Green Light for

Green

NASA’s Contributions 

to Environmentally 

Flight Responsible Aviation

Peter W. Merlin

Green Light for

Green

NASA’s Contributions 

to Environmentally 

Flight Responsible Aviation

Peter W. Merlin
Library of Congress Cataloging-in-Publication Data

Names: Merlin, Peter W., 1964– author. | United States. National Aeronautics and Space 

Administration, issuing body. 

Title: Green light for green flight : NASA’s contributions to environmentally responsible 

aviation / Peter W. Merlin.

Other titles: NASA’s contributions to environmentally responsible aviation | NASA aeronautics book series. 

Description: Washington, DC : NASA, [2020] | Series: NASA aeronautics book series | 

Includes bibliographical references and index. | Summary: “NASA’s Environmentally 

Responsible Aviation (ERA) project began in 2009 to explore and document the feasibility, benefits and technical risks of advanced vehicle concepts and enabling technologies 

for reducing aviation’s overall impact on the environment. Its goals included reducing community noise footprints, fuel burn, and nitrogen oxide emissions. This book 

reviews the advanced aircraft design concepts, construction technologies, and propulsion 

advancements that were researched by the ERA project”—Provided by publisher. 

Identifiers: LCCN 2020004753 (print) | LCCN 2020004754 (ebook) | 

ISBN 9781626830554 (hardback) | ISBN 9781626830561 (paperback) | 

ISBN 9781626830578 (epub)

Subjects: LCSH: NASA Environmentally Responsible Aviation Project (U.S.) | 

Aeronautics—Environmental aspects—United States. | Green technology—United 

States. | Aeronautics—Research—United States. | Airplanes—United States—Testing. 

Classification: LCC TD195.A27 .M47 2020 (print) | LCC TD195.A27 (ebook) | DDC 

629.1300286—dc23 | SUDOC NAS 1.120:EN 8 

LC record available at https://lccn.loc.gov/2020004753

LC ebook record available at https://lccn.loc.gov/2020004754

Copyright © 2020 by the National Aeronautics and Space Administration.

The opinions expressed in this volume are those of the authors and do not necessarily reflect 

the official positions of the United States Government or of the National Aeronautics and 

Space Administration.

On the cover: Boeing’s Subsonic Ultra Green Aircraft Research (SUGAR) Volt advanced 

aircraft concept (NASA)

This publication is available as a free download at 

http://www.nasa.gov/ebooks

National Aeronautics and Space Administration

Washington, DC
iii

Table of Contents

Chapter 1: Birthing the Environmentally Responsible Aviation Initiative .......... 1

Chapter 2: The ERA Advanced Vehicle Concepts Study...................................41

Chapter 3: Pursuing Damage-Tolerant Composite Structures ........................63

Chapter 4: The Adaptive Compliant Trailing Edge Investigation......................93

Chapter 5: Reducing Airframe Noise..............................................................121

Chapter 6: Integrated Propulsion System Technologies................................145

Chapter 7: ERA and the Boeing 757 ecoDemonstrator..................................177

Afterword: End of an Era: Conclusions and Lessons Learned.......................197

Appendix: Environmentally Responsible Aviation Technical Overview.........203

Acronyms and Abbreviations 245

Acknowledgments 249

Selected Bibliography 251

About the Author 281

Index 283
iv

The Active Flow Control system was tested on a full-sized tail in the National Full-Scale 

Aerodynamics Complex (NFAC). (NASA)
1

CHAPTER 1

Birthing the

Environmentally Responsible 

Aviation Initiative

For over a century, NASA and its predecessor, the National Advisory Committee 

for Aeronautics (NACA), have undertaken research and technology development that directly contributed to establishing air transportation as a cornerstone of our way of life. These efforts led to advances across the spectrum of 

study, from a basic understanding of the science of flight to practical engineering achievements that transformed concepts into reality. Although the shape 

and speed of commercial airliners have not changed significantly since the 

late-1950s, many aspects of performance—such as range and fuel efficiency, 

and environmental impacts including noise and emissions—have improved 

tremendously. By the beginning of the 21st century, however, it became clear 

that more work was necessary. Heightened sensitivity to, and understanding 

of, the impact of aviation on the environment and the reduced availability of 

low-cost energy placed the spotlight directly on aircraft efficiency and reducing 

environmental impacts. This realization spawned one of NASA’s most ambitious aviation projects to date.

NASA researchers believe that in the second quarter of this century, commercial airline companies could save as much as $250 billion thanks to 

so-called “green” aviation technology pioneered by the Agency and industry 

partners under NASA’s Environmentally Responsible Aviation (ERA) project. This 6-year effort, which concluded in 2015, focused on development 

of technologies that will help aircraft manufacturers to reduce fuel consumption, exhaust emissions, and aircraft noise by increasing engine efficiency 

and improving overall aircraft design. The Agency initiated the ERA project 

to explore, mature, and document the feasibility, benefits, and technical 

risks of vehicle concepts and enabling airframe and propulsion technologies originally identified in the Agency’s Fundamental Aeronautics Program 

(FAP) and, in particular, the Subsonic Fixed Wing (SFW) project in order 
Green Light for Green Flight

2

to mitigate the impact of aviation on the environment. NASA ultimately 

contributed more than $400 million to the project, which also received 

approximately $250 million from industry partners including Boeing and 

Pratt & Whitney.

The ERA project began in 2009 as part of the NASA Aeronautics Research 

Mission Directorate’s Integrated Systems Research Program. Currentgeneration aircraft already benefit from NASA investments in aeronautical 

research of past decades. The development of digital fly-by-wire flight controls, supercritical airfoils, and winglets during the early 1970s and 1980s 

improved flying safety, controllability, and fuel efficiency, but most important, 

they became standard features of many modern aircraft. Once fully matured, 

technologies developed through the ERA project promise to become standard 

features of future generations of commercial air transports.

Forecasts call for the Nation’s air transportation system to undergo significant expansion up to 2035. Unless new technologies are introduced, adverse 

environmental impacts from increased air operations could curtail the ability 

of the Next Generation Air Transportation System (NextGen) to accommodate 

projected growth in demand for air transportation. To offset these impacts, 

NASA implemented the ERA project to pioneer technologies that might be 

sufficiently mature to meet mid-term goals—within the first 5 to 10 years—for 

reducing community noise footprints, fuel burn, and nitrogen oxide (NOx) 

emissions. Researchers would also attempt to determine the potential benefits 

of various advanced aircraft configurations beyond the conventional tube-andwing design that has been standard since the earliest days of commercial air 

transportation. If successful, NASA scientists believe, such new technologies 

and configurations could cut airline fuel consumption in half, reduce pollution by as much as 75 percent, and drop noise pollution to nearly one-eighth 

of current levels.1

Simultaneously, the Federal Aviation Administration (FAA) is working 

to implement significant improvements to the National Airspace System 

between 2012 and 2025. The advent of NextGen is transforming America’s 

air traffic control system from a land-based radar network to one using Global 

Positioning System (GPS) satellite technology and replacing radio communications with digital data transfer. FAA officials expect this approach to 

shorten routes, save time and fuel, reduce traffic delays, increase airport 

1. Pia Bergqvist, “NASA’s ERA Project Could Save Airlines Billions,” Flying, January 13, 2016. at 

https://www.flyingmag.com/nasas-era-project-could-save-airlines-billions/ (accessed August 

21, 2019).
Birthing the Environmentally Responsible Aviation Initiative

3

capacity, and permit controllers to monitor and manage aircraft with greater 

safety margins.2

Project Goals and Relevance

For planning purposes, ERA project managers defined first-generation technology (i.e., circa 2015) as N+1, second-generation (that is, circa 2020) as N+2, 

and third generation (the target date of about 2035) as N+3. Researchers were 

challenged to mature a variety of promising new technologies and to study vehicle 

concepts that together might simultaneously meet NASA Subsonic Transport 

System Level Metrics for noise, emissions, and fuel burn within the N+2 timeframe. To that end, the ERA project challenged researchers to focus on technologies in four research areas. These included innovative aerodynamic flow-control 

concepts for drag reduction, improvements in composite materials for reducing 

aircraft structural weight, advanced ultra-high-bypass (UHB) engine designs for 

improving fuel efficiency and noise reduction, and advanced combustor designs 

for reducing NOx emissions. Airframe and propulsion technologies sufficiently 

mature for full-scale development by 2020 could enter into service by 2025.

The Aeronautics Research Mission Directorate’s (ARMD) focus on longterm, cutting-edge research that expands the boundaries of aeronautical knowledge for the benefit of the broad aeronautics community directly supported 

NASA’s mission to pioneer scientific discovery, aeronautics research, and space 

exploration. At the time, both the Agency’s aeronautics and space research and 

technology activities were focused on lightweight integrated structures and 

environmentally friendly, high-performance propulsion systems. Additionally, 

such factors as continuing growth in air traffic volume, the vital role of air transportation on the global economy, and concerns about the overall environmental impacts of aviation added focus to the National Aeronautics Research and 

Development Policy that was established by President George W. Bush in 2006.

To address such concerns in the context of that policy, NASA set aggressive goals for noise reduction, emissions, and energy consumption. Originally 

developed under the SFW project, these goals were updated jointly with the 

ERA project to be consistent with the goals of the National Plan for Aeronautics 

Research and Development, which had also been approved by President Bush 

in 2007. As a result, ERA project research would have far-reaching effects on 

2. U.S. House of Representatives Comm. on Science, Space, and Technology, 112th Cong. (2011) 

(statement of Peter H. Appel, Administrator, Research and Innovative Technology Administration, 

U.S. Dept. of Transportation). https://www.hq.nasa.gov/legislative/hearings/2011%20hearings/

9-8-11_APPEL.pdf (accessed August 21, 2019).
Green Light for Green Flight

4

civil and military aviation with a focus on energy efficiency, national security, 

aviation safety, and enhanced mobility.3

ERA Goals, Objectives and System Level Metrics

ERA Goal

N+1 = 2015**

Technology Benefits 

Relative to a Single 

Aisle Reference 

Configuration

N+2 = 2020**

Technology Benefits 

Relative to a Large 

Twin Aisle Reference 

Configuration

N+3 = 2025**

Technology Benefits

Noise below Stage 4 –32 dB –42 dB –71 dB

LTO NOx Emissions 

below CAEP 6 –60% –75% better than –75%

Performance: 

Aircraft Fuel Burn –33% –50% better than –70%

Performance: 

Field Length –33% –50% exploit metro-plex* concepts

** Technology Readiness Level for key technologies = 4–6. ERA undertook a time phased approach, 

TRL 6 by 2015 for “long-pole” technologies

* Concepts that enable optimal use of runways at multiple airports within the metropolitan area

Goals for the ERA project focused primarily on maturing technologies that could enter service by 

the year 2020. (NASA)

The ERA project and its partners, including the Air Force Research 

Laboratory and the FAA, commercial aircraft industry, and academia were 

united in a common goal of reducing the environmental impact of aviation. In 

large part, this was due to similarities in military and civil transport capability 

requirements that offered significant opportunities for leveraging and, where 

possible, cost sharing technology development. In fact, the Department of 

Defense (DOD), FAA, and the airframe and engine companies were already 

aggressively pursuing energy efficient, environmentally friendly technologies. 

The FAA Continuous Lower Energy Emissions and Noise (CLEEN) program, 

for example, focused on integrated technology demonstrations to facilitate 

N+1 technology transition into the commercial air fleet. The DOD identified 

energy efficiency as a near-term strategic initiative to enable effective mobility. 

3. Fayette Collier and Gaudy Bezos-O’Connor, “Technology Development Project Plan: Environmentally 

Responsible Aviation Project,” ERA-01-0001, Rev. B, NASA Aeronautics Research Mission 

Directorate, September 30, 2013, 9–14.
Birthing the Environmentally Responsible Aviation Initiative

5

Though industry generally looks to near-, mid-, and far-term horizons like 

the ERA project, the emphasis is more typically geared toward producing and 

marketing the next generation of aircraft. The ERA project’s focus provided a 

technology pull well beyond the next generation of vehicle systems.4

Management by Technical Challenges

At its formulation, planners organized and managed the ERA Phase 1 portfolio around four major subprojects. The Airframe Technology subproject 

focused on research into lightweight structures, flight dynamics and control, 

drag reduction, and noise reduction. The Propulsion Technology subproject 

included research into improving jet engine combustors, propulsor concepts, 

and the gas generator core (engine section containing the compressor, combustor, and turbine). The Vehicle Systems Integration (VSI) subproject focused 

on research into systems analysis, airframe-propulsion integration, propulsionairframe aero-acoustics, and advanced vehicle concepts. A fourth subproject 

concerned overall management of the ERA project.

The ERA project defined metrics for measuring technical progress at the 

technology, airframe and propulsion systems, vehicle, and fleet levels as aligned 

with the ARMD’s framework of managing the NASA aeronautics portfolio 

by technical challenges and their corresponding progress indicators or key 

performance parameters. Use of this concept of Management by Technical 

Challenges enabled ERA managers to provide context and a compelling case 

for why the project’s technical content was needed, what they were trying 

to achieve, and how they intended to execute the technical plan. Managers 

defined specific technical challenges and identified key performance parameters for each to quantify progress in technology maturation by establishing 

performance targets, and by tracking progress as major testing campaigns were 

completed and system-level assessments conducted. In preparation for Phase 

2, the ERA Project mapped airframe, propulsion, and vehicle system integration research activities to five technical focus areas and evaluated the projected 

individual technology contribution of each.5

The Phase 2 Integrated Technology Demonstration (ITD) Portfolio was 

consistent with ARMD’s principles of Management by Technical Challenges.6

Phase 2 technical focus areas and corresponding technical challenges are:

4. Ibid.

5. Ibid.

6. Ibid.; Pamela A. Davis, Steven B. Harris, Dawn C. Jegley, and Thomas K. Rigney, “Environmentally 

Responsible Aviation Project Status of Airframe Technology Subproject Integrated Technology 

Demonstrations,” presented at the AIAA SciTech Conference, Kissimmee, FL, January 5, 2015.
Green Light for Green Flight

6

• TFA1: Innovative Flow Control Concepts for Drag Reduction

– TC1: Demonstrate drag reduction of 8 percent, contributing 

to the 50 percent fuel burn reduction goal at the aircraft system 

level, without significant penalties in weight, noise, or operational 

complexity.

• TFA2: Advanced Composites for Weight Reduction

– TC2: Demonstrate weight reduction of 10 percent compared to 

composites, contributing to the 50 percent fuel burn reduction 

goal at the aircraft system level, while enabling lower drag airframes and maintaining safety margins at the aircraft system level.

• TFA3: Advanced UHB Engine Designs for Specific Fuel Consumption 

and Noise Reduction 

– TC3: Demonstrate UHB efficiency improvements to achieve 

15 percent Thrust Specific Fuel Consumption (TSFC) reduction, contributing to the 50 percent fuel burn reduction goal at 

the aircraft system level, while reducing engine system noise and 

minimizing weight, drag, NOx, and integration penalties at the 

aircraft system level.

• TFA4: Advanced Combustor Designs for Oxides of Nitrogen Reduction

– TC4: Demonstrate reductions of landing and takeoff NOx emissions by 75 percent relative to the stringent standards established at the International Civil Aviation Organization’s (ICAO) 

Committee on Aviation Environmental Protection, Sixth Meeting 

(CAEP6), held in Montreal, Canada, in 2004, and reduce cruise 

NOx by 70 percent while minimizing the impact on fuel burn at 

the aircraft system level, without penalties in stability and durability of the engine system.

• TFA5: Airframe and Engine Integration Concepts for Community 

Noise and Fuel Burn Reduction 

– TC5: Demonstrate reduced component noise signatures leading 

to an Effective Perceived Noise in decibels (EPNdB) decrease by 

42 EPNdB to Stage 4 noise margin for the aircraft system while 

minimizing weight and integration penalties to enable 50 percent 

fuel burn reduction at the aircraft system level.7

7. Collier and Bezos-O’Connor, “Technology Development Project Plan,” 14–17.
Birthing the Environmentally Responsible Aviation Initiative

7

ERA Project — Key Technical Focus Areas and Technical Challenges

1. Innovative Flow Control Concepts for Drag Reduction

TC: Reduce fuel burn by 6 percent while minimizing maintenance issues

2. Advanced Composites for Weight Reduction

TC: Reduce aircraft weight by 10 percent over SOA composites while 

maintaining safety margins at the aircraft system level

3. Advanced UHB Engine Designs for Specific Fuel Consumption and Noise 

Reduction

TC: Reduce fuel burn by 20 percent while reducing engine system noise 

and while minimizing weight, drag and integration penalties at AC 

system level

4. Advanced Combustor Designs for Landing Takeoff Oxides of Nitrogen 

Reduction

TC: Reduce L TO NOX by –75 percent while reducing fuel burn by 50 

percent at the aircraft system level

5. Airframe and Engine Integration Concepts for Community Noise Reduction

TC: Reduce component noise signatures while minimizing weight and 

integration penalties at aircraft system level to achieve 42 EPNdB 

margin to Stage 4

After selecting five key technical focus areas, ERA planners designated specific technical challenges to drive progress forward. (NASA)

ERA Formulation and Planning

In the Formulation Phase of the project (dubbed Phase 0), a planning team 

used prior research results and inputs from stakeholders across the aviation 

community, to develop an overarching plan along with an effective approach 

to achieve the project’s goals. To begin, researchers considered how to best 

exploit some of the concepts and technologies already under investigation 

through NASA’s SFW project. Each of these technologies was evaluated to 

determine its respective Technology Readiness Level (TRL), a measurement 

used to assess maturity.

NASA uses nine Technology Readiness Level rating levels. The lowest 

is TRL-1, designated when scientific research is just beginning, and results 

are being translated into future research and development. This represents a 

transition from basic science to applied research and explores the essential 
Green Light for Green Flight

8

characteristics of systems and architectures. A rating of TRL-2 is assessed 

once the basic principles have been studied and it is clear that initial findings 

have practical applications. Technology at the TRL-2 level is still considered 

very speculative, as there is as yet little or no experimental proof of concept 

for the technology. Once active research and design efforts begin, a technology may be elevated to TRL-3. This generally requires both analytical and 

laboratory studies to determine whether a technology is viable and ready to be 

advanced further through the development process. During TRL-3, researchers typically demonstrate technical feasibility using a simple proof-of-concept 

model. Next, the new technology is advanced to TRL-4. During this phase, 

multiple components or subsystems are integrated and tested in a laboratory environment. Research under TRL-5 involves more rigorous testing and 

thorough validation under environmental conditions that are as realistic as 

possible. Once this has been completed, the technology is ready to advance 

to TRL-6 in the form of a fully functional prototype or representational 

model. Engineering feasibility is demonstrated in a suitably relevant end-toend environment (ground, air, or space). By the time the technology reaches 

TRL-7, it is ready for a system prototyping demonstration in an operational 

environment. At this point, a working model or prototype should be at or 

near the scale of the proposed operational system, with most functions available for demonstration and testing. TRL-8 technology has been tested and 

“flight qualified,” and is ready for implementation. Finally, once a technology 

has been “flight proven” through successful operational experience, it can be 

elevated to TRL-9.8

NASA’s Aeronautics Research Mission Directorate (ARMD) is tasked 

with advancing U.S. technological leadership in aeronautics through partnership with industry, academia, and other Government agencies that conduct aeronautics-related research. This includes addressing the fundamental 

research needs of NextGen in partnership with member agencies of the NASA 

Joint Planning and Development Office. In 2008, members of the NASA 

Advisory Committee (NAC) called for ARMD to plan and develop candidate 

systems-level research projects consistent with the Agency’s National Policy and 

Plan and to leverage NASA’s unique expertise and competencies, to advance 

8. NASA, “Definition of Technology Readiness levels,” undated fact sheet https://www.nasa.gov/

directorates/heo/scan/engineering/technology/txt_accordion1.html (accessed August 21, 2019);

John C. Mankins, Advanced Concepts Office, Office of Space Access and Technology, NASA,

“Technology Readiness Levels, a White Paper,” April 6, 1995. http://www.artemisinnovation.com/

images/TRL_White_Paper_2004-Edited.pdf (accessed August 21, 2019).
Birthing the Environmentally Responsible Aviation Initiative

9

state-of-the-art capabilities in key disciplines and facilitate transition of results 

to the aviation community.9

Definition of Technological Readiness Levels

TRL 1 Basic principles observed and reported

TRL 2 Technology concept and/or application formulated

TRL 3 Analytical and experimental critical function and/or characteristic proof of concept

TRL 4 Component and/or breadboard validation in laboratory environment

TRL 5 Component and/or breadboard validation in relevant environment

TRL 6 System/subsystem model or prototype demonstration in a relevant environment 

(ground or flight)

TRL 7 System prototype demonstration in a ground or flight environment

TRL 8 Actual system completed and “flight qualified” through text and demonstration 

(ground or flight)

TRL 9 Actual system “flight proven” through successful mission operations

Technology readiness was rated on a scale of nine levels, from lowest (TRL-1) to highest (TRL-9). 

(R.P. Hallion)

In support of this goal, NASA created the Integrated Systems Research 

Program (ISRP) to conduct research at an integrated system level on promising concepts and technologies and to explore, assess, and demonstrate the 

benefits in a relevant environment. All research in this program was coordinated with ongoing long-term, foundational research, as well as with efforts 

being undertaken by other Government agencies such as the FAA. In order 

to meet the projected growth in demand for air transportation, the ISRP was 

designed to focus primarily on maturing and integrating major vehicle system 

and subsystem technologies for accelerated transition to practical application. 

However, the greatest foreseeable obstacles to increasing the number of flight 

operations at many of the Nation’s largest airports were environmental concerns over noise and emissions that would necessarily limit the growth capacity 

of those airports, and therefore limit the capacity of the entire air transportation system. Based on these parameters, the U.S. National Research Council 

(NRC); Congress; the Executive Office of the President; and the NAC issued 

several mandates, directives, and recommendations to NASA. Some of these 

9. Dr. Jaiwon Shin, “Aeronautics Research Mission Directorate System-Level Research,” presented at 

the Meeting of Experts, Aeronautics and Space Engineering Board Meeting of the National Research 

Council, Washington, DC, May 14, 2009.
10

Langley Research Center Project Scientist

Dr. Richard A. Wahls. (NASA)

Green Light for Green Flight

cited the need for NASA to develop a 

“green aircraft initiative” and to advance 

the development of technologies and 

operational procedures to decrease the 

significant environmental impacts from 

aviation.10 This set the stage for creating 

the ERA project.

About 2 weeks after the 2008 

presidential election, ARMD associate 

administrator Dr. Jaiwon Shin called 

SFW project scientist Dr. Richard A. 

Wahls at Langley Research Center 

(LaRC) to discuss preliminary plans for 

a two-pronged project to advance an 

environmentally responsible approach 

to aircraft design and operations. 

One team would focus on a segment, 

dubbed ERA-Vehicle (ERA-V), that 

would explore and assess new aircraft design concepts and enabling technologies through system-level experimentation to simultaneously reduce total fuel 

burn, carbon emissions, and airport area noise. The other, ERA-Operations 

(ERA-O), would investigate air traffic control system technologies that could 

significantly improve safety, capacity, and efficiency on runways and in the 

Nation’s skies, as envisioned under NextGen, while simultaneously providing 

environmentally friendly procedures for reducing fuel burn, emissions, and 

noise.11 Ultimately, the ERA-V planning activity became the first and only 

project in the new ISRP, while selected activities from the ERA-O planning 

team were integrated into a revamped Airspace Systems Program.12

Wahls was assigned to serve as activity planning lead for the FAP, reporting directly to the FAP director, Dr. Ajay Misra. He pulled together approximately 30 people in multiple disciplines from all four of the NASA Aeronautics 

Centers to formulate the overall ERA concept. With a new administration 

coming into the White House, this challenge was particularly daunting. “At 

that time,” he recalled, “NASA was dealing with the presidential transition 

10. Aeronautics Research Mission Directorate, program overviews and descriptions, https://www.nasa.

gov/pdf/428150main_Aeronautics_Research.pdf, circa 2008 (accessed August 3, 2016).

11. Dr. Richard A. Wahls interview with the author, August 4, 2016.

12. Thomas B. Irvine, “Research and Technology Project Formulation Authorization Document:

Environmentally Responsible Aviation (ERA) Project,” Integrated Systems Research Program, NASA 

Aeronautics Research Mission Directorate, June 9, 2009, 2.
Birthing the Environmentally Responsible Aviation Initiative

11

team and trying to figure out what the budget would be, and whether [ERA] 

would even make it into the [new NASA] budget.”13

Only a handful of people within NASA were initially aware of what was 

being planned. Information regarding the fledgling ERA project was otherwise 

embargoed from being disclosed to industry or media representatives. “We 

weren’t allowed to talk about it because it was still in the budgeting process,” 

said Wahls. “Basically, from November 2008 through May 2009, there were 

people inside NASA that knew that there was planning going on, but nobody 

outside knew what was going on.”14

Eventually, the effort grew to the point where so many people were involved 

that others began to suspect what was afoot. “We were using all the knowledge 

of all the partnerships we already had, and of all the technologies that were 

available at the time, to formulate a construct that we were pretty sure people 

would buy into, and they did,” Wahls remembered, “It was a pretty interesting time.”15

Planning Guidance 

According to official planning guidance, as of November 2008, ARMD leadership still envisioned two distinct system-level activities, the ERA-V and ERA-O 

efforts, which would be distinct yet coupled. From a technical approach perspective, development of a multifunctional testbed vehicle would serve as a 

centralized airborne test facility for systems integration research. The proposed 

experimental vehicle testbed, or XVT, was expected to provide significant collaborative opportunities between the ERA-V and ERA-O activities. It soon 

became apparent, however, that the anticipated ERA budget would not support acquisition of an XVT. Nevertheless, the idea endured. Early ERA-related 

studies included analysis to inform project leaders as to the potential scope, 

cost, and benefits to be derived from such an asset, thereby providing a basis 

for future decisions.16

Throughout the initial formulation, the technical scope of ERA remained 

focused primarily on N+2 concepts at technologies at the airframe, propulsion, and vehicle systems level. From a schedule perspective, planners held to 

a nominal 6-year timeframe with definitive start and end points. The ERA 

project notionally had two 3-year phases, with Phase 1 setting the stage for 

Phase 2. Originally, the second phase emphasized construction and utilization 

13. Wahls interview.

14. Ibid.

15. Ibid.

16. Irvine, “Research and Technology Project Formulation Authorization Document: Environmentally 

Responsible Aviation (ERA) Project,” 2.
12

Green Light for Green Flight

of the multifunctional airborne testbed for research and development. Without 

the XVT, the two-phase approach remained but with some necessary adjustments and overlap.17

From a budget perspective, the original ERA planning guidance covered 6 

years beginning in FY 2009 and totaling $451.2 million. This included $215.6 

million for design and fabrication of the XVT and associated ERA experiments. Original estimates were based on an annual budget ranging from $35 

million in FY 2009 to a peak of $133 million in FY 2013. However, based 

on the President’s FY 2010 budget, the ERA project had a much flatter level 

in the $60 to $65 million range, and totaling at about $318.8 over 5 years.18

According to Richard Wahls, the overall shape and structure of ERA 

depended entirely on the availability of funding. “To put it in context,” he 

explained, “back in the 1990s with the Advanced Subsonic Technology and 

High Speed Research programs, the NASA aeronautics budget was up in the low 

billion-dollar range, and when those projects ended, the budget started a steady 

decrease through the late 1990s until it was only at the $450 million level.”19

Wahls noted that prior to 2007, there had not been a sustained increase in 

the NASA aeronautics budget in roughly 10 years. Around that time, Congress 

added about $30 to $40 million, but the additional money was of the sort 

that had to be spent within the fiscal year for which it was allocated, making it 

nearly impossible to develop a long-range plan. “So, when President Obama’s 

5-year budget was announced, aeronautics was increased to something like

$510 million, and that was the first sustained increase in NASA aeronautics

in over a decade.”20

Meeting of Experts 

After the president’s new budget became public, ERA planners arranged for a 

meeting of experts from the Aeronautics and Space Engineering Board (ASEB), 

National Research Council (NRC) to gather feedback on the project’s technical 

feasibility. Ilan Kroo, a distinguished professor of aeronautics and astronautics 

at Stanford University and a member of the NAC Aeronautics Committee, 

chaired the 2-day meeting, which began on May 14, 2009, in National Harbor, 

Maryland. This event was open to the general public.

Dr. Jaiwon Shin led off with an overview of ARMD plans and goals for 

system-level research in environmental impact mitigation. John A. Cavolowsky, 

acting director of the Airspace Systems Program (ASP), and NextGen 

17. Ibid.

18. Ibid.

19. Wahls interview.

20. Ibid.
Birthing the Environmentally Responsible Aviation Initiative

13

integration manager Barry Sullivan 

gave presentations on technologies for 

NextGen systems analysis, integration, 

and evaluation. Anthony Strazisar, 

acting director of FAP, presented a broad 

overview of the proposed ERA project 

followed by a much more in-depth technical description by Wahls. The ERA 

Planning Team Lead later described the 

pressure he felt as he prepared to make 

his presentation. “The night before, I 

remember cramming for my 2-hour 

presentation like it was my Ph.D. dissertation,” he recalled. “There was so 

much riding on it, and if those experts 

had said, ‘You’re way off base, this is 

garbage,’ ERA would probably not 

have happened.”21 Wahls’ presentation, 

entitled “Environmentally Responsible 

Aviation Technical Overview,” represented not only his own viewpoint, but 

the contributions of a multicenter ERA planning team including Dr. Fayette 

S. “Fay” Collier Jr., Dr. Rubén Del Rosario, Dennis Huff, Laurence “Larry” 

Leavitt, Patrick “Pat” Stoliker, and Anthony “Tony” Strazisar. So incisive was 

this survey, and so positive was its reception, that it served as the foundational 

document underpinning the entire ERA effort and, for that reason, is included 

as Appendix 1 of this work.

Dr. Jaiwon Shin, NASA Associate 

Administrator for the Aeronautics Research 

Mission Directorate. (NASA)

The following morning was devoted to breakout sessions, discussions 

between presenters and experts, and comments from general public attendees. 

According to Wahls, there was a great deal of useful feedback that coupled with 

budgetary allocations helped drive the final planning stages. Early on, there 

had been some discussion of building a large-scale experimental demonstrator 

aircraft, or X-plane. Such an endeavor would have added to the complexity, 

and therefore the overall cost, of the ERA project. Unfortunately, funding 

on that scale was not available, so planners opted to focus on a number of 

critical technologies in the first phase of the project, and then select the most 

promising ones for more detailed study in the second phase. As Wahls put it, 

“We were looking into whether we could do an X-plane, and then the projected budget went down and ERA ended up becoming a Phase 1 portfolio 

21. Ibid.
Green Light for Green Flight

14

of a number of activities, and then a down-select to a Phase 2 that focused on 

certain technologies.”22

Phase 1 focused on studies involving:

• Stitched composite technology for weight reduction and damage tolerance,

• Laminar flow technology for drag reduction,

• Flight dynamics and control technology for enabling alternate aircraft

configurations,

• Engine combustor technology for low emissions,

• Propulsion technology and integration for improved specific fuel consumption (SFC) and noise reduction, and

• Propulsion shielding for noise reduction.

These concepts and technologies would be explored and assessed with respect 

to feasibility, potential benefits, interdependencies, and risks. Researchers were 

tasked to refine existing concepts and develop new ideas, and to uncover unexpected multidisciplinary interactions. Based on the results, they would then 

prioritize the various technologies for further study. Phase 1 results, system 

studies, and stakeholder input would determine which technologies were most 

promising, or best suited to meeting ERA project goals. Eight of these were 

eventually selected for more detailed exploration as Phase 2 integrated technology demonstrations, which began in FY 2013 and continued throughout the 

remainder of the ERA project.23

Having secured validation of the basic ERA concept, it was time to formalize the project’s organization and structure. Management personnel were drawn 

from the SFW project through which a variety of advanced aircraft concepts 

including a hybrid wing-body, truss-braced wings, and others was already being 

investigated. Dr. Fay Collier was SFW principal investigator and Dr. Rubén 

Del Rosario served as project manager. As project scientist, Wahls primarily 

acted as a senior technical advisor. “I’m pretty sure that Fay kind of endorsed 

me to peel away from SFW to lead the planning for ERA,” he said.24

Collier assumed leadership of the ERA effort immediately following completion of the meeting of experts. His team formulated detailed project life cycle 

events to be executed over the remainder of FY 2009 and Collier conducted 

22. Ibid.

23. Dr. Richard A. Wahls, “Environmentally Responsible Aviation Technical Overview,” presented at

National Research Council Aeronautics and Space Engineering Board meeting, National Harbor, MD, 

May 14, 2009, https://www.hq.nasa.gov/office/aero/pdf/2009_05_14_nrc_rich_wahls_508.pdf

(accessed August 3, 2016).

24. Wahls interview.
Birthing the Environmentally Responsible Aviation Initiative

15

major planning reviews with NASA 

senior leadership. These included an 

acquisition strategy meeting with 

associate administrator Christopher 

J. Scolese in August 2009 and a baseline review the following month, 

during which Dr. Shin granted the 

authority to proceed with Phase 1 

starting October 1. 

Dr. Fayette S. “Fay” Collier. (NASA)

After ERA went public and 

Collier moved over from SFW to 

become the ERA project manager, 

Del Rosario was placed in charge 

of SFW. It was also a busy time for 

Wahls. “For the next 6 months, I 

served as technical advisor to both 

of them. We then worked through 

the rest of the summer to formalize 

the ERA program so that we could start in October [the beginning of FY 2010]. 

Around November or December, they had mercy on me and let me choose 

which project I would work with, to do one or the other.”25

At that point, Anthony E. “Tony” Washburn became chief technologist for 

ERA and Wahls went back to serving full time as chief technical advisor and 

strategist for SFW. “It was a real hard decision for me because in my heart and 

soul I had been in all the ERA planning, but I was equally invested in SFW,” 

said Wahls. “I decided to go the SFW route because I felt that I could better 

complement Rubén.”26

As the structure of the new project also began to come into focus, budget 

realities affected plans to include air traffic management operations research as 

part of ERA along with the vehicle-related efforts. According to Wahls, “Their 

budget was always going to be a little bit lower than ours, and around about 

the middle of April 2009 we were told that we didn’t have enough money to 

do both of those things.” So, while ERA went on to focus on vehicle technologies, the ASP effort spun off on its own using SFW funds. “So, there was 

this dynamic of one project that was kind of losing money, but the money for 

subsonic vehicles was going up because ERA was bigger than what was being 

pulled out of the Subsonic Fixed Wing project, so it was a win all the way 

around,” Wahls explained. Around that time, Congress passed the American 

25. Wahls interview.

26. Ibid.
Green Light for Green Flight

16

Recovery and Reinvestment Act of 2009. “It was a stimulus package that came 

right in the middle of February when we were planning everything, and we 

were looking into how that might feed into [ERA],” said Wahls. “It was a different color of money, and there were certain amounts that could be applied 

to get things started.”27

Prior

Research

KDP-1

$56.9M $65.1M $74.2M $70.5M $70.1M $69.7M

Formu- KDP-2

lation

External

Input

Phase 1 Investigations

Input from Fundamental Programs, NRAs, Industry, Academia, Other Government Agencies

Phase 2 

Planning

FY09 FY10 FY11 FY12 FY13 FY14 FY15

Advanced Vehicle Concept Study

Integrated Technology

Demonstrations (ITD)

ERA Project Flow with key decision points at the beginning of Phase 1 and Phase 2. (R.P. Hallion)

Wahls noted that of all the many challenges the ERA faced at its onset, the 

most significant—and ultimately the most significant early accomplishment—

was securing funds for the project in the President’s budget submission and 

winning the approval of the panel of experts. Additionally, the unusually rigid 

two-phase project structure with set starting and ending points was a real test 

for NASA. As Wahls explained, “From the beginning, Jaiwon had said that 

this was not going to be an enduring project; we were going to start it and... 

stop it, and... show that we can complete a project and hold a schedule.” 

Normally, he added, the project managers would designate a specific set of 

milestones spanning 5 or 6 years, and that would constitute the project plan. 

“But we made the case that we wanted to have this decision point [at the end 

of Phase 1] and then down-select technologies for Phase 2,” he said. “That was 

a whole programmatic challenge in itself that kind of set the stage for some of 

the other projects that followed. From there, a lot of what happened in Phase 1 

was involved with technologies that were ready to graduate to higher technology readiness levels. So we picked a suite of things that we thought were ready, 

and that’s how the first phase of ERA started.”28

27. Ibid.

28. Ibid.
Birthing the Environmentally Responsible Aviation Initiative

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ERA Phase 1 (FY 2010–FY 2012)

Phase 1 spanned the first 3 years of implementation and served two primary 

purposes. First, during this phase a significant NASA Research Announcement 

(NRA) was defined with the primary purpose to conduct N+2 vehicle system 

studies, develop enabling technology roadmaps, and scope key enabling system-level experiments, ranging from those focused on integrated airframe 

systems and integrated propulsion systems to the development of appropriate experimental vehicle testbed, or X-plane, concepts. The scope of Phase 1 

included trades on technology suites, scale, cost and schedule. This NRA also 

enabled solicitation of ready-to go, system-level experiments that could be 

initiated in short order.

Second, several promising technology solutions were matured from ARMD 

foundational projects. In general, these larger-scale experiments were selected 

from the existing ARMD technology portfolio because they provided both 

a significant system benefit and were already at a TRL level that warranted 

conducting a large-scale demonstration to validate the technology.29

One of the most promising concepts for simultaneously meeting all of the 

subsonic transport system level metrics was the hybrid wing-body (HWB) 

configuration, also known as the blended wing body (BWB). Proving the 

viability of the HWB concept was at the forefront of the ERA project and was 

dependent on two “long pole” items. First was the requirement to demonstrate 

low-speed flight controls and handling characteristics. Second was successful demonstration of an advanced composite manufacturing technique called 

Pultruded Rod Stitched Unitized Structure (PRSEUS). Additionally, researchers hypothesized that the HWB would be the best configuration for achieving 

the very difficult goal of simultaneously meeting fuel burn and noise goals. 

The emergence of advanced open rotor propulsion systems promised increased 

propulsive efficiency and reduced noise. Research ultimately demonstrated that 

combining the HWB airframe with open rotor propulsion came very close to 

meeting all ERA goals. 

During Phase 1, engineers and scientists assessed dozens of environmentally 

friendly aircraft technologies. These included: 

• Advanced vehicle design studies,

• Open rotor technology for reduced engine noise,

• Non-stick coatings for low-drag wing designs intended to achieve 

drag reduction via laminar flow, 

29. Fayette S. Collier, “Technology Development Project Plan: Environmentally Responsible Aviation 

Project,” NASA Integrated Systems Research Program, October 21, 2009, 11.
Green Light for Green Flight

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• Testing advanced fabric composite manufacturing techniques,

• Reducing mission fuel burn and community noise (including airframe noise and propulsion noise from the fan, core, and jet of gas 

turbine engines, and minimizing propulsion-airframe aeroacoustics 

via tailored airframe-propulsion integrated design and shielding),

• Integration of advanced engines, cowlings, nacelles, and pylons for 

zero installation drag, 

• Advanced engine combustor development,

• Ultra-quiet HWB wind tunnel demonstrations and systems analysis, 

and 

• Test flights of a subscale remotely piloted HWB research vehicle. 

Many of these concepts showed great potential for reducing aircraft noise 

and carbon footprints. By the end of Phase 1, ERA project teams were ready to 

take the valuable lessons learned over the first 3 years and begin implementing 

the most promising technologies during Phase 2.

Key enablers for beginning Phase 1 included stable funding coupled with a 

number of promising concepts and technologies that had strong potential for 

further system-level maturation. With the endorsement of the panel of experts, 

the NAC called upon ARMD leadership to plan and develop a number of 

candidate research projects consistent with the National Aeronautics Policy 

and Plan and leverage NASA’s unique expertise and competencies. The primary 

goal was to advance state-of-the-art capabilities in several key disciplines and 

facilitate transition of results to the aerospace community. At the time when 

ERA was formulated, there was strong support from industry for new systemlevel research for improving aircraft efficiency.30

Advancing the State of the Art

The ERA Phase 1 research portfolio focused on exploring, assessing, and demonstrating the benefits of promising concepts and technologies, and expanding 

upon previous work. Phase 1 efforts took full advantage of previous groundwork laid at the foundational research level and moved it from a TRL of at least 

4 or 5 (component and/or breadboard validation in a laboratory or relevant 

environment, respectively) to a desired TRL of at least 6 (system prototype 

demonstration in an operational environment). The ERA planning team began 

by identifying technologies already investigated in ARMD programs that had 

30. Craig L. Nickol and William J. Haller, “Assessment of the Performance Potential of Advanced 

Subsonic Transport Concepts for NASA’s Environmentally Responsible Aviation Project,” AIAA2016-1030, presented at American Institute of Aeronautics and Astronautics SciTech, 54th AIAA 

Aerospace Sciences Meeting, San Diego, CA, January 6, 2016, 1.
Birthing the Environmentally Responsible Aviation Initiative

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the potential to reduce subsonic transport emissions, fuel burn and noise, yet 

met the minimum TRL requirements.31

Technologies under consideration had to meet several criteria in order to 

be selected for research. First, a candidate technology had to have attained 

sufficient maturity in the foundational research program that it merited more 

in-depth evaluation at an integrated system level. Second, systems analysis 

had to indicate that the technology in question had the maximum potential 

for contributing to the simultaneous attainment of all ERA technical goals. 

Finally, any research had to be appropriate for NASA to conduct and could 

not duplicate that being done by other Government agencies.32

For the most part, ERA efforts focused on the N+2 technologies that had 

the potential to mature to a TRL level of 4–6 by 2020. By August 2010, 

researchers had assembled an ERA N+2 technology database analysis examining the interaction of 65 different technologies, including 19 related to airframe 

design and manufacturing processes, and 46 related to engine performance. 

According to Craig Nickol, ERA vehicle systems integration element lead, 

researchers took into account current and projected TRLs and studied the 

interactions of various technologies using a compatibility matrix. Analysts 

then estimated projected benefits and impacts on fuel burn, noise, and emissions using models developed with NASA’s Aircraft Noise Prediction Program 

(ANOPP), Flight Optimization and Performance Sizing (FLOPS) tool, and 

Numerical Propulsion Simulation System/Weight Analysis of Turbine Engines 

(NPSS/WATE) modeling programs. Under contract to NASA, researchers at 

Georgia Institute of Technology integrated this software toolset into their own 

computer-based Environmental Design Space (EDS) tool to make deterministic and probabilistic assessments of the ERA technology package most likely 

to result in the best overall performance.33

For baseline purposes, researchers used the Boeing 777 as an ideal example 

of an advanced, long-range, large twin-aisle (LTA) tube-and-wing (TAW) airliner configuration. The optimal technology package for this class of aircraft 

included a combination of 14 new airframe technologies and 25 engine technologies. Many of these came directly from the Phase 1 portfolio. Among the 

new airframe advances were stitched composite materials, laminar flow control 

(LFC) airfoils and nacelles, active flow control rudder, continuous mold-line 

31. Collier and Bezos-O’Connor, “Technology Development Project Plan,” 11.

32. Nickol and Haller, “Assessment of the Performance Potential of Advanced Subsonic Transport 

Concepts,” 1–2.

33. Nickol and Haller, “Environmentally Responsible Aviation (ERA) Project: Assessing Progress toward 

Simultaneous Reductions in Noise, Fuel Burn and NOx,” presented at the 49th AIAA Aerospace 

Sciences Meeting, Orlando, FL, January 4, 2011, passim.
Green Light for Green Flight

20

(CML) link flaps, and aerodynamic landing gear fairings. New engine technologies included active compressor/turbine flow control, active film cooling, 

highly loaded compressor/turbine systems, ceramic matrix composites (CMC), 

metal-foam liners and engine vanes, and advanced combustors. Analysts also 

studied noise and fuel-burn tradeoffs and compared optimized performance 

points to goals established in the ERA project plan.34

Advanced Tube and Wing Hybrid Wing-Body

Engine Options:

Advanced direct drive

Geared turbofan

Open rotor

• Potential ERA airframe and engine technology packages installed on both conventional and advanced configurations

• Fuel burn, noise, and emissions are estimated using models developed in NASA’s 

standard toolset (NPSS/WATE, FLOPS, ANOPP) which has been integrated into Ga 

Tech’s Environmental Design Space (EDS) tool

• EDS can feed global tools in AEDT for fleet level global impact estimates

• Seeking additional technology collector advanced configurations through NRA and 

in-house efforts

NASA planners employed Notional Advanced Tube-and-Wing and HWB concepts as initial 

technology baseline “collectors” for evaluating ERA engine and airframe combinations, and for 

modeling emissions, noise, and fuel burn. (NASA) 

Individually, each of these goals presented a significant challenge. Because 

ARMD leadership had decreed that all goals be met simultaneously, it was 

much more difficult to make tradeoffs or compromises with regard to each 

parameter. The bar had been set intentionally high because NASA analysts 

determined that without major improvements, environmental and related cost 

concerns would significantly impede the growth of future civil and military air 

transport operations. Rising oil prices alone dictated a necessity for improving 

fuel economy. In 2008, when aviation fuel cost approximately $3 per gallon, 

U.S. commercial carriers alone spent $59 billion. At the same time, there was 

an increasing focus on the environmental effects of aircraft exhaust emissions. 

Annual fuel consumption for Defense Department purposes was roughly 4.6 

billion gallons, pumping as much as 250 million tons of carbon dioxide (CO2) 

into the atmosphere. Additionally, population growth in areas surrounding 

34. Ibid.
Birthing the Environmentally Responsible Aviation Initiative

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existing airports and increased demand for air transportation heralded the need 

to reduce aircraft noise, which would also act as a restraining factor on growth 

of the National Airspace System (NAS).35

The ERA initiative was well timed to leverage these environmental and 

economic concerns and accelerate new commercial engine and airframe developments. Project researchers anticipated that new technologies would yield 

a high payoff for a multitude of commonly used commercial platforms such 

as the Boeing 737 and 777 series, Airbus A320, and the Canadair Regional 

Jet (CRJ) family of aircraft. Moreover, the ERA technology portfolio would 

have a significant impact on the wide-body replacement market regardless of 

whether new aircraft were designed using conventional tube-and-wing or more 

advanced configurations. A year into Phase 1, project manager Fay Collier 

expressed confidence in meeting the expected challenges. “People are asking 

how we are going to do this,” he said. “We’ve done some experiments that lead 

us to believe that although this is difficult, we think it is achievable.”36

Phase 1 Research Portfolio

The ERA project’s Phase 1 technology investigations were approved for implementation at the first key decision point, known as the KDP-1 Formulation 

Review. At this time, the Phase 1 research portfolio was organized and managed 

around the three major technical subprojects: Airframe Technology, Propulsion 

Technology, and Vehicle Systems Integration. The technology portfolio within 

each subproject was divided into the following research elements:

• Airframe Technology Subproject

– Lightweight Integrated Structures Element 

– Flight Dynamics and Control Element

– Drag Reduction Element

– Noise Reduction Element

• Propulsion Technology Subproject

– Combustor Technology Element

– Propulsor Technology Element

– Core Technology Element 

• Vehicle Systems Integration Subproject

– Systems Analysis Element

35. Guy Norris, “‘Green’ Airliner Targets Achievable by 2025, Says NASA,” Aviation Week & Space 

Technology, http://aviationweek.com/awin/green-airliner-targets-achievable-2025-says-nasa, 

April 18, 2011 (accessed July 21, 2016).

36. Ibid.
Green Light for Green Flight

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– Propulsion Airframe Integration Element

– Propulsion Airframe Aero-Acoustics Element

– Advanced Vehicle Concepts Element 

Concept Modeling Summary

Regional 

Jet

SingIe 

AisIe

SmaII 

Twin AisIe

Large Twin 

AisIe

Very 

Large

Baseline 

Vehicle CRJ900 737-800 767-300ER

777-

200ER 747-400

Engine CF34-8 CFM56-7B27 CF6-80 GE90-94B PW4056

Passengers 86 174 210 301 416

2025 Tube+Wing

Fuel Burn –42.0% –40.8% –47.3% –44.3% –41.0%

Noise (dB cum 

below Stage 4) 30.5 24.0 27.1 27.3 22.6

Emissions –75.0% –75.0% –75.0% –75.0% –75.0%

2025 HWB

Fuel Burn N/A N/A TBD –50.2% TBD

Noise (dB cum 

below Stage 4) N/A N/A TBD 43.6 TBD

Emissions N/A N/A TBD –75.0% TBD

This ERA concept modeling summary compares late 1990s baseline commercial transport 

aircraft to proposed future concepts. (NASA)

Airframe Technology Subproject

The Airframe Technology subproject was designed to conduct system level 

experiments on airframe technologies, particularly with a view toward addressing the fuel burn and noise reduction goals. The Phase 1 airframe technology 

portfolio was built upon technologies that would take full advantage of previous groundwork laid at the foundational research level, and in Phase 1 move 

from a TRL of 3, in most cases, to at least a TRL of 4 (component and/or 

breadboard validation in a laboratory).37

Planners selected numerous key airframe technologies for Phase 1 investigations based on projected vehicle-level benefits. The following technologies 

were of paramount interest:

37. Collier and Bezos-O’Connor, “Technology Development Project Plan,” 10–17.
Birthing the Environmentally Responsible Aviation Initiative

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• Lightweight Integrated Structures 

– PRSEUS composite technology system that includes

· Stitching 

· Resin infusion 

· Unitized structures 

· Damage tolerant structures 

· Post-buckled structures 

• Flight Dynamics and Control 

– Stability and control characteristics of low-noise HWB vehicle 

configurations

• Drag Reduction 

– Active flow control rudder 

– Natural laminar flow ground test capability 

– Hybrid laminar flow control via discrete roughness elements

– Adaptive Compliant Trailing Edge (ACTE) flaps

• Noise Reduction

– Flap edge and landing gear fairings noise reduction concepts 

Lightweight Integrated Structures Element

The Lightweight Integrated Structures investigation focused on a new structural concept called PRSEUS. This lightweight, damage-tolerant, stitched 

composite system offered benefits with regard to manufacturing a wide variety 

of aircraft components from wings to cargo doors, but primarily served as an 

enabling technology for non-circular pressurized fuselage assemblies. Aircraft 

designers considered the ability to build a non-circular pressurized fuselage as 

key to enable vehicle concepts with a reduced overall wetted area (surface area 

in direct contact with external airflow). Reductions in wetted area directly 

addressed the fuel-burn goal through reduced overall skin friction drag.

The PRSEUS concept introduced both new materials and manufacturing 

processing methods in a structural concept with high potential for both weight 

reduction (10.3 percent compared to advanced composite sandwich structures 

analytically applied to a non-circular center body shell) and manufacturing cost 

savings (based on experience with a similar but non-primary structural application). However, to be a viable approach, these new materials and processes 

needed to be validated.

In Phase 1, researchers adopted a building block approach to analysis and 

testing, from coupons to structural elements and subcomponents. These efforts 

culminated during Phase 2 in full-scale PRSEUS testing involving a full test 

matrix of specimens with large cutouts, impact damage, and discrete-source 

damage subjected to representative loadings. Data requirements included 
Green Light for Green Flight

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information on repair methodology, material durability, combined load 

conditions, and long-term structural performance, as well as safety margins, 

design methodology, inspection techniques, and understanding of load paths. 

Additionally in Phase 1, researchers investigated PRSEUS acoustic transmission characteristics to determine cabin noise impacts. Investigators also identified structural design strategies for minimizing the coupling of cabin and 

engine noise.38

Flight Dynamics and Control Element

During Phase 1, NASA, Boeing and the Air Force Research Laboratory (AFRL) 

collaborated on flight-testing an 8.5-percent-scale, remotely piloted aircraft 

of a potential, full-scale HWB-type aircraft called the X-48. The plane was 

equipped with a computerized “fly-by-wire” system and flown remotely from 

a ground control station. Flight-testing consisted of a multi-step research program aimed at ascertaining the low-speed handling and flying qualities of 

this type of aircraft. Researchers experimented with two leading-edge configurations, with wing slats either extended or retracted. Flying with the slats 

extended allowed the plane to fly slower with greater lift, representative of 

takeoff and landing conditions. The high-speed, slats-retracted, configuration 

represented the aircraft in cruise mode. Additionally, two different stabilizer 

and engine arrangements were evaluated.39

Designed by Boeing and built by Cranfield Aerospace, the initial X-48B 

configuration was flown in partnership with NASA at Armstrong Flight 

Research Center (AFRC) beginning in July 2007. The aircraft weighed 

about 500 pounds and spanned just over 20 feet. Initially powered by three 

52-pound-thrust JetCat P200 turbojets, the demonstrator was capable of a 

top speed of 140 miles per hour and a maximum altitude of around 10,000 

feet. Flight-testing through the summer of 2010 focused on defining the lowspeed, low-altitude flight characteristics of the HWB configuration, including 

engine-out control, stall characteristics, and handling qualities.

Following its 92nd flight, the aircraft underwent extensive modification 

to the X-48C configuration. External changes entailed relocating wingtip 

winglets inboard next to the engines, effectively turning them into a widely 

spaced twin V-tail. The three noisy turbojets were replaced with two quieter 

89-pound-thrust JetCat SPT15 ducted fan engines. The center-body’s aft deck 

was extended approximately 2 feet and the engines positioned between the tails 

to provide further sound baffling.

38. Ibid.

39. Ibid.
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The X-48C configuration, more highly refined than the B configuration, 

repeated many of the same test points as the previous X-48B. The major differences between the two configurations were optimized to reduce the predicted 

noise footprint at full scale. Flight data were obtained on both configurations at 

the same flight conditions to determine which one provided better low-speed, 

high angle-of-attack handling characteristics.40

Drag Reduction Element

For conventional commercial aircraft, the two main contributors to overall drag 

are skin friction drag and lift-induced drag. In ERA Phase 1, researchers focused 

on technologies related to the reduction of the overall viscous drag, specifically 

to reductions in wetted area or skin friction. Viscous drag contributes on the 

order of 50 percent of the total drag for a typical transonic transport vehicle 

and is highly dependent on the wetted area of the aircraft, the boundary layer 

state, and flight conditions. In particular, for a given wetted area, a turbulent 

boundary layer causes significantly more skin friction drag than does a laminar 

boundary layer. Laminar flow is one of the key aerodynamic technologies with 

the potential to provide large reductions in fuel burn (roughly 5 to 15 percent, 

depending on configuration). To achieve significant total projected fuel burn 

reductions on the order of 40 percent required by the N+2 goals, researchers’ 

projections indicated that laminar flow needed to be obtained over 60 percent 

of the chord on the upper wing surface, simultaneously with 50 percent of the 

chord on the lower wing surface.41

From an aerodynamic perspective, multiple approaches to achieve laminar 

flow are available. In Phase 1, the ERA project focused, in part, on validating 

the NASA LaRC National Transonic Facility (NTF) as a viable ground-test 

facility for natural laminar flow (NLF) testing. Researchers also investigated 

the possibility of achieving hybrid laminar flow control by applying discrete 

roughness elements (DREs), small micro-bumps spaced at specific intervals 

along a swept leading edge of an airfoil. The DRE investigation in Phase 

1 enabled a better understanding of the application of DREs at moderate 

Reynolds numbers (the ratio of inertial forces to viscous forces within a fluid 

which is subjected to relative internal movement due to different fluid velocities) and assessed their feasibility at higher-chord Reynolds numbers typical 

of large subsonic transports.

Some Phase 1 research was devoted to the study of engineered airfoil 

surfaces. For example, because resistance to insect impacts is important to 

establishing and maintaining laminar flow, researchers identified and tested 

40. Ibid.

41. Collier and Bezos-O’Connor, “Technology Development Project Plan,” 14–15.
Green Light for Green Flight

26

low-energy surface coatings for composite wings that might make manufacturing tolerances considerably lower in the leading-edge area. Another approach 

to reduce viscous cruise drag was to reduce the wetted area due to improved 

aerodynamic efficiency through the use of active flow control. This technique 

enables more aggressive surface performance though the use of low-energy 

interactions in the boundary layer for the purpose of gust load alleviation, 

buffet control, and high-lift enhancement. At the time, the concept of using 

active separation control to increase rudder/aileron effectiveness for the purpose of reducing the size of control surfaces was ready to be pursued at the 

system level for technology integration. Phase 1 wind-tunnel investigations 

applied active separation control to the rudder and vertical tail for the purpose 

of reducing the tail surface area required during a hypothetical engine-out contingency landing. Preliminary system analysis indicated a cruise drag reduction 

benefit of 1 to 2 percent through the application of this technology on the 

vertical tail alone. Additionally, at the start of Phase 1, NASA and the AFRL 

formed a partnership to conduct a “bolt-on” ACTE flight experiment using a 

modified Gulfstream G-3 aircraft. Both the DRE and ACTE flight experiments 

were classified as potential ITD candidates for Phase 2.42

Noise Reduction Element

During landing, propulsion noise is typically reduced because engine power is 

cut back significantly. As a result, the airframe contribution to community (airport environment) noise from conventional aircraft systems and configurations 

is approximately equal to that of the engines. ERA researchers expected N+2 

vehicle configurations would employ engine noise shielding to further reduce 

community noise, along with quieter high-bypass-ratio engines. Therefore, the 

primary source of noise during future landings would result from airframerelated elements, specifically the landing gear and flap edges. Researchers considered using both aerodynamic fairings to mitigate landing gear noise, and 

CML technology to eliminate gaps between flaps, ailerons, and wings.

Testing of CML concepts revealed some structural weaknesses requiring 

further study of new structural concepts. This pushed Phase 1 research to focus 

on the development of noise reduction fairings for airframe-related elements. 

This included wind tunnel testing of a semi-span scale model of a G550 business jet baseline configuration without noise reduction fairings. Resulting data 

were used to validate aerodynamic performance models and baseline flight 

test data to determine scaling effects for Phase 2 test results. As part of Phase 

2, researchers planned to use aero-acoustic wind tunnel data to determine 

the most promising noise reduction concepts to be integrated onto the main 

42. Ibid.
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landing gear and flap edges of a full-scale Gulfstream G550 and conduct a 

series of flight tests.43

Propulsion Technology Subproject

All three of ERA’s N+2 goals were addressed in Phase 1 investigations within 

the Propulsion Technology subproject. Advanced combustor designs focused 

on NOx reduction and required a different approach than what was used to 

meet NASA subsonic transport system level metrics for N+1. The ERA noise 

reduction goal required new propulsor configurations, including engine noise 

shielding in an advanced propulsor. Fuel-burn reduction goals required a higher 

bypass-ratio propulsion system to improve propulsive efficiency. Investigations 

also addressed highly loaded front block compressor designs to enable higher 

thermal efficiency.44

In Phase 1, the Propulsion Technology Subproject addressed the following 

propulsion technologies:

• Combustor technology element 

– Advanced combustor concepts

– Active combustion control 

– Lightweight CMC liners 

• Propulsor technology element 

– Open rotor 

– UHB turbofan with noise reduction technologies including: 

· Lip liner 

· Over the rotor metal foam liner 

· Soft vane 

· Zero splice inlet 

– Shape memory alloy variable area nozzle

• Core Technology Element 

– High temperature erosion coatings for CMC vanes and exhaust 

nozzles 

– High operating pressure ratio (OPR) compressor/turbine 

Combustor Technology Element

As part of Phase 1, researchers sought to achieve landing and takeoff NOx 

reduction goals by maturing several new combustor concepts in partnership with engine manufacturers and fuel injector companies. Their strategy 

43. Ibid.

44. Ibid.
Green Light for Green Flight

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aimed toward improving injector designs and developing innovative concepts 

to improve fuel/air mixing into the combustor. This was accomplished in 

combination with higher temperature materials that required less cooling air, 

namely coated CMC, along with fuel flexibility and improved fuel injector 

timed-modulation control.

Researchers considered designs for conventional and alternative fuels. 

Alternative fuels with higher thermal stability allowed for improved injector 

designs without choking concerns. As part of Phase 1, advanced combustor 

concepts were developed and screened through flame tube tests (matured to 

TRL 4) and sector tests (matured to TRL 5). Industry cost-shared the flame 

tube and sector tests with NASA. Advanced combustor testing was conducted 

using the Advanced Subsonic Combustion Rig (ASCR) facility and advanced 

fuel flexible injectors were tested at the CE-5 Facility, both located at the NASA 

Glenn Research Center (GRC). Additional testing took place in industry test 

facilities. A promising advanced combustor concept was selected for further 

engine development as part of Phase 2.45

Propulsor Technology Element

Propulsive efficiency has a significant impact on an aircraft’s SFC, and increasing the bypass ratio of a turbofan engine provides benefits for both fuel-burn 

reduction and noise reduction. The ability to increase bypass ratio is limited by 

engine size, weight, aerodynamic drag and installation issues. ERA researchers considered several advanced propulsion systems for future aircraft having 

higher bypass ratios, including open-rotor systems and UHB geared turbofans. 

For N+2 vehicle concepts, the integration of these propulsion systems was a 

key technical issue addressed by the ERA project.

Researchers investigated near-term and far-term advanced propulsor configurations in Phase 1. Near-term investigations focused on wind tunnel testing 

of the best available propulsor systems, both open rotor and UHB turbofans, 

that offered the desired performance and acoustic characteristics. Through 

high-fidelity model-scale experiments, Phase 1 propulsor technology investigations assessed system level performance and acoustics. The results were critical 

for anchoring system studies and identifying appropriate propulsion/airframe 

configurations that warranted further study in Phase 2.46

Testing of a low-noise, open-rotor system, initiated under the SFW Project, 

was completed during FY 2010 in the 9-by-15-foot and 8-by-6-foot wind 

tunnels at GRC for acoustic and aerodynamic performance. The open rotor 

propulsor was tested both in isolation and with simulated installed effects 

45. Ibid.

46. Ibid.
Birthing the Environmentally Responsible Aviation Initiative

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for candidate N+2 vehicle concepts. Candidate technologies tested for lower 

noise included increased rotor spacing, lower blade count, lower tip-speed 

rotors with larger diameters, clipped aft rotors, and optimized spanwise rotor 

loading. Researchers used particle-image velocimetry, pressure-sensitive paint, 

and phased-array data to develop a complete validation dataset for open-rotor 

aerodynamic and acoustic code assessment and development. This investigation explored the design space for lower noise while maintaining the high 

propulsive efficiency provided by a counter-rotating open-rotor system.

As with the open-rotor configuration, subscale UHB turbofan models were 

tested in the GRC wind tunnels in both isolation and with simulated installed 

effects in late FY 2011 and early FY 2012. These tests provided the opportunity 

to investigate the impact of increasing the bypass ratio to approximately 15 

to 18. Additionally, a test for a distortion-tolerant, integrated-inlet fan investigated cruise performance impacts and helped establish a database for code 

development. Researchers studied installation effects such as pylon, fuselage, 

and wing integration along with features that minimized interaction noise 

sources. Technologies investigated included shape memory alloy variable-area 

nozzles, acoustically treated (“soft”) stator vanes, over-the-rotor acoustic treatment, low-distortion short inlets, active stall control integrated with the variable area nozzle, and a distortion-tolerant fan design.47

Core Technology Element

The thermal efficiency of the engine core that provides power to the propulsor is an important parameter for fuel consumption. Researchers found 

that although there is a perception that the optimum thermal efficiency has 

almost been reached for gas turbine engine core components, there are still 

opportunities for optimizing the system for fuel burn reduction by increasing 

the turbine inlet temperatures and increasing the overall pressure ratio of the 

compression system. In Phase 1, several advanced technologies were selected 

for maturation that promised benefits in the near term to virtually any core 

engine system. ERA researchers performed low-level exploratory work with 

CMC materials and structures and investigated an advanced highly loaded 

front block compressor concept. They concluded that CMC materials are ready 

for application in the N+2 timeframe for engine components such as turbine 

vanes and core exhaust nozzles.

The greatest advantage of CMCs over existing materials is that their lightweight structures can withstand very high temperatures. For the ERA project, 

researchers investigated the properties of both turbine vanes and panels for core 

nozzles made from CMC capable of withstanding 2,400  ̊F. Testing proved 

47. Ibid.
Green Light for Green Flight

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these materials highly effective as a practical cooling strategy for ceramic turbine 

vanes. Studies involving turbine vane design, fabrication, and testing explored 

materials and structures, coatings, ceramics durability, and heat-transfer characterization at realistic engine temperatures. Two different fabrication techniques were pursued by the end of FY 2010 that led to a verification of the 

manufacturability of CMC turbine vanes and coatings. CMC nozzles were 

developed and demonstrated using lightweight/high temperature static structures to reduce weight of the nacelle/exhaust stream. Researchers concluded that 

high-temperature ceramic composites with integral acoustic treatments could 

save considerable weight while reducing noise simultaneously. During Phase 1, 

a CMC exhaust nozzle was designed, and representative panels were fabricated 

for coupon durability assessments in FY 2010. In FY 2011, key subcomponents were built to verify fabrication methods. Additionally, NASA established 

a partnership with industry to test key nozzle subcomponents in FY 2012.48

Generally, an engine with a high number of low-pressure turbine stages 

tends to be long and heavy. For an N+2 powerplant, it was considered desirable 

to keep the engine as short and light as possible. In Phase 1, NASA researchers 

and designers from General Electric (GE) worked together to develop a highly 

loaded, high aerodynamic efficiency design that used steady blowing in the exit 

guide vanes to enable more rear-stage loading and fewer stages for the same 

work. Designers employed three-dimensional optimization in the design for 

the first time, which resulted in improved non-axisymmetric contouring of the 

hub end wall, ultimately contributing to higher efficiency. The advanced compressor was fabricated at GE and the first stage configuration of the compressor 

was tested in the GRC W7 facility to verify the design. Phase 1 investigations 

validated this design approach and were subsequently implemented in testing 

of an advanced compressor during Phase 2.49

Vehicle Systems Integration Subproject

The Vehicle Systems Integration subproject combined airframe technologies 

and propulsion technologies into larger subsystems, multicomponent, and 

vehicle experiments to address progress towards the simultaneous achievement 

of N+2 subsonic transport system level metrics with regard to emissions, fuel 

burn, and noise reductions. For Phase 1, this subproject also included multidisciplinary systems analysis activities.

In Phase 1, the focus of the four elements of the VSI Subproject included:

48. Ibid.

49. Ibid.
Birthing the Environmentally Responsible Aviation Initiative

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• Systems Analysis Element 

– Individual technology maturation assessment against ERA N+2 

metrics 

– Vehicle-level assessment of the ERA technology suite against ERA 

N+2 metrics 

– Fleet-level assessment on the technology insertion timeframe for 

all vehicle classes across N+1, N+2, and N+3 aircraft and propulsion system generations 

– Airframe and propulsion system integration studies on the most 

promising technologies based on predicted performance against 

ERA N+2 metrics 

• Propulsion-Airframe Integration (PAI) Element

– Subscale, high-speed UHB turbine powered simulator PAI 

test on an advanced tube-and-wing vehicle concept in cruise 

configuration

• Propulsion-Airframe Aeroacoustic (PAA) Element

– Subscale, low-speed, open rotor PAA testing of HWB vehicle 

configuration

– Subscale, low-speed, UHB turbine powered simulator PAA testing of HWB vehicle configuration

• Advanced Vehicle Concepts Element

– Over-the-wing nacelle vehicle configuration study

– N+2 Advanced Vehicle Concepts study

Systems Analysis Element

At formulation, a suite of potential ERA project technologies and concepts 

were identified and assessed utilizing an “analysis of alternatives” approach. 

Researchers conducted a broad survey of technologies with the goal of identifying technologies with a potential to reach TRL 4–6 in the 2015 timeframe. Additionally, these technologies provided positive system-level benefits 

toward simultaneously meeting the N+2 noise, emissions, and fuel burn goals. 

Unconventional air vehicle concepts and configurations were also included as 

part of the technology survey. Subsystem technologies were then integrated 

onto advanced vehicle configurations to assess system-level benefits. This initial 

assessment informed the development of the ERA Phase 1 investigations.50

Phase 1 airframe and propulsion technology performance results from subsystem and component testing were extrapolated, when feasible, to estimate fullscale, integrated performance at the airframe system, propulsion system, vehicle, 

and fleet level against N+2 subsonic transport system metrics for emissions, fuel 

50. Ibid.
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burn and noise reduction. At the end of Phase 1, a common set of metrics was 

established and validated across the airframe, propulsion, and vehicle integration research areas of the ERA project. Academic, industry, and NASA results 

were used to validate the analysis of alternatives approach. Research teams also 

conducted sensitivity analysis for critical technology elements.

Researchers produced semiannual updates and annual reports to communicate the assessment status and findings to ERA project leaders, the ISRP 

program, and ARMD. These technology and concept integration assessments supported the formulation of the Phase 2 investigation presented at 

the KDP-2 Review at the end of FY 2012. The systems analysis/integration 

function continued into Phase 2. Results were reported directly to the ERA 

Project Management Leadership Team, a multi-center NASA, academic, and 

industry team that served as an “honest broker” whenever it became necessary 

to set priorities.51

Propulsion-Airframe Integration Element

Aircraft engines are the single most significant contributor to aircraft community noise. Prior to the ERA project, virtually all large-scale installed engine/

airframe performance information came from conventional tube-and-wing 

configurations with engine pods hanging below the wings. Alternate configurations, such as the HWB with top-mounted engines, high-wing tube-and-wing 

concepts with conventional nacelle installations (as on military transports, for 

example), and low-wing tube-and-wing concepts having over-wing-mounted 

nacelles, provide shielding benefits that offer tremendous potential to reduce 

community noise.

Phase 1 research focused on understanding PAI challenges associated with 

integration of a UHB engine on a transonic transport configuration with some 

form of acoustic shielding installed. This investigation involved the optimization of multiple concepts with a candidate UHB engine design, and included 

the simultaneous design of wing airfoil shapes, pylons, and nacelle configurations. A semi-span high-speed wind-tunnel model was designed from outer 

mold line shapes developed during the design process. Researchers then modified an existing turbine power simulation with an advanced fan-and-stator 

design simulating an engine-bypass ratio on the order of 15-20. A transonic 

performance test was conducted in the Ames Research Center (ARC) 11-Foot 

Unitary Tunnel in FY 2011. The resulting data validated the design methodology and provided insights on the impact of large turbofan shapes on 

51. Ibid.
Birthing the Environmentally Responsible Aviation Initiative

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other aerodynamic considerations, such as control surfaces and variable-area 

fan nozzles.52

Propulsion-Airframe Aeroacoustic Element

Phase 1 investigations for the PAA Element provided valuable insight in both 

performance and noise characteristics of key propulsion concepts integrated 

with the HWB advanced airframe concept. This configuration provided builtin acoustic shielding by making efficient use of the large vehicle surface below 

the engine to prevent much of the engine noise from radiating to the ground, 

especially when coupled with improved designs of nacelles, nozzles, and novel 

engine/airframe installation effects. At the time, based on limited laboratory 

experiments and analytical predictions, it appeared that there was significant 

noise-reduction potential due to shielding of both fan and jet noise if optimum 

engine placement could be determined and methods of enhancing shielding 

with nozzle, pylon, and airframe edge treatments and devices studied.53

Two complex scale-model aeroacoustic wind-tunnel tests were completed 

in Phase 1. Both experiments provided high-fidelity acoustic data in a windtunnel environment to validate improved predictive capabilities. In addition, 

these tests provided direct quantitative evidence that the HWB configuration, 

specifically designed for significant noise shielding, did provide cumulative 

noise reductions on the order of 17 dB as predicted. These highly complex 

tests required hot-jet-propulsion simulation capability (to mimic the effects of 

aeroacoustic noise) that most low-speed wind-tunnel facilities lacked. Coupled 

with a new phased-array microphone technology, tests were undertaken in 

the LaRC 14-by-22-foot subsonic tunnel and the Boeing Low-Speed Aero 

Acoustic Facility (LSAF). The results quantified key installation parameters 

such as jet-airframe spacing, jet-flap interaction, pylon effects, sideline noise 

shielding from canted vertical tails, and other noise reduction concepts on 

the aeroacoustic signatures at low speeds. High-fidelity acoustic data acquired 

with microphone array technology developed for the HWB 14-by-22-foot 

wind-tunnel tests was incorporated into noise prediction tools to provide more 

accurate assessments on the acoustic characteristics of any low-noise vehicle or 

component technology during flight.54

Advanced Vehicle Concepts Element

Although elements of the Phase 1 research portfolio offered significant improvements in both aircraft noise and fuel burn, it soon became abundantly clear 

52. Ibid.

53. Ibid.

54. Ibid.
Green Light for Green Flight

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that even applying all of the N+2 technologies then under consideration to an 

advanced tube-and-wing configuration would not meet all ERA goals. Doing so 

would require exploring radically new and innovative aircraft configurations.55

The Phase 1 investigations in this element focused on identifying and 

characterizing advanced vehicle concepts with the potential of meeting the 

ERA Subsonic Transport System Level N+2 goals. The investigations were 

conducted via the N+2 Advanced Vehicle Concept NRA, with some in-house 

modeling support initially focused on the over-wing-nacelle concept in FY 

2010 and an alternative advanced concept in FY 2011. Various propulsion 

technologies (open-rotor, UHB, geared-turbofan [GTF], etc.) and vehicle 

configuration (HWB, high-wing, over-the-wing-nacelle, truss-braced-wing, 

etc.) concepts were evaluated under this NRA. The advanced vehicle concepts 

identified through the NRA process identified the critical technologies that 

needed to be matured to TRL 6 by 2015 to enable entry into service (EIS) by 

2025. These advanced vehicle concepts and their respective technology suites 

were incorporated into the Analysis of Alternative assessment and validation 

that informed the ERA Phase 2 Portfolio formulation.56

ERA Phase 2 Planning

The ERA project’s Phase 2 ITD portfolio was defined to begin after completion of Phase 1 investigations (scheduled for October 1, 2012) and conclude 

on September 30, 2015. Development of the portfolio required input from a 

broad range of sources, including the ERA Phase 1 project system assessment, 

Phase 1 technology maturation, the Phase 1 N+2 Advanced Vehicle Concepts 

NRA, ongoing ARMD project foundational research developments, and other 

national interests.

Phase 2 planning began in October 2011 with the formation of the Phase 2 

Tiger Team led by the ERA project chief technologist Tony Washburn and chief 

engineer Mark Mangelsdorf. Their team included subject matter experts leading major efforts in the airframe technology, propulsion technology, and vehicle 

systems integration subprojects. The Tiger Team identified ITD opportunities 

that were feasible in the FY 2013–FY 2015 timeframe through dialogue with 

existing partners from Phase 1 and dialogue with governmental partners (FAA 

CLEEN, AFRL, and Air Mobility Command) as well as from the N+2 AVC 

55. Nickol and Haller, “Environmentally Responsible Aviation (ERA) Project: Assessing Progress,” 

passim.

56. Collier and Bezos-O’Connor, “Technology Development Project Plan,” 25–31.
Birthing the Environmentally Responsible Aviation Initiative

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NRA results, specifically the FY 2013–FY 2015 critical technologies and TRL 

Maturation Roadmaps.57

Evaluating Candidate ITD Opportunities

Selected ITD opportunities were vetted both within NASA and with industry. 

The Tiger Team held a technology review in February 2012 with technical 

representatives from ARMD, ISRP, and the implementing NASA Centers. 

Another meeting of experts was held in March 2012, with representatives 

from the aviation industry, including aircraft and engine manufacturers, technology suppliers, academia, and other Federal laboratories to vet candidate 

technologies under consideration and the candidate ITD demonstrations being 

considered. Additionally, NASA released a Request for Information (RFI) in 

January 2012 requesting inputs from the aviation community regarding areas 

of interest in the conduct of collaborative, 50-50 cost share integrated technology demonstrations.

A rigorous technical and programmatic review and assessment process commenced at the completion of the internal and external technical reviews, and 

the RFI submission period closed. The candidate ITD Opportunities were 

evaluated based on the following criteria:

1. Assess the Candidate ITD Technical Benefit 

– Contribution to the NASA Subsonic Transport System Level 

Metrics 

– Scalability of the ITD Technologies, including level of broad 

applicability to the fleet classes 

– Ending TRL 

– Industry interest, i.e., partner cost share and Independent 

Research and Development (IRAD) 

2. Assess ITD Execution Risk 

– Technical Risks included: 

· ITD complexity with respect to technology integration challenges and number of interfaces 

· Intellectual property difficulty with respect to ease of negotiation of Government use and public data rights 

– Cost and Schedule Risks included:

· Partner contributions/cost share 

· Facility and workforce availability 

· Acquisition method 

57. Ibid.
Green Light for Green Flight

36

Project managers conducted a benefit, risk, and cost analysis of all candidate 

ITDs, considering the following factors:

• Technical Focus Area (TFA) balance 

• Individual system benefits contributions 

– Best fuel burn, emissions, and noise 

• Industry-Government partnerships 

• Center balance 

Other considerations included inputs from the ERA project subproject 

managers, Phase 2 Technical Tiger Team, the Phase 2 Technical Review Panel 

members, and the ISRP and ARMD Leadership. The project manager’s ITD 

portfolio recommendation was presented to ARMD leadership at the ERA 

Project Phase 2 Technology Tollgate meeting held on June 25, 2012. At this 

review, ARMD authorized the development of detailed implementation plans 

for the recommended ITD portfolio. The ERA deputy project manager led 

an ITD Detail Planning Process that resulted in development of ITD-driving 

technical requirements, a risk-informed budget and schedule with 80 percent 

confidence level, risk registries, and an ITD cost and schedule margin reserve.58

The Phase 2 portfolio achieved a significant and quantifiable measure of 

progress towards the National Research and Development Plan and ERA/ISRP 

integrated systems research goals. It encompassed a selection of ITDs that ERA 

planners believed would enable:

• Technology maturation in the N+2 timeframe; 

• Clear technology/product transition paths via industry and Other 

Government Agency (OGA) partnership cost share indicating high 

national interest, industry, and U.S. Government commitment to 

mature the technology suite over the 3 years of ERA Phase 2; 

• Broad applicability across vehicle configurations; and 

• High probability of transition into the fleet no later than 2025.

The Phase 2 key decision point (KDP-2) review was held at NASA 

Headquarters on September 26, 2012, following nearly 12 months of preparation. Satisfied with the results, Dr. Jaiwon Shin authorized the ERA project 

to proceed with the selected ITDs.

Each Phase 2 ITD was assigned an alphanumerical designation that identified the associated technical focus area and the technology demonstration 

concept proposed as shown in the two examples below. Note that the first 

58. Ibid.
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digit represented the primary TFA and ITD impacts, and the second digit 

represented the secondary TFA and ITD impacts.59

• 21A: 

– Technical Focus Area 2: Advanced Composites for Weight 

Reduction

– Technical Focus Area 1: Innovative Flow Control Concepts for 

Drag Reduction

– Technology Demonstration Concept A: The first concept formulated to address TFA 2 and 1 

• 21C: 

– Technical Focus Area: 2: Advanced Composites for Weight 

Reduction

– Technical Focus Area: 1: Innovative Flow Control Concepts for 

Drag Reduction

– Technology Demonstration Concept C: The third concept formulated to address TFA 2 and 1

The Phase 2 ITDs were managed under the same subproject structure as 

in Phase 1: 

• AT Subproject: 

– 12A+: AFC Enhanced Vertical Tail (Lead) and Advanced Wing 

Flight Experiment. Key partner: Boeing Commercial

– 21A: Damage Arresting Composite Demonstration. Key partner:

Boeing Commercial

– 21C: ACTE Flight Experiment. Key partners: FlexSys, Inc. and 

AFRL

• PT Subproject: 

– 30A: Highly Loaded Front Block Compressor. Key partner:

General Electric

– 35A: Second Gen UHB Propulsor Integration. Key partner: Pratt 

& Whitney

– 40A: Low NOx Fuel Flexible Combustor Integration. Key partner: Pratt & Whitney

• Vehicle Systems Integration Subproject: 

– 50A: Flap Edge and Landing Gear. Key partner: Gulfstream

– 51A: UHB Integration for HWB aircraft. Key partner: Boeing 

Commercial

59. Ibid.
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The ERA project manager was accountable to the ISRP director with overall 

technical and programmatic management responsibility, including strategic 

and tactical direction. The deputy project manager was responsible for execution of the Project Plan and providing oversight of day-to-day operations. In 

Phase 2, the position of chief technologist from Phase 1 was replaced with a 

Systems Engineering and Integration lead. The inclusion of a strong systems 

engineering leadership team was deemed critical to execution of Phase 2.60

Risk Management

The ERA project developed and approved a Continuous Risk Management/

Risk Informed Decision Making plan that tracked schedule, cost, and technical risks. The ERA risk management process tracked risks by milestone, 

annual performance goals, key performance parameters, technical maturation, 

and technical challenges. The ERA Risk Management Working Group served 

as the official forum for risk identification, assessment, and mitigation plan 

development. The ERA Risk Management Board was the project forum for 

risk evaluation, deliberation, classification, and control of project risks.

The Phase 1 ERA technology portfolio was focused on validating the performance of technologies at the system level to TRL 4. The design of experiments 

and system level assessments validated technical performance to “buy down” 

the inherent risks of these advanced technologies. In Phase 2, the ERA Project 

conducted ITDs that tested technologies at the system level, further maturing 

each new technology and validating technical performance at the airframe/

engine system level, which further reduced performance uncertainty as system 

complexity and scale increased. This further reduced overall risk.

The ITD structure of the project required more specialization for Phase 2. 

The realization that each ITD was a project unto itself meant that risk had to be 

tracked and managed at the project level, ITD level, and subproject level simultaneously. For this reason, ERA managers introduced the Risk Management 

Panel (RMPn) concept for Phase 2, and additional risk managers were added 

to the organizational structure to assist in the control of risk at multiple levels 

of the project.61

Technology Transfer

A coherent plan for technology transition approach was essential to the success 

of the ERA project. Technology transfer was the primary means by which project team members shared their research with stakeholders and with the aeronautics community at large. The ERA project pursued an overarching strategy 

60. Ibid.

61. Ibid.
Birthing the Environmentally Responsible Aviation Initiative

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of collaboration with industry, universities, and other Government agencies 

to advance the TRL of the Phase 2 technology portfolio and achieve measurable progress towards NASA’s N+2 Subsonic Transport System Level metrics.

Project results were appropriately disseminated and validated through a 

peer-review process. NASA transferred data and/or hardware to end-user customers for use throughout the life of the project. Data transfer took place via 

technology interchange meetings and membership on key technical working 

groups in industry, academia, and other Government agencies, in addition to 

specific requirements in agreements with partners.62

62. Ibid.
40

Boeing’s elegant Subsonic Ultra Green Aircraft Research (SUGAR) Volt concept was among the 

many early designs examined by the NASA Aeronautics Research Mission Directorate in April 

2010 for its NRA-funded studies into advanced aircraft that could enter service in the 2030–

2035 timeframe. (NASA)
41

CHAPTER 2

The ERA Advanced Vehicle 

Concepts Study

Despite obvious advances in aircraft design and manufacturing techniques, 

the basic configuration for a commercial air transport has changed very little 

over the past six decades. One need only compare the similarities that the 

Boeing 707, first introduced in December 1957, shares with the Boeing 787 

that debuted some 50 years later. Most other modern airliners share this configuration as well. The wings, invariably swept, are usually mounted low on 

the fuselage. Pylon-mounted gas turbine engines are typically attached to the 

wings or flank the aft end of the tubular, constant-diameter cylindrical fuselage. 

This elegantly simple configuration is attractive to manufacturers because it can 

be easily assembled using conventional techniques and materials. It may not, 

however, necessarily yield maximum aerodynamic efficiency or minimize fuel 

consumption. In fact, most airframes tend to be over-engineered for structural 

strength or to provide airfoil stiffness as a hedge against wing flutter. This design 

technique may result in excess structural weight as well as incorporate features 

that add drag, both of which reduce aerodynamic and fuel efficiency.

Commercial airliners typically have a cylindrical fuselage that is joined to 

either straight or swept-back wings; the wings can be placed either high or low 

on the fuselage; and the tail section can have its horizontal tail surfaces project 

either from the fuselage or be perched on the top of the vertical fin. During 

the ERA project, NASA and industry teams considered a wide variety of alternatives ranging from modified tube-and-wing designs featuring nonstandard 

engine placement to mitigate jet noise, un-ducted turbofans, V-tails, joined 

or box wings, high-aspect-ratio braced wings, and even tailless configurations. 

Engineers evaluated the potential effects of alternate fuselage shapes, wing 

and body integration, engine installation, and controls integration. Reduction 

of the wetted area of some configurations offered huge potential for viscous 

drag reduction.

Three areas in which commercial aircraft design has seen a great deal of 

improvement are materials, propulsion, and aerodynamic efficiency. Strong, 

lightweight composites offer opportunities to reduce overall gross weight. This 
Green Light for Green Flight

42

is crucial to improving fuel consumption, as are any technologies that reduce 

airframe drag. By far the greatest progress has been made in propulsion technology. Advanced high-bypass turbofan engines are much more efficient and 

more reliable than older turbojets and quieter as well. The incorporation of 

supercritical airfoils and winglets has also provided performance benefits. The 

resulting dramatic improvements in fuel economy have imparted better performance in terms of passenger miles flown per gallon of fuel expended (P-mpg).1

The 707-320 Intercontinental variant, powered by four turbofan engines, 

could carry a maximum of 189 passengers. This yielded approximately 45 

P-mpg. (By comparison, the 787 with two turbofans and 240 to 330 seats, 

performs at 120 P-mpg, mostly resulting due to improved engine efficiency.)2

These numbers may be further improved through advanced aerodynamic 

designs, engines, and manufacturing techniques.

New technologies developed during the ERA project can be used to significantly enhance future tube-and-wing designs as well as to help introduce 

radically different configurations. Though the traditional tube-and-wing configuration offers numerous advantages, such as decoupled bending and pressure 

loads, as well as circumferential or hoop stress to maintain the integrity of the 

pressure vessel, there are also attendant disadvantages. Foremost among these 

is the large amount of wetted area contributing to overall aerodynamic drag. 

The non-lifting tubular fuselage results in a large drop-off of lift at the center 

of the airframe, and cantilevered wings require a heavy wing box assembly to 

counteract bending loads.

Alternative configurations require many trade-offs as well, some of which 

can be minimized through the use of new technologies. The truss-braced wing 

concept, for example, may incorporate a lighter, thinner, airfoil with a longer 

span. But while this may reduce both structural weight and wave drag, it also 

increases parasitic drag and wetted area, and introduces interference drag from 

truss attachment points as well as compressive loading on the truss assemblies. A 

multiple-fuselage approach may provide shielding for engine noise, but suffers 

from interference drag and, potentially, from non-linear elastic effects. Flying 

wing and blended wing body (BWB) designs have a greatly reduced wetted 

area and increased inherent lift. Greater wing thickness results in lower bending 

stress, and powerplant placement can significantly reduce engine noise. The 

elimination of a conventional tail assembly requires special attention to flight 

controls and, because pressure loads are not carried in hoop stress, the pressure 

1. P-mpg = (payload weight/total gross weight) × (lift/drag) × engine efficiency.

2. Mark Drela, “Making an Extraordinary Machine Better,” presented at TEDxNewEngland, Boston, 

MA, October 24, 2012, 56 (accessed September 3, 2019).
The ERA Advanced Vehicle Concepts Study

43

vessel needs to be assembled using unconventional means.3 NASA and industry 

teams had an opportunity to explore these problems through the ERA N+2 

Advanced Vehicle Concepts study.

NASA LaRC engineers used this 15-percent-scale semi-span model designed by Boeing to 

assess the aeroelastic qualities of an unusual truss-braced wing configuration in the Transonic 

Dynamics Tunnel. (NASA)

Developing the ERA’s Preferred System Concept (PSC)

NASA invited proposals from throughout the aircraft industry to perform a 

study to determine aircraft systems (configuration, technologies, and airspace 

requirements) and a time-phased technology development plan that would 

meet NASA’s N+2 subsonic system-level metrics, based on entry into service 

(EIS) circa 2025. NASA project managers would use the results to guide technology investments during ERA Phase 2.

Objectives of the study as defined by a NASA Research Announcement (NRA) 

included development of a preferred system concept (PSC), with several variants, 

designed to meet or exceed N+2 metrics. Those companies offering proposals 

were encouraged to describe their view on potential 2025 NextGen National 

3. Eric M. Boekeloo, Anthony Favaloro, Timothy Harris, Luke Humphrey, Brandon Johnson, Troy Lake, 

Collin McAtee, Kimberly Scheider, Yukiko Shimizu, and Barrett Tirey, “Integrated Systems Design 

of a Cargo Aircraft with Environmentally Responsible Goals,” AIAA-2012-1759, presented at 53rd 

AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, Honolulu, 

HI, April 24, 2012, 4.
Green Light for Green Flight

44

NAS environmental challenges and constraints. NASA requested that proposers 

evaluate how their PSC would operate within the NextGen NAS with regard to 

noise profiles, landing and takeoff (LTO) and cruise NOx output, carbon emissions, and operational trajectories. Additional objectives included development 

of time-phased technology maturation plans to quantify beginning and ending 

TRLs, and to prioritize which technologies would have to be developed within 

the FY 2013–2015 timeframe in order to enable each PSC. The NRA specified 

that approximately $9 million would be split among two to three awards. Results 

of the 18-month effort would do much to determine the outcome of the FY 2012 

decision point for Phase 2. One of the most important goals, according to Mark 

Mangelsdorf, was “to foster strategic partnerships between NASA and the awardees 

for collaborative research and development of innovative concepts and ideas.”4

Proposals included relevant qualifications, capabilities, and experience of 

each lead organization and team members, as well as work plans that outlined 

schedules with milestones and measurable metrics. Because NASA wanted to 

maximize public access to the results, intellectual property rights were negotiated on a case-by-case basis. In order to minimize paperwork, the science and 

technology management section of each proposal was limited to 50 pages. 

Proposals were evaluated on the basis of the following four criteria: 

• Relevance to ERA objectives (20 percent), 

• Technical merit (35 percent), 

• Effectiveness of the proposed work plan (20 percent), and 

• Team qualifications (25 percent). 

The final NRA was posted on March 1, 2010, with proposals due by April 

15. Final selections were complete by May 1, and the awardees were on contract by June 30.5

Winners were Boeing, Lockheed Martin, and Northrop Grumman. From 

December 2010 through December 2011, these three industry teams derived 

and then evaluated new aircraft configurations designed to burn as much as 50 

percent less fuel than comparable aircraft that entered service in 1998, emit 75 

percent fewer harmful pollutants, and reduce the size of areas affected by objectionable airport noise by as much as 83 percent. The Boeing 777-200LR, with 

a 291-seat capacity, cruising speed of 550 miles per hour, and a 10,000-mile 

range, served as a baseline reference. The Advanced Vehicle Concepts (AVC) 

4. Mark F. Mangelsdorf, “Overview of ERA’s Advanced Vehicle Concepts NRA,” presented at the N+2 

Advanced Vehicle Concepts and Quick Starts NRA Pre-Proposal Meeting, February 19, 2010, https://

www.hq.nasa.gov/office/aero/pdf/era_preproposal_overview2010.pdf (accessed September 3, 2019).

5. Ibid.
The ERA Advanced Vehicle Concepts Study

45

study was broken down into five tasks, the first four relating to a planned fullsized concept and the fifth specifically related to proposals for a subscale test 

vehicle (STV) that would serve as a technology demonstrator.6

AVC Contract Timeline

2010 2011

Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Contract

Start

PSC

Review

Conceptual

Design Review

Independent Assessment by 

NASA supporting ERA Phase II

Final Contract

Presentations

AIAA Nashville

3 Month

Technical

Interchange

Kick-Off

NASA held the various contractor teams to a strict schedule to ensure that the Advanced Vehicle 

Concept study produced the desired results within the ERA timeframe. (NASA)

Task 1 was to develop a future scenario beginning with evaluating how each 

of the contenders would best fit into the FAA’s NextGen airspace plan. Task 2 

focused on meeting PSC performance requirements. Specifications for notional 

commercial transports called for a maximum speed of between 0.70 and 0.85 

Mach, a range of 8,000 nautical miles, and a 50,000-pound passenger/baggage 

payload. Industry teams were also encouraged to develop a freighter variant 

capable of transporting a 100,000-pound payload over a range of 6,500 nautical miles. Task 3 detailed a 15-year technology maturation road map for each of 

the critical technologies, specifying research, analysis, tool and method development as well as necessary ground or flight tests. Each technology maturation 

plan accounted for cost, schedule, and technical outcome, and was expected to 

be useful for advocacy beyond the ERA project timeline. Task 4 would focus 

on the “long poles,” or the critical enabling technology demonstrations to be 

used for the second half of the ERA program in FY 2013–FY 2015. Researchers 

had to consider the scalability of these technologies for use in the proposed 

STV, and the potential ramifications of cost, complexity, schedule, and technical risk. Task 5 entailed conceptual design of the STV that, if built, would 

6. Mark F. Mangelsdorf, “Environmentally Responsible Aviation N+2 Advanced Vehicle Concepts NRA 

Status,” presented at the ASME Turbo Expo conference, Vancouver, BC, June 6–10, 2011, 1–20.
Green Light for Green Flight

46

have to be large enough to simultaneously demonstrate ERA noise, emissions 

and fuel-burn goals. The half-scale-or-larger STV was to have the same overall 

configuration as the full-scale PSC, the same cruising speed, retractable landing 

gear, and be adaptable for use in additional future demonstration efforts. The 

size of the vehicle was necessary to provide realistic aero-acoustic data. Planners 

projected that the STV would have a 20-year service life.7

NASA managers were eager to take advantage of improvements in airframe and powerplant designs as quickly as possible and sought ways to apply 

technology to existing airframes that were then in service or expected to be 

introduced within the next decade. As Mangelsdorf noted, “We’re also interested in technologies that apply to N+1—[these are] nearer term single-aisle 

concepts or ones that can be retrofitted to current 737/CRJ-class [Boeing 737 

and Bombardier CRJ] models.”8

Key factors in both the N+1 and N+2 studies were benefit gains versus a 

specific class of aircraft. While N+1 focused on smaller technology gains applicable to short-range, domestic flights in 737/CRJ-class aircraft, N+2 focused 

on long-range, international flights in 777-class transports. “There’s a difference 

in range and seating capacity, and also in Mach number,” said Mangelsdorf. 

“The 777 cruises between approximately 0.82 to 0.85 Mach, while the 737 

cruises at around 0.78 to 0.80 Mach.” Some ERA researchers were considering slower aircraft concepts because they burned less fuel, but there was point 

at which this benefit began to decrease. “You burn less fuel per minute, the 

slower you go,” Mangelsdorf pointed out, “but if you slow down too much, 

you burn more fuel per mile because the number of minutes is stacking up, so 

your total fuel burn ultimately increases.”9

While ERA managers were trying to decide how best to focus their efforts, 

the SFW program was looking at several revolutionary concepts that were 

more attuned to N+1 or even N+3. In 2010, a team led by Boeing proposed 

the Subsonic Ultra Green Aircraft Research (SUGAR) Volt, a twin-engine 

concept with a hybrid propulsion system combining gas turbine and battery 

technology, a tube-shaped body, and a truss-braced wing mounted to the top of 

the aircraft. It was designed to fly at Mach 0.79 while carrying 154 passengers 

3,500 nautical miles.

Researchers at the Massachusetts Institute of Technology submitted a design 

called the D8 “Double Bubble” based on a modified tube and wing with a very 

7. Guy Norris, “‘Green’ Airliner Targets Achievable by 2025, Says NASA,” Aviation Week & Space 

Technology, http://aviationweek.com/awin/green-airliner-targets-achievable-2025-says-nasa, 

April 18, 2011 (accessed July 21, 2016).

8. Ibid.

9. Mark F. Mangelsdorf, interview with the author, October 19, 2016.
The ERA Advanced Vehicle Concepts Study

47

wide fuselage to provide extra lift, with narrow-chord low-sweep wing offering reduced drag and weight. Optimized for domestic service, the D8 series 

aircraft could fly at Mach 0.74 while carrying 180 passengers 3,000 nautical miles in a coach cabin roomier than that of a Boeing 737-800. Another 

team, from California Polytechnic State University, submitted the Advanced 

Model for Extreme Lift and Improved Aeroacoustics (AMELIA), a possible 

future subsonic HWB vehicle with short takeoff and landing capabilities. 

Of the three, only the AMELIA was suited to N+2, but according to Rich 

Wahls, “The powers that be didn’t want ERA to become driven strictly by an 

HWB configuration.”10

10. Wahls interview.

Boeing’s elegant Subsonic Ultra Green Aircraft Research (SUGAR) Volt concept was among the 

many early designs examined by the NASA Aeronautics Research Mission Directorate in April 

2010 for its NRA-funded studies into advanced aircraft that could enter service in the 2030–

2035 timeframe. (NASA)
Green Light for Green Flight

48

Alternative Lockheed-Martin (left bottom), Northrop-Grumman (left top), and Boeing (right) consortium approaches to meet NASA’s goals for making future aircraft burn 50 percent less fuel 

than aircraft that entered service in 1998, emit 75 percent fewer harmful emissions, and shrink 

the size of geographic areas affected by objectionable airport noise by 83 percent.

Low-speed designs like the SUGAR Volt and D8 introduced a number 

of economic variables extraneous to ERA management’s interest at the time, 

which remained firmly fixed on technical issues. “We decided to reduce those 

variables and go for a Mach number of 0.85,” said Mangelsdorf, “because we 

know that 0.85 Mach is a speed that works well for long-distance flights.”11

Configurations like the truss-braced wing and Double Bubble were far better 

suited to the domestic 737-class market than was the HWB. Moreover, he 

noted, “Our studies showed that the HWB does not scale well at or below a 

737-class airframe. “As you shrink it, your packaging challenge gets tougher 

and by the time you fit the people and the landing gear in, your wetted area 

per passenger gets too high and you begin to lose the benefit over a tube-andwing configuration.”12

As all of these different ideas evolved, Dr. Jaiwon Shin and others were 

examining possibilities for NASA to fund new X-plane projects, leading to 

the AVC study contract awards. “We had the NASA subsonic metrics chart 

11. Mangelsdorf interview.

12. Mangelsdorf interview.
The ERA Advanced Vehicle Concepts Study

49

that outlined N+1, N+2, N+3 fuel-burn versus noise-reduction goals for those 

timeframes,” said Mangelsdorf, “and that provided guidance as to what it would 

take to get a 777-class vehicle to meet those requirements, both in terms of 

passenger count and range.” Those targets for simultaneous achievement of a 

50-percent fuel burn reduction and 75-percent NOx reduction, as well as a 

–40 db noise reduction, drove the size the vehicle proposed in each AVC study.13

Three Teams = Three Very Different Approaches

Design studies by each of the industry teams yielded three very different configurations, and one of these—the Northrop Grumman entry—was ultimately 

discarded in favor of a different design altogether. ERA managers planned to 

down-select the best two in December 2011, and eventually choose one for 

the STV.

Lockheed Martin: Looking Toward the Joined Wing

The Lockheed Martin Skunk Works (LMSW) team, which included RollsRoyce and the Georgia Institute of Technology, offered an unconventional 

“joined wing” concept for improved structural and aerodynamic efficiency. It 

incorporated advanced technologies in the areas of propulsion for significant 

fuel burn and noise reduction, new lightweight composite materials, laminar 

wing aerodynamics, and other efficiency technologies. Designers envisioned 

that the concept would be integrated into existing airport infrastructure without significant changes and provide a passenger experience consistent with the 

best contemporary airliners.14

Starting with a standard tubular fuselage, designers abandoned traditional 

wings and tails for the joined wing configuration. In place of wingtips, swept 

wings turned upward to join with a second set of airfoils that met at the vertical 

stabilizer. The result looked like a futuristic biplane, except that the continuous 

surface of the non-planar airfoil would act to eliminate wingtip vortices that 

are a major component of wake turbulence and induced drag. By having what 

amounted to two sets of wings, it was possible to use airfoils with a narrower 

chord than on conventional wings. To further reduce drag, the joined wing 

was also to be designed for laminar flow control. According to Bruce McKay, 

LMSW ERA project chief engineer and propulsion lead, “[The joined wing] 

uses two [sets of] very long, skinny wings which are aerodynamically more 

13. Ibid.

14. Mangelsdorf, “Environmentally Responsible Aviation N+2 Advanced Vehicle Concepts NRA 

Status,” 1–20.
Green Light for Green Flight

50

efficient than traditional wings, and the aft wing is actually mounted high 

on the airplane, which allows me to put much larger, more efficient engines 

on it.”15

For propulsion, the airplane was to be equipped with two Rolls Royce 

LibertyWorks UltraFan engines, each with a bypass ratio (flow of air around 

engine compared to through the engine) nearly five times greater than thencurrent engines. The unusual configuration called for suspending the power 

plants from beneath the upper aft set of airfoils. According to McKay, Rolls 

Royce designed the ultra-high-bypass engines to push the limits of turbofan 

technology to maximize efficiency. “We get about 20 percent to 25 percent 

better fuel efficiency with those,” he noted.16

Advances in strong, lightweight composite materials were crucial to enabling 

construction of a joined wing airliner. LMSW leveraged its experience with 

the Lockheed-Martin X-55 Advanced Composite Cargo Aircraft (ACCA) that 

demonstrated the feasibility of designing and manufacturing large, bonded 

unitized structures featuring low-temperature, out-of-autoclave curing. First 

flown in 2009, the X-55 employed the cockpit, wings, engines, and horizontal tail of a Dornier 328J airliner. The LMSW team then constructed the 

advanced composite fuselage in two large half-section subassemblies (upper 

and lower) using honeycomb-sandwich construction with carbon composite 

skins and Nomex core. Instead of the numerous frames, stiffeners, and metal 

fasteners commonly used in traditional aircraft, the composite components 

were bonded together with adhesive and ply overlays along the longitudinal 

seam. Compared to the basic Dornier 328J airframe, the ACCA structure used 

only around 300 structural parts versus the original 3,000 metallic parts and 

approximately 4,000 mechanical fasteners compared to 40,000. A new vertical 

tail was designed using tailored stiffness technology.17 As applied to advanced 

aircraft concepts, these technological leaps would allow manufacturers to save 

time during both the design and construction phases, reduce aircraft structural 

weight, and reduce corrosion and metal fatigue problems.

The joined wing design is optimized for low drag and reduced fuel burn. 

LMSW program manager Kenneth Martin noted that a full-scale airframe 

based on the team’s PSC would be 181 feet long with a 171-foot wingspan. 

A lightweight composite airframe would allow for a maximum takeoff weight 

15. Bruce McKay interview with Johnny Alonso, NASA 360, S03E22, NASA podcast, December 5, 

2011, transcript at http://www.nasa.gov/multimedia/podcasting/nasa360/nasa360-0322.html, 

(accessed September 3, 2016).

16. Ibid.

17. Lockheed Martin, “The Carbon Comet: X-55 Advanced Composite Cargo Aircraft,” http://www.

lockheedmartin.com/us/100years/stories/acca.html, date unknown (accessed September 3, 2016).
The ERA Advanced Vehicle Concepts Study

51

(MTOW) of around 365,900 pounds versus the 550,400 pound maximum 

gross weight of an equivalent 1998 technology-standard aircraft. Designers 

anticipated a similar reduction in fuel weight to less than half that of the 

250,000 pounds required by the baseline design. Advanced composite construction also made possible a high-aspect-ratio wing configuration that would 

allow steep landing approaches to help contain noise within airport boundaries. 

This contributed to a noise reduction of –35 decibels. Martin noted that, with 

potential military roles in mind, the LMSW joined wing configuration also 

offers “scalability from tactical to strategic, as well as reduced span for compatibility with the existing infrastructure.”18

The Lockheed-Martin X-55 ACCA taking off from Air Force Plant 42 at Palmdale, CA, on its first 

flight, June 2, 2009. The X-55 marked an important step forward in understanding and realizing 

the potential of advanced composites. (NASA)

ERA managers were suitably impressed with the 63,600-pound-thrust 

Rolls-Royce UltraFan engine. This revolutionary powerplant is a hybrid 

between current advanced turbofans and open-rotor engines. With a diameter of approximately 140 inches, the geared fan assembly is extremely large 

compared to conventional turbofans. Rolls-Royce encased the UltraFan in a 

slender, natural laminar-flow nacelle without a thrust reverser. Additionally, 

18. Guy Norris, “Future-Airliner Concept Contenders Reveal Design Surprises,” Aviation Week & Space 

Technology, http://aviationweek.com/awin/future-airliner-concept-contenders-reveal-designsurprises, January 16, 2012 (accessed September 3, 2016).
Green Light for Green Flight

52

data indicated that engine emissions beat the target by coming in at –89 percent relative to CAEP6 standards. Fay Collier noted enthusiastically that, “We 

thought we knew where things were going with the engine companies, but the 

UltraFan concept came out of the blue.”19

For the flight demonstration program, LMSW proposed building a 

50-percent-scale STV measuring 125 feet in length and spanning 99 feet, 

with a maximum takeoff weight of 162,500 pounds. Powered by unspecified 45,000-pound-thrust engines, the demonstrator was to have a cockpit 

configuration based on that of the Lockheed Martin C-130J transport with 

open-architecture mission systems to support avionics and system upgrades. 

Design flexibility allowed for further optimizations including such airframe 

noise-reduction technology as continuous-mold line flaps, aerodynamic landing-gear fairings, slat fillers, and shape-memory alloy serrations on the engine 

bypass-duct exit.20

Northrop Grumman: Extending its Heritage of Pure Flying Wings

Northrop Grumman initially proposed a notional double-fuselage configuration, but expected a final—and probably very different—concept to evolve over 

the course of the study. The unusual twin-hulled aircraft sported two identical bodies, each with its own V-tail. A crew cabin and cockpit were centrally 

located at the apex of gently swept wings bridging the top of the double fuselage. Placement of two pylon-mounted high-bypass turbofan engines on either 

side of the crew compartment and beneath the wing provided some measure of 

acoustic shielding. As with the LMSW effort, the Northrop Grumman team 

sought reductions in weight and drag coupled with improvements in engine 

efficiency. Eventually, the Northrop Grumman team, which also included 

Rolls-Royce, Wyle Laboratories, and Iowa State University, abandoned the 

radical twin-fuselage configuration in favor of a flying wing based directly on 

the Northrop Grumman B-2A bomber design heritage.21

Northrop Grumman had a long history with flying-wing designs dating to 

aviation pioneer John K. “Jack” Northrop. By 1947, his company was flying 

a prototype eight-engine, jet-powered flying wing bomber, the YB-49A, with 

a span of 172 feet, a top speed of more than 400 miles per hour, and a range 

of nearly 3,000 miles with a 10,000-pound payload. Northrop also explored 

the possibility of producing an 80-passenger commercial transport variant, 

though it was never built. Northrop Grumman’s entry for the ERA design 

19. Ibid.

20. Ibid.

21. Mangelsdorf, “Environmentally Responsible Aviation N+2 Advanced Vehicle Concepts NRA 

Status,” 1–20.
The ERA Advanced Vehicle Concepts Study

53

competition echoed this “Flying Wing airliner of tomorrow” concept but with 

all the advantages of modern technology.22

Northrop developed a notable and frontier-breaking family of small and large flying wings. Here 

is the largest, the jet-powered eight-engine YB-49A, shown during a 1948 test flight over the 

Muroc bombing range. While visually impressive, the YB-49A—like all early flying wings—was 

impractical, with serious stability and control deficiencies. Modern digital electronic flight control 

technology, coupled with advances in composite structural materials, makes such aircraft practical. (USAF)

As Northrop Grumman ERA program manager Aaron Drake noted, this 

approach came very naturally to the company’s design team. “The airplanes of 

today look very similar to how they’ve looked for 50 or 60 years, and because of 

that, there has been a lot of time to optimize those airplanes,” he said, explaining that the tube-and-wing configuration has undergone continual improvement to maximize efficiency. “What we’re looking at [now] is taking advantage 

of configurations that are different than the conventional transport airplanes, 

22. For Northrop’s extensive studies of flying wings, see Tony Chong’s book Flying Wings and Radical 

Things: Northrop’s Secret Aerospace Projects and Concepts 1939–1994 (Forest Lake, MN: 

Specialty Press, Inc., 2016), 10–16, 25–34, 39–42, 49–50, 55–63, 69–70. Chong is the recipient of the History Manuscript Award of the American Institute of Aeronautics and Astronautics.
Green Light for Green Flight

54

ones that maybe draw on sort of unique military heritage—airplanes that were 

designed for other missions—and then take what’s been learned from that.”23

In particular, Drake noted the low-drag aerodynamic advantages of the 

flying wing configuration. “Flying wings offer a lot of inherent efficiency 

advantages, largely because every part of the aircraft actually is performing 

the function of flying,” he said. “You’re not carrying extra structure, fuselage, 

whatever, just to carry passengers; it’s all working toward the goal of flying 

efficiently.” He added that his team had identified a number of technologies in 

various levels of maturity that might, with additional work over the next few 

years, be incorporated into the design of a flying-wing transport to improve its 

efficiency. “One of the technologies that we’re looking at is swept-wing laminar 

flow control, basically technology to make the aerodynamics better and reduce 

drag,” he said. “Essentially what it means is designing the wing so that the air 

that passes over the wing does so more smoothly [so that] you get less drag.” 

This technique has already been applied to airplanes with long, thin, straight 

wings, such as the Northrop Grumman RQ-4 Global Hawk. “Applying it to 

a transport like the ERA aircraft is more difficult than our past applications 

because of the wing sweep, and because of the size of the airplane.”24

Northrop Grumman’s PSC called for building the airframe using a combination of conventional materials and advanced composites. “Our concept 

makes extensive use of composites, but not in a way that is particularly risky,” 

said Drake. “These are sort of conventional approaches to composites that have 

been well proven in military applications, [the benefits of which can now] be 

extended into transport aircraft.”25

When designing a flying wing configuration, the passenger/cargo compartment layout “drives the center-body, and the propulsion system is integrated 

with the flow path and side clearances,” Drake explained. The 224-seat passenger version of the company’s PSC had a 260-foot span and a wide centerbody cabin that was 119 feet in length. The Northrop Grumman team also 

proposed a freighter variant, spanning 230 feet, with a slenderer center-body 

and a 100,000-pound cargo capacity. “We’re focusing a lot also on the cargo 

applications of it, because that’s just as important for transport efficiency,” 

Drake said. He emphasized that despite the airplane’s size and unusual configuration, the large flying wing could be easily integrated into existing airport 

environments. “It still fits on conventional taxiways because we haven’t pushed 

23. Aaron Drake interview with Johnny Alonso, NASA 360, S03E22, NASA podcast, December 5, 

2011, transcript at http://www.nasa.gov/multimedia/podcasting/nasa360/nasa360-0322.html, 

(accessed September 3, 2016).

24. Ibid.

25. Ibid.
The ERA Advanced Vehicle Concepts Study

55

beyond what some of the very largest airplanes flying today have in terms of 

the sort of space that they take up at the airport.” In fact, taking advantage 

of such advanced technologies as laminar flow control that promote reduced 

fuel consumption means that the airplane can be made smaller that it might 

be otherwise since less airframe space for fuel tanks is required.26

The flying-wing configuration also offered opportunities for jet noise reduction. Unlike most conventional transport aircraft, where the engines are located 

beneath the wings or on either side of the fuselage, the powerplants would be 

buried inside the wing itself. As on the B-2A, exhaust gases would vent across 

wide, flat channels on the aft upper surface, allowing the aircraft structure to 

block some of the noise generated by the engines. The inherent size of the airfoil itself would also allow for good performance at low speeds, such as during 

takeoff and landing, without the need for the kinds of high-lift devices typically found on the leading and trailing edges of conventional wings. Airflow 

around these devices is a leading source of airframe-generated noise. “By not 

having those, we make our airplane a lot quieter,” said Drake. He added that 

the team’s concept was built around manufacturers’ descriptions of expected 

future powerplants to be developed over the next decade or so that will have 

improved efficiency and reduced noise levels. “So, this is an airplane that in 

operational service would have something like 40 percent less fuel consumption than the current airplanes that are out there today, and it would be much, 

much quieter.” So much so, he insisted, that, “If you were outside the airport 

boundaries, you probably wouldn’t hear it during takeoff.”27

By the end of the Advanced Vehicle Concepts study, the Northrop Grumman 

PSC had a predicted noise reduction of around –74.7 decibels, emissions 88 

percent below current levels, and fuel burn 41.5 percent below the 1998 baseline. Among the team’s key design concepts were composite wing structures, 

integrated high-bypass engines, advanced inlets, maneuver-load alleviation, 

and carbon-nanotube data cables. Northrop Grumman proposed to build a 

55 percent scale STV spanning 143 feet and powered by four General Electric 

Passport (formerly called TechX) high-bypass turbofans. “The biggest benefit 

is in the advanced propulsion,” Drake stated, “which provides 20 percent of 

overall improvement; second is swept-wing laminar flow, which contributes 

around 8.3 percent.”28

26. Ibid.

27. Ibid.

28. Norris, “Future-Airliner Concept Contenders Reveal Design Surprises.”
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56

Boeing Consortium: Advancing the Hybrid Wing-Body

Boeing led a consortium composed of Pratt & Whitney, Rolls-Royce, the 

Massachusetts Institute of Technology, and Cranfield Aerospace (the latter a 

British partner). Despite Boeing’s long history with tube-and-wing airliners, 

the company chose to pursue a cutting-edge HWB reflecting the work of noted 

aerodynamicist Robert Liebeck, powered by either two GTF engines or three 

UHB open-rotor propfans (turbine engines featuring contra-rotating fan stages 

not enclosed within a casing). The design incorporated a variety of technologies to reduce noise and drag, and long-span wings to improve fuel efficiency.

At first glance, the HWB looks much like a flying wing in that there is no 

clear dividing line between the wings and the main body of the aircraft. But 

upon closer examination it becomes clear that though the wings are smoothly 

blended into the body, the airframe is composed of distinct wing and body 

structures. Unlike a flying wing, which has no distinct fuselage, the HWB 

features a central crew/passenger/cargo section that is clearly more body than 

airfoil. With a relatively wide center-body chord and a narrow wing chord, the 

HWB has been described as resembling a manta ray.

The Boeing team realized that the HWB planform offered significant 

improvements in lift-to-drag (L/D) ratio versus a tubular fuselage—because 

lift is distributed over a broader area—as well as opportunities to take advantage of such enabling technologies as lightweight, damage-arresting composite structures, laminar-flow-control techniques, acoustic shielding, low-speed 

flight controls, and high-efficiency engines. 

The Boeing team’s advanced vehicle concept PSC featured pylon-mounted 

engines positioned between the tails and forward of the trailing edge. Boeing 

ERA program manager John Bonet explained that this decision was based 

solely on a desire to minimize the airplane’s acoustic footprint. Earlier design 

iterations featured engines embedded within the trailing edge, which reduced 

drag but not noise because there was nothing to shield the exhaust. “If it were 

only [a matter of] aerodynamic efficiency, we would have them hanging off 

the back, but you wouldn’t have any shielding of the engines and the [HWB] 

would be just as loud as a tube-and-wing airplane,” Bonet said.29

Boeing engineers assumed a 14 percent fuel-burn benefit based on detailed 

analysis of operating in NextGen airspace. Choice of engine was also important, which is why the team considered two powerplant options for the HWB. 

The first configuration, featuring twin high-bypass-ratio GTF engines, was 

projected to achieve a 52 percent reduction in fuel burn, beating the nominal 

29. John Bonet interview with Juan Alonso, NASA 360, S03E22, NASA podcast, December 5, 2011, 

transcript available at http://www.nasa.gov/multimedia/podcasting/nasa360/nasa360-0322.html, 

(accessed September 3, 2016).
The ERA Advanced Vehicle Concepts Study

57

target, but it fell short of NASA’s noise goal by realizing only a 34-decibel 

reduction. The second and more unconventional configuration was powered 

by three General Electric/CFM Leap-X-based propfans. This arrangement 

afforded even lower fuel consumption through better engine performance and 

reduced emissions, but the open-rotor was 8 decibels noisier than the GTFpowered version.30

Minimizing engine noise revealed a host of other acoustic sources that 

would have to be dealt with, but also afforded the opportunity to meet the 

–42-decibel goal through reduction of airframe noise. “We noticed that jet 

noise is so low on the HWB that other sources become dominant,” said Bonet, 

noting that advanced landing gear and slat-noise reduction could make up the 

difference. “Airframe noise reductions are the only ones that will allow us to 

meet the noise goals.”31

Boeing proposed building a 65-percent-scale version of the team’s preferred concept. The 83-foot-long, 149-foot-span STV was to be powered by 

twin 24,000-pound-thrust Pratt & Whitney PW1000G GTF engines. The 

HWB airframe configuration necessitated a modular, stitched resin-infused 

composite structure, but cost reduction could be realized through the use of 

commercial off-the-shelf (COTS) landing gear and a modified business-jet 

flight deck with modular electronics. The wings would not initially be equipped 

for drag-reducing laminar flow, though it would be possible to introduce the 

feature later on.32

Subscale HWB Testbed

Although not part of the AVC study, the Boeing/Cranfield team leveraged 

lessons learned from their HWB design efforts by flight-testing the subscale 

remotely piloted X-48B/C research vehicle. The demonstrator performed 92 

flights in the X-48B configuration during Phase 1 and an additional 30 flights 

between August 2012 and April 2013, after being modified to the X-48C 

configuration.

This second phase of testing directly supported the ERA project by demonstrating a noise-shielding configuration. “We have accomplished our goal 

of establishing a ground to flight database and proving the low speed controllability of the concept throughout the flight envelope,” Collier proclaimed 

at the conclusion of X-48C flight-testing, adding, “The hybrid wing body 

30. Norris, “Future-Airliner Concept Contenders Reveal Design Surprises.”

31. Ibid.

32. Ibid.
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58

has shown promise for meeting all of NASA’s environmental goals for future 

aircraft designs.”33

Flight-testing at NASA AFRC of the subscale Boeing-Cranfield X-48C Remotely Piloted Aircraft 

demonstrated the low-speed handling characteristics of a proposed HWB transport. (NASA)

The Study Teams’ Final Reports

When Lockheed Martin, Boeing, and Northrop Grumman submitted their 

final reports in early January 2012, each team offered a preferred system concept that either met or closely matched NASA’s stringent noise, emissions, and 

fuel-burn targets for airliners entering service in the late 2020s. Each team 

was also asked to sketch out a 15-year technology maturation road map and 

propose critical technology demonstrations for the second half of the ERA 

program. NASA officials had expected each concept to score high marks, but 

the teams presented a surprisingly wide array of unanticipated technologies and 

innovations. In addition to the unconventional flying-wing and HWB designs 

from Northrop Grumman and Boeing, and an innovative Rolls-Royce engine 

33. Gray Creech, “X-48 Research: All good things must come to an end,” http://www.nasa.gov/topics/

aeronautics/features/X-48_research_ends.html, April 17, 2013, (accessed September 5, 2016).
The ERA Advanced Vehicle Concepts Study

59

powering Lockheed Martin’s joined wing concept, the AVC studies unexpectedly highlighted significant benefits that would result from flying advanced 

airliners within the FAA’s NextGen airspace system. Fay Collier noted, “There 

were a number of things I hadn’t anticipated, and one was the benefit of 

[NextGen improvements to the] national airspace system; that’s a low-hanging 

fruit, maybe, and confirms a number that came out of our colleagues in the 

SFW program.”34

To maximize the STV’s value as a research platform, NASA asked the AVC 

study teams for designs with a 10,000-hour, 20-year service life. Additionally, 

following the ERA program, the optionally manned STV was to be flown 

autonomously as part of a research effort to integrate unmanned aircraft systems into the national airspace system in 2020–2025. The STV might then 

finish its career as a testbed for ERA’s sister effort, the SFW fundamental 

research program, from 2025–2030.35

By the time the AVC studies were completed, however, according to 

Mangelsdorf and Rich Wahls, NASA was facing a looming budget squeeze that 

forced the Agency to reconsider its ambitions for funding a flying, full-scale 

STV demonstrator in the near term.36 “It quickly became apparent that ERA 

was not going to have a budget profile to do a half-scale X-plane,” Wahls said.37

Ultimately, the AVC study only represented about 5 percent of the total 

ERA Phase 1 investment, but successes during this phase provided ARMD 

decision makers with the necessary confidence to proceed through the next 

key decision point and gain authority to undertake Phase 2. Results of the 

AVC contracts were then used as a foundation upon which to build the case 

for an eventual X-plane program sometime after the conclusion of the ERA 

project. In fact, the project management tools and rigor established in ERA 

and subsequent projects, along with information first generated in the AVC 

studies, were crucial to building ARMD confidence in such X-Plane programs 

as the low sonic boom flight demonstrator. 

NASA Reaches Key Decision Point

Toward the end of ERA Phase 1, as the key decision point approached, NASA 

established a Tiger Team for down-selecting key technologies to be explored 

in greater detail during the second phase. Substantial progress on the broader 

aspects of Phase 1 informed the development of technology road maps and 

34. Norris, “Future-Airliner Concept Contenders Reveal Design Surprises.”

35. Norris, “Future-Airliner Concept Contenders Reveal Design Surprises.”

36. Mangelsdorf interview.

37. Wahls interview.
Green Light for Green Flight

60

priority targets for Phase 2, which were shaped, in part, by the results of the 

vehicle concept studies.38 The resulting plan for the remainder of the program 

focused on eight integrated technology demonstrations (ITDs) that were completed by NASA researchers and industry partners overseen by project managers (named in parentheses) at LaRC, AFRC, and GRC. These ITDs included:

• Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS):

Assessment of a low-weight, damage-tolerant, stitched composite 

structural concept for stitching together large sections of lightweight 

composite materials that could be used in uniquely shaped future aircraft that weighed up to 20 percent less than a similar all-metal aircraft. (LaRC: Dawn Jegley)

• Adaptive Compliant Trailing Edge (ACTE): A radical new morphing wing technology that allows an aircraft to seamlessly extend its 

flaps, leaving no drag-inducing, noise-enhancing gaps for air to flow 

through. (AFRC: Tom Rigney)

• Flap and Landing Gear Noise Reduction Flight Experiment:

Analysis, wind-tunnel and flight tests to design quieter flaps and landing gear without performance or weight penalties and develop new 

design tools to aid engineers in reducing noise from deployed wing 

flaps and landing gear during takeoff and landing. (LaRC: Mehdi 

Khorrami)

• Highly Loaded Front Block Compressor Demonstration: Tests 

to show ultra-high-bypass (UHB) or advanced turbofan efficiency 

improvements of a two-stage, transonic high-pressure engine compressor, and refine the design of a General Electric open rotor compressor stage of a turbine engine to improve its aerodynamic efficiency. 

(GRC: Ken Suder)

• 2nd Generation UHB-Ratio Propulsor Integration: Collaboration 

with Pratt & Whitney on the company’s geared turbofan jet engine to 

improve propulsion efficiency and reduce noise. (GRC: Ken Suder)

• Low NOx Fuel Flexible Engine Combustor Integration: Testing an 

improved design for a jet engine combustor to reduce NOx emissions. 

(GRC: Ken Suder)

• UHB Engine Integration for a Hybrid Wing-Body: Computer 

modeling and wind-tunnel verification of HWB airframe/powerplant 

integration concepts to reduce noise and fuel consumption. (LaRC: 

Greg Gatlin)

38. Norris, “Future-Airliner Concept Contenders Reveal Design Surprises.”
The ERA Advanced Vehicle Concepts Study

61

• Boeing ecoDemonstrator 757: Use of a full-scale flying laboratory to 

demonstrate an active flow control (AFC) enhanced vertical tail flight 

experiment that could enable future aircraft to fly with smaller tails, 

thus reducing weight and drag, and an Insect Accretion Mitigation 

(IAM) to test wing surface coatings designed to minimize drag 

caused by bug residue building up on the leading edge. (LaRC: Mike 

Alexander)

Environmentally Responsible Aviation

Integrated Technology Demonstrators Partner

AFC Enabled Vertical Tail and Advanced Wing Flight Experiment Boeing

Damage Arresting Composites Demonstration Boeing

Adaptive Compliant Trailing Edge Flight Test AFRL/FlexSys

Highly Loaded Front Block Compressor Demonstration General Electric

2nd Generation UHB Propulsor Integration Pratt & Whitney

Fuel Flexible, Low NOx Combustor Integration Pratt & Whitney

Landing Gear and Flap Edge Noise Reduction Flight Test Gulfstream

UHB Integration on Hybrid Wing-Body Aircraft Boeing

Work during ERA Phase 2 focused on eight integrated technology demonstrations undertaken 

with several industry partners, and with the Air Force Research Laboratory (AFRL).

The challenge the ERA faced with these eight ITDs was simple but formidable: winnow down the number of possible ITDs and then build manageable work efforts to achieve them within the remaining 3 years. Such schedule 

consciousness may have shocked those accustomed to working on programs 

with flexible end dates, but was, in fact, a rediscovery of the time-and-schedule 

consciousness the Agency had earlier displayed, in the heyday of the Space 

Race in the 1960s.
62

Researchers intentionally damaged a critical portion of a PRSEUS composite panel to observe 

whether the damage progressed under stress. Only after being subjected to stresses well 

beyond those expected during flight did it finally fail, resulting in the tear seen here. (NASA)
63

CHAPTER 3

Pursuing Damage-Tolerant 

Composite Structures

The Phase 2 ITDs aggressively pushed the technological state of the art. Thus, 

although innovative airframe concepts like the HWB offered characteristics 

that might dramatically reduce fuel consumption, they also presented significant design challenges. For example, constructing a non-circular pressure 

vessel capable of meeting the necessary reduced structural-weight requirements 

required a novel approach. Researchers at NASA and Boeing teamed up to 

advance a new structural concept called Pultruded Rod Stitched Efficient 

Unitized Structure (PRSEUS) for stitching together large sections of damagetolerant, lightweight composite materials that could be used to build uniquely 

shaped future aircraft weighing as much as 20 percent less than similarly sized 

all-metal airframes. During ERA Phase 2, researchers assessed structural test 

articles assembled from integrally stiffened PRSEUS panels designed to maintain residual load-carrying capabilities under a variety of damage scenarios.1

NASA-led research and development of advanced composite structures 

began during the 1970s in response to rising fuel costs and perceived requirements for more energy-efficient commercial transports. Launched at LaRC in 

1976, the Aircraft Energy Efficiency (ACEE) program sought to dramatically 

reduce airline fuel consumption through improved aerodynamic efficiency and 

lighter structures as well as development of improved engines. Composites 

research became a centerpiece of the ACEE program, with the primary goal 

of accelerating the application of composite primary structures in future civil 

air transport aircraft designs. Although this goal was never achieved by the 

time ACEE ended in 1985, contracts with Boeing, McDonnell Douglas, and 

Lockheed provided the aircraft industry with important new technology. 

Without the impetus of a NASA technology program, industry players lacked 

confidence to proceed with such a high-risk investment as using composites 

1. Dawn C. Jegley and Alexander Velicki, “Development of the PRSEUS Multi-Bay Pressure Box for a 

Hybrid Wig Body Vehicle,” AIAA-2015-1871, presented at 56th AIAA/ASCE/AHS/ASC Structures, 

Structural Dynamics, and Materials Conference, Kissimmee, FL, January 8, 2015, 1.
Green Light for Green Flight

64

for primary structural components. There was not yet sufficient evidence that 

composite structures could be produced more economically than aluminum 

assemblies, or that conventional laminated composites could withstand the 

rigors of routine flight operations with minimal damage.2

New manufacturing techniques were necessary to overcome these hurdles, 

and so researchers turned to methods similar to those used by the textiles 

industry. Advanced composite assemblies would need to be made using 

woven, knitted, braided, or stitched bundles of carbon filaments (called 

tows) to make dry preforms. These would then be subjected to resin transfer 

molding, or resin film infusion (RFI), to produce a composite with throughthe-thickness reinforcement. As with conventional composites, the epoxy 

resin was cured using heat and pressure inside an autoclave. The process could 

be facilitated by using sheets of material that have been pre-impregnated 

(“pre-preg”) with epoxy resin that could be stored in bulk in a cool area until 

use. Additionally, in order to reduce labor-intensive operations, production 

methods would need to be automated to the greatest extent practical. After 

examining several potential methods for manufacturing preforms, NASA 

researchers settled on a method involving stitching. Compared to other processes (such as weaves, knits, and braids), stitching offered the greatest potential for cost-effective manufacturing of damage-tolerant structures. In fact, 

several military programs already employed stitched carbon/epoxy pre-preg 

with Kevlar thread to enhance the structural integrity and damage tolerance 

of thin composite panels. Unfortunately, in the mid-1980s, these methods 

were of limited use for stitching the thick pre-pregs that would be required 

for use in large wing structures.3

NASA researchers at LaRC explored the potential of several textile processes 

for use in cost-effective production of damage-tolerant structures. Despite 

known deficiencies in shear stiffness (resistance to deformation in response to 

lateral strain), biaxial woven and knitted fabrics were considered simply because 

they were readily available from commercial sources. A tri-axial weave would 

have been better, but at the time, such a thing was not commercially available. 

In fact, the textile industry did not see a sufficient market for woven carbon fabrics to make production a worthwhile investment. Instead, researchers began 

looking at a process to manufacture tri-axial warp-knit fabric. This technique 

combined warp knitting with stitching for through-the-thickness reinforcement to create useful preforms. Testing at LaRC provided data for identifying 

2. Marvin B. Dow and H. Benson Dexter, “Development of Stitched, Braided and Woven Composite 

Structures in the ACT Program and at Langley Research Center (1985 to 1997),” NASA TP-97-

206234, November 1997, 1, 1–8.

3. Ibid., 1–2.
Pursuing Damage-Tolerant Composite Structures

65

knitted preforms with the best combination of strength and damage tolerance. 

McDonnell Douglas later used these data in selecting a warp-knit fabric that 

could be used in the fabrication of a composite wing structure. NASA researchers also looked at braiding carbon tows to create a multi-axial preform that 

would be useful for producing damage-tolerant composite laminates. They 

discovered, however, that the same reinforcement feature of the braids that contributed to low damage-tolerance also reduced the laminate’s in-plane strength. 

Moreover, existing commercial braiding machines could not economically produce large-area preforms. Lockheed Martin and Northrop Grumman conducted in-house research demonstrating the usefulness of braided composites 

for smaller structures including window belts, curved fuselage frames, and 

wing stiffeners, where flexibility and damage tolerance are essential. Similarly, 

McDonnell Douglas adopted braided preforms for blade-section stiffeners on 

wing covers.4

 The greatest promise, however, lay in stitching.

Advanced Composite Technology

In 1987, NASA issued an NRA seeking proposals for innovative approaches 

to cost-effective composite fabrication, enhanced damage tolerance, and 

improved analysis methods. The response from industry and academia 

included 48 proposals, of which 15 were accepted for contract awards. The 

following year, these contracts became the basis for the Advanced Composite 

Technology (ACT) program, which focused on developing composite primary 

structure for fuselage and wing assemblies, and provided impetus for a rapid 

transition of this technology to industry. ACT program managers specified a 

goal of reducing the structural weight of a future commercial transport aircraft 

by 30 to 50 percent while also reducing manufacturing costs by as much as 

25 percent. The resulting primary wing and fuselage structures had to behave 

(structurally) in a predictable manner, meet FAA requirements for certification 

(including with regard to damage tolerance), and be repairable in a manner 

acceptable to the airlines. Another important objective of the ACT program 

was development of an integrated, affordable composites technology database 

to foster a rapid and timely transition of this technology into production 

airframes. Administrative management for the ACT program was assigned 

to the Structures Technology Program Office at LaRC, and each company 

that received a contract had its own focus. Boeing concentrated on lowcost, automated fabrication techniques. Several other contractors investigated 

new RTM materials and processes. Lockheed Martin, Northrop Grumman, 

4. Ibid., 2–3.
Green Light for Green Flight

66

and McDonnell Douglas worked on design and fabrication of composite 

aircraft structures.5

McDonnell Douglas had been investigating a revolutionary process that 

involved stitched dry carbon fabric preforms and reinforced composite laminates. Best of all, the resulting assemblies demonstrated outstanding damage 

tolerance, acceptable fatigue performance, and good strength properties. 

The company employed an integrated approach that balanced compromises 

between design and manufacturing in order to simplify fabrication tools, lessen 

thermal distortion, improve accuracy during assembly, and prevent separation 

due to out-of-plane loads.6

To showcase the process, McDonnell Douglas demonstrated a buildingblock approach to assembling a wing stub box with a 12-foot span and 8-foot 

chord. The stub box featured stitched upper and lower covers including skin, 

blade-stiffeners, spar caps, and intercostals (the supporting structures between 

load-bearing members) as integral structures. Technicians at the McDonnell 

Douglas plant in Long Beach, California, fabricated large tension and compression panels, which later underwent testing at LaRC. Evaluation of various 

epoxy resins led to the conclusion that a formula called Hercules 3501-6 had 

the best properties and cost advantages. Fabrication of the stub box successfully 

demonstrated a full-scale stitched/RFI process for assembling an integral wing 

cover incorporating heavy spar caps, intercostals, and stiffeners with runouts. 

The build team also took advantage of lessons learned during process development. Replacing uni-weave fabric, for example, with multi-axial warp-knit 

fabric preforms eliminated many handling and lay up problems.7

In July 1995, the fully assembled wing stub box arrived at LaRC, where 

it was bolted to a massive steel backstop in the Materials Laboratory for a 

series of static loading tests. These included tests building up to the design 

limit load (DLL), the maximum load factor authorized during operational service. Researchers also gauged the results against the calculated design ultimate 

load (DUL), the point at which catastrophic failure was expected to occur. 

Objectives included demonstrating that stitched panels could meet stringent 

FAA damage and repair criteria, and particularly that damaged composite 

panels could be restored to design ultimate strength. Prior to the DLL test, 

technicians purposely inflicted visible damage at a critical location. To promote realism, all repairs were made by aircraft maintenance technicians from 

American Airlines using mechanically fastened plates. This work was completed before subjecting the test article to the DUL test, during which the stub 

5. Ibid., 4.

6. Ibid., 6.

7. Ibid., 6.
Pursuing Damage-Tolerant Composite Structures

67

box failed at a load equivalent to 143 percent DLL. Notably, failure occurred 

close to the test-fixture mounting point, a metal assembly located some distance 

from the repair site.8

The McDonnell Douglass-fabricated composite wing stub box undergoing loads testing at LaRC 

in 1995. McDonnell Douglas subsequently merged with Boeing in August 1997. (NASA)

By this time, work was already under way to design, build, and test a 42-foot 

semi-span composite wing. Researchers from both NASA and McDonnell 

Douglas considered this a necessary step in the building-block approach to 

fabricating and testing a full span stitched composite wing-and-center-box 

assembly of the type that could be used on a 220-seat, single-aisle aircraft. 

In terms of size and complexity, the semi-span wing test article represented a 

major step forward. Whereas the stub box was a simple structure with flat cover 

panels, the wing would require aerodynamically contoured covers, as well as 

simulated control surfaces, engine pylon attachment, and landing gear fairings. 

Meeting the challenge of these requirements called for advanced tooling concepts and new computer modeling techniques.9

 Boeing assumed responsibility 

for these efforts following the company’s merger with McDonnell Douglas in 

August 1997.

One objective of the semi-span wing development was to demonstrate technology readiness through processing, scale-up, and structural testing. Researchers 

8. Ibid., 6.

9. Ibid., 6.
Green Light for Green Flight

68

used the results to develop and verify techniques to be used in the design, 

manufacture, and testing of a follow-on full-scale aircraft wing. First, designers 

established specifications for a representative composite wing box structure as 

part of efforts to develop detail design features along with the associated analytical and manufacturing techniques. This wing box was derived from an aircraft 

concept representative of a next-generation twin-engine, 220-passenger commercial aircraft equipped with a supercritical airfoil wing with an aspect ratio 

of 12:1 that was optimized using composite material properties.10

For test purposes, the semi-span box represented only the first 42 feet of 

the wing starting from the aircraft side-of-body splice outward toward the 

wingtip. It consisted of an upper and lower stitched/RFI cover, two spars, 

and 18 ribs spaced approximately 30 inches apart. Composite stringers were 

spaced 7.6 inches apart, compared to 6.5-inch spacing for stringers used on 

typical aluminum wing panels. Taking advantage of the stitching technology, 

many components were integrated into the cover panels. This reduced the 

assembly part count and eliminated thousands of fasteners and their associated 

costs and weight penalties. Each cover panel consisted of multiple stacks of 

uni-axial warp-knit carbon fiber material stitched together to form the wing 

skin. Additional structural details stitched into the skin panel included blade 

stiffeners to give added structural stability and interleaved spar caps and intercostal clips for attaching various substructural components. This substructure 

consisted of ribs and bulkheads made of conventional tape lay up carbon 

composite pre-preg and spar webs of stitched/RFI multi-axial warp knit material. Stiffening elements were fabricated by bonding pre-cured stiffeners to 

pre-cured flat webs. To complete the box assembly, mechanical fasteners were 

used to attach the cover panels to the substructure. Once completed, the dry 

preform assembly was placed into rigid tooling and infused with resin in an 

autoclave.11

Boeing shipped the completed McDonnell Douglas–legacy wing box to 

LaRC, where engineers mounted it to a laboratory wall for testing. Researchers 

introduced loads using hydraulic jacks to simulate representative aircraft design 

requirements. Although the semi-span test article did not fully represent an 

optimized wing box design, it included many important design features that 

emerged as potential solutions to issues that needed addressing in the design 

of a stitched/RFI composite wing box for commercial aircraft applications.12

Following these tests, Boeing continued efforts to develop cost-effective 

composite manufacturing processes in support of the NASA Airframe 

10. Ibid., 24.

11. Ibid., 25–26.

12. Ibid., 25–26.
Pursuing Damage-Tolerant Composite Structures

69

Materials and Structures element of the Advanced Subsonic Technology 

(AST) program. Earlier ACT research results showed great promise for reducing both manufacturing-cost and damage-tolerance barriers to the application 

of stitched/RFI materials in primary structures for commercial transports. 

Replacing thousands of mechanical fasteners with a highly automated stitching process had the potential to significantly reduce manufacturing costs of 

composite structures while simultaneously reducing stress-induced damage 

and airframe weight. Such advances in composite structure fabrication represented a significant advantage over metallic designs. Toughened-resin systems 

used during ACT efforts in the 1980s showed promise for improving the 

damage tolerance of carbon fiber composites, but high costs offset the benefits. 

Development of through-the-thickness stitching of dry preforms provided a 

more affordable alternative.13

McDonnell Douglas researchers focused their early composite wing structure research toward 

application to future derivatives of the firm’s MD-90 series of commercial transports; here is a 

Delta Airlines Boeing 717 (N978AT) originally ordered by Air Tran as an MD-95. (R. P. Hallion)

Goals of the AST program included making composite wing structures 25 

percent lighter than current aluminum wing designs, reducing fabrication costs 

by 20 percent, and reducing airline operating costs by approximately 4 percent. Preliminary design studies by Boeing under LaRC contract NAS1-20546 

showed that these goals were achievable. Using a wing-torque-box design 

13. Ibid., 1.
Green Light for Green Flight

70

applicable to an MD-90-40X commercial passenger transport, the company 

conducted a weight-trade study that verified the weight savings a stitched/RFI 

structure would offer compared to an identical wing box design built from aluminum alloys. Under the AST Composite Wing program, Boeing planned to 

build and test a full-scale, full-span, wing-box/fuselage section to demonstrate 

the maturity of stitched/RFI technology, but due to program scope reductions 

the full-scale structural test article was never built.14

Among the most significant results of the ACT program were development of automated stitching equipment for fabricating an integral wing skin 

and stiffener concept and improved understanding of the structural mechanics of stitched composites, damage containment, and failure effects. NASA 

awarded Boeing a contract to develop a high-speed, multi-needle Advanced 

Stitching Machine (ASM) capable of stitching entire wing covers for large 

commercial transport aircraft. The Ingersoll Milling Machine Company of 

Rockford, Illinois, was selected to design and build the ASM under subcontract 

to Boeing. Pathe Technologies, Inc., of Irvington, New Jersey, designed and 

built the ASM’s advanced stitching heads. In a cost-sharing effort, NASA spent 

$10 million on development of the ASM and Boeing paid for renovations at 

the company’s Marvin B. Dow Stitched Composites Center in Huntington 

Beach, California, which underwent extensive modification to accommodate 

the ASM.15

Equipped with four stitching heads, the ASM combined high speed with 

advanced automation, allowing manufacturers to assemble large complex wing 

structures without manual intervention. The ASM was capable of stitching 

single-piece aircraft wing cover panels 40 feet long, 8 feet wide, and 1.5-inches 

thick, at a rate of 3,200 stitches per minute. The stitching heads were capable 

of making eight stitches per inch with precision row spacing of just 0.2 inches. 

Achieving this rate required development of an automated thread gripper and 

cutting mechanism and a pivoting needle mechanism, as well as a cooling 

system to prevent excessive needle temperature buildup and bending. Prior to 

stitching a wing panel, a laser projection system precisely located the dry fabric 

wing skin preforms and any secondary materials, such as stiffeners. Computer 

controls directed and confirmed the stitching pattern and allowed for 38 axes 

of motion. Automated controls then synchronized the movements of the stitching heads with each of 50 lift tables necessary to control stitching over the 

contoured shapes of the airfoil. Researchers demonstrated that the ASM was 

14. Ibid. 1–2.

15. Darryl R. Tenney, John G. Davis Jr., R. Byrin Pipes, and Norman Johnson, “NASA Composite 

Materials Development: Lessons Learned and Future Challenges,” presented at NATO RTO AVT164 Workshop on Support of Composite Systems, Bonn, Germany, October 19, 2009, 19.
Pursuing Damage-Tolerant Composite Structures

71

capable of stitching wing 

cover panels in a single twoshift operation that saved 

days over conventional 

composite manufacturing 

processes. Moreover, subsequent cost analyses indicated that a reduction of 20 

percent could be achieved 

over equivalent wings built 

from aluminum alloys, and 

the desired weight savings 

goal was achieved as well.16

The automated NASA-Boeing-Ingersoll Advanced 

Stitching Machine made it possible to assemble large 

complex wing structures quickly and without manual 

intervention. (NASA)

In order to support 

future FAA certification of 

composite wing structures 

for commercial transports, 

ACT researchers had to 

develop new test procedures and analyses databases. Numerous material samples, sub-elements, and elements underwent extensive testing prior to trials 

involving larger test articles. Researchers performed pre- and post-test analyses 

on each item and test article in order to understand failure modes and validate 

analytical methods. These efforts culminated in testing of the 41-foot-long 

stitched/RFI composite semi-span wing assembly in 2000. Over the course of 

eight trials in the LaRC Structures and Materials Laboratory, this structural 

test article was subjected to multiple specified load conditions under simulated positive and negative g-loading to incrementally build up to and exceed 

the DLL.17

Researchers installed a total of 466 strain gages on the edge of critical access 

holes at the midplane, but not on the cover panel surface. Additional gages 

were placed on the skin and stringer-blade surfaces. Resulting data indicated 

that local nonlinear deformations occurred in the upper cover panel in an 

unsupported region behind the rear spar. High strain levels were also detected 

at access holes on the lower cover panel. One surprise was that larger local 

displacements and strains occurred during the test than had been predicted by 

nonlinear finite element model (FEM) analysis. Post-test analyses suggested 

that further refinements to the FEM might provide a better agreement between 

analytical results and test data. Otherwise, experimental and analytical results 

16. Ibid., 19–20.

17. Ibid., 20.
Green Light for Green Flight

72

were in generally good agreement. This further validated the importance of a 

building-block approach to developing and understanding the behavior and 

failure modes of composite structures.18

After successfully completing the first six tests, researchers inflicted discrete 

source damage on the upper and lower cover panels of the wing by making 

7-inch-long saw cuts to both the upper and lower cover panels. Each cut ran 

through two stinger bays and cut through a stringer. The airfoil was then loaded 

to 70-percent DLL in a 2.5g up-bending condition and unloaded (relaxed). 

Once the airfoil was relaxed, technicians repaired the damaged area to restore 

the wing to full load-carrying capability. Repairs consisted of a metal plate that 

conformed to the wing contours on the outer surface of the cover panels, and 

internally spliced stringers. All parts of the repair assembly were attached to 

the wing using conventional mechanical fasteners. Researchers then inflicted 

six impacts on the test article. First, a 25-pound, 1.0-inch-diameter falling 

weight was dropped three times from a height of 4 feet, resulting in barely 

visible damage to the upper cover panel. The depth of the resulting damage 

ranged from 0.01 to 0.05 inches. Next, an air-propelled steel projectile was 

used to inflict three impacts to the lower cover panel with an energy level of 

83–84 foot-pounds. The 0.5-inch-diameter steel sphere was accelerated to a 

speed of approximately 545 feet per second, resulting in clearly visible damage 

with indent depths up to 0.135 inches. The wing was then loaded to failure 

in a 2.5g up-bending load condition. Ultimately, the test article withstood 97 

percent of the DUL prior to failing through a lower-cover access hole, which 

resulted in the loss of the entire lower cover panel.19

These results were quite good, and the research team came away with many 

valuable lessons. Among these was that the building-block approach based 

on tests and analyses of materials and components that make up the structure imparted significant risk reduction, as well as providing important data 

and analyses to support the FAA certification process. Researchers noted that 

composite structures tended to fail in quasi-brittle mode, and that out-ofplane loads (often ignored when manufacturing metal structures) must be 

considered. Applying loads to areas with perforations such as fastener holes 

and access openings, stiffener run-out, and sites of discrete damage (even when 

barely visible) have potential for delamination and failure. The ACT program 

identified issues in design, analyses, fabrication, and testing of built-up structure that formed the basis for identifying important thrusts for composite 

18. Ibid., 21.

19. Ibid., 20–21.
Pursuing Damage-Tolerant Composite Structures

73

fabrication techniques and provided insight into the potential payoff of new 

technology development.20

Researchers intentionally damaged a critical portion of a PRSEUS composite panel to observe 

whether the damage progressed under stress. Only after being subjected to stresses well 

beyond those expected during flight did it finally fail, resulting in the tear seen here. (NASA)

Perhaps surprisingly, many years passed following completion of the ACT 

program before the first stitched-composite production part flew on an airplane. In 2003, Boeing added a composite fairing to the aft fuselage of its 

C-17 Globemaster III cargo transport, but it experienced only light loading, and did little to demonstrate the structural advantages of stitching. It 

was, however, an important step in establishing the manufacturing benefits of 

stitched/RFI technology. It was not until 2007 that more innovative one-piece 

multi–rib-stiffened box structures were produced in the form of new landing 

gear doors for the C-17. The complex preforms were stitched together, infused 

with resin, and cured at atmospheric pressures in an oven. To suppress outof-plane de-laminations that were common to the bonded production doors 

they replaced, all the rib caps and perimeter lands on the new door assemblies 

were reinforced with through-the-thickness stitching. This allowed operation 

20. Ibid., 21–22.
Green Light for Green Flight

74

of the doors further into the post-buckled regime than was possible with the 

earlier bonded design.21

PRSEUS

Following the end of the ACT program, Boeing continued to work with stitched 

composites in conjunction with the Air Force Research Laboratory (AFRL) 

in Dayton, Ohio. The most promising result was a highly integrated stitched 

concept in which an arrangement of dry, warp-knit fabric preforms, pultruded 

rods, and foam core materials are assembled and then stitched together to create 

an optimal structural geometry for fuselage loading.22 The PRSEUS concept 

eventually became a major component of ERA Phase 2 research.

Invented in the 1950s by W. Brandt Goldsworthy, a plastics engineer at 

Douglas often credited as the “father of composites,” pultrusion (for “pull” and 

“extrusion”) is a process whereby dry, continuous fibers are pulled through a 

resin bath and then through a heated die that cures the resin to set the fiber 

bundle into its final shape. “Goldsworthy’s invention of the pultrusion process 

in the 1950s,” historian Stephen Trimble has written, “would make durable 

and high-strength composites affordable for a range of applications, from cars 

to aircraft parts to fishing rods.”23

For their part, Boeing researchers discovered that adding pultruded rods to 

the top of each stiffener in a stitched composite assembly allowed the components to be stronger in bending and more structurally efficient, enabling use 

of lighter-weight structures than would normally be required.24 This was of 

great interest to NASA because it would help achieve ERA project goals with 

regard to reducing overall aircraft structural weight.

Early testing began with small samples (called “coupons” in materialstesting parlance) that were developed to a point where researchers were confident about moving on to larger scale test articles. By the time NASA got 

involved with PRSEUS, Boeing and AFRL had significantly advanced the art 

of composite fabrication. Previously, resin-infused materials had to be cured 

using hard metal tooling and the high pressures and temperatures that could 

be achieved only with an autoclave. According to Dawn C. Jegley, a senior 

aerospace engineer in the LaRC Structural Mechanics and Concepts Branch, 

21. Ibid., 22.

22. Ibid., 23.

23. Stephen Trimble, “Evolving the Modern Composite Airplane,” in Richard P. Hallion (ed.), NASA’s 

Contributions to Aeronautics, v.2: Flight Environment, Operations, Flight Testing and Research, 

NASA SP-2010-570-Vol 2 (Washington, DC: NASA, 2010), 379.

24. Dawn C. Jegley, interview with the author, July 28, 2016.
Pursuing Damage-Tolerant Composite Structures

75

Boeing devised a method that eliminated the need for an autoclave altogether; 

composite lay ups could be cured with just an oven and vacuum pressure. 

“That was really helpful as we began making larger and larger parts,” she said, 

“because we no longer needed to worry about whether we had an autoclave 

available and we were no longer limited based on the size of the autoclave.” It 

also helped reduce some of the uncertainties inherent in the process. When 

ready for use, pre-preg sheets were removed from the freezer and thawed. At 

that point, there was only a limited amount of time—usually no more than 30 

days—before the lay-up had to go into the autoclave before the epoxy resin set 

up. “If something goes wrong during 

that phase where you’re laying up all 

the pieces, you risk having [the process] go past that 30 days,” Jegley said. 

“If your autoclave breaks down when 

you’re partway through laying up the 

part, then you’re going to have parts 

backing up on the production line 

and your pre-preg is going to go bad 

before you have a chance to cureit.”25

Dawn C. Jegley, senior aerospace engineer in 

the LaRC Structural Mechanics and Concepts 

Branch. (NASA)

Another advantage of PRSEUS 

was the elimination of conventional 

fasteners (rivets, screws, bolts, etc.) 

and a reduction in the number of 

parts needed for each assembly. 

“Instead of using fasteners,” Jegley 

said, “you just stitch the whole thing 

together and then you don’t have to 

drill holes; you don’t have to keep 

track of all those fasteners.” In traditional metal aircraft assemblies, 

drilled holes and fastener-stress can 

cause imperfections that later result in cracking or other damage. All drill holes 

have to be inspected repeatedly throughout the airplane’s service life, a timeconsuming and costly process. A reduction in the required number of metal 

fasteners promotes structural integrity while reducing inspection costs and, not 

incidentally, aircraft weight. It does, however, have an impact on disassembly 

and access to internal spaces. “It becomes much more difficult to disassemble 

the parts,” Jegley noted. “That’s where there is a tradeoff and why you wouldn’t 

want to stitch the whole airplane together because you do need to be able to 

25. Ibid.
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get inside [for maintenance], but at the same time it allows you to build some 

very large assemblies using a smaller total number of parts.”26

A stitched composite wing assembly, for example could be fabricated from 

root to tip using single-piece cover panels with integral stiffeners. Similarly, a 

fuselage or HWB center-body could be constructed from just behind the cockpit to just forward of the tail in one piece with all stiffeners in both directions 

built in. Instead of being assembled in cylindrical barrel sections, the lower half 

could be fabricated first and packed with all of the necessary hydraulics, electrical systems, and other equipment, and then the floor stitched in place while 

the top is still open. Eventually, the upper half would be installed. Systems 

designers see this as an advantage because not only does this eliminate all the 

joints from one barrel to the next, but it also eliminates the need for joints 

between the different hydraulic components and electrical components from 

one barrel section to the next. “So you have integrated all of it, and you have 

easy access [to equipment spaces] before you put the floor in,” Jegley said. It 

also allows for larger single-piece subassemblies. “That way, when you get to 

final assembly, you’re now bringing together very few pieces; when you put 

them all together, you still have real joints and metal fittings and fasteners, but 

you’re bringing together a much smaller number of parts.”27

PRSEUS technology also made it possible to get away from pre-preg by 

instead using dry warp-knit fabric stitched together with Kevlar or Vectran, 

and then curing it later in an oven. The greatest advantage of using dry fabric 

materials was being able to store them almost indefinitely at room temperature. Then, according to Jegley, “You can just push everything off to the side 

and wait to put it into the oven.” This technique came in handy while making 

some of the parts for a large test article. “Because of timing, we were making 

up panels and then stacking them off to the side in the lab at Boeing while we 

waited for the oven to become available so we could do all the curing,” said 

Jegley. “With pre-preg, you could never do that; [the new method] helped us 

get some of the panels laid up and ready to go and move forward with the 

schedule without being affected by a short period of time when we didn’t have 

the oven available to us.”28

ERA researchers recognized PRSEUS technology as a key enabler for manufacturing future HWB airframes. It was clear that requirements for ensuring 

pressure integrity of a passenger cabin with a non-circular cross section would 

result in significant weight penalties if the aircraft were assembled using conventional methods. In fact, this would have been equally true using what were 

26. Ibid.

27. Ibid.

28. Ibid.
Pursuing Damage-Tolerant Composite Structures

77

then state-of-the-art methods for fabricating composite materials. Certain 

regions of the pressure vessel are subject to out-of-plane loading conditions, 

in which traditional layered-material composite techniques would require 

thousands of mechanical attachments to suppress de-laminations and to join 

structural elements, ultimately leading to fastener pull-through problems in 

the thin-gauge skins. Such fasteners and attachments would necessarily contribute to airframe weight. Another argument against conventional composite 

fabrication involved high manufacturing costs associated with a highly contoured airframe. Building the HWB using traditional means would require 

complex outer-mold-line (OML) tooling as well as individual toolsets for all 

of the interior stringers and frame members, which would further drive up 

costs. PRSEUS technology provided the means to fabricate complex aircraft 

structures that were both effective in out-of-plane loading scenarios and affordable to produce.29

Not only is the flattened geometry of the HWB subject to secondary bending stresses during pressurization, but the shell also experiences a unique biaxial load pattern during maneuver loading conditions. Researchers discovered 

that these load magnitudes are more nearly equal in each in-plane direction 

than is typically found in conventional tube-and-wing fuselage arrangements 

where the cantilevered fuselage is more highly loaded in the Nx (streamwise, 

or fuselage-bending) direction, along the stringers, than in the Ny (spanwise, 

or wing-bending) direction, along the frames. This characteristic dictates a 

structural concept in which the optimum surface panel geometry must provide 

continuous load paths in both directions in addition to efficiently transmitting internal pressure loads (Nz). Additionally, a conventional panel built up 

in a skin-stringer-frame arrangement would typically include discontinuous 

frame-shear-clip members to allow stringers to pass through uninterrupted in 

the primary longitudinal loading direction.30

Such an arrangement in an HWB would be less effective in bending and 

axial loading than a continuous frame design attached directly to the skin. In 

contrast, the PRSEUS approach replaces conventional laminated and bonded 

assembly techniques with a single piece, co-cured panel design with seamless 

transitions and damage-arresting interfaces.31 The highly integrated nature 

of the PRSEUS stiffened-panel design promotes unprecedented potential 

for structural optimization through fiber tailoring and load-path continuity 

29. Dawn C. Jegley and Alex Velicki, “Status of Advanced Stitched Unitized Composite Aircraft 

Structures,” AIAA-2013-0410, presented at the 51st AIAA Aerospace Sciences Meeting, Grapevine, 

TX, January 7–10, 2013, 1–2.

30. Ibid., 2.

31. Ibid., 2.
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between individual structural elements. The PRSEUS structural concept was 

made possible through advances in fabric manufacturing, out-of-autoclave 

resin infusion processing, through-thickness stitching technology, and singlesided stitching.32

In PRSEUS panel geometry, load-path continuity at the stringer-frame 

intersection is maintained in both directions by passing the rod through a small 

keyhole aperture in the frame web. The presence of the rod increases the local 

strength and stability of the stringer section while simultaneously enhancing 

the panel’s overall bending capability. Frame elements, placed directly on the 

inner mold line skin surface, are designed to take advantage of carbon fiber 

tailoring by placing bending and shear-conducive lay-ups where they will be 

most effective. The stitching is used to suppress out-of-plane failure modes, 

enabling a higher degree of tailoring than would be possible using conventional 

laminated materials. This configuration results in a bi-directionally stiffened 

panel that is highly efficient in all three loading directions. Although this design 

is ideal for the HWB pressure cabin, it is also applicable to cylindrical fuselage 

sections with thin skins as well as composite wing structures. The stitching 

approach would allow thin fuselage skins to safely buckle while causing minimal disruption of transverse stiffener elements, allowing the stringer to pass 

through a frame or wing rib cap.33

A Crucial Milestone: 

Fabricating and Proof-Testing a Multi-Bay Box

The key to maturing stitched composite manufacturing technology for possible use in constructing a future HWB aircraft involved a building-block 

approach to development and validation of the PRSEUS concept. Over a 

roughly 4-year period, from late 2009 through 2013, researchers took their 

work from TRL-3 to TRL-5, demonstrating construction of tension and compression panels, a pressure panel and pressure cube, and then a multi-bay box, 

the latter demonstrating over a 10 percent benefit in weight reduction relative 

to sandwich composites.

The first step in designing an effective pressure vessel was to evaluate the 

effect of pressure on a test article called the Internal Pressure Box (IPB). This 

TRL-4 activity demonstrated the capability of a minimum-gauge PRSEUS 

panel to carry limit loads of 1P (equal to a normal operating pressure of 9.2 

psi) and 2P, which represents the 18.4 psi maximum overpressure condition. Next, the team built a single pressurized cube as a risk-reduction test 

32. Jegley and Velicki, “Development of the PRSEUS Multi-Bay Pressure Box,” 3.

33. Ibid., 3.
Pursuing Damage-Tolerant Composite Structures

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article to examine a new integral cap joint concept. Finally, lessons learned 

from these tests led to fabrication of a large-scale test article representing a 

section of an HWB fuselage that could be tested under combined axial and 

pressure loading.34

As tested on the IPB, the 108-by-48-inch PRSEUS panel had 20-inch 

frame spacing, 6-inch stringer spacing, and a 0.052-inch skin thickness. Prior 

to testing, engineers conducted both linear and nonlinear static analyses using 

a model with a combination of shell and beam finite elements. The test panel 

was then bolted to a metallic pressure vessel and subjected to pressure loads 

while a combination of instruments and sensors monitored displacements 

and strains. Results showed that the pristine pressure panel was capable of 

withstanding the required 2P internal overpressure loading condition with no 

evidence of damage. Researchers then inflicted barely visible impact damage 

(BVID) to a primary load-carrying member, the rod region of a stringer, and 

ran the tests again. Even with slight damage, the panel withstood the 2P load 

condition as well as higher pressures up to 28.44 psi before suffering initial 

failure through the center stiffener. Technicians arrested the damage before it 

could reach the skin by stitching the stiffener, and the panel was then loaded 

to 30 psi without sustaining additional damage or loss of pressure integrity. 

Because initial failure occurred at a load significantly higher than that required 

for commercial transport aircraft, researchers concluded that pressure loading is 

not a critical load condition for a minimum gauge PRSEUS panel. Therefore, 

the minimum gauge panel geometry of the pressure panel was also applied to 

the panels used for constructing the pressure cube test article.35

The IPB consisted of six composite PRSEUS panels assembled using aluminum fittings and an untested stitched integral-cap-joint concept. The cube 

assembly was designed to represent a portion of a pressurized HWB fuselage 

section incorporating the upper cover skin (crown) panel, two side ribs, two 

side bulkheads, and a pressurized floor section. Because the crown panel was 

representative of the upper surface of the baseline aircraft, there were few 

fasteners protruding through the OML, where they would be exposed to the 

airstream. Two pairs of opposing panels, arranged symmetrically to represent 

rib and bulkhead panel regions, formed the sides of the pressure cube. These 

were representative of the outer cabin pressure-carrying ribs and the rear pressure bulkhead of the baseline aircraft.36

34. Nicolette Yovanov, Andrew E. Lovejoy, Jaime Baraja, and Kevin Gould, “Design, Analysis and Testing 

of a PRSEUS Pressure Cube to Investigate Assembly Joints,” presented at the 2012 Aircraft 

Airworthiness and Sustainment Conference, Baltimore, MD, April 2, 2012, 1–2.

35. Ibid., 3.

36. Ibid., 4.
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The PRSEUS pressure cube was a representative section of the eventual HWB large-scale test 

article. (NASA)

In order to accommodate an access door and instrumentation pass-through, 

the floor panel was not strictly representative of the baseline aircraft, but was 

designed using available panel tooling. Where necessary, components were 

secured using aluminum fittings and titanium bolts. Stitched T-shaped integral caps were manufactured into the panels to reduce the complexity and 

number of metallic fittings required to assemble the panels. This integral cap 

joint design, incorporated around all four edges of the crown panel to provide 

a means of attaching the side panels, was the main focus of the pressure cube 

risk-reduction test. The pressure cube was also the first test specimen in which 

PRSEUS panels were joined together to create a 90-degree corner. Researchers 

needed to verify that the joint concept could hold an adjusted 2P load case 

scaled up to account for the subscale dimensions of the cube. Prior to shipping 

the cube assembly to LaRC, technicians applied a coat of flat white paint to 

the interior surfaces and the crown panel OML, and gray paint with a speckled pattern to the rib and bulkhead panel exterior surfaces to help engineers 

visualize the path of panel delamination and cracks during pressure testing.37

Once again, researchers began by making a detailed FEM to obtain linear 

analysis predictions and nonlinear analysis verification. They created detailed 

local FEMs for joint analysis, and, when necessary, employed additional 

37. Ibid., 4.
Pursuing Damage-Tolerant Composite Structures

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analysis to predict the 

response of specific 

local regions of the 

cube. This detailed 

analysis was required 

to predict failure loads 

and to verify the analytical methods that 

would later be used 

for design and analysis 

of the large-scale test 

article. The research 

team completed linear 

analysis prior to pressure testing in order to 

determine panel strains 

and displacements for 

correlation during the 

test (to predict critical 

panel locations and 

failure modes), and to 

demonstrate that the 

overall specimen strength would meet the 2P load requirements.38

The PRSEUS pressure cube test setup at LaRC. Loads were 

measured with direct-current differential transformers and 

acoustic emission sensors. High-speed video cameras recorded 

the results. (NASA)

As with the earlier pressure panel test, the design pressure limits for the cube 

were 1P, with a 2P maximum overpressure condition. After testing began, the 

cube was subjected to several pressure loads at various levels while still in pristine 

condition, and was later pressure loaded to failure with BVID imparted to the 

exterior of the cube at one of the rib integral cap web locations. Initially, researchers conducted two checkout tests at 4.6 psi (0.5P) to verify proper operation of 

all data acquisition systems and the pressure control system. The pristine cube 

was cycled up to 1P pressure and then completely unpressurized. Additionally, 

the pristine cube was cycled up to 20.15 psi (2.2P) prior to being unpressurized 

to ensure that no failure would occur for the overpressure condition, but with 

an additional margin of 10 percent included for safety. Following these pressure 

cycles, inspectors examined the cube using ultrasonic non-destructive inspection (NDI) techniques. Researchers then turned the cube on its side and, using 

a 1-inch spherical drop weight with an impact-energy of 100 foot-pounds, 

imparted BVID to an integral cap where it attached a rib and bulkhead. After 

inspectors performed additional NDI in the vicinity of the BVID, the cube was 

rotated back to the test position and then pressurized until catastrophic failure 

38. Ibid., 4.
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occurred. Afterward, they performed a final NDI on what remained of 

the cube.39

The PRSEUS pressure cube is seen here after being tested to failure. 

One panel has been completely blown off, some rib frames were fractured, and a metal fitting failed due to local buckling of the composite 

structure. (NASA)

Upon completion of the pressure cube tests, researchers determined that 

the combined pretest and post-test analytical methods used correlated well 

with actual test results. Assessment of strain gauge, video image correlation, 

and NDI data demonstrated that key failure modes and locations had been 

accurately predicted. In order to effectively serve as a risk-reduction specimen, loads in the joints of the pressure cube had to be accurately scalable 

to demonstrate the higher loads expected in the large-scale test article when 

subjected to the 2P overpressure condition. Researchers, therefore, conducted 

additional post-test correlation to relate the bending moments of the pressure 

cube to those of the planned Multi-Bay Box (MBB) large-scale test article. 

39. Ibid., 6–7.
Pursuing Damage-Tolerant Composite Structures

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This comparison yielded a factor of 2.35 difference in the bending moments 

due to geometric considerations. This meant that to scale up to the required 

bending moment, the pressure cube needed to meet a loading condition of 

4.7P, or approximately 43 psi. This demonstrated that a pressure cube failure at 

48 psi (5.2P) correlated to an MBB test article failure at 20 psi (2.2P), which 

met the overpressure requirement.40

At 80-percent-scale, the PRSEUS Multi-Bay Box represented a significant portion of the HWB 

center-body assembly. This schematic shows the internal structural arrangement of the MBB. 

(NASA)

The final step in the PRSEUS technology building block series involved 

extensive testing of an 80-percent-scale MBB representing a portion of the 

center section of an HWB transport aircraft capable of withstanding bending and internal pressure loadings representative of operational conditions. 

NASA partnered with Boeing to evaluate the MBB test article using the LaRC 

Combined Loads Test System (COLTS) facility. Boeing fabricated the test 

article primarily using PRSEUS cover panels, pressure bulkheads, and floor 

structures assembled into a double-deck test article measuring approximately 

30 feet wide, 14 feet high, and 7 feet deep.41

40. Ibid., 8–9.

41. Dawn C. Jegley, Marshall Rouse, Adam Przekop, and Andrew E. Lovejoy, “The Behavior of a LargeScale, Stitched Composite Multi-Bay Pressure Box,” NASA TM-2016-218972, April 2016, 1–2.
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Following completion at Boeing’s Long Beach facility, the PRSEUS MBB is prepared for shipment 

to LaRC. (NASA)

The MBB test article was assembled at the Boeing C-17 manufacturing 

plant in Long Beach. It was sized to represent an 80-percent-scale section of 

the most heavily loaded portion of the HWB center-body. This made the MBB 

large enough to be representative of a full-scale structure while still allowing 

the largest composite panels to fit inside the available oven for curing, and 

also permitting the assembled structure to fit within the COLTS test chamber. 

The MBB structural arrangement consisted of 11 PRSEUS panels forming the 

exterior shell and floor members, along with four interior ribs. Boeing first 

fabricated the crown panel, which was the first 30-foot-long PRSEUS panel 

ever made. As such, there was necessarily somewhat of a learning curve to the 

manufacturing process. Increasing the scale of the panel resulted in imperfections caused by motion of metal plates used to transmit normal pressure and 

temperature, and provided a smooth surface for the finished laminate during 

the lay up process. During resin infusion and curing, these plates shifted in 

such a way as to create dents in the OML surface. Because the panel skin was 

only 0.052 inches thick in some places, manufacturers were concerned that 

these dents might harm the load-carrying capability of the crown panel in 
Pursuing Damage-Tolerant Composite Structures

85

compression. It became necessary to add bonded patches over the dents to 

ensure that the crown panel would not fail prematurely.42

After the MBB arrived at LaRC, researchers subjected it to a series of loadings in the COLTS facility. As with the IPB experiments, testing was first conducted with the structure in pristine condition, then with intentional minor 

damage, and then finally pressurized to failure. Data were monitored and 

recorded using several types of instrumentation including 262 linear and 36 

rosette strain gauges, 15 linear variable displacement transducers, 4 pressure 

transducers, 4 fiber optic wires, 4 video digital image correlation systems, 26 

acoustic emission sensors, and 9 video cameras used to record the behavior 

of the test article and the COLTS system. Researchers initially installed strain 

gauges on all panels and on most load-introduction elements. They added 

more strain gauges following application of BVID to track the progression 

of damage emanating from the impact site. Video cameras inside each of the 

MBB’s six bays recorded cracks and deformations of the bulkheads, crown, and 

keel. Researchers monitored additional video cameras outside the test article 

to obtain a global view of the structure.43

Once the MBB had been installed inside the COLTS, researchers applied 

mechanical loads to simulate critical flight conditions and internal pressure 

loads to represent normal cabin pressure. Four actuators provided mechanical 

loads while internal pressure was introduced through a valve in an upper bulkhead panel access door. Holes in the floor ensured that the pressure remained 

constant in both the upper and lower sections of the test article. The test series 

included runs during which mechanical loads were applied alone, pressure was 

applied alone, and combinations of internal pressure and mechanical loads 

were applied simultaneously. In each case, loading was quasi-static and was 

applied slowly enough to ensure that mechanical actuators were synchronized 

with one another and with the pressure load.44

Once again, researchers conducted DLL and DUL loadings of the pristine 

structure before running the same tests with intentional damage. In all cases 

where the load factor was less than or equal to DUL and pressure loading was 

applied simultaneously with the mechanical load, the pressure and actuator 

systems were programmed to ramp together from zero to maximum loading. 

When the programmed mechanical loading exceeded predicted DUL, the pressurization system was programmed to not exceed the DUL condition for pressure. During each test run, researchers ramped loads from zero to maximum 

with short pauses at intervals to compare test data with predictions. Once the 

42. Ibid., 6.

43. Ibid., 7–8.

44. Ibid., 9.
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maximum load value was attained, it was held briefly to allow for data collection and then the structure was unloaded at a steady but relatively rapid rate.45

For the next series of test runs, the COLTS proved to be a valuable tool for 

ERA researchers in evaluating the MBB’s damage-tolerance characteristics. 

Several experiments were devised to examine the PRSEUS structure’s ability 

to withstand the types of minor damage that might be incurred during routine 

flight line operations. “Some of the things that could impact the plane are rocks 

or debris on the runway when it’s taking off or landing,” said NASA research 

aerospace engineer Andrew Lovejoy, “or you can have a mechanic hit it with a 

tool, or a vehicle driving by could hit it.”46

Any of those events—extremely common occurrences at airports—could 

cause damage that might be barely visible only to crewmembers doing a walkaround inspection. One of the goals of designing damage-arresting composites 

was so that an aircraft would be capable of sustaining operational loads even 

with that damage in place. Once again, it was necessary to conduct impact 

tests to intentionally cause BVID, but with such a large test article inside the 

COLTS facility this proved especially challenging.47

To solve this problem, NASA technicians designed and built a unique test 

rig. “If you are going to impact the top of something, you would just have a 

free-falling weight that would come down and hit,” said Lovejoy. “You have a 

mass and a height, so that’s a fairly straightforward calculation of the energy; 

if you wanted to hit on the side, you have a spring-loaded impactor.” But 

researchers needed to strike upward at the keel, or underside, of the MBB. 

“There is very limited space in COLTS,” Lovejoy explained. “You can have an 

air-driven projectile, or a spring-loaded one, but those are less controllable, so 

we came up with the ‘roller-coaster’ impactor.” In order to produce a controlled 

impact, it was necessary to propel a weight down a track that curved upward 

until the impactor was oriented in a vertical direction. “We didn’t have any 

device to do that,” he added, “so we built from scratch an impactor and a track 

to make it go where we wanted it to go; two pieces of track encapsulated it 

just before impact to guarantee that we’re getting that vertical impact on the 

bottom of the keel.”48

Damage testing consisted of three impacts to the interior of the structure 

on the stiffened side of the upper bulkhead, and three impacts to the exterior 

of the structure on the unstiffened side of center keel. Researchers used a 

45. Ibid., 9–10.

46. Andrew Lovejoy interview, NASA X, “End of an ERA—Part 1,” NASA TV, November 24, 2015. 

Subsequent Lovejoy quotes are from this interview unless otherwise noted.

47. Ibid.

48. Ibid.
Pursuing Damage-Tolerant Composite Structures

87

spring-loaded impactor at locations at the top of a stringer along the upper 

edge of a frame, and at a mid-bay location between stiffeners to inflict BVID 

to the MBB interior. These impacts represented a range of locations and the 

type of damage possible due to service events such as tool drops. Exterior 

damage was inflicted using the gravity-fed “roller-coaster” apparatus to strike 

locations at the flange edge of a stringer, at the flange edge of a frame, and at 

a mid-bay skin location between stiffeners. These strikes were imparted to an 

area of the structure that would likely buckle during loading so as to evaluate 

whether typical exterior impacts would degrade the performance of buckled 

structure. In each case, researchers employed a weight with a 1-inch-diameter 

hemispherical tup.49

The COLTS control room team gathers data during a test run. (NASA/David C. Bowman)

Researchers noted that BVID results for the interior sites corresponded to 

20 foot-pounds for the top of the stiffeners, which caused little damage but 

is the maximum energy allowed for internal impacts to commercial aircraft, 

and 15 foot-pounds for the skin mid-bay location, where visible damage was 

clearly evident. BVID for the exterior sites corresponded to energy levels of 

60 foot-pounds, 50 foot-pounds, and 15 foot-pounds for the frame flange, 

stringer flange, and the mid-bay locations, respectively. On one of the exterior 

tests, the tup slightly missed the planned impact site, striking the thin-skin 

region instead of directly at the adjacent flange. As a result, the damage was 

more severe than intended. The tup created a through-hole that was clearly 

visible from both the exterior and interior. Engineers evaluated the damage at 

49. Jegley, et al., “The Behavior of a Large-Scale, Stitched Composite Multi-Bay Pressure Box,” 10.
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this location and indicated that it would not reduce the ability of the structure 

to sustain mechanical load but might reduce the structure’s ability to support 

internal pressure loads. Technicians effected repairs by taping a non-structural 

patch over the hole on the inner, stiffened side of the center keel. Inspectors 

conducted ultrasonic scans immediately before and after each impact so the 

extent of the damage could be quantified. These scans indicated that although 

delamination occurred at the keel skin and flange impact sites, it was successfully arrested at the stitch line closest to the impact site. Scans of the bulkhead 

stiffener impacts found no damage, but inspection of the interior skin impact 

revealed delamination running from the point of impact to the nearest stitch 

line, located at the edge of the adjacent flange.50

COLTS engineers repeated the DLL and DUL loadings with the final BVID 

test to a load greater than DUL in both the up-bending and up-bendingplus-pressure conditions. Loads were applied using the same methodology as 

in the earlier tests, but the pressure was held constant while the mechanical 

load was increased by 10 percent. Next, the mechanical load was decreased to 

DUL and held constant while the pressure load was decreased to zero, leaving 

the test article at DUL in the up-bending condition without pressure. Finally, 

the mechanical load was increased to 10 percent greater than DUL and held 

briefly before being removed.51 Researchers calculated that the ultimate load 

factor was 1.5 times the DLL. With testing and data acquisition complete, they 

concluded that the PRSEUS large-scale test article had performed beautifully 

under conditions of multiple and extreme stresses. “In fact,” said Lovejoy, “the 

MBB exceeded expectations, performing well beyond the predicted DUL.” 

From these results, researchers concluded that PRSEUS technology offered 

an opportunity to lighten the HWB structure even more, potentially making 

future aircraft even more efficient.52

PRSEUS Results

Testing of PRSEUS technology during the ERA project was the culmination 

of more than two decades of effort to develop technology that would improve 

damage tolerance and reduce the weight of composite structures for commercial transport aircraft applications through the use of through-the-thickness 

stitching. The partnership between NASA and Boeing under the ERA project 

further advanced this technology in an attempt to encourage and enable nextgeneration aircraft configurations such as the HWB.

50. Ibid., 10–11.

51. Ibid., 11.

52. Lovejoy interview.
Pursuing Damage-Tolerant Composite Structures

89

Analytical modeling and engineering experiments conclusively demonstrated that PRSEUS technology effectively suppressed delamination, arrested 

damage, and reduced or eliminated the need for fasteners in the acreage of 

composite panels. A traditional layered assembly would require thousands of 

mechanical fasteners to join structural elements and suppress delamination. 

Disadvantages of using metal bolts and rivets to join layers of thin-gauge 

composite skins include added weight, localized stress fractures, and fastener 

pull-through (a critical failure mode). Reducing the number of fasteners 

eliminates the need to drill large numbers of holes, reduces the necessity to 

add doubler plates to mitigate stress concentrations around those holes, and 

minimizes the time required to inspect fastener holes throughout the service 

life of the aircraft.53

The PRSEUS panel architecture constituted a significant step beyond current state-of-the-art conventional layered composite systems. The addition 

of a pultruded rod to the stringer, and a tall foam-filled frame perpendicular 

to the stringer, improved bending stiffness in both directions compared to 

traditional construction, a characteristic critical to the HWB configuration. 

PRSEUS also provided efficient load paths because all panel elements are 

integrated into a single one prior to curing, eliminating the need for shear 

clips and other elements that add weight to the structure. The pultruded rod 

increased local strength and stability of the stringer section while shifting the 

neutral axis away from the skin to further enhance overall panel buckling characteristics. Frame elements were stitched directly onto the skin surface to take 

advantage of carbon fiber tailoring by placing bending and shear-conducive 

lay-ups where they are most effective. The integral panel design exploited 

the orthotropic nature of carbon fibers and suppressed out-of-plane failure 

modes with through-the-thickness stitching. These two features enable applying PRSEUS technology as an effective damage-arresting design approach for 

composite structures.54

Another advantage expected of the PRSEUS lightweight composite concept 

is a dramatic overall reduction in airframe weight. This feature was particularly 

significant when designing the HWB pressure cabin, where the design was 

largely driven by out-of-plane loading considerations. In addition to secondary bending stresses experienced during pressurization, another key difference 

between the highly contoured HWB shell and the traditional cylindrical fuselage is a unique bi-axial loading pattern that occurs during maneuver loading 

53. Jegley, et al., “The Behavior of a Large-Scale, Stitched Composite Multi-Bay Pressure Box,” 

25–26.

54. Tenney, et al., “NASA Composite Materials Development: Lessons Learned and Future 

Challenges,” 23.
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conditions. Load magnitudes for the HWB are nearly equal in each in-plane 

direction (Nx and Ny), in contrast to the type of loading typically found in 

conventional tube-and-wing fuselage configurations, where the cantilevered 

fuselage is more highly loaded in the Nx direction, along the stringer, than in 

the Ny direction, along the frame. This dictates that the optimum structural 

panel geometry for the HWB should have continuous load paths in both 

directions (Nx and Ny), in addition to efficiently transmitting internal pressure 

loads (Nz).55

For a conventional panel built up in a skin-stringer-frame arrangement, the 

frame shear clip is typically discontinuous to allow the stringer to pass through 

the frame. If such an arrangement were used to assemble the HWB centerbody, the frame would be less effective in bending and axial loading than a 

continuous frame attached directly to the skin. Additionally, the resulting panel 

assembly would necessarily be heavier to provide structural strength. To overcome the inherent weight penalties of the non-circular pressure cabin, aircraft 

manufacturers could instead use PRSEUS technology to design a lightweight 

bi-directionally stiffened panel, where the wing bending loads are carried by 

the frame members and the fuselage bending loads are carried by the stringers. Such a panel arrangement could be optimized to include continuous load 

paths in both directions, highly tailored stringer and frame laminates, thin 

skins designed to operate well into the post-buckled regime, and crack-arresting 

features designed to minimize damage propagation.56 Research results indicate 

that the PRSEUS concept would be approximately 10.3 percent lighter than a 

conventional aluminum honeycomb sandwich assembly in the pressure cabin 

of a large BWB aircraft.57

Beginning with tests of small sample coupons and ending with a 30-footlong large-scale pressure box, the PRSEUS ITD successfully demonstrated the 

viability of both the technology itself and the use of PRSEUS construction 

techniques to build the center-body for a proposed HWB transport aircraft. 

This building-block approach showed that designs could be refined and the 

risk of premature failure reduced as more complex assemblies were introduced. 

The final test series involving the MBB validated flight-maneuver load conditions and internal pressurization loads to demonstrate that the technology 

was capable of meeting the structural weight goals established for the HWB 

airframe. The test article demonstrated anticipated post-buckling behavior, 

and preliminary evaluations showed no damage growth from impact sites. 

55. Jegley, et al., “The Behavior of a Large-Scale, Stitched Composite Multi-Bay Pressure Box,” 2–3.

56. Ibid., 3.

57. Tenney, et al., “NASA Composite Materials Development: Lessons Learned and Future Challenges,” 

23.
Pursuing Damage-Tolerant Composite Structures

91

While this ITD was primarily aimed at demonstrating PRSEUS viability for 

the HWB, the benefits demonstrated could also be applied to traditional tubeand-wing aircraft, other advanced configurations, spacecraft, and any structures where weight and through-the-thickness strength are significant design 

considerations.58 From a production standpoint, PRSEUS is also attractive 

because no autoclave is required, and therefore larger composite parts can be 

fabricated. “PRSEUS is broadly applicable to fuselages and wings of any shape; 

it is lightweight, damage-tolerant and built with fewer parts,” said Fay Collier, 

adding “It could be a game changer.”59

58. Jegley, et al., “The Behavior of a Large-Scale, Stitched Composite Multi-Bay Pressure Box,” 25–26.

59. Guy Norris, “‘Green’ Airliner Targets Achievable by 2025, Says NASA,” Aviation Week & Space 

Technology, http://aviationweek.com/awin/green-airliner-targets-achievable-2025-says-nasa, 

April 18, 2011 (accessed July 21, 2016).
92

NASA-Armstrong/Air Force Research Laboratory tests of ACTE flaps installed on a NASA 

Gulfstream Aerospace G-III validated that the seamless design with its advanced lightweight 

materials could reduce wing structural weight, improve fuel economy and efficiency, thereby 

reducing environmental impacts. (NASA)
93

CHAPTER 4

The Adaptive Compliant 

Trailing Edge Investigation

Another important element of NASA’s green aviation research involved developing a wing surface capable of changing shape in flight, which would make 

airplanes quieter and more fuel-efficient. This played strongly to NASA’s traditional excellence in aerodynamic research, which dated to the earliest days of its 

predecessor, the National Advisory Committee for Aeronautics. The Adaptive 

Compliant Trailing Edge (ACTE) is an imaginative morphing wing technology 

that allows an aircraft to seamlessly extend its flaps, leaving no drag-inducing, 

noise-enhancing gaps for air to flow through, creating energy-robbing (hence 

fuel-robbing) turbulence and vortices. Compliant structures are used to change 

the wing trailing edge shape to maintain smoothly curved surfaces along the 

flow direction, thereby avoiding abrupt discontinuities, particularly those created by conventional hinged control surfaces such as roll-controlling ailerons, 

lift-enhancing flaps and slats. Compliant structures allowed smooth variation 

of the trailing edge shape in the spanwise direction, making it possible to seal 

any gaps at the edges of deflected control surfaces. These characteristics can 

improve control surface effectiveness and also provided the ability to tailor 

spanwise aircraft load distribution to enhance aerodynamic efficiency, reduce 

structural loads, and generate control forces.1

A compliant structure is a monolithic joint-less mechanism that exploits 

the elasticity of material to produce a desired functionality such as force or 

motion transmission, motion guidance, shape changing, and/or energy storage 

and release. As applied to a device such as an aircraft flap, compliant structures are optimized to distribute localized strain to change the shape of the 

control surface as needed during flight. Instead of using a series of mechanical 

actuators, a compliant structure deforms as a whole, thus avoiding high-stress 

concentrations in localized regions. Known as distributed compliance, this 

1. Sean Wakayama and Edward V. White, “Evaluation of Adaptive Compliant Trailing Edge Technology,” 

AIAA 2015-3289, presented at the 33rd AIAA Applied Aerodynamics Conference, AIAA Aviation 

Forum, Dallas, TX, June 25, 2015, 1.
Green Light for Green Flight

94

design concept offers additional benefits because the entire adaptive structure 

can reshape itself into complex predetermined positions with minimal force 

and can be locked in place at any desired configuration. Although such structures are generally described as flexible, they are actually optimized to resist 

deflection under significant external aerodynamic loading and provide the same 

stiffness and structural strength as a conventional flap.2

NASA-Armstrong/Air Force Research Laboratory tests of ACTE flaps installed on a NASA 

Gulfstream Aerospace G-III validated that the seamless design with its advanced lightweight 

materials could reduce wing structural weight, improve fuel economy and efficiency, thereby 

reducing environmental impacts. (NASA)

Compliant trailing-edge flaps are configured to have a seamless surface contiguous with the primary wing surface. Elimination of surface discontinuities 

results in both lower drag and higher control authority than provided by conventional hinged flaps because a compliant flap increases camber under load, 

generating more lift, and is more effective in roll and gust-load-alleviation per 

degree of deflection. Additional benefits of seamless surfaces include making 

the flaps less susceptible to icing and fouling from debris, and the seamless 

transition between the fixed and movable portions of the wing reduces noise 

2. Sridhar Kota, Russell Osborn, Gregory Ervin, Dragan Maric, Peter Flick, and Donald Paul, “Mission 

Adaptive Compliant Wing – Design, Fabrication and Flight Test,” RTO-MP-AVT-168, NATO Research 

and Technology Organization, 2009, 2–5.
The Adaptive Compliant Trailing Edge Investigation

95

associated with the turbulent airflow generated by discontinuous surfaces at 

the flap ends when the high-lift devices are deployed for landing.3

The Air Force-NASA AFTI F-111A (SN 63-9778) Mission Adaptive Wing (MAW) testbed on one of 

its test flights. (NASA)

There was historical precedent for NASA’s work on compliant trailing 

edge flaps. In 1978, as part of the joint Air Force-NASA Advanced Fighter 

Technology Integration (AFTI) program, the Air Force Flight Dynamics 

Laboratory (AFFDL, now enfolded within the Air Force Research Laboratory) 

launched a joint Air Force-NASA program to build and test a so-called “mission 

adaptive wing” (MAW). The MAW, an outgrowth of aerodynamicist Richard 

T. Whitcomb’s brilliant conceptualization of the so-called “supercritical wing” 

(SCW) earlier tested on a modified Vought TF-8A Crusader, was a smooth 

variable-camber relatively low-aspect-ratio wing which, via series of internal 

actuators and linkages, could be adjusted for optimum transonic and supersonic flight performance. Following tests of competing industry designs in the 

Langley 8-ft Transonic Pressure Tunnel, Boeing received a 1979 contract to fabricate the new wing, which was subsequently installed on a modified General 

Dynamics F-111A airplane (SN 63-9778) already flying with an experimental NASA-developed SCW as part of the Air Force-NASA Transonic Aircraft 

Technology Program. Over 59 flights totaling 145 flight-test hours from 1985 

3. Ibid., 5–7.
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96

through 1988, the AFTI F-111A MAW demonstrated that such a wing could 

afford significant aerodynamic and performance advantages—cruise performance showed an approximately 7 percent reduction in drag, supersonic range 

at low altitude increased by 25 percent, and the plane had 30 percent greater 

range at high altitude than a conventional F-111A—though weight and complexity of the mechanical actuation system hindered its further development at 

that time, and some promised modes of camber control remained unexplored.4

From the mid-1980s through the early 1990s, a succession of wind tunnel 

test programs had shown that an aeroelastic wing—one thin- and flexibleenough to have reduced torsional stiffness, in contrast to a conventional wing, 

which deliberately has a robust structure to minimize loads-induced deformation—would also have greatly enhanced control power: the ability to be flexed 

as an entire surface for enhanced roll control and also for loads alleviation. 

Additionally, gross weight could be reduced significantly, perhaps by as much 

as 20 percent. As a consequence, in 1996, NASA, in conjunction with the Air 

Force and Boeing (to whom Lockheed-Martin, British Aerospace, and Moog, 

Inc., were subcontractors), began the Active Aeroelastic Wing (AAW) development and flight validation program, a major step forward towards creating a 

bird-like “morphing” wing, one where the aeroelastic response of the wing itself 

could be exploited to “deform” the wing’s shape into one furnishing optimum 

aerodynamic performance and flight loads distribution across the range of an 

aircraft’s flight envelope.5

As they had earlier with the AFTI F-111A MAW, Boeing engineers, in 

conjunction with their military and NASA counterparts, took key structural 

and other components from the wings of a retired NASA F/A-18A: the former 

4. Eric J. Miller, Josue Cruz, Shun-Fat Lung, Sridhar Kota, Gregory Ervin, Kerr-Jia Lu, and Pete Flick, 

“Evaluation of the Hinge Moment and Normal Force Aerodynamic Loads from a Seamless Adaptive 

Compliant Trailing Edge Flap in Flight,” AFRC-E-DAA-TN28829 (January 21, 2016), 5; Richard 

P. Hallion and Michael H. Gorn, On the Frontier: Experimental Flight at Dryden (Washington, DC: 

Smithsonian Books, 2001), 271–273; Joseph R. Chambers, Partners in Freedom: Contributions 

of the NASA Langley Research Center to U.S. Military Aircraft of the 1990s, NASA SP-2000-4519 

(Washington, DC: NASA, 2000), 81; Sheryll Goecke Powers, Lannie D. Webb, Edward L. Friend, 

and William A. Lokos, “Flight Test Results from a Supercritical Mission Adaptive Wing with Smooth 

Variable Camber,” NASA TM-4415 (November 1992), 2–3.

5. Ed Pendleton, Pete Flick, Donald Paul, Dave Voracek, Eric Reichenbach, and Kenneth Griffin, “The 

X-53: A Summary of the Active Aeroelastic Wing Flight Research Program,” AIAA 2007-1855, 

presented at the 48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials 

Conference, April 23–26, 2007, 1–2.
The Adaptive Compliant Trailing Edge Investigation

97

The NASA-AFRL-Boeing F/A-18 Active Aeroelastic Wing testbed (later designated X-53) 

employed “wing-warping” reminiscent of that employed by the Wright brothers at the dawn of 

powered, winged flight. Here it is demonstrating its ability to undertake a full-stick-deflection 

360-degree roll. (NASA)

High-Alpha Research Vehicle (HARV), NASA 840.6 They added a drive system 

for cycling independent outboard and inboard leading edge flaps, and replaced 

the plane’s thick wing panels with thinner panels fabricated from aluminum 

and titanium with solid composite skins, reducing the wing’s torsional stiffness 

by approximately 17 percent over the baseline F/A-18A. (In effect, they were 

“restoring” the F/A-18 wing to the original very elastic “thin” structural design 

that had characterized early production models). The modified wing incorporated new high-rate actuators that, driven by a new flight control system, could 

deflect the wing surface similar to the “wing warping” the Wright brothers had 

employed on their 1903 Kitty Hawk Flyer. The new wings were then joined 

to a Navy-furnished airframe, the resulting “new” F/A-18 AAW becoming the 

6. Alpha (which often appears as its Greek letter α) means “angle of attack” in aerospace engineering 

shorthand.
Green Light for Green Flight

98

NASA 853. (In 2006, the F/A-18 AAW was designated the X-53, though it is 

best-remembered by its original name).7

Extensive ground tests, including tests of a scale F/A-18 AAW model in the 

NASA Langley Transonic Dynamics Tunnel in July and August 2004, preceded 

Phase I flight testing, which included functional check flights, aeroservoelastic 

clearance flights, aerodynamic and loads model development, and envelope 

expansion. Phase I commenced in November 2002, and concluded in April 

2003 after 50 test missions, during which the F/A-18 AAW flew to Mach 

1.32, approximately 914 miles per hour, at 24,800 feet. Following extensive 

preparations including exploiting Phase I-acquired data to update aerodynamic and loads databases, and others necessitated by the complex nature of 

the closed-loop control laws testing to come, Phase II commenced in midDecember 2004 and continued through the end of March 2005. Testing went 

smoothly, including full-deflection 360-degree rolls validating full-scale AAW 

performance, and concluded in less than 4 months, after a total of 34 Phase II 

flights. Overall, the F/A-18 AAW was a great success, proving the practicality 

and benefits of active aeroelastic control.8

Though undertaken for very different purposes, the F-111A MAW and 

F/A-18A AAW thus constituted important predecessor steps before the onset 

of the ACTE investigation. It began in the summer of 2014, when researchers 

at AFRC replaced the conventional aluminum flaps, speed-brakes, and spoilers 

of the Armstrong Flight Research Center’s Gulfstream Aerospace G III (NASA 

804) with advanced, shape-changing assemblies that formed seamless bendable 

and twistable surfaces. A series of flight tests from November 2014 through 

April 2015 explored the feasibility of using such flexible trailing-edge wing 

flaps to improve aerodynamic efficiency and reduce noise generated during 

takeoffs and landings. Employment of ACTE technology would also result in 

size and weight reductions for aircraft wings, leading to reductions in fuel burn 

and greenhouse gas emissions. Other potential aerodynamic benefits of ACTE 

include increased control effectiveness and alleviation of structural loads. ERA 

researchers concluded that the results of flight-testing successfully increased 

the ACTE technology TRL to 6.9

7. Ibid., 4–9.

8. Ibid., 10–20. See also Robert Clarke, Michael J. Allen, Ryan Dibley, Joseph Gera, and John 

Hodgkinson, “Flight Test of the F/A 18 Active Aeroelastic Wing Airplane,” NASA TM-2005-213664 

(August 2005), 3–35.

9. Craig L. Nickol and William J. Haller, “Assessment of the Performance Potential of Advanced 

Subsonic Transport Concepts for NASA’s Environmentally Responsible Aviation Project,” AIAA2016-1030, presented at American Institute of Aeronautics and Astronautics SciTech, 54th AIAA 

Aerospace Sciences Meeting, San Diego, CA, January 6, 2016, 4.
The Adaptive Compliant Trailing Edge Investigation

99

The ACTE ITD was a joint 

effort between NASA and 

AFRL, using flaps designed and 

built by FlexSys, Inc., of Ann 

Arbor, Michigan. With AFRL 

funding available through 

the Air Force’s Small Business 

Innovation Research (SBIR) 

program, FlexSys developed a 

variable geometry airfoil system 

called FlexFoil that could be 

retrofitted to existing airplane 

wings or integrated into new 

airframes. During test efforts at 

Armstrong, FlexSys founder and 

chief executive officer Dr. Sridhar 

Kota (an engineering professor 

at the University of Michigan 

and former Assistant Director 

for Advanced Manufacturing 

at the White House Office of 

Science and Technology Policy) 

expressed the hope that testing with a modified Gulfstream G-III would confirm his design’s flightworthiness and open doors to future applications and 

commercialization.

According to Kota, “The aerospace community has known for a long time 

that if you have a seamless wing that can be morphed in flight to maximize 

performance, then you can get significant fuel efficiency.” In the earliest days 

of aviation, various designers—the Wrights, Louis Blériot, Louis Béchéreau, 

Robert and Léon Morane, Igo Etrich, Tony Fokker, Geoffrey de Havilland, 

and Igor Sikorsky, among many others—used wing warping for controlling 

their wood-and-fabric aircraft. Later use of metal alloys for structural strength 

precluded designing aircraft with morphing wings, but the advent of strong, 

lightweight flexible materials made it possible to revisit the concept. While 

looking at this problem in the early 1990s, Kota came up with an idea he called 

“compliant design” that used techniques borrowed more from nature than from 

traditional mechanical design.10

In conventional aircraft construction, everything that is strong is also 

very rigid. Mechanical functionality requires multiple parts and complex 

10. Sridhar Kota interview, NASA X, “End of an ERA – Part 1,” NASA TV, November 24, 2015.

Dr. Sridhar Kota, founder and CEO of FlexSys, and the 

former Assistant Director for Advanced Manufacturing 

at the White House Office of Science and Technology 

Policy, from 2009–2012. (NASA/FlexSys)
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100

mechanisms. “Designs in nature are different,” he explained. “They are strong, 

but they are compliant, they’re flexible; you can see countless examples in 

nature of intricate mechanical motion without conventional joints.” In Kota’s 

FlexFoil system, every part of the structure shares a small portion of the total 

load; stresses are evenly distributed. “You have large deflections and small 

strains, so you can do this multiple times [throughout the life of the aircraft], 

you can do millions of cycles and still not fail.” Although the exact technique 

remains a trade secret, Kota explained that a proprietary algorithm minimizes 

the force it takes to morph the wing into a prescribed shape. This technique 

also has inherent mechanical advantages in that it remains very rigid toward 

external loads and although the design is incredibly strong, it does not require 

large, heavy motors to actuate the control surface.11

Specific details regarding the design configuration and materials that make 

up the ACTE control surfaces remain proprietary, but the potential benefits 

are known. A morphing surface has the potential to cut cruise drag by around 

3 percent on retrofitted aircraft and up to 12 percent on all-new designs. The 

shape-changing mechanism exploits the inherent elasticity of the composite 

material from which it is made, and by using evenly distributed devices rather 

than flexural hinges, the ACTE flap requires less power to actuate than conventional flaps. The seamless transition region with the wing eliminates a major 

source of airframe noise, which is most noticeable during takeoff and landing. 

Additionally, the ACTE flap provides larger system-level weight benefits for allnew designs (as opposed to retrofits) because it could be used to twist a specially 

designed wing spanwise to reduce loading as well as wing-bending moment.12

In the mid-1990s, Kota unveiled his unique concept to Air Force engineers 

at the AFRL at Wright-Patterson AFB, Ohio, eventually securing SBIR funding 

through AFRL. According to Pete Flick, ACTE project manager at AFRL, the 

laboratory’s investment from 1998 to 2015 totaled around $20 million. NASA 

ACTE project manager Thomas Rigney said that his Agency contributed about 

$25 million starting in 2009. The NASA portion primarily supported flighttesting and modifications to the G-III testbed that included fitting it with 

special instrumentation and a new power system.13

11. Ibid.

12. Guy Norris, “NASA-led Team Completes Morphing Flap Tests,” Aerospace Daily & Defense Report, 

May 5, 2015, http://aviationweek.com/technology/nasa-led-team-completes-morphing-flap-tests

(accessed October 1, 2016).

13. Jerome Greer Chandler, “Flexible Flap Test Program Looks to Take ‘Next Step,’” Aviation Pros, July 

16, 2016, http://www.aviationpros.com/article/12079146/flexible-flap-test-program-looks-totake-next-step (accessed November 1, 2016).
The Adaptive Compliant Trailing Edge Investigation

101

According to Rigney, “The 

reason we needed to do this on 

a full-scale aircraft was to take 

the readiness level from TRL-5 

to TRL-6 in a relevant flight 

environment; for that you need 

a full-scale aircraft.” This was 

important because Government 

and industry team members 

were convinced that ACTE 

technology had the potential 

to be a real game changer for 

the aeronautics community, 

particularly with regard to its 

impact on fuel savings. “The 

amount of potential fuel savings 

is significant,” said Rigney, “not 

just for the economy, but for the 

environment.”14

Project manager Tom Rigney, right, briefs NASA 

Administrator Charlie Bolden on the progress of ACTE 

in 2012. (NASA)

Researchers selected the 

AFRC-based Gulfstream G-III 

Subsonic Research Aircraft 

(SCRAT) as the ACTE testbed 

because the airplane’s baseline flight characteristics were well understood, and 

the modification process would be relatively uncomplicated. Additionally, the 

aircraft was already equipped with data acquisition and telemetry systems for 

transmitting data to the AFRC control room, where researchers and engineers 

monitor experiments and safety-related information. Necessary modifications 

included removal of both the left and right aluminum Fowler flaps, each 19 feet 

long. Technicians replaced these with flexible ACTE flaps, which were mated 

to the same attach points on the wing as the original flaps. The main portion of 

each ACTE assembly was blended seamlessly into the wing structure on both 

ends via flexible transition sections. These had to be both strong and flexible 

enough to withstand aerodynamic loads and maintain the integrity of the outer 

mold-line while being exercised through large deformations.15

14. Tom Rigney interview, NASA X, “Revolutionary Evolution – ERA,” NASA TV, February 17, 2014.

15. Claudia Y. Herrera and Shun-Fat Lung, “Aeroelastic Response of the Adaptive Compliant Trailing 

Edge Transition Section,” presented at 54th AIAA Aerospace Sciences Meeting, San Diego, CA, 

January 4, 2016, 4.
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102

ACTE Flight Experiment Flap Replacement

• Compliant flap replacing both aircraft flaps in their entirety

• Ground spoilers, flight spoilers/speed-brakes and flaps removed to make room for ACTE

• Target flap geometry approximately 19-ft in-span for each surface

The ACTE trailing edge flaps replaced the G-III’s conventional flaps, spoilers, and speed-brakes. 

(NASA)

ACTE Ground Tests

Ground testing was an essential element of the ACTE project development 

approach, which incorporated conventional design practices and a build-up 

test and model-validation approach. Throughout the project, researchers conducted a wide variety of ground-based tests to ensure airworthiness of the 

structure and to mitigate potential risks to schedule and mission success. These 

experiments included material characterization testing, structural proof testing, 

structural qualification testing, fatigue testing, and ground vibration testing 

(GVT). Project engineers employed a methodical building-block approach 

to ground testing so that any design and fabrication flaws could be detected 

and corrected early in order to minimize schedule delays and cost increases. 

According to AFRC aerostructures engineer Claudia Herrera, this method also 

“provided opportunities for the project team to gain early fundamental insight 

into the compliant structure technology.” Before any hardware was subject 

to evaluation, engineers first used a computational tool called FEM analysis to build confidence in the validity of their mathematical models. FlexSys 

then manufactured two prototype test articles representative of the ACTE flap 

design and fabrication process. Ground testing of these articles provided the 

data necessary to support an accurate airworthiness assessment of the flight 

article. This was crucial to the success of the project. Not only did these tests 
The Adaptive Compliant Trailing Edge Investigation

103

provide confidence in the aeroelastic analyses, they also helped determine best 

practices for testing the flight article.16

Diagram of the ACTE building-block testing approach. (NASA)

Ground testing included analyses of aeroelastic response through the transition sections used to integrate the ACTE flap to the G-III wing in a continuous mold-line. Researchers evaluated the results to ensure that the combined 

SCRAT/ACTE system was safe to fly within the desired flight envelope. Per 

AFRC aeroelastic guidelines, the testbed with its integrated ACTE flaps was 

required to demonstrate a 20 percent flutter margin to satisfy project requirements for airworthiness. To satisfy this requirement, engineers performed 

vibration testing and used the resulting data to develop a detailed FEM. In 

order to minimize impacts to the flight schedule while accurately modeling 

the flight article, FlexSys built two sets of prototype test articles, designated 

Prototype 2 (P2) and Prototype 3 (P3) prior to fabricating the ACTE flighttest articles. The differences between P2 and P3 were essentially minor design, 

fabrication, and manufacturing process changes that allowed P3 to minimize 

strain levels when subjected to large deflections. The P3 design possessed the 

full chord-wise size of the flight-test article but represented only a section of 

the spanwise size. Prototypes containing components designated as “A” were 

representative of a portion of the main flap section, while those with components designated “B” were full-scale chord-wise transition sections.17

16. Ibid., 4.

17. Ibid., 4–5.
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The engineering team performed GVTs on test articles P2.2B and P3.2B 

as a proof-of-concept for testing the compliant structure. NASA personnel 

conducted both tests at the FlexSys facility in Ann Arbor. The results provided 

confidence in test procedures as well as data for understanding the modal characteristics of the transition sections needed for validation of the FEM. Both 

test articles represented the right-hand inboard transition section with only 

slight variations. Conducted in September 2012, the P2.2B GVT was the first 

opportunity to experimentally evaluate the structural dynamic response of the 

FlexSys compliant flap structure. Goals included measurement of vibration 

frequencies and mode shapes at several deflections and two different boundary 

conditions. The engineers wished to consider any apparent change in structural stiffness due to changing the flap deflection. They also needed to evaluate several factors including the use of accelerometers as instrumentation on 

flexible structures, various types of excitation methods and instrumentation 

mass-loading effects, and a variety of analytical FEM techniques. The results 

provided a comprehensive set of lessons learned on how best to model the 

ACTE flap and how to test compliant structures, and also helped identify 

possible design variables for updating the FEM. For the P3.2B GVT in April 

2013, the test article was a reproduction of the right inboard transition section 

of the ACTE flight article. It included a 3-inch section simulating the main 

flap portion of the ACTE flight article, a 5.75-inch section to simulate the 

fixed wing structure, and a truncated section of the ACTE flap spar with an 

attachment simulating how the flap would connect to the SCRAT airframe.18

Engineers analyzed the ACTE structure’s modal response by deflecting the 

ACTE FEM analytically using the ANSYS and NASTRAN software packages 

to match the planned physical deflections of the structure. To do this, FlexSys 

first developed a mathematical relation between the amount of input applied at 

the flap actuation points and the amount of control-surface deflection relative 

to the airplane’s fixed wing section outer mold-line. These same inputs were 

applied at the actuation locations in the FEM. The engineering team then analyzed the deflected structure for natural mode shapes and frequencies using a 

non-linear analysis method. During the P2.2B test, researchers noticed that the 

resulting deflected FEMs did not match the test results as well as expected, and 

it was necessary to validate each of the deflected shapes against the measured 

deflected structure to determine which software performed best for creating the 

deflected FEM of the ACTE structure. As a result, validation of the analytical 

deflected shapes against the measured deflected shapes became an important 

objective to be satisfied during the P3.2B GVT. Other test objectives for the 

P3.2B GVT included quantifying changes in frequencies and mode shapes as a 

18. Ibid., 5–6.
The Adaptive Compliant Trailing Edge Investigation

105

function of flap deflection, evaluating various types of excitation methods, and 

determining what design variables to use in potential future FEM updates.19

The various ACTE flap components, as seen from aft of the wing. (NASA)

Results of the P3.2B GVT indicated that ANSYS consistently produced 

more accurate analytical deflection frequencies and mode shapes that better 

matched the GVT data. Because they took a conservative approach with the 

FEM in the flutter analysis, the analytical results were lower than GVT results 

for critical modes. Although a post-test FEM update was not required, the 

team performed one in order to more accurately determine material properties for the flexible structure. After being validated, these properties were 

inserted into the full-flap FEM, which was also validated and updated using 

the full-flap GVT results. While conducting these tests, engineers noted that 

a geometry change caused by applying different flap deflections significantly 

altered the internal loading of the structure. This created an apparent change 

in stiffness that was manifested in the test frequencies and mode shapes. It 

became necessary to apply multiple types of excitation to locations on the 

simulated wing and main flap sections to verify that the modes of interest 

identified through earlier analysis were adequately measured. The resulting 

19. Ibid., 7.
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GVT data were used to ensure correlation with analytical models. Testing also 

produced some unexpected results. Unexpectedly high damping values had 

to be empirically estimated due to some unique characteristics of the flexible 

structure, and researchers discovered an unpredicted mode at 30 degrees deflection. Engineers concluded that the truncated main flap section of the P3.2B 

test article probably engendered a lack of stiffness in the spanwise direction 

for the highly deflected flap.20

The team performed flutter analyses using ZAERO, a powerful engineering tool developed by Zona Technologies Inc., in Scottsdale, Arizona, that 

integrates essential disciplines required for advanced aeroelastic design and 

analysis. In order to facilitate this, researchers at AFRC first developed a baseline SCRAT aerodynamic model based on a half-model supplied by Gulfstream 

Aerospace and updating it to reflect the full aircraft configuration complete 

with ACTE flaps. Using a finished model that contained 3,021 flat-panel elements, researchers analyzed two fuel conditions (full and empty) and three flap 

settings from –2 to 30 degrees. Flutter frequency response was calculated for 

speeds ranging from 0.6 to 0.8 Mach.21

According to Tom Rigney, the greatest challenge was integrating the ACTE 

flap with the SCRAT testbed. “It was a major modification,” he said, “and it 

was absolutely critical to get the new flap to fit exactly right; it had to be very 

closely coupled with the wing.” There was little room for error because if either 

flap was just one degree out of alignment, it could make the airplane difficult 

to control. “When the ACTE flaps first arrived at Armstrong, they didn’t fit 

just right and we had to work very closely with the people in the machine shop 

to adjust the flaps and compensate for gaps, and things like that,” said Rigney. 

“We were eventually able to meet the requirements, but it wasn’t easy.”22

Testing of the complete SCRAT aircraft with flaps installed took place in 

the AFRC Loads Laboratory. Having access to a comprehensive set of data and 

accurate loads equations resulted in more precise flight-test data and enhanced 

safety of flight because researchers were able to expand the flight-test envelope 

without exceeding the aircraft’s structural limits. Before flying the airplane 

in the ACTE configuration, however, technicians first had to conduct specialized tests that involved applying loads to the aircraft via hydraulic jacks. 

Scientifically calibrated strain gages provided researchers with highly accurate 

measurements of the applied stresses, enabling them to predict the structural 

performance of the aircraft in flight. According to chief test engineer Larry 

Hudson, “Doing a test of this nature enables us to understand on the ground, 

20. Ibid., 8.

21. Ibid., 14.

22. Thomas K. Rigney, interview with the author, January 17, 2017.
The Adaptive Compliant Trailing Edge Investigation

107

by applying certain loads into the airplane, what loads will be experienced in 

flight under similar conditions.”23

Aeronautical engineer William Lokos monitors a wing loading test of the NASA Gulfstream G-III 

SCRAT at the NASA Armstrong Flight Research Center’s Loads Laboratory. (NASA) 

Lifting the airplane off the loads lab floor with the three inflatable airbags 

isolated the airframe from any potential influence the landing gear might 

exert upon strain-gage data. According to principal investigator Bill Lokos, 

the center’s loads lab had never previously employed this method. Technicians 

positioned the airbags beneath the wings and aft fuselage to keep the main 

gear tires off the floor thus ensuring that loading across the aircraft’s center 

wing box structure remained constant. This was necessary, Lokos explained, 

because increased loading on the wings typically resulted in decreased loads on 

the gear, which affected the strain-gage measurements and skewed the results 

of the preliminary equations. Supporting the G-III with standard aircraft jacks 

was not an option, he noted, because the effects were even more pronounced 

than those produced by the gear alone. Alternatively, using a cradle system 

would have been costly and time-consuming. Airbags of the type used to lift 

23. Peter W. Merlin, “Pumping It Up: Airbags Take the Weight in ACTE G-III Loads Tests,” June 16, 

2014, http://www.nasa.gov/centers/armstrong/Features/ACTE_G-III_loads_test.html (accessed 

September 18, 2016).
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aircraft in the field following gear-up landings were already available as proven, 

off-the-shelf hardware.24

Once the G-III was properly positioned, lab technicians applied loads with 

hydraulic jacks positioned underneath the wings. Structures lead Eric Miller 

explained that combining known loads values with strain-gage responses in the 

lab helped researchers develop a database for validating or correcting existing 

load equations. “We correlate these data so that we can drive our own load 

equations and be able to monitor flight loads in real time during ACTE flight 

tests,” he said.25

Flight-Testing ACTE

Now, the team was ready to begin collecting flight-test data to validate preflight 

modeling. Based on these analyses, both ACTE flap assemblies had been fully 

instrumented to capture the in-flight aeroelastic response for comparison to a 

set of predictions for each deflection test point. Engineers in the AFRC mission 

control room monitored the instrumentation suite via telemetry throughout 

every flight. Flight-test points that could be compared directly to analytical predictions were considered anchor points, but the ACTE flaps were also 

deflected to positions that were not analyzed. The results of these spot checks 

could be used only to verify trends between anchor points.26

Prior to takeoff, an aircraft crewmember manually excited the transition 

sections with a few sharp taps, and the response was measured through the 

onboard data collection system. While flying, engineers used response data provided during various maneuvers to estimate vibration frequency and damping. 

While airborne, excitation was provided primarily by light turbulence encountered during flight over the Edwards Air Force Base test ranges. After each test 

mission, the engineers evaluated mode symmetry by comparing recordings of 

accelerometer signal data.27

For the sake of simplicity and safety, the actuation system was not exercised during flight. Instead, the ACTE research team developed a build-up 

approach to flight-testing that entailed clearing the ACTE flight envelope 

through a series of specific test points in such a manner as to strategically 

increase dynamic pressure and Mach number throughout the performance 

envelope. Initial flights took place at low altitudes and speeds, followed by 

24. Ibid.

25. Ibid.

26. Herrera and Lung, “Aeroelastic Response of the Adaptive Compliant Trailing Edge Transition 

Section,” 16–17.

27. Ibid., 17.
The Adaptive Compliant Trailing Edge Investigation

109

tests at high-altitude and low speeds. The next step included high-altitude and 

high-speed tests and concluded with low-altitude and high-speed tests. “It was 

a build-up approach,” Rigney explained. “Each time, we increased the flap 

angle and airspeed parameters, and we expanded the [performance] envelope 

on every flight.”28

Gulfstream G-III SCRT ACTE testbed N804NA takes off on an early test flight, accompanied by a 

NASA AFRC F/A-18 chase airplane. (NASA)

During each flight, the AFRC mission control center was staffed to monitor 

mission-critical, safety-of-test, and safety-of-flight parameters. The SCRAT 

crew included two pilots and one engineer, and each mission was required 

to be accompanied by a safety chase aircraft. Each test mission consisted of 

a set of specific flight-test maneuvers to validate stability and control predictions, aerodynamics models, structural analyses, and aeroelastic predictions. 

The high-speed/high-altitude envelope provided flight limits for small flap 

deflections, and the low-speed/low-altitude envelope provided flight limits 

for large flap deflections.29

During the inaugural ACTE flight, the experimental control surfaces were 

locked at a 0-degree setting. Although the flaps were not actuated during flight, 

28. Rigney interview with author.

29. Herrera and Lung, “Aeroelastic Response of the Adaptive Compliant Trailing Edge Transition 

Section,” 16.
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110

NASA’s SCRAT testbed in flight. Note ACTE flap deflection. (NASA)

Flight research engineers monitor an ACTE test flight from the AFRC mission control room. (NASA)

a set of incremental fixed-flap settings were later employed on subsequent test 

missions to collect a variety of data demonstrating the capability of the flexible 

surfaces to withstand a real flight environment. Data from each of these test 

points were combined to extrapolate the behavior of the flap while extending 

and retracting throughout the flight envelope. “We have progressed from an 
The Adaptive Compliant Trailing Edge Investigation

111

innovative idea and matured the concept through multiple designs and windtunnel tests, to a final demonstration that should prove to the aerospace industry that this technology is ready to dramatically improve aircraft efficiency,” 

said AFRL program manager Pete Flick following the initial test.30

Mach number

ACTE flap 0: all

Altitude × 1,000 ft

0.2 0.4 0.6 0.8 1

55

50

45

40

35

30

25

20

15

10

5

0

1.3

Vs – ACTE

30 deg – 44 klb

350 KCAS

340 KCAS

300 KCAS

250 KCAS

220 KCAS

150 KCAS

170 KCAS

200 KCAS

Large

deflections

Small

deflections

1.3 Vs – ACTE 5 deg – 44 klb

G-III flight envelope with ACTE flaps installed. (NASA)

“The first flight went as planned; we validated many key elements of the 

experimental trailing edges,” said Tom Rigney, noting that now the team faced a 

variety of challenges including a tight schedule. “We had to minimize the downtime between flights, and the airplane had not been flown for a long time prior 

to the ACTE project, which created a lot of maintenance problems up front,” 

he recalled. The NASA maintenance crew thoroughly inspected the aircraft and 

made necessary repairs. Problems had to be addressed rapidly, and there were 

some surprises during the first few flights. “Once those were overcome,” Rigney 

noted, “we had to deal with new schedule pressures because we had less time to 

complete all of our flights.” He said that between flights, the team focused on the 

30. Peter W. Merlin, “ACTE Takes Flight,” The Armstrong X-Press, Vol. 56, Issue 7, November 2014, 

1–2.
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time it took to reduce the data, and made every effort to optimize the amount 

of work done between flights. “Normally, you have milestones that you monitor 

to make sure everything is on track,” he explained, “but I created what I called 

‘inch-stones,’ where everything is monitored very tightly to make sure we were 

on schedule and doing all the flights that we needed to.” He also kept AFRC 

managers apprised of potential conflicts, so they could resolve them as early as 

possible. “Overall, we hit our mark in terms how many flights we wanted and 

the amount of time we wanted them done in,” he said. “It’s hard to convey the 

amount of effort it took when there were so many people working hard at what 

they do, because when you’re successful, you make it look easy.”31

Every aspect of the flight-test process was designed to prove the radical new 

technology with a minimum of risk. Each incremental test point expanded 

the flight envelope enough to move on to the next step. The decision, made 

early in the project, to use fixed flap positions instead of actuating the flaps in 

flight was based on risk reduction and also allowed the team to move forward 

at an accelerated pace. “Actuators for ACTE will be developed using existing 

technology,” said Rigney. “We didn’t need to test that.” Backup instrumentation systems made it possible to continue flying even if one or more sensors 

failed during a flight. “The ERA project was really good at making sure we 

had enough emphasis on risk management,” he said. “It was a key factor in 

our success.”32

The ACTE project achieved a major milestone at AFRC on February 

18, 2015, when the modified G-III completed a flight with 15 degrees flap 

deflection, thus successfully meeting all of the project’s primary requirements. Throughout the flight test series, data were taken at points ranging 

from –2 degrees (up) to 30 degrees (down). Although the flexible ACTE flaps 

are designed to morph throughout the entire range of motion, each test was 

conducted at a single fixed setting in order to collect incremental data with a 

minimum of risk. Following the milestone flight, NASA ARMD Integrated 

Aviation Systems program director Ed Waggoner declared, “Reaching our minimum success criteria for the ACTE Integrated Technology Demonstration is 

a testament to the exceptional cooperation and collaboration toward the success 

of this flight campaign” between NASA, AFRL and FlexSys. “Every milestone 

we achieve helps us to better understand how these enabling technologies 

reduce aviation’s impact on the environment.”33

31. Rigney interview with author.

32. Ibid.

33. Peter W. Merlin, “Shape-Changing Flap Project Meets First Milestone,” NASA Armstrong News 

Features, March 11, 2015, http://www.nasa.gov/centers/armstrong/Features/acte_milestone.html

(accessed September 18, 2016).
The Adaptive Compliant Trailing Edge Investigation

113

Flight test of ACTE flap at 25 degrees deflection. (NASA)

By the end of April 2015, the team had successfully completed 23 ACTE 

test flights aimed at proving the airworthiness of the flexible structure and its 

ability to withstand high dynamic pressures at speeds up to 0.8 Mach and aerodynamic loads up to 11,500 pounds per flap segment at high deflection angles. 

Despite the complexity of the task, all primary and secondary objectives for 

the test series were successfully completed on schedule and within budget. “It 

was a very successful demonstration in that we accomplished all of our primary 

and secondary goals,” said Rigney, “we didn’t miss any requirements at all.”34

Moreover, he explained, ACTE technology is expected to have far-reaching 

effects on future aviation. Advanced lightweight materials will reduce wing 

structural weight and give engineers the ability to aerodynamically tailor the 

wings to promote improved fuel economy and more efficient operations, while 

reducing environmental impacts. “It also has the potential to save hundreds of 

millions of dollars annually in fuel costs,” he said.35

Researchers incorporated the results of the ACTE flight tests in subsequent 

LaRC design trade studies for future large transport aircraft. “Armstrong’s work 

with ACTE is a great example of how NASA works with our Government 

and industry partners to develop innovative technologies that make big leaps 

34. Rigney interview with author. 

35. Ibid.
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114

in efficiency and environmental performance,” said Jaiwon Shin. “This is 

consistent with the Agency’s goal to support the nation’s leadership in the 

aviation sector.”36

36. J. D. Harrington and Leslie Williams, “NASA Successfully Tests Shape-Changing Wing for Next 

Generation Aviation,” NASA News Release 15-072, April 28, 2015.

Overview of ACTE Flights Conducted by NASA

Flap Setting Cleared Alt. (ft) Max Q (psf) Max Accel (Gs)

20 High Speed Taxi NA NA

0 10,000 206 1.7

0 20,000 206 1.7

0 40,000 310 1.85

0 40,000 384 1.8

2 20,000 215 1.74

2 40,000 213 1.91

2 40,000 370 1.8

5 30,000 210 1.7

10 20,000 210 1.8

12.5 20,000 210 1.96

15 20,000 206 1.8

17.5 20,000 100 1.7

20 20,000 100 1.7

25 20,000 100 1.7

5 40,000 200 1.8

5 40,000 304 1.97

–2 20,000 206 1.7

–2 40,000 206 1.7

–2 40,000 245 1.7

–2 40,000 300 1.7

30 20,000 101 1.7

15 20,000 200 1.7
The Adaptive Compliant Trailing Edge Investigation

115

Evaluating Future ACTE Applications

Every now and then, NASA research leads to innovations that become industry 

standards. Two excellent examples include winglets and supercritical wings. 

Both represent simple, yet elegant, solutions to the problem of making airplanes 

more aerodynamically efficient. When first introduced in the early 1970s these 

technologies seemed revolutionary, yet today they are commonplace, having 

been integrated into a wide range of civil and military aircraft. Based upon its 

success in testing and its relatively high TRL, it is likely that ACTE technology 

will be similarly received by the aviation industry.

Several analyses of ACTE and similar technologies reported aerodynamic 

advantages derived from adaptive trailing edges. Sridhar Kota and others at 

FlexSys and AFRL projected a 3.3 percent improvement in cruise lift-to-drag 

ratio for a conventional medium-range transonic transport retrofitted with 

ACTE flaps.37 NASA researchers at LaRC conducted a study using the center’s 

Airborne Subscale Transport Aircraft Research (AirSTAR) testbed, a 5.5 percent dynamically scaled, remotely piloted, twin-turbine, swept-wing, generic 

transport model designed to provide an experimental flight-test capability for 

research experiments pertaining to dynamics modeling and control beyond the 

normal flight envelope. In this case, the AirSTAR was used to demonstrate flap 

camber shapes optimized with a vortex lattice code at multiple cruise conditions. The results showed induced drag improvements ranging from 1.2 percent 

to 9.9 percent depending on flight condition and the rate at which camber 

was allowed to change in the spanwise direction.38 A study at the University of 

Michigan examined the application of morphing trailing edges to a conventional long-range, twin-aisle transport configuration. Using a Navier-Stokes 

based aerodynamic optimization of a variable camber trailing edge flap on a 

777-size aircraft, researchers projected drag reductions from one percent at 

on-design conditions to five percent at off-design conditions.39

In May 2013, NASA sponsored a Boeing study evaluating the comprehensive effects of ACTE technology on a variety of commercial transport aircraft 

applications. Study goals included quantifying changes in weight, drag, and 

37. Kota, et al., “Mission Adaptive Compliant Wing - Design, Fabrication and Flight Test,” 18–19.

38. James Urnes, Nhan Nguyen, Corey Ippolito, Joseph Totah, Khanh Trinh, and Eric Ting, “A MissionAdaptive Variable Camber Flap Control System to Optimize High Lift and Cruise Lift-to-Drag 

Ratios of Future N+3 Transport Aircraft,” AIAA 2013-0214, presented at the 51st AIAA Aerospace 

Sciences Meeting, Grapevine, TX, January 7–10, 2013, 1–6.

39. Zhoujie Lyu and Joaquim Martins, “Aerodynamic Shape Optimization of an Adaptive Morphing 

Trailing Edge Wing,” AIAA 2014-3275, 52nd AIAA Aerospace Sciences Meeting, January 13–17, 

2014, 1–14.
Green Light for Green Flight

116

fuel burn caused by the introduction of ACTE technology to three aircraft 

configurations: a 224-seat HWB, a 222-seat conventional wide-body transport, and a 154-seat conventional narrow-body transport. Both the HWB and 

wide-body transport were designed for 8,000 nautical mile range at 0.85 Mach 

cruise speeds. The narrow-body transport was designed for 3,500 nautical mile 

range at an average cruising speed of 0.785 Mach. For the purpose of the study, 

conventional elevon and aileron controls on the outboard wing of the HWB 

were replaced with ACTE surfaces. Researchers converted the aft portions of 

the flaps on the tube-and-wing wide-body and narrow-body configurations to 

ACTE surfaces, allowing the flaps to be extended as single-slotted Fowler flaps 

in low-speed conditions to preserve high-lift characteristics.40

NASA’s AirSTAR subscale flight research vehicle was used to demonstrate optimized flap camber 

shapes at multiple cruise conditions. (NASA)

With flaps retracted for high-speed flight conditions, ACTE allowed the 

flap trailing edges to both control roll and alleviate aerodynamic loads. The 

latter thus made possible tailoring span-loading over a wide range of flight 

conditions and thus achieving more structurally efficient span-loadings in 

critical structural design conditions, as well as achieving more aerodynamically efficient span-loads in cruise. Having the capability to reduce bending 

moments through load alleviation made it possible to incorporate wings with 

lower structural weight than is typical, and improvements in cruise span-loads 

40. Wakayama and White, “Evaluation of Adaptive Compliant Trailing Edge Technology,” 2.
The Adaptive Compliant Trailing Edge Investigation

117

resulted in lower drag. Both of these characteristics contributed to reductions 

in fuel consumption. The study also revealed some penalties derived from 

using ACTE technology. These included higher weights for control surface 

structures, control surface actuators, and hydraulic systems needed to power 

the larger actuators. These weight estimates were based partially on results from 

the ACTE G-III flight demonstration at AFRL and might be offset by future 

improvements to the technology.41

Each configuration in the Boeing study was evaluated with and without 

application of ACTE technology. Because each model had a fixed wing area 

and engine thrust, the results demonstrated only the direct effects of ACTE, 

but not the augmentation of effects that would result from changing these 

variables. The benefits derived from the use of ACTE varied among the different aircraft, and were largely driven by the effectiveness of control surface 

arrangements and the importance of wing-weight reduction for each aircraft 

configuration. Researchers planned to use these results in subsequent studies to 

model the integrated effects of ACTE in combination with other technologies, 

as well as in scaling effects.42

Results of the initial study showed that the 224-seat HWB configuration 

derived the least amount of benefit from ACTE. Researchers found that the 

benefits of ACTE on span loading and load alleviation could have been accomplished just as easily with less weight penalty using the original control surfaces. 

Although some additional aerodynamic benefits might have been achieved by 

closing control-surface gaps, these effects were beyond the analysis methods 

used in this study. The most significant benefit demonstrated was a mere 0.7 

percent fuel burn reduction.43

ACTE surfaces were added to the aft portion of the flaps on the 154-seat 

narrow-body transport model. These flaps were configured with typical single 

slotted Fowler motion to provide high-lift capability, but the ACTE surfaces 

could be actuated even with the flaps stowed for high-speed flight conditions. 

In this case, the ACTE surfaces were used primarily for load alleviation and 

roll control. Researchers discovered that when taking some responsibility for 

roll control away from the ailerons, the ACTE surfaces freed the ailerons to be 

partially used for load alleviation. Additionally, use of the ACTE surfaces to 

transfer loads for roll control inboard effectively reduced bending moments 

during critical rolling maneuvers. In this configuration, ACTE technology 

41. Ibid., 2.

42. Ibid., 2.

43. Ibid., 11.
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provided a net benefit of 0.9 percent reduction in fuel burn, which was somewhat better than in the HWB configuration.44

Researchers found that ACTE provided smaller benefits to HWB configurations than to conventional tube-and-wing aircraft. (NASA)

The greatest benefits resulted from the 222-seat wide-body transport with 

ACTE surfaces added to the aft portion of the flaps. These modified surfaces 

worked with the outboard ailerons to provide load alleviation while the inboard 

ailerons were responsible for roll control. Although the original control surfaces 

could have provided effective load alleviation by themselves, the application 

of ACTE technology demonstrated larger benefits because the control surface 

arrangement was favorable for load alleviation, and because wing weight reduction had greater vehicle-level effects. Altogether, this configuration resulted 

in a 2.6 percent reduction in empty weight, 2.4 percent reduction in takeoff 

weight, and a 3.0 percent reduction in fuel burn.45

The Boeing researchers concluded that the arrangement of control surfaces 

may have a strong influence on the benefits of ACTE, and that optimization of 

the sizing and placement of control surfaces would better define the limits of 

this technology. They also noted that trading load alleviation benefits against 

penalties for control surface structure and actuator weight could also allow 

44. Ibid., 11.

45. Ibid.,12.
The Adaptive Compliant Trailing Edge Investigation

119

aircraft designers to size control surfaces in such a way as to provide greater load 

alleviation benefits. Most important, they felt that with further work ACTE 

technology could contribute significantly to fuel-burn reduction.46

46. Ibid., 12.
120

NASA researchers simulated the radiated sound field produced by a full-scale Gulfstream G-III 

aircraft during landing. (NASA)
121

CHAPTER 5

Reducing Airframe Noise

NASA’s major ERA goals included reducing and mitigating aircraft noise 

in communities adjacent to airports, a quest rendered more significant still 

because analysts predicted an average growth in U.S. carrier passenger flights 

of 2.2 percent per year between 2013 and 2033, with slightly higher-thanaverage growth in the first 5 years. An increase in takeoff and landing cycles 

could be expected to trigger an increase in noise complaints in communities 

surrounding affected airports.

The NASA Boeing 747 SOFIA (Stratospheric Observatory for Infrared Astronomy) flying low over 

the NASA Ames Research Center. Aircraft noise during takeoffs, climb-outs, approaches, and 

landings has a significant impact on quality of life in communities adjacent to airports and has 

been a long-standing research interest of NASA scientists, engineers, and technologists. (NASA)

For decades—well over a half-century—reducing and mitigating aircraft 

noise had been a NASA (and NACA before it) area of interest. Traditionally, the 
Green Light for Green Flight

122

greatest source of aircraft noise has been propulsion systems, particularly as the 

growl of piston-and-propeller systems gave way to the roar of turbojets. Over 

the past 50 years, air traffic increased dramatically and communities rapidly 

encroached on airport boundaries. Noise complaints soon became the numberone issue reported to FAA public liaison offices around the country. Solutions 

ranging from restricting the number of flights allowed per day to nighttime 

curfews for commercial air carriers had an adverse economic impact. Noiseabatement approaches and departure trajectories have mitigated the problem to 

some degree, but the most significant improvement was the advent of quieter 

turbofan engines. Advances in quiet engine technologies have resulted in such 

significant reductions in engine noise during takeoff and landing operations 

that, for current state-of-the-art aircraft, engine and airframe noise are nearly 

comparable. NASA researchers, therefore, sought to decrease airframe noise in 

order to minimize community exposure to aircraft acoustics. Primary sources 

of airframe noise include the wing high-lift system (slats and flaps) and the 

undercarriage (nose and main landing gear). Secondary sources include slat 

brackets, flap actuators, and gaps between airframe and control surfaces.1

According to Mehdi Khorrami, lead project investigator at LaRC and 

ERA noise-reduction element lead, a complete understanding of the problem 

requires full consideration of both engine noise and airframe noise contributions to an airplane’s acoustic signature. “You have to look at the problem 

from a system level,” he said, “because if you can reduce overall noise by 3 or 4 

decibels, you have effectively cut it in half.” ERA goals for N+2 civil transports 

called for a reduction of 32 decibels below current FAA Stage 4 standards, and 

a reduction of -42 decibels for aircraft entering service in the N+3 timeframe. 

“Our dream is to see Stage 4 minus 71 decibels,” said Khorrami, “because by 

then you have essentially confined aircraft noise to within airport boundaries.”2

To begin making these goals a reality, ERA managers established the Flap 

Edge and Landing Gear Noise Reduction ITD. Objectives for this effort 

included development of advanced modeling tools and capabilities that would 

enable aerodynamic and acoustic design considerations to be integrated concurrently early in the airframe design process, and innovations in effective 

noise-reduction concepts. One such concept consisted of modifications to 

the side edges (tips) of the flaps to mitigate noise source regions by altering local steady and fluctuating flow fields. For landing gear, the addition of 

1. Craig L. Nickol and William J. Haller, “Assessment of the Performance Potential of Advanced 

Subsonic Transport Concepts for NASA’s Environmentally Responsible Aviation Project,” AIAA2016-1030, presented at American Institute of Aeronautics and Astronautics SciTech, 54th AIAA 

Aerospace Sciences Meeting, San Diego, CA, January 6, 2016, 4.

2. Mehdi R. Khorrami, interview with the author, August 17, 2016.
Reducing Airframe Noise

123

aerodynamic fairings altered the flow fields impinging on various gear components in a way that minimized pressure fluctuations on the gear surfaces.3

Airflow over spoilers, flaps, slats, and associated actuator hardware is an important source of 

aerodynamic noise. (Peter Merlin)

During approach and landing, when engines are throttled back to reduce 

thrust, the noise generated by various parts of the airframe may be louder 

than that produced by the propulsion system. In order to effectively mitigate 

this problem, it is necessary to lessen the combined acoustic signatures from 

all major contributors concurrently. “As you reduce noise from one source,” 

Khorrami explained, “another one becomes more prominent.” The task is especially complicated because aircraft of different sizes and configurations offer 

different challenges. “Any protuberance in an area where it creates turbulent 

flows will generate airframe noise,” Khorrami said. “Turbulent vortex filaments 

interacting with the flaps and landing gear merge together and become stronger, creating pressure fluctuations on the aircraft surface that we hear as noise.”4

Initially, Khorrami led studies of the acoustic effects of specific structures 

such as the nose gear and flaps. “I was working with Fay Collier under the 

Subsonic Fixed Wing program and we made some measurements with a 

3. Nickol and Haller, “Assessment of the Performance Potential of Advanced Subsonic Transport 

Concepts for NASA’s Environmentally Responsible Aviation Project,” 4.

4. Khorrami interview.
Green Light for Green Flight

124

G550 aircraft in 2006,” he recalled. 

Researchers at LaRC then conducted 

further investigations with a smallscale, semi-span model of a G550 

to look at several aircraft components and try to understand their 

effect on airframe noise characteristics. In 2009, the team decided to 

expand the research from just looking at individual components to 

visualizing the entire airplane as an 

integrated system. “When the ERA 

project was created, we fully migrated 

[our research] to ERA because it was 

a technology development project,” 

Khorrami said, “and we wanted to 

develop new technologies to reduce 

airframe noise.” He added that, 

“During Phase 1 of ERA, our major 

focus was trying to see how much 

data we could get from scale models and with the same [G550] testbed we 

used in 2006, and in Phase 2 we made a full-blown effort to push our computational models.”5

NASA Langley research scientist Mehdi 

Khorrami. (NASA)

The first step toward designing viable noise-reduction technologies was 

to accelerate the development of high-fidelity simulation tools with accurate 

predictive capabilities. To this end, NASA partnered with Gulfstream Aviation 

for a comprehensive computational effort designed to advance the state of 

the art in airframe noise prediction from scale models to system-level, fullsize aircraft configurations. “First, we simulated airflow over a semi-span scale 

model of a G550 in landing configuration to accurately capture the prominent 

noise sources at both flap tips for the baseline configuration,” Khorrami said. 

“We then evaluated the effectiveness of noise-reduction technologies applied 

to the flap tips.”6

Advanced computing software was key to success. Researchers used both 

the NASA LaRC FUN3D unstructured Navier-Stokes flow solver and Exa 

Corporation’s PowerFLOW Lattice-Boltzmann flow solver to perform timeaccurate simulations. As efforts continued, the subscale half-span model was 

replaced with a full-span, full-scale baseline aircraft in landing configuration 

5. Ibid.

6. Ibid.
Reducing Airframe Noise

125

with wing flaps deflected 39 degrees and the main landing gear deployed. Once 

again, simulated airflow over the model’s virtual surface revealed the primary 

airframe noise sources associated with the G550. Khorrami explained, “We 

focused our efforts on accurately resolving the local flow fields at three locations: the flap tips, the regions near the main landing gear, and the interaction 

zones between these two areas.”7

7. Ibid.

Two visualizations of the simulated flow-field for a Gulfstream aircraft high-lift configuration 

depict complex, unsteady flow features at both flap tips. The upper image shows the formation 

of vortex filaments and their roll-up into a single, prominent vortex at each tip. The lower image 

shows the corresponding surface pressure fluctuation field (noise sources), corroborating windtunnel measurements obtained at NASA LaRC. (NASA)
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NASA researchers simulated the radiated sound field produced by a full-scale Gulfstream G-III 

aircraft during landing. (NASA)

High-fidelity simulations enabled researchers to probe into the underlying 

noise-generating mechanisms associated with airflow over the flaps and landing 

gear, and to develop potential solutions. Because these simulations presented 

numerous challenges due to geometric complexities and requirements for a 

wide range of spatial and temporal resolution, all computations had to be 

performed on Pleiades, one of the world’s most powerful supercomputers. This 

vital resource was located at ARC, where the NASA Advanced Supercomputing 

Division’s visualization team created renderings from the resulting datasets. 

According to Khorrami, even medium-resolution simulations required as many 

as 4,000 processors and 1.5 million processor hours and generated 40 to 50 

terabytes of data. “The knowledge gained from these computations allowed 

us to develop and improve several concepts that reduce the noise generated by 

these components,” he said.8

Wind-Tunnel Test Setup

Several methods for mitigating airframe noise underwent aeroacoustic testing 

in the NASA LaRC 14-by-22-foot subsonic wind tunnel during three test 

series, each lasting 4 to 5 weeks. The test article was an all-metal 18-percent 

scale, semi-span, high-fidelity model of a Gulfstream G550 aircraft. It was 

designed, fabricated, instrumented, and integrated at NASA LaRC based on a 

8. Ibid.
Reducing Airframe Noise

127

set of airframe geometry files provided by Gulfstream. Model details included 

a 185.4-inch-long half-span fuselage, a wing measuring 104.5 inches from 

the tunnel floor to the tip, a flow-through engine nacelle, engine pylon, and 

high-fidelity scale replicas of the flap and main landing gear.9

The initial test series, completed in November 2010, documented the 

model’s basic aerodynamic characteristics. At this time, researchers acquired 

measurements of such global forces as lift and drag, along with steady and 

unsteady surface pressure measurements. The second series of tests took place 

in February and March 2013. These experiments were dedicated to conducting comprehensive aeroacoustic testing of the model in a landing configuration, with and without flap/gear noise-reduction devices installed. The third 

and final test series was completed in April 2013. These tests were devoted to 

collecting off-surface flow measurements from the model in baseline landing 

configuration with emphasis on gear-flap flow interactions.10

Acquisition of acoustic measurements and determination of flyover sound 

directivity patterns for flap/gear noise sources necessitated that the wind-tunnel 

facility be operated in an open-wall mode in which the ceiling and the sidewalls 

of the tunnel test section were raised. This had been determined during the 

2010 series, when tests demonstrated that the model’s overall aerodynamic 

characteristics were largely unaffected regardless of whether the measurements 

were conducted with the test section in a closed-wall or open-wall mode. In 

order to reduce the adverse impact of background noise and improve the 

quality of the collected acoustic data, the tunnel had to be modified to operate in a semi-anechoic acoustic mode. In this configuration, the test-section 

floor, raised ceiling, and control room blast wall were all treated with soundabsorbing foam wedges and the sidewalls were covered with perforated mesh 

skins backed by foam. Even the floor area outside of the test section was covered 

with soft foam slabs to absorb sound.11

Acoustic measurements were obtained using a traversing microphone array 

constructed specifically for use in the LaRC wind tunnel. The phased array 

consisted of 97 individual Bruel and Kjaer (B&K) Model 4938 quarter-inch 

pressure-field microphones that were flush-mounted within an 8-foot-diameter 

flat fiberglass honeycomb plate. An operational frequency range of approximately 1.5 to 80 kHz was achieved by placing the microphones in an irregular 

9. Mehdi R. Khorrami, William M. Humphreys Jr., David P. Lockard, and Patricio A. Ravetta, 

“Aeroacoustic Evaluation of Flap and Landing Gear Noise Reduction Concepts,” AIAA Paper 

2014-2478, presented at the 20th AIAA/CEAS Aeroacoustics Conference, Atlanta, GA, June 

16–20, 2014, 2.

10. Ibid., 1–2.

11. Ibid., 9.
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circular pattern comprised of 16 “arms” with six microphones in each, and 

a single microphone positioned at the center of the array. The array panel, 

positioned at a lateral distance of 17.5 feet from the tunnel center line, was 

mounted on a rigid frame attached to two sets of 44-foot-long linear traversing 

rails. These rails were positioned in such a way as to allow the array to face the 

pressure side of the G550 model and permit smooth movement of the array 

panel in the streamwise direction along the full length of the test section. An 

additional set of six microphones was positioned around the phased-array panel 

for use in characterization of noise directivity for various model configurations. 

Given that airframe noise during landing operations was of primary interest, 

a 39-degree flap setting with the main landing gear alternately installed and 

removed were the most heavily tested configurations.12

Test setup in the LaRC 14-by-22-foot wind tunnel with the 18-percent-scale semi-span G550 

model and microphone array. (NASA)

Flap Noise Mitigation

In 2013, the research team experimented with several types of flap modifications to reduce noise. Because the model flap had been designed in a modular 

fashion, with removable inboard and outboard tips, it was possible to evaluate 

12. Ibid., 9–11.
Reducing Airframe Noise

129

more than 30 permutations of five different design concepts. These included 

a porous skin treatment, a variety of bulk and locally reactive acoustic liners, 

Flap Edge Noise Reduction Fins (FENoRFins) with both circular and noncircular cross sections, FLEXible Side-Edge Link (FLEXSEL), and Reactive 

Orthotropic Lattice Diffuser (ROLD). For all concepts tested, the extent of 

the tip area receiving treatment was carefully tailored to remain very small 

relative to the total flap surface area and the spanwise extent of treated area 

scaled with local maximum flap thickness. To maintain the flap’s overall aerodynamic efficiency, the first 10 percent of the flap chord at the leading edge 

remained unaltered.13

The porous tip concept involved substituting the upper, lower, and side 

surfaces of the flap with a wire mesh skin having a specific acoustic resistance. 

Treated surfaces produced a cavity at each tip that allowed interaction between 

the external flow and the internal cavity fluid (air). Computational models 

and earlier studies guided the design of the porous surface treatment, which 

was tailored to minimize aerodynamic performance penalties while achieving 

substantial noise reduction. To achieve these goals, technicians fabricated cagelike structures at each tip to accommodate a wire-mesh skin that was carefully 

spot-welded to the cage to avoid creating surface imperfections. This resulted in 

a treated area that was continuous with the remaining flap surfaces. Researchers 

produced four sets of treated inboard and outboard tips with respective skin 

surface resistances of 150, 270, 450, and 570 Rayl (a unit of acoustic impedance). To determine whether the concept performed differently depending on 

the size of the treated area, they also tested an additional set of 270-Rayl tips 

where the spanwise extent of the treatment had been reduced by 33 percent 

on both upper and lower flap surfaces.14

Researchers experimented with bulk and locally reactive acoustic liners, 

also applied to the flap tips. In one configuration, they fabricated a sixth set 

of porous tips with an extremely low resistive skin (20 Rayl mesh) and then 

filled the interior volume of the cage with bulk absorbent materials. Tests were 

conducted using such fill materials as Nomex Felt Model 4485R and FeCrAlY 

(iron-chromium-aluminum-yttrium) metal foam of varying densities. Experts 

at LaRC also designed and tested a locally reactive acoustic liner that featured 

a series of serpentine channels of various lengths terminating within the interior volume adjacent to the flap tips. These channels were designed to achieve 

optimal absorption over a wide range of acoustic frequencies. Due to the small 

scale of the test model, it was impossible to obtain the most efficient design 

13. Ibid., 3.

14. Ibid., 3.
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for maximizing sound absorption, but these tests served as useful proof-ofconcept demonstrations.15

FENoRFins are modifications consisting of an array of densely packed 

comb-like structures (fins) in a three-dimensional arrangement extending 

inward in the spanwise direction from the flap side edge to reduce airframe 

noise generated by the interaction between unsteady airflow and the flap edge 

surface. Limiting control action to the steady and fluctuating fields within a 

very small region near the edges leaves the gross aerodynamic characteristics 

of the flap unaltered, minimizing aerodynamic penalties. The aeroacoustic 

environment created by the FENoRFins alters the effective boundary condition 

at the surface of the flap, significantly reducing the steady-pressure differential 

experienced by the flap edge. As a result, the vortex formation process at the 

edge may be delayed or substantially weakened.16 For subscale testing, the 

spanwise extent of the fin region (that is, the length of each FENoRFins element) followed a slanted pattern with the shortest fins applied near the leading 

edge and the longest toward the trailing edge of the flap. Researchers evaluated 

several FENoRFins concepts and discovered that circular cross-section fins with 

the smallest diameter tested produced the best noise reduction.17

Researchers also experimented with a continuous-mold-line flap configuration. They conducted extensive testing of FLEXSEL, a structural-link design 

in which the flap side edge is connected to the wing edge adjacent to the 

flap with stretchable hyper-elastic materials. In order to effectively evaluate 

its noise reduction capability, the team tested both rigid polycarbonate and 

soft elastomeric versions of the FLEXSEL concept. An additional set of rigid 

FLEXSEL flap tips with a 40 percent larger transition section was fabricated 

to determine the effects of more extensive spanwise treatment on aerodynamic 

and acoustic performance.18

The final flap modification, known as ROLD, consisted of a honeycomblike, interconnected structure added to the region of the flap that experiences 

the highest degree of sound-generating airflow instability. ROLD combined 

several attributes of the porous skin treatment and liner concepts. All three 

surfaces in the flap tip region were porous, and the internal structure was comprised of passages or perforations orthogonally aligned in all three directions. 

The intersection of passages from each direction created a three-dimensional 

lattice structure that was orthotropic in nature (that is, having elastic properties 

15. Ibid., 4.

16. Khorrami interview.

17. Khorrami et al., “Aeroacoustic Evaluation of Flap and Landing Gear Noise Reduction Concepts,” 5.

18. Ibid., 5–6.
Reducing Airframe Noise

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in two or three planes perpendicular to each other).19 Researchers tested three 

sets of ROLD flap tips with different-sized holes, spaced evenly. Acoustic performance of the ROLD concept improved as the size of the holes decreased, 

with the smallest diameters producing the highest noise reduction.20

Each of the flap modifications tested could be easily retrofitted to existing 

aircraft structures, and several designs demonstrated significant noise reduction 

without adversely affecting aerodynamics. The porous characteristics of ROLD 

and the gaps between FENoRFins fin elements changed the boundary condition at the surface of the flap, reducing the steady pressure differential experienced along the flap edge. This reduced turbulent flow and delayed the type 

of vortex formation that causes noise during landing approach. Aerodynamic 

penalties were minimized by limiting the control action to the steady and 

fluctuating fields within a very small region near the flap edge. Testing of the 

18 percent scale model thoroughly validated earlier computational modeling. 

According to Mehdi Khorrami, “Comparisons of fluctuating surface pressures 

between untreated and treated flaps showed an order of magnitude reduction 

of pressure fluctuation amplitude by three to 5 decibels.”21

Main Landing Gear Noise Mitigation

Mitigating noise from the main landing gear posed a greater challenge because 

an aircraft undercarriage comprises a complex system of subcomponents of 

various shapes and sizes. Whereas all flaps are generally similar in shape, landing 

gear configurations vary widely from aircraft to aircraft depending on size and 

mission requirements. Consequently, researchers realized, no single concept 

could be expected to produce the desired noise-reduction levels. It quickly 

became apparent that they would have to approach the problem of gear noise 

reduction using multiple, complementary concepts tailored to specific noise 

sources associated with individual subcomponents.22

Installation of aerodynamic and acoustically absorbent fairings over various gear components offered a relatively simple and effective solution. NASA 

researchers pursued initial design studies under a collaborative effort with 

industry partners Goodrich Landing Gear Services, Exa Corporation, and 

Gulfstream. During the subsequent developmental phase, Goodrich employed 

19. Ibid., 6.

20. Mehdi R. Khorrami, William M. Humphreys Jr., and David P. Lockard, “An Assessment of Flap and 

Main Landing Gear Noise Abatement Concepts,” AIAA Paper 2015-2987, presented at the 21st 

AIAA/CEAS Aeroacoustics Conference, Dallas, TX, June 22–26, 2015, 3.

21. Khorrami interview.

22. Khorrami, et al., “Aeroacoustic Evaluation of Flap and Landing Gear Noise Reduction Concepts,” 7.
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high-fidelity computer simulations to obtain a set of viable fairing concepts. 

Designers then used these same computational models to further refine the 

most promising concepts to achieve designs that provided at least the minimum 

desired noise-reduction goals for each gear component.23

Upper fairings

Forward 

drag fairing

Spindle spin 

fairing

Upper plug 

fairing

Door rod fairing

Porous knee and brake fairings

Porous knee 

fairing

Original brake 

fairing

Main landing gear noise-reduction concepts employed a variety of aerodynamic and acoustically 

absorbent fairings over multiple components. (NASA)

To accurately evaluate the acoustic performance of the landing gear noisereduction concepts using the 18 percent scale model, technicians replaced the 

baseline Gulfstream landing gear with a modified set fabricated by Goodrich. 

Prior to installation, however, researchers placed the Goodrich gear into the 

Virginia Tech Stability Tunnel as a stand-alone test article. From September 

2011 until July 2012, they tested the effectiveness of numerous permutations 

of seven independent noise-reduction concepts. The results guided researchers 

in identifying the most effective technologies to be further evaluated on the 

half-span model in the 14-by-22-foot wind tunnel at LaRC in 2013.24

These technologies were simple but effective. First, technicians installed 

streamlined housings (collectively called “upper fairings”) over several existing components to mitigate turbulent-flow fluctuations across the gear strut. 

A plug filled the void between the forward drag brace and the strut, and the 

23. Ibid., 7.

24. Ibid., 7.
Reducing Airframe Noise

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landing gear door connecting rod was covered with an 

aerodynamic cover. Next, 

a more complex structure 

called a porous knee fairing 

covered the upstream main 

post, and a streamlined, 

tight-fitting fairing covered 

brake components located 

between the two wheels. 

Researchers were pleased 

to find that the knee fairing 

significantly diminished the 

noise-producing flow interaction between the front and 

rear struts.25

Wheel fairing

Porous 

wheel fairing

Solid and porous aerodynamic fairings designed were 

installed over the main wheels. (NASA)

An early brake fairing design performed well 

acoustically but was deemed 

impractical for application 

in a realistic environment at 

full scale due to brake cooling and maintenance issues. Instead, it was replaced 

with a larger aerodynamic fairing that partially covered the front portion of 

the two main wheels. Testing of both solid and partially porous wheel fairings 

proved very effective in reducing aerodynamic noise, while avoiding the pitfalls 

of the earlier brake fairing concept. The only challenge left was to reduce the 

low frequency noise associated with the wheel well cavity. The solution involved 

installing a stretchable mesh to cover the cavity opening. During tests on the 

isolated Goodrich gear in the Virginia Tech tunnel, mesh cover reduced lowfrequency gear noise by more than 2 to 3 decibels.26

Researchers also strove to understand the complex phenomenology of noise 

produced by interactions between airflow and two or more closely aligned airframe components. The test model was representative of many types of aircraft 

in use today, wherein the deployed main landing gear is located underneath the 

wing and aligned with the inboard tip of the deflected flap. In this configuration, the turbulent wake of the main gear typically impinges on the flap edge 

at the inboard tip. The research campaign provided an excellent opportunity 

for a detailed examination of gear-flap interactions in a realistic setting and 

25. Ibid., 7.

26. Ibid., 7.
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their effects on the airplane’s acoustic signature. Using various combinations of 

low-noise gear modifications and flap, researchers demonstrated that flap noise 

was largely unaffected by the deployed gear. The reduction in aerodynamic 

noise produced by the main landing gear was primarily attributed to a slowing 

of the flow impinging on the gear caused by flap down wash. As Khorrami 

noted, “We showed that installation effects have a pronounced impact on gear 

noise, which was substantially reduced when the flap was deflected downward 

[as on landing approach].”27

Developing Predictive Tools

Testing new noise-reduction technologies and hardware was only one aspect of 

the Flap and Landing Gear Noise Reduction ITD. The other involved development of new design tools to aid engineers in modifying existing wing flaps 

and landing gear—or designing new ones—that would produce significantly 

reduced acoustic signatures during takeoff and landing. In the latter, ERA 

researchers pushed the state of the art in airframe noise-prediction modeling 

that could be applied to everything from subscale test articles to a full-scale 

aircraft with all its complex geometries. “That was, I think, the most important 

thing to come out of our ITD,” Khorrami said. “By the end, we had developed 

a number of technologies that resulted in four or five patents, but I am most 

proud of the computational simulation. It was truly world class.”28

The complexity of the problem created a serious challenge for engineers 

and programmers. Among the most daunting tasks was the development of a 

virtual full–scale geometry model from very old airframe design data provided 

by Gulfstream. “These were data that had to be converted so we could use it in 

CAD [computer-aided design] models for both the wind-tunnel and computer 

simulation,” Khorrami explained, “and that took a lot of detailed effort.” But 

the resulting computational modeling software exceeded the team’s wildest 

expectations. “It was something most of us only dreamed of,” he said, “because 

we always thought it would happen perhaps 20 or 30 years from now. We put 

a lot of resources into it and a can-do mentality that we were going to push it 

until it couldn’t be pushed anymore.”29

Researchers compared preliminary computations to test results from the 

LaRC wind tunnel, which had originally been built in the 1970s as an aerodynamic testing facility. It was later modified to allow aeroacoustic testing 

of rotary wing configurations and, under the auspices of the ERA project, 

27. Khorrami, et al., “An Assessment of Flap and Main Landing Gear Noise Abatement Concepts,” 24.

28. Khorrami interview.

29. Ibid.
Reducing Airframe Noise

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underwent significant improvements to facilitate airframe noise testing of both 

conventional and unconventional aircraft configurations. In advance of tests 

with the G550 model, researchers measured tunnel background noise inside 

an empty test section, with and without airflow, employing an array of microphones positioned at specified flyover directivity angles that would later be used 

to acquire the model’s acoustic signature. Additionally, a series of pyrotechnic 

charges was detonated along the center line of the empty tunnel in front of the 

microphone array to identify significant unwanted sound reflections that might 

cause spurious returns. With careful planning and extensive acoustic treatments 

applied to the test-section floor, sidewalls, and ceiling, such interference was 

effectively eliminated.30

There were, however, still obstacles to overcome. When acoustic engineers 

compared integrated sound pressure levels from the microphone array (computed over the entire measurement grid) with the spectrum obtained from 

the individual microphone mounted at the center of the array, they discovered elevated tunnel background noise at both low and high frequencies. Jet 

shear-layer interaction with the flow collector lip, which was not acoustically 

treated, seemed the most likely cause of the higher background noise at low 

frequencies. Elevated background noise at high frequencies was mostly due to 

roughness of the tunnel floor, which was formed from wire cages containing 

sound-absorbing foam wedges. This background noise rendered measurements 

with individual microphones unreliable for determining absolute noise levels 

for the model and had to be accounted for when processing the final test data.31

Through direct comparison of computer-based models with wind-tunnel 

test data and acoustic measurements obtained during the 2006 flight test, 

Khorrami’s team successfully demonstrated that high-fidelity computer simulations could accurately predict the very complex fluid dynamic processes that 

generate airframe noise. “It was like a puzzle,” he said. “We used all the pieces 

to shed some light on the problem, and to benchmark our computational tools; 

to find out how we were doing, we looked at how the computer modeling compared to the wind-tunnel results and validated the results with flight-testing. 

It was a step-by-step, or building-block, approach.” The ultimate goal was to 

develop technologies that could be applied to most current and future aircraft, 

but, more important, to have the ability to predict the noise both in terms of 

a baseline aircraft as well as for whatever future configurations emerged, and 

to see how much improvement had been made.32

30. Khorrami, et al., “An Assessment of Flap and Main Landing Gear Noise Abatement Concepts,” 12.

31. Ibid., 12.

32. Khorrami interview.
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136

Modeling the acoustic signatures of flaps and slats was relatively easy because 

these aerodynamic devices have a high degree of universality. Whether installed 

on a regional jet, a 767, or something else, the basic configuration is very 

similar. “Obviously,” said Khorrami, “the noise-prediction tools that we have 

developed can be applied regardless of aircraft geometry, whether it is a tubeand-wing or a hybrid wing-body, and once you have the prediction tool, you 

can apply it to any concept, any configuration, and you can develop [noise 

reduction] technology for that particular configuration.”33

Gulfstream aircraft during landing (flaps deployed), showing significant reduction in fluctuation 

amplitudes with the application of noise reduction technology. Top image: untreated inboard flap 

tip; bottom image: tip with porous surface treatment. (NASA)

The problem of landing-gear noise is much more complex because each 

type of aircraft has a different undercarriage geometry. Depending on the size 

and weight of the aircraft, the nose gear may have one or two wheels and the 

main gear may have as many as three on each gear truck. The more complex 

the gear arrangement, the more auxiliary components are necessary. “All those 

little details can have an impact on how much noise is generated and how that 

33. Ibid.
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noise propagates,” Khorrami explained. “With a good prediction tool, you can 

see how the noise gets generated and figure out what other areas you have to 

go after, and then you can tailor your design as necessary.”34

Computational software for the project was adapted from an existing program developed by Exa Corporation for automotive applications. At the time 

that ERA began, Exa was seeking to expand the company’s business from 

automotive to aerospace. “It was a convergence of the right people at the 

right time,” said Khorrami. “We also used some of our own in-house code as 

a benchmark to make sure their software worked for what we wanted to do.” 

The Exa software proved well suited to highly complex geometries that challenged other codes. “During our project, [the Exa code] was greatly expanded 

and improved because we pushed the state of the art,” he added.35

Simulations performed for the ERA study constituted the first attempt to 

predict airframe noise radiating from a full-scale aircraft in landing configuration with most of the geometrical details included. NASA research results 

subsequently stimulated such aerospace industry giants as Boeing, Airbus, 

Gulfstream, Bombardier, Embraer, and others to consider applying these new 

predictive tools to current and future aircraft designs. Engine manufacturers 

also expressed interest. According to Khorrami, “Our study will help NASA 

mature existing technologies and eventually produce new ones that will confine 

aircraft noise footprints within airport boundaries.”36

ERA planners originally intended to follow up modeling and wind-tunnel 

testing with further flight tests of the G550 at NASA’s Wallops Flight Facility, 

Virginia, to gather steady-state surface pressures on the wing and flaps, as well 

as unsteady pressures on the flap edges, nose, and main landing gear. To accomplish this, researchers developed a ground-based microphone array to measure 

the sound footprint of an airplane in flight. Dubbed an “acoustic camera,” it 

was designed to provide detailed information on the location and intensity of 

various noise sources as the airplane passed overhead. “My only regret is that 

the flight tests for Phase 2 were not accomplished due to funding constraints 

at Gulfstream,” Khorrami lamented. “We were going to fly in 2014, but after 

the Critical Design Review something happened and the flights got cancelled.” 

After that happened, NASA Headquarters continued to express interest in 

conducting a full-scale demonstration at some point in the future. “So in 2016, 

after ERA, we moved the array to Armstrong Flight Research Center, where 

NASA has a Gulfstream G-III testbed aircraft,” said Khorrami. “It’s quite a 

34. Ibid.

35. Ibid.

36. Ibid.
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capable system,” he added, “so we are now trying to transfer that technology 

we developed under ERA to a flight demonstration at Armstrong.”37

Frequency (Hz)

1⁄3-Octave SPL (db)

102 103 104

Simulated (2014 PowerFLOW)

Measured (2006 Flight Test)

10 db

This comparison between measured and simulated far-field noise spectra for a full-scale 

Gulfstream aircraft during landing shows that high-fidelity computer simulations can accurately 

predict the very complex fluid dynamic processes that generate airframe noise. (NASA)

This setback forced Khorrami’s research team to push the computational 

simulation to its fullest capability. Fortunately, the structure of the ERA project 

provided the necessary resources to do this. “ERA was an unusual program 

because we knew exactly when it would start and when it would finish,” he 

said. “Headquarters was fully behind it throughout the 6-year span, and one 

of the reasons that ERA was so successful was that we had stable funding 

throughout.”38

37. Ibid.

38. Ibid.
Reducing Airframe Noise

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Sound Advice

Stephen Rizzi, a senior researcher for aeroacoustics at LaRC, led another team 

whose mission was to create technological tools capable of predicting and simulating sounds of flying machines still in the conceptual phase. Using computer 

models, flight measurements, and wind-tunnel data, they developed methods 

for predicting the sonic characteristics that would be produced by aircraft of 

any given configuration. These data were then turned into a set of synthesized 

sounds that were played for people who volunteered to be subjects in what were 

dubbed psychoacoustic tests—measurements of how humans react to different 

levels of aircraft noise. Rizzi’s system produced scientifically valid simulations 

of whirring rotors, roaring jet engines, and even the sounds of wind rushing 

over flaps and landing gear. Programmers had to consider such elements as 

distance, motion, the Doppler effect, and atmospheric influences on sound.39

This process of studying a test subject’s reaction, called “auralization,” is 

intended to help aircraft designers take noise into consideration when imagining new shapes and configurations. According to Rizzi, the automotive industry has been conducting similar tests for many years, but until recently there 

has not been a similar capability for aircraft designers. “By putting these pieces 

of prediction and auralization together,” he explained, “we have [created] a 

new capability.”40

The new noise-predicting software tool is called the NASA Auralization 

Framework (NAF). It incorporates a set of computer codes to transform noise 

predictions from numerical data into actual sounds. This is especially important if future aircraft are to be designed so as to cut noise to nearly one-eighth 

of what is currently allowed, and to confine the most annoying noise levels to 

within an airport’s outer fence line. Until now, NASA researchers have relied 

on a number of older computer-based tools including the ANOPP, which was 

first developed during the mid-1970s. This program allowed acoustic engineers 

to calculate how much noise would be made by aircraft components such 

as engines, flaps, slats, landing gear, and any other protuberances sticking 

out into the slipstream and causing localized, noise-making turbulence. The 

advent of unconventional aircraft configurations such as the HWB necessitated 

39. Sam McDonald, “New Acoustics Techniques Clear Path for Quieter Aviation,” October 14, 2014, 

http://www.nasa.gov/larc/new-acoustics-techniques-clear-path-for-quieter-aviation (accessed 

August 7, 2016).

40. Ibid.
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development of a new, more versatile version called ANOPP2 to analyze noise 

produced by aircraft that don’t follow the conventional tube-and-wing design.41

By 2013, it was abundantly clear that numerical predictions alone were 

insufficient. The NAF, therefore, was developed by NASA and its partners as a 

complementary tool to ANOPP. Rizzi explained that with the NAF, researchers 

input all the ANOPP/ANOPP2 aircraft noise prediction calculations, process 

the information and, within minutes, produce an electronic audio file that can 

be played on any compatible sound system.

“The vision is that an engineer working on the aeroacoustics of a new airplane would be able to readily generate an auralization output in the form of 

a calibrated [sound] file,” he said.42

Having a realistic sound file available provides several major benefits. The 

first concerns how hearing the sound helps verify and validate the engineering processes used to make the prediction in the first place. “If my prediction method is producing something that I know doesn’t sound realistic—for 

example, if I am trying to auralize an aircraft already flying today to use as a 

reference and the file sounds nothing like the real thing—then that tells me 

something is wrong or missing in the process,” Rizzi explained.43

Sound files can also be used to evaluate how people respond to aircraft noise 

from various configurations. Using a concept called “perception-influenced 

design,” aircraft manufacturers can use these data to make an airplane quieter 

and less of an annoyance to communities surrounding affected airports. For 

example, a listener may hear two different sounds at the same decibel level but 

perceive them very differently. According to Rizzi, it doesn’t matter how loud 

or quiet the sound is if it is perceived as annoying.

With airplanes, two different engines may be equally loud in terms of decibels but may sound differently because of the way they are designed, making 

one more annoying than the other. As Rizzi noted, “What we’re trying to do 

is not only lower the decibel level to meet future noise regulations, but we also 

want to make sure that the resulting sound isn’t objectionable to the public.”44

Rizzi believes the third major benefit is the capability to use sound files as 

a communication or public relations tool aimed at anyone who has a vested 

interest in how much noise an airplane makes near or at an airport. Interested 

parties include airport or airline officials, Federal regulators, state and local 

41. Jim Banke, “NASA Researchers Turn Noise Predictions into Sound—and Video,” June 13, 2016, 

https://www.nasa.gov/aero/nasa-researchers-turn-noise-predictions-into-sound-and-video

(accessed August 7, 2016).

42. Ibid.

43. Ibid.

44. Ibid.
Reducing Airframe Noise

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lawmakers, and most especially members of the general public living in close 

proximity to an airport. “With this tool we can share these sound files with a 

community so they can hear first-hand what a new jet engine or airplane design 

might sound like,” said Rizzi. “We can tell people we’ve made a reduction of 42 

decibels in noise levels, but what does that sound like? How does that compare 

with current levels? Now we can demonstrate that with realistic sound.”45

In May 2016, during an international aeroacoustics conference in France, 

NASA used the NAF to demonstrate how several noise-reducing technologies 

evaluated during the ERA project would deliver on the promise of cutting 

noise to nearly one-eighth of today’s standards. Prior to the conference, NASA 

researchers partnered with AMA Studios in London to combine the noise prediction sound files with flight-simulator-quality, computer-generated imagery 

representing two of the proposed future aircraft configurations studied for 

ERA. The result was a startlingly lifelike depiction that enabled designers to see 

as well as hear their work in action. Rizzi noted that the combined audiovisual 

presentation could also serve as a valuable tool to showcase to the public the 

dramatic reductions in perceived noise levels achieved by new aircraft designs.46

In the initial set of videos, the two different aircraft were shown from the 

perspective of someone standing near a runway as the airplane was taking off, 

as well as near the end of the runway directly under the final-approach path 

of the airplanes. One aircraft represented a typical twin-engine tube-and-wing 

configuration similar to a wide-body Boeing 777. The second aircraft was an 

HWB configuration with twin engines mounted on top of the airplane near the 

tail. “If you watch the videos and compare the sound of the two aircraft, both 

in terms of the overall decibel level and the annoyance factor, there’s no debate 

that the hybrid wing-body represents a huge reduction in perceived noise,” 

Rizzi said, adding that “The videos were well received by those attending the 

conference, with many [in the audience] noting how helpful it was to be able 

to compare the sounds of the two different aircraft shown.”47

Auralization studies have continuing at a steady pace since 2001, with support from a number of the Agency’s ARMD programs, including the Rotary 

Wing and Aeronautical Sciences projects of the Fundamental Aeronautics 

Program and ERA. Researchers at LaRC recruit test subjects as often as four 

times per year and pay them a modest amount to listen to sound simulations 

and register their reactions. Subjects sit in a small, theater-like room outfitted 

with 27 speakers and 4 subwoofers and answer questions on an electronic tablet 

while listening to simulated aircraft sounds. “We conduct a test, we analyze the 

45. Ibid.

46. Ibid.

47. Ibid.
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data, and we report on it,” Rizzi said. Research results will become increasingly 

valuable in years ahead due to predicted growth in commercial air traffic and 

the advent of new technologies such as drones for delivering consumer goods. 

“We’re in a leadership position on this,” Rizzi noted, describing his team’s 

forward-thinking acoustics research. “There is no organization I know of that 

has the capabilities that we do; it’s a one-stop-shop for this type of work.”48

Dr. Stephen Rizzi, NASA Langley Research Center’s Senior Researcher for Aeroacoustics, monitoring aircraft noise simulations played for test subjects. Much of the software used to simulate and 

test aircraft flight noise has been developed in-house at NASA’s Langley Research Center. (NASA)

He noted that his team had come a long way from initially focusing on 

making recordings of aircraft and reproducing those sounds in a controlled lab 

setting for test subjects. “That works well if the aircraft you’re interested in is a 

real aircraft,” he said, “but we work for NASA, so we’re more interested in the 

future and there are no recordings of paper planes; that led us down the path 

48. McDonald, “New Acoustics Techniques Clear Path for Quieter Aviation.”
Reducing Airframe Noise

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of having to synthesize the sound.” Rizzi knows what he’s talking about. He is 

a musician in his spare time and he sees a clear relationship between the science of noise and the art of melody. “There’s a lot of overlap in the techniques 

used to generate the musical kinds of sounds and those used with aircraft noise 

synthesis,” he said. It’s why he enjoys his work so much. “It’s new and there 

aren’t a lot of other people doing it.”49

In the spring of 2016, Rizzi’s team demonstrated an NAF simulation of a 

proposed open-rotor engine for a group of engineers from General Electric. 

They were evidently excited to hear the sound of the latest power plant, even 

before it actually flew. “They really started thinking, ‘Wow, what can we do 

[to change the character of the sound and make it more acceptable]?’” Rizzi 

said. “When you get a reaction like that based on your work, that’s a pretty 

gratifying experience.”50

49. Ibid.

50. Ibid.
144

National Full-Scale Aerodynamics Complex (NFAC) 40-by-80-foot test section with a Boeing 

HWB model. The size of the NASA personnel gives a sense of the immense size of this 70-yearold facility. (NASA)
145

CHAPTER 6

Integrated Propulsion 

System Technologies

ERA researchers pursued several technologies for improving aircraft jet engine 

performance in ways that would simultaneously reduce fuel-burn, noise, and 

emissions to meet national objectives. Project goals for fuel burn called for an 

immediate reduction of 33 percent relative to optimum performance standards 

as of 2005, and of at least 50 percent by the year 2020. Within these same 

timeframes, researchers hoped to reduce NOx emissions by 60 percent and 75 

percent, respectively, relative to CAEP 6 standards. Goals for noise reduction 

were based on effective perceived noise levels measured in decibels (EPNdB), 

a measure of the relative loudness on an individual aircraft during a 10-second 

duration pass-by event. In the immediate, researchers strove to reduce aircraft 

noise footprints by 32 EPNdB relative to FAA Stage 4 requirements, and by 

42 EPNdB by 2020.1

Working in collaboration with Pratt & Whitney, General Electric, and the 

FAA, GRC completed three engine-related integrated technology demonstrations. Promising technologies selected for maturation included:

• An improved two-stage transonic high-pressure engine compressor 

for an advanced UHB turbofan,

• Integration of a second-generation ultra-high-bypass-ratio propulsor 

(mechanism consisting of fan, stators, and nacelle) to improve propulsion efficiency and reduce the noise of a geared turbofan engine,

• An improved design for a jet engine combustor to reduce NOx emissions, and

• Verification of HWB power plant and airframe integration concepts 

that would allow fuel consumption reductions in excess of 50 percent 

while simultaneously reducing noise on the ground.

1. Dale E. Van Zante and Kenneth L. Suder, “Environmentally Responsible Aviation: Propulsion 

Research to Enable Fuel Burn, Noise, and Emissions Reduction,” ISABE 2015-20209, presented 

at the 22nd International Symposium of Air Breathing Engines, Phoenix, AZ, October 25, 2015, 13.
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These demonstrations successfully advanced the maturation of compressor, 

propulsor, and combustor technologies for the next generation of jet engines.2

Inside the 8-by-6-foot wind tunnel at NASA GRC, engineers tested a fan and inlet design, commonly called a propulsor, which uses 4 to 8 percent less fuel than today’s advanced aircraft. 

Designed to be embedded in the aircraft’s body, it would ingest the slower flowing boundarylayer air that normally develops along an aircraft’s surface and use it to help propel the aircraft 

more efficiently. (NASA)

Turbine Engine Design and Performance

Gas turbine engines have been a fundamental part of aviation since the beginning of the Jet Age in the early 1940s. The two most common variants are the 

turbojet, which produces thrust from the direct impulse of exhaust gases, and 

the turbofan, in which a set of ducted fan blades produces additional thrust. 

Turbofans tend to be inherently less noisy than turbojets and are thus better 

suited to requirements for quieter aircraft.

In a jet engine, the compression system feeds high-pressure air into the combustion chamber (combustor), where it is heated before passing through a set 

of nozzle guide vanes to the turbine. Most modern designs are swirl-stabilized, 

creating a low-pressure zone and generating turbulence in the flow to rapidly 

mix fuel with air. However, the higher the turbulence, the higher the pressure 

2. Ibid., 13.
Integrated Propulsion System Technologies

147

loss for the combustor, so the swirler must be carefully designed so as not to 

generate more turbulence than necessary to sufficiently facilitate combustion.3

The combustor converts chemical energy bound in the fuel into thermal 

energy to drive the compressor. The amount of thrust produced by a turbofan 

engine is the product of the mass of air per unit time and the change in velocity 

imparted to that mass. It is more efficient to impart a smaller velocity change on 

a large volume of air than to make a large velocity change to a smaller volume of 

air; the more efficient the engine, the less fuel is consumed. For these reasons, 

designers of turbofan technology have been developing engines with higher 

bypass ratios (BPR) and lower fan pressure ratios (FPR) in order to produce 

more efficient power plants with a high operating pressure ratio (OPR). One 

significant obstacle is the necessity to increase fan diameter to maintain consistent thrust. Increasing fan size adds weight and produces requirements for 

larger engine nacelles that add drag at higher airspeeds. More thrust is needed 

to overcome the additional drag.4

Engine core size is another important factor because fan speed must be kept 

as low as possible to reduce its noise signature. However, since a common drive 

shaft connects the core components and the fan, lower fan speeds necessitate 

lower compressor and turbine component speeds in the engine core as well. 

A specific amount of power is required to drive the fan for a given amount of 

thrust. Therefore, as the fan speed drops, the core components must increase 

in size, both in number of stages and in stage diameter, to provide the necessary power since the core component speeds cannot increase. Larger components, or increased numbers of stages, add more weight to the engine, thus 

requiring increased fuel burn to carry that weight around. At some point, fan 

size increases to a point where the added weight of the fan and nacelle, the 

additional nacelle drag, and the increase in core weight overcomes the fuel 

advantage of the higher BPR and lower FPR design.5

The fuel-burn trend line for a given engine cycle eventually drops to a 

minimum where increasing engine BPR beyond a certain point yields negative 

results, with higher fuel burn as increases in engine weight and size overcome 

the benefits of a high-BPR engine cycle. With the technology available at the 

start of the ERA project, this phenomenon resulted from the conventional 

high-bypass engine cycle known as direct drive, in which both fan and core 

3. Christopher E. Hughes, Dale E. Van Zante, and James D. Heidmann, “Aircraft Engine Technology 

for Green Aviation to Reduce Fuel Burn,” NASA TM-2013-217690, June 2011, 3.

Van Zante and Suder, “Environmentally Responsible Aviation: Propulsion Research to Enable Fuel 

Burn, Noise, and Emissions Reduction,” 3.

4. Ibid., 3.

5. Ibid., 3–4.
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components are rotating on the same shaft. Because the fan, compressor, and 

turbine all rotate at the same speed, system operation is constrained by the 

lowest-speed component, which is the fan in the propulsor. A low-speed fan 

design forces the core to run at slower, less-efficient speeds. As a result, more 

compressor and turbine stages are required to produce the necessary power to 

drive the fan and provide sufficient thrust at a given operating speed. Adding 

more stages increases engine weight.6

Basic operation of turbojet, turbofan, and geared turbofan engines. (NASA)

Pratt & Whitney designers sought to solve this problem by introducing a 

paradigm-shifting advanced technology, the geared turbofan (GTF). In the 

GTF, the fan and the core components are separated by a gear system that 

allows the fan and the core to operate at different speeds. This allows the core 

to operate more efficiently and produce desired thrust levels at the fan stage 

6. Ibid., 4.
Integrated Propulsion System Technologies

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using fewer compressor and turbine stages compared to a direct-drive engine. 

The net result is reduced engine weight and, therefore, greater fuel economy.7

Pratt & Whitney developed the first-generation GTF in partnership with 

NASA. Work began in 2006 with testing of a scale model in the 9-by-15-foot 

low-speed wind tunnel at GRC. These tests validated the predicted aerodynamic, acoustic, and aeroelastic design characteristics of the GTF. Pratt & 

Whitney conducted the first full-scale GTF engine demonstration in 2008, 

followed shortly thereafter by the first flight-test demonstration using the company’s modified Boeing 747SP flying testbed. The first-generation GTF first 

entered service in January 2016 as the PW1100G-JM. During development, 

NASA and Pratt & Whitney collaboratively investigated a low-speed/lowpressure-ratio fan, fan gear system, low-emissions combustor, and a compact 

high-speed low-pressure spool. These technologies enabled higher fan efficiencies by pushing engine BPR into the UHB ratio range of 12 or greater while 

allowing for FPR reduction of between 1.3 and 1.4.8

The PW1100G represented N+1 technology. At this level, researchers 

expected the GTF to once again reach a maximum benefit point as the design 

BPR continued to increase while FPR decreased. In order to achieve ERA 

N+2 system-level environmental goals, this necessitated a second technological 

paradigm shift to significantly extend the beneficial fuel-burn trend line. To 

this end, NASA and Pratt & Whitney again partnered to investigate secondgeneration GTF propulsor technologies to enable a BPR up to 18 and an FPR 

of between 1.25 and 1.3. Researchers projected the second-generation GTF 

would provide a 25- to 30-percent reduction compared to an Airbus A320 

powered by two Pratt & Whitney V2500 engines.9

Another unconventional propulsion technology considered during ERA 

was the prop-fan, or “open rotor,” engine. NASA had previously explored this 

concept, also known as an “un-ducted fan” (UDF), as early as the mid-1980s 

but it was never adopted by the aircraft industry. Superficially, the front end 

of the UDF resembled that of a turbojet or turbofan, featuring a conventional 

circular air inlet. At the back end of the nacelle, however, it sported an unshrouded, contra-rotating propeller consisting of a large number of short, 

twisted fan blades.10

After initial flight testing, the UDF was largely shunned due to the excessive cabin noise it generated compared with turbofan engines. ERA researchers chose to revisit this technology because it offered the potential for large 

7. Ibid., 4.

8. Ibid., 4.

9. Ibid., 6.

10. Ibid., 7.
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increases in propulsion efficiency and, therefore, significant reduction in fuel 

burn compared with then-current high-BPR turbofans. Open rotor engines 

represent the ultimate in high-BPR propulsion, typically providing BPRs 

between 40 and 80 as well as very low FPRs (below 1.1) compared with other 

types of aircraft power plants, and thus offering very high propulsive efficiency 

overall. Designers at General Electric Aviation predicted that their open rotor 

concept had the potential to reduce fuel burn by as much as 25 to 30 percent compared to such modern turbofan engines as the company’s own GE 

CFM56-5B.11

With renewed emphasis on reducing the environmental impact of commercial aircraft, NASA partnered with General Electric to investigate open 

rotor propulsion for ERA N+2 generation aircraft systems. As a starting point, 

the company built upon its experience with the GE36 proof-of-concept UDF 

demonstrator that had been successfully flight-tested in 1986 and 1987, winning the prestigious Collier Trophy. By 2009, designers of a new generation of 

open rotor engines had access to tremendous increases in aerodynamic design 

capability in computational fluid dynamics, advanced high-speed computers, 

and strong lightweight-materials technology that had been developed over the 

preceding two decades. General Electric designers could now take advantage 

of advanced tailored-construction techniques to optimize fan blade shapes for 

maximum performance and minimum noise. In 2010, this effort spawned 

an extensive series of tests at GRC of a new generation of advanced fan blade 

technology, sponsored through both the SFW and ERA projects.12 This led to 

three ERA propulsion ITDs that were focused on increased thermal efficiency, 

increased propulsive efficiency and noise reduction, and low-NOx combustor 

designs for high pressure ratio engines.13

Highly Loaded Front Block Compressor Demonstration

The first demonstration, ITD30A, sought to improve overall aircraft engine 

efficiency with a view toward reducing TSFC and total fuel burn, as well as those 

enabling technologies needed for developing high-OPR core engines. TSFC 

quantifies the fuel efficiency of an engine design with respect to thrust output. 

Researchers hoped to increase both efficiency and core pressure by 30 percent 

relative to ERA baseline engine specifications in order to achieve a 2.5 percent 

TSFC reduction. To this end, the Highly Loaded Front Block Compressor 

11. Ibid., 7.

12. Ibid., 8.

13. Van Zante and Suder, “Environmentally Responsible Aviation: Propulsion Research to Enable Fuel 

Burn, Noise, and Emissions Reduction,” 3.
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151

Demonstration included two test and analysis campaigns to improve both 

propulsive efficiency and engine core thermal efficiency. In order to meet ERA 

goals, designers had to increase engine OPR and efficiency without adversely 

affecting system weight, length, diameter, or operability. In the first campaign, 

which took place during ERA Phase 1, researchers investigated the performance of the two front stages of a legacy high-OPR six-stage core compressor. 

ERA Phase 2 focused on two major variants of a new compressor design.14

For ITD30A, NASA researchers partnered with designers at General 

Electric to refine the design of the open rotor compressor stage of a UHB 

turbofan engine. This work mainly focused on increasing engine core thermal 

efficiency by boosting the engine’s OPR via a pressure ratio increase in the 

high-pressure compressor (HPC). Boosting OPR by adding more compressor 

stages would also increase the engine’s weight and complexity, so the team 

chose a novel approach. They opted to enhance the capability of the HPC’s 

“front block” (comprising the first three stages) through increased stage loading 

while simultaneously maintaining compressor efficiency comparable to that of 

state-of-the-art HPCs. For the purposes of the ERA demonstration, technology development and maturation activities focused on improving the pressure 

rise across the first three stages of a 30:1 class HPC. According to James D. 

Heidmann, project engineer for the ERA Propulsion Technology subproject, 

the primary objective was to validate innovative engineering concepts and 

models for higher OPR in an advanced technology development compressor 

(ATDC) testbed to achieve NASA fuel burn and NOx emissions goals.15

Phase 1 Demonstration

In preparation for ERA Phase 1 testing, NASA collaborated and developed cost-sharing protocols with industry partners, academia, and other 

Government agencies to create and operate the ATDC testbed in the W7 

Multi-Stage Compressor Test Facility at NASA Glenn Research Center. During 

FY 2010–2011, this necessitated refurbishing the test rig’s straddle-mounted 

driveline, balancing a 640-pound five-stage checkout rotor, and installing a 

new high-temperature throttle valve. The facility’s 15,000-hp synchronousdrive motor was capable of operation between 300 and 3,600 rpm and was 

equipped with a 5.21:1-ratio gearbox enabling a maximum compressor shaft 

speed of nearly 20,000 rpm. NASA researchers worked with Pratt & Whitney, 

General Electric, and Rolls-Royce to define the basic operating envelope for the 

W7 compressor rig while also designing and fabricating the ATDC Phase A 

14. Ibid., 3–4.

15. James Heidmann, “NASA’s Current Plans for ERA Propulsion Technology,” presented at the 48th 

AIAA Aerospace Sciences Meeting, Orlando, FL, January 4–7, 2010, 1–8.
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test article.16 Ultimately, NASA was responsible for refurbishing the W7 facility and conducting the ATDC tests. General Electric received a contract to 

provide an advanced two-stage compressor test article and additional test support. Research goals focused on making steady and unsteady flow measurements, initially for the first stage by itself and then subsequently after adding 

the second stage. This would enable detailed evaluation of the performance 

and losses in each stage.17

Schematic rendering of the W7 Test Facility at GRC. (NASA)

Baseline and developmental testing of the state-of-the-art transonic highpressure compressor began in FY 2012. Researchers primarily sought to 

understand the flow physics that typically limit stage loading, characterize 

interactions between the various rows of compressor blades, and validate the 

design methodology and capability of predictive tools through comparison 

with experimental results. Initial checkout of the W7 facility driveline was 

conducted in June 2012. When technicians operated the driveline motor up 

16. Ibid.

17. Kenneth L. Suder, “Overview of the NASA Environmentally Responsible Aviation Project’s Propulsion 

Technology Portfolio,” AIAA-2012-4038, presented at 48th AIAA/ASME/SAE/ASEE, Joint Propulsion 

Conference, Atlanta, GA, July 33–August 1, 2012, 16.
Integrated Propulsion System Technologies

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to 9,000 rpm, they detected a vibration in the gearbox that was later diagnosed as an issue with the coupling between the gearbox and compressor shaft. 

Modifications to the coupling installation eliminated the problem.18

In September, General Electric delivered the compressor test rig to GRC, 

where it was installed in the W7 facility for testing. Researchers then validated 

the bi-directional capability of the W7 motor drive, and worked to increase 

flow quality and reduce measurement uncertainty within the multistage compressor facility. The results of pretest computational fluid dynamics (CFD) 

simulations were compared to experiment data to determine the breakout of 

the Stage 1 and Stage 2 blade-row interaction effects that limit stage loading 

and high-OPR capability. General Electric performed system studies that indicated the highly loaded front block compressor would provide a 25 percent 

increase in OPR relative to that of the GE90, then the world’s most powerful 

turbofan engine. Notably, this would result in a 2.0 to 2.5 percent reduction 

in TSFC.19

When these tests concluded in 2015, the results indicated that highly loaded 

core compressor technology developed under the ERA project had successfully 

realized fuel-burn reduction goals by increasing compressor OPR to increase 

the engine’s thermal efficiency. Contemporary turbofan engines then in service 

operated at an OPR of 30 to 45. The new research results potentially paved the 

way for engines with an OPR between 60 and 70, nearly doubling the compressor pressure ratio performance while retaining a high level of efficiency. Because 

previous General Electric test experience of a similar two-stage compressor had 

failed to meet high-speed efficiency goals due to unpredicted pressure losses, the 

new test article was designed to run in both one-stage and two-stage configurations in separate tests to assess whether interaction between the bow shock of 

the second rotor and the upstream stage contributed to the anomaly, or if the 

anomaly was due to interaction between the first-stage rotor and stator sections. 

Losses encountered in the earlier tests were not fully understood and had not 

been predicted by sophisticated CFD simulations, including multi-blade-row 

unsteady modeling of the inlet guide vane through Rotor 2.20

The primary research goal of the next set of tests was enabling engineers to 

fully understand first-stage performance under isolated and multistage conditions. Secondary goals included developing a detailed set of aerodynamic data 

18. Ibid., 16.

19. Ibid., 16–17.

20. Patricia S. Prahst, Sameer Kulkarni, and Ki H. Sohn, “Experimental Results of the First Two Stages 

of an Advanced Transonic Core Compressor Under Isolated and Multi-Stage Conditions,” GT2015-

42727, presented at the ASME Turbine Technical Conference and Exposition, Montreal, Canada, 

June 2015, 1–2.
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for CFD model validation. Researchers initially ran the compressor in a onestage configuration to fully characterize and understand Stage 1 in isolation, 

and then subsequently ran both stages together. This allowed them to isolate 

the effect of the Rotor 2 bow shock as it impinged on the upstream blade rows. 

Advanced diagnostic instrumentation provided data that allowed the team to 

fully understand the loss mechanisms, thus permitting designers to develop 

highly loaded front stages that mitigate the identified losses and permit the 

core compressor to reach target efficiency levels.21

General Electric provided a test article that included an air inlet and the 

first two stages of a highly loaded axial compressor. The first test assembly 

consisted of an inlet, fan frame struts, guide vanes, Rotor 1, and Stator 1, 

with a downstream de-swirl vane to maintain axial flow. The configuration 

for the second test consisted of all of these components plus a transition duct 

from the low-pressure compressor to the high-pressure compressor, a second 

rotor and stator, and no de-swirl vane. The inlet guide vanes and both stators 

were of a variable geometry configuration and were designed to follow a vane 

schedule corresponding to the rotation speed. Researchers acquired data at 

various off-schedule vane angles in order to assess performance at different 

rotor-loading values.22

Technicians installed a variety of instrumentation to obtain performance 

maps during the single-stage compressor test. Most of these data were acquired 

at 97 percent and 100 percent Nc (that is, corrected speed, the speed at which 

a component would rotate if the inlet temperature corresponded to ambient 

conditions at sea level, on a standard day). Pressure rakes mounted at mid-pitch 

of the struts in five circumferential locations allowed researchers to establish 

inlet total pressure and temperature profiles. Steady-state static pressure ports 

located above the rotor tips identified rotor start and unstart conditions and 

captured rotor shock and tip vortex data. Two rows of high-response pressure transducers measured the unsteady pressure over the rotor to provide a 

detailed perspective of the rotor static-pressure field. Two stator vanes were 

instrumented with total pressure probes along five radial locations on the 

leading edges to obtain rotor performance data, and two others were similarly 

instrumented with temperature sensors. Five circumferentially spaced rakes 

situated downstream of Stator 1 at the leading edge of the de-swirl vane measured stator exit flow. Additional data were provided by sensors that captured 

casing and hub static pressures along the entire flow path from the inlet through 

the diffuser section. Researchers determined overall performance of the onestage configuration by measuring the difference between readings taken at the 

21. Ibid., 2.

22. Ibid., 2.
Integrated Propulsion System Technologies

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inlet and exit rakes. Detailed measurements with a traversing probe averaging 

data taken at four points across the diameter of the exit duct provided finer 

data definitions.23

CFD model of the flow through an inlet and through rotor and stator blades. (NASA)

The research team then installed the second stage on the test article and 

repeated the data runs. This design configuration pushed the design to higher 

blade loading levels (pressure rise per stage) but failed to meet efficiency goals. 

Test results indicated that Stage 2 was choking at a mass-flow rate that prevented Stage 1 from reaching peak efficiency. Upon reviewing the data from 

both configurations to determine the most likely cause of this stage mismatch, 

researchers found that Stator 1 performed equally well with or without the 

presence of the second stage. They concluded that the performance deficit was 

likely from a first-stage loss not predicted by available design tools. Phase 1 

testing yielded a vast amount of high quality data, but much work remained. 

Most important, the resulting data set could be used to validate CFD models 

and help determine how to redesign the compressor system while accounting 

for loss mechanisms.24

23. Ibid., 2–4.

24. Ibid., 10–11.
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CFD visualization illustrating rotor tip leakage. (NASA)

Phase 2 Demonstration

Flow across the span of the front two compressor stages was transonic. As a 

result, stage performance was very sensitive to changes in the effective flow 

area. Such changes can affect flow separation, as well as cause low momentum 

and loss due to passage shock and/or blade row interactions. During Phase 

1 testing, researchers sought to understand the flow physics that resulted in 

high losses, characterize blade row interactions and their impact on loss, and 

validate the design methodology and capability of the prediction tools through 

comparisons with experimental results. Phase 2 testing involved a completely 

new core compressor design strategy that leveraged lessons learned from the 

Phase 1 compressor design. This new compressor was designed for increased 

efficiency and higher blade loading.25

The primary goals of Phase 2 were to realize higher efficiency levels than 

those of Phase 1 and increase blade-loading levels relative to those of the best 

design then available, but not to the higher levels of blade loading attempted in 

the Phase 1 design. The Phase 2 compressor test program was undertaken in the 

third and fourth quarters of FY 2015. It consisted of two tests designated Build 

1 and Build 2, where the primary difference was that Build 2 was designed to 

achieve higher compressor blade loading at the same efficiency levels as Build 1. 

The higher blade loading of Build 2 provided an overall system benefit because 

25. Van Zante and Suder, “Environmentally Responsible Aviation: Propulsion Research to Enable Fuel 

Burn, Noise and Emissions Reduction,” 4.
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157

compressor bleed locations could be moved further upstream, reducing the 

compressor work required to provide the bleed flow.26

A Pratt & Whitney next-generation ultra-high-bypass-ratio turbofan model undergoing testing in 

the NASA Glenn Research Center’s 9-by-15-foot Low Speed Wind Tunnel. (NASA)

Second-Generation Ultra-HighBypass Propulsor Integration

During another demonstration (ITD35A) in late 2011, NASA researchers 

worked with Pratt & Whitney to mature several low-FPR, UHB propulsor 

technologies through a series of collaborative tests of a scale model secondgeneration GTF in the NASA GRC 9-by-15-foot low-speed wind tunnel. Prior 

to ERA, first-generation GTF testing demonstrated the potential efficiency 

gains that could be achieved with low-FPR, geared-fan propulsor systems. 

Designers developed a fan architecture that served as a basis for the PW1500G 

engine, but the success of the demonstration at GRC motivated the company 

to evolve the technology for use in larger thrust-class engines. One of the 

most important improvements involved the addition of a gear to the fan drive 

26. Ibid., 6.
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system to allow the low-tip speed, low-FPR fan to be coupled to a smaller, 

more efficient high-speed core. This lowered the minimum fuel burn FPR 

but resulted in a necessarily larger fan diameter so as to produce an equivalent 

amount of thrust. This was significant because increased fan size could result 

in prohibitively large high-drag nacelles. Shortening the length of the nacelle 

would increase the acoustic signature, so designers also had to explore noisereduction technologies.27

27. Van Zante and Suder, “Environmentally Responsible Aviation: Propulsion Research to Enable Fuel 

Burn, Noise and Emissions Reduction,” 6–7.

An open-rotor engine model undergoes testing in the GRC 8-by-6-foot high-speed tunnel. (NASA)
Integrated Propulsion System Technologies

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Acoustic liners offered one possible solution, but engines with large-diameter 

fans and short nacelles provide limited internal surface area for acoustic liners, 

and the effectiveness of these liners is also decreased due to the less optimal 

lift-to-drag ratio of the bypass duct. NASA ERA researchers increased the 

acoustic treatment area in the propulsor through development of two advanced 

liner concepts that came to be known as over-the-rotor (OTR) and soft vanes 

(SV). The OTR concept featured an acoustically designed casing treatment 

installed above the rotor tip region. The proprietary design absorbed pressure 

fluctuations at the source before the sound could propagate for any appreciable 

distance. The SV concept employed cylindrical folded passages inside the fan 

exit guide vanes to absorb pressure fluctuations at their source.28

Results were mixed. Researchers conducted separate tests of the OTR and 

SV concepts employing a legacy fan with a 1.5 FPR, both in a rotor-only configuration to analyze any performance impact as well as inside a productiontype nacelle configuration to measure the resulting acoustic characteristics. 

Noise measurements taken of the rotor alone demonstrated minimal and 

acceptable efficiency losses resulting from the OTR treatment. Acoustic results 

from SV tests with the nacelle installed achieved a noise reduction of 1.5 dB, 

but there was no comparable noise reduction using the OTR concept in the 

same configuration. Test data suggested that OTR deficiencies likely resulted 

from manufacturing difficulties as well as from acoustic design limitations for 

the rotor tip flow-field conditions.

Bypass duct pressure losses have a greater influence on engine TSFC for lowFPR engines cycles than relative to legacy engines. Pratt & Whitney and NASA 

researchers explored this phenomenon while testing concepts for low-loss fan 

exit guide vanes (FEGVs) and methods for optimizing duct end-wall contouring in combination with axial spacing changes to limit total nacelle length. 

These tests validated a design concept resulting in lower duct/FEGV pressure 

losses for advanced propulsor configurations. A final series of tests in 2014 

and 2015 focused on a scale model of the FAA-sponsored CLEEN engine, 

an open-rotor UHB power plant designed by Pratt & Whitney for increased 

performance and reduced noise comparable to existing turbofan levels. The 

wind-tunnel model used for the integrated systems test featured a drooped 

inlet, bifurcated bypass duct, exit guide vanes, and a non-axisymmetric bypass 

duct. One of the primary objectives of the experiment, which concluded in 

June 2015, was to compare model-scale acoustic results to those acquired from 

later full-scale engine static testing. ERA performance goals for ITD35A called 

for a nine percent reduction in TSFC and a 15 EPNdB cumulative noise reduction relative to the baseline engine. NASA and Pratt & Whitney team members 

28. Ibid., 7.
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were pleased to conclude that the test results validated both the aerodynamic 

and acoustic performance of the new propulsor section technologies.29

Low NOx Fuel Flexible Combustor Integration

A series of increasingly stringent Government NOx emission standards imposed 

by the ICAO CAEP over the years has limited aviation emissions below 3,000-

foot altitudes. CAEP standards cover the landing, takeoff, descent, and taxiing 

phases of engine operation in a prorated fashion. Goals set for ERA included 

a demonstration of a low-NOx, fuel-flexible combustor designed to provide a 

75 percent reduction in emissions below CAEP6 standards without increasing 

particulate matter, and with minimal impact on fuel-burn and noise targets. 

ERA researchers recognized that the primary technical challenge centered on 

the fact that to meet fuel-burn reduction targets, advanced engines must operate at higher pressures and temperatures that encourage NOx production. New, 

advanced injector designs and air/fuel-mixing concepts, such as lean direct 

injection (LDI), were required to meet emissions targets and provide fuel flexibility. Leaner-burn concepts, however, tend to provide less stability margin and 

require fuel-staging and combustion control. ERA planners, therefore, established the Low NOx Fuel Flexible Combustor Integration Demonstration, 

ITD40A, to reduce technical risks and mature a new fuel-flexible combustor 

concept from Pratt & Whitney that would maintain low-NOx emissions at 

the higher cycle conditions expected of future engines.30

“Emissions during landing and takeoff affect local air quality, and above 

3,000 feet they account for 92 percent of total ozone,” said ERA combustor 

task lead Chi-Ming Lee. But, he added, “The problem is that the NOx emissions increase as overall pressure ratio increases, particularly above 50:1, so it 

is a tremendous challenge for us.” The answer lay in balancing an advanced 

combustor design with an improved fuel-air mixer and a lean direct-injection 

combustion system. Lee observed: “Every time we improve fuel mixing, the 

NOx drops.”31

For the ERA Phase 1 demonstration in 2012, Lee’s team began by testing a single-injector flame tube before progressing to multi-injector sector 

29. Ibid., 6–8.

30. Joanne C. Walton, Clarence T. Chang, Chi-Ming Lee, and Stephen Kramer, “Low NOx Fuel Flexible 

Combustor Integration Project Overview,” NASA TM-2015-218886, NASA Glenn Research Center, 

Cleveland, OH, October 2015, 2.

31. Guy Norris, “‘Green’ Airliner Targets Achievable by 2025, Says NASA,” Aviation Week & Space 

Technology, http://aviationweek.com/awin/green-airliner-targets-achievable-2025-says-nasa, 

April 18, 2011 (accessed July 21, 2016).
Integrated Propulsion System Technologies

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combustor trials using arc-shaped partial combustor rings and, ultimately, a 

full annular (ring-shaped) combustor test aimed at substantial NOx reductions of up to 80 percent by 2015. Researchers focused on maturing injector 

design, active combustion, and advanced liner technologies from TRL3 to 

at least TRL5. Among the active combustor concepts studied were devices 

designed to carefully control combustion instability and incorporate an intelligent fuel/air management system. They also studied advanced liners made from 

fiber-reinforced silicon carbide matrix ceramic composites (known as SiC/SiC 

CMCs) capable of reducing combustor cooling-air requirements. “We need 80 

percent of the air in front of the combustor to get fuel/air mixing going and 

that’s going to come from the combustor liner,” said Lee. His team also sought 

to further refine existing fuel injectors to accommodate alternative jet fuels such 

as those produced through the Fischer-Tropsch process, in which fuel is synthesized from nonpetroleum sources such as coal, natural gas, and renewable 

biomass. According to Lee, “This provides more opportunities for emissions 

reductions because [Fischer-Tropsch fuel] has no aromatics, no sulfur and its 

distillation profile is different; it can vaporize quicker than jet fuel, and the 

viscosity is less. This, in turn, means droplet size is 10 to 20 percent smaller.”32

Pratt & Whitney’s experimental axial stage combustor (ASC) was designed to 

operate on the lean-lean concept, in which the engine burns fuel with an excess 

of air as compared with conventional engines. This would enable increased 

fuel combustion while simultaneously decreasing hydrocarbon emissions. Two 

types of fuel injectors were employed during different thrust conditions. When 

operating at low-power conditions, the ASC concept used a pilot injector at the 

front of the combustor, while additional main injectors were used in addition to 

the pilot injector for high-power conditions. The key to maintaining low NOx 

production at N+2 cycle conditions was to keep the fuel-air mixture as lean as 

practical throughout the entire axial length of the combustor.33

During ERA Phase 1, researchers from NASA, General Electric, and Pratt 

& Whitney validated lean-burn combustor performance through a series of 

flame tube, sector, and full annular tests in the GRC ASCR facility. Prior 

to testing, the ASCR was upgraded to provide entrance conditions of 900 

pounds per square inch absolute (psia) and temperatures up to 1,300 degrees 

Fahrenheit. To meet ERA goals researchers pursued multiple concepts including a lean partial-mixed combustor and lean direct multi-injection system. The 

results of flame tube tests were used to select the most promising candidates for 

additional sector rig and annular combustor testing. An integrated CMC and 

32. Ibid.

33. Van Zante and Suder, “Environmentally Responsible Aviation: Propulsion Research to Enable Fuel 

Burn, Noise and Emissions Reduction,” 9.
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environmental barrier coating (EBC) liner system made it possible to supply 

more air for fuel/air mixing because less cooling air was required than for an 

engine with a conventional metallic liner.34

NASA researchers conduct an experiment in the Advanced Subsonic Combustor Rig test facility 

at GRC. (NASA)

Under the ERA project, engine designers sought to develop combustor concepts having a high-OPR engine cycle that met specified N+2 noise, fuel burn, 

and emissions goals as part of an overall aircraft system. Advanced lean-burn 

concepts offered by General Electric were based on previous efforts tested under 

the NASA-sponsored Ultra-Efficient Engine Technology (UEET) program 

of the 1990s. This led to development of the twin annular premixing swirler 

(TAPS) combustor, most recently included in the GEnx-1B and GEnx-2B 

engines that power the Boeing 787 and 747-8 wide-body aircraft, respectively. 

Features of the TAPS design included independently controlled, swirl stabilized, annular flames for low-power (pilot) and high-power (main) operation. 

34. Kenneth L. Suder, John Delaat, Chris Hughes, Dave Arend, and Mark Celestina, “NASA 

Environmentally Responsible Aviation Project’s Propulsion Technology Phase I Overview and 

Highlights of Accomplishments,” AIAA 2013-0414, presented at the AIAA 51st Aerospace Sciences 

Meeting, Grapevine, TX, January 7–10, 2013, 4.
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In this configuration, a concentric main flame holder surrounded the central 

pilot flame. By itself, the pilot flame not only provided good low-power operability, but also resulted in reduced carbon monoxide and hydrocarbon emissions. The main burner was optimized to produce low NOx emissions during 

high-power operation. Incorporating advanced liner materials benefitted the 

combustion system in terms of both durability and emissions by decreasing 

cooling air requirements and enabling a higher fraction of combustion air in 

the main mixer for lower NOx emissions.35

In a jet engine, the flame tube is designed to allow a percentage of the air 

that enters the combustion chamber to mix with the fuel inside the combustion chamber. It also controls the ignition process, so the flame never actually 

touches the walls of the chamber or extends back into the turbine section. 

Researchers tested several advanced TAPS injector concepts in a flame tube 

configuration to evaluate emissions, combustion dynamics, and auto-ignition 

margins up to full operating conditions. The results demonstrated acceptable 

operability margins at takeoff conditions and indicated that landing and takeoff 

emissions would likely exceed ERA targets. Based on analysis of test results, the 

research team selected the most effective injector design for incorporation into 

an advanced five-cup sector rig in the ASCR. Data obtained during the latter 

half of 2012 over the entire flight envelope including high-power operation 

met the conditions required for calculating the ICAO landing and takeoff NOx 

emissions levels for engines with an overall pressure ratio of 50:1. Preliminary 

results indicated that the General Electric combustor concept has the potential 

to meet ERA goals.36

Engineers at Pratt & Whitney and United Technologies Research Center 

(UTRC), in East Hartford, Connecticut, employed modern tools to improve 

fuel/air mixing at individual injection points in order to reduce the number of 

injection points by a factor of two compared to previous designs, and to investigate a variety of advanced concepts. This effort resulted in several configurations that featured lean-staged multipoint designs, radially staged swirlers, rich 

quench lean (RQL) combustors, and axially staged combustors. These concepts 

offered simplicity, operability, durability, and emissions levels that ultimately 

made the RQL family of combustors a staple in Pratt & Whitney engines. The 

company’s TALON X combustor was developed with support from NASA 

under the UEET program and was selected for use in a GTF engine slated for 

several future Airbus, Bombardier, and Mitsubishi aircraft.37

35. Ibid., 5.

36. Ibid., 5–6.

37. Ibid., 6.
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Initial testing at UTRC used an idealized single-nozzle rig at 7 and 30 percent power settings for a variety of injector configurations and fuel/air ratios. 

The results demonstrated that all of these concepts could produce emissions 

results below the ERA goals set by NASA. A few of the concepts not only 

performed very well with regard to NOx emissions but also demonstrated 

excellent efficiency. Following additional testing and analysis, one injector 

concept was selected for evaluation in an advanced three-cup sector rig in the 

ASCR facility in 2012. As with the General Electric combustor design, results 

were extremely encouraging.38

For lean combustion cycles, up to 70 percent of the total combustor airflow 

has to be pre-mixed with fuel before entering the combustion chamber. Cooling 

flow must therefore be reduced accordingly to provide sufficient air for mixing. 

An optimized fuel/air mixture is key to lowering flame temperatures and reducing thermal NOx formation. At the beginning of ERA Phase 1, researchers 

considered a 75 percent reduction goal for landing and takeoff NOx emissions 

to be a significant challenge for partial pre-mix combustor configurations such 

as those being pursued by both Pratt & Whitney and General Electric. As an 

additional hedge, in case the partial pre-mix systems demonstrated unresolvable 

auto-ignition issues at the higher inlet pressure and temperature conditions, 

ERA researchers also pursued lean direct injection concepts from industry 

partners Goodrich, Woodward FST, and Parker Hannifin.39

Although limited in scope, these efforts aggressively explored radical new 

ideas such as burning blends of up to 80 percent alternative fuel to 20 percent 

ordinary jet fuel. These designs pushed multipoint LDI concepts that NASA 

had been working on for more than 2 decades to new levels of maturity. LDI 

is ideal for ultrahigh pressure engine operation in which the flame front moves 

closer to the injectors than it does in conventional power plants. Some of these 

concepts also incorporated fuel-flow control features to prevent instability. 

After evaluating several designs, Goodrich engineers down-selected their best 

concept and began lean blow-off testing in the NASA high-pressure flame 

tube facility at GRC. Unfortunately, testing had to be temporarily suspended 

due to a fuel leak and was scheduled for completion at a later date. Woodward 

FST researchers completed light-off and lean blow-off testing of their injector 

concept using their own facilities as well as NASA’s. Parker Hannifin tested the 

38. Ibid., 7.

39. Van Zante and Suder, “Environmentally Responsible Aviation: Propulsion Research to Enable Fuel 

Burn, Noise and Emissions Reduction,” 9–10.
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fuel-spray and lean blow-off characteristics of a new miniaturized fuel valve 

actuator designed for fast response time for combustion control.40

The ERA combustor technology maturation plan involved parallel research 

activities at different TRL levels. For example, low-TRL flame tube tests of 

several swirler concepts were ongoing throughout much of Phase 2, with the 

most promising concepts being down-selected for further demonstration in 

sector tests. Even before all these latter tests were completed, engineers had to 

freeze the swirler design that was to be tested using the full annular rig. This 

technique kept things moving along and allowed researchers to demonstrate the 

best available technology to the highest possible TRL allowed by the design/

test schedule.41

Test results from ERA Phase 1 in 2012 indicated that partial pre-mix concepts from both Pratt & Whitney and General Electric had the potential to 

meet NASA NOx goals without LDI, active combustion control, or alternative 

fuels. Researchers conducted tests involving conventional and blended alternative fuels in all three test rigs (flame tube, arc sector, and full annular) to build 

a state-of-the-art emissions, performance, and fuel flexibility database that 

would eventually enable development of combustors that might be integrated 

into commercial fleets by 2025. At the end of Phase 1, the Pratt & Whitney 

ASC concept was selected for continued technology maturation from TRL4 

to TRL5 during Phase 2. Plans for these tests called for using a sector combustor rig to demonstrate improved operability over Phase 1 results for a full 

range of operational conditions including ambient inlet temperatures as high 

as 1,300 degrees Fahrenheit, compressor pressures up to 50 atmospheres, and 

a maximum flame temperature of 3,000 degrees.42

UTRC and GRC arc sector testing performed during the first year of Phase 

2 focused on system integration and fuel flexibility. Initial testing with the 

UTRC sector rig validated performance of second-generation concepts previously explored in a single-nozzle rig by researchers from NASA, UTRC, 

Georgia Institute of Technology, and the University of Connecticut. Here, 

researchers sought to obtain low-power emissions points and determine how 

to best operate and stage the combustor configuration. Experiments included 

designs featuring combustor liners, cooling techniques, and a variety of different fuel nozzles. In order to substantiate and refine the design, researchers 

assessed system integration aspects across a range of operational temperatures 

40. Suder, et al., “NASA Environmentally Responsible Aviation Project’s Propulsion Technology Phase I 

Overview and Highlights of Accomplishments,” 7.

41. Van Zante and Suder, “Environmentally Responsible Aviation: Propulsion Research to Enable Fuel 

Burn, Noise and Emissions Reduction,” 10.

42. Walton, et al., “Low NOx Fuel Flexible Combustor Integration Project Overview,” 7–8.
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and pressures. They also measured low-power efficiency, lean blowout, and 

emissions performance, and explored an acoustic boundary condition to 

explore dynamic stability of the design refinements with respect to pilot-main 

fuel flow splits. Such realistic engine conditions were necessary for defining 

combustor inlet pressure and temperature levels based on a Pratt & Whitney 

advanced engine concept capable of meeting the NASA N+2 noise, emissions, 

and performance goals.43

In the second year of Phase 2 testing, Pratt & Whitney designers incorporated those features from Phase 1 that appeared to best meet production 

requirements and emissions and performance goals. The company then provided these improved combustors to NASA for further evaluation. Sector testing of these concepts in the ASCR facility represented a critical risk-reduction 

element in the development process. It validated performance and emissions 

at realistic full-engine pressures and temperatures and flame temperatures, 

enabling performance to be measured over the complete range of operating conditions rather than having to extrapolate from limited data. Pratt & 

Whitney fabricated a full annular ring test article representing the secondgeneration combustor configuration. This underwent 100 hours of testing in 

the company’s X960 rig at Middletown, Connecticut, to validate combustor 

emissions, pattern and profile factors (thermal gradient variations), lighting, 

lean blowout, operability, dynamics, and heat loading.44

The process of designing, fabricating, and assembling the full annular combustor gave Pratt & Whitney the opportunity to address mechanical design 

and packaging issues unique to the ASC architecture, including a complete 

fuel system. This allowed designers to accelerate the maturity of the concept 

and enable its incorporation into future engines. Emissions measurements were 

correlated with those made while testing the arc sector test article at UTRC 

and GRC. Researchers learned from experience that for rich-burn, quickmix, lean-burn (also known as rich quench lean, or RQL) combustors, NOx 

emissions measured in arc sector rigs are quantitatively predictive of measurements taken from full annular assemblies as well as from complete engines. 

The full-annular test also enabled measurement of stage-to-stage transition 

characteristics. Researchers used these data to improve combustor operation 

readiness. Additionally, the long-duration test provided durability data to finalize the cooling characteristics of combustor liners in preparation for full-scale 

engine testing. Inclusion of a full fuel system allowed researchers to evaluate 

43. Ibid., 7–8.

44. Ibid., 9–10.
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techniques for the mitigation of traveling-wave tangential-mode combustion 

acoustics, thereby enabling quieter engines.45

Prior to ASCR high-pressure sector testing, researchers pre-screened injector/swirler concepts in flame tube tests at GRC and UTRC, and also conducted 

a lower-pressure sector test at UTRC. Back in the ASCR, the same sector hardware was subsequently evaluated throughout the full operating envelope from 

sea-level takeoff to relight and lean blowout. The full annular combustor test, 

using the same injectors and swirlers as in the ASCR sector test, was completed 

in June 2015. Preliminary analysis of results confirmed the ASCR NOx data 

from ASCR and demonstrated good emission performance using standard 

Jet-A fuel. Additional sector testing with a 50/50 blend of FT alternative fuel 

demonstrated combustor emissions performance and operability characteristics 

nearly identical to the results with Jet-A.46

These successful results indicated performance acceptable for commercial 

airline operation and advanced the Pratt & Whitney combustor concept from 

TRL4 to TRL5. Most important, ITD40A was declared fully successful when 

the full annular test rig achieved 75 percent landing and takeoff NOx reduction and 70 percent cruise-level NOx reduction over a 2005 state-of-the-art 

engine. Total system-level impacts, however, could only be thoroughly validated through testing of a complete full-scale engine. The results of ITD40A 

established that the Pratt & Whitney ACS combustor was ready for such 

testing and provided critical operational and performance data necessary to 

develop test plans and technology development roadmaps for advancing the 

technology from TRL5 to 6.47

UHB Engine Integration for Hybrid Wing-Body Concepts

Although most ERA propulsion technology demonstrations were applicable 

to a wide variety of turbine-driven aircraft configurations, one ITD specifically addressed how best to integrate UHB engines with a hybrid wing-body 

concept. Conventional tube-and-wing configurations are typically equipped 

with engine pods mounted beneath the wings. For the purpose of noise reduction, ERA researchers proposed installing two or more engines on top of the 

HWB airframe at the aft end, between the tails. While this arrangement offered 

shielding benefits for reducing community noise, it also posed questions with 

regard to aerodynamics and performance. Carried out under the ERA Vehicle 

45. Ibid., 9–10.

46. Ibid., 9–10; Van Zante and Suder, “Environmentally Responsible Aviation: Propulsion Research to 

Enable Fuel Burn, Noise, and Emissions Reduction,” 9–10.

47. Ibid., 10–11.
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168

Systems Integration subproject, ITD51A addressed the need to quantify the 

impact of engine/airframe integration on HWB system performance as well as 

noise levels across key on- and off-design conditions. This directly supported a 

technical challenge to demonstrate reduced component noise signatures totaling 42 EPNdB noise reduction for the entire aircraft system while simultaneously minimizing weight and integration penalties to enable an overall 50 

percent fuel-burn reduction at the aircraft system level.48

Hybrid Wing-Body (HWB) aircraft powered by UHB turbofan engines promise remarkable flight 

efficiency coupled with design elegance. Here is one promising Boeing concept evaluated by 

NASA. (NASA)

For purposes of this subproject, researchers focused on a twin-engine HWB 

configuration equipped with UHB turbofan engines. Airframe/power plant 

integration was considered critical to success from performance (drag and 

stability and control), engine operability, and noise shielding perspectives. 

The two major areas of interest were aerodynamic efficiency and engine operability. Such details as nacelle size and location relative to oncoming airflow at 

cruise and low-speed conditions, placement and size of the vertical tails, and 

distance between the engines and aft deck would influence interference drag 

48. Jeffrey D. Flamm, Kevin D. James, and John T. Bonet, “Overview of ERA Integrated Technology 

Demonstration (ITD) 51A Ultra-High Bypass (UHB) Integration for Hybrid Wing Body (HWB),” presented at AIAA SciTech 2016, 54th AIAA Aerospace Sciences Meeting, San Diego, CA, January 

4–8, 2016, 1–2.
Integrated Propulsion System Technologies

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effects as well as overall stability and control characteristics. ERA researchers 

also wanted to explore airflow dynamics at low speeds, high angle-of-attack, 

and during crosswind operation to characterize their effects on the operability 

of the inlets, fans, and nozzles before the HWB concept could be considered 

a viable technology option for commercial transport vehicles.49

During ITD51A, NASA partnered with Boeing to design and validate a 

concept for the HWB that minimized adverse propulsion/airframe-induced 

interference effects that might result in high drag or poor aerodynamic characteristics. Designers used CFD modeling and wind-tunnel tests to quantify 

key design trade space issues that could impact UHB engine operability in 

HWB configurations and minimize adverse effects. Key objectives included 

characterizing airframe-dominated flows on the operability of UHB engines 

at key off-design conditions (low speeds, high angles-of-attack, and sideslip), 

and characterizing the performance (drag, lift, stability and control, propulsion 

induced effects, etc.) of the resulting HWB propulsion/airframe integration 

design throughout the Mach number range. In order to fully advance the 

knowledge and TRL of UHB engine integration on the HWB, researchers also 

evaluated methods for integrating a large diameter fan/nacelle configuration 

with various N+2 vehicle concepts. This included investigations into how best 

to address nacelle weight for large-diameter fans, the viability of shorter inlets 

to reduce nacelle drag, and the use of thrust reversers, variable-area nozzles, 

and low-noise fan designs.50

Wind-tunnel models for this demonstration were based on Boeing’s PSC 

from ERA Phase 1, updated with design refinements to address all key performance metrics and ERA goals as well as potential issues uncovered during prior 

HWB design studies. Boeing developed an HWB configuration designated 

N2A-EXTE under a NASA NRA in 2012, but initial low-speed wind-tunnel 

testing revealed slight problems with airframe-generated inlet flow distortion. 

Boeing subsequently revised the design in the areas of planform, propulsion 

aerodynamic integration, high-lift systems, and propulsion system sizing. The 

updated PSC design also addressed fundamental requirements for weight and 

balance, and stability and control. Designers focused particularly on low-speed 

inlet distortion and recovery, engine installation drag penalty at cruise conditions, noise effects resulting from engine position relative to the body trailing 

edge, maximum lift coefficient (CL max) at takeoff and landing, and cruise liftto-drag ratio (L/D). These design trades resulted in significant changes to the 

original planform and wing leading edge sweep to improve both stability and 

control and center-of-gravity characteristics. Additionally, designers discovered 

49. Ibid., 5.

50. Ibid., 2–5.
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National Full-Scale Aerodynamics Complex (NFAC) 40-by-80-foot test section with a Boeing 

HWB model. The size of the NASA personnel gives a sense of the immense size of this now-70-

year-old facility. (NASA)
Integrated Propulsion System Technologies

171

that integration of the propulsion system above the wing body posed challenges 

for both low-speed operability and high-speed cruise drag. After observing 

shock interactions between the nacelles and the body at high speeds, Boeing 

engineers performed a rigorous optimization study to minimize installed drag 

of the engine nacelle at transonic conditions.51

NASA researchers tested three configurations of a 5.75-percent-scale HWB 

model in the Langley 14-by-22-foot subsonic wind tunnel, and in the 40-by80-foot test section of the National Full-Scale Aerodynamics Complex (NFAC) 

at Ames. These configurations included an HWB with flow-through nacelles, 

one with ejector-powered inlets, and another with turbine-powered engine 

simulators (TPS). Researchers first performed flow-through nacelle testing 

while optimizing a high-lift system for takeoff and landing conditions. This 

involved the use of Krueger flaps, lift-enhancement devices that may be fitted 

to the leading edge of an aircraft wing. This model configuration was also used 

for force and moment testing. Next, they replaced the flow-through nacelles 

with ejector-powered inlets to simulate scaled mass conditions at the engine 

inlet. The objectives of this test were twofold. First, researchers needed to 

characterize inlet flow distortion, particularly that induced by peculiarities of 

HWB airframe/propulsion integration. Second, they attempted to mitigate 

such adverse effects by varying inlet height and Krueger settings. Researchers 

felt that because the engines were mounted on the aft body upper surface, 

the inlets might be susceptible to vortex ingestion originating from the wing 

leading edge at high angles of attack and sideslip, and from separated wing/

body flow. Finally, devices that simulated scaled exhaust flow were installed to 

characterize the power-on effects of engine exhaust flow on increased pitching 

moment and elevon effectiveness.52

As originally outlined by ERA planners, all wind-tunnel testing for ITD51A 

was to be accomplished in the LaRC subsonic tunnel, a closed circuit, singlereturn, atmospheric wind tunnel capable of producing a maximum speed of 

348 feet per second. Unfortunately, this facility suffered a failure of the main 

fan drive in September 2014 at the beginning of the first series of ejector 

test runs. Investigators estimated it would take as much as 1 year to repair 

the motor, a delay that would have extended the wind-tunnel test campaign 

beyond the scheduled end of the ERA project in September of 2015. In order 

to remain on schedule, all subsequent testing was relocated to the NFAC, and 

testing resumed in January 2015.53

51. Ibid., 6.

52. Ibid., 8–12.

53. Ibid., 8.
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Moving these experiments to the NFAC provided an unexpected opportunity to acquire acoustic data to refine noise estimates. Originally, there were no 

plans to take direct acoustic measurements of the HWB model during tunnel 

runs in the LaRC subsonic facility. Instead, the project plan called for all such 

estimates to be done computationally. With its much larger test section, the 

NFAC had room for a phased array acoustic measurement system. Prior to runs 

with the flow-through nacelles, researchers installed a traversing array below 

the left wing of the HWB model to measure Krueger flap noise under various conditions. Pre-test CFD modeling results determined the most effective 

positioning for the acoustic array to provide high signal-to-noise ratios without 

inducing adverse aerodynamic effects on the test article. Researchers used the 

array to acquire data for a number of different Krueger flap configurations, 

dynamic pressure sweeps ranging from 20 to 60 psf, angle-of-attack sweeps 

from 0 to 16 degrees, and emission angles from 60 to 120 degrees. Preliminary 

results indicated that noise generated by the Krueger flaps largely depended on 

the size of the gap between the flap and the wing leading edge. High-resolution 

beam-form images showed the acoustic research team that flap brackets were 

the primary noise sources on the leading edge, and that a sealed-gap configuration produced high-lift noise comparable to the baseline cruise configuration.54

Researchers also used CFD modeling to validate the Boeing HWB design 

would meet fuel-burn performance goals by assessing the vehicle’s transonic 

performance characteristics. Although plans originally called for this to be 

accomplished using a combination of CFD and wind-tunnel test data, program 

constraints resulted in the elimination of high-speed, transonic testing from the 

project. Consequently, all transonic performance characterization of the HWB 

design was done exclusively using predictive models. To build confidence in 

the CFD predictions, NASA and Boeing performed independent assessments. 

The NASA team made computations using an unstructured grid code called 

USM3D, and Boeing performed calculations using a structured grid code 

called OVERFLOW, both of which are fully turbulent, Reynolds-averaged 

Navier-Stokes flow solvers. NASA’s goal was to assess Boeing’s overall process 

for determining interference drag and develop a database of independent CFD 

solutions for comparison. In general, agreement between the two independent 

simulations was excellent.55

Because mounting the engines on the upper aft fuselage of the HWB created the potential for flow distortion from the forebody to be ingested into the 

engines when flying at high angles-of-attack, Pratt & Whitney performed an 

assessment of inlet distortion effects on engine and fan operability. The severity 

54. Ibid., 13.

55. Ibid., 14.
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of this phenomenon, ranging from flow angularity and swirl distortion to total 

inlet pressure loss, was a function of the BWB forebody design, including highlift devices such as Krueger flaps as well as aerodynamic operating conditions 

(Mach, angle of attack, and sideslip). Researchers used both computational 

modeling and experimental data obtained from the project to identify such 

potential threats to the engine as fan stall, low-pressure compressor stall, and 

fan blade vibratory stress. Although any significant inlet distortion affected 

engine performance to some degree, this represented an off-design condition 

that did not affect the overall mission fuel-burn assessment. At this point, the 

team concluded that the technology readiness level for the operability and blade 

stress assessment was for all intents and purposes TRL4.56

A Boeing HWB model in the 14-by-22-foot subsonic wind tunnel at NASA Langley Research 

Center. (NASA)

In order to evaluate Boeing’s PSC, the company supplied data for inlet 

distortion at a number of limiting conditions derived from CFD of the complete aircraft configuration at full scale. For all inlet distortion cases that fell 

within the expected operational envelope of the PSC configuration, engine 

operability and fan blade stress metrics were determined to be within acceptable limits. Therefore, designers at Boeing saw no reason to modify the PSC 

engine/airframe integration concept. The only marginal or unacceptable 

56. Ibid., 19.
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assessments resulted from inlet distortion analysis cases that were well outside 

the predicted operating envelope of the PSC aircraft. These results provided 

confidence that the operability assessment methodologies were producing 

reasonable results.57

Finally, ERA researchers conducted a system-level assessment to quantify 

the overall integrated vehicle performance of the HWB against ERA project 

goals. Vehicle system-level assessment metrics for ERA Phase 2 called for simultaneously meeting mission fuel-burn reduction of 50 percent, cumulative community noise levels of 42 EPNdB below Stage 4, and 70 percent lower engine 

NOx emissions to validate technology maturation of the overall HWB aircraft 

performance. To meet these goals, Boeing refined the PSC configuration as 

new data became available throughout the ERA project. Design modifications 

included changing the wing position, adjusting the engine cycle, and altering 

the planform shape and the overall size of the airframe. Additional high-fidelity 

analysis resulted in further refinements to the aerodynamic lines of the nacelles, 

wing, body, and control surfaces. Designers used results from wind-tunnel 

tests and CFD modeling to update the configuration, making design tradeoffs 

between noise reduction and fuel-burn reduction where necessary. All windtunnel data were based on a design configuration designated ERA-0009GM, 

the outer mold lines of which had been frozen in the first year of the project 

in order to fabricate hardware for the 5.75-percent geometrically scaled model 

in time to meet testing milestones.58

The updated configuration used for the system-level assessment, dubbed 

ERA-000H1, incorporated fuselage shaping for transonic performance 

improvements. The key difference between the ERA-0009GM and ERA0009H1 configurations involved fuselage shaping around the engine nacelles, 

which designers did not expect to affect the aircraft’s subsonic performance 

characteristics. For purposes of this assessment, the ERA-0009H1 was assumed 

to be equipped with Pratt & Whitney GTF engines. Changes to the PSC configuration resulted in the ERA-0009H1 achieving a fuel-burn level greater than 

53 percent better than the reference configuration. Additionally, assessment 

of the ERA-0009H1 predicted a 1.8 percent lower fuel-burn than the original 

PSC, due primarily to a five percent increase in initial cruise L/D. An assessment of predicted noise levels for the updated PSC configuration, equipped 

with aerodynamic landing gear fairings and noise-reducing nozzle technologies, produced a cumulative margin of 37 EPNdB below Stage 4.59 “We see a 

57. Ibid., 19–20.

58. Ibid., 21.

59. Ibid., 21.
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strong technical path to meeting the –42 dB goal,” said Russ Thomas, HWB 

community noise team leader. “It’s the last 10 dB that’s really tough.”60

60. Norris, “‘Green’ Airliner Targets Achievable by 2025, says NASA.”
176

NASA Deputy Administrator Dava Newman addresses dignitaries, technical staff, and 

other guests on June 17, 2015, at ceremonies during a two-day visit of Boeing’s 757 

ecoDemonstrator to NASA’s Langley Research Center. (NASA)
177

CHAPTER 7

ERA and the Boeing 757 

ecoDemonstrator

NASA contracted Boeing to test two ERA technologies on the 757 

ecoDemonstrator. The first of these focused on active flow control (AFC) 

to improve airflow over the rudder and maximize its aerodynamic efficiency. 

Preliminary research indicated that AFC could improve aerodynamic efficiency 

and potentially allow for the use of smaller vertical tails on future airplanes. 

NASA also tested non-stick coatings on the 757’s right wing to reduce drag from 

insect residue; this would enable more laminar flow by smoothing the airflow on 

the surface of the wing. With the exception of Boeing’s proprietary technology, all 

NASA knowledge gained in collaboration with Boeing from ecoDemonstrator 

research became publicly available to benefit the aviation industry.1 “Both [technologies] are designed to improve the airflow over the surface and ultimately 

reduce drag,” said Fay Collier. “Increased drag means increased fuel consumption, which results in more pollutants in the atmosphere.”2

The 757 ecoDemonstrator was a highly modified Boeing airliner that served 

as a testbed for a variety of technologies. The project was built upon previous work by Boeing and various Government and private partners to accelerate and leverage new technologies to reduce emissions and noise, improve 

airlines’ gate-to-gate efficiency, and help meet other environmental goals. The 

ecoDemonstrator’s origins dated to Boeing’s Quiet Technology Demonstrator 

program: the company worked with Rolls-Royce in 2001 to develop a quieter 

turbofan engine incorporating saw-toothed chevrons on the aft end of the nacelle 

and exhaust nozzle. Additional testing in 2005 allowed designers to refine the 

chevron design and validate an acoustically treated inlet. Boeing applied these 

technologies to both the 747-8 and 787, providing dramatic noise reduction.

1. Jessica Kowal and Bret Jensen, “The Boeing ecoDemonstrator Program” fact sheet, Boeing 

Commercial Airplanes, Seattle, Washington, December 2015, 3–4.

2. Kathy Barnstorff, “NASA Tests Green Aviation Technology on ecoDemonstrator,” April 2, 2015, 

http://www.nasa.gov/aero/nasa-tests-green-aviation-technology-on-boeing-ecodemonstrator.

html (accessed July 21, 2016).
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The distinctive saw-tooth engine chevron, an imaginative effort to reduce engine noise. (NASA)

Building on this success, the company began the ecoDemonstrator program in 2011, in cooperation with American Airlines and the FAA. The earlier efforts, which had begun as a partnership among Boeing, the Federal 

Aviation Administration’s CLEEN, and American Airlines, expanded over time 

to include Japan Air Lines, NASA, Rolls-Royce, General Electric, Rockwell 

Collins, Honeywell, Panasonic, Stifel Bank and the TUI Group. Flight evaluations were undertaken with both ecoDemonstrator 737 and 787 jetliners. 

It was hoped that technologies and processes validated by ecoDemonstrator 

evaluation would be then incorporated into existing production models and 

made available for in-service fleets as well as new airplane development programs, supporting long-term sustainable aviation growth.

In 2012, the ecoDemonstrator program began with a Boeing 737-800 

(N897NN, loaned from American Airlines) demonstrating 15 advanced technologies (including a variable-area fan nozzle, active engine vibration control, 

flight trajectory optimization, a regenerative hydrogen fuel cell, blended biofuel, and carpet made from recycled materials). Most impressively, it validated 

the aerodynamic performance of an NLF winglet that improved fuel efficiency 

by up to 1.8 percent.3

3. Jessica Kowal and Bret Jensen, “The Boeing ecoDemonstrator Program” fact sheet, December 

2015, 1–2.
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In 2014, using an ecoDemonstrator Boeing 787-8 Dreamliner (N7874, 

owned by the company) researchers tested a high-strength, heat-resistant 

ceramic composite nozzle designed to enable jet engines to operate at higher 

temperatures, improving fuel efficiency while decreasing emissions and noise. 

Altogether they evaluated more than 25 new technologies aimed at making 

airplanes quieter and more fuel-efficient. These included aerodynamic and 

flight control improvements, special coatings to reduce ice accumulation on 

wing surfaces, and software applications and connectivity technologies for 

improved flight planning and fuel-load optimization. NASA researchers tested 

a flight interval management system to help achieve precise spacing between 

aircraft upon landing. Known as Airborne Spacing for Terminal Arrival Routes 

(ASTAR), this technology is intended to increase landing frequency and reduce 

holding patterns, thus saving fuel and time and reducing NOx emissions.4

Toward the end of the year, Boeing demonstrated the world’s first flight 

using “green diesel,” a sustainable biofuel already widely available and used in 

ground transportation. For initial testing, the ecoDemonstrator 787 was fueled 

with a blend of 15 percent green diesel and 85 percent petroleum jet fuel in the 

left engine only. The same blend was later used in both engines on subsequent 

test flights. Boeing offered data generated by these flights to support industry 

approval of green diesel for commercial aviation. Other technologies tested on 

the ecoDemonstrator 787 included touch-screen displays in the flight deck and 

outer wing access doors made from recycled 787 carbon fiber material. Boeing 

researchers also introduced wireless sensors in an effort to reduce the amount 

of wiring in the airplane.5

The ecoDemonstrator Program

In 2015, the ecoDemonstrator Program tested more than 15 technologies 

on a modified 757-222 (N757ET) that had been retired after years of service 

with United Airlines, and which was owned by Stifel Bank’s finance group. 

Project collaborators included the NASA ERA project and the Europe-based 

TUI Group, an integrated tourism consortium that includes six airlines (and 

which includes the 757 ecoDemonstrator in its fleet). The first series of tests 

involved Boeing-proprietary technologies applied to the airplane’s left wing 

to see if they would reduce environmental effects on natural laminar flow and 

improve aerodynamic efficiency. During these experiments, high-resolution 

infrared cameras attached to the top of the fuselage monitored laminar flow 

over the wing while Boeing test pilots flew the ecoDemonstrator through 

4. Ibid., 2–3.

5. Ibid., 3.
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various environmental conditions to study the potential effects of those conditions on aerodynamics. The most noticeable modification to the airplane 

was the installation of a 22-foot span variable-camber Krueger (VCK) flap 

designed to protect the leading edge of the wing from insect strikes because 

insect residue adversely affects airflow over the airfoil surface. Previous attempts 

to use Krueger flaps as insect mitigation screens resulted in additional drag. 

To avoid this problem, Boeing’s new VCK was designed to retract seamlessly 

into the lower wing surface.6

The Boeing 757 ecoDemonstrator’s deceptively conventional appearance masked the very 

advanced technologies it demonstrated. (NASA-Boeing)

In June 2015, Boeing flew the 757 ecoDemonstrator using a 5 percent 

blend of domestically produced green diesel to support ongoing biofuel and 

aviation industry efforts to have this biofuel approved for use in commercial 

aviation. Additional Boeing-sponsored demonstrations included solar and 

thermal “energy harvesting” to power electronically dimmable windows that 

would reduce wiring, weight, fuel use, and carbon emissions; and a 3D-printed 

aisle stand made from carbon fiber left over from 787 production, which was 

an example of the company’s efforts to re-purpose aerospace-grade carbon fiber 

and thus reduce both airplane weight and factory waste.7

6. Guy Norris, “Bug Smasher—Wing Protection System Tests Could Help Unlock Benefits of Laminar 

Flow,” Aviation Week & Space Technology, March 30–April 12, 2015, 37. https://archive.

aviationweek.com/issue/20150331 (accessed September 4, 2019)

7. Kowal and Jensen, “The Boeing ecoDemonstrator Program,” 3–4.
ERA and the Boeing 757 ecoDemonstrator

181

NASA Deputy Administrator Dava Newman addresses dignitaries, technical staff, and other guests 

on June 17, 2015, at ceremonies during a two-day visit of Boeing’s 757 ecoDemonstrator to 

NASA’s Langley Research Center. (NASA)

(L-R) NASA Langley Research Center Director David Bowles, NASA Deputy Administrator Dava 

Newman, and Dr. Edgar “Ed” Waggoner, chief of NASA’s Integrated Aviation Systems Program, 

receive a briefing while touring the 757 ecoDemonstrator during its Langley visit, June 17, 

2015. (NASA)
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182

Active Flow Control

On a modern multi-engine commercial transport aircraft, the vertical tail 

is employed primarily to provide directional stability and control at all airspeeds across the aircraft’s performance envelope. But it is particularly significant during takeoff and landing. A large vertical stabilizer is indispensable in 

addressing crosswind conditions, as well as in the event of an engine failure 

that results in the remaining “healthy” power plants generating unbalancing 

asymmetrical thrust or torque effects. The vertical stabilizer’s size is optimized 

for low-speed flight, where directional stability and control (the latter via the 

rudder) is particularly critical. When the aircraft is cruising at altitude the same 

large, heavy tail is no longer required. Additionally, designers typically size the 

vertical tail for the shortest version in an aircraft’s model family but use the 

same tail for every variant to reduce production costs. Thus, the ideal vertical 

tail for the 777-200 is effectively oversized for the longer 777-300.8

The optimum vertical tail area is determined by the fuselage length/tail 

moment arm; as this value increases with fuselage length, less tail area is 

required for stability and control. Sized in this way, the drag and weight of 

the tail increases fuel consumption for the entire aircraft family. One way to 

compensate for a reduction in tail size is through the use of active flow control, which is the commanded manipulation of fluid flows with the addition 

or subtraction of energy from the fluid (in this case, air). By employing AFC 

technology on the shortest variant of an aircraft family to facilitate both lowspeed control and the ability to react to sudden changes in flow conditions, 

smaller tail sizes may be use on the stretched versions.9

For the AFC Enhanced Vertical Tail Flight Experiment, NASA worked with 

Boeing to install dozens of tiny, sweeping jet actuators capable of manipulating, on demand, the airflow over the 757 ecoDemonstrator’s vertical tail. This 

action was calculated to delay flow separation over a highly deflected rudder 

and to increase the rudder’s side force capability. If successful, this technology 

would enable designers to incorporate a smaller vertical tail that provides the 

necessary control authority during an emergency situation using AFC, while 

operating in a conventional manner throughout the rest of the flight envelope. 

8. Craig L. Nickol and William J. Haller, “Assessment of the Performance Potential of Advanced 

Subsonic Transport Concepts for NASA’s Environmentally Responsible Aviation Project,” AIAA2016-1030, presented at American Institute of Aeronautics and Astronautics SciTech, 54th AIAA 

Aerospace Sciences Meeting, San Diego, CA, January 6, 2016, 3.

9. Ibid., 3.
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183

Reducing tail size would also lower the airplane’s overall airframe weight and 

aerodynamic drag and decrease fuel consumption.10

Prior to the ERA project, researchers had demonstrated the capabilities 

of AFC only on small-scale airplane and component models in laboratory 

environments. Wind-tunnel experiments conducted at Rensselaer Polytechnic 

Institute in Troy, New York, applied eight synthetic jet actuators to a 7 percent 

scale vertical stabilizer model, where actuators placed just upstream of the 

rudder hinge line produced a side force increase of nearly 20 percent at moderate rudder deflections. Researchers at the California Institute of Technology 

(Caltech) in Pasadena, California, collaborated with Israel Wygnanski, a professor of aerodynamics and fluid mechanics at the University of Arizona (considered the “father of active flow control”), to install sweeping jet actuators on 

a 14 percent scale vertical tail model. In tests carried out using Caltech’s 5-by6-foot Lucas Wind Tunnel facility, AFC actuation was applied to the rudder 

and the stabilizer main element’s trailing edge. Experiments involving multiple 

flap deflections and spanwise actuator configurations resulted in a side force 

increase of as much as 50 to 70 percent depending on the free stream velocity 

and momentum input.11

Installed just beneath the outer skin of the tail along the stabilizer’s vertical 

length, the AFC devices deliver a strong burst of sweeping air just along the 

rudder, equivalent to the amount of airflow that would normally be encountered by the tail and rudder at higher speeds. Because these jets of air sweep 

back and forth over the length of the tail rather than blasting a single, linear 

burst of air, researchers discovered that they could increase airflow over the 

entire tail with just six of the AFC devices. Caltech aerospace research project 

manager Emilio Graff suggested that, using such devices, airplane manufacturers could reduce the size of airplane tails by as much as 20 percent, with the 

sweeping jet actuators needing to be activated only during takeoff and landing. Fuel savings are derived not only from reduced drag due to the smaller 

tail size but also from weight savings and structural advantages from having a 

shorter vertical stabilizer. Even if the AFC system itself used a relatively large 

amount of energy, it would be activated only for short periods during the 

takeoff and landing phases of flight. “When you take off or land, the air jets 

will be on—just in case an engine fails,” he said, “but on a 12-hour flight, if 

10. Marlyn Y. Andino, John C. Lin, Anthony E. Washburn, Edward A. Whalen, Emilio C. Graff, and Israel 

J. Wygnanski, “Flow Separation Control on a Full-Scale Vertical Tail Model using Sweeping Jet 

Actuators,” AIAA-2015-0785, presented at the 53rd AIAA Aerospace Sciences Meeting, AIAA 

SciTech Forum, Kissimmee, FL, January 4–8, 2015, 2.

11. Ibid., 2.
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184

you’re only using the system for 30 minutes, you’re still saving gas during 11 

hours and 30 minutes.”12

The Active Flow Control system was tested on a full-sized tail in the NFAC. (NASA)

Next, as part of NASA’s ERA project, Graff and his colleagues designed and 

built a scaled-up AFC system to test the effects of sweeping jet actuators on a 

full-sized airliner tail with the help of a multi-institutional team that included 

engineers from Boeing Research and Technology and NASA LaRC. The test 

article was an actual 757 vertical tail that had been removed from a retired 

airliner in storage at Pinal Airpark in Marana, Arizona. With the help of the 

Boeing test and evaluation team, Advanced Technologies, Inc., of Newport 

News, Virginia, modified and refurbished the tail into a wind-tunnel model, 

and installed 31 AFC devices. Because it was nearly 27 feet tall, the next stage 

of the experiment was moved to the NFAC—one of the world’s largest wind 

tunnels—located at NASA ARC in Mountain View, California. This allowed 

researchers to realistically simulate wind conditions similar to those that would 

be experienced during takeoff and landing.13

The 2-month-long test series began in early September 2013. Data from fullscale testing confirmed that sweeping jet actuators could sufficiently increase 

the airflow around the rudder to steer the plane in the event of an engine failure. 

12. Jessica Stoller-Conrad, “Sweeping Air Devices for Greener Planes,” October 20, 2014 http://www.

caltech.edu/news/sweeping-air-devices-greener-planes-43987 (accessed February 5, 2017).

13. Ibid.
ERA and the Boeing 757 ecoDemonstrator

185

According to Graff, this flow control technique is not new; it has previously 

been used for augmented takeoffs and landings in military applications. But, 

he added, existing systems are not energy-efficient, “and if you need a third 

engine to power the system, then you may as well use it to fly the plane.” By 

comparison, the AFC system designed by Graff and his colleagues is small and 

efficient enough to be powered by an airliner’s auxiliary power unit (the small 

engine that powers the cabin’s air conditioning and lights at the gate). “We 

were able to prove that a system like this can work at the scale of a commercial 

airliner, without having to add an extra engine,” he said.14

Sweeping jet actuators were developed more than 50 years ago at the Harry 

Diamond Research Laboratories, where they were initially considered for use 

in analog computers and as fluidic amplifiers. Other applications include 

oscillating windshield washers on cars, shower heads, and irrigation systems 

that use liquid (mostly water) as the working fluid. Only recently were these 

devices considered for aeronautical experiments for the purpose of delaying 

separation on airfoils. They are ideal devices for AFC because they introduce 

spanwise unsteady (oscillatory) blowing with no moving parts, requiring only 

a steady supply of compressed air. In this sense, the system is similar to steady 

blowing, but uses less air to provide similar levels of effectiveness. Each actuator is a solid-state unit with no moving parts and an internal feedback loop 

that causes a continuous jet of air to sweep back and forth across an arc. This 

action reenergizes the separated flow, reattaching it to the rudder even at higher 

deflection angles. ERA researchers chose to implement a sweeping jet AFC 

system on the trailing edge of the main tail element for ease of integration. 

Successful demonstration served as a major risk-reduction step toward a flight 

demonstration planned for 2015.15

These experiments also provided, for the first time on a commercial aircraft, 

the opportunity to assess design and scaling issues for full-scale application of 

AFC and validation of subscale and CFD observations regarding the sensitivities and effects of AFC on a vertical tail. The wind-tunnel tests enabled the 

research team to observe “a wide array of flow control configurations across 

the whole low-speed flight envelope of the vertical tail,” said Boeing Research 

and Technology program manager Ed Whalen. Based on wind-tunnel results, 

the team selected the most efficient and effective flow control configuration 

for flight-testing on the 757 ecoDemonstrator. According to Fay Collier, the 

AFC test campaign was a key component of ERA. “The maturation of technologies such as active flow control, which will benefit aviation by improving 

fuel efficiency, reducing emissions and noise levels, is what NASA’s aeronautics 

14. Ibid.

15. Andino, et al., 3.
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186

research is all about [and] will have an impact on future ‘green’ aircraft designs,” 

he said.16

Wind-tunnel results demonstrated the effectiveness of AFC jets for increasing side force by 20 to 30 percent, which would allow aircraft designers to 

scale-down the vertical tail of a 757-class commercial transport by at least 

17 percent and reduce fuel usage by as much as 0.5 percent.17 This technology promised other potential benefits as well. In addition to reduced airframe 

weight and improved fuel economy, this type of device could also allow future 

aircraft to use shorter runways—or perhaps no runways at all. “We can possibly 

take off from a football field–sized area with some sort of rotor-type airplane, 

maybe a tilt-rotor, and we can fly at maybe 300 to 350 knots, and do this very 

efficiently,” said Israel Wygnanski. “If we do it from downtown to downtown, 

from a parking lot, then that’s much more effective than to do it the way we 

do today [from airport to airport] because you spend so much time in the airports.” He added, “There are also a lot of smaller airports with runways of less 

than 5,000 feet that are closer to city centers that can be used, so some sort of 

short-takeoff airplane that can use this technology would be very effective.”18

Now it was time to validate much of the research data in flight. This phase 

of testing had three primary objectives. First, demonstrate the team’s ability to 

integrate a prototype AFC system into an airframe and thereby highlight key 

integration challenges. Second, demonstrate AFC impact on rudder effectiveness in flight using available APU flow rates. And, finally, collect in-flight data 

for comparison to full-scale wind-tunnel results and CFD predictions. To meet 

these objectives, planners outlined a number of system design requirements. 

These included minimal modifications to existing structures and systems and 

minimal relocation of flight-critical systems. All installation of test equipment had to be accommodated within the 757’s tail assembly. The airplane’s 

auxiliary power unit (APU) powered the AFC system’s sweeping jets. APU 

compressor air temperatures had to be kept below 130 degrees Fahrenheit to 

protect the integrity of the rudder skin. All externally mounted components 

had to be bonded in such a way as to mitigate static electricity and electromagnetic emissions risks. In order to power the AFC system, the APU had to be 

disconnected from the airplane’s pneumatic bleed air system, which meant it 

was not available for starting the engines nor could it provide bleed air for the 

16. Michael Braukus, John M. Foley, and Daryl Stephenson, “NASA, Boeing Finish Tests of 757 Vertical 

Tail with Advanced Technology,” November 14, 2013, https://www.nasa.gov/content/nasa-boeingfinish-tests-of-757-vertical-tail-with-advanced-technology (accessed February 5, 2017).

17. Barnstorff, “NASA Tests Green Aviation Technology on ecoDemonstrator.”

18. Israel J. Wygnanski interview, NASA X “End of an ERA—Part 2,” NASA TV, December 18, 2015.
ERA and the Boeing 757 ecoDemonstrator

187

air-conditioning packs. Once AFC testing was completed, the airplane’s bleed 

air system was restored to its original production configuration.19

Following completion of testing in the NFAC, the modified tail was installed 

on the 757 ecoDemonstrator, and testing continued at Boeing Field in Seattle. 

Integrating AFC system hardware was challenging because the airplane had 

already been built. Ducting inside the vertical fin had to be installed in multiple 

segments due to limited access and bends and branches needed to fit within 

the existing geometry to minimize cutting any part of the aircraft structure. If 

the system were installed in a new airplane design, designers could incorporate 

longer tubes and better-optimized flow paths, thus reducing both installation time and the number of coupling adapters. In addition to routing the 

plumbing around structures, it was also necessary to mitigate interference with 

existing systems. Pressurized air, supplied by the APU, was pre-cooled from 

its 380-degree exit temperature by running it through an externally mounted 

heat exchanger beneath the aft fuselage. The heat exchanger itself was taken 

from the 757’s original environmental control system and plumbed into a 

duct running along both the front and rear spars of the stabilizer to ensure an 

even supply of air to the actuators. In addition, a special duct was calibrated 

to provide engineers with a primary means of calculating system mass flow. 

Pressure and temperature sensors installed at key locations allowed researchers 

to monitor system performance and provided a means to detect a system leak 

or duct burst. Boeing engineers also had to conduct thorough design and stress 

analyses to ensure that the new ducting would be able to withstand maximum 

loading during flight without leakage and be able to accommodate thermal 

expansion due to the heated APU air. Loads analysis ensured the vertical tail 

would be able to withstand the additional weight of the ducting and side forces 

generated during testing.20

Prior to the first flight, in March 2015, Boeing conducted a series of laboratory and ground tests to verify the overall functionality of the heat exchanger, 

system controller, and AFC system. Company technicians also conducted a 

ground vibration test to establish the heat exchanger’s frequency response and 

vibration-level limitations. Since many ducting joints were required, technicians performed periodic leak checks at key points during installation to 

ensure proper sealing. As leaks were discovered they took appropriate corrective actions until the leaks stopped. After the AFC system assembly was 

19. Michael G. Alexander, F. Keith Harris, Marc A. Spoor, Susannah R. Boyland, Thomas E. Farrell, and 

David M. Raines, “Active Flow Control (AFC) and Insect Accretion and Mitigation (IAM) System Design 

and Integration on the Boeing 757 ecoDemonstrator,” AIAA-2016-3746, presented at the AIAA 

Aviation and Aeronautics Forum and Exposition, Washington, DC, June 15, 2016, 4.

20. Ibid., 8.
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188

fully integrated into the aircraft, a system leak check was completed with no 

further concerns. This disciplined approach saved time by eliminating the 

need to “chase leaks” after installation had been completed, when access to 

repairs would have been much more difficult. Finally, Boeing completed a total 

system functionality check that included measuring AFC actuator output and 

flow output from the sweeping jets actuators. This also ensured that the AFC 

system was controllable, instrumentation operated nominally, and the system 

operation process was established.21

The AFC system, mounted on the right side of the fin only for the tests, 

comprised 31 sweeping jet actuators positioned ahead of the rudder’s leading 

edge. The NFAC experiments employed 37, but those closest to the tip of the 

tail proved less effective than desired, according to Doug Christensen, Boeing’s 

ecoDemonstrator program manager. The airplane was equipped with two Pratt 

& Whitney PW2037 engines leased from Delta Airlines. To maximize the 

asymmetric forces on the rudder, Boeing modified the left engine via Service 

Bulletin to increase the available thrust from 37,000 to 40,000 pounds. The 

ecoDemonstrator flew test flights with and without the AFC system operating. The rudder was fully instrumented so engineers could determine rudder 

hinge moments and see how much force was being generated. Chase pilots 

observed flow cones fixed to the rudder to see how much of the flow remained 

attached to the rudder.22

The flight conditions that were necessary to demonstrate AFC effectiveness 

necessitated that all testing be performed over water under visual flight rules 

(VFR) conditions. Mission planners, therefore, selected a work area above 

the Strait of Juan de Fuca, a nearby body of water separating the state of 

Washington from Vancouver Island, British Columbia, Canada. This site was 

chosen based on proximity, meteorological analysis of historical weather patterns, cost effectiveness for flight crew and test monitors, and compatibility 

and availability of the chase plane for flow visualization photography.23

After 4 days of AFC flight-testing were completed, comprising six flights 

during which the crew performed various maneuvers to evaluate the effectiveness of the AFC jets. Maneuvers included simulated engine failures and 

variations in AFC jet arrangements and flow rates. According to Jeanne Yu, 

Environmental Performance Director for Boeing Commercial Airplanes, these 

experiments were important for moving the technology forward, and because 

of the significant savings the technology may potentially afford aircraft operators and the flying public. “When you think about the worldwide fleet of 

21. Ibid., 8–9.

22. Norris, “Bug Smasher,” 37.

23. Alexander, et al., 10.
ERA and the Boeing 757 ecoDemonstrator

189

airplanes, there are about 18,000 that fly today and that’s projected to [increase] 

to about 36,000 airplanes in the next 20 years,” Yu said; “If you can get a little 

bit of improvement on one airplane and multiply it by 36,000 in the next 20 

years, it’s a huge improvement with regard to reducing overall fuel burn.”24

Active and laminar flow control are NASA’s two main areas of investigation 

for reducing skin friction, which accounts for an estimated 48 percent of the 

drag on current airliner designs. “The intent is to increase its [laminar flow 

control systems] technology readiness level so that it could be applied to any 

generic wide-body or single-aisle hinged rudder,” said Tony Washburn, NASA’s 

ERA chief technologist. In the near term, successful results of AFC flighttesting could be applied to development of a drag-reducing hybrid laminar 

flow-control system for Boeing’s stretched 787-9.25

Insect Accretion Mitigation

The second ERA technology maturation task involving the 757 ecoDemonstrator 

was the Insect Accretion Mitigation (IAM) experiment. Studies have shown 

that keeping the airflow smooth, or laminar, over a wing can reduce fuel consumption by roughly 5 to 6 percent, on average. Among the major challenges 

to maintaining laminar flow over wing surfaces is roughness induced by insect 

contamination. Even something as seemingly inconsequential as bug remains 

spattered on a wing’s leading edge can cause turbulent wedges that interrupt 

laminar flow, resulting in an increase in both drag and fuel consumption. The 

IAM research effort involved an investigation of insect protection coating technologies for mitigating wing and leading edge insect residue adhesion in order 

to enhance and maintain natural laminar flow over the ecoDemonstrator’s 

right wing.26

Success depended on consultation with entomologists and extensive study 

of bug chemistry and analysis of what happens when an insect strikes a surface at a high velocity. “We learned when a bug hits and its body ruptures 

the blood starts undergoing some chemical changes to make it stickier,” said 

Mia Siochi, senior materials scientist at Langley. Then the materials scientists 

turned to nature—specifically, lotus leaves—to create the right combination 

of chemicals and surface roughness for the test coatings. “When you look at a 

lotus leaf under the microscope the reason water doesn’t stick to it is because it 

24. Jeanne Yu interview, NASA X “End of an ERA—Part 2,” NASA TV, December 18, 2015.

25. Guy Norris, “‘Green’ Airliner Targets Achievable by 2025, Says NASA,” Aviation Week & Space 

Technology, http://aviationweek.com/awin/green-airliner-targets-achievable-2025-says-nasa, 

April 18, 2011 (accessed July 21, 2016).

26. Nickol and Haller, “Assessment of the Performance Potential of Advanced Subsonic Transport 

Concepts for NASA’s Environmentally Responsible Aviation Project,” 3.
Green Light for Green Flight

190

has these rough features that are pointy,” said Siochi. “When liquid sits on the 

microscopically rough leaf surface, the surface tension keeps it from spreading 

out, so it rolls off,” she added. “We’re trying to use that principle in combination with chemistry to prevent bugs from sticking.”27

“Bug team” researcher John Gardner lies inside Langley’s Basic Aerodynamics Research Tunnel, 

or BART, to prep the leading edge of a scale model wing for a blast of bugs from the tunnel’s 

“bug gun.” (NASA)

During ERA Phase 1, NASA engineers developed and tested more than 

200 nonstick coating formulations at LaRC. Initial testing used a small wind 

tunnel to propel insects onto a simulated wing leading edge at approximately 

150 mph. Coating performance success criteria were based on the insect residue coverage area as measured by profilometry, a quantification of surface 

roughness. Researchers exposed potential IAM coating samples to sunlight and 

weather to measure durability and the effects of ultraviolet radiation. They then 

conducted another low-speed wind-tunnel test in the LaRC Basic Aerodynamic 

Research Tunnel (BART) to further down-select the final coatings for flighttesting. Following the BART test, an assortment of coatings were affixed to 

27. Kathy Barnstorff, “NASA Tests Aircraft Wing Coatings that Slough Bug Guts,” June 1, 2015, https://

www.nasa.gov/press-release/nasa-tests-aircraft-wing-coatings-that-slough-bug-guts (accessed 

February 5, 2017).
ERA and the Boeing 757 ecoDemonstrator

191

the wing of a NASA Dassault HU-25C Guardian Falcon (a militarized Falcon 

business jet formerly used by the Coast Guard as a maritime patrol airplane) 

for a short series of low-altitude flight tests. Based upon the results, the five 

most promising candidates were chosen for use in the 757 ecoDemonstrator 

IAM flight demonstration.28

One of NASA’s two ex-Coast Guard Dassault HU-25C Guardian Falcons used for a variety of 

earth sciences and other research missions. (NASA)

The next step was to select a bug-infested area in which to flight-test the 

treated surfaces, and that met the NASA-defined insect density criterion of 25 

strikes per square foot. Because insect life is rarely a factor above 10,000 feet, 

the 757 would have to be flown at low level in a bug-infested area. Researchers 

installed high-definition cameras in the airplane’s forward cabin to view the test 

sections on the wing through optically clear windows and record the number 

and size of the insect strikes during each flight. In order to determine the best 

location for carrying out the experiment, test planners listed all airports capable 

of handling a 757 around the country, and then matched this with information from entomologists about insect breeding and migration patterns.29 A 

team that included members from NASA, Boeing, University of CaliforniaDavis, and the Department of Transportation’s Volpe National Transportation 

Systems Center narrowed a list of 86 airports in 15 states to six airports in three 

28. Alexander, et al., 13.

29. Norris, “Bug Smasher,” 37.
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192

states. Eventually, Shreveport Regional Airport in Louisiana was chosen due to 

a combination of temperature, geology, runway length, and the potential for 

large numbers of insects. To capture insect accumulation rates, initial test flights 

established a baseline using uncoated surfaces. Researchers then installed eight 

panels treated with samples of each of the coatings onto the ecoDemonstrator’s 

second and third leading-edge slats. Among the engineering goals was to test 

how durable the coatings were; treated surfaces will be effective as drag reducers 

only if they can withstand the harsh flying environment. Any degradation of 

the coatings will result in excess drag.30

The five anti-adhesion surface treatments chosen for the final evaluation 

were sprayed in a 0.012-inch thick layer onto substrate panels made from 

27-by-30-inch 7075-T6-clad aluminum panels. Researchers called this combination an engineered surface, or ES, panel. These ES panels were positioned 

on the wing edge adjacent to accompanying control test articles, which were 

substrate panels without coatings. Researchers arranged the ES and control 

panels in an alternating pattern along slats 8 and 9 on the right wing of the 

ecoDemonstrator. Prior to flight, Boeing conducted a series of spray tests to 

develop and verify application procedures. These were done to address specific 

concerns and questions regarding material quantities needed for producing the 

coated panels for the flight test, mixing and application protocols to produce 

consistent and uniform coatings, application tools and methods, and cure rates. 

Lessons learned from these spray tests ensured the successful application of 

IAM coatings to the 37 ES panels used in the flight demonstration.31

As part of the evaluation, Boeing conducted a trade study to determine the 

optimum method for attaching the ES and control panels to the wing slat’s 

outer surface. The best solution involved applying double-sided adhesive tape 

to the backside of the aluminum substrate panel. This tape demonstrated sufficiently low adhesion strength to allow for removal of the substrate without 

deforming the plastic coating, yet was strong enough to withstand aerodynamic 

loading during flight. This allowed the experimental panels to be removed 

without degrading insect residue data. The tape also had to withstand exposure 

to solar radiation and temperature variations. A small piece of copper tape 

on each panel satisfied grounding requirements for mitigating static electricity generated by the striking of rain, snow, hail, dust, or other atmospheric 

particles on the airplane’s surface. Technicians also sealed the edges of the ES 

panels with tape to prevent edge peeling due to aerodynamic forces. During 

ES installation procedural development, researchers noticed some amount of 

lower surface creep and de-bonding from the slat surface resulting from the 

30. Barnstorff, “NASA Tests Green Aviation Technology on ecoDemonstrator.”

31. Alexander, et al., 13–14.
ERA and the Boeing 757 ecoDemonstrator

193

stiffness of the substrate panel. To alleviate this, they added a 2-inch strip of 

high-strength double-sided, pressure-sensitive adhesive tape. Because this portion of the panel was not subject to insect accretion, some plastic deformation 

during removal was acceptable.32

NASA researchers check results following preliminary flight tests of nonstick anti-insect 

coatings. (NASA)

Test planners selected flight profiles so as to maximize insect accretion while 

flying a variety of takeoff conditions to simulate actual airline operations. To 

achieve a range of climb rates and speeds, the flight crew performed takeoffs 

at a range of trailing edge flap angles. The leading edge Krueger flaps on both 

wings were fully deployed and were not retracted during these tests. A total of 

15 IAM test flights were conducted to maximum altitudes of between 1,500 

and 10,000 feet. Selected crewmembers aboard the 757 ecoDemonstrator 

monitored onboard camera data during each flight, recording insect accretion 

counts by panel and slat zone. A 1 square foot template was used to aid in 

the counting to ensure that each test run met an insect density criterion of 25 

strikes per square foot, though this device proved difficult to use. Markings 

32. Ibid., 14.
Green Light for Green Flight

194

on the uncoated substrate panels split each into equivalent frontal area zones 

to assist onboard accumulation monitoring.33

Ultimately, NASA and Boeing created a new assessment process for counting the number of insect strikes in zones 2 and 3, and changed the insect density 

criterion from 25 strikes per square foot to a total count between 53 to 62 hits 

in both zones per panel. This process reduced the time needed to confirm the 

density count. During periodic pauses between test runs, with the flight crew 

still on board, NASA and Boeing researchers on the ground verified whether 

the insect strike density on the IAM control panels had been met. If not, the 

flight crew conducted additional sorties until onboard data indicated the density criterion was met before returning to Shreveport. Afterward, all of the ES 

and control panels were photographed, underwent an in situ accretion count, 

and were removed from the slats before being delivered to LaRC for a detailed 

insect count. In addition, an entomologist identified the various insects that 

had impacted the panels. Of the five different surface treatment coatings, the 

best (IAMC-5B) demonstrated up to a 37 percent average reduction in insect 

accretion over the control panels.34

The research team was very happy with the results, as insect accretion meant 

drag, and drag meant higher fuel burn. In the complex world of aircraft operations, with billions of passengers flying in jetliners each year, saving even a 

small percentage of fuel burn adds up to massive savings in operating costs and, 

more important still, reduces harmful emissions that threaten the environment. 

“This has been about a 5-year effort; we started in the lab, knowing that the 

problem is that when bugs stick to aircraft wings it affects laminar flow and 

fuel efficiency,” said Mia Siochi. “We’ve taken samples of over 200 different 

compositions from the lab to wind-tunnel tests to flying the best five on an 

actual aircraft; if at least one of those works in practical application, where we 

have significantly reduced the number of bugs sticking to the surface in critical 

locations where they affect the flow, then this has been a success.”35

Following completion of the 757 ecoDemonstrator program in August 

2015, the airplane met an unusual fate for an experimental testbed. Boeing 

collaborated with Stifel Bank’s aircraft finance division, which owned the airplane, as well as with the Aircraft Fleet Recycling Association and an airplane 

demolition company in Spokane, Washington, to dismantle and recycle the 

757 using environmental best practices. Including parts and materials, about 

90 percent of the airplane by weight was reused or recycled, with only 10 

33. Ibid., 16.

34. Ibid., 16–17.

35. Mia Siochi interview, NASA X “End of an ERA—Part 2,” NASA TV, December 18, 2015.
ERA and the Boeing 757 ecoDemonstrator

195

percent being disposed of in a landfill.36 This was entirely in keeping with the 

project’s theme of environmental responsibility.

The airplane flew to Moses Lake Airport in Washington, where technicians 

from Aircraft Demolition, LLC, demolished it with an industrial excavator 

while Boeing engineers and members of the news media watched. Boeing participated in order to study more efficient recycling techniques and determine 

how to extract the most value from materials and parts on future airplanes. The 

demolition company removed valuable parts from the airframe and sold them 

for Stifel Bank. Included were such items as landing gear, avionics, lavatories, 

and ductwork, and high-value metals. Many of the components were overhauled, re-certified, and returned to service. “We are using this opportunity to 

look at how we can design our airplanes better in the future for end-of-service 

recycling,” said Jeanne Yu. “Watching this has given us insight about using 

better materials to build the airplane, with recycling in mind, or better design 

that allows the airplane to be disassembled easier.”37

After 2 months of extracting valuable components, all that remained was 

recyclable aluminum and unusable material. All of the aluminum scrap went 

to a company in Tacoma, Washington, where it was sold for re-use in Asia 

and elsewhere. Boeing engineers used the opportunity to evaluate how more 

of the airplane-grade aluminum might be recycled for use in future aircraft. 

“Designing for producibility and safety have always been high priorities as 

we draw up our next new airplane,” said Mike Sinnett, Boeing Commercial 

Airplanes vice president for product development. “The ecoDemonstrator program is showing us that looking at the end-of-service of an airplane should 

be a consideration also to create higher residual value, economically and 

environmentally.”38

36. Kowal and Jensen, “The Boeing ecoDemonstrator Program,” 3-4.

37. Boeing, “ecoDemonstrator 757 disassembled for final recycling research project,” September 8, 

2015, http://www.boeing.com/company/about-bca/washington/ecodemonstrator-757-

disassembled-for-final-recycling-research-project-09-08-2015.page (accessed January 14, 

2017).

38. Ibid.
196

Hybrid Wing-Body (HWB) aircraft powered by UHB turbofan engines promise remarkable flight 

efficiency coupled with design elegance. Here is one promising Boeing concept evaluated 

by NASA. (NASA)
197

AFTERWORD

End of an ERA: 

Conclusions and Lessons Learned

NASA’s leadership realized when the ERA project began that researchers 

would have difficulty achieving its stated goals. NASA executives had tasked 

researchers with reducing aircraft drag by 8 percent, overall structural weight 

by 10 percent, airframe and engine noise by more than 12 percent, enginespecific fuel consumption by 15 percent, exhaust NOx emissions by 75 percent, and aircraft noise to nearly one-eighth of today’s standards, all by the 

year 2025. Compounding the 

difficulty of making such dramatic improvements was the 

fact that many in the aviation 

industry believed that conventional tube-and-wing aircraft 

had already reached the pinnacle of efficiency. Talented 

experts from several NASA 

field centers, along with highly 

skilled teams from industry 

and academia, set out to prove 

naysayers wrong and fulfill the 

promise of making air travel 

more environmentally friendly. 

NASA Integrated Aviation 

Systems Program (IASP) director Dr. Edgar “Ed” Waggoner, 

sees the ERA project as a clear success. “We [developed] new airframes and we 

will be delivering technologies that meet all of those goals; every one of our 

partners is getting something out of it.”1

1. NASA X, “End of an ERA – Part 1,” NASA TV, November 24, 2015.

NASA IASP director Dr. Edgar “Ed” Waggoner lauded 

the ERA project for meeting its goals and delivering 

revolutionary new “green” technology. (NASA)
Green Light for Green Flight

198

The 6-year endeavor progressed from innovative ideas to multiple designs, 

analytical models, wind-tunnel tests, ground-based experiments, and testing of 

actual flight hardware in a relevant environment. ERA project teams employed 

an incremental process to achieve their ambitious goals in an orderly and 

logical fashion. Throughout Phase 1, researchers strove to determine whether 

certain technologies were, in fact, viable. Lessons learned from more than 30 

of these smaller experiments paved the way for selection of the eight integrated 

technology demonstrations undertaken during Phase 2. The ERA project was 

organized so as to mature the most promising technologies and advance aircraft configurations to meet midterm goals for community noise, reduced fuel 

burn, and NOx emissions. Researchers were also tasked with determining the 

potential impact of introducing these advances into the NextGen air transportation system. The solutions they have achieved will undoubtedly reduce fuel 

consumption by up to several percentage points for the aviation community. 

That may not sound like much, but shaving aircraft fuel consumption by 

even a few percentage points will save millions of dollars and help protect the 

environment from harmful emissions.2

Work within the ERA project was coordinated with research performed 

by other programs within NASA’s Aeronautics Research Mission Directorate 

as well as other Federal Government agencies. NASA also put mechanisms in 

place to engage academia and industry, including working groups and technical 

interchange meetings, and Space Act Agreements for cooperative partnerships. 

In addition, the NASA Research Announcement process provided for full and 

open competition for the best and most promising ideas. With the exception 

of proprietary data, the ERA Project disseminated all research results to the 

widest practical extent. “Most of the technologies we looked at, like low-noise 

landing gear or adaptive trailing edges or engine technologies, are broadly 

applicable to a number of different aircraft configurations,” said Rich Wahls. 

“Throughout the 6-year span of ERA, we were looking at tradeoffs between 

better technologies and different aircraft configurations.”3

These new technologies and concepts may soon start appearing on production airplanes. In 2016, FlexSys formed a joint venture with Aviation Partners, 

Inc., of Seattle, Washington, to commercialize the wing morphing technology. 

Additionally, NASA and Air Force researchers have begun a second phase of 

ACTE testing at AFRC. Future efforts in the FAA CLEEN program may 

advance PRSEUS into TRL6 and TRL7. Advanced turbofan and combustor 

technologies evaluated during the ERA project are also moving forward. 

2. NASA X “End of an ERA – Part 2,” NASA TV, December 18, 2015.

3. Dr. Richard A. Wahls interview with the author, August 4, 2016.
End of an ERA: Conclusions and Lessons Learned

199

Projected Integrated Portfolio Benefits

Mission Fuel Burn/Carbon Emissions

Projected benefits of ERA technologies on future conventional and HWB transport designs. (NASA)

Although most ERA technologies were not intended to be configurationspecific, a great deal of effort focused on development of the HWB concept. 

This revolutionary configuration was the first modern commercial transport 

design to completely move away from the traditional tube-and-wing paradigm. 

“You take an airplane from, say, 1950, and an airplane from today, and lay 

them on top of each other, and they have almost exactly the same outline,” said 

Kevin James, ERA project manager at ARC. He added that in order to realize 

dramatic improvements in fuel economy, emissions, and acoustics, “We have to 

keep looking and sometimes think outside the box; with a hybrid wing body, 

depending on how you control the wake and how you put the engines on it, we 

have significant improvements in fuel burn and also improvements in noise.”4

Testing of the X-48B and X-48C successfully demonstrated HWB lowspeed flight control characteristics. Controllability and construction methods 

for building a flat-sided pressure vessel were only two of the key enablers for 

designing a practical HWB. Engine operability was another. Wind-tunnel testing at LaRC helped researchers determine how to integrate modern engines 

4. Kevin James interview, NASA X, “End of an ERA – Part 2,” NASA TV, December 18, 2015.
Green Light for Green Flight

200

with the HWB airframe. Work by various manufacturers on BWB and HWB 

configurations continues.5

Year

Jet Fuel Use (Billion Gallons)

Projected Impact of ERA on the Fleet

15

2005 2010 2015 2020 2025 2030 2035 2040 2045 2050

20

25

30

35

40

45

50

55

RTC

BAU

ITD

ITD+

N2

N2+

Through 2050 the cumulative delta between RTC to ITC 

is 88 billion gallons = 264 billion dollars

BAU: Business as usual, no technology insertion

RTC: Potential impact of technology available prior to ERA

ITD: Potential impact of ERA Integrated Technology Demos

Application of ERA technologies to future airline fleets will reduce operational costs by billions of 

dollars. (NASA)

By the time the ERA project concluded on January 7, 2016, NASA had 

invested more than $400 million, with a further $250 million of in-kind 

resources contributed by industry partners. The cost may have been worth 

it, according to Dr. Jaiwon Shin. “If these technologies start finding their 

way into the airline fleet, our computer models show the economic impact 

could amount to more than $255 billion in operational savings between 2025 

and 2050.”6

“The mandate was a 50 percent reduction in fuel burn and a 42 db reduction in noise in a cumulative fashion from a 1999-target airplane, and where 

we ended up was really, really close,” said Kevin James. “When we start talking 

about numbers, even if it’s cumulative, that is a huge improvement; it really 

is in terms of quality of life, especially for people living around airports.” He 

took the opportunity to articulate that, among so much else, the ERA project had involved a lot of hard work. “There have been some frustrations, but 

that’s kind of what makes the job fun sometimes, [when you are] solving the 

5. Collier interview.

6. Pia Bergqvist, “NASA’s ERA Project Could Save Airlines Billions,” Flying, (January 13, 2016): https://

www.flyingmag.com/nasas-era-project-could-save-airlines-billions/ (accessed August 21, 2019).
End of an ERA: Conclusions and Lessons Learned

201

challenges.” He said that he enjoyed seeing all the pieces come together in terms 

of project management, vehicle concepts, and systems integration. “At the end 

of the program, it was just incredible seeing the progress that was made and 

the deliverables that we’re getting from all the components,” he said. “As they 

come together, we’re going to be in a position to say that this works, and that 

this is where we should be headed.”7

7. Kevin James interview.

203

APPENDIX

Environmentally Responsible 

Aviation Technical Overview

The following appendix is a seminal May 2009 presentation given by Rich 

Wahls before a Meeting of Experts sponsored by the Aeronautics and Space 

Engineering Board of the National Research Council. It is included in this work 

because it had far-reaching influence on the subsequent ERA effort.

Readers who wish to view it online can access it here: https://www.hq.nasa.

gov/office/aero/pdf/2009_05_14_nrc_rich_wahls_508.pdf.
204

Environmentally Responsible Aviation Environmentally Responsible Aviation Technical Overview Technical Overview

Presented by Presented by

Rich Wahls

Contributors ContributorsFay Collier, Ruben Fay Collier, Ruben DelRosario elRosario, Dennis Huff, Larry Leavitt, Pat Dennis Huff, Larry Leavitt, Pat Stoliker toliker, Ton

multi-center planning team

Meeting of ExpertsAeronautics and Space Engineering Board MeetingNational Research CouncilWashington, DCMay 14-15, 2009
205Environmentally Responsible Aviation 

• Overview

– Vision, Mission, Scope, Goals– Alternate Vehicle Concepts and Technologies

• Technical Approach

– Project Framework/Schedule– Critical Technology Areas

• Concluding Remarks
206

Environmentally Responsible Aviation 

ERA Project 

ERA Project 

• Vision

– ERA will expand the viable and well-informed trade space for 

decisions enabling simultaneous realization of National noise, 

performance goals

– ERA will enable continued aviation growth while reducing or e

effects on the environment

• Mission

– Perform research to explore/assess the feasibility, benefits, in

and risks of vehicle concepts and enabling technologies ident

potential to mitigate the impact of aviation on the environment

– Transfer knowledge outward to the aeronautics community, a

fundamental aeronautics projects

• Scope

– N+2 vehicle concepts and enabling technologies

– System/subsystem research in relevant environments
207Environmentally Responsible Aviation 

ERA Proj

ERA Proj

National Pla

• Mobility, Security/Defense, Safety, Energy & En

– Enable growth in Mobility/Aviation/Transportation– Dual use with Security/Defense– Safety and Cost Effectiveness are pervasive factors

• Energy and Environment goals are central to E

– Energy Diversity

• use of alternative fuels, not creation of alternative fuels

– Energy Efficiency– Environmental Impact

• reduction of impacts, not reducing scientific uncertainties 
208

Environmentally Responsible Aviation 

Subsonic Fixed Wing System 

.... technology for improving noise, em

Approach- Enable Major Changes in Engine Cycle/Airframe Configurations

- Reduce Uncertainty in Multi-Disciplinary Design and Analysis Tools a

- Develop/Test/ Analyze Advanced Multi-Discipline Based Concepts an

Noise

-60% -75% 

-33%** -40%** 

-33% -50% ex

N+1 (2015)***

Generation

Conventional Configurations

relative to 1998 reference

N+2 (2020)***

Generation

Unconventional Configurations

relative to 1998 reference

Ad

relati

LTO NOx Emissions

(below CAEP 6)

Performance:

Aircraft Fuel Burn

Performance:

Field Length

-32 dB

(cum below Stage 4)

-42 dB

(cum below Stage 4)

CORNERS OF THE

TRADE SPACE

***Technology Readiness Level for key technologies = 4-6

** Additional gains may be possible through operational improvements

* Concepts that enable optimal use of runways at multiple airports within the metropolitan area
209Environmentally Responsible Aviation 

Alternate Configuratio

Alternate Configuratio

Many ideas, but...What combination of configuration and technology can What is possible in the N+2 timeframe?

Boeing NRA

FAP Annual Mtg 10/08

Airbus

Aviation Week 1/15/01 

Airbus

Aviation Week 1/15/01 NASA VSP

2003

NASA- M Moore

2009

easyJet ecoJet

Reuters 6/14/07
210

Environmentally Responsible Aviation 

Underlying T

Underlying T

• Technology enablers - broadly applicable

– less visible than configuration features– applicable to alternate and advanced conventional c– Noise: continuous mold lines, increasing ducted BP– Emissions: low NOx combustion, reduced fuel burn – Fuel Burn: lightweight structure, reduced drag, and 

Velocity

TSFC

Lift

Drag

ln

Wfuel

WPL + WO

=

•Aerodynamics • Empty Weight •Engine Fuel Consumption

Aircraft

Range 1 +

Velocity

TSFC

Lift

Drag

ln

Wfuel

WPL + WO

=

•Aerodynamics • Empty Weight •Engine Fuel Consumption

Aircraft

Range 1 +
211Environmentally Responsible Aviation 

Alternate Configuratio

Alternate Configuratio

a case study to a case study to 

• Many ideas, but most concepts remain on paper

– Hybrid Wing Body (HWB) concept has been explored in more detail– 1989 Origins: NASA Advanced Concepts Workshop challenges aero

– 1990s System Concept Studies, Technology Challenges identified

33% wetted area reduction

offers huge viscous drag reduction potential

• Greater fuel e• Reduced Env• Operational F

• Noncylindrica• Edge-of-the-e• Propulsion-A

Benefits

Challeng
212

Environmentally Responsible Aviation 

Alternate Configuratio

Alternate Configuratio

a case study to a case study to 

• Many ideas, but most concepts remain on paper

– Hybrid Wing Body (HWB) concept has been explored with mo

– 2000s

Research addressing technology challenges, ongoing system 

• Provides a framework for advancement of broadly applicable te

– Today

Continues to show potential of simultaneously meeting the N+
213Environmentally Responsible Aviation 

Potential Reduction in Fuel C

Potential Reduction in Fuel C

2) N+2 HWB

-91,900 lbs

-38.8%

2 3) N+2 HWB + more aggress

-107,200 lbs

-45.2%

3

Reference Fuel Burn = 237,100 lbs

1997 Technology Large Twin Aisle Vehicle “777-200E

1) N+2 Advanced "tube-and-w

-75,200 lbs

-31.7%

1

Fuselage – com

Wing – compos

Adv Composite 

Adv Propulsion

HLFC - wing/na

Embedded eng

LFC - centerbo
214

Environmentally Responsible Aviation 

N+2 Potential Noise 

N+2 Potential Noise 

Thomas, Berton, et al

Includes estimate of maximum propulsion noise shielding

delta dB below Stage 4

0.0

10.0

20.0

30.0

40.0

50.0

-10.0

11.4 dB baseline

1.1 dB chevrons

Best Cumulative Estimate

Adv Tube & Wing

Stage 4 - 26 dB

HWB HWB HWB HW

19.9 dB shielding

22.3 dB baseline
215Environmentally Responsible Aviation 

Market Needs and Des

Market Needs and Des

A sweet spot for noise and fuel burn

...but lower cruise speed may change technologi

Market will ultimately determine outcome

...but concepts and technologies enable o

...as might payload/range (vehicle size)

... or relative emphasis on noise & fuel burn
216

Environmentally Responsible Aviation 

Design Trades and Dep

Design Trades and De

Our focus is Noise, Energy Efficiency, and Emis

...but airplane design is a balance among 

Madden, IC
217Environmentally Responsible Aviation 

The W

The W

• System research to bridge the gap between fundament

(TRL 1-4) and product prototyping (TRL 7)

– Identify vehicle concepts with the potential to simultaneously m

for noise, emissions, and fuel burn in the N+2 timeframe

– Understand the concept and technology feasibility/risk vs pote

– Understand the concept and technology trades and interdepe

fidelity in relevant environments

– Determine safety implications of new technologies and config

• Technology investments guided by

– matured in fundamental program and worthy of more in-depth 

system level in relevant environment

– systems analysis indicates most potential for contributing to s

attainment of N+2 goals

– identified through stakeholder input as having potential for con

simultaneous attainment of N+2 goals
218

Environmentally Responsible Aviation 

• Overview

– Vision, Mission, Scope, Goals– Alternate Vehicle Concepts and Technology

• Technical Approach

– Project Framework/Schedule– Critical Technology Areas

• Concluding Remarks
219Environmentally Responsible Aviation 

Research F

Research F

1.0 Project Management2.0 Airframe Technology

2.1 Lightweight Structures2.2 Flight Dynamics and Control2.3 Drag Reduction2.4 Noise Reduction

3.0 Propulsion Technology

3.1 Combustor Technology3.2 Propulsor Technology3.3 Core Technology

4.0 Vehicle Systems Integration

4.1 Systems Analysis4.2 Propulsion Airframe Integration4.3 Propulsion Airframe Aeroacoustics

4.4 Advanced Vehicle Concepts

Natural MetricsML/D, Empty Weight, Airframe Nois

investigations where propulsion system 

SFC, Engine Noise, Emission Index

investigations where airframe system is 

ML/D, Weight, SFC, Emission Index

investigations where propulsion/airframe 
220

Environmentally Responsible Aviation 

FY09 FY10 FY11 FY12 FY13 FY14 

Technical input from Fundamental Programs, NRAs, Industry, Academia

Initial NRAs

External

Input

ERA P

ERA P

Phase 1 Investigations

Phase 2 Investigatio

Key Decisionsfor Phase 2

Prior

Research Planning

$62.4M $64.4M $67.1M $64.4M $60.5
221Environmentally Responsible Aviation 

Initial NRA Topics Under D

• Topic 1 - N+2 Advanced Vehicle Concepts

– Concept development and technology roadmaps

– Scope key system Investigations to inform Phase 2 decisions

• Topic 2 - Low NOx Combustors

– Concept development and technology roadmaps

– Initial flametube experiments

– Inform Phase 2 decisions

• Topic 3 - Quick-Start System Research Investigations

– Complementary to Phase 1 investigations– Early technical progress/results toward ERA goals– Inform Phase 2 decisions

Bidders Conference Prior to Solicitation
222

Environmentally Responsible Aviation 

Phase 1 Inv

Phase 1 Inv

• Scope

– Concepts and technologies from fundamental projects ready for syst– System integration and multidisciplinary risks/barriers– 2-3 years

• Critical Technology Focus

– Stitched composite technology for low weight and damage tolerance– Laminar flow technology for drag reduction– Flight dynamics & control technology enabling alternate configuration– Combustor technology for low emissions– Propulsion technology and integration for SFC and noise reduction– Propulsion shielding for noise reduction

• Outcome

– Selected concepts and technologies explored/assessed with respect 

benefits, interdependencies, and risks - uncover unexpected multidis

– New and/or refined ideas emerge– Detailed information to update systems studies, and for prioritization 

Phase 2 investigations
223Environmentally Responsible Aviation 

Phase 2 Inv

Phase 2 Inv

• Key Decisions

– FY12 timeframe plus/minus 1 year - not a specific point in tim

• Scope

– Similar to Phase 1, plus further exploration of Phase 1 concep

technologies as appropriate

– 3-4 years

• Technology Focus

– Informed by Phase 1 progress/results, system studies, stakeh

– Envision investigations which integrate results from Phase 1, 

sources

• Outcome

– Selected concepts and technologies explored/assessed with r

feasibility, benefits, interdependencies, and risks - trade space 
224

Environmentally Responsible Aviation 

Loading

Damage

Size

x

x

“safe-life”

conv composites

damage tolerance

“fail-safe”

metallic & stitched composite

Lightweight 

Lightweight 

Advanced Stit

• Can the same load limits as metal be applied to a lower weight comp

• Can structural weight be reduced while meeting certification/safety re

• Can cabin noise be acceptable with lightweight structure, particularly 

propulsion noise shielding?

Stit

Pultruded Ro

Stitches

Stitched Composites - enabling weight reduction with load limits of metals

Damage Tolerance, Durability, Flexibility, Cabin Noise

Adapted from

Velicki 2009 Aging A/C Conf

Energy Efficiency
225Environmentally Responsible Aviation 

Lightweight 

Lightweight 

• Objective

– Explore/validate/characterize/document new

stitched composite structural concept under realistic loads

• Approach

– Building block experiments on sub components, joints, cutouts

– Explore repair/maintenance, NDE methods

– Large scale pressurized multi-bay fuselage section under comb

– Incorporation of IVHM sensors in large scale COLTS test

• Benefit

– Validate damage-arresting characteristics under realistic loads.

Expected 20% reduction in weight and cost of conventional com

concepts. Extensible to wings, etc.

PRSEUS Pressure

Panel Test Complete

FY10 FY11 FY12 FY13 F

Large-Scale PRSEUS

Test in COLTS Complete

possibilities

• PRSEUS wing• alternate structu

• integrate with ot

• enable unconve

Energy Efficiency

concept understood

for large integrated component

Noise Transmission

Assessment

Design Criteria

for Low Noise Lt Wt Structure
226

Environmentally Responsible Aviation 

Flight Dynamic

Dynamic

• Can alternative vehicle concepts meet Federal airworthiness require

without negating performance/acoustic benefits?

• Can alternative vehicle concepts meet passenger ride quality expec

without negating performance/acoustic benefits?

• Can advanced controls enable performance and safety improvemen

beyond simply enabling a new vehicle concept?

Unconventiona

provide uniq

Enabling

Flight Controls - enabling alternate vehicle conceptsHandling/Ride Quality, Safety of Flight

Regulatory acceptance

Market acceptance

Performance benefit
227Environmentally Responsible Aviation 

Flight Dynamics a

Flight Dynamics a

• Objective

– Explore/assess dynamics and control design space for unconventiona

flexible wing vehicle, w/ extensibility to other advanced aircraft designs

• Approach

– Utilize extensive HWB database to develop full-scale piloted

motion-based simulation for advanced HWB concept; establishcontrol system design requirements and guidelines for HWB aircraft

– Complete X-48B flight test– Explore/assess a broad range of handling, ride quality, control authorit

load alleviation, upset recovery, aero-servoelastic control concepts/cha

• Benefit

– Advanced/adaptive control law technology for handling, ride quality, an

extensible to a range of advanced vehicle concepts

Full-scale

Piloted Sim

for Handling

Qualities Complete

FY10 FY11 FY12 FY13 F

Adv Control Laws for

Lightly Loaded and/or

Flexible Wing Concepts

Validated in Simulators

X-48B Flight

Tests Complete

X-48C Tests

In 30x60 Complete

possibilities

• flight experiments with ada

• other control concepts in p

• enable investigation of ligh

• enable unconventional flig

Enabling

full envelop HWB control and dynamics understood;

advanced controls explored
228

Environmentally Responsible Aviation 

Drag 

Drag 

• Aerodynamic/drag benefits are known, and depend on application (c

Challenges

• Integration trades for high-lift performance, and suction systems for 

• Robustness to contamination and structural/surface imperfection

• Ability ground test/assess across full flight envelop at relevant condi

Laminar Flow - breaking down technical barriers to practical laminar flow aSystem integration trades, robustness, pre-flight assessment

Energy Efficiency

Laminar flow yet to be exploited on

transonic transport aircraft

Ac

Pass

Conce
229Environmentally Responsible Aviation 

G

Drag 

Drag 

FY10 FY11 FY12 FY13 F

Evaluate Ground

Test Capability

For NLF HLFC Flight Test

completed

DRE Glove Flt Teston G-IIB completed Flt Test Assessment of

Low Energy Coatings

completed

• Objective

– Enable practical laminar flow application for transport aircraft

• Approach

– Mature multiple approaches to laminar flow to enlarge trade space

– Address critical barriers to practical laminar flow application

– Explore synergy with other advanced technologies

(e.g. composite structure, cruise slots, novel high lift systems, inte

• Benefit

– Validated passive and active drag control technologies capable of 

reductions in fuel burn. Expanded design trade space with higher 

information. Expanded database (higher Rn) for validation of trans

possibilities

•

“in-service” flight t

• integrate with othe

- re-wing resea

• incorporate in des

• other drag reductio

Energy Efficiency

confidence to proceed to

highly integrated flight test experiment
230

Environmentally Responsible Aviation 

Propulsion system improvements require advances in propulsor and 

Alan Ep

Pratt & Whitney Ai

Core Improvements(direct impact on LTO NOx)

Propulsor improveme

Propulsio

Propulsio
231Environmentally Responsible Aviation 

Core/Combustor T

Core/Combustor T

Low NOx combustor concepts for high OPR environmentIncrease thermal efficiency without increasing NOx emissions

LTO NOx

• Improved fuel-air mixing to minimize hot spots that create additional NOx

• Lightweight liners to handle higher temperatures associated with higher OPR• Fuel Flexibility• DoD HEETE Program is developing higher OPR compressor technology

.... ERA will focus on new combustor technology for reduced NOx format

Injector Conc• Partial Pre-M

• Lean Direct 

Enabling Tec

• lightweight C

• advanced in
232

Environmentally Responsible Aviation 

Combustor T

Combustor T

• Objective

– Extend maturation of technologies for reducing

LTO NOx. Concepts must ensure fuel flexibility.

• Approach

– Pursue 3 concepts: Lean Partial-Mixed Combustor,

Lean Direct Multi-Injection, TBD from NRA.

– Flametube, sector, and annular combustor tests.– Select single concept for engine tests.– Assume 50% cost share with industry.

• Benefit

– Technologies to reduce LTO NOx by 75% below

CAEP/6.

Select Low-NOx

Combustor Concepts

(downselect 3 to 1)

FY10 FY11 FY12 FY13 F

Complete

Flametube

Experiments

Fu

Stag

Initiate Low-NOx

Combustor Concept

Studies

Increasing

integration/complexity

LTO NOx
233Environmentally Responsible Aviation 

Core T

Core T

• O

bje

ctiv

e

– Explore core architectures and develop key

technologies needed for N+2 propulsion

• A

p

pro

a

c

h

– Use NRA to explore core engine concepts;

specific technologies TBD but will integrateexisting work on high OPR compressors fromVAATE, turbine cooling work in SFW.

– Pursue technologies like Ceramic Matrix

Composite (CMC) materials that will benefit anygas turbine engine concept; early workassesses fabrication methods for cooled vanesand nozzles

• B

e

n

efit

– Technologies to increase thermal efficiency that

enable higher BPR propulsion (either turbofans,open rotors, or embedded engines)

F

Y10

F

Y11 F

Y12

F

Y13

F

Assess C

M

C Fab

Methods, Durability

& Heat Transfer

Identify Core

N+2 Concepts

Advanced C

(

• high OPR compressor

• increase T4 Incre

asing

integration/co

mple

x

Energy

Efficiency

LT

O

N

O

x
234

Environmentally Responsible Aviation 

Propulsor 

Ultra high bypass ratio propulsorDucted v Unducted trade, noise v efficiency

Energy Efficiency

Noise Reduction

Concepts

• Ducted UHB

• short inlets, laminar flow nacelles

• SMA variable area nozzle• soft vane, over-the-rotor treatment

• Unducted UHB (Open Rotor)

• increased rotor spacing, lower blade count

• Embedded for boundary layer ingestion

• inlet flow control, distortion tolerant fan

Challenges• Open Rotor - reduced noise while

maintaining high propulsive efficiency

• Ducted UHB - nacelle weight & drag with

increasing diameter
235Environmentally Responsible Aviation 

Propulsor Propulsor T

• Objective

– Explore propulsor (bypass flowpath) configurations for

N+2 vehicle concepts to expand and better define thetrade space between performance and noise reduction.

• Approach

– Investigate feasibility of higher BPR propulsion

systems: UHB Turbofans, Open Rotors and TBDAdvanced Propulsor identified from NRA.

– Evaluate UHB & Open Rotor for N+2; isolated and

partially installed simulations in wind tunnel tests;Handoff to VSI for full installation experiments.

• Benefit

– Propulsor concepts identified and validation data

available for noise & performance trades.

Select UHB &Open Rotor

Concepts

FY10 FY11 FY12 FY13 F

Select Adv.Propulsor

Concept

For N+2

Complete

UHBTurbofan

Tests

Complete

Isolated

Open Rotor

Tests

UHB 

O

Energy Efficiency

Noise Reduction

possibilities

• isolated and partially instal

ground tests similar to pha

• integrate with other techs (

• flight test propulsion conce

• incorporate in design of flig
236

Environmentally Responsible Aviation 

Propulsion Airframe 

Propulsion Airframe 

Lord, Sepulveda, et al

Fuel Burn Fuel Burn

Fan Diameter

Noise

(Higher FPR) (Lower FPR)

Low

High

Noise

TSFC

Fuel Burn

GTF

Adv GTF

~ 2018

Weight& Drag

Turbofan

PAI Challenges Increase

UHB Installation that minimizes or avoids performance penaltiesIncreased size of system may drive need for alternate configurations

Energy Efficiency

• Increasingly large diameters present increasingly difficult installations for con

configurations, and may require alternate configurations/installations to take 

 propulsive efficiency

.... significant vehicle level trade space to explore
237Environmentally Responsible Aviation 

Propulsion Airfram

• Objective

– Understand synergistic performance/efficiency coupling

potential between advanced propulsor and airframe concepts

• Approach

– Explore/assess (large-scale testing) performance benefits

thru integration of advanced low noise/efficient open rotor

and UHB propulsors

– Quantify installed performance benefits of alternate engine

airframe integrations (e.g. boundary layer ingestion)

• Benefit

– Enlarged PAI design trade space with new open rotor and

UHB propulsors (and integrations) with advanced N+2

airframes (15-25% fuel burn reduction)

FY10 FY11 FY12 FY13 F

Performance Assessment

of installed UHB

Performance Assessment

of installed Open Rotor

HWB Low-Speed

Performance

Assessment

Pressure Sensiti

Paint results

Possibilities

• flight tests o

• integration o

• integrations 

flow control, 

• other low no

Note:• Advanced large scale

isolated/proximity UHB and reduced

noise OR propulsor experiments

being conducted in 9x15. Results

will feed integrated experiments.

Energy Efficiency
238

Environmentally Responsible Aviation 

Propulsion Airframe A

• Objective

– Understand synergistic acoustic coupling potential between

advanced propulsor and airframe concepts

• Approach

– Explore/quantify (large-scale testing) airframe shielding

benefits thru integration of advanced low noise/efficient open

rotor and UHB propulsors

– Quantify aeroacoustic benefits of alternate engine airframe

integrations (e.g. boundary layer ingestion)

• Benefit

– Enlarged PAA design trade space for new open rotor and

UHB propulsors (and integrations) with advanced N+2

airframes (15-20 dB cum reduction to Stage 4)

FY10 FY11 FY12 FY13 F

Noise Assessment of

Installed UHB

Hot Jet Test

Technique and

Acoustic Upgrades

14x22

HWB Noise

Shielding Eval

In 14x22

Possibilities

• flight tests of se

• integration with 

• integrations with 

flow control, comp• other low noise 

Noise Assessment of

Installed Open Rotor

Noise Reduction
239Environmentally Responsible Aviation 

Airframe Nois

• Landing gear designed for performance/weight,

but generate much more noise

• High lift system gaps and exposed flap edges help perfo

but generate noise

• Currently cannot accurately account for all aircraft sourc

interactions with other components, and installation e

Noise Reduction

Quiet flaps and landing gear without performance penaltiesLow airframe noise technologies conflict with low drag/weight
240

Environmentally Responsible Aviation 

Airframe Nois

• Objective

– Understand/research synergistic coupling/multidisciplinary

aspects of integrated adv. airframe noise reduction

technologies

• Approach

– Flight test of CML flap on NASA G-IIB aircraft

– Wind tunnel and flight test campaign on large business jet

(LBJ) targeting landing gear and flap edge noise as well as

gear/flap interactions. Improved microphone array

technology.

• Benefit

– Quantified technologies for airframe noise reduction on the

order of 5-10 dB cum; enlarged design trade space for adv.

low noise configurations

FY10 FY11 FY12 FY13 F

Valid. Adv. Low Noise Gear and/or

Flap Edge Noise Concepts

on LBJ Flt Test

Assess CML

Perf & Acoustic

Benefits on G-IIB

Possibilities

• Characterize & sim

large-scale multi-ba

• Large-scale or fli

vehicle with adv. N

gear fairings or low 

flap edge devices, 

Low Noise Concepts

on LBJ in 14x22

Noise Reduction
241Environmentally Responsible Aviation 

Phase 2 Investigations 

Phase 2 Investigations 

• Key Decision Point(s) for Phase 2 in the FY12 t

• Noted several times today the idea of an exper

vehicle testbed (XVT) as centralizing focus for 

systems research on an unconventional configu

• The XVT would (very) likely require a significan

increase and/or significant cost sharing partner

• Initial NRA Topic 1 may inform us as to the pos
242

Environmentally Responsible Aviation 

XVT - Experimental Vehic

XVT - Experimental Vehic

• Drivers for a (large) Flight Research Vehicle

– Appropriate scale for aerodynamics validation

• High Reynolds number to minimize scaling issues

• High speed - compressibility effects

• Geometric fidelity

– Appropriate scale for acoustics flight test

• Geometry fidelity

• Physics of the noise sources require they be of the same 

• Scale required to understand noise attenuation and shield

– Appropriate scale for aero-elasticity and flight dynam– Capability to assess advanced flight controls conce
243Environmentally Responsible Aviation 

XVT - Experimental Vehic

XVT - Experimental Vehic

• Additional Benefit of a Flight Research Vehicle

– Validate simultaneous progress toward N+2 goals through technolog

testbed

• Gain understanding of technology interdepencies/interactions and hardw

• Ability to validate multiple off-nominal data points through full envelope 

• Flight Reynolds number with real world effects

– Produce and disseminate high quality data for technology characteriz

validation

– Collect actual flying qualities, passenger ride quality, and cabin noise – Ability to operate in the National Air Space for integration Airspace/O– Testbed for future technology concepts including propulsion systems

• AN IDEA ....

Baseline Aircraft Embedded Engine A
244

Environmentally Responsible Aviation 

Concludin

Concludin

• Explore/demonstrate the feasibility, benefits, and risks o

concepts and enabling technologies identified to have p

mitigate the impact of aviation on the environment

• Expand viable and well-informed trade space for vehicle 

decisions enabling simultaneous realization of National emissions, and performance goals; identify challenges ffoundational research

• Alternative configurations w/ advanced technology will b

simultaneously achieve the N+2 goals; technologies wilapplicable and tradable

• Systems research in relevant environment
245

Acronyms and Abbreviations

AAW Active Aeroelastic Wing 

ACCA Advanced Composite Cargo Aircraft 

ACEE Aircraft Energy Efficiency

ACT Advanced Composite Technology

ACTE Adaptive Compliant Trailing Edge 

AFC active flow control

AFFDL Air Force Flight Dynamics Laboratory

AFRC Armstrong Flight Research Center

AFRL Air Force Research Laboratory

AirSTAR Airborne Subscale Transport Aircraft Research

AMELIA Advanced Model for Extreme Lift and Improved Aeroacoustics

ANOPP Aircraft Noise Prediction Program

APU auxiliary power unit

ARC Ames Research Center

ARMD Aeronautics Research Mission Directorate

ASC axial stage combustor

ASCR Advanced Subsonic Combustion Rig

ASEB Aeronautics and Space Engineering Board

ASM Advanced Stitching Machine

ASP Airspace Systems Program

AST Advanced Subsonic Technology

ASTAR Airborne Spacing for Terminal Arrival Routes

ATDC advanced technology development compressor

AVC Advanced Vehicle Concepts

BART Basic Aerodynamic Research Tunnel

BPR bypass ratio

BVID barely visible impact damage

BWB blended wing body

CAD computer-aided design

CAEP Committee on Aviation Environmental Protection

CFD computational fluid dynamics

CLEEN Continuous Lower Energy Emissions and Noise

CL max maximum lift coefficient
Green Light for Green Flight

246

CMC ceramic matrix composite

CML continuous mold line

COLTS Combined Loads Test System

COTS commercial off-the-shelf

CRJ Canadair Regional Jet

DLL design limit load

DOD Department of Defense

DRE discrete roughness element

DUL design ultimate load

EBC environmental barrier coating

EDS Environmental Design Space

EIS entry into service

EPNdB effective perceived noise levels measured in decibels

ERA Environmentally Responsible Aviation 

ES engineered surface

FAA Federal Aviation Administration

FAP Fundamental Aeronautics Program 

FEGV fan exit guide vane

FEM finite element model

FENoRFins Flap Edge Noise Reduction Fins

FLEXSEL FLEXible Side-Edge Link

FLOPS Flight Optimization and Performance Sizing

FPR fan pressure ratios

GE General Electric

GPS Global Positioning System

GRC Glenn Research Center

GTF geared-turbofan

GVT ground vibration testing

HARV High-Alpha Research Vehicle

HPC high-pressure compressor

HWB hybrid wing-body

IAM Insect Accretion Mitigation

IASP Integrated Aviation Systems Program

ICAO International Civil Aviation Organization

IPB Internal Pressure Box

IRAD Independent Research and Development

ISRP Integrated Systems Research Program

ITD integrated technology demonstration

KDP key decision point

LaRC Langley Research Center

L/D lift-to-drag
Acronyms and Abbreviations

247

LDI lean direct injection

LFC laminar flow control

LMSW Lockheed Martin Skunk Works

LSAF Low-Speed Aero Acoustic Facility

LTA large twin-aisle

LTO landing and takeoff

MAW mission adaptive wing

MBB Multi-Bay Box

MTOW Maximum Takeoff Weight

NACA National Advisory Committee for Aeronautics 

NAF NASA Auralization Framework

NAS National Airspace System

NextGen Next Generation Air Transportation System 

NDI non-destructive inspection

NFAC National Full-Scale Aerodynamics Complex

NLF natural laminar flow

NOx nitrogen oxide

NPSS Numerical Propulsion Simulation System

NRA NASA Research Announcement

NRC National Research Council

NTF National Transonic Facility

OGA Other Government Agency

OML outer-mold-line

OPR operating pressure ratio

OTR over-the-rotor

P&W Pratt & Whitney

PAA Propulsion-Airframe Aeroacoustic

PAI Propulsion-Airframe Integration

P-mpg passenger miles flown per gallon of fuel expended

PRSEUS Pultruded Rod Stitched Efficient Unitized Structure

PSC preferred system concept

psia pounds per square inch absolute

RFI Request for Information

RFI resin film infusion

RMPn Risk Management Panel

ROLD Reactive Orthotropic Lattice Diffuser

RQL rich quench lean

SBIR Small Business Innovation Research

SCRAT Subsonic Research Aircraft

SCW supercritical wing

SFC specific fuel consumption
Green Light for Green Flight

248

SFW Subsonic Fixed Wing

SiC/SiC CMC silicon carbide/silicon carbide ceramic matrix composite

STV subscale test vehicle

SUGAR Subsonic Ultra Green Aircraft Research

SV soft vane

TAPS twin annular premixing swirler

TAW tube-and-wing

TFA Technical Focus Area

TPS turbine-powered engine simulator

TRL Technology Readiness Level

TSFC Thrust Specific Fuel Consumption

UDF un-ducted fan

UEET Ultra-Efficient Engine Technology

UHB ultra-high-bypass

UTRC United Technologies Research Center

VCK variable-camber Krueger

VFR visual flight rules

VSI Vehicle Systems Integration

WATE Weight Analysis of Turbine Engines 

XVT experimental vehicle testbed
249

Acknowledgments

The author would like to thank the many people who helped make this book 

possible; however, the judgments and views expressed are his alone and do 

not necessarily reflect the opinions or conclusions of the people who helped 

him, nor are they the official positions of the National Aeronautics and Space 

Administration. 

First of all, as always, I greatly appreciate the steadfast support and encouragement of Anthony “Tony” Springer, of NASA Aeronautics Research Mission 

Directorate (ARMD) and Director (Acting) for the Integrated Management 

Office, who sponsored this project. Additional thanks go to Gaudy M. BezosO’Connor, deputy project manager, Advanced Air Transport Technology 

Project, and Karen L. Rugg, Aeronautics Communications and Education, 

lead NASA ARMD.

I am grateful as well for the efforts of series editor Dr. Richard Hallion, as 

well as Karl Bender and the technical library staff at NASA Armstrong Flight 

Research Center and others. 

I owe special thanks to Barbara Bullock, Maxine Aldred, Michele Ostovar, 

Jennifer Way, and the Communications Support Service Center (CSSC) editorial and layout staff for preparing the manuscript for publication. Additional 

thanks to Kristin Harley for developing the index.

Special thanks as well to the subject matter experts who reviewed the material for technical accuracy—and, especially, to Sarah Merlin for copyediting 

the final manuscript.

251

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NASA/CR NAS1-NNL04AA11B. Task NNL06AB63T. The Boeing 

Company, 2006.

Guo, Yueping P., Casey L. Burley, and Russell H. Thomas. “Landing Gear 

Noise Prediction and Analysis for Tube-and-Wing and Hybrid Wing Body 

Aircraft.” AIAA-2016-1273. Presented at the 54th AIAA Aerospace Sciences 

Meeting in San Diego, CA, January 4–8, 2016.

Guo, Yueping P., Casey L. Burley, and Russell H. Thomas. “Modeling and 

Prediction of Krueger Device Noise.” AIAA-2016-2957. Presented at the 

22nd CEAS/AIAA Aeroacoustics Conference in Lyon, France, May 30–

June 1, 2016.

Guo, Yueping P., Craig L. Nickol, and Russell H. Thomas. “Noise and Fuel Burn 

Reduction Potential of an Innovative Subsonic Transport Configuration.” 

AIAA-2014-0257. Presented at the 52nd AIAA Aerospace Sciences Meeting 

in National Harbor, MD, January 13–17, 2014.

Guo, Yueping P., Casey L. Burley, and Russell H. Thomas. “On Noise 

Assessment for Blended Wing Body Aircraft.” AIAA-2014-0365. Presented 

at the 52nd AIAA Aerospace Sciences Meeting in National Harbor, MD, 

January 13–17, 2014.

Guynn, Mark D., Jeffrey J. Berton, Kenneth L. Fischer, William J. Haller, 

Michael T. Tong, and Douglas R. Thurman. “Analysis of Turbofan 

Options for an Advanced Single Aisle Transport Aircraft.” AIAA-2009-

6942. Presented at the 9th AIAA Aviation Technology, Integration, and 

Operations Conference in Hilton Head, SC, September 21–23, 2009.

Guynn, Mark D., Jeffrey J. Berton, Kenneth L. Fisher, William J. Haller, 

Michael T. Tong, and Douglas R. Thurman. “Refined Exploration of 

Turbofan Design Options for an Advanced Single-Aisle Transport.” NASA 

TM-2011-216883. NASA Langley Research Center, 2011.

Hallion, Richard P. (ed.). NASA’s Contributions to Aeronautics, v.2: Flight 

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Green Light for Green Flight

258

Hallion, Richard P., and Michael H. Gorn. On the Frontier: Experimental Flight 

at Dryden. Washington, DC: Smithsonian Books, 2001.

Heidmann, James. “NASA’s Current Plans for ERA Propulsion Technology.” 

Presented at the 48th AIAA Aerospace Sciences Meeting in Orlando, FL, 

January 4–7, 2010.

Herkes, William H., Ronald F. Olsen, and Stefan Uellenberg. “The Quiet 

Technology Demonstrator Program: Flight Validation of Airplane NoiseReduction Concepts.” AIAA-2006-2720. Presented at the 12th AIAA/

CEAS Aeroacoustics Conference in Cambridge, MA, May 8–10, 2006. 

Herrera, Claudia Y., and Shun-Fat Lung. “Aeroelastic Response of the Adaptive 

Compliant Trailing Edge Transition Section.” Presented at the 54th AIAA 

Aerospace Sciences Meeting in San Diego, CA, January 4–7, 2016.

Herrera, Claudia Y., Natalie D. Spivey, Shun-Fat Lung, Gregory Ervin, 

and Pete Flick. “Aeroelastic Airworthiness Assessment of the Adaptive 

Compliant Trailing Edge Flaps.” #46-23. Presented at the 46th Annual 

Society of Flight Test Engineers International Symposium in Lancaster, 

CA, September 14–17, 2015.

Hill, Geoffrey A., and Russell H. Thomas. “Challenges and Opportunities 

for Noise Reduction Through Advanced Aircraft Propulsion Airframe 

Integration and Configurations.” Presented at the 8th CEAS Workshop 

on Aeroacoustics of New Aircraft and Engine Configurations in Budapest, 

Hungary, November 11–12, 2004.

Hoffman, Krishna. “Air Vehicle Technology Integration Program (AVTIP), 

Delivery Order 0059: Multi-role Bomber Structural Analysis.” AFRL-VAWP-TR-2006-3067, 2006.

Horne, Michael R., and Eric I. Madaras. “Evaluation of Acoustic Emission 

SHM of PRSEUS Composite Pressure Cube Tests.” NASA TM-2013-

217993. NASA Langley Research Center, 2013.

Horne, Michael R. and Eric I. Madaras. “Evaluation of Acoustic Emission 

SHM of PRSEUS Multi-Bay Box Tests.” NASA TM-2016-218976. NASA 

Langley Research Center, 2016.
Selected Bibliography

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Huff, Dennis L. “Progress in Aircraft Noise Reduction.” Presented at the GARDN 

2nd Annual Conference in Toronto, Canada, September 25–16, 2012.

Hughes, Christopher E., “Aircraft Engine Technology for Green Aviation to 

Reduce Fuel Burn.” NASA TM-2013-217690. June 2011.

Hughes, Christopher E. “Geared Turbofan Technology.” Presented at the NASA 

Green Aviation Summit in Mountain View, CA, September 8–19, 2010.

Hughes, Christopher E. “NASA Collaborative Research on the Ultra High 

Bypass Engine Cycle and Potential Benefits for Noise, Performance and 

Emissions.” ISABE -2009-1274. Presented at the 19th ISABE in Montreal, 

Canada, September 7–11, 2009.

Hughes, Christopher E., and Envia Edmane. “Overview of Recent Ultra High 

Bypass Engine Cycle -Based Scale Model Test Results.” Presented at the 

Fundamental Aeronautics Program 3rd Annual Review, September 2009.

Hughes, Christopher E., and Wesley Lord. “NASA/Pratt & Whitney 

Collaborative Partnership Research in Ultra High Bypass Cycle Propulsion 

Concepts.” E-16905. Fundamental Aeronautics Program 2nd Annual 

Review, NASA Glenn Research Center, 2008.

Hughes, Christopher E., and Steven Smith. “The Promise and Challenges of 

Ultra High Bypass Ratio Engine Technology and Integration.” HQ-STI-11-

012. Presented at the AIAA 49th Aerospace Sciences Meeting in Orlando, 

FL, January 7–11, 2011.

Hughes, Christopher E., Dale E. Van Zante, and James D. Heidmann. 

“Aircraft Engine Technology for Green Aviation to Reduce Fuel Burn.” 

NASA TM-2013-217690. NASA Glenn Research Center, 2013.

Humphreys, William M., Mehdi R. Khorrami, David P. Lockard, Dan H. 

Neuhart, and Christopher J. Bahr. “Characterization of Flap Edge Noise 

Radiation from a High-Fidelity Model.” AIAA-2015-2991. Presented 

at the 21st AIAA/CEAS Aeroacoustics Conference in Dallas, TX, June 

22–26, 2015.

Humphreys, William M., David P. Lockard, Mehdi R. Khorrami, William 

G. Culliton, Robert G. McSwain, Patricio A. Ravetta, and Zachary Johns. 

“Development and Calibration of a Field-Deployable Microphone Phased 
Green Light for Green Flight

260

Array for Propulsion and Airframe Noise Flyover Measurements.” AIAA2016-2898. Presented at the 22nd AIAA/CEAS Aeroacoustics Conference 

in Dallas, TX, June 22–26, 2016.

Hutcheson, Florence V., Thomas F. Brooks, Casey L. Burley, Christopher J. 

Bahr, Daniel J. Stead, and Dennis S. Pope. “Shielding of Turbomachinery 

Broadband Noise by a Hybrid Wing Body Aircraft Configuration.” AIAA2014-2624. Presented at the 20th AIAA/CEAS Aeroacoustics Conference 

in Atlanta, GA, June 16–20, 2014.

Irvine, Thomas B. “Research and Technology Project Formulation Authorization 

Document: Environmentally Responsible Aviation (ERA) Project.” 

Integrated Systems Research Program, NASA Aeronautics Research Mission 

Directorate, June 9, 2009. (Gaudy Bezos-O’Connor e-mailed this document to the author.)

Jaskowiak, Martha, James A. DiCarlo, Ramakrishna T. Bhatt, Dongming Zhu, 

Ronald Phillips, Richard Rauser, and Daniel Gorican. “SiC/SiC Composites 

Evaluated for Turbine Vane Applications under NASA’s ERA Program.” 

Presented at the 36th Annual Conference on Composites, Materials, and 

Structures in Cocoa Beach, FL, January 28–31, 2012.

Jegley, Dawn. “Behavior of Frame-Stiffened Composite Panels with Damage.” 

Presented at the 54th AIAA Structures, Structural Dynamics, and Materials 

Conference in Boston, MS, April 8–11, 2013.

Jegley, Dawn. “Influence of Impact Damage on Carbon-Epoxy Stiffener 

Crippling.” American Society for Composites Meeting in Dayton, OH, 

September 20–22, 2010.

Jegley, Dawn. “The Influence of Restraint Systems on Panel Behavior.” 

Presented at the Society of Experimental Mechanics Annual Conference 

in Uncasville, CT, June 2011.

Jegley, Dawn. “Structural Efficiency and Behavior of Pristine and Notched 

Stitched Structure.” Presented at the SAMPE Fall Technical Conference in 

Fort Worth, TX, October 17–20, 2011.

Jegley, Dawn C. “Damage Arresting Composites Using Interlaminar Stitches.” 

Presented at the Composites Consortium in Hampton, VA, April 

29–30, 2015.
Selected Bibliography

261

Jegley, Dawn, Adam Przekop, Marshall Rouse, Andrew Lovejoy, Alex Velicki, 

Kim A. Linton, Hsi-yung Wu, Jaime Baraja, Patrick Thrash, and Krishna 

Hoffman. “Development of Stitched Composite Structure for Advanced 

Aircraft.” Presented at the American Society for Composites, 30th Technical 

Conference in East Lansing, MI, June 2–3, 2015.

Jegley, Dawn C., Marshall Rouse, Adam Przekop, and Andrew E. Lovejoy. “The 

Behavior of a Large-Scale, Stitched Composite Multi-Bay Pressure Box.” 

NASA TM-2016-218972. April 2016.

Jegley, Dawn C., Marshall Rouse, Adam Przekop, and Andrew E. Lovejoy. 

“Testing of a Stitched Composite Large-Scale Multi-Bay Pressure Box.” 

Presented at the AIAA SciTech Forum and Exposition in San Diego, CA, 

January 4–8, 2016.

Jegley, Dawn C., and Alexander Velicki. “Development of the PRSEUS 

Multi-Bay Pressure Box for a Hybrid Wing Body Vehicle.” AIAA-2015-

1871. Presented at the 56th AIAA/ASCE/AHS/ASC Structures, Structural 

Dynamics, and Materials Conference in Kissimmee, FL, January 8, 2015.

Jegley, Dawn C., and Alexander Velicki. “Status of Advanced Stitched Unitized 

Composite Aircraft Structures.” AIAA-2013-0410. Presented at the 51st 

AIAA Aerospace Sciences Meeting in Grapevine, TX, January 7–10, 2013.

Jegley, Dawn C., Alexander Velicki, and Daniel A. Hansen. “Structural Efficiency 

of Stitched Rod-stiffened Composite Panels with Stiffener Crippling.” 

Presented at the 49th AIAA/ASME/ASCE/SHS/ASC Structures, Structural 

Dynamics, and Materials Conference in Schaumburg, IL, April 7–10, 2008.

Jimenez, Hernando, Holger Pfaender, and Dimitri N. Mavris. “Systemwide Fleet Assessment of NASA Environmentally Responsible Aviation 

(ERA) Technologies and Concepts for Fuel Burn and CO.” AIAA-2011-

6882. Presented at the 11th AIAA Aviation Technology Integration, and 

Operations Conference in Virginia Beach, VA, September 20–22, 2011.

Jimenez, Hernando, Jeff S. Schutte, and Dimitri N. Mavris. “System Readiness 

and Risk Assessment for Advanced Vehicle Concepts – Discussion of 

Fundamental Concepts.” AIAA-2011-0423. Presented at the 49th AIAA 

Aerospace Sciences Meeting in Orlando, FL, January 4–7, 2011.
Green Light for Green Flight

262

Johnston, Patrick H. “Ultrasonic Nondestructive Evaluation of PRSEUS 

Pressure Cube Article in Support of Load Test to Failure.” NASA TM-2013-

217799. NASA Langley Research Center, 2013.

Johnston, Patrick H., and Peter D. Juarez. “Nondestructive Evaluation of 

PRSEUS during Large-Scale Load Testing and Rod Push-Out Testing.” 

NASA TM-2016-218978. NASA Langley Research Center, 2016.

Jones, Michael G., Brian M. Howerton, and Earl Ayle. “Evaluation of ParallelElement, Variable-Impedance, Broadband Acoustic Liner Concepts.” AIAA2012-2194. Presented at the 18th AIAA/CEAS Aeroacoustics Conference 

in Colorado Springs, CO, June 4–6, 2012.

Jones, Michael G., Tony L. Parrott, Daniel L. Sutliff, and Christopher E. 

Hughes. “Assessment of Soft Vane and Metal Foam Engine Noise Reduction 

Concepts.” AIAA-2009-3142. Presented at the 15th AIAA/CEAS 

Aeroacoustics Conference in Miami, FL, May 11–13, 2009.

Karal, Michael. “AST Composite Wing Study – Executive Summary.” NASA/

CR-2001-210650. The Boeing Company, 2001.

Kawai, Ron T., Derell Brown, D. Roman, and R. Olde. “Acoustic Prediction 

Methodology and Test Validation for an Efficient Low-noise Hybrid Wing 

Body Subsonic Transport.” NASA Contract NNL07AA54C. Phase I Final 

Report PWDM08-006A, 2008.

Kawai, Ron T., Douglas M. Friedman, and Leonel Serrano. “Blended Wing 

Body (BWB) Boundary Layer Ingestion (BLI) Inlet Configuration and 

System Studies.” NASA CR-2006-214534. NASA Langley Research 

Center, 2006.

Kestner, Brian K., Jeff S. Schutte, Jonathan C. Gladin, and Dimitri N. Mavris. 

“Ultra High Bypass Ratio Engine Sizing and Cycle Selection Study for a 

Subsonic Commercial Aircraft in the N+2 Timeframe.” Presented at the 

ASME Turbo Expo in Vancouver, Canada, June 6–10, 2011.

Khorrami, Mehdi R., Benjamin Duda, Andreas Hazir, and Ehab Fares. 

“Computational Evaluation of Airframe Noise Reduction Concepts at Full 

Scale.” AIAA-2016-2711. Presented at the 22nd AIAA/CEAS Aeroacoustics 

Conference in Lyon, France, May 30–June 1, 2016.
Selected Bibliography

263

Khorrami, Mehdi R., and Ehab Fares. “Simulation-Based Airframe Noise 

Prediction of a Full-Scale, Full Aircraft.” AIAA-2016-2706. Presented at 

the 22nd AIAA/CEAS Aeroacoustics Conference in Lyon, France, May 

30–June 1, 2016.

Khorrami, Mehdi R., and Ehab Fares. “Towards Full Aircraft Airframe Noise 

Prediction: Lattice Boltzmann Simulations.” AIAA-2014-2481. Presented 

at the 20th AIAA/CEAS Aeroacoustics Conference in Atlanta, GA, June 

16–20, 2014.

Khorrami, Mehdi R., Judith A. Hannon, Dan H. Neuhart, Gregory A. 

Markowski, and Thomas Van de Ven. “Aeroacoustic Study of a High-Fidelity 

Aircraft Model: Part 1 – Steady Aerodynamic Measurements.” AIAA-2012-

2233. Presented at the 18th AIAA/CEAS Aeroacoustics Conference in 

Colorado Springs, CO, June 4–6, 2012.

Khorrami, Mehdi R., William M. Humphreys Jr., David P. Lockard. “An 

Assessment of Flap and Main Landing Gear Noise Abatement Concepts.” 

AIAA Paper 2015-2987. Presented at the 21st AIAA/CEAS Aeroacoustics 

Conference in Dallas, TX, June 22–26, 2015.

Khorrami, Mehdi R., William M. Humphreys Jr., David P. Lockard, and 

Patricio A. Ravetta. “Aeroacoustic Evaluation of Flap and Landing Gear 

Noise Reduction Concepts.” AIAA-2014-2478. Presented at the 20th 

AIAA/CEAS Aeroacoustics Conference in Atlanta, GA, June 16–20, 2014.

Khorrami, Mehdi R., and Raymond E. Mineck. “Towards Full Aircraft 

Airframe Noise Prediction: Detached Eddy Simulations.” AIAA-2014-

2480. Presented at the 20th AIAA/CEAS Aeroacoustics Conference in 

Atlanta, GA, June 16–20, 2014.

Khorrami, Mehdi R., Raymond E. Mineck, Chungsheng Yao, and Luther 

N. Jenkins. “A Comparative Study of Simulated and Measured Gear-Flap 

Flow Interaction.” AIAA-2015-2989. Presented at the 21st AIAA/CEAS 

Aeroacoustics Conference in Dallas, TX, June 22–26, 2015.

Khorrami, Mehdi R., and Dan H. Neuhart. “Aeroacoustic Study of a HighFidelity Aircraft Model: Part 2 – Unsteady Surface Pressures.” AIAA-2012-

2234. Presented at the 18th AIAA/CEAS Aeroacoustics Conference in 

Colorado Springs, CA, June 4–6, 2012.
Green Light for Green Flight

264

Kimmel, William M. “Systems Analysis Approach for the NASA Environmentally 

Responsible Aviation Project.” AIAA-2011-3528. Presented at the 3rd 

AIAA Atmospheric and Space Environments Conference in Honolulu, 

HI, June 27–30, 2011.

Kirby, Michelle R., Peter A. Barros Jr., and Dimitri N. Mavris. “Enhancing the 

Environmental Policy Making Process with the FAA’s EDS Analysis Tool.” 

AIAA-2009-1262. Presented at the 47th AIAA Aerospace Sciences Meeting 

in Orlando, FL, January 5–8, 2009.

Kirby, Michelle R., Keith Becker, Steven Isley, Graham Burdette, and Dimitri 

N. Mavris. “Development of an Interactive Capability to Trade off New 

Technologies and Future Aircraft to Reduce Aviation Environmental 

Impacts.” Presented at the 27th International Congress of the Aeronautical 

Sciences in Nice, France, September 19–24, 2010.

Kota, Sridhar, Peter Flick, and Fayette S. Collier. “Flight Testing of the FlexFoil 

Adaptive Compliant Trailing Edge.” AIAA-2016-0036. Presented at the 

54th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum in San 

Diego, CA, January 4–8, 2016.

Kota, Sridhar, Russell Osborn, Gregory Ervin, Dragan Maric, Peter Flick, and 

Donald Paul. “Mission Adaptive Compliant Wing – Design, Fabrication 

and Flight Test.” RTO-MP-AVT-168. NATO Research and Technology 

Organization, 2009.

Kresja, Eugene A., and James R. Stone. “Enhanced Fan Noise Modeling for 

Turbofan Engines.” NASA CR-2014-218421. NASA Glenn Research 

Center, 2014.

Leavitt, Laurence D., and Fayette S. Collier. “Integrated Systems Research 

Programme: Environmentally Responsible Aviation (ERA) Project.” 

Presented at the ICAO Colloquium on Aviation and Proceedings Climate 

Change Proceedings in Montreal, Canada, May 11–14, 2010.

Leone, Frank A. “Pultruded Rod/Overwrap Testing for Various Stitched 

Stringer Configurations.” NASA TM-2016-218975. NASA Langley 

Research Center, 2016.

Leone, Frank A., Dawn C. Jegley, and Kim A. Linton. “Compressive Loading 

and Modeling of Stitched Composite Stiffeners.” AIAA-2016-2179. 
Selected Bibliography

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Dynamics, and Materials Conference in San Diego, CA, January 4–8, 2016.

Li, Victor P., and Kim A. Linton. “Hybrid Wing Body (HWB) Aircraft 

Design and Optimization using Stitched Composites.” AIAA-2015-3289. 

Presented at the 33rd AIAA Applied Aerodynamics Conference in Dallas, 

TX, June 22–26, 2015.

Li, Victor P., and Alexander Velicki. “Advanced PRSEUS Structural Concept 

Design and Optimization.” AIAA-2008-5840. Presented at the 12th 

AIAA/ISSMO Multidisciplinary Analysis and Optimization Conference 

in Anchorage, AK, September 10–12, 2008.

Liebeck, Robert H. “Design of the Blended-Wing-Body Subsonic Transport.” 

AIAA-2002-0002. Presented at the 40th AIAA Aerospace Sciences Meeting 

in Reno, NV, January 14–17, 2002.

Lim, Dongwook, Taewoo Nam, Graham Burdette, Michelle Kirby, Dimitri 

N. Mavris, Phillipe A. Bonnefoy, R. John Hansman, James I. Hileman, Ian 

A. Waitz, and Brian Yutko. “An Investigation of the Potential Implications 

of a CO2 Emission Metric on Future Aircraft Designs.” Presented at the 

27th International Congress of the Aeronautical Sciences in Nice, France, 

September 19–24, 2010.

Linton, Kim A., Alex Velicki, Krishna Hoffman, Patrick Thrash, Robert Pickell, 

and Robert Turley. “PRSEUS Panel Fabrication Final Report.” NASA 

CR-2014-218149. The Boeing Company, 2014.

Lockard, David P., William M. Humphreys, Mehdi R. Khorrami, Ehab 

Fares, Damiano Casalino, and Patricio A. Ravetta. “Comparison of 

Computational and Experimental Microphone Array Results for an 18%-

Scale Aircraft Model.” AIAA-2015-2990. Presented at the 21st AIAA/CEAS 

Aeroacoustics Conference in Dallas, TX, June 22–26, 2015.

Lopes, Leonard V., and Casey L. Burley. “Design of the Next Generation Aircraft 

Noise Prediction Program: ANOPP2.” AIAA-2011-2854. Presented at the 

17th AIAA/CEAS Aeroacoustics Conference in Portland, OR, June 5–8, 

2011.
Green Light for Green Flight

266

Lovejoy, Andrew E. “Optimization of Blended Wing Body Composite Panels 

Using Both NASTRAN and Genetic Algorithm.” NASA-CR-214515. 

NASA Langley Research Center, 2006.

Lovejoy, Andrew E. “PRSEUS Pressure Cube Test Data and Response,” NASA 

TM-2013-217795. NASA Langley Research Center, 2013.

Lovejoy, Andrew E., and Frank A. Leone Jr. “T-Cap Pull-Off and Bending 

Behavior for Stitched Structure.” NASA TM-2016-218971. NASA Langley 

Research Center, 2016.

Lovejoy, Andrew E., and Adam Przekop. “Imparting Barely Visible Impact 

Damage to a Stitched Composite Large-Scale Pressure Box.” AIAA-2016-

2178. Presented at the 57th AIAA/ASCE/AHS/ASC Structures, Structural 

Dynamics, and Materials Conference in San Diego, CA, January 4–8, 2016.

Lovejoy, Andrew E., Marshall Rouse, Kim A. Linton, and Victor P. Li. 

“Pressure Testing of a Minimum Gauge PRSEUS Panel.” AIAA-2011-

1813. Presented at the 52nd AIAA/ASME/ASCE/AHS/ASC Structures, 

Structural Dynamics and Materials Conference in Denver, CO, April 

4–7, 2011.

Lucas, Nathaniel, Lutz Taubert, René Woszidlo, Israel J. Wygnanski, and 

Michael A. McVeigh. “Discrete Sweeping Jets as Tools for Separation 

Control.” AIAA-2008-3868. Presented at the 4th AIAA Flow Control 

Conference in Seattle, WA, June 23–26, 2008.

Lyu, Zhoujie, and Joaquim Martins. “Aerodynamic Shape Optimization of 

an Adaptive Morphing Trailing Edge Wing.” AIAA 2014-3275. Presented 

at the 52nd AIAA Aerospace Sciences Meeting in National Harbor, MD, 

January 13–17, 2014.

Mangelsdorf, Mark F. “Overview of ERA’s Advanced Vehicle Concepts NRA.” 

Presented at the N+2 Advanced Vehicle Concepts and Quick Starts NRA 

Pre-Proposal Meeting in Washington, DC, February 19, 2010.

Mangelsdorf, Mark F. “Environmentally Responsible Aviation N+2 Advanced 

Vehicle Concepts NRA Status.” Presented at the ASME Turbo Expo 

Conference in Vancouver, Canada, June 6–10, 2011.
Selected Bibliography

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Advanced Concepts Office. Office of Space Access and Technology, 1995.

Martin, Kenneth C., and Bruce G. McKay. “Environmentally Responsible 

Aviation Advanced Subsonic Transport Study and Conceptual Design of 

a Subscale Testbed Vehicle.” NASA CR-2013-218394. Lockheed Martin 

Aeronautics Company, 2013.

McGowan, David M., Damodar R. Ambur, and Stephen R. McNeil. “Full-field 

Structural Response of Composite Structures: Analysis and Experiment.” 

AIAA-2003-1623. Presented at the 44th AIAA/ASME/ASCE/AHS 

Structures, Dynamics and Materials Conference in Norfolk, VA, April 

7–10, 2003.

Mead, Craig, and Owen Kenning. “Noise and Flow Studies of Coaxial 

Jets at Incidence.” AIAA-2003-3213. Presented at the 9th AIAA/CEAS 

Aeroacoustics Conference in Hilton Head, SC, May 12–4, 2003.

Miller, Eric J., Josue Cruz, Shun-Fat Lung, and Peter Flick. “Evaluation 

of the Hinge Moment and Normal Force Aerodynamic Loads from a 

Seamless Adaptive Compliant Trailing Edge Flap in Flight.” AFRC-EDAA-TN28829. NASA Armstrong Flight Research Center, January 21, 

2016.

Miller, Eric J., Josue Cruz, Shun-Fat Lung, Sridhar Kota, Gregory Ervin, KerrJia Lu, and Pete Flick. “Evaluation of the Hinge Moment and Normal 

Force Aerodynamic Loads from a Seamless Adaptive Compliant Trailing 

Edge Flap in Flight.” AFRC-E-DAA-TN28829. NASA Armstrong Flight 

Research Center, January 21, 2016. 

Miller, Eric J., Andrew C. Holguin, Josue Cruz, and William A. Lokos. “Strain 

Gage Load Calibration of the Wing Interface Fittings for the Adaptive 

Compliant Trailing Edge Flap Flight Test.” AIAA-2014-0277. Presented at 

the 52nd Aerospace Sciences Meeting in National Harbor, MD, January 

13–17, 2014.

Miller, Eric J., William A. Lokos, Josue Cruz, Glen Crampton, Craig A. 

Stephens, Sridhar Kota, Gregory Ervin, and Pete Flick. “Approach for 

Structurally Clearing an Adaptive Compliant Trailing Edge Flap for Flight.” 

Presented at the 46th Annual Society of Flight Test Engineers International 

Symposium in Lancaster, CA, September 14–17, 2015.
Green Light for Green Flight

268

Moore, Jason P., Adam Przekop, Peter D. Juarez, and Mark C. Roth. “Fiber 

Optic Rosette Strain Gauge Development and Application on a Large-Scale 

Composite Structure.” NASA TM-2016-218970. NASA Langley Research 

Center, 2016.

Mukhopadhyay, Vivekanand. “Blended-Wing-Body (BWB) Fuselage 

Structural Design for Weight Reduction.” AIAA-2005-2349. Presented at 

the 46th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics 

and Materials Conference in Austin, TX, April 18–21, 2005.

Mukhopadhyay, Vivekanand, Jaroslaw Sobieszczanski-Sobieski, Iku Kosaka, 

Gary Quinn, and Chris Charpentier. “Analysis Design and Optimization 

of Non-cylindrical Fuselage for Blended-Wing-Body (BWB) Vehicle.” 

AIAA-2002-5664. Presented at the 9th AIAA/ISSMO Symposium on 

Multidisciplinary Analysis and Optimization in Atlanta, GA, September 

4–6, 2002.

Nagel, A.L, W. J. Alford, and J. F. Dugan. “Future Long-Range Transports: 

Prospects for Improved Fuel Efficiency.” NASA TM-X-72659. NASA 

Langley Research Center, 1975.

Neuhart, Dan H., Judith A. Hannon, and Mehdi R. Khorrami. “Aerodynamic 

Measurements of a Gulfstream Aircraft Model with and Without Noise 

Reduction Concepts.” AIAA-2014-2477. Presented at the 20th AIAA/

CEAS Aeroacoustics Conference in Atlanta, GA, June 16–20, 2014.

Nickol, Craig L. “Technologies and Concepts for Reducing the Fuel Burn of 

Subsonic Transport Aircraft.” NF1676L-15121. Presented at the NATO 

AVT-209 Workshop on Energy Efficient Technologies in Lisbon, Portugal, 

October 22–24, 2012.

Nickol, Craig L., and Peter Frederic. “Conceptual Design and Cost Estimate 

of a Subsonic NASA Testbed Vehicle (NTV) for Aeronautics Research.” 

AIAA-2013-4270. Presented at the AIAA Aviation Technology, Integration, 

and Operations Conference in Los Angeles, CA, August 12–14, 2013.

Nickol, Craig L., and William J. Haller. “Assessment of the Performance Potential 

of Advanced Subsonic Transport Concepts for NASA’s Environmentally 

Responsible Aviation Project.” AIAA-2016-1030. Presented at the American 

Institute of Aeronautics and Astronautics SciTech, 54th AIAA Aerospace 

Sciences Meeting in San Diego, CA, January 6, 2016. 
Selected Bibliography

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Nickol, Craig L., and William J. Haller. “Environmentally Responsible 

Aviation (ERA) Project: Assessing Progress toward Simultaneous Reductions 

in Noise, Fuel Burn and NOx.” Presented at the 49th AIAA Aerospace 

Sciences Meeting in Orlando, FL, January 4, 2011.

Nickol, Craig L., and Linwood A. McCullers. “Hybrid Wing Body 

Configuration System Studies.” AIAA-2009-0931. Presented at the 47th 

AIAA Aerospace Sciences Meeting in Orlando, FL, January 5–8, 2009.

Pendleton, Ed, Pete Flick, Donald Paul, Dave Voracek, Eric Reichenbach, and 

Kenneth Griffin. “The X-53: A Summary of the Active Aeroelastic Wing 

Flight Research Program.” AIAA 2007-1855. Presented at the 48th AIAA/

ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials 

Conference in Honolulu, HI, April 23–26, 2007.

Powers, Sheryll Goecke, Lannie D. Webb, Edward L. Friend, and William A. 

Lokos. “Flight Test Results from a Supercritical Mission Adaptive Wing 

with Smooth Variable Camber.” NASA TM-4415. NASA Dryden Flight 

Research Facility, November 1992.

Prahst, Patricia S., Sameer Kulkarni, and Ki H. Sohn. “Experimental Results 

of the First Two Stages of an Advanced Transonic Core Compressor Under 

Isolated and Multi-Stage Conditions.” GT2015-42727. Presented at the 

ASME Turbine Technical Conference and Exposition in Montreal, Canada, 

June 2015.

Przekop, Adam. “Repair Concepts as Design Constraints of a Stiffened 

Composite PRSEUS Panel.” Presented at the 53rd AIAA/ASME/ASCE/

AHS/ASC Structures, Structural Dynamics and Materials Conference in 

Honolulu, HI, April 23–26, 2012.

Przekop, Adam, and Dawn C. Jegley. “Testing and Analysis Validation of a 

Metallic Repair Applied to a PRSEUS Tension Panel.” AIAA-2013-1735. 

Presented at the 54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural 

Dynamics and Materials Conference in Boston, MA, April 8–11, 2013.

Przekop, Adam, Dawn C. Jegley, Marshall Rouse, and Andrew E. Lovejoy. 

“Finite Element Analysis and Test Results Comparison for the Hybrid 

Wing Body Center Section Test Article.” NASA TM-2016-218973. NASA 

Langley Research Center, 2016.
Green Light for Green Flight

270

Przekop, Adam, Dawn C. Jegley, Andrew E. Lovejoy, Marshall Rouse, and 

Hsi-Yung T. Wu. “Testing and Analysis of a Composite Non-Cylindrical 

Aircraft Fuselage Structure, Part I: Ultimate Design Loads.” AIAA 2016-

2176. Presented at the 57th AIAA/ASCE/AHS/ASC Structures, Structural 

Dynamics, and Materials Conference in San Diego, January 4–8, 2016.

Przekop, Adam, Dawn C. Jegley, Andrew E. Lovejoy, Marshall Rouse, and HsiYung T. Wu.“Testing and Analysis of a Composite Non-Cylindrical Aircraft 

Fuselage Structure, Part II: Severe Damage.” AIAA-2016-2177. Presented at 

the 57th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics 

and Materials Conference in San Diego, CA, January 4–8, 2016.

Przekop, Adam, Hsi-Yung T. Wu, and Peter Shaw. “Nonlinear Finite Element 

Analysis of a Composite Non-Cylindrical Pressurized Aircraft Fuselage 

Structure.” AIAA-2014-1064. Presented at the 55th AIAA/ASME/ASCE/

AHS/ASC Structures, Structural Dynamics and Materials Conference in 

National Harbor, MD, January 13–17, 2014.

Rathay, Nicholas, Matthew Boucher, Michael Amitay, and Edward A. Whalen. 

“Performance Enhancement of a Vertical Stabilizer using Synthetic Jet 

Actuators: No Sideslip.” AIAA-2012-0071. Presented at the 50th AIAA 

Aerospace Sciences Meeting in Nashville, TN, January 9–12, 2012.

Rawdon, Blaine K., Darrell L. Brown, Ron T. Kawai, Joshua Clough, and 

Daniel Vetters. “Blended Wing Body Configuration for Energy Efficiency.” 

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Aerospace Sciences Meeting including the New Horizons Forum and 

Aerospace Exposition in Orlando, FL, January 4–7, 2011.

Seele, Roman, Emilio Graff, Morteza Gharib, Lutz Taubert, John Lin, and 

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Selected Bibliography

271

Seele, Roman, Emilio Graff, John Lin, and Israel J. Wygnanski. “Performance 

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Green Light for Green Flight

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Van Zante, Dale E., and John Wojno. “The NASA/GE Open Rotor Research 

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Velicki, Alex. “Damage Arresting Composites for Shaped Vehicles.” NASA 

CR-2009-215932. The Boeing Company, 2009.

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274

Velicki, Alex, and Dawn C. Jegley. “PRSEUS Development for the Hybrid 

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Velicki, Alex, Dawn C. Jegley, and Patrick J. Thrash. “Airframe Development 

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Velicki, Alex, Kim A. Linton, Krishna Hoffman, Patrick Thrash, Robert Pickell, 

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Velicki, Alex, and Patrick J. Thrash. “Advanced Structural Concept 

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the 49th AIAA/ASME/ASCE/SHS/ASC Structures, Structural Dynamics, 

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Velicki, Alex, and Patrick J. Thrash. “Blended Wing Body Structural Concept 

Development.” Presented at the Aircraft Structural Design Conference in 

Liverpool, England, October 14–16, 2008.

Velicki, Alex, Nicolette P. Yovanof, Jaime Baraja, Kim A. Linton, Victor P. 

Li, Arthur Hawley, Patrick J. Thrash, Steve DeCoux, and Robert Pickell. 

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Vicroy, Dan D. “Blended-Wing-Body Low-Speed Flight Dynamics: Summary 

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47th AIAA Aerospace Sciences Meeting in Orlando, FL, January 5–8, 2009.

Wahls, Richard A. “Environmentally Responsible Aviation Technical 

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Space Engineering Board Meeting in National Harbor, MD, May 14, 2009. 

Wakayama, Sean, and Edward V. White. “Evaluation of Adaptive Compliant 

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Applied Aerodynamics Conference, AIAA Aviation Forum in Dallas, TX, 

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275

Walton, Joanne C., Clarence T. Chang, Chi-Ming Lee, and Stephen Kramer. 

“Low NOx Fuel Flexible Combustor Integration Project Overview.” NASA 

TM-2015-218886. NASA Glenn Research Center, Cleveland, Ohio, 

October 2015.

Wang, John, Ray Grenoble, and Robert Pickell. “Structural Integrity Testing 

Method for PRSEUS Rod-Wrap Stringer Design.” Presented at the 53rd 

AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and 

Materials Conference in Honolulu, HI, April 23–26, 2012.

Whalen, Edward A., Douglas Lacy, Emilio Graff, and Israel J. Wygnanski. 

“Performance Enhancement of a Full-Scale Vertical Tail Model Equipped 

with Active Flow Control.” AIAA-2015-0784. Presented at the 53rd AIAA 

Aerospace Sciences Meeting in Kissimmee, FL, January 5–9, 2015.

Wilsey, Craig. “Continuous Lower Energy, Emissions and Noise (CLEEN) 

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Session in Atlanta, GA, November 8, 2012.

Wu, Hsi-Yung T., Peter Shaw, and Adam Przekop. “Analysis of a Hybrid 

Wing Body Center Section Test Article.” AIAA-2013-1734. Presented at 

the 54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics 

and Materials Conference in Boston, MA, April 8–11, 2013.

Yao, Chungsheng, Luther N. Jenkins, Scott M. Bartram, Jerome Harris, Mehdi 

R. Khorrami, and W. Derry Mace. “Flow-Field Investigation of Gear-Flap 

Interaction on a Gulfstream Aircraft Model.” AIAA-2014-2479. Presented 

at the 20th AIAA/CEAS Aeroacoustics Conference in Atlanta, GA, June 

16–20, 2014.

Yovanof, Nicolette P. “Advanced Structural Stability of a Non-Circular 

BWB-Shaped Vehicle.” Presented at the 50th AIAA/ASME/ASCE/AHS/

ASC Structures, Structural Dynamics, and Materials Conference in Palm 

Springs, CA, May 4–7, 2009.

Yovanof, Nicolette P., and Dawn C. Jegley. “Compressive Behavior of FrameStiffened Composite Panels.” AIAA-2011-1913. Presented at the 52nd 

AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and 

Materials Conference in Denver, CO, April 4–7, 2011.
Green Light for Green Flight

276

Yovanof, Nicolette P., Andrew Lovejoy, Jaime Baraja, and Kevin Gould. “Design, 

Analysis and Testing of a PRSEUS Pressure Cube to Investigate Assembly 

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Conference in Baltimore, MD, April 2, 2012.

Yovanof, Nicolette P., Alex Velicki, and Victor P. Li. “Advanced Structural 

Stability Analysis of a Nonlinear BWB-Shaped Vehicle.” AIAA-2009-2452. 

Presented at the 50th AIAA/ASME/ASCE/AHS/ASC Structures, Structural 

Dynamics, and Materials Conference in Palm Springs, CA, May 4–7, 2009.

Zhu, Dongming, Robert A. Miller, and Dennis S. Fox. “Thermal and 

Environmental Barrier Coating Development for Advanced Propulsion 

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281

About the Author

Peter W. Merlin (NASA)

Peter W. Merlin has been a NASA contract aerospace historian since 1997.

He has authored a number of books, 

including several NASA Special Publications on aeronautical research projects, as 

well as two volumes on aerospace safety. 

He has also written several technical papers for the American Institute of 

Aeronautics and Astronautics, and numerous journal articles on aerospace history 

and technology. 

In addition, he served as contributing 

editor for historical publications at NASA 

Armstrong Flight Research Center and has 

appeared in more than a dozen documentary television programs for Discovery, the 

History Channel, National Geographic, 

and others. 

He holds a Bachelor of Science degree in aviation management from 

Embry-Riddle Aeronautical University.

283

Page numbers in bold indicate pages with illustrations.

A

Active Flow Control (AFC) system, iv, 26, 177, 

182–189, 184

rudder, 19, 23

vertical stabilizer (tail), 61

Advanced Subsonic Combustion Rig (ASCR) 

facility, 28, 161–164, 162, 166–167

Advanced Technologies, Inc., 184

Aeronautics and Space Engineering Board 

(ASEB), 12

Aeronautics Research Mission Directorate 

(ARMD), 141

Advanced Vehicle Concepts (AVC) study, 

59–60

ERA Project goals, 20

ERA Project research projects, 18

foundational projects, 17–19

metrics, 5

Phase 2 Tiger Team, 34–36

research and development, 34

system-level ERA-V and ERA-O, 10–11

updates, 32

Air Force, U.S.

ACTE flaps, 94, 111

ACTE flight experiments, 26, 92, 94, 103, 

108–114, 198

ACTE testing, 98–119

ACTE wing loading test, 107–108

Active Aeroelastic Wing (AAW), 96

Advanced Fighter Technology Integration 

(AFTI) program, 95–96

composites (stitched), 74

Edwards Air Force Base, 108

ERA Project Phase 1 flight testing, 24

F/A-18 AAW testbed, 97–98, 97

Flight Dynamics Laboratory (AFFDL), 95

GE F-111A MAW, 95, 98–100

Gulfstream Aerospace G-III, 98–100

Gulfstream G-III Subsonic Research Aircraft 

(SCRAT), 101

Loads Laboratory, 106

mission adaptive wing (MAW), 95–96

mission adaptive wing (MAW) testbed, 95

Research Laboratory (AFRL), 4, 24, 26, 61, 

74, 95

Small Business Innovation Research (SBIR), 

99–100

Transonic Aircraft Technology Program, 

95–96

Wright-Patterson AFB, 100

Air Mobility Command, 34

Airbus, 21, 137, 149, 163

aircraft configurations. See also individual aircraft

707-320 Intercontinental, 42

737-800 aircraft, 178

737/CRJ, 46

747-8 wide body aircraft, 162, 177

747SP flying testbed, 149

777 airliner, 115

777-200 and 777-300 compared, 182

787 airliner, 162, 177

787-9 stretched aircraft, 189

1903 Kitty Hawk Flyer, 97

ACTE compared to HWB, 117–118

Index
Green Light for Green Flight

284

active flow control (AFC), 183

Airborne Subscale Transport Aircraft 

Research (AirSTAR) testbed, 115, 116

Airbus A320 airliner, 21, 149

alternatives, 48

AMELIA (Advanced Model for Extreme Lift 

and Improved Aeroacoustics), 47

angle of attack, 25, 97n6, 169, 172–173

ATDC testbed, 151–156

B-2A bomber concept, 52–55

BWB concept, 17, 42, 90, 173, 200

change, 41

combustor, 146–147

commercial off-the-shelf (COTS) landing 

gear, 57

commercial transports, 182

D8 “Double Bubble,” 46–48

Dassault Aviation HU-2C Guardian Falcon, 

191

Dassault HU-2C Guardian Falcon, 191

Dornier 328J airliner, 50

double-fuselage configuration, 52

Drake, Aaron, flying wing concept, 53–55

ecoDemonstrator 737, 178

ecoDemonstrator 757, 176, 177–195

ecoDemonstrator 787, 178–179

engine core, 147

F/A-18, 109

F/A-18 AAW testbed, 97–98, 97

flaps, 136

flying wing concept, 52–56

G550 aircraft, 26, 124–125

GE F-111A, 95–96

Gulfstream Aerospace G-III, 98–100

Gulfstream G550, 126–128, 128

Gulfstream G-III Subsonic Research Aircraft 

(SCRAT), 101, 102, 120, 126

High-Alpha Research Vehicle (HARV), 97

HWB, 20, 22

HWB compared to ACTE, 117–118

HWB compared to tube-and-wing (TAW), 199

HWB concept, 20, 32, 34, 56–58, 63, 88, 

91, 118, 136, 139–140, 162, 170

HWB ERA concept modeling, 22

HWB lift-to-drag (L/D) ratio, 56

HWB stitched composite wing assembly, 

76–88

innovations, 34, 41–42

joined wing concept, 49–51

Kitty Hawk Flyer, 97

landing gear, 131, 132, 133

large twin-aisle (LTA), 19

maximum lift coefficient (CLmax), 169

MD-90-40X commercial passenger 

transport, 70

mission adaptive wing (MAW), 95–96

NASA 804, 98

NASA 840, 97

NASA 853, 98

NASA F/A-18A, 96–97, 97, 109

Notional Advanced Tube-and-Wing, 20

outer-mold-line (OML), 32, 77, 101, 104, 174

over-the-wing nacelle, 31

propulsor, 5, 27–29, 145–146, 146, 

148–149, 157–160

slats, 24, 93, 122, 123, 136, 139–140, 194

SOFIA (Stratospheric Observatory for Infrared 

Astronomy), 121

strength, 41

SFW program, 1–3, 14–15, 28, 59, 150

supercritical wing (SCW), 95

swept-wing concept, 41, 54

swirler, 147, 162–165, 167

tail assembly, 42–43

truss-braced wings, 14

tube-and-wing (TAW), 19, 20, 21, 22, 48, 

118, 136, 197

tube-and-wing (TAW) compared to HWB, 199

tube-and-wing (TAW) ERA concept modeling, 

22
Index

285

tube-and-wing (TAW) high-wing concept, 32

tube-and-wing (TAW) low-wing concept, 32

tube-and-wing (TAW) modifications, 41–42, 

46–47

undercarriage geometry, 136–137

vertical stabilizer (tail), 182, 184

Vought TF-8A Crusader, 95

V-tail, 52

wing hardware, 123

X-48, 24

X-48B demonstrator, 24–25, 57, 199

X-48C demonstrator, 24–25, 57–58, 58, 

199

X-53, 97, 98

X-55 Advanced Composite Cargo Aircraft 

(ACCA), 50–52, 51

XVT (experimental vehicle testbed), 11–13

YB-49A, 52

Aircraft Demolition, LLC, 195

Aircraft Fleet Recycling Association, 194

Airframe Technology subproject, 11, 21–23

Airspace Systems Program, 10

Airspace Systems Program (ASP), 12, 15

Alexander, Mike, 61

American Airlines

737-800 aircraft, 178

ecoDemonstrator 757, 178

Ames Research Center (ARC), 121, 199

National Full-Scale Aerodynamics Complex 

(NFAC), iv, 144, 170

Pleiades (supercomputer), 126

transonic performance test, 32

Armstrong Flight Research Center (AFRC)

ACTE flaps, 94, 111

ACTE flight experiments, 92, 94, 99, 

109–111, 113

ACTE wing loading test, 107

Adaptive Compliant Trailing Edge (ACTE), 61

control room, 109–110, 110

Gulfstream Aerospace G-III, 98–100

Gulfstream G-III Subsonic Research Aircraft 

(SCRAT) milestone, 112

microphone array, 137–138

partnerships, 112–114

Phase 2 Tiger Team, 34

X-48B demonstrator, 24

X-48C demonstrator, 58

Aviation Partners, Inc., 198

B

Béchéreau, Louis, 99

blended wing body (BWB) concept, 17, 42, 90, 

173, 200. See also hybrid wing-body (HWB) 

concept

Blériot, Louis, 99

Boeing, 2

707 airliner, 41

737 airliner, 21, 46, 48

737-800 aircraft, 178

747-8 wide body aircraft, 162, 177

747SP flying testbed, 149

777 airliner, 19, 21, 46, 49, 141

777-200LR airliner, 44

787 airliner, 41–42, 162, 177

787-9 stretched aircraft, 189

ACTE technology benefits, 118–119

ACTE technology studies, 115–119

Active Aeroelastic Wing (AAW), 96

active flow control (AFC), 177

Advanced Stitching Machine (ASM), 70–71, 

71

AFC Enabled Vertical Tail and Advanced Wing 

Flight Experiment, 61

AFC Enhanced Vertical Tail Flight Experiment, 

182–189, 184

AFC leak tests, 187–188

AFC total system functionality check, 188

APU design and stress analysis, 187

AST Composite Wing program, 70

Boeing Field, 187
Green Light for Green Flight

286

C-17 Globemaster III cargo transport, 73

C-17 manufacturing plant, 84, 84

Commercial Airlines, 188, 195

composites, 63, 65, 70–74, 76

composites fabrication, 65

computer modeling, 137

Damage Arresting Composites 

Demonstration, 61

ecoDemonstrator 757, 61, 176, 177–195, 

180, 181

ecoDemonstrator 757 IAM airports, 

191–192

ecoDemonstrator 757 IAM flight 

demonstration preparation, 191–193

ERA Project Phase 1 flight testing, 24

ERA Projects goals, 69–70

ES panel trade study, 192–193

“green diesel,” 179–180

hybrid wing-body (HWB) concept, 55–58, 

170, 196

hybrid wing-body (HWB) model, 144

IAM assessment process, 194

innovations, 179

ITD51A testing, 171–175

lift-to-drag ratio (L/D), 169

Low-Speed Aero Acoustic Facility (LSAF), 33

maximum lift coefficient (CLmax), 169

McDonnell Douglas merger, 67

mission adaptive wing (MAW), 95–96

N2A-EXTE concept, 169

NASA F/A-18A, 96–97

non-stick coating spray tests, 192

non-stick coatings, 177

OVERFLOW, 172

partnerships, 56, 63, 83, 88, 177–178

Preferred System Concept (PSC), 44, 48, 49, 

56–58, 58–59, 169

proprietary technologies, 179–180

PRSEUS (Pultruded Rod Stitched Unitized 

Structure), 63

PRSEUS multi-bay box (MBB) testing, 83–84

Quiet Technology Demonstrator program, 177

recycled materials, 195

Research and Technology program, 185

SOFIA (Stratospheric Observatory for Infrared 

Astronomy), 121

solar energy, 180

subscale test vehicle (STV), 49, 57–59

SUGAR (Subsonic Ultra Green Aircraft 

Research) Volt concept, 40, 46, 47, 48

thermal energy, 180

tube-and-wing (TAW) configuration, 56

UHB Engine Integration for a HWB, 61

UHB-HWB drag reduction study, 171

UHB-HWB integration, 169, 171

variable-camber Krueger (VCK) flap, 180

X-48B/C demonstrator, 57

Bolden, Charlie, 101

Bombardier, 46, 137, 163

Bonet, John, 56–57

Bowles, David, 181

British Aerospace, 96

Bush, George W., 3, 12

C

California Institute of Technology (Caltech), 

183–185

California Polytechnic State University, 47

cameras

“acoustic camera” (microphone), 137–138

ecoDemonstrator 757, 179, 191, 193

PRSEUS cube testing, 81, 81

PRSEUS MBB, 85

Canadair Regional Jet (CRJ), 21

Cavolowsky, John A., ASP, 12

Christensen, Doug, 188

Coast Guard, U.S., 191

Collier, Jr., Fayette S., 15, 20, 177

AFC Enhanced Vertical Tail Flight Experiment, 

185–186
Index

287

ERA Project challenges, 21

ERA Project management, 15

ERA Project planning, 13–15

hybrid wing-body (HWB) concept, 57–58

NextGen advantages, 59

PRSEUS applications, 91

Rolls-Royce Liberty Works UltraFan, 52

SFW program, 123–124

Collier Trophy, 150

commercial airline industry

ACTE technology studies, 115–116

AFC Enhanced Vertical Tail Flight Experiment, 

185

aircraft configurations, 41

costs (fuel), 20

ERA Project, 4–5

flight increases, 189

hybrid wing-body (HWB) concept, 199

partnerships, 35–36, 39

passenger experience, 49

passenger flights, 121

risk investment, 63–64

composites, 17–20, 65

Advanced Composite Technology (ACT) 

program, 65–74

Advanced Stitching Machine (ASM), 70–71

Air Force Research Laboratory (AFRL), 74

building block approach, 78

building block fabrication approach, 67

C-17 Globemaster III cargo transport, 73

carbon fiber, 69

ceramic matrix composites (CMC), 20, 

27–30, 161–162

costs, 64

curing, 76

dry warp-knit fabric, 74, 76

environmental barrier coating (EBC), 162

ERA Project goals, 74

FAA certification, 65, 71–72

fabrication, 64–65, 76–77

Hercules 3501-6, 66

hybrid wing-body (HWB) concept, 91

Internal Pressure Box (IPB), 78–79

Lockheed Martin Skunk Works (LMSW) 

Team, 50

Marvin B. Dow Stitched Composites Center, 

70

NASA F/A-18A, 97

NASA research, 63–65

Northrop Grumman PSC, 54

outer-mold-line (OML), 77, 79, 80, 84–85

PRSEUS (advantages), 89–91

PRSEUS (Pultruded Rod Stitched Unitized 

Structure), 23, 23–24, 62, 73

PRSEUS multi-bay box (MBB), 78–88, 

83–88, 83

PRSEUS multi-bay box (MBB) testing, 84

PRSEUS panels, 83–84, 89

resin-infused, 74–75

risk management, 72

skins, 97

stitched, 65–76, 78

stitched composites wing assembly, 76, 78

testing, 71–73, 80, 81, 82

wing stub box, 66–67

Congress, U.S., 9, 12, 15–16

Continuous Lower Energy Emissions and Noise 

(CLEEN) Program, 4, 34, 159, 178, 198

continuous mold-line (CML) technology, 19, 26

Cranfield Aerospace, 24, 56–58

D

Dassault Aviation HU-2C Guardian Falcon, 191

data

acoustic, 33, 127, 136, 172

ACTE flight experiments, 108–114

ACTE ground vibration testing (GVT), 

102–106, 103

Advanced Composite Technology (ACT) 

program, 71
Green Light for Green Flight

288

aerodynamic, 153–155

AFC Enhanced Vertical Tail Flight Experiment, 

184–186

airflow dynamics, 169

airframe noise reduction, 134–138

analytical modeling, 89, 124–126, 198

auralization studies, 141–143

baseline flight test, 26

building block approach, 135

computational fluid dynamics (CFD), 

153–155, 155, 156, 169, 172–173, 

185–186

data transfer, 39

drag measurements, 127

ecoDemonstrator 757 cameras, 193

ecoDemonstrator 787 flight tests, 179

engine inlet distortion, 173

ERA-000H1, 174

ERA-0009GM data, 174

F/A-18 AAW testbed, 98

finite element model (FEM) analysis, 71–72, 

80–81, 89, 102–105

flight data (X-48B/X-48C), 25

Gulfstream G-III Subsonic Research Aircraft 

(SCRAT) milestone, 112

lift measurements, 127

N+2 technology analysis, 19

NASA Advanced Supercomputing Division, 126

noise simulation, 120, 124–126, 126, 

132–143, 136, 138

non-destructive inspection (NDI), 81–83

partnerships, 39

perception-influenced design, 140–141

Pleiades (supercomputer), 126

proprietary technologies, 179–180, 198

PRSEUS multi-bay box (MBB) testing, 85

requirements for ERA Project Phase 1, 

23–24

Rolls-Royce Liberty Works UltraFan, 52

scale models, 124

systems analysis element, 31–32

TAPS combustor, 163

transfer, 39

transonic performance test, 32

validation dataset, 29

vertical stabilizer (tail) AFC, 185–186

wind tunnel demonstrations, 26–27

de Havilland, Geoffrey, 99

decibels, 175

Effective Perceived Noise in decibels 

(EPNdB), 6, 145, 159–160, 168, 174

ERA Project goals, 57, 122

FAA certification, 145

FAA standards, 122

flap noise reduction tests, 131

gear noise, 133

Grumman PSC, 55

high-bypass-ratio GTF engines, 57

joined wing concept, 51

noise (perceived), 140–141

open-rotor propulsion, 57

reductions, 200

soft vanes (SV) technology, 159

Delta Airlines, 69, 188

Department of Defense (DOD), 4, 20

Department of Transportation (DOT), 191–192

Dornier 328J airliner, 50

Drake, Aaron, flying wing concept, 53–55

E

efficiencies, 166

ACTE technology, 115–116

ACTE flaps, 111

aerodynamic, 26, 30, 41–42, 49, 56, 129

aerodynamic fairings, 132, 133

aerodynamic noise, 123

aerodynamic wings, 49–50

Aircraft Energy Efficiency (ACEE) program, 63

alternative fuels, 28, 161, 165, 167, 179–180

ATDC technology, 151–157
Index

289

commercial transports, 63

compliant structures, 93–94

control, 168

emission standards, 160

engine operability, 168

engines, 52, 55, 56, 150–175

engines (advanced high-bypass turbofan), 

42, 60

engines (quiet engine technologies), 122

engines (turbofan), 50

engines (types compared), 122

engines (UHB), 60

ERA Project goals, 152–155

Fischer-Tropsch process, 161

flaps, 98

flying wing concept, 52–54

fuel, 1–4, 41, 50, 56, 178, 199

fuel (wing warping), 99

fuel economy, 186

improvements, 111, 113–114

lift-to-drag ratio (L/D), 174

loads (internal pressure), 90

non-stick coatings, 190–192

propulsion, 17–18, 27–29, 60, 150

PRSEUS (Pultruded Rod Stitched Unitized 

Structure), 60, 74–78, 88–89

recycled materials, 179–180, 194–195

reused materials, 194–195

runways, 10, 186

span-loading, 116–117

stability, 168

thermal, 29–30, 150, 153

transport, 54

tube-and-wing (TAW), 197

UHB, 6

UHB-HWB integration Mach numbers, 169

Embraer, 137

energies, green, 1, 10

alternative fuels, 28, 161, 165, 167, 

179–180

solar energy, 180

thermal, 180

engines, 28, 30, 55

707-320 Intercontinental turbofans, 42

advanced injector designs, 160–161

advanced technology development 

compressor (ATDC), 151–156

advanced turbofan, 60, 198

AFC Enhanced Vertical Tail Flight Experiment, 

188

auxiliary power unit (APU), 186–187

bypass ratio, 28, 32

CFM56-5B, 150

chevron (saw-tooth), 178

CLEEN Program, 159

core element, 29–30

direct drive, 147–148

efficiencies, 179

ERA Project Phase 2 development, 28

failures, 182, 184

failures (simulated), 188

fan pressure ratio (FPR), 147, 149

fuel burn reductions, 160

G-III-ACTE integration, 111

gas turbine, 29, 41

GE90, 153

geared turbofan (GTF), 34, 56, 60, 145, 148,

163

General Electric (GE) CFM Leap-X-based 

propfans, 57

GEnx-1B, 162

GEnx-2B, 162

high-bypass-ratio (BPR), 26, 147, 149–150

high-bypass-ratio (BPR) GTF, 56–57

high-bypass turbofan, 52

high-OPR engine, 162

high-pressure compressors (HPC), 151

HWB integration, 199–200, 199

inlet distortion, 172–174

innovations, 20, 46
Green Light for Green Flight

290

ITD51A testing, 172

jet, 146–147, 163, 179

JetCat P200 turbojet, 24

JetCat SPT15, 24

lean direct injection (LDI), 160–161, 164

lean-lean concept, 161

noise (compared), 146

noise (perceived), 140–143

NPSS/WATE, 19

open-rotor propulsion, 17, 19, 28–29, 34, 

60, 143, 149–150, 158

open-rotor UHB, 159

operability, 168

operating pressure ratio (OPR), 27, 147, 151

OPR compressor/turbine, 27

propulsion systems, 31

propulsive efficiency, 28–29

PW advanced engine concept, 166

PW1100G-JM, 149

PW1500G, 157

PW1000G GTF, 57

PW2037, 188

pylon-mounted, 56

Rolls-Royce, 58–59

Rolls-Royce Liberty Works UltraFan, 50–52

simulated, 143

specific fuel consumption (SFC), 28

TAPS combustor, 162–163

trailing edge embedded, 56

turbine (gas), 146

turbine-powered engine simulators (TPS), 

171

turbofan, 146–147, 148, 153

turbofan (Boeing-Rolls-Royce QTD program), 

177

turbofan (high-bypass), 42, 60

turbojet, 42, 146, 148

twin engine HWB, 168

UHB, 3

UHB efficiency, 6

UHB turbofan, 27, 28, 145, 149, 151, 168, 

196

UHB-HWB integration, 167–175

ultra-high-bypass (UHB), 60, 145

ultra-high-bypass (UHB) open-rotor propfans, 

56

un-ducted fan (UDF), 149–150

V2500, 149

Environmentally Responsible Aviation (ERA) 

project

2nd Generation UHB-Ration Propulsor 

Integration, 60

acoustic liners, 129–130, 159

ACTE control surfaces, 100

ACTE flaps, 105–108, 105, 111, 113, 114, 

115

ACTE flight experiment goals, 104

ACTE flight experiments, 94, 98, 102, 103, 

108–114, 109, 110, 113, 114

ACTE ground tests, 102–108, 103

ACTE proprietary designs, 100

ACTE technology benefits, 117–119, 118

ACTE technology studies, 115–119

ACTE wing loading test, 107–108, 107

active flow control (AFC), 177, 182–189

active flow control (AFC) system, 184

Adaptive Compliant Trailing Edge (ACTE), 60

advanced combustor concepts, 27–28, 198

advanced fan blade technology, 150

advanced technology development 

compressor (ATDC), 151–156

advanced turbofan, 198

Advanced Vehicle Concepts (AVC) Element, 

33–34

Advanced Vehicle Concepts (AVC) study, 16, 

31, 43–49, 45, 55, 57–59

Advanced Vehicle Concepts study timeline, 

45

aerodynamic performance models, 26

air traffic management, 15
Index

291

Airborne Subscale Transport Aircraft 

Research (AirSTAR) testbed, 116

airflow dynamics, 169

airfoil surfaces, 25–26

airframe and propulsion performance results, 

31–32

airframe noise subsonic wind tunnel tests, 

131–138

Airframe Technology subproject, 5

alternative fuels, 28, 161, 165, 167, 

179–180

baseline flight test, 22

beginning, 147, 197

Boeing ecoDemonstrator 757, 61

budget, 11–12, 15–16, 44

building block approach, 23–24, 67, 83, 

102, 103, 198

BWB compared to PRSEUS, 90

challenges, 16, 61, 63, 123, 131, 160, 168, 

189, 200–201

CML technology, 26–27

combustor technology maturation plan, 165

compliant structures, 93–94, 99, 104

Concept Modeling Summary, 22

continuous mold-line (CML) link flaps, 130

costs, 20, 35–36, 200

Critical Design Review, 137

Dassault HU-2C Guardian Falcon, 191

described, 1–2

development, 10–14, 18

ecoDemonstrator 737, 178

ecoDemonstrator 757, 176, 177–195

ecoDemonstrator 757 crewmembers, 

193–194

ecoDemonstrator 757 IAM flight 

demonstration preparation, 189–193

ecoDemonstrator 757 IAM flight 

demonstrations, 191, 193–195

ecoDemonstrator 757 preliminary flight test 

results, 193

ecoDemonstrator 757 recycling, 194–195

ecoDemonstrator 757-222 retired aircraft, 

179–180

ecoDemonstrator 787, 178

effects, 3–4

ending, 137, 171, 200

ERA-000H1, 174

ERA-0009GM data, 174

ERA-Operations (ERA-O), 10–11

ERA-Vehicle (ERA-V), 10

failures, 155

Flap and Landing Gear Noise Reduction 

Flight Experiment, 60

Flap Edge and Landing Gear Noise Reduction 

ITD, 122–126

Flap Edge Noise Reduction Fins (FENoRFins), 

129–131

FlexFoil system, 99–100

FLEXible Side-Edge Link (FLEXSEL), 129–130

FlexSys Inc., 102–104

flight testing, 198

fuel alternatives, 179–180

funding, 12–14, 138

G-III-ACTE integration, 111

geared turbofan (GTF) engines, 148–150

goals, 3, 4, 14, 17, 20, 58, 147, 151, 

161–165, 169, 197

goals (ATDC), 152–153

goals (composites), 69–70

goals (drag), 23, 25

goals (emissions), 160

goals (fuel burn), 22, 145

goals (ITD51A testing), 174

goals (noise), 22–23, 27, 121–122

goals (performance), 159–160

ground-based testing, 198

Gulfstream G-III Subsonic Research Aircraft 

(SCRAT), 101–114, 102, 107, 109, 110

Gulfstream G-III Subsonic Research Aircraft 

(SCRAT) flight envelope, 111
Green Light for Green Flight

292

Gulfstream G-III Subsonic Research Aircraft 

(SCRAT) milestone, 112

Gulfstream G-III Subsonic Research Aircraft 

(SCRAT) sound field, 120, 126

high-OPR engine, 162

Highly Loaded Front Block Compressor 

Demonstration, 60, 150–157

HWB aerodynamic testing, 144

HWB center body, 90

HWB innovations, 199–200

HWB pressure cabin, 89–90

hybrid wing-body (HWB) concept, 63, 

199–200, 199

hybrid wing-body (HWB) stitched composite 

wing assembly, 80, 83, 88

impact, 199

innovations, 17–18, 19–20, 20, 40, 

113–115, 117, 198–201

insect accretion mitigation (IAM), 177, 

189–195

installation parameters (wind tunnels), 33

Internal Pressure Box (IPB), 78–79, 85

KDP, 16

KDP-1 Formulation Review, 21, 32, 59–61

KDP-2 Review, 36

knowledge sharing, 177, 198

landing gear noise reduction materials, 132, 

133

landing gear noise reduction tests, 131–134

lean direct injection (LDI), 164

Lightweight Integrated Structures Element, 

23–24

Low NOx Fuel Flexible Combustor 

Integration, 60, 160–167

mated SCRAT and ACTE, 103

metrics, 4, 5, 7, 17

metrics (N+2), 31

metrics (performance), 169

metrics (Phase 1), 32

metrics (subsonic transport system), 32

milestones, 16, 112

noise (causes), 123, 123

noise (perceived), 140–141

noise reduction (flaps), 128–131

noise shielding, 26

non-stick coatings, 177, 193

open-rotor propulsion, 149–150

open-rotor propulsion testing, 158

operating pressure ratio (OPR), 153

over-the-rotor (OTR) technology, 159–160

partnerships, 11, 34–35, 38–39, 101

peer review, 39

Phase 0 (formulation), 7–11

Phase 1, 11, 13–14, 16, 17–34, 151

Phase 1 accomplishments, 23–24

Phase 1 Boeing PSC, 169

Phase 1 Boeing X-48B demonstrator, 57

Phase 1 data, 124

Phase 1 flight testing, 24–25

Phase 1 ITD40A, 160–167

Phase 1 lean combustion testing, 164

Phase 1 lessons learned, 18, 198

Phase 1 non-stick coating formulations, 

190–191

Phase 1 Portfolio, 21–34, 38

Phase 1 propulsion systems, 28

Phase 1 scope, 17

Phase 1 subprojects, 5, 21–23

Phase 1 technologies, 27

Phase 1 test results, 165

Phase 1 testing (ATDC), 151–156

Phase 2, 11–12, 14, 16

Phase 2 advanced compressor concepts, 30

Phase 2 computational models, 124

Phase 2 design, 151

Phase 2 hierarchy, 38

Phase 2 Integrated Technology 

Demonstration (ITD) Portfolio, 5–7

Phase 2 ITDs, 59–61, 61, 63, 198

Phase 2 metrics, 174
Index

293

Phase 2 planning, 5, 16, 32, 34–35

Phase 2 swirler concepts, 165

Phase 2 testing, 18, 58, 171–175

Phase 2 testing (ATDC), 156–157

Phase 2 Tiger Team, 34–37, 59–61

planning, 12–13, 16

Preferred System Concept (PSC), 43–49

Project Flow, 16

project management, 5

projected impact, 199

propulsion, 145–175

propulsion technology demonstrations, 

145–175

Propulsion Technology subproject, 5, 27, 151

Propulsion-Airframe Aeroacoustic (PAA) 

Element, 31, 33–34

Propulsion-Airframe Integration (PAI) 

Element, 31–33

PRSEUS (Pultruded Rod Stitched Unitized 

Structure), 59, 78–91, 198

PRSEUS cube testing, 78, 80, 80, 81, 82

PRSEUS multi-bay box, 78–88

PRSEUS multi-bay box (MBB) testing, 78–88, 

83, 84, 90

PRSEUS panel testing, 62, 63, 73, 78–79, 81

PRSEUS panels, 83–84, 89

PRSEUS test results, 88–91

Reactive Orthotropic Lattice Diffuser (ROLD), 

129–131

reductions results, 197–198

risk management, 38, 112

risks, 35–36

schedule, 11–12, 35, 38, 61, 138, 171

SCRAT-ACTE integration, 103

secrecy, 11

SOFIA (Stratospheric Observatory for Infrared 

Astronomy), 121

soft vanes (SV) technology, 159–160

successes, 112, 134, 138, 159–160, 164, 

167, 197–201

systems redundancy, 112

TAPS combustor, 162–163

technical focus areas (TFAs), 37

Technology Readiness Levels (TRLs), 18–19

technology solutions, 17

technology solutions criteria, 19

technology transfer, 38–39

testing, ground, 25

timeline, 138

timing, 21

UHB Engine Integration for an HWB, 60

UHB-HWB integration challenges, 171

Vehicle Systems Integration (VSI) subproject, 

5, 167–175

wind tunnel demonstrations, 198

wind tunnel setup, 126–128

wind tunnel testing, 26–27

wing (morphing/warping), 96–97, 99–100, 

198

wings (ACTE), 93–119

wings (aeroelastic), 96

X-48C demonstrator, 58

Etrich, Igo, 99

Exa Corporation, 124, 131, 137

experimental vehicle testbed (XVT), 11–13

F

Federal Aviation Administration (FAA), 9

CLEEN Program, 4, 34, 159, 178, 198

composites certification, 65, 71–72

ecoDemonstrator 757, 178

ERA Project, 4

National Airspace System, 2–3

Next Generation Air Transportation System 

(NextGen), 44–45, 59

noise complaints, 122

noise standards, 122, 145

propulsion efficient technology, 145

FlexSys Inc., 37, 112

ACTE flight experiments, 99
Green Light for Green Flight

294

Adaptive Compliant Trailing Edge (ACTE), 61

Kota, Sridhar, 99

lift-to-drag ratio (L/D), 115

partnerships, 198

wing (morphing/warping), 198

Flick, Pete, 100, 111

flying wing concept, 42, 52–56, 53, 58

Fokker, Tony, 99

Fundamental Aeronautics Program (FAP), 1–2, 

10–11, 141

G

Gardner, John, 190

Gatlin, Greg, 60

General Electric (GE), 143

advanced compressor concepts, 30

ATDC testbed, 151–154

Aviation, 150

CFM Leap-X-based propfan, 57

CFM56-5B engine, 150

ecoDemonstrator 757, 178

F-111A, 95–96

failures, 153, 155

GE90 engine, 153

GEnx-1B, 162

GEnx-2B, 162

high-bypass turbofan, 55

Highly Loaded Front Block Compressor 

Demonstration, 61

lean-burn concept, 161–164

open-rotor compressor, 60

open-rotor propulsion, 150–151

partnerships, 151

Phase 1 test results, 165

propulsion efficient technology, 145

TAPS combustor, 162–164

un-ducted fan (UDF), 150

Georgia Institute of Technology, 19, 49, 165

Glenn Research Center (GRC)

acoustic liners, 158

advanced fuel injectors, 28

Advanced Subsonic Combustion Rig (ASCR) 

facility, 161–163, 162

advanced technology development 

compressor (ATDC), 151–156

flame tube facility, 164

geared turbofan (GTF) engines, 149

geared turbofan (GTF) testing, 157–160

injector/swirler concepts testing, 167

lean-burn concept, 161

Low Speed Wind Tunnel, 157

open-rotor propulsion, 28–29

open-rotor propulsion testing, 158

Phase 2 single nozzle rig testing, 165

propulsion efficient technology, 145–150

UHB turbofan testing, 29, 157

W7 Multi-Stage Compressor Test Facility, 

151–153, 152

wind tunnel demonstrations, 146

Goldsworthy, W. Brandt, 74

Goodrich, 131–133, 164

Graff, Emilio, 183–185

green aviation concepts, 1

alternative fuels, 28, 161, 165, 167, 

179–180

“green aircraft initiative,” 10

“green diesel,” 179–180

solar energy, 180

thermal, 180

Gulfstream Aerospace, 106

aircraft noise, 138

budget, 137

computer modeling, 137

Flap and Landing Gear Noise Reduction 

Flight Experiment, 61

flap noise reduction tests, 136

G-3 aircraft, 26

G550 aircraft, 26, 126–128, 128, 135, 137

G550 simulated flow-field, 125

G-III-ACTE integration, 111
Index

295

G-III Subsonic Research Aircraft (SCRAT), 

101, 102, 107, 109, 110, 112

G-III Subsonic Research Aircraft (SCRAT) 

sound field, 120, 126

G-III testbed, 92, 94, 137–138

Gulfstream G-III Subsonic Research Aircraft 

(SCRAT) flight envelope, 111

landing gear reduction tests, 131–133

software, 134

H

Harry Diamond Research Laboratories, 185

Heidmann, James D., 151

Herrera, Claudia, 102

Honeywell, 178

Hudson, Larry, 106–107

Huff, Dennis, ERA Project planning, 13

hybrid wing-body (HWB) concept, 14, 20, 32, 34, 

58, 139–140, 162, 168, 170, 196, 199–200. 

See also blended wing body (BWB) concept

ACTE flight experiments, 118

ACTE technology, 116–118

AMELIA (Advanced Model for Extreme Lift 

and Improved Aeroacoustics), 47

ANOPP2 software, 139–140

center body assembly, 83–84, 83

CFD modeling, 172–174

challenges, 63

compared to 777 airliner, 141

compared to Double Bubble, 48

compared to tube-and-wing (TAW) 

configuration, 22, 199

concept modeling, 22

development, 17–18, 55–58

fuel burn reduction, 145

importance of, 199–200

noise reduction, 23, 32, 136, 140–141

noise-prediction tools, 136–137

open-rotor propulsion, 17, 31

propulsion efficient technology, 145

Propulsion-Airframe Aeroacoustic (PAA) 

Element, 33

PRSEUS (Pultruded Rod Stitched Unitized 

Structure), 63, 76–79, 80, 81, 88–91

research vehicle, 18

scale, 48

testbed, 57–58

testing, 31, 33, 173

transport concept, 58

turbine-powered engine simulators (TPS), 

171–175

twin engine, 168

UHB turbofan integration, 31, 37, 60–61, 61, 

167–175, 168, 196

wind tunnel demonstrations, 18

X-48, 24–25

I

Ingersoll Milling Machine Company Advanced 

Stitching Machine (ASM), 70, 71

Integrated Systems Research Program (ISRP), 2, 

9–10, 32, 35–36, 38

Integrated Technology Demonstration (ITD) 

Portfolio, 4–5, 16, 35, 60, 179–180

35A, 157–160

35A performance goals, 159–160

40A, 160–167

ACTE testing, 98–119

benefits, 199

challenges, 5–6, 7

engines, 145

Flap Edge and Landing Gear Noise Reduction, 

122–126

impacts, 37

ITD30A, 150–157

ITD51A, 167–175

NASA Request for Information (RFI) 

candidates, 35–38

Phase 2, 14, 26, 58–59, 198
Green Light for Green Flight

296

PRSEUS (Pultruded Rod Stitched Unitized 

Structure), 78–91

reductions, 36

risks, 35–36

technical focus areas (TFAs), 5–6, 7

thermal efficiency, 150

International Civil Aviation Organization’s 

(ICAO) Committee on Aviation Environmental 

Protection

landing and takeoff NOx emissions levels, 163

Sixth Meeting (CAEP6), 6

Sixth Meeting (CAEP6) emission standards, 

160

Iowa State University, 52

J

James, Kevin, 199–201

Japan Air Lines, 178

Jegley, Dawn C., 60, 74–77, 75

K

Khorrami, Mehdi, 124

computational modeling, 126, 137–138

computer modeling, 134–136

Flap and Landing Gear Noise Reduction 

Flight Experiment, 60

flap noise reduction tests, 130–131

G550 simulated flow-field, 124–125

G550/G-III flight tests, 137–138

landing gear noise reduction tests, 134

noise, 122

noise (causes), 123

noise-prediction tools, 136

SFW program, 123–124

Kota, Sridhar, 99–100, 99, 115

Kroo, Ilan, 12

L

laminar flow control (LFC)

airfoil surfaces, 25–26

airfoils, 19

discrete roughness elements (DRE), 25–26

drag reduction, 57

ERA Project Phase 1, 25

hybrid wing-body (HWB) concept, 56

joined wing concept, 49

natural laminar flow, 25, 189

non-stick coatings, 177

swept-wing concept, 54–55

landing gear, 37, 48, 57, 73, 107

aerodynamic fairings, 19, 23, 52, 67, 132, 

133, 174

Flap Edge and Landing Gear Noise Reduction 

ITD, 60, 61, 122–126, 134

noise reduction, 26–27, 60–61, 122–139, 

198

recycled materials, 195

retractable, 46

Langley Research Center (LaRC), 142

777 airliner, 115

acoustic liners, 129–130

ACTE flight experiments, 113

AFC Enhanced Vertical Tail Flight Experiment, 

184

Airborne Subscale Transport Aircraft 

Research (AirSTAR) testbed, 115

Aircraft Energy Efficiency (ACEE) program, 63

airframe noise reduction, 135

auralization studies, 141–143

Basic Aerodynamic Research Tunnel (BART), 

190–191

Bowles, David, 181

Combined Loads Test System (COLTS) 

facility, 83–88, 87

composites, 63–65

composites fabrication, 64–65

composites testing, 66

design studies, 113

drag reduction, 115

ecoDemonstrator 757, 176, 181
Index

297

ERA Projects goals, 69–70

failures, 171

G550 aircraft, 124–125

insect accretion mitigation (IAM), 189–191

insect count, 194

ITD51A testing, 171–175

Khorrami, Mehdi, 122

National Transonic Facility (NTF), 25

Navier-Stokes flow solver, 124

non-stick coatings, 190–191

PRSEUS cube testing, 80–81, 81

PRSEUS multi-bay box (MBB), 83–85

semi-span wing test article, 68

Structures and Materials Laboratory, 68–69, 

71, 74

Structures Technology Program Office, 65

subsonic tunnel testing, 33, 126–128, 

134–135, 171

subsonic wind tunnel, 128, 132

Transonic Dynamics Tunnel, 98

Transonic Pressure Tunnel, 95

Wahls, Richard, 10

wind tunnel demonstrations, 199–200

wing stub box, 66–67, 67

Leavitt, Laurence, ERA Project planning, 13

Lee, Chi-Ming, 160–161

Liebeck, Robert, 56

Lockheed Martin

Active Aeroelastic Wing (AAW), 96

C-130J transport, 52

composites, 63–65

composites fabrication, 65–66

joined wing concept, 58–59

Preferred System Concept (PSC), 44, 48, 

58–59

X-55 Advanced Composite Cargo Aircraft 

(ACCA), 50, 51

Lockheed Martin Skunk Works (LMSW) Team

composites, 50

joined wing concept, 49–51

subscale test vehicle (STV), 49, 52

Lokos, William, 107, 107

Lovejoy, Andrew, 86, 88

M

Mangelsdorf, Mark, 34, 44–49, 59

Martin, Kenneth, 50–51

Massachusetts Institute of Technology, 46, 56

McDonnell Douglas, 63, 65, 67, 74

semi-span wing test article, 67–69

wing stub box, 66–67, 67

McKay, Bruce, 49–50

microphones, 137–138

airframe noise subsonic wind tunnel setup, 

127–128, 128

airframe noise testing, 135

PAA Element testing, 33

Miller, Eric, 108

Misra, Ajay, 10–11

Mitsubishi, 163

Moog, Inc., 96

Morane, Léon, 99

Morane, Robert, 99

Moses Lake Airport, 195

N

National Aeronautics and Space Administration 

(NASA)

ACTE budget, 100

ACTE flaps, 94, 111

ACTE flight experiments, 26, 92, 94, 114, 

198

ACTE technology studies, 115–116

ACTE testing, 98–119

ACTE wing loading test, 107

Active Aeroelastic Wing (AAW), 96

active flow control (AFC), 177, 189

advanced combustor concepts, 28

Advanced Composite Technology (ACT) 

program, 69
Green Light for Green Flight

298

Advanced Fighter Technology Integration 

(AFTI) program, 95–96

advanced fuel injectors, 28

Advanced Stitching Machine (ASM), 71

Advanced Subsonic Technology (AST) 

program, 69

Advanced Supercomputing Division, 126

Advanced Vehicle Concepts (AVC) study, 45

Aeronautics Research Mission Directorate 

(ARMD), 2–3, 8–9, 40, 198

AFC Enhanced Vertical Tail Flight Experiment, 

182–189

AFC Enhanced Vertical Tail Flight Experiment 

preliminary testing, 184

Airborne Spacing for Terminal Arrival Routes 

(ASTAR), 179

Aircraft Noise Prediction Program (ANOPP), 

19

Ames Research Center (ARC), 121

ATDC testbed, 151–156

Auralization Framework (NAF), 139

budget, 12

composite wing, 67

composites fabrication, 65

composites research, 63–65

computer modeling, 137–138

Critical Design Review, 137

Dassault HU-2C Guardian Falcon, 191

ecoDemonstrator 757, 177–195

ecoDemonstrator 757 IAM airports, 

191–192

ecoDemonstrator 757 preliminary flight test 

results, 193

Environmentally Responsible Aviation (ERA) 

project, 1–2

ERA Project goals, 20, 58, 121–122, 164, 

197

ERA Project Phase 1 flight testing, 24

ERA Project research projects, 19

ERA Project successes, 159–160

F/A-18 AAW testbed, 97–98, 97

F/A-18 chase plane, 109

flame tube facility, 164

Flight Optimization and Performance Sizing 

(FLOPS) tool, 19

Fundamental Aeronautics Program (FAP), 

1–2, 141

GE F-111A MAW, 95, 98–100, 100

geared turbofan (GTF) engines, 149, 

157–160, 163

“green aircraft initiative,” 10

Gulfstream Aerospace G-III, 92, 94, 98–100

Gulfstream G-III testbed, 137–138

High-Alpha Research Vehicle (HARV), 97

hybrid wing-body (HWB) concept, 196

IAM assessment process, 194

innovations, 46, 115, 142–143, 177

insect density criterion, 191, 194

Integrated Aviation System Program (IASP), 

181, 197

Integrated Systems Research Program 

(ISRP), 2, 9

ITD51A testing, 171–175

knowledge sharing, 177, 198

LaRC National Transonic Facility (NTF), 25

LaRC subsonic wind tunnel setup, 126–128

lean direct injection (LDI), 164–165

lean-burn concept, 161

low-loss fan exit guide vanes (FEGVs), 159

metrics (subsonic transport system), 27, 30, 

35, 48–49

mission adaptive wing (MAW), 95–96

mission adaptive wing (MAW) testbed, 95

NASA 804, 98

NASA 840, 97

NASA 853, 98

National Advisory Committee for Aeronautics 

(NACA), 1

National Advisory Committee (NAC), 8–10, 

18, 121
Index

299

Navier-Stokes flow solver, 124

Next Generation Air Transportation System 

(NextGen), 2, 43–44, 56

noise mitigation, 121–122

non-stick coatings, 177, 193

Numerical Propulsion Simulation System/

Weight Analysis of Turbine Engines (NPSS/

WATE), 19

organizational culture, 61

over-the-rotor (OTR) technology, 159–160

partnerships, 18, 29–30, 32, 44, 63, 83, 88, 

112–114, 149, 151, 197–198

partnerships (OGA), 36, 39, 113–114

personnel, 87, 110, 111, 144, 162, 170, 

176, 181, 193

Phase 2 advanced engine concept, 166

Phase 2 single nozzle rig testing, 165

Phase 2 Tiger Team, 34–37, 59–61

Preferred System Concept (PSC), 43–49

PRSEUS (Pultruded Rod Stitched Unitized 

Structure), 63, 74–79

PRSEUS multi-bay box (MBB) testing, 83, 

86

Request for Information (RFI), 35

research and development, 1

Research Announcement (NRA), 17, 34–35, 

43, 47, 65, 169, 198

risk investment, 63–64

Rotary Wing and Aeronautical Sciences, 

141

semi-span wing test article, 67–69

SOFIA (Stratospheric Observatory for Infrared 

Astronomy), 121

soft vanes (SV) technology, 159–160

Space Act Agreements, 198

subscale test vehicle (STV), manned, 59

Subsonic Fixed Wing (SFW) program, 1–3, 

7, 59

Subsonic Transport System Level Metrics, 3

TALON X combustor, 163

TAPS combustor, 162–163

time-and-schedule project planning, 61

Transonic Aircraft Technology Program, 

95–96

UHB turbofan testing, 157

UHB-HWB integration, 169

Ultra-Efficient Engine Technology (UEET) 

program, 162–163

USM3D code, 172

W7 Multi-Stage Compressor Test Facility, 

151–156, 152

Wallops Flight Facility, 137

wings (ACTE), 93, 95

X-48B demonstrator, 24

X-48C demonstrator, 58

X-53, 97, 98

National Aeronautics Policy and Plan, 18

National Aerospace System (NAS), 21

National Full-Scale Aerodynamics Complex 

(NFAC), iv, 144, 170, 171–175, 187–188

National Research Council (NRC), 9, 12

Navy, U.S., 97

Newman, Dava, 176, 181

Next Generation Air Transportation System 

(NextGen), 2, 8, 10–13, 43–45, 56, 59, 88, 

198

Nickol, Craig, 19

Northrop, John K., 52–53

Northrop Grumman

B-2A bomber concept, 52–55

composites, 65

composites fabrication, 65–66

double-fuselage configuration, 52

flying wing concept, 52–55

partnerships, 52

Preferred System Concept (PSC), 44, 48, 49, 

52–55, 58–59

subscale test vehicle (STV), 49, 55

YB-49A, 52, 53
Green Light for Green Flight

300

O

Obama, Barack, 12, 16

P

Panasonic, 178

Parker Hannifin, 164–165

Pathe Technologies, Inc., 70

Pleiades (supercomputer), 126

population growth, 20–21

powerplants. See engines

Pratt & Whitney, 2

2nd Generation UHB-Ration Propulsor 

Integration, 61

ATDC testbed, 151

axial stage combustor (ASC), 161–167

axial stage combustor (ASC) technology, 166

engine inlet distortion, 172–174

ERA Project successes, 159–160

full annular ring test article, 166

geared turbofan (GTF) engines, 60, 148–150, 

174

hybrid wing-body (HWB) concept, 56–58

lean-burn concept, 161, 164, 166–167

low-FPR UHB propulsor testing, 157–160

Low NOx Fuel Flexible Engine Combustor 

Integration, 61, 160

low-loss fan exit guide vanes (FEGVs), 159

open-rotor UHB, 159

partnerships, 56, 149

Phase 1 test results, 165–166

Phase 2 advanced engine concept, 166

propulsion efficient technology, 145

PW1000G GTF, 57

PW1100G-JM, 149

PW2037 engines, 188

rich quench lean (RQL) combustors, 163, 

166–167

successes, 167

TALON X combustor, 163

turbofan (UHB), 157

UHB turbofan testing, 157

V2500 engine, 149

X960 rig, 166

Propulsion Technology subproject, 5, 11, 21, 

27, 151

PRSEUS (Pultruded Rod Stitched Unitized 

Structure), 17, 63, 73, 198

defined, 23–24, 60

ERA Project Phase 1, 23–24

multi-bay box (MBB), 83, 84

Phase 2, 23

pressure cube, 80, 81

pressure cube testing, 82

R

reductions, 7, 125

acoustic shielding, 56–57

ACTE flight experiments, 92, 98

airframe noise subsonic wind tunnel setup, 

126–128

alternative fuels, 28

composites, 69

drag, 23, 47, 56, 94, 115, 147, 168–169, 

192, 197–198

drag (cruise), 100

drag (insect accretion), 177, 189–195

drag (parasitic), 42

drag (tail), 182–183, 186, 189

drag (viscous), 25–26, 41, 42

drag (wave), 42

emissions, 2–3, 19, 27–28, 31–32, 57, 145, 

151, 160–166, 162, 174, 179, 197–198

environmentally friendly aircraft technologies, 

17–18

EPNdB, 6, 145, 159–160, 168

ERA concept modeling summary, 22

ERA Project goals, 58

ERA Project Phase 1 studies, 14

ERA Project Phase 2 technical focus areas 

(TFAs), 6, 7
Index

301

factory waste, 180

fan pressure ratio (FPR), 147, 149, 158–159

Flap and Landing Gear Noise Reduction 

Flight Experiment, 60

flying-wing concept, 55

fuel, 101

fuel burn, 2–3, 19, 22, 27, 31–32, 55–57, 

118–119, 145–146, 153, 160, 177, 

197–198, 200

fuel burn (aircraft system level), 168

fuel burn (bypass ratio), 28

fuel burn (insect accretion), 189

fuel burn (ITD51A testing), 172–174

fuel burn (low FPR), 158

fuel burn (tail-influenced), 182–183, 186

fuel burn (total), 150–151

fuel weight, 51

General Electric (GE) CFM Leap-X-based 

propfans, 57

HWB load magnitudes, 90

hybrid wing-body (HWB) concept, 56–58, 63

insect accretion mitigation (IAM), 189–195

integration penalties, 168

ITD candidates, 36

lift-to-drag ratio (L/D), 115, 159

loads, 117, 119

loads (pressure), 42–43

mission adaptive wing (MAW), 96

multidisciplinary systems analysis, 30–31

Next Generation Air Transportation System 

(NextGen), 44

noise, 2–3, 19–20, 22, 29, 31–32, 94–95, 

158–159, 166, 200

noise (ecoDemonstrator 757), 178

noise (airframe), 121–143, 128, 197

noise (Boeing-Rolls-Royce QTD program), 

177

noise (bypass ratio), 28

noise (causes), 123, 123, 125

noise (engine), 42, 140–141, 145, 197–198

noise (ERA Project Phase 1), 26–27

noise (far-field), 138

noise (flaps), 128–131, 134, 136

noise (importance of), 121–122

noise (landing gear), 131–134, 132, 133

noise (PAA Element), 33

noise (perceived), 6, 140–143

noise (propulsion), 123

noise (public opinion), 140–141

noise (simulated), 138

noise (types of engines), 122

noise (wind tunnel background), 135

Northrop Grumman PSC, 55

obstacles, 147

ozone, 160

Phase 2 ITDs, 60–61

pollution, 2–3, 20–21, 177

Preferred System Concept (PSC), 46, 49

PRSEUS (Pultruded Rod Stitched Unitized 

Structure), 89–90

runway dimensions, 186

sonic boom, 59

sound baffling, 24

Thrust Specific Fuel Consumption (TSFC), 6, 

150–151, 153, 159

vortex filaments, 125

weight, 47, 168

weight (aircraft structural), 74, 180, 

197–198

weight (empty), 118

weight (engine), 148

weight (tail), 182–183, 186

weight (takeoff), 118

weight (wing structural), 113

wetted area, 41–42

wetted area (per passenger), 48

X-55 Advanced Composite Cargo Aircraft 

(ACCA), 51

Rensselaer Polytechnic Institute, 183

Reynolds numbers, defined, 25
Green Light for Green Flight

302

Rigney, Thomas, 60, 100–101, 101, 106, 109, 

111–113

Rizzi, Stephen, 139–143, 142

Rockwell Collins, 178

Rolls-Royce, 49, 52

ATDC testbed, 151

ecoDemonstrator 757, 178

HWB development, 56, 58–59

joined wing concept, 49

Liberty Works UltraFan engine, 50–52

partnerships, 52, 177

Quiet Technology Demonstrator program, 

177–178

UltraFan engine, 51

W7 compressor rig, 151

Rosario, Rubén Del, 13–15

S

Scolese, Christopher J., 15

Shin, Jaiwon, 10, 12, 13, 15–16, 36, 48–49, 

113–114, 200

Shreveport Regional Airport, 192

Sikorsky, Igor, 99

Sinnett, Mike, 195

Siochi, Mia, 189–190, 194

software, 124–126

aerodynamic performance models, 26

ANOPP, 139–143

ANOPP2, 140

ANSYS, 104–105

CAD (computer-aided design), 134

computational modeling, 139–143, 

172–173

computer modeling, 137–138, 169

Environmental Design Space (EDS) tool, 19

Exa code, 137–138

full-scale geometry modeling, 134

modeling, 129, 131–138, 138

NASA Auralization Framework (NAF), 

139–143

NASTRAN, 104

Navier-Stokes flow solver, 124

OVERFLOW, 172

PowerFLOW Lattice-Boltzmann flow solver, 

124

USM3D code, 172

ZAERO, 106

Stanford University, 12

Stifel Bank, 178–180, 194–195

Stoliker, Patrick, 13

Strazisar, Anthony, 13

Subsonic Fixed Wing (SFW) program, 1–2, 

14–15, 57, 123

advanced fan blade technology, 150

Airspace Systems Program (ASP), 15

ERA Project goals, 3

ERA Project partnership, 14

ERA Project precedents, 7, 28, 150

fan blade technology, 150

innovations, 46

open-rotor propulsion, 28–29

Rosario, Rubén Del, 15

Shin, Jaiwon, 10

SUGAR (Subsonic Ultra Green Aircraft 

Research) Volt concept, 46, 47

Wahls, Richard, 15

Suder, Ken, 60

Sullivan, Barry, 13

T

Technology Readiness Levels (TRLs), 9

ACTE flight experiments, 98, 101

ACTE technology, 115

advanced combustor concepts, 28

Airframe Technology subproject, 22

axial stage combustor (ASC), 165, 167

beginning, 44

combustor technology maturation plan, 165

defined, 7–8

ending, 35, 44
Index

303

ERA Project Phase 1, 34, 38

ERA Project technology solutions, 17

ITD51A testing, 173

Low NOx Fuel Flexible Engine Combustor 

Integration, 161

Maturation Roadmaps, 35

PRSEUS (Pultruded Rod Stitched Unitized 

Structure), 78, 198

UHB-HWB integration, 169

TechX, 55

Thomas, Russ, 175

Trimble, Stephen, 74

truss-braced wings, 14, 34, 42, 46, 48

TUI Group, the, 178–180

turbulence, 49, 93, 108, 139–140, 146–147

U

United Airlines, 179–180

United Technologies Research Center (UTRC), 

163–167

University of Arizona, 183

University of Connecticut, 165

University of Michigan, 99, 115

V

Vehicle Systems Integration (VSI) subproject, 5, 

11, 19, 21–23, 30–31, 34, 37

Virginia Tech Stability Tunnel, 132–133

Volpe National Transportation Systems Center, 

191–192

W

Waggoner, Edgar, 112, 181, 197, 197

Wahls, Richard A., 10, 13, 198

American Recovery and Reinvestment Act of 

2009, 16

ERA Project challenges, 16

ERA Project funding, 12, 15–16, 59

ERA Project Phase 1, 15

FAP, 10–11, 13

hybrid wing-body (HWB) concept, 47

SFW program, 10, 15

X-plane, 13–14, 59

Washburn, Anthony E., 15, 34, 189

Whalen, Ed, 185

Whitcomb, Richard T., 95

White House, the, 9–10, 99

Woodward FST, 164

Wright brothers, 97, 99

Wygnanski, Israel, 183, 186

Wyle Laboratories, 52

X

X-planes, 11–13, 17, 48–49, 59

Advanced Vehicle Concepts (AVC) study, 

48–49

Boeing X-48B demonstrator, 57

Boeing X-48C demonstrator, 57

ERA Project Phase 2, 59

NASA budget, 59

X-48, 24

X-48B demonstrator, 24–25, 57, 199

X-48C demonstrator, 24–25, 57–58, 58, 199

X-53, 97, 98

X-55 Advanced Composite Cargo Aircraft 

(ACCA), 50–52, 51

XVT (experimental vehicle testbed), 11–13

Y

YB-49A aircraft, 52, 53

Yu, Jeanne, 188–189, 195

Z

Zona Technologies, Inc., 106

National Aeronautics and Space Administration

Washington, DC

NASA SP-2020-646