Austin Weber // Senior Editor // webera@bnpmedia.com

The basic design of commercial airplanes hasn’t changed much in decades. Companies such as Airbus and Boeing have traditionally relied on a wing-and-tube configuration, where wings are attached underneath the fuselage. But, engineers are now studying the pros and cons of an old idea where the wing is located above the fuselage.

Extra-long, thin wings stabilized by diagonal struts could make future commercial airliners lighter than traditional designs. When combined with expected advancements in propulsion systems, materials and systems architecture, a single-aisle airplane equipped with a transonic truss-braced wing (TTBW) could reduce fuel consumption and emissions by up to 30 percent.

The concept has piqued the interest of aerospace engineers in Europe and the United States looking for new ways to improve the efficiency and sustainability of midsized aircraft, the workhorse of airlines worldwide.

Boeing and NASA are developing a TTBW design that features two jet engines and a high “T” tail. Meanwhile, a team at ONERA, the French national aerospace research center, is working on a plane that uses a pair of open-rotor engines fueled by hydrogen.

Diagonal struts used in both designs support the long wings and help add lift. The unique shape of the wings creates less drag in the air. That would enable a plane to burn less fuel than an aircraft equipped with a traditional wing design.

According to Greg Hyslop, chief engineer and executive vice president of engineering, test and technology at Boeing, “ultrathin wings braced by struts with larger spans and higher-aspect ratios could eventually accommodate advanced propulsion systems that are limited by a lack of underwing space in today’s low-wing airplane configurations.”

The idea of using high wings equipped with struts is not new. In the early 1950s, a French manufacturer called Hurel-Dubois produced the HD.31 airplane. It featured a high-aspect ratio design with a 150-foot wing that was more than twice the length of its fuselage.

In the U.S., Werner Pfenninger, Ph.D., a leading expert on laminar flow control technology, proposed a TTBW concept in the mid-1950s. In 1979, he also sketched a design for a jumbo-sized transport aircraft equipped with four jet engines and a 420-foot truss-supported wing.

And, in the mid-1990s, engineers at Virginia Tech University studied a long-haul jet similar to a Boeing 777 that featured truss-supported wings and tip-mounted engines. The objective of the project was to minimize the take-off gross weight of jetliners.

At the recent Paris Air Show, ONERA displayed a model of its Gullhyver concept. The 200-seat aircraft features a double-bubble cross section and an upward-sloped nose to generate lift.

The thin wing has an aspect ratio of 20, which is twice as much as current short- and medium-range commercial aircraft. The configuration is being evaluated as part of the EU’s U-Harward project, which is studying the pros and cons of truss-braced wings.

Unlike their European counterparts, who are proposing to build an entirely new aircraft, American aerospace engineers plan to modify an existing plane by adding a new center fuselage and wings. They’ve been studying the pros and cons of the concept for more than a decade.

Despite numerous advantages, TTBW airframes pose challenges. For instance, the unconventional configuration leads to complex airflow phenomena, such as transonic buffet, separated flow and a turbulent wake.

Extensive wind tunnel tests have examined these issues. During recent load testing at NASA’s Armstrong Flight Research Center, engineers observed the interaction of a model strut and wing, as well as the forces affecting each.

They previously had no calculations to estimate how forces transferred from the main wing to the strut. Information gathered from using a six-foot model in the wind tunnel will enable engineers to calculate what will happen when they build a larger wing.

“We identified early that we needed to learn more about how these structures with the strut respond to load and to see what additional information we may need for a calibration of a bigger structure,” says Frank Pena, mock wing test director at the NASA Armstrong Flight Loads Laboratory. “We decided to use the load cell between the strut and the main wing to help us track down some of this missing information that otherwise could not be obtained.”

Pena and his colleagues plan to use the six-foot model data to guide the design of a 10-foot version, in coordination with NASA’s Langley Research Center, which has also been studying the TTBW concept. The larger wing will have a swept-back angle closer to the design developed at Langley.

“It differs from the smaller wing version, which focused on testing instrumentation and methods,” explains Pena. “The larger wing will also have more representative connections between the fuselage and the strut and wing.”

Engineers at NASA’s Ames Research Center recently conducted additional wind tunnel tests using an 11-foot-long half-model equipped with hundreds of sensors. They’re using the data to find the upper limits that the aircraft can perform at—in terms of factors such as speed and altitude—before the vibrations caused by buffet become excessive.

The engineers are using new methods for simulating buffet and air flow to better predict the performance of truss-braced wings. Eventually, they’ll move to more complex TTBW configurations, studying deep stall and high-lift configurations where devices such as slats and flaps 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, preventing 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 critical to accurately predicting this behavior.

The next step in the multiyear development process will be to construct a full-scale prototype aircraft. Technologies tested as part of the sustainable flight demonstrator (SFD) program will inform future designs and could lead to new breakthroughs in aerodynamics and fuel efficiency gains.

“The SFD program has the potential to make a major contribution toward a sustainable future,” claims Hyslop. “It represents an opportunity to design, build and fly a full-scale experimental plane, while solving novel technical problems.

For the demonstrator vehicle, Hyslop and his colleagues plan to use elements from existing aircraft and integrate them with all-new components. They recently took delivery of a former Delta Air Lines MD-90. It will be modified with a 145-foot truss-braced wing, which will be 50 percent longer than the cantilever wing used on traditional single-aisle airplanes such as the Boeing 737. Test flights will be conducted at the Armstrong Flight Research Center later this decade.

Boeing envisions a future family of TTBW aircraft that would enter service in the 2030s. The VS-1 would seat 130 to 160 passengers, while the larger VS-2 would feature 180 to 210 seats. The latter model would sport a bigger wing and engines. Folding wing tips would help the aircraft adapt to existing airport infrastructure, especially facilities that have tight space.

“How this type of aircraft could fit within existing airports would be a big challenge,” says George “Nick” Bullen, an internationally recognized expert on aircraft manufacturing. “A lot of airports have space constraints and collisions with ground support equipment are already a problem. The struts would add an additional element of danger.”

Extra infrastructure burden on airports has doomed some aircraft in the past, such as the Airbus A380. While Bullen says the TTBW concept is interesting, he believes that it may never become commercially viable, just like other jaw-dropping designs from the past, such as Boeing’s Sonic Cruiser prototype from two decades ago.

“The use of struts will stabilize the wings and provide some extra lift, which allows for better efficiency,” explains Bullen. “You don’t have as much drag, because you don’t have as much surface area for the wing to run across.

“However, there are many challenges associated with using a radical concept and a new type of wing structure,” warns Bullen. “Elasticity and flutter can be liabilities on long, thin wings. And, engineers will have to figure out new ways to store fuel, mount engines to wings and mount landing gear to struts.

“To save weight, I assume that the wing would be made out of composite materials,” notes Bullen. “The length and thinness of the wing could present manufacturing issues. There also could be challenges attaching the wing and truss to the fuselage.”

“A company with Ford’s scale can really influence the supply chain and business practices across our entire industry,” adds Sue Slaughter, purchasing director at Ford Motor Co. “It is so important that we not only think about how [we] can use our purchasing power to fuel our business needs, but also to advance sustainability.”

Because the automotive supply chain is extremely complex, the Guiding Principles contain expectations about business ethics, working conditions, human rights, health and safety, environmental leadership and supply chain due diligence for suppliers at all tiers. All suppliers are expected to uphold these standards and enforce them throughout their supply chain.

The Guiding Principles are based on fundamental elements of social, environmental and governance responsibility that are consistent with applicable laws and international standards created by organizations such as the United Nations.

Topics covered under the revised guidelines include the following:

        Business ethics, including counterfeit parts and data protection.

        Environmental issues, such as air quality, carbon neutrality, chemical management, circularity and water management.

        Health and safety issues, such as personal protective equipment and workspace.

        Human rights and working conditions, such as benefits, wages and working hours.

        Responsible supply chain management, such as ethical sourcing of raw materials.

The BMW Group has implemented several projects in its packaging logistics unit to help the environment and conserve resources. The goal of the initiative is to work closely with suppliers to reduce carbon emissions and adhere to the principles of a circular economy.

BMW’s European assembly plants are using more recycled material in their packaging. For newly awarded contracts, the proportion of recycled material in reusable packaging for logistics purposes will almost double this year from around 20 percent to over 35 percent.

Using alternative sustainable materials, reducing single-use packaging, introducing lightweight packaging in certain areas and reducing transport volumes will also help cut carbon emissions.

BMW is monitoring the impact of individual measures via a CO₂ calculator for packaging. The automaker’s overall aim is to reduce CO₂ emissions in the supply chain by 20 percent per vehicle compared to 2019.

“Our re:think, re:duce, re:use, re:cycle approach is being implemented consistently in packaging logistics,” says Michael Nikolaides, head of production network and logistics at BMW Group. “We’re using innovative strategies to consistently reduce the volume of resources we use, thus reducing our carbon footprint.

“We are also doing our part to get the BMW iFACTORY up and running, with a particular focus on the ‘green’ side of things…with an emphasis on flexibility and efficiency, sustainability and digitalization,” explains Nikolaides. “It provides an answer to the challenges involved in the transformation to e-mobility and [leverages] the latest technologies to create a production process that uses minimal resources.”

According to Nikolaides, BMW is using more recycled material, such as expanded polypropylene (EPP) packaging. “Our newly developed EPP packaging already contains 25 percent recycled material,” he points out. “EPP is used in special containers, as its shape can be adapted to the components being packaged, allowing them to be transported safely.

“Around 360,000 of these containers are needed each year,” claims Nikolaides. “Using 25 percent recycled material allows us to save almost 280 tons of CO₂ annually. There are plans to increase this proportion of recycled material even further, with the first pilot schemes with 100 percent recycled material currently underway. If these tests are successful, this configuration will become standard for new contracts from 2024.

“An additional 680 tons of carbon emissions savings can be made every year by using covers and so-called small load carriers with 50 percent recycled contents,” says Nikolaides. “As things stand, these measures are focused within the European markets due to the current waste management situation and available recycling infrastructure. But, we are working toward expanding to our locations in China, Mexico and the United States.”

BMW also plans to use folding large load carriers in place of traditional pallet cages made of steel. The plastic alternatives will be made from over 90 percent recycled material. They work in a similar way to the collapsible shopping crates that most people are familiar with.

When they’re empty, the carriers can be folded up, making them easier to transport. Nikolaides claims that using 15,000 of these new containers will reduce CO₂ by around 3,000 tons per year.

“When it comes to packaging, the sky’s the limit,” says Nikolaides. “We’re launching pilot projects using bio-based materials to replace oil-based substances such as polyethylene and polypropylene.

“We are also investigating whether and in what ways we can use materials from recycled household appliances in our packaging,” explains Nikolaides. “In the long term, our aim is to use alternatives to raw materials across the board.”