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A Transonic Truss Braced Wing aircraft concept being tested in a virtual wind tunnel, showing how its wings interact with the air around them. (Image: NASA)

NASA recently issued an award to The Boeing Company for the agency’s Sustainable Flight Demonstrator project, which seeks to inform a potential new generation of green single-aisle airliners. Under a Funded Space Act Agreement, Boeing will work with NASA to build, test, and fly a full-scale demonstrator aircraft and validate technologies aimed at lowering emissions.

Artist concept of commercial aircraft families with a Transonic Truss-Braced Wing configuration from the Sustainable Flight Demonstrator project. (Image: Boeing)

Over seven years, NASA will invest $425 million, while the company and its partners will contribute the remainder of the agreement funding, estimated at about $725 million. As part of the agreement, the agency also will contribute technical expertise and facilities.

Single-aisle aircraft are the workhorse of many airline fleets, and due to their heavy usage, account for nearly half of worldwide aviation emissions. NASA plans to complete testing for the project by the late 2020s, so that technologies and designs demonstrated by the project can inform industry decisions about the next generation of single-aisle aircraft that could enter into service in the 2030s.

Through the Sustainable Flight Demonstrator project, Boeing and its industry team will partner with NASA to develop and flight-test a full-scale Transonic Truss-Braced Wing (TTBW) demonstrator aircraft.

The TTBW concept involves an aircraft with extra-long, thin wings stabilized by diagonal struts. This design results in an aircraft that is much more fuel efficient than a traditional airliner due to a shape that would create less drag — resulting in its burning less fuel.

The unconventional configuration of the TTBW, which includes a high aspect ratio wing, in addition to wing and jury struts, leads to complex flow phenomena such as transonic buffet, separated flow, and a turbulent wake.

Current industry best practices tend to employ Reynolds-Averaged Navier-Stokes (RANS)-based computational fluid dynamics (CFD) analysis for buffet onset prediction, but accurate prediction of the onset of buffet and the development of the separated flow may require more accurate scale-resolving CFD simulations. To provide more insight into the best practices for using scale resolving simulations to predict transonic buffet onset, among other challenges, NASA’s Advanced Air Transport Technology Project launched a collaborative multi-center effort to develop new methods for simulating the TTBW to better predict its performance and that of similar truss-braced wing configurations.

Researchers in the NASA Advanced Supercomputing (NAS) Division’s Computational Aerosciences Branch are simulating the TTBW’s Mach 0.8 cruise configuration to validate and develop their scale-resolving CFD models. A wind tunnel experiment was conducted on a half-span model of this configuration in the NASA Ames Unitary Plan 11- by 11-foot Transonic Wind Tunnel in January 2022, and large angle-of-attack sweeps at different Mach numbers were run. This data is being used to validate the simulations, with a goal to be able to accurately reproduce experimental results using CFD.

The NAS Division’s Launch, Ascent, and Vehicle Aerodynamics (LAVA) team initially chose Hybrid RANS/Large Eddy Simulations (HRLES) for the scale-resolving simulation approach. As the name suggests, this hybrid method models turbulence inside the boundary layer to solve the RANS equations, while the turbulence is resolved outside the boundary layer with LES. The dominant transonic buffet phenomena can largely be captured by steady-state RANS or unsteady RANS. However, neither is suitable for investigating the phenomena in the turbulent wake (including deep stall and high-lift configurations) due to the excessive dissipation of turbulent motion, and the team has demonstrated that RANS alone is an unreliable tool when simulating the maximum value of lift (CLmax) and the onset of stall. Scale-resolving simulation approaches like HRLES are better able to resolve the turbulent content and enable more accurate predictions.

The LAVA team showed that utilizing RANS-type grids for HRLES simulations would likely produce a result that is inferior to that of a RANS simulation, so they purpose-built HRLES grids with low aspect ratio grid cells appropriate for LES outside the boundary layer. The aerodynamic loads (lift, drag, and pitching moment) and surface pressure data from the HRLES simulations were then compared with steady and unsteady RANS simulations and the experimental wind tunnel data.

The team discovered that at higher angles-of-attack, the RANS simulations tended to overpredict the location of the shock in the chordwise direction in both the midboard and outboard regions of the wing when compared to the experiment, while the HRLES simulations showed much better agreement with the experiment in the predicted shock location. These initial simulations provided important insight into the behavior of transonic buffet — in particular, the spanwise (root-to-tip) development of the unsteady shock motion—and indicated where the computational grids need to be refined further.

The high-performance computing resources at the NAS facility have made it possible to run the HRLES simulations within a short turnaround time. Scale-resolving simulations are computationally expensive due to the small timestep size and grid cell sizes needed to resolve the unsteady turbulent flow, and HRLES approaches have a lot of tuning parameters. Being able to run a parameter study to test their sensitivity using the efficient throughput provided by NAS systems enabled quick decision-making and accelerated the development of simulation best practices.

The LAVA team will move to more complex TTBW configurations, including studying deep stall, and high lift configurations where devices such as slats and flaps on the aircraft are deployed. Due to the T-tail configuration of the empennage proposed for the TTBW, it may be prone to deep stall — where the turbulent wake from the stalled main wing and strut blankets the tailplane and renders the elevators ineffective and prevents the aircraft from recovering. Accurately capturing the turbulent wake coming from the main wing and strut of the TTBW and maintaining the turbulent fluctuations until reaching the tail will be paramount in accurately predicting this behavior.

“NASA is working toward an ambitious goal of developing game-changing technologies to reduce aviation energy use and emissions over the coming decades toward an aviation community goal of net-zero carbon emissions by 2050,” said Bob Pearce, NASA’s Associate Administrator for the Aeronautics Research Mission Directorate. “The Transonic Truss-Braced Wing is the kind of transformative concept and investment we will need to meet those challenges and, critically, the technologies demonstrated in this project have a clear and viable path to informing the next generation of single-aisle aircraft, benefiting everyone that uses the air transportation system.”

NASA’s goal is that the technology flown on the demonstrator aircraft, when combined with other advancements in propulsion systems, materials, and systems architecture, would result in fuel consumption and emissions reductions of up to 30 percent relative to today’s most efficient single-aisle aircraft, depending on the mission.

For more information about NASA’s Sustainable Aviation efforts, visit here .