AeroAstro Magazine Highlight

SUBSONIC CIVIL TRANSPORT AIRCRAFT FOR 2035

By Elena de la Rosa Blanco and Edward M. Greitzer

Aviation is a critical aspect of modern society, moving people and goods throughout the world and fostering economic growth. From 1981 to 2006, the demand for air transportation in North America grew by a factor of three; while forecasts for the next 25 years vary, they present a strong message that this trend will continue.

MIT's designs for two future aircraft: the D Series (left), a double-bubble configuration, which is a 180-passenger plane, and the H Series, a blended-wing body which is a larger 350-passenger plane. Both would offer major fuel, noise, engine emissions, and runway length reductions compared to aircraft of today. (MIT/Aurora Flight Sciences image)

In 2008, NASA awarded four research contracts to define advanced concepts and enabling technologies for subsonic aircraft, in the 2035 timeframe, that could address the challenges posed by this increased demand. The research was part of the NASA N+3 program, where N+3 refers to aircraft three generations beyond those currently flying. The awards were to teams led by Boeing, Northrop Grumman, GE, and MIT. The MIT team, the only one led by a university, included Aurora Flight Sciences and Pratt & Whitney as partners. The collaboration among these three organizations resulted in the development of innovative conceptual designs, with the potential for step changes in capabilities, for future subsonic commercial transports.

PROJECT TARGETS: NOISE, EMISSIONS, FUEL, RUNWAY LENGTH

NASA set four targets as metrics for the design concepts: aircraft noise, engine emissions (as expressed in terms of nitrogen oxides produced during landing and takeoff), fuel burn, and runway length. The targets were aggressive; for example, a reduction of 70 percent in fuel burn for a reference aircraft and a noise goal comparable with that of the MIT-Cambridge University Silent Aircraft Initiative of several years ago, namely aircraft noise imperceptible beyond the airport perimeter. The team added a fifth metric as part of its design evaluation: the global average surface temperature change due to aircraft emissions, which reflects aviation's impact on climate change.

The multidisciplinary MIT-Aurora-P&W team had as an objective the rigorous definition of the potential for improvements in noise, emissions, fuel burn, climate, and airport use for subsonic transport aircraft. The project incorporated assessments of technologies in aerodynamics, propulsion, operations, and structures to ensure that a full spectrum of improvements was identified, plus a system-level approach to find integrated solutions that offer the best balance in performance enhancements. The assessment was enabled by a first-principles methodology that allowed simultaneous optimization of the airframe, propulsion system, and operations. The conceptual design exercise also included evaluations of the risks and contributions associated with each enabling technology, and roadmaps for the steps needed to develop the levels of technology required.

As the initial task — to frame the type of aircraft that would be most appropriate — the team defined a scenario for 2035 aviation based on estimates of passenger demand, airline operations, fuel constraints, airport availability, environmental impact, and other parameters. This scenario, plus the NASA targets, led to two conceptual aircraft designs. The missions of the two were selected from different market segments, but they were chosen so that, together, the two aircraft would represent a substantial fraction of the commercial fleet. This, in turn, implied that adoption of such designs could have a significant impact on fleet-wide fuel burn, noise, emissions, climate, and airport use.

DESIGNS FOR DIFFERENT MARKETS

A major result of the program was development of the two conceptual aircraft designs. One of these is aimed at the domestic market, flights from 500 nautical miles up to coast-to-coast across the United States. This design represents a 180-passenger aircraft, in the Boeing 737 or Airbus A320 class, which makes up roughly a third of the current fleet. We named this concept the "D Series" because of its "double bubble" fuselage cross-section. The other aircraft, which we call the "H Series" for hybrid-wing-body, is defined for international routes. This latter design, envisioned as a Boeing 777 aircraft replacement, features a triangular hybrid wing body that blends into the wings, accommodation of 350 passengers in a multiclass configuration with cargo, and a range of at least 7,000 nautical miles.

Rendering and performance assessment of two MIT aircraft designs relative to the four N+3 NASA targets. The circles on the small graphs represent the 50 percent, 75 percent and 100 percent target levels targets, and the solid symbols indicate the performance of the concept aircraft. (Click image to enlarge)

The D Series configuration was calculated to meet fuel burn, engine emissions, and runway length targets, and to provide a substantial step towards achieving the noise target. The H Series was calculated to meet engine emissions and runway length targets, and is markedly improved compared to current aircraft for fuel burn and noise.

For both designs, the engines ingest the relatively slower moving air from the fuselage boundary layer (the air flowing next to the aircraft's body), providing a higher propulsive efficiency and, thus, an advantage from a fuel burn perspective. However, the flow into the engines consists of fluid from both within and outside of the boundary layer, so there is a non-uniform velocity into the engines. This is different from current engines, which hang in front of the wing and thus encounter virtually uniform flow. Integration of the aircraft and this unconventional propulsion system is one of the main technical challenges. The D Series flies about 10 percent slower than the 737, so the wings on the former, which have a much higher aspect, or length-to-width, ratio (29 vs. 10), require less sweepback than those on the latter. (The sweepback is to address deteriorations in airfoil performance that can occur on an unswept wing at higher velocities.) The lower speed also allows other changes that result in a lighter, more efficient aircraft, leading to the 70 percent fuel burn reduction mentioned earlier.

The fuselage is shorter and wider than a 737's, and the D Series configuration gives numerous structural, aerodynamic, and propulsion system benefits, which contribute to the much reduced fuel burn. While both aircraft can be classed as "tube and wing," the D Series features two parallel tubes in a double-bubble fuselage cross-section accommodating two aisles, a possible time saver for passenger loading and unloading. The lifting fuselage allows smaller and lighter wings. The nose-up pitching moment from the upturned nose, and the twin vertical tails, reduce the size and weight of the horizontal tail. The D Series has three engines placed above the aircraft between the vertical tails, shortening and lightening the landing gear and enabling smaller and lighter vertical tails. The configuration also provides acoustic shielding and, therefore, a reduction in the engine noise that propagates to the ground.

The D Series aircraft fuselage (left) is shorter and wider than that of a 737 (right). It provides three times the percentage of the overall lift on the aircraft, compared to that of the 737 fuselage (approximately 6 percent for 737). (Click images to enlarge)

Compared to current aircraft, the double-bubble configuration offers a greater fuel reduction at the 737 payload and range than at higher payload missions. In contrast, the hybrid wing body achieves its best fuel burn at the 777 payload and range. Yet, even at the larger payload (and aircraft size), the double-bubble configuration offers essentially the same performance (NASA metrics) as the hybrid-wing body. Therefore, a second major finding is that although both configurations offer substantial benefits compared to the baselines, for the aircraft considered, the double-bubble configuration exhibits better performance (or equal performance for large payload/range) compared to the hybrid wing body.

chart

(left) Performance of the D and H series aircraft compared with NASA N+3 targets.

(right) The D series configuration offers major performance benefits even with current technology. This chart shows the effect of configuration change on the NASA targets (the top three bars) compared with the benefits brought by changes in technology (the bars in all the other categories).

A third result stems from our investigation of specific contributions to the performance of the D Series aircraft. The benefits of the N+3 concepts are from two sources. One is advances in specific technologies, such as stronger and lighter materials, higher efficiency engine components, and turbine materials with increased temperature capability. The other is the inherent benefit of the aircraft configuration. In other words, even limited to existing technologies (aluminum wings and fuselage, current technology engines with current bypass ratios, etc.), the configuration alone offers major performance benefits.
The step change in capability calculated for the D Series configuration is perhaps this project's most important finding. It implies that an aircraft configuration change has the potential to alter the face of commercial aviation, and that this change could occur on a much shorter time scale than required for maturation of many separate technologies. At this writing we await information on the second phase of the NASA N+3 program, under which we hope to take the next steps in bringing the D Series closer to service.

UNIVERSITY-INDUSTRY COLLABORATION

Two aspects of the university-industry collaboration are particularly important. The first was the virtually seamless interaction between the different organizations. The second, enabled by the first, was the emphasis on what is perhaps best described as the primacy of ideas rather than of organization or hierarchy. In other words, concepts and suggestions were considered directly on merit (e.g., content, strategic value, or impact) rather than the originator of the idea, or the legacy of the idea. From the start of the project, this was emphasized and fostered in team discussions. The consequence was that the team functioned with open-mindedness to new ideas and, as a direct corollary, a willingness to subject even cherished concepts to in-depth scrutiny. Our goal was to create a team in which "the whole was greater than the sum of the parts" because of strong interactions among participants. The achievement of this goal in an enterprise involving students, staff, faculty, and engineers in industry from a number of fields, with benefits to all parties involved, is also a major outcome of the project.

Team members
It cannot be emphasized too strongly that the project was a team effort, involving numerous MIT AeroAstro faculty and staff, as well as engineers from Aurora and Pratt & Whitney, taking major roles. MIT faculty and staff participants were Mark Drela, John Hansman, James Hileman, Jack Kerrebrock, Robert Liebeck, and Choon Tan. Jeremy Hollman and Wesley Lord were the team leads at Aurora Flight Sciences and Pratt & Whitney, respectively. The analyses and design information described came from all of these, from students Chris Dorbian, David Hall, Jonathan Lovegren, Pritesh Mody, Julio Pertuze, and Sho Sato, and from many others at Aurora and Pratt & Whitney.

Elena de la Rosa Blanco is a research engineer in the AeroAstro Gas Turbine Laboratory. A University of Cambridge PhD, she was a member of the Cambridge-MIT Silent Aircraft Initiative project before coming to MIT. She may be reached at edlrosab@mit.edu.

Edward Greitzer, the N+3 project principal investigator, is the H. N. Slater Professor of Aeronautics and Astronautics and former director of the Gas Turbine Laboratory. He is a Fellow of the ASME and AIAA, a member of the National Academy of Engineering, and the International Fellow of the Royal Academy of Engineering. He may be reached at greitzer@mit.edu.