundary layers measured to be on the order of one-third of the inlet height, confirming previous CFD 

estimates. The S-duct geometry caused boundary-layer separation. 

• August 2000 

Roman, D.; Allen, J. B.; and Liebeck, R. H.: “Aerodynamic Design Challenges of the Blended Wing Body 

Subsonic Transport,” AIAA-2000-4335, August 2000.
59

 

 Challenges include: volume requirements (led to center wing t/c of approximately 17 percent); deck 

angle versus cruise trim trade study; secondary power requirements for control surfaces; lower wing 

loading required for acceptable approach speeds; buffet and stall characteristics driven by outboard 

airfoils; high alpha during approach; propulsion airframe integration; and manufacturing issues of 

complex 3D shapes. Other proprietary constraints hinted at but not detailed. CFL3D utilized for clean 

wing CFD calculations, compared with NTF data (1998 test). OVERFLOW utilized for PAI. First 

generation BWB was 800 passenger, second generation was 450 passengers (referred to as current 

generation in paper). New class of transonic airfoils were designed to reduce t/c and improve 

manufacturability. Longitudinal trim problem solved using TE camber and wing twist. Problems 

remain with deck angle, secondary power, and PAI. Mail slot BLI, isolated submerged BLI, isolated 

with boundary-layer diffusion (BLD) have all been deemed unacceptable. Current concept is strutmounted engines. 

• September 2000 

Wakayama, S.: “Blended-Wing-Body Optimization Problem Setup,” AIAA-2000-4740, September 2000.

 WingMOD applied to next iteration of BWB, focusing on cabin geometry, balance, and stability and 

control issues. Detailed description of how WingMOD is used to model and design a BWB. 

Optimization added wing sweep (35.7 to 42.8 deg) to balance the aircraft. 

• January 2001 

Pambagjo, T. E.; Nakahashi, K.; Obayashi, S.; and Matsushima, K.: “Aerodynamic Design of a Medium 

Size Blended-Wing-Body Airplane,” AIAA-2001-0129, January 2001.

 200-passenger, 2000-nm-range BWB studied for aerodynamic feasibility. Minimum cabin height of 2 

m (78 in.) resulted in maximum of 15 percent t/c. Results indicate that this size BWB may have greater 

wetted area than conventional counterpart. Inverse design process and new smoothing algorithms 

applied. L/D = 18.9. 

• January 2002 

Liebeck, R. H.: “Design of the BWB Subsonic Transport,” AIAA-2002-0002, January 2002.

 2002 Wright Brothers Lecture Paper. Gives background of BWB formulation (CR 4624, Sep. 1994) 

and initial development (1994–1997 NAS1-20275, CRAD-9405-TR-3780, and AIAA-98-0438). After 

this, in-house preliminary design study initiated on BWB-450 concept (468? 478? passenger, 7750 

nm? 9000 nm? range). WingMOD used to size this concept. Podded engines and BLD embedded 

engines studied; podded engines selected, nose-down thrust moment deemed acceptable. Center-body 

structural concept skin/stringer outer surface with 5- to 6-in. deep stringers, internal Y-braced ribs. 

Claims this weighs little more than a conventional fuselage, but table shows 21 percent heavier than 

A380 fuselage? 

• February 2002 

Green, J. E.: “Greener by Design – The Technology Challenge,” The Aeronautical Journal, Vol. 106, No. 

1056, February 2002, pp. 57–113.

 Simplistic parametric sizing studies, no numerical analysis. Proposes multiple segments of shorter 

range aircraft instead of single segment of a long range aircraft. BWB one of eleven concepts studied, 

recommended as a key area of future research. 

• April 2002 

Willcox, K. and Wakayama, S.: “Simultaneous Optimization of a Multiple-Aircraft Family,” AIAA-2002-

1423, April 2002.
60

 

 Presents development of framework to simultaneously optimize two common aircraft in a family, a 

475-passenger and a 272-passenger BWB, both with 8550-nm range. Weight savings were in the tenths 

of a percent of TOGW. Focus on MDO formulation. 

• September 2002 

Laban, M.; Arendsen, P.; Rouwhorst, W.; and Vankan, W.: “A Computational Design Engine for MultiDisciplinary Optimisation with Application to a Blended Wing Body Configuration,” AIAA-2002-5446, 

September 2002.

 Computational Design Engine (CDE) for multidisciplinary, multifidelity, multilevel, multisite design 

and analysis is presented, with sample application of BWB in aero (panel codes to Euler to full NavierStokes), structures (bending-beam theory to full blown FEM), and flight mechanics (low-fidelity 

stability and control to a full handling qualities analysis of closed-loop aircraft system). “CDE system 

applied to the Breguet range optimization of the BWB for constant MTOW.” Focuses on CDE process 

more than BWB design. 

• September 2002 

Qin, N.; Vavalle, A.; Le Moigne, A.; Laban, M.; Hackett, K.; and Weinerfelt, P.: “Aero Studies for 

Blended Wing Body Aircraft,” AIAA-2002-5448, September 2002.

European BWB concept used for CFD studies; 80-m span, propulsion system not included. On 

baseline, outer wing stalls at cruise M = 0.85 and α = 4 deg (cruise α = 3 deg). Twist distribution 

redesigned to alleviate high outboard loading. 

• September 2002 

Mukhopadhyay, V.; Sobieszczanski-Sobieski, J.; Kosaka, I.; Quinn, G.; and Charpentier, C.: “Analysis 

Design and Optimization of Non-cylindrical Fuselage for Blended-Wing-Body Vehicle,” AIAA-2002-

5664, September 2002.

 Deep sandwich panel and ribbed shell concepts shown to be structurally inefficient. These flat panel 

concepts weighed 25–38 kg/m2

. Alternative approach using multibubble fuselage with outer ribbed 

shell was more promising (20–30 percent lower weight than flat-panel concepts); however, still twice 

as inefficient as a cylindrical fuselage. 

• September 2002 

Rodriguez, D. L.: “A Multidisciplinary Optimization Method for Designing Boundary Layer Ingesting

Inlets,” AIAA-2002-5665, September 2002.

 CFL3D, NEPP (engine analysis code) are linked, and NPSOL (gradient-based nonlinear optimizer) is 

used for optimization. Fuel burn was minimized while maintaining cruise lift. Emphasis of paper on 

MDO method, not BWB design results. 

• September 2002 

Gilmore, R.; Wakayama, S.; and Roman, D.: “Optimization of High-Subsonic Blended-Wing-Body 

Configurations,” AIAA-2002-5666, September 2002.

 BWB cruise speed pushed from M = 0.85 to 0.95; M = 0.93 configuration was studied in more detail. 

Best option was M = 0.90, but propulsion airframe interference was not assessed. “BWB sonic 

cruiser” study. 

• January 2003 

Gilmore, R.; Wakayama, S.; and Roman, D.: “Aerodynamics of High-Subsonic Blended-Wing-Body

Configurations,” AIAA-2003-0554, January 2003.
61

 

 Similar to September 2002 paper. CFL3D N-S CFD results used to calibrate WingMOD. The M = 

0.93 BWB-6-250B configuration optimized (propulsion not modeled). 

• June 2003 

Pilczer, D.: “Noise Reduction Assessments and Preliminary Design Implications for a Functionally-Silent 

Aircraft”, Massachusetts Institute of Technology, M.S. Thesis, June 2003.

Introduces “silent” aircraft concept utilizing “quiet lift” using clean BWB wing and seamless TE, 

“silent thrust” using embedded propulsion (nine engines) with high-aspect-ratio nozzles and low jet 

velocities, and a “hidden TE” by ingesting the boundary layer avoiding the scattering of turbulent 

boundary-layer flow across the TE. BLI improves range by 5.4 percent assuming 60 percent of engine 

mass flow stems from the boundary layer. Concept predicted to reduce approach and takeoff EPNL by 

30 dB and 22.5 dB, respectively. “Silent drag” requirements quantified, but no solutions proposed. 

• July 2003 

Wakayama, S.; Gilmore, R.; and Brown, D.: “Design Trades for a Large BWB Freighter,” AIAA-2003-

2503, July 2003.

Design BWB to carry smaller sized versions of standard shipping containers (intermodal containers). 

5000-nm range. Measuring efficiency as (Wpayload*range*cruise Mach)/TOGW, the “BWB-8-1000” 

offers significant improvement (44 percent) over baseline (current freighters). 

• July 2003 

Liebeck, R. H.: “BWB Design Challenges,” AIAA-2003-2659, July 2003.

Proposes common airframe for both military (tanker, cargo, bomber) and commercial application to 

make development viable. States “BWB must exceed performance of existing airplanes.” Challenges 

are commonality, speed and flight mechanics. Breguet range equation used for performance estimates, 

BWB compared with A380F, C-5A, KC-10A, B-52, and B-2. 

• July/August 2003 

Willcox, K. and Wakayama, S.: “Simultaneous Optimization of a Multiple-Aircraft Family,” Journal of 

Aircraft, Vol. 40, No. 4, July/August 2003, pp. 616–622.

Journal article of AIAA-2002-1423, April 2002. See above for summary. 

• November 2003 

Ko, A.; Leifsson, L. T.; Mason, W. H.; Schetz, J. A.; Grossman, B.; and Haftka, R. T.: “MDO of a 

Blended-Wing-Body Transport Aircraft with Distributed Propulsion,” AIAA-2003-6732, November 2003.

 Examines effect of distributed propulsion using eight embedded engines and a hybrid jet flap/jet 

wing/conventional exhaust approach. Shows 5.4-percent TOGW improvement compared with 

conventional BWB (four embedded engines). Effect of jet wing on pressure distribution of body not 

included. The jet is expected to entrain the flow over the body and increase drag. Breguet range 

equation used for performance estimates. Refreshingly good paper relative to many others mentioned 

above (acknowledgements to Kimmel and Guynn). 

• December 2003 

Daggett, D. L.; Kawai, R.; and Friedman, D.: “Blended Wing Body Systems Studies: Boundary Layer 

Ingestion Inlets with Active Flow Control,” NASA CR-2003-212670, December 2003.

 Sponsored by the Ultra Efficient Engine Technology (UEET) project, Karl Geiselhart was NASA 

technical monitor. Utilized BWB 450-1U design as baseline. Overall performance compared with 747-

400. Study concluded BLI with active flow control (pulsating air jets) showed 5.5 percent fuel 
62

 

efficiency benefit compared with conventional pylon-mounted engine. Assumes active flow control 

technology works and requires negligible power. 

• January/February 2004 

Liebeck, R. H.: “Design of the Blended Wing Body Subsonic Transport,” Journal of Aircraft, Vol. 41, No. 

1, January/February 2004, pp. 10–25.

 Journal article version of 2002 Wright Brothers lecture and paper. Good summary of concept 

development. BWB-450 (primary structure assumed to be mostly composite) compared with Airbus 

A380-700 (primary structure assumed to be mostly aluminum), BWB showed 32-percent lower fuel 

burn per seat. 

• May 2004 

Manneville, A.; Pilczer, D.; and Spakovszky, Z. S.: “Noise Reduction Assessments and Preliminary Design 

Implications for a Functionally Silent Aircraft,” AIAA-2004-2925, May 2004.

 Study funded by Russ Thomas, NASA Langley Research Center. Explores the technological barriers to 

silent aircraft and assesses potential noise-reduction concepts and technologies. Aerodynamically 

smooth lifting surfaces (no flaps or slats), hidden TE, BLI, UHB engines (BPR = 40), distributed 

propulsion/high-aspect-ratio nozzles, and silent drag with engine air brakes were the main ideas 

proposed. Gear noise was neglected by assuming gear deployment very close to the ground. Idea for 

common core that drives multiple fans proposed. Concludes BWB-type aircraft with embedded 

propulsion can yield large noise reductions. 

• July/August 2004 

Mukhopadhyay, V.; Sobieszczanski-Sobieski, J.; Kosaka, I.; Quinn, G.; and Vanderplaats, G. N.:

“Analysis, Design and Optimization of Noncylindrical Fuselage for BWB Vehicle,” Journal of Aircraft, 

Vol. 41, No. 4, July/August 2004, pp. 925–930.

 Journal article version of AIAA-2002-5664. See summary above. 

• September 2004 

Hill, G. A.; Brown, S. A.; Geiselhart, K. A.; and Burg, C. M.: “Integration of Propulsion-AirframeAeroacoustic Technologies and Design Concepts for a Quiet Blended-Wing-Body Transport,” AIAA-2004-

6403,September 2004.

Range of noise reduction technologies as applied to a 300-passenger, GE90-powered BWB 

configuration were studied using two evaluation frameworks. These evaluation frameworks were 

applied to rank PAA (Propulsion Airframe Aeroacoustics) technologies. Results show all technologies 

have performance penalties that become tolerable only if noise requirements are design drivers. 

Second, the concept was sized and analyzed using NPSS/WATE, FLOPS, and ANOPP assuming BLI 

inlets with AFC. Various nozzle and PAA technologies examined and several recommended for 

further study. 

• November 2004 

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

Aircraft Propulsion Airframe Integration and Configurations,” 8th CEAS Workshop Paper, November 2004.

 General PAA discussion with focus on airframe shielding. The forward shielding of the BWB concept 

was highlighted; however, aft shielding remains a challenge. 

• January 2005 
63

 

Campbell, R. L.; Carter, M. B.; Pendergraft, O, C.; Friedman, D. M.; and Serrano, L.: “Design and Testing 

of a Blended Wing Body with Boundary Layer Ingestion Nacelles at High Reynolds Numbers,” AIAA2005-0459, January 2005.

 A knowledge-based aerodynamic design method (CDISC), coupled with an unstructured-grid N-S flow 

solver (USM3D), was used to improve PAI for a BWB with BLI nacelles. An NTF model was built 

and tested to validate predictions. Sponsored by UEET in 2004 timeframe. 

• February 2005 

Diedrich, A. J.: “The Multidisciplinary Design and Optimization of an Unconventional, Extremely Quiet 

Transport Aircraft,” Massachusetts Institute of Technology, M.S. Thesis, February 2005.

 In-house tools and the Boeing WingMOD program was used to design a silent BWB. Results showed 

direct operating costs equal to that of B-747-400. Assumed technology levels for 2030 EIS. BWB 

configuration appears to be selected a priori. Design mission is 250-passenger, 4000-nm range. Inhouse model utilized Breguet range equation for cruise performance. Various parametric studies 

performed using in-house model and WingMOD. Early SAX-10 configuration proposed here. Specific 

fuel consumption was 0.54 lbm/lbf-hr. 

• April 2005 

Beran, P. S.; Snyder, R. D.; Strong, D.; Bryson, D.; Hur, J.; and Strganac, T.: “Static Nonlinear Aeroelastic 

Analysis of a Blended Wing Body,” AIAA-2005-1944, April 2005.

Static aeroelastic analysis of BWB for Sensorcraft application performed using NASTRAN to obtain 

deflections caused by linearly computed airloads. These deflections were then transferred to highfidelity methodology (Air Force Air Vehicles Unstructured Solver (AVUS)) to compute nonlinear 

aerodynamics. Computational framework is MDICE (Multidisciplinary Computing Environment), 

which is designed to link high-fidelity tools, in this case aerodynamics and structures. Paper focuses on 

geometry representation for BWB and grid generation and deformation. 

• April 2005 

Mukhopadhyay, V.: “Blended Wing Body Fuselage Structural Design for Weight Reduction,” AIAA-2005-

2349, April, 2005.

 Lessons learned from earlier 800-passenger BWB studies, and current efforts on 480-passenger version 

described. Use of rapid FEM tools to design and analyze Y-braced fuselage derivatives were presented. 

Double-bubble, triple-bubble, and four- and five-bubble design concepts were presented and shown to 

be superior to the vaulted-shell concepts of the earlier generation. Finally, a Y-braced box-type 

fuselage concept using stitched resin-film-injected (RFI) composites with foam cores was presented as 

a practical alternative to ease manufacturing concerns. 

• May 2005 

Reimann, C. A.; Tinetti, A. F.; and Dunn, M. H.: “Noise Prediction Studies for the Blended Wing Body

Using the Fast Scattering Code,” AIAA-2005-2980, May 2005.

 Fast Scattering Code (FSC) validated by comparisons of predicted values with measured values from 3 

percent scale model acoustic tests performed at NASA Langley Research Center. Results show that 

FSC can successfully predict measured acoustic behavior. 

• September 2005 

Green, J. E.: “Air Travel—Greener by Design,” The Aeronautical Journal, September 2005.

 Update on the 2002 publication. Mentions BWB and SAI concept in areas of noise reduction and 

increased L/D for reduced fuel burn. Recommends further study of BWB concept. 
64

 

• January 2006 

Hileman, J. I.; Spakovszky, Z. S.; Drela, M.; and Sargeant, M. A.: “Aerodynamic and Aeroacoustic ThreeDimensional Design for a ‘Silent’ Aircraft,” AIAA-2006-0241, January 2006.

 Presents design methodology for BWB airframe shape. States, “To be viable, the aircraft [SAI 

concept] requires a fuel burn comparable to modern conventional aircraft.” Mission is 215 passengers 

and 5000-nm range. Paper addresses problem of obtaining low noise approach with minimal impact to 

cruise L/D. Suggested approaches are to increase centerbody LE camber, utilize thrust vectoring for 

pitch trim on approach, use quiet LE high-lift devices such as drooped LE, use quiet perforated spoilers 

to produce drag on approach, and use landing-gear fairings. 

• January 2006 

Diedrich, A.; Hileman, J.; Tan, D.; Willcox, K.; and Spakovszky, Z: “Multidisciplinary Design and 

Optimization of the Silent Aircraft,” AIAA-2006-1323, January 2006.

 MDO design tool development and application for BWB is described. 2030 EIS date postulated, but 

baseline is the Boeing 787. 215 passengers, 5000-nm range. Tool combines a modified version of 

WingMOD with acoustics models and semi-empirical propulsion, aero, weights, and mission modules. 

WingMOD provides an initial input to the other modules, which then iterate on thrust and fuel needed 

to complete the mission. Acoustic modules presented. Cruise module uses Breguet range equation. 

SAX-12 design presented here. SFC is 0.50 lbm/lbf-hr. Noise performance compared with 767-300. 

• January 2006 

Wakayama, S.: “Subsonic Multi-Role Aircraft,” AIAA-2006-1513, January 2006.

 Paper examines a conceptual tanker/bomber aircraft (BWB configuration), describes compromises 

needed for commonality, and explores potential capabilities. Performance of BWB concepts compared 

with existing aircraft (i.e., B-52, KC-10). BWB tanker/bomber concept of operations developed. 

Results show 60 percent fuel-burn savings over current aircraft, plus a 40 percent reduction in 

development cost compared with two independent development efforts. 

• July 2006 

Stone, J. R.; Krejsa, E. A.; Berton, J. A.; and Kim, H. D.: “Initial Noise Assessment of an EmbeddedWing-Propulsion Concept Vehicle,” AIAA-2006-4979, July 2006.

 Cruise efficient short takeoff and landing (CESTOL) vehicle concept with embedded-wing-propulsion 

utilized by consultants to Diversitech for NASA GRC. The study concept is a product of a Boeing 

configuration definition study by Kawai for a 170-passenger, 180,000-lb TOGW, twelve-engine BWB 

design with internally blown flaps for STOL performance. Noise-prediction capability for this concept 

was developed based largely on NASA’s FOOTPR code. Significant noise reduction relative to current 

state of the art is predicted. 

• September 2006 

Kim, H. D.; Berton, J. J.; and Jones, S. M.: “Low Noise Cruise Efficient Short Take-Off and Landing

Transport Vehicle Study,” AIAA-2006-7738, September 2006.

 Presents overall CESTOL concept from NASA-sponsored study. Twelve-engine, 170 passenger, 3000-

nm range BWB from Boeing WingMOD folks (Kawai, Wakayama). 

• September/October 2006 
65

 

Campbell, R. L.; Carter, M. B.; Pendergraft, O, C.; Friedman, D. M.; and Serrano, L.: “Design and Testing 

a Blended Wing Body with Boundary-Layer Ingestion Nacelles,” Journal of Aircraft, Vol. 43, No.5, 

September-October 2006, pp. 1479–1489.

 Journal version of AIAA-2005-459. (See notes above.) 

• November 2006 

Peigin, S. and Epstein, B.: “CFD Driven Optimization of BWB Aircraft,” AIAA Journal, Vol. 44, No.11, 

November 2006, pp. 2736–2745.

Multipoint, multiconstrained optimization of BWB for minimum drag using genetic algorithms and a 

combination of full N-S computations and reduced-order methods. Large drag reductions shown. 

• December 2006 

Kawai, R. T.; Friedman, D. M.; and Serrano, L.: “Blended Wing Body (BWB) Boundary Layer Ingestion 

(BLI) Inlet Configuration and System Studies,” NASA CR-2006-214534, December 2006.

 More detailed follow-on from 2003 study (see NASA CR-2003-212670 above). Previous study 

predicted 5.5-percent fuel efficiency benefit for BLI with AFC, but utilized simplified method to 

account for first-order airframe integration effects. This study utilized fully viscous N-S analysis. The 

BWB 450-1U configuration is utilized (three UEET direct-drive fan engines in podded nacelles). 

Results show a 10-percent fuel efficiency benefit. Details (and potential installation penalties) of AFC 

system not presented. 

• January 2007 

Staelens, Y. D.; Blackwelder, R. F.; and Page, M. A.: “Novel Pitch Control Effectors for a Blended Wing 

Body Airplane in Takeoff and Landing Configuration,” AIAA-2007-0068, January 2007.

 Introduces the concept of belly flaps to enhance the lift and pitching moment of a BWB during takeoff 

and landing. Wind-tunnel tests of a model patterned after the BWB-450 concept show an increase of 

35 percent for takeoff CL and an increase of 10 percent for pitching moment with the belly flap 

deployed to 90 deg. 

• January 2007 

Plas, A. P.; Sargeant, M. A.; Madani, V.; Crichton, D.; Greitzer, E. M.; Hynes, T. P.; and Hall, C. A.: 

“Performance of a Boundary Layer Ingesting (BLI) Propulsion System,” AIAA 2007-0450, January 2007.

Utilized a power-saving coefficient (PSC), which is the difference in power required between 

traditional non-BLI podded engines (Pref) and BLI engines (Pbli), divided by Pref. Three models of 

increasing fidelity were used to examine the distortion transfer and propulsion system performance. 

Low distortion transfer through the fan will lead to higher PSC values. Results indicate BLI will 

decrease fuel burn up to 3.8 percent. Engine aeromechanical response issue noted briefly, no solutions 

proposed. More research needed. 

• January 2007 

Hileman, J. I.; Reynolds, T. G.; de la Rosa Blanco, E. R.; Law, T. R.; and Thomas, S.: “Development of 

Approach Procedures for Silent Aircraft,” AIAA-2007-0451, January 2007.

 Approach trajectory optimized for low noise to include a flight path angle of 3.9 deg, velocity of 60.8 

m/s (118 knots), and a threshold displacement of 1.2 km (3937 ft). Noise footprint was then calculated 

given these assumptions, resulting in 61 dBA at the airport perimeter; EPNL was 71.9 EPNdB. Trim 

and drag production on approach (α = 15.6 deg) are accomplished by a combination of elevon 

deflection (18.5 deg up) and thrust vectoring (30 deg down). Approach high-lift configuration includes 
66

 

a 27-deg drooped LE, elevator deflection of –16 degrees and α of 21.9 deg (not sure how this relates to 

trim conditions listed above). 

• January 2007 

Hileman, J. I.; Spakovszky, Z. S.; Drela, M.; and Sargeant, M. A.: “Airframe Design for ‘Silent Aircraft,’” 

AIAA-2007-0453, January 2007.

 Main focus of this paper is the aerodynamic shaping of the airframe centerbody, but also includes a 

summary of the evolution of the SAX concepts and various design assumptions, such as 10-percent 

structural weight reduction as a result of advanced composites (2025), and fuel burned during climb 

was 2 percent of maximum takeoff weight. 

• January 2007 

de la Rosa Blanco, E. R.; Hall, C. A.; and Crichton, D.: “Challenges in the Silent Aircraft Engine Design,” 

AIAA-2007-0454, January 2007.

 Several potential engine concepts studied. References to previous work published in Europe in 2005 

result in selection an of embedded engine with a single core driving three fans. Three engines were 

needed, resulting in nine fans, each 1.2 m in diameter. Variable-area 2D nozzles utilized, exit area is 45 

percent larger at takeoff than top of climb. Engine component design described (good reference for 

GRC evaluation). Engine weight table not complete. 

• January 2007 

Tam, R.; Belobaba, P.; Polenske, K. R.; and Waitz, I.: “Assessment of Silent Aircraft-Enabled Regional 

Development and Airline Economics in the UK,” AIAA-2007-0455, January 2007.

 London Heathrow and East Midlands airport are used to analyze relationships among airline 

operations, noise, local housing prices, and regional economic growth. Assumed purchase price of 

SAX-40 was $161M. Several regulatory scenarios studied, including “light green” and “dark green,” 

providing progressively greater operational and financial penalties for noisier aircraft. SAX-40 is the 

only profitable aircraft in the “dark green” scenario, assuming that purchase price and maintenance 

costs are no more than conventional alternatives (in 2020). 

• January 2007 

Crichton, D.; de la Rosa Blanco, E. R.; Law, T. R.; and Hileman, J. I.: “Design and Operation for Ultra 

Low Noise Take-off,” AIAA-2007-0456, January 2007.

 SAI target noise level was 60 dBA for 2025 aircraft technology (at the airport perimeter). Details given 

on takeoff profile design and performance. Analysis here indicates nozzle area should be 35 percent 

larger at takeoff than top of climb to achieve noise goal. Cumulative noise estimated to be 210 EPNdB, 

75 dB below Chapter 4 requirements. 
67

 

APPENDIX C : SAI Requirements 

Noise 

“Aircraft must be inaudible outside of a typical airport boundary.” 

Goal is 60 dBA. 

Airport assumptions: 

• 3000-m runway 

• 450 m from the side of the runway to the airport perimeter 

• 1000 m from both ends of the runway to the perimeter 

• 2200 m from perimeter (on the approach end of the runway) to the touchdown point 

Rationale: Aircraft noise is costly to society. Limiting aircraft noise to the airport boundary would result 

in significant benefits. The noise requirement is used to drive all aspects of the design to achieve the best 

possible result without unduly compromising performance. The goal of 60 dBA is equal to background 

noise inside an office or restaurant. 

Comments: The Effective Perceived Noise in Decibels (EPNdB) certification points are not located at the 

airport perimeter, so the SAI team estimated the certification cumulative noise to be 209.4 EPNdB, which 

is 75 dB below Chapter 4 noise requirements. This certification noise level can be considered an SAI 

requirement, along with the 60 dBA goal.

Technology Risk Assumption 

In general, a 2025 technology level (~TRL = 6) was assumed (e.g., for propulsion, materials). 

Rationale: To meet the requirements, low TRL technologies and aggressive technology assumptions are 

utilized. 

Mission 

Mission performance paramters: 

• 215 passengers in three-class configuration (assume 240 lb per passenger) 

• 5000 nm range 

• Cruise altitude of 40,000-45,000 ft 

• Cruise speed = 0.8 M 

• Reserve fuel for a 200-nm divert and a 45-minute loiter 

Rationale: The target is a midsize aircraft that best meets the noise goals. The cruise speed was selected 

to be similar to existing aircraft (737, 757, 767), and initially the worry about inlet design held the speed 

to 0.8 M. Subsequent work showed that the inlet could handle a higher cruise speed. 
68

 

Emissions 

Competitive with existing or next-generation aircraft. 

Rationale: Emissions characteristics are likely to be equal to or possibly greater in importance than noise 

for future air transports. 

Comments: No explicit emissions requirements were found; however, the general theme would be that 

emissions should be no worse than conventional transport aircraft in the same timeframe. 

Operations 

Airports: Heathrow and East Midlands 

Operational assumptions and characteristics that flow down from the noise requirement: 

• Thrust-managed takeoff 

• Approach speed of 60.8 m/s 

• CDA on a 3.9-deg glide slope 

• Displaced threshold of 1.2 km 

• FAR requirements met for go-around, engine-out, gusts, and so on 

Rationale: Low approach speeds (to enable displaced threshold landings), CDA on a glide slope that is 

slightly steeper than normal, and thrust-managed takeoff profiles all provide significant benefits for noise 

reduction. 

Cost 

No explicit cost requirements. Economics paper shows parametric sensitivity studies. Costs are 

extrapolated from existing aircraft cost data. 

Rationale: No explicit cost requirements were found; however, the general theme would be that Life 

Cycle Cost (LCC) should not be a showstopper. 
69

 

APPENDIX D: Risk Item List 

Risk 

ID Risk Title Risk Statement Complete? Likelihood Consequence

1 Cruise aero performance

If the actual SAX-40 cruise aerodynamic 

performance is less than predicted, then 

attainment of the mission requirements will 

be jeopardized.

Y4 3

2

Deployable drooped leadingedge and continuous mold 

line elevons

If the deployable drooped LE and CML 

elevon performance is less than predicted, 

then attainment of the noise and 

operational performance requirements will 

be jeopardized.

Y2 4

3 Stability and control

If the SAX-40 stability and control 

characteristics differ from the predictions, 

then attainment of the mission and 

operational requirements will be 

jeopardized.

N- -

4 BWB configuration: 

structures, weight

If the SAX-40 structural and subsystems 

weights are greater than predicted, then 

attainment of the mission requirements will 

be jeopardized.

Y4 4

5 BWB configuration: human 

factors

If the SAX-40 human-factors 

characteristics negatively impact the 

concept feasibility, then attainment of the 

requirements will be jeopardized.

Y3 5

6 BWB configuration: internal 

layout.

If the SAX-40 configuration layout does not 

result in a feasible design, then attainment 

of the requirements will be jeopardized.

Y1 3

7 Boundary-layer 

ingestion/inlet design

If the SAX-40 inlet design and use of BLI 

does not result in the predicted 

performance, then attainment of the 

mission and noise requirements will be 

jeopardized.

Y4 4

8 Variable-area exhaust thrustvectoring nozzle

If the variable-area/thrust-vectoring nozzle 

performance is less than predicted, then 

attainment of the mission and noise 

requirements will be jeopardized.

Y5 5

9 Transmission system

If the engine transmission system 

performance is less than predicted, then 

attainment of the mission requirements will 

be jeopardized.

Y3 3
70

 

Risk 

ID Risk Title Risk Statement Complete? Likelihood Consequence

10 Single-core/multiple-fan 

concept

If the singe-core/three-fan concept 

performance is less than predicted, then 

attainment of the mission requirements will 

be jeopardized.

Y3 3

11 Propulsion-airframe 

integration/buried engines

If the buried-engine concept presents 

propulsion/airframe integration issues that 

cannot be resolved, then attainment of the 

mission requirements will be jeopardized.

Y2 3

12 Low flight idle thrust

If the assumed low flight idle thrust is not 

feasible, then attainment of the noise and 

operational requirements will be 

jeopardized.

Y3 3

13 Low-speed fan with forward 

swept blades

If the fan performance is less than 

predicted, then attainment of the noise and 

mission requirements will be jeopardized.

Y3 2

14 High-pressure compressor 

design

If the HPC performance is less than 

predicted, then attainment of the noise and 

mission requirements will be jeopardized.

Y3 2

15 Low noise low-pressure 

turbine design

If the LPT performance is less than 

predicted, then attainment of the noise and 

mission requirements will be jeopardized.

Y3 2

16 Trailing-edge brushes

If the TE brush technology performance is 

less than predicted, then attainment of the 

noise requirements will be jeopardized.

Y3 3

17 Low noise undercarriage

If the low noise undercarriage 

performance is less than predicted, then 

attainment of the noise requirements will 

be jeopardized.

Y2 3

18 Quiet drag

If sufficient quiet drag cannot be 

generated, then attainment of the noise 

requirements will be jeopardized.

Y2 2
71

 

Risk 

ID Risk Title Risk Statement Complete? Likelihood Consequence

19 Long ducts with acoustic 

liners

If the integration of long ducts with 

acoustic liners results in acoustic 

penalties, then attainment of the noise and 

mission requirements will be jeopardized.

Y2 3

20 Airframe shielding

If the airframe shielding benefits are less 

than predicted, then attainment of the 

noise requirements will be jeopardized.

Y1 2

21 Thrust managed takeoff

If the thrust-managed takeoff is not viable, 

then attainment of the noise requirements 

will be jeopardized.

Y2 5

22

Displaced threshold and 

continuous descent 

approach with increased 

approach angle

If the use of a displaced threshold and 

continuous descent approach with an 

increased approach angle have penalties, 

then attainment of the noise and 

operations requirements will be 

jeopardized.

Y5 4

23 Cost

If the costs of developing, producing, and 

operating the SAX-40 outweigh the 

benefits, then attainment of the mission 

requirements will be jeopardized.

Y4 4

24 Overall noise estimates

If the overall noise performance is worse 

than predicted, then attainment of the 

noise requirements will be jeopardized.

Y4 4

25 Aeroelasticity

If the aeroelastic characteristics of the 

SAX-40 result in penalties, then attainment 

of the mission requirements will be 

jeopardized.

N- -

26 Emissions

If the emissions characteristics of the SAX40 are undesirable, then attainment of the 

mission requirements will be jeopardized.

Y2 4

27 Experimental

If the attributes (e.g., assumptions, 

approximations, scaling) of any of the 

experimental investigations that were 

conducted in support of the SAX-40 

concept are questionable, then attainment 

of the mission requirements will be 

jeopardized.

Y3 4
72

 

APPENDIX E: Additional Data Required to Support Further Analysis 

As part of the risk-assessment process, the following list of questions was developed by the NASA risk-assessment 

team and provided to SAI. Because of resource limitations, SAI was unable to respond other than to provide 

previously published references. 

Weights: 

[Unless otherwise designated, all of the questions in this section pertain directly to Risk ID #4 and indirectly to 

many other items that apply to weights, such as Risk ID #’s 1, 2, 8, 9, 10, 16, 17, and 19.] 

1. Do you have a short group weight statement for the SAX-40? The standard format is the Society of Allied 

Weights Engineers Recommended Practice No. 8A (SAWE RP #8A). Part I is relevant; Part II is too detailed for this 

application. This format is generally used to show a build-up to weight empty, then useful load, and then gross 

weight. If you don’t have this type of breakdown, can you provide component weight estimates or assumptions (e.g., 

inlet and nozzle duct weights, nozzle variable geometry and thrust-vectoring mechanism weight, transmissionsystem gear and shaft weight)? 

2. What database or methods were used for the WingMod response-surface-based weight estimation? 

3. If any empirical weight estimation data exists, are these data available and how do they compare with this data? 

4. What are the load cases that were used for the structural design? 

5. Is a finite-element model (FEM) available for the SAX-40 internal structural layout? Is a FEM under 

development? What is the priority for such an effort compared with the noise reduction efforts? 

6. Has any work has been done in the area of aeroelasticity, such as free-body flutter, tumbling, wing-flutter 

margins, and so on? [Risk ID #25] 

7. What are the perceived risks that are associated with the weight escalation in the absence of a detailed FEM 

analysis design? 

8. How did you determine the assumed 10-percent reduction in weight with application of advanced composite 

materials? 

Aerodynamic Performance: 

[All questions in this section pertain to Risk ID #1.] 

1. What is your minimum allowable L/D to meet your mission goals? 

2. In view of the lower performance that was predicted by viscous CFD, have you done anything to improve the 

performance following the SAX-30 iteration? 

3. Your span loading is much more elliptical than typical transports or the Boeing BWB. Has this been evaluated for 

loads, weights, off-design (e.g., approach, buffet)? 
73

 

4. Your airfoils are fairly aft-loaded. Have they been assessed for control deflection performance and actuator 

loads/sizing? 

5. Do you have any predictions/estimates of the impact of adding the winglets and nacelles, and, if so, what method 

was used to generate the estimates? 

6. Have you considered the additional performance losses that a real airplane might have? 

Stability and Control:

[All questions pertain to Risk ID #3.] 

1. For ramp and taxi operations, did you estimate stability during ground maneuvering in terms of tip-back, rollover, and so on? 

2. For takeoff, what is the predicted trim control setting and control flap deflection margin (i.e., degrees of deflection 

remaining) for longitudinal stability? What roll and yaw rates can be sustained? What is the predicted rotation 

speed? What is the predicted control setting to cause rotation? 

3. For engine-out takeoff, what is the predicted minimum control speed (Vmc)? What is the predicted control setting 

to trim one-engine-inoperative (OEI) takeoff? Did you account for second-segment climb gradient regulation? 

4. For climb, what is the predicted trim control setting and control flap deflection margin (i.e., degrees of deflection 

remaining) for longitudinal stability? What roll and yaw rates can be sustained? 

5. For cruise, what is the predicted trim control setting and control flap deflection margin (i.e., degrees deflection 

remaining) for longitudinal stability? What roll and yaw rates can be sustained? Have you identified a Mach buffet 

margin? 

6. For descent and landing, what is the predicted trim control setting and control flap deflection margin (degrees 

deflection remaining) for longitudinal stability? What roll and yaw rates can be sustained? Do you have both an 

equilibrium configuration at the nominal operating point and a margin to allow off-nominal operation (e.g., higher 

descent angles and different speeds)? 

7. Does the design have a tendency to pitch up on stall? If so, is the control power sufficient to recover? 

8. Have you accounted for the “jet-flap” effect of the vectored thrust? 

General Airframe: 

1. Please provide any drawings or assumptions on the internal layout of the SAX-40. The economics paper states 

that the passenger compartment was designed for 215 passengers in a “two-class international configuration.” Can 

you provide more details? Are the two classes business and economy? How many seats of each? What about 

windows, aisles, doors, galleys, lavatories, and so on? How much cargo volume was assumed? What is the fuel tank 

layout, and how much fuel volume was assumed? Avionics bays? APU? Landing gear stowage? We’re in the 

process of creating an internal layout using ProE, so any insight you can give us per your assumptions in this area 

would be helpful. [Risk ID #5, 6] 
74

 

2. Did you make any assumptions for the cabin/center-body/pressure-vessel materials/layers so that we can make an 

internal noise assessment? Were acoustic liners assumed for this area? [Risk ID #5] 

3. Can you provide us full configuration drag polars for up-and-away cruise and low-speed takeoff and approach 

configurations? Can you provide a breakdown of these polars into zero-lift drag and induced-drag components? For 

the zero-lift drag, can you provide a further breakdown of each contributor to CD0, including any estimates or 

assumptions for interference drag, excrescence and protuberance drag, and so on? [Risk ID #1] 

High Lift System: 

[All questions in this section support Risk ID #2.] 

1. Where on the vehicle LE will the droop capability exist? 

2. How will the droop mechanism work? Do you have a diagram of the mechanism? 

3. If the droop capability exists inboard of the straight LE section, how will the droop mechanism work in this 

curved, concave area? 

4. What demonstrated concept indicates that LE droop will be attainable on a BWB type configuration? Where are 

CML LE flaps being developed or used? You mentioned the A380? Can you provide more details on that research, 

development, and integration effort to show applicability to the SAX-40 concept? Are you familiar with any other 

applications, particularly with composite materials? 

5. What CLmax and α are needed for takeoff? For approach? What data support this? 

6. How much LE droop is needed? 

7. How do you know that a drooped LE and CML elevons with brushes will be able to provide the lift levels 

required? 

8. Will CML capability allow elevon deflections to large enough angles? 

9. How will flutter or skin-rippling difficulties be avoided on the CML elevons? 

10. How will fatigue fracture of CML elevon skins be avoided? 

11. How will adequate frequency response of the elevons be ensured? 

12. How do you ensure that deflected CML elevons will return to the correct undeflected external shape? 

13. How do you ensure that maximum deflected CML elevons will still produce the required low noise levels? 

14. How will an elastic flap, split flap, and TE brushes all work together effectively at the same time? 

15. Where are drooped LEs, CML elevons, and TE brushes being developed? How was the weight estimated for 

these items? 
75

 

Propulsion: 

1. Please provide a full set of the efficiencies that were used in the engine-cycle analysis. Was a chart was shown 

during the March 2007 workshop at MIT with this type of data? [Risk ID #10] 

2. Please discuss the assumption that the distortion produced by the BLI inlets can be tolerated by the fans. Highcycle fatigue caused by inlet dynamic distortion is an issue, and the assumption that it can be tolerated by the threefan system downstream seems very optimistic. Did the engine companies express concern about this? Has active 

and/or passive flow control been considered as a means for reducing inlet distortion? [Risk ID #7] 

3. How did you handle having three fans share a common inlet duct? Were any losses or inefficiencies assumed to 

result from interference, or were the three fan inlets somehow segregated inside the inlet? Were any other issues 

considered given this unconventional design? [Risk ID #7] 

4. How finely were you able to design the inlet (i.e., were elements like the fineness of lip, the lip contraction ratio, 

and external forebody shaping factored into the design)? [Risk ID #7] 

5. If flow control is required to achieve acceptable levels of dynamic distortion, do you have any concerns that the 

flow control devices will perform/behave differently in the presence of the liners? [Risk ID #7] 

6. In the cruise condition, is a normal shock generated upstream of the inlet? If so, how is it addressed? [Risk ID #7] 

7. Low noise LPT design: The low Reynolds number in the LPT can result in separation bubbles on the airfoils as a 

result of the boundary-layer profile growth, which results in losses. How much efficiency was traded off for the 

benefit of reduced noise? Do you have a flow analysis for the LPT conceptual design that can be made available to 

us? [Risk ID #15] 

8. Low-speed fan: How large are the incidence swings that are experienced by the OGV during the flight envelope? 

In addition to the variable fan nozzle, is a variable OGV being considered? What about flow control on the guide 

vane surfaces and end walls? Do you have a flow analysis for the fan and an OGV conceptual design that can be 

made available to us? [Risk ID #13] 

9. High-pressure compressor: During vehicle maneuvers, do large variations occur in the boundary-layer thickness 

on the wing surface in comparison with some nominal boundary layer thickness? If so, could the fan and the HPC 

be subject to large swings in corrected mass flow rate? What is the variable-geometry schedule for the compressor 

inlet guide vanes and stators along the operating line? Does the centrifugal compressor require variable geometry to 

match the axial compressor at all operating conditions? What are the design-point pressure-ratio requirements for the 

HPC? What are the loading levels per blade row? Do you have a flow analysis for the HPC conceptual design that 

can be made available to us? [Risk ID #14] 

10. AIAA-2007-450 quotes a 3.8-percent fuel burn reduction and a 16.6-percent airframe drag reduction as a result 

of BLI (~25.1 cruise L/D). The paper also states that "the installation downstream of the fan was assessed using loss 

estimates based on clean flow." [Risk ID #7] 

• Were friction losses in the acoustically lined S-duct accounted for in the inlet pressure recovery? 

• Were aerodynamic and friction losses in the acoustically lined duct that leads to the nozzle accounted for? 
76

 

• Were aerodynamic, leakage, and friction losses in the variable-area, thrust-vectoring nozzle accounted for? 

• Was the external (boat-tail) drag of the variable-area, thrust-vectoring nozzle accounted for? 

• Was the gear-box cooling system drag accounted for? 

If the answer to any of the above questions is yes, please quantify. 

11. During thrust reversal, how is the flow directed: up, down, or both? [Risk ID #8] 

12. Were any weight penalties assessed for acoustic treatment of the passenger compartment or acoustically induced 

structural fatigue and vibration? [Risk ID #11] 

13. In AIAA-2007-450, you recognize distortion as a high-risk area and you account for it by designing the fan to 

transfer/attenuate it downstream. Do any penalties result in the fan efficiency (for increasing the required surge 

margin) or weight (due to high-cycle fatigue) as a result of the circumferential distortion that enters the fan? If yes, 

please quantify. Do you believe a fan can even be designed to tolerate that level of distortion for 30,000 hours? 

[Risk ID #7] 

14. Did you resize the engine for different levels of BLI in your studies? Increased BLI would decrease the engine 

mass flow for a given inlet size, as well as cause a drop in the mass-averaged pressure recovery of the inlet flow. 

This result should lead to a change (loss) in overall engine thrust. This loss is somewhat offset by the decrease in 

ram drag, but it is unlikely that these two values match. [Risk ID #10] 

15. The CFD of the flow through the inlet ducts looks very symmetric within the duct, albeit with circumferential 

distortion. I would expect to see more asymmetry because the inlets are not symmetric. A lack of any vortical 

structures that might be expected is also noted. Is the CFD of the actual configuration or a surrogate? [Risk ID #10] 

16. Can you provide additional information (e.g., efficiencies, pressure ratios, geometry, weight, performance) on 

the alternate lower risk propulsion system design (three podded engines). This would aid in our assessment of the 

SAX-40 system. [Risk ID #10] 

17. Do you have a design or assumption for the mechanism that controls the variable-area nozzle and/or the thrustvectoring system? Have you accounted for any power requirements to operate these systems? [Risk ID #8] 

18. Can you provide more details on the engine transmission system design, such as gear and shaft weights, 

reliability, horsepower ratings, lubrication and cooling requirements, vibration, noise, and so on? [Risk ID #9] 

19. Can you provide the engine deck (thrust and fuel flow as a function of Mach, altitude, and power setting) that 

you utilized for your performance estimates? [Risk ID #1] 

Operations: 

1. Can the SAX-40 use a conventional approach speed of 140 knots to 145 knots? [Risk ID #22] 

2. Quantify both separately and cumulatively the loss of the noise benefit that results from not using CDA, from not 

using a displaced threshold, and from not using a lower speed approach. Can the SAX-40 still provide a useful 

reduction in noise even if it is flown with conventional approaches and departures at speeds that are compatible with 

existing large aircraft? [Risk ID #22] 
77

 

3. Does any wake benefit result from this design—is the wake likely to be classified as "small" or "large"? [Risk ID 

#22] 

4. A 45-percent engine design speed is required during landing approach. Have the engine resonance issues been 

considered? (Normally you accelerate quickly through any resonant point and maintain an operational speed well 

away from resonance.) [Risk ID #12] 

5. What consideration has been given to stage matching in the compressor during extended operation at 45-percent 

speed? Do the stators have to be rotated so far as to cause a wide clearance issue during cruise? [Risk ID #12] 

6. Has engine response time during landing been considered? This could be an issue as a result of the low spool 

speed. [Risk ID #12] 

Noise: 

 [Questions 1-5 pertain to Risk ID #19.] 

1. According to AIAA-2007-454, the current SAX-40 design allows for a fan diameter of 1.2 m and an engine 

length of 2.46 m. 

• What is the axial length of the lined region in the engine nacelle? 

• What is the duct height (inner to outer wall) of this lined section of the engine nacelle? 

2. The exhaust duct is a complex propagation path (S-shaped). How does this complexity affect the requirement for 

splices for the liners? Will they be wider or will there be more of them? If this has been considered, what are the 

predicted effects? 

3. According to AIAA-2006-2525, numerical optimization of the liner parameters was accomplished by developing 

and solving the appropriate eigenvalue problems based on in-house routines. The paper also states that single-layer 

liners were used in an optimization study and that any scattering effects that were caused by impedance changes at 

the interfaces were neglected. Also, the optimization used liners with depths up to 8 in. 

• Have any estimates been made regarding the positive and negative effects of scattering on the far-field 

attenuation? 

• Has any consideration been given to mode interaction effects (that result from modes of different phases 

that are encountering the LE of the liner)? 

• Because the fans will be smaller and the noise spectrum will shift upward in frequency, what is the purpose 

of including frequencies as low as 50 Hz in the optimization? Also, conventional aircraft typically only 

allow liner depths of up to ~3 in. Will the SAX-40 be able to relax this constraint to allow for significantly 

deeper liners? What are the impacts if this constraint is relaxed? 

4. According to AIAA-2007-453, improved increases in attenuation will be possible with more advanced liners. 

• Have any of these advanced liners already been used for attenuations predictions, or is this just a plan for 

the future? 

5. Can you provide any further details on your decisions related to the application of TE brushes beyond what is in 

AIAA-2007-451? Have you looked at any potential challenges that are associated with the use of TE brushes (e.g., 

material, reliability, maintainability)? Do you have a drawing or concept of the installation and mechanisms utilized 

for TE brush deployment? Did you account for the power that will be required to operate this system? 
78

 

[Risk ID #16] 

6. How much drag is required for landing (both nominal and off-design)? [Risk ID #18] 

7. What is the noise benefit that is estimated for the CML surfaces? [Risk ID #18] 

8. What is your assumed noise contribution for induced drag? Is induced drag truly quiet? [Risk ID #18] 

9. What is the noise budget for the quiet drag? [Risk ID #18] 

10. Do you have a baseline quiet drag device in mind? If it is an engine air brake, do you have an estimate of the 

noise impact during operation? What are the possible noise sources (mechanisms)? How well will the swirl tube 

work with BLI? If using engine braking, how does this work with thrust vectoring for trim? [Risk ID #18] 

11. If you are using perforated drag plates, what is the expected noise impact and why? [Risk ID #18] 

 

Emissions: 

1. Can you provide an explanation for how you estimated your emissions performance? Numbers for CO2 and NOx

were quoted in the slide presentation. What correlations were utilized? [Risk ID #26] 

Cost: 

 [All questions in ths section pertain to risk ID #23.] 

1. In the economics paper (AIAA-2007-455), under Section V, Airline Business Case Analysis, on page 9, you 

mention comparisons with “competitors in the same seat class.” Could you provide more information on these 

competitor concepts, that is, which ones (if any others besides the 767-300ER), what range/payload was assumed, 

fuel burn, and so on. How did you define the current technology (2006) midsize international aircraft? Can you 

provide more details on that aircraft as well? 

2. Can you provide a copy of Ref. 26 in the economics paper (AIAA-2007-455), by Belobaba, P., “Airline Fleet 

Planning Overview,” Lecture Notes, The Airline Industry (MIT 16.71J), October 10, 2001. 

3. Can you provide the data and process/assumptions used in the extrapolation of “average price per seat” 

information to estimate purchase prices (page 10 of the economics paper, AIAA-2007-455)? What year dollars are 

these estimates? 

Experimental: 

1. Is any information available on the design of the phased microphone arrays that were used for component testing, 

as well as a description of the experiments and results? A list of references would be fine. [Risk ID #27] 
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Silent Aircraft Initiative Concept Risk Assessment

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Nickol, Craig L.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center 

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

L-19438

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

An electronic version can be found at http://ntrs.nasa.gov

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 05

Availability: NASA CASI (301) 621-0390 

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

A risk assessment of the Silent Aircraft Initiative’s SAX-40 concept design for extremely low noise has been performed. A NASA team developed a list of 27 

risk items, and evaluated the level of risk for each item in terms of the likelihood that the risk would occur and the consequences of the occurrence. The 

following risk items were identified as “high risk,” meaning that the combination of likelihood and consequence put them into the top one-fourth of the risk 

matrix: structures and weight prediction; boundary-layer ingestion (BLI) and inlet design; variable- area exhaust and thrust vectoring; displaced-threshold and 

continuous descent approach (CDA) operational concepts; cost; human factors; and overall noise performance. Several advanced-technology baseline concepts 

were created to serve as a basis for comparison to the SAX-40 concept. These comparisons indicate that the SAX-40 would have significantly greater research, 

development, test, and engineering (RDT&E) and production costs than a conventional aircraft with similar technology levels. Therefore, the cost of obtaining 

the extremely low noise capability that has been estimated for the SAX-40 is significant. The SAX-40 concept design proved successful in focusing attention 

toward low noise technologies and in raising public awareness of the issue. 

15. SUBJECT TERMS

BLI; BWB; Embedded; Noise; Silent; Transport

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83

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(301) 621-0390

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561581.02.08.07.13.05 

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

NASA/TM-2008-215112

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01- 02 - 2008