UK experts imagine the future of passenger flight – a hybrid, bio-inspired green airliner for sustainable aviation that could cut fuel burn by up to 50%. TIM ROBINSON reports on the Airbus ‘Bird of Prey’.

The Bird of Prey incorporates bleeding-edge aerospace technology to create an airliner able to slash fuel burn by 80%. (Airbus)

Airbus has revealed a visionary concept from its engineers in Filton for an ultramodern green airliner of the future – one possible outcome of where today’s advanced technology may take us when designers’ imaginations are truly unleashed.

The Bird of Prey is a 80-seater, 1,500km range hybrid-electric regional airliner that incorporates the latest thinking in aerodynamics and flight control, structures and distributed propulsion to create the greenest ever future airliner. Using technology now under development, the Bird of Prey could provide a 30-50% reduction in fuel burn compared to equivalent aircraft today – a major leap in efficiency.

Says Jeremy Greaves FRAeS, Vice President, UK Corporate Affairs and Strategy, Airbus: “This is a visioneering project where we asked our talented flight physicists to imagine what the future could look like.” The Bird of Prey then, is an example of critical High Value Design (HVD) that combines preliminary engineering with creativity to define a product. Explains Martin Aston, Senior Manager, Airbus and project lead for Bird of Prey: “It’s that combination of imagination, innovation and awareness of science, then converting that into a viable product.”

Thought up by Airbus engineers at the UK’s Filton site who were given a brief to imagine the future of flight without any limits, the Bird of Prey was brought to life by a British superyacht and industrial designer – Rob McPherson of BezierLab. McPherson remarked: “When I was approached to see if I’d be interested in creating a vision for a future airliner I jumped at the chance” adding: “The brief was to create a concept which would be technically possible if we pushed ourselves to the limits, but futuristic enough to inspire. It was delightful to be encouraged to think freely and could only be done with such a visionary client!”

The company stresses that the Bird of Prey should not be seen as a firm Airbus programme, “or even as having an entry-into-service date assigned to it.” Says Greaves: “It may not be real in current Airbus strategy or financial planning but it provides a headmark for future inspirations.”

As well as Airbus, the initiative (which includes a large 2.5m wingspan static model to be unveiled at RIAT today) is supported by the UK’s Air League, Aerospace Technology Institute, IET, Whitehall’s ‘Britain is Great’ campaign and the Royal Aeronautical Society. This model will make for an impressive talking point and centrepiece at future aviation events and functions. A ‘Phase 2’ of the initiative, says Airbus, could see this non-flying model transformed into a flying prototype as an exciting student design project for control systems.

Says Aston on the model and Bird of Prey concept: “What we want to do is get people to look at that and say, ‘right, how would I design that’? What technologies would I put on there? How would I make it work? How would I make it? How would I maintain it, how would I make sure that it’s reliable?”

The aviation industry has set itself tough targets to reduce its environmental impact – such as the EU’s Flightpath 2050 – which calls for a reduction in CO 2 by 75%, NO x by 95% and noise by 65% by 2050. Concepts such as Bird of Prey will help turn these aspirations into reality by showing what might be possible and inspiring a new generation of aerospace professionals to dream big.

Biomimicry as a design theme

Biomimicry is not new, but modern materials and technology open up amazing possibilities. (Airbus)

“The birds fly a lot better than we do” observes Leslie Howard as legendary Spitfire designer Reginald J Mitchell in the fictional film ‘First of the Few’, “See how they wheel and bank and glide. Perfect. And all in one: wings, body, tail. All in one. And when we try, we build something all stuck together with strings and struts and wires. But, you wait. Someday, I’m going to build a plane that will move just like a bird.” That aircraft, of course, was the iconic Spitfire, a paradox in being perhaps one of the world’s most beautiful aircraft but built with a deadly purpose.

‘Strings and struts and wires’ may have been replaced today by ‘nacelles, engine pylons and fuselage belly fairings’, but, some 83 years after Spitfire’s first flight, modern design tools, advanced materials, 3D printing, brought together with ‘high value design’ (HVD) may allow tomorrow’s aircraft engineers to pursue Mitchell’s vision of a perfectly streamlined ‘bird-like’ aircraft to its ultimate form for a more peaceful role, the Bird of Prey.

Says industrial designer McPherson: “During the initial meeting at Airbus I was briefed on the technology that was currently being developed and one aspect was biomimicry. The talk was about making wings more flexible/adaptive like a bird's wing. Wing tip ‘feathers’ were discussed which led me to create a couple of concept sketches of a more natural looking wing. I wanted something that would look powerful and purposeful so decided to look at birds of prey for inspiration.”

This is no mere aesthetics but would result in improved aerodynamics by taking cues from nature. An albatross, with wing aspect ratio of 18:1 compared to around 9.5 on an A320 is able to travel 1,000s of kilometres by barely using any energy.

Blending the wing and fuselage in an arch, mirrors how a bird’s body blends into its wings and provides for maximum aerodynamic efficiency. Instead of hinged ailerons or flaps, the Bird of Prey would use highly-efficient wing-tip feathers to ‘morph’ and control its flightpath, as well as shaping the wing for various phases of flight. Control using these active wingtip devices and distributed propulsion would also allow the tail empennage to be reduced in size and weight and radically reimagined into a bird’s tail feathers to provide precision control. No vertical tail, as in the B-2 stealth bomber and various UAVs, would allow for reduced drag and increased aerodynamic efficiency.

As Mitchell’s Spitfire demonstrates, taking ideas from nature is nothing new for aerospace, but todays scientists are constantly discovering new applications of biomimicry. Sharks and whales, for example have rough skin that paradoxically reduces drag in the water. That adaptation has been investigated by Airbus in a series of operational trials using test patches of ‘riblets’ applied to aircraft surfaces. A further development is that these rough drag-reducing skins may be ‘spray painted’ on using today’s automated systems.

Moveable wingtips

Airbus AlbatrossOne is a UK-developed subscale demonstrator for semi-aeroelastic wingtips. (Airbus)

Perhaps one of the most radical aspects of the Bird of Prey is in its individual wingtip ‘feathers’ – which combine the drag-reduction function of traditional winglets, seen on many aircraft today, with the control and stability function of a bird’s feathers, replacing traditional ailerons and flaps. Only recently an idea like this might have seemed like science fiction. However, advances in active and passive ‘morphing’ using shape-memory materials has now brought this advanced control system technology, a call-back to the Wright brothers ‘wing-warping’, closer to practical applications.

As well as NASA in the US, the UK is making strides in this field. Earlier this year Airbus revealed that it had flown AlbatrossOne – a sub-scale demonstrator developed by its British Filton engineers which featured a movable hinged wingtip. The ‘semi-aeroelastic hinge’ on the AlbatrossOne allows the wingtips to freely flap – cutting down on turbulence and gusts, while also reducing overall weight. Airbus Filton engineer Tom Wilson explains: “The Airbus demonstrator is the first aircraft to trial in-flight, freely-flapping wing-tips to relieve the effects of wind gusts and turbulence,” adding: “We drew inspiration from nature – the albatross marine bird locks its wings at the shoulder for long-distance soaring but unlocks them when wind-gusts occur or manoeuvring is required.”

“The AlbatrossOne model will explore the benefits of unlockable, freely-flapping wing-tips – accounting for up to a third of the length of the wing – to react autonomously during inflight turbulence and lessen the load on the wing at its base, so reducing the need for heavily reinforced wing boxes.”

Combining these wingtip functions of gust response, drag and load reductions, with roll control functions, as well as locking for long-range cruise, thus brings an aeroplane close to the ultra-aerodynamic efficiency of birds that are able to constantly reshape their wings for different phases of flight. Bird of Prey could be thought of as the ultimate extrapolation of AlbatrossOne.

Hybrid-electric power

E-Fan X prototype, set to fly in 2021, will trial high-power hybrid propulsion technology. (Airbus)

The Bird of Prey would use a next-generation hybrid electric propulsion system for eco-efficient power, driving four A400M-style scimitar-shaped propellers. The benefits of hybrid-electric propulsion is that kerosene-powered engines (whether turbofans or turboprops) can be resized and optimised for the cruise phase of flight, rather than take-off and climb where electric power is added to boost performance. Fuel savings could thus be as much as 30% compared to today’s designs – a massive jump compared to the average 1% that engine manufacturers can squeeze out of turbofan architectures each year.

Airbus is already leading the charge towards hybrid-electric propulsion with its E-Fan X demonstrator which it is set to fly in 2021 with partner Rolls-Royce. That ‘X-Plane’ which will fly with the world’s largest (2MW) airborne turbogenerator as a demonstrator for this large and powerful hybrid-electric system.

Moving to a hybrid-electric propulsion system also allows aircraft designers to decouple the ‘propulsor’ (fan, ducted fan or propeller) from the ‘engine’ (turbogenerator or battery), opening up radical new configurations in distributed propulsion that optimise aerodynamics, increase redundancy and boost fuel efficiency. “We are at the dawn of a new age in aviation” says Rolls-Royce’s Chief Technology Officer, Paul Stein.

While long-haul, twin aisle aircraft will still need to rely on kerosene-powered aircraft, Airbus’ Chief Technology Officer, Grazia Vittadini, notes that the benefits of hybrid-electric power are such that: “It is definitely one of the configurations we are exploring injecting into our core products” when Airbus finally comes to design a replacement for its short-haul A320neo family.

Distributed propulsion

Distributed propulsion could make for a super-quiet STOL regional airliner. (Airbus)

Distributed propulsion, too, may have other benefits. Much like an A400M or C-130J can perform short landings and take-offs using its propellers, so too could a propeller-driven hybrid airliner. This would enable it to operate into smaller airports and airfields. Lower fuel burn would also translate into lower operating costs, lower ticket prices for passengers and thus potentially open up wider networks of regional airports for point-to-point travel.

As well as its incredible fuel efficiency, the Bird of Prey would also be a greener neighbour, as its short-take-off capability would also allow it to climb out steeper, thus reducing the noise footprint on the ground. With distributed power, the propellers, too, could potentially act as yaw controllers, allowing smoother and more stable landings in crosswinds.

Indeed, with distributed hybrid-electric propulsion there may even be ways to ‘tune’ the propellers as a form of ‘active noise control’ to redirect or reduce noise downwards to the ground. Germany’s DLR lab is already investigating this concept for a small regional airliner with distributed propulsion. An ultra-quiet regional airliner would open up the possibility of 24hour operations or increased frequencies at a far wider number of airports.

Finally, a distributed hybrid-electric propulsion system would also provide additional levels of system redundancy and safety. A (highly unlikely) propeller failure on one, two or even three propulsors could just see the system reroute power to the other remaining propellers. Four-engined aircraft (such as the B-17) have landed before on single engines but a distributed propulsion system able to reconfigure and reroute power dynamically as needed would provide far higher margins of safety.

3D printing and geodesics

Geodesics may allow future airliners to dispense with fuselage frames and stringers in favour of organic-looking ultra-lightweight 3D-printed 'skeletons'. (Airbus)

Another advance that could be used on the Bird of Prey is 3D printing and advanced composites to create an ultra-lightweight, almost ‘bird-like’ skeleton and structure in place of traditional fuselage frames, or wing spars and structures. Parts made by additive layer manufacturing (ALM) or 3D printing have the potential to be up to 55% lighter than traditional parts. Already small 3D printed parts are appearing on aircraft such as the Airbus A350 – and these will continue to expand in numbers and applications.

Even more excitingly, as well as using 3D printing to replace existing parts on conventional configuration aircraft, ALM can free designers from thinking about traditional shapes for structures, ushering in a new era of organic-looking aircraft. Airbus has already hinted at future possibilities with its 2050 concept airliner which features a geodetic-style fuselage structure.

Interestingly, the Bird of Prey’s geodesic (or geodetic) structure harks back to another famous aircraft built at Airbus UK’s Broughton site in the 1940s – the Vickers Wellington bomber. Geodetic technology, developed by Barnes Wallis, produced a light but immensely strong and durable structure able to take large amounts of punishment and damage yet still remain airworthy.

Twenty-first century 3D printing and computer design make it feasible that this lattice-work style structure could return in a more elegant form, combining beautiful organic shapes with extreme strength.

The need for high-value design (HVD)

Chinese HVD - a COMAC subscale demonstrator for an efficient airliner with a V-tail. (COMAC)

To help develop an aircraft like the Bird of Prey, the UK needs to refresh and reinvigorate its shrinking HVD pool of talent. It is estimated, for example, that between 1990 and 2015 UK HVD capabilities declined by around 30%. This is a relatively recent development. For the UK, its HVD contribution to Airbus has centred around wings, engines (via Rolls-Royce) and landing gears. Research, begun decades ago with wind tunnels and design tools has allowed Airbus to soar to a position where it commands a duopoly with Boeing.

HVD − or the ability to apply advanced engineering from concept to market, is the ‘noble work’ that truly creates long-term value, growth and jobs in aerospace, rather than sub-system design or ‘build to print manufacture’ where work can be easily shifted around the world to cheaper workers.

Airbus’ Aston explains: “In 1998 the Ford Motor Company produced a study. What they found is that 70% of a product's complete life cycle costs are locked in the preliminary or HVD phase. Which means that if you control that part of the process, you are very, very influential. If we don’t do that, all we do is become a build-to-print nation, take other people’s designs and make them. Whereas if we control the product definition, we control what comes next.”

However, that position (and the UK’s part in it) is not guaranteed. Leaving the politics of Brexit aside, it is clear that the UK has been ‘living on past glories’ where key decisions and research made years ago have allowed Britain to retain its crown as the designer and manufacturer of Airbus wings. The incremental improvements to airliners such as the A320neo and A330neo have resulted in conservative safe design choices, rather than radical configurations. Even the A350 XWB, while sporting elegant winglets, conforms to the ‘dominant configuration’ of tube, wings and engines set in the 1950s by the Boeing 707. Says Aston: “We’ve taken the Wright brothers aeroplane and got it about as far as it’s going to get for now. It now needs the next generation to come along and do something really clever.”

Today though, there are new challenges. The retirement of skilled and experienced aerospace engineers and intense competition from new sectors such as IT or web start-ups have stoked fears of a future skills gap for aerospace engineers. There is also competition from new entrants, such as China, which is innovating at an incredible pace. China, although its new C919 airliner looks highly conventional, is already looking to a future replacement with a potential V-tailed airliner model. It has to be remembered that it was Airbus’ willingness to take risks with a widebody twinjet, composites, a two-person flightdeck and FBW that allowed it to leap-frog over US competitors and see off both Lockheed and McDonnell Douglas as commercial rivals. Stagnating is a sure-fire way to decline. Says Aston of the UK’s position: “We’re on the edge. I think at the moment there is still real excellence in UK engineering, certainly in aerospace. But unless we actually invest in that capability and actually put it ahead of the global competition, we risk going into decline.”

Neglect HVD and the consequences may not just be economic and the gradual decline and slow death of the UK aerospace industry as it fades into irrelevance, but can also prove fatal and result in lives being lost. The current 737 MAX crisis, for example, goes back to a HVD engine-airframe integration challenge where a more complex and expensive (longer landing gear) solution was rejected in favour of what seemed at the time to be a low-risk cheap fix.

The retirement of these engineers, the decline of these HVD skills and increased competition, comes at a critical time for the UK as it wrestles with its place in a post-Brexit world. ‘Disruptive’ technology in the form of 3D printing, hybrid-electric propulsion, AI, self-healing smart materials, VR/AR and digitisation means that tomorrow’s aerospace engineers will require new skills, education and training to be able to harness HVD for the next generation of aircraft – which will require diverse, creative teams to think about the whole design. Tomorrow’s aircraft designers will need to view aircraft as an integrated ‘whole’ rather than systems or subsystems such as wings, fuel or landing gear. For radical configurations like the Bird of Prey, or even nearer term advances such as short-lip engine nacelles, integration between airframers and engine manufacturers will become even closer.

There are bright spots on the horizon; for example, the UK’s Aerospace Technology Institute, the Brunel Challenge, launched earlier this year and the Team Tempest consortium are all working towards a common goal of revitalising and re-energising HVD in the UK where for many years the sector has been content to ‘play it safe’ and focus on its strengths.

The cusp of an aerospace revolution?

One of the early sketches of the Bird of Prey. (Airbus/Rob McPherson)

Although the Bird of Prey may just be CGI and a model at the moment, progress in aviation, particularly in electric/hybrid electric technologies and advanced structures, is now moving faster than many might grasp. The recent Paris Air Show, for example, showed several examples of distributed propulsion which, when combined with hybrid electrics, biomimicry, advanced composites and 3D printing, has the potential to unlock a revolution in eco-efficient flight. Says Aston: “It was engineering excellence that is why we are where we are today. And that’s why, when the next generation of aircraft come through, we want to be in the same position."

Jeremy Greaves from Airbus UK is optimistic of the possibilities: “Today we can’t say how much of the reality of this ‘visioneering’ will come to fruition but we can say that even if only 20% of ideas come to reality, we will be at the cutting edge of tomorrow’s technology.”

But make no mistake, for those at school, university, or college or just entering the aerospace industry, they are about to enter a new age of aerospace innovation where the sky is literally the limit and over half a century of established textbook wisdom of ‘this is what an airliner should look like’ is set to be thrown out of the window. The next generation of engineers, empowered with new tools, materials and digital savvy are thus set to ‘re-engineer engineering’.

For budding aerospace engineers who dream of designing aeroplanes that fly like a bird – your time has come!

Tim Robinson

