Emerging Configurations for Sustainable Aviation

Climate change has become a concern for society now. Aviation contributes two per cent of human-made CO₂ emissions and has challenged itself to reduce net emissions even while demand for air travel and transport has grown significantly. The Air Transport Action Group has pushed the aviation industry to become the world’s first industrial sector to set an ambitious target: reduce CO₂ emissions to half of year 2005 levels by 2050 and to limit the growth of net CO₂ emissions by 2020.

There are three major technological elements to sustainable aviation are:

• Continuing to develop aircraft and engine design and technology for improvements in fuel efficiency and reduced CO₂ emissions.
• Supporting the commercialisation of sustainable, alternate aviation fuels.
• Developing radically new aircraft and propulsion technology and accelerating technologies that will enable the ‘third generation' of aviation.

Other factors, such as efficient air traffic management and aircraft routing that minimises fuel consumption also have a vital part to play. Additionally, the industry has made some significant progress on reducing noise and other environmental impacts.

AIRCRAFT AND ENGINE DESIGN AND TECHNOLOGY

For the last 40 years, aircraft and engine technology has reduced CO₂ emissions by one per cent per passenger mile. This has been the result of significant investments in R&D in materials, aerodynamic efficiency, digital design and manufacturing methods, turbo-machinery developments and aircraft systems optimisation.

SUSTAINABLE AVIATION FUELS

Aviation will continue to rely on liquid fuels as the fundamental energy source for larger and longer-range aircraft for the foreseeable future. Even under the most optimistic forecast for electric-powered flight, regional and single-aisle commercial airplanes will continue operating with jet fuel for decades to come. Therefore, the development of Sustainable Aviation Fuels that use recycled rather than fossil-based carbon and meet strong, credible sustainability standards is an essential component of a sustainable future. What is needed is government support around the world for technology development, production facility investment, and fuel production incentives. The global aviation industry is closely working with fuel producers, operators, airports, environmental organisations and government agencies to bring these fuels into widespread aviation use well ahead of 2050.

THE THIRD GENERATION OF AVIATION

Aviation’s third generation leap is enabled by advances in new architectures, advanced engine thermodynamic efficiencies, electric and hybrid-electric propulsion, digitisation, artificial intelligence, materials and manufacturing. Larger aircraft will begin to benefit from novel designs that will further improve efficiency through management of aircraft drag and distributing propulsion in new ways. New materials will enable lighter aircraft, further improving efficiency. This third generation in aviation promises a positive impact on lives around the globe.

Aircraft entering service in the next few years will have the same overall configuration as their predecessors. However, they will be equipped with retrofits, serial upgrades and newly designed components and systems, which allow them to have a higher fuel efficiency performance. Another group of new fuelefficient technologies are under development and planned to be used in new aircraft types in the near future and do not require radically new aircraft configurations.

INDIVIDUAL TECHNOLOGIES

The main contributions to fuel efficiency in evolutionary aircraft design are in the areas of aerodynamics, new engine architecture and systems.

AERODYNAMICS

Aerodynamic technology has been progressing continuously throughout the past decades to produce new designs with significantly reduced drag. An aerodynamic technology that has been pursued over many years and has recently made new progress in development is Laminar Flow Control. This technology allows considerable drag reduction by preventing turbulences in the airflow over the surface of the aircraft.

Natural Laminar Flow (NLF) Control achieves laminar flow only by designing the surfaces of the wings and other aircraft parts with a suitable shape. This technology is being studied in the project Breakthrough Laminar Aircraft Demonstrator in Europe. Since September 2017, flight tests have been conducted with an A340 test aircraft on which the outer wing sections, of about ten metres width, were replaced by laminar profiles. The size of the laminar sections is representative of the wing dimensions of typical narrow-body aircraft that are likely to apply the laminar flow technology first. From the test flight results, the fuel saving potential of NLF for an 800-nautical mile flight would be around 4.6 per cent.

Another way to create laminar flow conditions is Hybrid Laminar Flow Control (HLFC), which uses boundary-layer suction to maintain laminar flow over the aircraft surface. This technology is particularly suited for swept wings and fins. NASA, in the framework of its Environmentally Responsible Aviation (ERA) research programme, carried out a series of test flights on a Boeing 757 equipped with a HLFC system to evaluate the dependence of laminar conditions on factors such as surface manufacturing, suction devices and surface coatings to prevent contamination.

NEW ENGINE ARCHITECTURE

The most significant contribution to fuel burn reduction comes from new engine technologies. These engines have higher By-Pass-Ratios (BPRs) than previous engine models. In the case of regional jets and singleaisle aircraft such as the MRJ, the Embraer E2 family, A220 (formerly C series), A320neo and 737 MAX, new engines operating on these aircraft have a BPR of 9 to 12, which allow fuel burn reduction of about 15 per cent compared to earlier engines with a BPR of typically five to six. New engines for wide-body aircraft including A330neo and Boeing 777-9 reduce fuel burn by ten per cent compared to previous engines. The GE9X engine, which powers Boeing’s new 777X aircraft, is considered to improve fuel efficiency by ten per cent relative to the GE90-115B engine.

The aviation industry is committed to driving the sustainability of aviation to make the world a brighter and safer place

Rolls-Royce is working on two new efficient designs planned for launch in 2020 and 2025, respectively: The Advance and the UltraFan engines. The Advance engine presents a three-shaft architecture with a new high-pressure core. The UltraFan is a step further using the Advance core but with a two-shaft configuration coupled to a geared turbofan. The Advance Engine is expected to have at least 20 per cent reduction in fuel burn and CO₂ emissions relative to the Trent 800.

Safran is working on Ultra-High-Bypass Ratio (UHBR) turbofan engines with a bypass ratio of at least 15. The design makes extensive use of lightweight composite materials. The UHBR is expected to have five to ten per cent fuel efficiency relative to the LEAP engine used as for example, in the A320neo family, or 20 to 25 per cent to conventional engines in the narrow-body category. In the US, new engine designs are being produced under the CLEEN II national research programme. Currently, the open-rotor design, boundarylayer ingestion and electric aircraft are the most prominent innovations when it comes to aircraft propulsion technologies.

AIRCRAFT SYSTEMS

Advances in aircraft systems can reduce fuel consumption considerably in current aircraft configurations. Aircraft taxiing offers high fuel saving potential through new aircraft systems. Safran has developed an electric taxiing system called - Electric Green Taxiing System (EGTS) consisting of electric motors mounted in the main landing gear wheels, which allow taxiing without using the main engines or a towing vehicle. An Auto Transformer Rectifier Unit is envisaged as a low-weight power supply for the EGTS which converts 115V AC produced by the aircraft’s Auxiliary Power Unit (APU) to 540V DC required by the EGTS motors.

Fuel cells produce electric energy from oxygen in ambient air and gaseous hydrogen. These are environmentally friendly as they generate neither noise nor pollutant emissions; only clean water. Fuel cells could replace the APU and power various onboard systems such as engine start-up, ventilation, flight controls, lighting, ovens and in-flight entertainment. These can robustly operate over the duration of a long-haul flight in all conditions. Safran is currently working on the development of this technology for commercial flight applications. Depending on operational conditions, fuel savings are predicted to be between one and five per cent. However, fuel cells require a regular supply of hydrogen as fuel. Therefore, the implementation of fuel cells for onboard power supply in future aircraft will be possible only once a worldwide hydrogen supply structure is being built up as a clean energy source for the industry. With the current progress in renewable energy, this may well happen within the next decade.

NOVEL AIRCRAFT CONCEPTS

The major trends in the development of future more efficient aircraft are in novel aircraft configurations as well as revolutionary propulsion technologies, materials and structures. While all current commercial aircraft have a conventional tube-and-wing configuration, novel configurations with higher fuel efficiency are also considered for future airframes. Design concepts currently seen as most promising by research establishments include strut-braced wing, blended wing body, double-bubble and box-/joined-wing aircraft. All these designs are significantly more environmentally friendly than conventional aircraft. Traditional aircraft manufacturers such as Airbus and Boeing, as well as specialised start-up companies such as Zunum Aero and DZYNE, research institutions and academia are working on a variety of novel concepts.

In October 2020, Airbus announced that it had completed Phase 2 of its Albatross ONE programme with revolutionary wing tips. The concept behind its implementation on the large-scale model of an Airbus airliner is simple. The tips, instead of falling to the hazards of turbulence and wind shear, adjust in real time to the air like the Albatross, preventing the tips from approaching stall thereby stabilising flight in the entire flight envelope from high-speed cruise down to approach. The benefits are improved fuel economy, fewer and less extreme bumps for the passengers and improved safety margins.

REVOLUTIONARY STRUCTURE AND MATERIALS

While a considerable amount of weight reduction has been achieved through the use of composite materials and light metal alloys, researchers are looking into revolutionary materials that would offer even better weight efficiency and increased aircraft performance. In its Span-wise Adaptive Wing project, NASA is currently investigating aircraft wings that can adapt to each flight phase by modifying the shape of different parts of the wings, with the aim of reducing weight and drag, thus improving fuel efficiency. A revolutionary material is the Shape Memory Alloy, a nickel-titanium alloy that can be “trained” to return to its initial shape after a deformation when heated.

Another promising technology is the morphing wing that is under study by NASA and MIT. This new wing architecture could greatly simplify the manufacturing process and reduce fuel consumption by two to eight per cent by improving the wing aerodynamics, as well as its manoeuvering capabilities. The morphing mechanism would comprise the whole wing, which would be covered by a skin made of overlapping pieces reminiscent of scales or feathers. The ability to de-form a wing shape to do pure lift and roll would increase flight efficiency and in turn reduce fuel burn. In addition, wind-tunnel tests of this structure showed that it matches the aerodynamic properties of a conventional wing at about one-tenth of its weight.

CALL TO MAKE THIS FUTURE TOGETHER

The future of aviation is bright. There must be public and private commitment to establish a sound regulatory foundation to address the widespread SAFs commercialisation as a low hanging fruit. The aviation industry is committed to driving the sustainability of aviation to make the world a brighter and safer place. Yet, in addition to the significant efforts by the industry it will depend on the coordinated support from policymakers, regulators and governments working together to make aviation sustainable and play an even bigger role in global community.