It's more than 100 years since
the Wright Brothers. But humanity
has never stopped dreaming of flying
free like a bird.
In the Greek myth, inventor Daedalus fashioned wings from willow branches, feathers and wax to create a means of escape from prison for himself and his son.
They flew free, but his wayward son, Icarus, soared too close to the sun in his excitement. The wax melted, and Icarus tumbled to a watery death.
Was this a warning against human arrogance, or a prophecy about the inevitable conquest of the skies?
If it was a warning, it was not heeded.
Humanity has always dreamed of flying. Leonardo da Vinci sketched out his idea for a “helical screw” flying machine in the 15th Century. But it wasn’t until Orville & Wilbur Wright made the first successful powered flight in 1903 that the dream showed signs of becoming a reality.
Future-gazing city planners always envisaged flying cars and planes - life in three dimensions. In Aldous Huxley’s Brave New World (1932) his genetically “pure” overlords rise vertically into the sky in their flying machines.
The invention of the jet engine in the 1930s and 40s made transatlantic air travel impossibly glamorous and luxurious. The world shrank and became more easily accessible – to a wealthy few at least.
Then as turbofan engines came in and plane production became more efficient, costs fell and aviation became more accessible to the masses.
While supersonic jets like Concorde retained the glamour of long-haul travel, the trend was always towards a democratisation of the air. Low-cost airlines such as Ryanair, EasyJet, AirAsia and Southwest Airlines brought short-haul destinations within reach of almost everyone.
Catching a plane became like catching a bus, and the number of air passengers ballooned.
More recently climate change reared its head and environmental imperatives – the need to reduce noise and carbon dioxide emissions – began to take precedence.
Dreams of hypersonic travel – New York to Shanghai in two hours rather than the more typical 15 – began to fade.
But necessity is the mother of invention, and this recent environmental driver has spawned a new era of innovation which Daedalus would have appreciated.
As Prof Iain Gray, director of aerospace at the UK’s Cranfield University, says: “We’re seeing new materials.. We’re seeing new technologies around autonomy [self-flying aircraft] and artificial intelligence for the first time applied in aerospace. And we’re seeing really exciting opportunities around electric propulsion.
Standing on an airstrip on a cold grey day in Donauwörth, southern Germany, watching Airbus's scale model octocopter rise into the air, it’s hard to imagine the radical transformation in urban mobility that this prototype – and others like it - could herald.
But Marius Bebesel, chief engineer of this CityAirbus concept, believes fleets of his pilotless fully electric “vertical take-of or landing” (VTOL) sky taxis will be shuttling four passengers at a time between airports and the world's mega-city centres within five years.
Booking one will be as easy as booking a taxi on a smartphone, he says, and the CityAirbus will be just one element in a seamless, integrated transport network.
Dedicated air traffic management systems will ensure safety, and people will soon become comfortable with the idea of autonomous aircraft and the benefits they can bring – greater convenience, less pollution, less congestion.
The craft is mechanically much simpler than a conventional helicopter, says Mr Bebesel, and will have a range of about 60km (37 miles) at speeds of up to 120km/h (75mph). Recharging the batteries will take about 15 minutes, he says.
VTOL craft are much more suited to urban environments because they don't need long runways and could land in tight spaces and on skyscraper roofs.
But will people really feel confident enough to board a pilotless aircraft?
"In the beginning we will have a pilot on board just to increase public acceptance," he says, but there won’t really be much for them to do. In an emergency, the central automated air traffic management system would take over, he says.
In fact, removing pilots – and the likelihood of human error – will actually increase safety, he says.
It's a point backed up by Prof Iain Gray.
"Without a driver you can fly further, faster and more economically," he says. "Autonomy on aircraft has been with us for 40 years. We just need to adapt air traffic management systems to unmanned aircraft."
Trust will come, Prof Gray believes, once a track record of safety has been demonstrated.
Marius Bebesel's CityAirbus team, which is benefiting from tens of millions of euros in development cash from its aircraft manufacturing parent Airbus, aims to have a full-size demonstrator up and flying by the end of 2018.
Meanwhile a couple of hundred kilometres away in the small town of Bruchsal, start-up Volocopter's ambition, according to chief executive Florian Reuter, is to become the "the first all-electric air taxi in the world".
The Volocopter has 18 propellers in a distributed circular array above the two-seater cockpit.
"It's all about safety," says Mr Reuter. "Up to nine propellers could stop working and it would still fly."
Designed for short-hop urban flights, the battery-powered Volocopter will have a range of around 16km-32km (10-20 miles), he says, and will eventually be fully autonomous once there's sufficient public confidence.
But this is no pie-in-the-sky fantasy of the company's entrepreneurial founder Alexander Zosel; the start-up has attracted serious money – 30 million euros from big hitters such as the owner of Mercedes-Benz, Daimler, and tech giant Intel.
The craft undertook its first fully autonomous test flight in Dubai in September 2017 - "the first in history", says Mr Reuter - and he envisages commercial production within five years.
Plenty of other companies are developing sky taxis, including China's Ehang, ride-sharing giant Uber and California-based Vimana.
But will this futuristic vision of urban mobility surmount the formidable regulatory obstacles?
Prof Gray says yes, eventually.
"We could see smaller sky taxis within the next 10 years, once regulation has caught up with the technology. Artificial intelligence will play a huge role in optimised air traffic management."
But others are more cautious.
"By 2022 we'll certainly see some people being transported by air taxis, at least by emergency services," says Prof Harry Hoster, director of energy at Lancaster University.
"But they won't replace car taxis any time soon. There are just too many rules governing airspace and how many aircraft that can be in the air at any one time."
And while battery powered sky taxis promise to reduce localised pollution and ease megacity congestion, he points out that total energy consumption could actually rise as more people use them – as would the number of accidents.
Sky taxis would use double the amount of energy compared with electric cars and have about half the range, Prof Hoster suggests. This wouldn't be an issue for climate change if all the electricity came from renewable sources, but recharging all these electric vehicles - on the road and in the air – could put a huge strain on electricity grids, he says.
Then there's the noise issue. Electric engines are undoubtedly quieter than combustion engines, but collectively, a sky full of clever copters could still make a racket and provoke public opposition.
"They looked at us as a bunch of crazy people!" says Ashish Kumar, co-founder and chief executive of Zunum Aero, a US-based start-up developing a range of hybrid-to-electric planes.
In 2013 when the business was founded, the concept may have seemed far-fetched. But now, the industry is taking electric engines for passenger planes very seriously indeed.
Commercial aviation contributes about 2% to global man-made CO2 emissions, according to International Air Transport Association (IATA) statistics. By 2050, the European Commission wants the industry to have reduced CO2 emissions by 60%, nitrous oxides by 90% and noise by 75%.
This can't be achieved using current technologies, most experts agree.
So Zunum (pictured) - funded by Boeing HorizonX, Jet Blue Technology Ventures and the State of Washington Clean Energy Fund - is one of several firms developing electric engines that promise to reduce the costs, noise and pollution associated with short-haul flights.
Two-seater electric planes are becoming more common with companies like Slovenian Pipistrel and Germany’s Siemens demonstrating their feasibility.
Pipistrel is also one of the companies involved in the 4-seater, German-designed HY4, which is powered by hydrogen fuel cells.
But the challenge is to develop commercially viable electric aircraft capable of taking multiple passengers.
Zunum is currently developing a 12-passenger hybrid-electric plane it hopes will be ready by 2022, while Israeli firm Eviation is designing a nine-passenger pure electric version.
Break free of jet fuel? Well, not completely…not quite yet.
A conventional gas turbine will always be required so long as aviation regulators require reserve power for 45 minutes of flight to cover emergencies, admits Mr Kumar.
The limitations of battery power mean electric planes will fly more slowly and at lower altitudes, he says, but this means they will need less maintenance and will not have to be as strong and heavy as aircraft designed to go higher and further.
Light, thermoplastic materials will help make the planes cheaper to make and operate.
"We forecast at least an 80% reduction in fuel costs," he says. Given that the industry will spend more than $150bn (£112bn) on fuel in 2018 (source: IATA), which is about 20% of an airline's total running costs, such savings should prove attractive, Mr Kumar hopes.
On landing, maintenance crew could simply swap the drained batteries with recharged ones.
Larger aircraft capable of carrying 100-plus passengers will need batteries that can deliver 17-20 megawatts of power, says Mr Kumar. "That's a hard challenge".
But it's one that Jeff Engler of California-based engineering firm Wright Electric is determined to overcome. He wants to build an electric-powered passenger airliner the size of an A320 within 10 years. This could carry around 180 people.
Short hop distances of around 335 miles (539km) are perfectly possible in electrically-powered passenger planes, believes Mr Engler.
His engineering firm is working on an electric power unit for a plane that could transport nine people. But he admits that “batteries today are not good enough” – they’re too heavy, they get too hot and they don’t produce enough power.
He’s hoping for a technological breakthrough.
“Every 10 to 20 years a new chemistry comes out, the way lithium-ion replaced nickel-cadmium, for example. Solid state batteries could be the next step, although we’re probably 10 years away from that.”
The advantage of electric engines is that “five to 10 small engines will be just as efficient as one big one”, he says, giving designers many more options over plane design. The turbines could be incorporated into the wing, for example, or placed above the wing at the back of the plane.
Long-haul flights may always be impossible for fully-electric aircraft, unless there’s a radical improvement in battery technology.
Along with many other airlines, low-cost carrier EasyJet is exploring ways to reduce fuel costs and CO2 emissions and has an understanding with Wright Electric, although no formal agreement exists between the two companies as yet.
And aviation industry heavyweights Airbus, Siemens and Rolls-Royce recently announced a joint project to develop a hybrid-electric engine and incorporate it into a BAe 146 short-haul jet by 2020. The E-Fan X, as they’re calling it, will demonstrate the feasibility of the technology and encourage airlines to take it up, its makers hope.
A gas turbine in the back of the plane will power a generator to create electricity for the one electric fan engine complementing three conventional turbofans. Any excess electricity will be saved to batteries and this stored power will be used for take off and climbing.
Such hybrid-electric formations could reduce fuel burn by about 10%, says Airbus, and be much less noisy on take-off and landing and cause less localised pollution.
Dr Frank Anton, head of Siemens' e-aircraft project, says: "We are opening the door to a new world of aviation. This is as disruptive as the gas turbines of the Forties and the turbofans in the Sixties."
The dream of personalised flight is still vivid in the minds of many inventors, some developing cycle powered craft, others ploughing money into jetpacks.
But the flying car has always remained the ultimate symbol of personal transport freedom.
In 1940 Henry Ford said:
Nine years later Moulton Taylor had designed and flown his Aerocar, proving the viability of Ford's concept (pictured above). But only three models were ever built.
Since then the flying car has remained as fantastical as Chitty Chitty Bang Bang.
Companies such as Terrafugia, recently bought by Volvo owner Geely, the Chinese automotive giant; Dutch Pal-V, with its gyrocopter concept; and Slovakian AeroMobil, have all produced prototypes than can drive on roads and fly.
And Airbus has a futuristic modular concept involving a passenger capsule that can be detached from the road-going chassis and picked up by a quadcopter flying machine.
But all these concepts are massively expensive, require safety certification standards for road and air, need dual controls, involve complex folding wings and propellers, and have to be flown from air-strips.
So they are likely to remain rich people's playthings rather than practical transport solutions for the masses.
“A car that takes off from some London street and lands in another random street is unlikely to happen,” says Prof Gray. "Sky taxis are much more likely."
But that won’t stop inventors dreaming up new ways to fly and trying to persuade investors to back their sometimes outlandish schemes.
Civilian aviation is being disrupted, not by the age-old desires for speed, romanticism and glamour, but by the pressing need to respond to a changing climate. New electric engines coupled with artificial intelligence and autonomous systems will contribute to a more efficient, integrated transport system that is less polluting and less noisy.
That may sound prosaic, but as Prof Gray says:
Now that would be progress.