Seventy years ago, Boeing was secretly preparing to gamble on whether to spend two-thirds of its post-war net profits on a radical new jet-powered long-range transport demonstrator—the Model 367-80.

The company’s May 1952 decision to approve the project, dubbed the Dash 80, became a defining moment in aviation history. Targeting the airline and military air tanker markets, the turbojet-powered aircraft would evolve into the 707/KC-135, establishing the blueprint for virtually every modern swept-wing transport with podded engines developed to this day as well as laying the foundation for Boeing’s 700-series jet airliner dynasty.

Fast-forward to early 2022 and Boeing is once more preparing to propose a new, potentially game-changing configuration. Although the concept is not yet aimed specifically at a new product, the high-aspect-ratio transonic truss-braced wing (TTBW) is targeted instead at a NASA demonstrator X-plane intended to prove airframe technology for a future highly efficient single-aisle airliner by the mid-2030s.

• New NASA X-plane RFP due in 2022
• Boeing seeking industry input for sustainable demo bid
• TTBW to fly on modified McDonnell Douglas T-tail fuselage

While other advanced configurations, such as the blended wing body (BWB), are in the frame for the X-plane—also known as the sustainable flight demonstrator (SFD)—Boeing’s TTBW proposal is widely expected to be the front-runner. Not only has Boeing been working on the configuration with NASA for more than a decade, but it is also actively soliciting suppliers to bolster broader industry participation in its bid.

For Boeing, the timing of the competition for the X-plane has assumed even more importance than the research opportunity it presents. Hit by the 737 MAX groundings, the market downturn caused by the COVID-19 pandemic and delays to deliveries of the 787 and 777X, Boeing has had to put near-term plans for all-new commercial aircraft development projects on the back burner. The X-plane therefore offers a chance to continue with tests of key technologies that could play a role in future single-aisle programs semi-independently of the company’s market-driven product-development strategy.

But as Boeing deliberates over its next market moves, ranging from long-term 737 replacements to a potential twin-aisle midsize family, the competition is stirring. The SFD has become increasingly timely in the face of Airbus’ plans to develop an all-new sustainable 100-plus-seat airliner in the 2030s.

If all goes according to plan, the new X-plane—which likely will be the largest purpose-built experimental aircraft in the seven-decade-old X series—is expected to begin flight tests in late 2026 and will help mature design, structures and systems technology timed for full-scale development in the early 2030s. The X-plane also forms a key part of NASA’s Sustainable Flight National Partnership (SFNP) plan with industry, researchers and academia, which will support the push toward new airliner technology across a broader front that includes developments in smaller engine cores, electrified aircraft propulsion and high-rate composite aircraft manufacturing.

“In 2022, we expect to put out a request for proposal [RFP] for the design and build of the flight demonstrator, and this is really a big, big deal,” says Rich Wahls, strategic technical advisor for the Advanced Air Vehicles Program at NASA’s Aeronautics Research Mission Directorate. “When was the last time we did a transport-class [aircraft-level] architecture change?” he asks. “My mind goes back to when Boeing did the Dash 80 with swept wings and underslung jet engines. That’s what we’re looking at here—a large-scale honest-to-God, prove-it kind of aircraft that would demonstrate key aspects that you can only do in flight.”

In the run-up to the RFP, NASA awarded study contracts for demonstrator plans to five unidentified companies and is working in parallel with two others on risk-reduction studies—one for the BWB and another with Boeing on the TTBW. “We’ve had wind tunnel tests [of the TTBW] that have gone on since 2013, and there have been aeroelastic tests, high-speed performance tests and low-speed integration tests,” Wahls says. “Often you take these concepts, and as you dig down to the next layer of detail, the benefit goes away. So far, it’s not going away on the truss-braced wing or really on the blended wing. We haven’t found that thing that completely stops either yet.”

Clarifying NASA’s role in the further advancement of sustainable concepts, Wahls adds: “We’re not about product development and doing the next airplane. Industry has their next baseline airplanes on their drawing boards. We’re trying to identify those technologies that are just beyond their risk threshold, both financially and technically, then use those as demonstrations. If successful, we bring them forward into that next generation. If they had enough confidence to put them on the next airplane, then we would have to start looking beyond that. So we’re trying to accelerate insertion of advanced technology into these game-changing architectures across all these projects we’re doing.”

Following a planned first flight in late 2026, NASA says the SFD research campaign will last six months and be completed in 2027. Design, ground test and flight research data from the SFD will be used to measure the winning contractor’s “vision system” performance relative to a set of midterm performance objectives set out by NASA for future subsonic transport aircraft in the 2025-35 time frame.

These targets call for technology readiness levels of 5 to 6 (ready to transition to production development) for an aircraft capable of cumulative noise levels of 32-42 dB below Stage 4 and landing and takeoff nitrogen oxide (NOx) emissions 80% below the International Civil Aviation Organization’s CAEP/6 standards. The requirements also call for cruise NOx emissions to be 80% lower relative to a 2005 best-in-class benchmark, and aircraft fuel and energy consumption levels to be 50-60% lower relative to the same 2005 standard.

As its name suggests, the TTBW configuration is all about maximizing wing efficiency and at the same time opening the aperture for a wide variety of potential future propulsion options, ranging from advanced turbofans and open rotors to hybrid engines and even a tail-mounted boundary-layer-ingesting fan. First developed in 2010 under the Boeing and NASA Subsonic Ultra Green Aircraft Research program to study ultraefficient airliner concepts for the 2035 time frame, the TTBW has continued to evolve into a flexible and practical configuration.

Despite many tweaks over the past decade, the design continues to hinge on the benefits of a high-aspect-ratio wing to minimize drag. The increased span lowers lift-induced drag because the wing is slender, while its reduced thickness ratio decreases profile and transonic drag due to its thinness. The wing is braced by trusses to minimize the weight penalty of the longer span.

The X-plane was originally designed with an unswept wing to cruise at a fuel-saving speed of Mach 0.75, but Boeing is basing its X-plane proposal on a revised wing configuration revealed in early 2019. The newer design is optimized around a 20-deg. swept wing to enable a higher Mach 0.8 cruise speed more typical of current jet airliners. The increase in sweep angle necessitated a redesign of the truss, which has increased chord at the fuselage and forward sweep at the trailing edge and tapers toward the junction with the wing. A small jury strut that connects the truss to the wing has also been moved farther outboard and closer to its junction with the wing. The changes have allowed the truss to generate lift, further maximizing performance.

With an aspect ratio of 19.6, the 170-ft.-span wing of any production TTBW version also will incorporate a 777X-like wing-fold feature. The fold, which is positioned outboard of the truss attachment point, is designed to enable the TTBW to use smaller gates, like those used by the 118-ft.-span 737. For the SFD bid, Boeing expects to modify the fuselage of a donor MD-80 or 717, but it is unclear if these precise wingspan dimensions will be reproduced for the demonstrator, which is also unlikely to include the folding feature.

Boeing is, meanwhile, canvassing industry for potential risk-share involvement in the modification of the T-tail fuselage into the X-plane. The company declined to comment on details of the plan, saying it would be premature to discuss its proposal prior to NASA’s RFP. Boeing did add, however, that it “enthusiastically supports NASA’s vision for a public-private partnership to enhance aviation sustainability under the umbrella of the Sustainable Flight National Partnership, which focuses industry and government on the critical challenges for products being introduced in the 2030s.”

Details of the proposed modification plan seen by Aviation Week show that a significant number of changes and additional systems and structure will be required to transform a McDonnell Douglas-heritage fuselage into the basis for the new X-plane. The biggest of these will involve the design and build of a composite wing with full-span slats and single-slotted flaps. The wing, which will be joined at the centerline above the fuselage, also will incorporate low- and high-speed ailerons.

To meet the required design length of the demonstrator, Boeing plans to remove an unspecified number of fuselage frames as well. This suggests the preferred donor fuselage may be from an MD-80 rather than the shorter DC-9-30-series-size 717. The fuselage will be reinforced with internal bracing from the wing to the existing structure, too, and will utilize the in situ carry-through torque boxes for the nose and main landing gears. The existing gear will be supported by a new pylon and enclosed in a new fairing.

Other changes will include the relocation of the tail-mounted engines to an inboard underwing mounting, where they will be attached with a new pylon and enclosed in purpose-built nacelles and inlets. The engine’s existing thrust reversers will be locked out while the nacelle will feature a purpose-designed anti-ice system.

Several key system changes also will be required, including the development of a fly-by-wire flight control system for the wing-control surfaces. Flight control functions will be hosted in a triplex vehicle management system controlled from a two-crew flight deck that will be modified with an additional flight control computer interface. Among the system changes will be the rerouting of the engine bleed air ducts through the fuselage to the environmental control system packs and the addition of an extra central hydraulic system to augment the existing configuration.

Changes to the interior will include installation of a full flight-test instrumentation suite and accommodation for flight-test personnel as well as provision for a set of pallets for center of gravity ballast.