Ultra-efficient Subsonic Aircraft

Since the introduction of the legendary X-1 in 1946, scientists have used the X-plane designations to identify experimental aircraft and rockets used to explore new aerospace technologies.As of late 2018, an X designator for this designator to this aircraft is conjectural.

In 2016 NASA announced a 10-year plan by NASA Aeronautics to achieve ambitious goals in reducing fuel use, emissions, and noise by the way aircraft are designed, and the way they operate in the air and on the ground. One exciting piece of this 10-year plan is New Aviation Horizons – an ambitious undertaking by NASA to design, build and fly a variety of flight demonstration vehicles, or “X-planes.” The New Aviation Horizons X-planes will typically be about half-scale of a production aircraft, although some may be smaller or larger, and are likely to be piloted. Design-and-build will take several years, with vehicles going to flight starting around 2020 depending on funding.

One X-plane will demonstrate specific technologies related to ultra-efficient subsonic aircraft designs in flight – possibilities include very long but narrow wings, a double-wide fuselage, or engines embedded into the vehicle. In October 2008, NASA asked industry and academia to imagine what the future might bring and develop advanced concepts for aircraft that can satisfy anticipated commercial air transportation needs while meeting specific energy efficiency, environmental and operational goals in 2030 and beyond. The studies were intended to identify key technology development needs to enable the envisioned advanced airframes and propulsion systems.

NASA's goals for a 2030-era aircraft, compared with an aircraft entering service today, are:

• A 71-decibel reduction below current Federal Aviation Administration noise standards, which aim to contain objectionable noise within airport boundaries.
• A greater than 75 percent reduction on the International Civil Aviation Organization's Committee on Aviation Environmental Protection Sixth Meeting, or CAEP/6, standard for nitrogen oxide emissions, which aims to improve air quality around airports.
• A greater than 70 percent reduction in fuel burn performance, which could reduce greenhouse gas emissions and the cost of air travel.
• The ability to exploit metroplex concepts that enable optimal use of runways at multiple airports within metropolitan areas, as a means of reducing air traffic congestion and delays.

The teams were led by General Electric, Massachusetts Institute of Technology, Northrop Grumman and The Boeing Company. The 18-month NASA research effort to visualize the passenger airplanes of the future produced some ideas that at first glance may appear to be old fashioned. Instead of exotic new designs seemingly borrowed from science fiction, familiar shapes dominate the pages of advanced concept studies which four industry teams completed for NASA's Fundamental Aeronautics Program in April 2010.

These are some of the common themes from the four reports:

• Slower cruising – at about Mach 0.7, or seven-tenths the speed of sound, which is 5 percent to 10 percent slower than today's aircraft -- and at higher altitudes, to save fuel.
• Engines that require less power on takeoff, for quieter flight.
• Shorter runways – about 5,000 feet long, on average – to increase operating capacity and efficiency.
• Smaller aircraft – in the medium-size class of a Boeing 737, with cabin accommodations for no more than 180 passengers – flying shorter and more direct routes, for cost-efficiency.
• Reliance on promised advancements in air traffic management such as the use of automated decision-making tools for merging and spacing enroute and during departure climbs and arrival descents.

With its 180-passenger D8 "double bubble" configuration, the Massachusetts Institute of Technology team strays farthest from the familiar, fusing two aircraft bodies together lengthwise and mounting three turbofan jet engines on the tail. Important components of the MIT concept are the use of composite materials for lower weight and turbofan engines with an ultra high bypass ratio (meaning air flow through the core of the engine is even smaller, while air flow through the duct surrounding the core is substantially larger, than in a conventional engine) for more efficient thrust. In a reversal of current design trends the MIT concept increases the bypass ratio by minimizing expansion of the overall diameter of the engine and shrinking the diameter of the jet exhaust instead. The team said it designed the D8 to do the same work as a Boeing 737-800. The D8's unusual shape gives it a roomier coach cabin than the 737.

Based on a modified tube and wing with a very wide fuselage to provide extra lift, its low sweep wing reduces drag and weight; the embedded engines sit aft of the wings. The D8 series aircraft would be used for domestic flights and is designed to fly at Mach 0.74 carrying 180 passengers 3,000 nautical miles in a coach cabin roomier than that of a Boeing 737-800. The D8 is among the designs presented in April 2010 to the NASA Aeronautics Research Mission Directorate for its NASA Research Announcement-funded studies into advanced aircraft that could enter service in the 2030-2035 timeframe.

In order to address growing concerns over the increasing global environmental impact of aviation and the resulting need for improved aircraft technologies, the National Aeronautics and Space Administration (NASA) initiated fundamental aeronautics research through its Subsonic Fixed Wing (SFW) and later Environmentally Responsible Aviation (ERA) projects. Focused primarily on applications beginning with generation after next (N+2) aircraft, NASA, industry and university researchers are pioneering new technologies which have the potential to significantly reduce utilization of the world’s finite petroleum resources, harmful atmospheric emissions and objectionable community noise levels.

The Robust Design for Embedded Engine Systems project is working to accomplish these goals through research into its embedded, boundary layer ingesting (BLI) propulsor concept integrated into a Hybrid Wing Body (HWB) or alternatively named Blended Wing Body (BWB) aircraft configuration. As its name implies, HWB aircraft combine desirable attributes possessed by conventional “tube and wing” and less often employed “flying wing” configurations in a way that mitigates their individual shortcomings. Much like a “flying wing” configuration, the entire form of an HWB aircraft is employed to generate lift, resulting in first order improvements in life-to-drag ratio. However, its shape and the inclusion of vertical stabilizers reduce its susceptibility to stability and control challenges. Although the focus of the initial effort was on HWB aircraft, it is anticipated that the technologies being researched would be applicable to other aircraft types as well.

Significant potential exists for the boundary layer ingesting propulsor concept. Indeed, these benefits were identified at least as early as 1947 by Smith. Many investigations have been conducted since this time. Notably, Smith12 and Drela provided methods for assessing and quantifying BLI benefits. A number of more recent aircraft systems studies have shown that embedded BLI propulsors are an important, enabling concept for meeting NASA’s goals.

The Robust Design for Embedded Engine Systems project first conducted a high-level vehicle system study based on a large commercial transport class hybrid wing body aircraft, which determined that a 3 to 5 percent reduction in fuel burn could be achieved over a 7,500 nm mission. Both pylon-mounted baseline and BLI propulsion systems were based on a low-pressure-ratio fan (1.35) in an ultra-high-bypass ratio engine (16), consistent with the next generation of advanced commercial turbofans.

NASA awarded six-month contracts to four companies, who will each define the technical approach, schedule, and cost for one or more large-scale, subsonic X-plane concepts. These concepts are in support of NASA’s ultra-efficient subsonic transport research goals. The companies are Aurora Flight Sciences Corporation of Manassas, VA; Dzyne Technologies Incorporated of Fairfax, VA; Lockheed Martin Aeronautics Company of Ft. Worth, TX; and The Boeing Company of Hazelwood, MO. The requested information is to be built around a plan that would see the selected experimental aircraft eventually flying no later than 2021.

The five X-plane concepts envisioned for possible further development and the contractor responsible for providing NASA with the required information include:

1. Aurora Flight Services for the D8 “Double Bubble,” a twin-aisle, largely composite airliner in which the fuselage is shaped to provide lift – enabling smaller wings – and the jet engines are mounted atop the rear tail area, which takes advantage of the air flow over the aircraft to both improve engine efficiency and reduce noise in the cabin and on the ground below.
2. Dzyne Technologies for a smaller regional jet-sized aircraft that features a blended wing body (BWB) design in which the lines of a traditional tube and wing airliner are shaped to become one continuous line in which the seam between the wing and fuselage is nearly indistinguishable. As an aerodynamic shape, this configuration increases lift and reduces drag.
3. Lockheed Martin for its Hybrid Wing Body, which includes features of the BWB on the forward part of the fuselage but has a more conventional looking T-shaped tail, with its jet engines mounted on the side of the hull but above the blended wing. Increased lift, reduced drag and quieter operations are all potential benefits.
4. Boeing for both its BWB concept – versions of which the company has flight tested with its subscale X-48 program in partnership with NASA – and a Truss-Braced Wing concept, which features a very long, aerodynamically efficient wing that is held up on each side by a set of trusses connecting the fuselage to the wing. Otherwise the aircraft appears more conventional than the other X-plane concepts under consideration.