Subsonic Ultra Green Aircraft Research (SUGAR)
The demand for green aviation is expected to increase with the need for reduced environmental impact of air travel. Most large transports today operate within the best cruise L/D range of 18-20 using the conventional tube-and-wing design. This configuration has led to incremental improvements in aerodynamic efficiency over this past century. In recent years, the use of lightweight materials, such as composites, has been shown to significantly reduce structural weight and trim drag, leading to improved energy efficiency. The Boeing 787 transport is an example of a modern airframe design that employs lightweight structures. High aspect ratio wing design can provide another opportunity for further improvements in energy efficiency.
Research and development of high aspect ratio wing transport designs has placed a greater emphasis on the studies of aeroelasticity and flutter owing to the increase in the wing flexibility as the wing aspect ratio increases. These studies have sought to develop methods and tools for aeroelasticity by laying the foundation for more modern high aspect ratio wing aircraft such as the Transonic Truss-Braced Wing (TTBW). The Subsonic Ultra Green Aircraft Research (SUGAR) TTBW aircraft concept is a Boeing-developed N+3 aircraft configuration funded by NASA Aeronautics Research Mission Directorate (ARMD) Advanced Air Transport Technologies (AATT) project.
The Transonic Truss-Braced Wing (TTBW) aircraft is designed to be aerodynamically efficient by employing an aspect ratio of about 19.55, which is significantly greater than those of conventional aircraft cantilever wings. The main idea is to use truss structures to alleviate the wing root bending moment, so that a significant increase in the wing aspect ratio could be afforded. The main wings are braced at approximately mid-span by two main struts. In addition, two jury struts, one on each wing, provide additional reinforcement. The additional braced structures will cause some aerodynamic impacts to the wing.
In January 2019 Boeing revealed the newest Transonic Truss-Braced Wing (TTBW), which researchers say will fly higher and faster than the previous TTBW concepts. The new configuration is designed to offer unprecedented aerodynamic efficiency while flying at Mach 0.80, which is consistent with the speed of many of today’s jetliners. From end-to-end, the folding wings measure 170 feet. The high wingspan is made possible by the presence of a truss, which supports the extended length of the ultra-thin wing.
Originally, the TTBW was designed to fly at speeds of Mach 0.70 – 0.75. To increase the aircraft’s cruise speed, the new concept now has an optimized truss and a modified wing sweep. By adjusting the wing sweep angle, the truss can carry lift more efficiently. The end result was a more integrated design that significantly improved vehicle performance. The new changes follow extensive wind tunnel testing at NASA Ames Research Center. For nearly a decade, Boeing and NASA have been studying the concept as part of the Subsonic Ultra Green Aircraft Research (SUGAR) program. The research focuses on innovative concepts that reduce noise and emissions while enhancing performance.
In the art of commercial airplanes, it is highly desirable to design airplane and engine configurations that yield reduced fuel burn per seat-mile, which is a metric of airplane fuel efficiency and carbon dioxide emissions. Carbon trading and Carbon tax regulations comparable to those already enacted in the European Union are also likely to be adopted in other industrialized nations including the United States. These environmental considerations become even more important in economic scenarios in which fuel cost increases. This motivates step-change technologies to reduce fuel consumption per passenger mile.
This need for reduced fuel burn per seat-mile may be in conjunction with anticipated near-term increases in stringency of community noise certification regulations. Current European workplace noise exposure regulations that affect allowable aircraft cabin noise work together with local airport environmental policies to also pose significant challenges to advanced propulsion design. Thus, improvements in community and cabin noise relative to existing airplanes are also desirable.
The emissions-based requirements motivate extremely high bypass ratio engines which can most easily be accomplished with un-shrouded engines. Some un-shrouded engines however might not have an optimized configuration for noise reduction. It is also an objective for commercial airplanes including their propulsors to be perceived in a positive way by the flying public, similar to how "jet airplanes" with turbofan propulsors are perceived in a positive way.
One existing approach to providing improved fuel efficiency or reduced fuel burn is to utilize turbofan engines with higher bypass ratios. However, very high bypass ratio turbofans suffer from large weight and drag penalties associated with their very large fan ducts. Very high bypass ratio turbofans also suffer from difficulties associated with achieving under-wing installations in low wing airplanes and difficulties in achieving simple lightweight thrust reversers.
Another existing approach to providing improved fuel efficiency or reduced fuel burn is to utilize a turboprop, propfan, or other "open rotor" types of propulsor. An open rotor propulsor is effectively a propeller with a six to ten discrete individual blades exposed at their tips, with a gas turbine core engine driving the propeller through a gearbox. Open rotor propulsors provide substantially better fuel burn through a higher effective bypass ratio and elimination of fan duct drag and weight, but may have airplane integration challenges, non-optimal community noise levels, and non-optimal cabin noise and vibration.
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