COMPOSITE MATERIALS

Carbon-fiber-reinforced polymer

Carbon-fiber composites have been used in aerospace applications since the 1960s.They can be likened to concrete, which is made by mixing cement, sand, and gravel with water. Alone, these ingredients have no particular strength, but when combined and bonded together by cement (the ‘matrix’) they are transformed into a material that is immensely strong. The idea is that in combining two or more materials it is possible to engineer a new one with distinct properties for specific uses.

Carbon-fiber-reinforced polymer (CFRP) composites have made lighter airframe components possible, contributing to at least a 14-15 per cent reduction in fuel consumption and carbon footprint. They can be formed into complex shapes, are structurally stronger and weigh much less than the same parts made from traditional aero metals, as well as lasting longer and being easier to repair when damaged.

Choosing the best material for a specific application has meant designers analyzing every area of the airframe to decide upon the best option, taking into account the operating environment and loads that a component experiences over the life of the structure.

As an example, aluminum – which has been the main aero metal for over 70 years – is susceptible to tension loads but deals with compression very well. On the other hand, composites are not as capable of dealing with compression loads but are exceptional at handling tension. Thus, the expanded use of composites, especially in the highly tension-loaded environment of the fuselage, considerably reduces maintenance due to fatigue when compared with an aluminum structure.

The Boeing 787 Dreamliner makes greater use of composite materials structure than any previous Boeing commercial aircraft. It is 50 per cent composite by weight, with the majority of the primary structure – most notably the fuselage – being made of CFRP and other composite materials. The result is an airframe offering weight savings of some 20 per cent when compared to more conventional aluminum designs, which translates into lower fuel consumption.

However, polymer composite technology does have its downside: it is normally made using petroleum products, while the manufacturing process can be time consuming and energy intensive. Landfilling has historically been the main disposal method for polymer composites, although Boeing has made significant progress with recycling over the last few years: “A method for vaporizing and dissolving resin developed by an industry partner company, UK-based ELG Carbon Fibre Ltd, has made possible a groundbreaking, 2018 Boeing partnership to recycle excess aerospace-grade composite materials. This is reducing solid waste sent to the landfill by more than 500 tons a year from 11 Boeing manufacturing sites,” commented Boeing.

Rotorcraft use of CFRP

Helicopter builders are in the vanguard of aerospace companies taking advantage of composites to give reduced airframe weight that will contribute towards lower fuel burn. The Airbus H160, unveiled in 2015, is the firstever, fully composite civil helicopter, which adds up to an airframe that is lighter in weight, more robust, resistant to corrosion and fatigue, while requiring less maintenance. Thanks to its Arrano engine by Safran Helicopter Engines, the H160 benefits from a 15-per cent reduction in fuel consumption. An additional boost to its environmental credentials is the use of its Blue Edge main rotor blades, which reduce exterior noise levels by 50 per cent (3dB).

The Sikorsky CH-53K was the first CH-53 model equipped with airframe side sponsons made from composite materials and a composite rotor blade system. In 2022, it was announced that the CH-53K was to receive a composite upgrade to its rear fuselage and tail rotor pylon assembly.

Leonardo’s single-engine Kopter SH09 boasts an all-composite airframe as well as all-composite main and tail rotor blades, and is expected to obtain European certification by the end of 2022.

Carbon ThreeSixty, Leonardo, and the National Composites Centre are collaborating to create an all-composite helicopter wheel using CFRP instead of aluminium alloy. The use of composites is expected to produce weight savings of about 1.5kg (3.3lb) with each 5kg (11lb) alloy wheel, equating to an overall weight saving of 6kg (13.2lb), which represents almost 4,546 litres (1,000 gallons) of fuel saved over a typical 12,000-hour service life, and around 12 tonnes of CO2.

Incremental design developments and improvements like the CFRP helicopter wheel and sponsons will pay dividends in sustainability by increasing efficiency and range, as well as delivering payload benefits. They will also provide an opportunity to explore the use of lower power density propulsion systems, such as electric batteries.

Bio-composites

Today, a new class of high-performance materials known as bio-composites is emerging, offering even more possibilities for improved environmental performance in future aerospace applications. Unlike CFRP, bio-composites are derived from natural renewable resources. They are lightweight, flexible, cost effective, and recyclable, which makes them attractive to a wide range of industrial uses.

Bio-sourced composite materials are obtained from biomass, plants, crops, micro-organisms, minerals, and bio-wastes, which are chemically or mechanically converted into bio-composites. The resulting bio-fiber is combined with a resin matrix (in the same way as CFRP composites), which then can be used alone or in complement to standard materials like carbon and/or glass fiber. In future aerospace products, bio-composites could potentially be used in primary and secondary airframe structures. 

THE NEXT GENERATION

Plane and rotorcraft builders are reimagining existing aircraft forms, creating new and innovative designs that take sustainability as their guide. European manufacturer Airbus is playing a leading role in some future rotorcraft concepts.

RACER program

Airbus Helicopters has been assisted in its Rapid and Cost-Effective Rotorcraft (RACER) demonstrator program by the EU-funded CA3TCH project. Using advanced simulation technology, it aims to predict the aerodynamic and aero acoustic behavior of essentially new rotorcraft configurations like the Airbus Helicopters RACER.

The futuristic-looking RACER will feature innovative environmental performance offering 15 per cent less fuel burn per nautical mile at 180kts (207mph) compared to a conventional helicopter at 130kts (150mph), as well as reduced sound pollution, a key factor being its airframe design, with its forward fuselage optimized for low drag. Its compound configuration departs from conventional helicopter design by featuring a main rotor, with wings and propellers for propulsion support instead of a conventional tail rotor.

RACER is expected to reach cruise speeds of more than 216kts (248mph). The cutting edge Safran Eco-Mode hybrid-electric system – which allows one of the two Aneto-1X engines to be switched to standby while in cruise flight – generates further fuel savings of up to 30 per cent. RACER specifically combines high speed and vertical take-off and landing (VTOL), allowing it to operate in remote areas, which could significantly extend the speed and range of HEMS and rescue services, improving chances of survival by reaching people within the first critical ‘Golden Hour’ of need.

It is hoped the RACER demonstrator proves that the compound rotorcraft configuration, which implements and combines cutting-edge technologies, will open up new mobility roles that neither conventional helicopters nor fixed-wing aircraft can presently cover in a sustainable way, for both operators and the industry.

eVTOL and Urban Air Transport

Most of the main helicopter builders are experimenting with electric/hybrid-electric propulsion and electric vertical take-off and landing (eVTOL) urban air-taxi concepts.

Balkiz Sarihan, Head of Urban Air Mobility (UAM) Strategy Execution and Partnerships at Airbus, said: “We’re testing many things like electrification and hybridization using our eVTOL platforms, which are contributing to net zero, and they have applications for our helicopters and larger commercial aircraft, too, so it’s a virtuous circle of technology investment.”

Engine builder Rolls-Royce is involved in developing a portfolio of all-electric and hybrid electric applications, in particular for the urban air mobility and regional air mobility markets. “Rolls-Royce powered electric flight is becoming a reality: e.g., the Tecnam/Wideroe regional plane will enter into service in the 2020s; Vertical Aerospace’s eVTOL is set for certification by 2024. We are continuously open to work with partners seeking to bring all-electric and hybrid-electric air vehicles to market,” a company spokesperson commented.

CityAirbus NextGen

As part of its drive to reduce emissions in aviation over its full product range, Airbus is developing an all-electric, four-seat eVTOL lift and cruise concept featuring a wing. Called CityAirbus NextGen, it boasts a 50-mile (80km) range and a cruise speed of 65kts (75mph), which makes it ideally suited to zero-emission flight operations for a variety of applications in major cities. Its eligibility for use in medical services roles is plain to see.

“Starting out in 2014 as a strategy exercise for Airbus, we began a dedicated program of research and innovation to understand more about eVTOL technology, and along the way we coined the term ‘urban air mobility’ as it hadn’t existed before,” explained Balkiz Sarihan.

“We created Airbus Urban Mobility because the concept and its technology were not directly related to our traditional aerospace or helicopter businesses. It’s a dedicated entity, because we wanted to build it in a different way, using a different application of aircraft architecture and technology. The latter is an area that’s evolving very fast – electrification, batteries, and hydrogen power. But we believe it’s not just about a technology push – we have to create the right service to society and do so in a meaningful way,” asserted Sarihan.

“It’ll be the end of 2023 when the prototype CityAirbus NextGen flies. We’ve already completed two eVTOL technology demonstrator programs – CityAirbus and Vahana – in which we really tested the technical capabilities of both vehicles against known future uses, establishing the relative balance between hover and range capabilities, with full eVTOL being an absolute requirement. Now we really believe the technology has matured, but we are still building the market and service around it,” she said.

And so, CityAirbus NextGen was born, and the lessons learned from the Airbus double-demonstrator approach have been essential to the creation of this new generation of eVTOL vehicle.

Bell Nexus 4EX

Bell’s Nexus 4EX e-VTOL four-duct tilt rotor (with echoes of the company’s V-22 Osprey VTOL tiltrotor) has been conceived as a solution to delivery and commuter challenges, carrying goods, people, and data to serve business or city-wide demands. “The Bell Nexus 4EX can operate as an electric-only vehicle or a hybrid-electric configured vehicle. With a hybrid platform, the Nexus 4EX promises an extended reach to travel farther or to more remote locations, based on your mobility needs,” said the manufacturer.

French engine maker Safran, which is building the hybrid-electric engine for the Nexus eVTOL vehicle, is also exploring uses for its electric and hybrid-electric engines on traditional helicopters.

Vertical VX4

The UK-based aerospace, and technology company Vertical Aerospace has partnered with Leonardo Helicopters for the design, testing, and manufacture of Vertical’s VX4 eVTOL vehicle. With capacity for a pilot and four passengers, the four tilt-rotor VX4 is projected to be capable of speeds up to 174kts (200mph), with a range of over 100 miles and zero operating emissions. Vertical is already considering medevac and HEMS roles for the VX4. The company has what it believes is the largest conditional pre-order book (by value) in the eVTOL industry, with up to 1,350 aircraft worth US$5.4 billion from the likes of American Airlines, Avolon and Bristow.

SMARTER OPERATING

The ‘perfect flight’

While the aerospace industry works hard to develop new engine technologies, sustainable aviation fuels, and design more energy-efficient aircraft, some plane makers and operators have been working on ways to fly more efficiently.

Express air freight carrier DHL has been identifying ways to reduce its carbon footprint by ‘smarter’ operating. By utilizing the latest technology, optimizing its route network, and choosing efficient carriers, DHL has found ways to save jet fuel and reduce its carbon footprint.

In July 2020, DHL partnered with its operating airline, European Air Transport GmbH, to plan the ‘perfect flight’. This ground-breaking demonstration flight was made from Leipzig, Germany, to New York-John F. Kennedy (JFK) International Airport, to test the effects of operating under optimal flight conditions. With air traffic considerably reduced due to Covid-19, the carrier planned and scheduled the most eco-friendly North Atlantic crossing possible, with all air traffic control sectors along the route supporting their efforts. DHL described how they achieved the ‘perfect flight’:

“To prepare for the flight, we identified more than 50 steps to minimize fuel consumption. These included washing the engine pre-flight to improve aerodynamics, optimizing the route using a state-of-the-art flight planning system, calculating the optimal take-off trajectory (unrestricted climb to cruising altitude), and determining the best possible descent procedure to JFK (constant angle with minimum engine power. A total of 13 agencies, such as the German Air Traffic Control, National Air Traffic Services UK (NATS), and the US Federal Aviation Administration, took part in the flight planning and operation.

“The results speak for themselves. Loaded with its standard payload (approx. 60 tons of freight), the Airbus A330-200F consumed 1,000kg less fuel, which reduced the flight’s CO2 emissions by 3,150kg. NATS reported that the flight through UK airspace scored 0.013 on its 3Di index, which they use to calculate each flight’s environmental efficiency. With zero being perfect, that’s about as close as you can get.”

Energy efficient flights

Launched in February 2021, Airbus is leading ALBATROSS – a large-scale initiative of major European aviation stakeholder groups to demonstrate the feasibility of implementing energy efficient flights in the short term. These are being made through a series of gate-to-gate live demonstration flights across Europe, combining several R&D technical and operational innovations.

Airbus commented: “ALBATROSS follows a holistic approach by covering all flight phases, directly involving all relevant stakeholder groups (such as airlines, Air Navigation Service Provider, network managers, airports, and industry) and addressing both operational and technological aspects of aviation and Air Traffic Management (ATM). Many solutions will be put into practice during the flight demonstrations, from new precision approach procedures to continuous climb and descent, a more dynamic management of necessary airspace constraints, sustainable taxiing, and sustainable aviation fuel (SAF) usage.

“Thanks to the transmission of four-dimensional trajectory data, ATM will be able to optimize and better predict an aircraft’s trajectory, thereby enabling it to immediately and concretely reduce a flight’s environmental footprint.”

These live trials began in September 2021 and will ultimately involve around 1,000 demonstration flights, showcasing real-life operating solutions and possibilities for making the ‘perfect flight’ with the potential to save fuel and reduce CO2 emissions. The first results are expected during 2022.

Smarter helicopter ops

Helicopter operations contribute a small percentage to greenhouse gas emissions – actually, less than one per cent of the aviation industry’s CO2 emissions – but they can be a significant source of local air pollution around airports and in urban areas. Research by the UK’s Cranfield University School of Engineering in 2011 into the environmental impact of the operation of conventional helicopters at mission level concluded that smarter use of rotorcraft by focusing on mission profile management can have positive effects on fuel burn and reduced emissions.

The future could be green

Developments in the fields of electric propulsion, small core gas turbines, smart design and composite materials have created a virtuous circle, which link to efficiency challenges related to aerodynamics, propulsion, and weight. Together and individually, they can improve overall performance and reduce fuel burn in fixed- and rotary-wing aircraft, leading to reduced carbon emissions, which in turn reduces running costs for the operators. Thus, if new aircraft that incorporate these features appeal to their users, then manufacturers will want to build them, improving balance sheets all round. This is good for the climate, good for sustainability and good for business – a win-win.