Making aircraft more energy-efficient
_{Caveat: incremental but important}

Change is necessary, and change is coming

As aircraft become more fuel-efficient, their fuel consumption per passenger mile continues to fall, but, as the total flight volume increases, total greenhouse gas (GHG) emissions continue to rise. This has been described as the ‘rebound effect’.

Broadly speaking, there has been continuous iterative progress over the last few decades on the aircraft configuration layout. Increasingly efficient, the design adopted on the majority of aircraft in commercial aviation has been optimised for both safety and performance. It sets a clear separation between the major functions of the aircraft and its major physical components, lift and fuel storage (wings), propulsion (engine), passengers or freight carrying (fuselage) and control (empennage).

The stability of this configuration over recent decades, the countless hours of flight test data and the longstanding collaborative culture among the aviation community have allowed us to reach an unprecedented and outstanding level of safety. The success of civil aviation can be seen, partly, in the confidence of airline passengers in the aircraft that transport them, including a familiarity with its general configuration layout. Might this confidence be affected by significant airframe redesigns? Possibly. But change is coming anyway.

For example, new energy sources are very likely to induce important changes in airframe architectures, at least for hydrogen-powered craft. SAF will not necessitate a redesign, which partly contributes to its use as a near-term solution. But even where there is no critical need for redesign, there are still plenty of improvements to be made to the airframe itself.

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Energy efficiency: alternative approaches

There are a number of possibilities to improve current aircraft to squeeze out those extra percentages of efficiency. We will look at some here.

Improved aerodynamics and ‘smart wings’

Currently, the adaptation of aircraft’s wing shape to its external conditions and flight phase is very limited. It relies on the slow and discontinuous motions of parts (slats and flaps), which makes it very far from the ideal motion of a bird, which can morph its wing shape and orientation in a highly efficient way.

An aircraft wing that could respond to airflow in a fashion similar to those seen in nature would be significantly more effective, but would require a fundamental redesign. This is the premise of a ‘smart wing’ – which could combine smart sensors all over the outside of the aircraft (to detect local airflow changes) and smart actuators that can react very quickly to these changes, driven by a control system capable of rapidly determining the optimum reaction in a very short timescale.

The collected sensor data could also feed back into the digital model of the airframe, and be used to improve it. Rethinking the traditional wing model in the manner described above increases opportunities to re-engineer the aircraft further.

Partial/full electrification

As discussed in the previous article, replacing hydraulics with electric control surfaces is a power-source agnostic way to reduce aircraft weight and increase efficiency. Electrification is also not a new idea, it began with the introduction of electrical flight control systems decades ago. Today, the trend is towards a growing number of electrical components – taking over more and more key flight functions. The refinement of battery technology is a major driver here, but we still have a long way to go.

New and improved materials

Weight is a massive driver of efficiency in aerospace. As such, any reduction in weight has huge value.Structural improvements include lightweight materials, like carbon fiber composites, that can be used, not only in the airframe, but eventually on aircraft’s subsystems. Other promising material groups include bio-composites, which are derived from biological and mineral-based sources, and graphene, which was only discovered in 2004.

The challenge here is scaling promising new materials to the point where they are cheap and plentiful; enough to be used at the scale required by the commercial aviation industry. And of course, successfully passing the rigorous certification process set by international authorities.

Instead of completely overhauling what major aircraft components are made from, we’re more likely to see an incremental approach, with the replacement of aircraft parts over time with lighter, carbon-fibre equivalents.

Additive manufacturing (AM)/3D printing

AM is promising. General Electric describes it as the potential of “Better performance from fewer parts”. It can, in theory, simplify designs (and manufacturing) – creating cost savings. Fewer parts mean a lighter airframe, and less fuel consumption. It also promises increased durability, as fewer welded joints and connected parts offer fewer points of potential failure.

Whilst not directly linked to in service efficiency, additive manufacturing nevertheless offers the potential of reduced waste: by avoiding overproduction (as components can be printed on demand limiting also storage), and the expenditure of energy and loss of material in milling and forming.

However, given aerospace’s Safety Assurance requirements, additive manufacturing is not yet suitable for all of an aircraft’s parts (and certainly not yet a 3D-printed major component). Other shortcomings include size limitations, production speed/scalability, a limited range of materials available, and the challenge of creating parts made from multiple materials. Again, an incremental approach is likely to be adopted.

Propellers

The physical configuration of aircraft is driven partly by its speed in the cruise phase. The current design aims to be as fast as possible, whilst remaining subsonic (as supersonic conditions create additional design constraints and performance issues).

However, if we prioritize efficiency over speed and slow down further, we create further opportunities for slower aircraft for shorter distances. In fact, we see this already in the success of turboprop designs, which tend to serve shorter routes and can outperform jets at lower speeds and altitudes. In fact, it’s worth mentioning that most efforts involving hydrogen-electric powerplants (eg. ZeroAvia, Universal Hydrogen) are being trialled on propeller platforms.

Moreover, and following a common vision from GE and Safran through their joint venture, CFM International, significant effort is being invested into a similar design – ‘open rotor engines’ or ‘propfans’. This propulsion technology uses counter rotating ranges of unducted and twisted blades, and allows aircraft to reach higher speeds than pure propeller designs, while presenting significant gains in terms of fuel consumption and emissions. Although these designs introduce additional difficulties around certification and acoustic performance, they have reached a level of maturity in recent years that makes them a serious and hopeful option.

What should companies do now?

Integrate Life Cycle Assessment and the Circular Economy into designs and redesigns

Making aircraft energy efficient isn’t just about how these aircraft perform when they fly – it’s also about ensuring that the plane itself (and by extension all of its constituent parts) are sourced in the most energy/carbon efficient way possible. For example, can recycled (and recyclable) materials be used? This requires a nuanced understanding of the supply chain for every part of the aircraft.

Build links with research institutions, academia and start-ups

The technologies that will help make the aircraft of tomorrow more energy efficient are (probably) already in their nascent forms in the lab. Startups with bold ideas and nimble processes may be able to develop new valuable technologies faster than in-house teams. Ergo, it is important to perform horizon scanning and build partner ecosystems so you know which labs, startups and institutions have the technology and IP to improve your designs.

Push for open standards and interoperability

There is also a pronounced need to develop (and support) non-proprietary digital engineering standards and interfaces, this will help to ensure that the advantages of digital engineering can be shared in complex, multi-vendor projects.

Learn from innovators in the automotive business

Aerospace could learn a few things from other sectors, particularly the automotive industry. For example, Tesla was able to create rapid and disruptive innovations by working in a cloud-based PLM, which was directly updated with vehicle sensor and test data. This provided real-time insight, and allowed them to use AI on that data to gather insights and build simulations. The variety of electrification problems overcome by Electric Vehicle (EV) manufacturers is also a highly valuable source of innovation for aeronautics.

Go as digital as you can

Most aviation players possess some level of digital engineering capability, but this may be an area in which ‘more is almost certainly always better’.

For example, the digital thread creates the opportunity for digital continuity between design, manufacturing and operation. After all, there is a 60-70 years aircraft lifecycle to consider, with a strict need to manage and maintain aircraft. This presents an ample timeframe to gather data. Such data can be fed back into the digital thread from operational and maintenance data to further optimise aircraft designs.

As previously stated, digital infrastructure is mature in major aerospace primes. The most advanced manufacturers (eg. Airbus and Boeing) have set up complex simulation infrastructures and fully virtual test environment ecosystems. This is done, for example, during the design phase, through the extensive use of advanced physics simulation, which allows manufacturers to understand, predict and validate complex physical behaviour in aerodynamics, structural dynamics, thermodynamics and acoustics with high fidelity.

This process can also be fully engaged in the Verification & Validation phase – from the complete aircraft down to the individual component level, allowing much of the work to be done before ‘steel is cut’, drastically reducing costs and flight testing required. This is both safer and cheaper. The smart combination of data from virtual simulation, ground testing and flight testing can be used to optimize and accelerate the certification journey.