W hen Chitty Chitty Bang Bang was released 50 years ago, flying cars were a flight of fancy. Now, these futuristic vehicles are entering the outer fringes of reality. According to a new study published in Nature, for some journeys flying cars could eventually be greener than even electric road cars, cutting emissions while also reducing traffic on increasingly busy roads.

However, gaps in necessary technology and practical uncertainties beyond the cars’ promising physics mean that they may not arrive in time to be a large-scale solution to the energy crisis and congestion – if at all.

How to make a car fly

It might at first seem crazy that a flying car could be more efficient than a road car, especially when conventional planes have such a reputation as gas guzzlers. But flying isn’t inherently inefficient – after all, birds can fly between continents without eating. Of course, a small, four-passenger car isn’t an albatross, but it isn’t a Boeing 737 either.

There are many ways to make a car fly, but most are too problematic to get off the ground. Perhaps the most promising option is that taken in this study, based on the physics of vertical take-off and landing (Vtol) aircraft. They’re pretty amazing beasts.

If you’ve heard of Vtol, something like a Harrier Jump Jet probably springs to mind, with two huge engines directing thrust that can be tilted vertically or horizontally. But these much smaller and lighter flying cars operate differently, with lots of tiny electric fans blowing air from many places. This fast-developing distributed electric propulsion (DEP) technology is key for efficiency when cruising, and it also creates possibilities for quieter take-off and hovering, as multiple small noise sources can be better managed.

Wing and propeller design can also be optimised to be long and thin with lots of moving surfaces, just as birds do to make their flying efficient. The aim of all of these technical enhancements is to achieve maximum lift for minimum drag – the force that opposes an object’s motion through air and slows it down. A better lift-to-drag ratio means lower power consumption and, therefore, lower emissions.

World’s first commercial flying car is now on sale

These energy-saving innovations make cruising a breeze – but they don’t help much with take-off, hovering or landing, which are still inherently inefficient. So while Vtol flying vehicles are still viable for short intra-city travel and pizza deliveries, they will not solve the energy crisis.

For 100km journeys, electric flying vehicles could be 35 per cent more efficient than a petrol-powered car – although, assuming the same number of passengers, still less efficient than an electric road car. However, it’s fair to assume that flying cars will serve primarily as taxi services in pre-defined air corridors and are, therefore, likely to consistently carry more people. Taking this into account, for a 100km journey flying car emissions could be 6 per cent less than those of electric road cars.

As journey distance increases, so too do the efficiency gains over stop-start road cars, which have to deal with rolling resistance and less efficient airflow. But unfortunately, range is the Achilles heel for electric aviation. The study looks at a range of up to about 200km and here flying cars could perform well. But while jet-fuelled planes can lose as much as 70 per cent of their weight during flight (albeit at a cost of 100kg of CO2 per passenger per hour), batteries don’t get lighter as they discharge. This means that beyond 200km or so, carrying batteries becomes a distinct disadvantage.

The accepted view is that electric planes will only ever be viable for short-haul flights. It’s energy density that matters, measured in watt-hours per kilogram. Right now, the best batteries provide around 250 W-h/kg, a mere shadow of jet fuel and gasoline’s 12,000 W-h/kg. Batteries could creep up to 800 W-h/kg by the middle of this century, increasing their feasible range to 700 miles – half of all global flights fall within this distance. But without more dramatic innovation in battery technology, biofuels and liquid fuel from air-capture of CO2 will likely need to play a substantial role in long-haul air travel.

100 years of the Ministry of Transport Show all 30 1 /30 100 years of the Ministry of Transport 100 years of the Ministry of Transport 1919 Motor lorries being used to transport passengers during a tube strike in London Getty 100 years of the Ministry of Transport 1919 A bank holiday crowd queuing to board a tram at Golder's Green Getty 100 years of the Ministry of Transport 1919 A London Motors bus en route to Dorking on Whitsun holiday Getty Images 100 years of the Ministry of Transport 1924 Mr Baldock from Clapham Depot Trams arrives at the Law Courts in London to attend an enquiry set up by the Ministry of Labour, into the bus and tram strike Getty 100 years of the Ministry of Transport 1939 The 'Royal Scot' pulling out of Euston Station, London, on the way to Blisworth, Northamptonshire, amongst the passengers was the Minister of Transport, Leslie Burgin Getty 100 years of the Ministry of Transport 1946 Labour politician Alfred Barnes ( right), the Minister of Transport, enjoys a cup of tea with Albert Stanley, 1st Baron Ashfield and a group of bus drivers at the LPTB headquarters Getty 100 years of the Ministry of Transport 1946 Lord Winster, Minister of Civil Aviation, making an address at Heathrow Airport, London beside Air Vice Marshal Donald Bennett (right), who formed and led the RAF's Pathfinder Force Getty 100 years of the Ministry of Transport 1948 A Vickers Viscount demonstrates near Weybridge, Surrey. The Viscount was the first civil transport plane powered exclusively with propeller-turbines PA 100 years of the Ministry of Transport 1958 The Minister of Transport and Civil Aviation Harold Watkinson, points out features of the new airport to Queen Elizabeth II and the Duke of Edinburgh, who were visiting Gatwick Airport for its official opening PA 100 years of the Ministry of Transport 1966 British Labour Minister of Transport, Barbara Castle with Maurice Holmes and parliamentary secretary to the minister, Stephen Swingler buying a ticket from a machine on board a conductorless bus Getty 100 years of the Ministry of Transport 1966 Barbara Castle, Minister of Transport, passes through the turnstile in one of London Transport's new Red Arrow buses that she inspected at the Ministry in Southwark, London PA 100 years of the Ministry of Transport 1966 British Labour Party politician Barbara Castle, the Minister for Transport, tries out the new 'Midge' Morrison-Electricar after opening the 1966 Commercial Motor Show at Earl's Court in London. The electric vehicle, was for use on private land, didn't require a driving licence or a tax disc Getty 100 years of the Ministry of Transport 1970 In between the the railway line are (left-right) Fred Mulley, Minister of Transport, George Thomas, Secretary of State for Wales, and Cledwyn Hughes, Minister of Agriculture, inspect the fire-damaged tubular Britannia Bridge over the Menai Straight PA 100 years of the Ministry of Transport 1970 Tory leader Edward Heath, fresh from his triumph in the Sydney-Hobart Ocean race, takes the helm of an exhibit during the International Boat Show at Earls Court, London PA 100 years of the Ministry of Transport 1980 Norman Fowler, British Conservative politician and Minister of Transport, with the prototype APT (Advanced Passenger Train) at Euston Station Getty 100 years of the Ministry of Transport 1980 Civil Aviation Authority crash investigators at the scene of the British Midland Boeing 737 air crash by the side of the M1 motorway near Kegworth, Leicestershire PA 100 years of the Ministry of Transport 1981 Queen Elizabeth II takes a ride on the Tyne and Wear Metro after the £280m project opened PA 100 years of the Ministry of Transport 1987 British Prime Minster Margaret Thatcher and Transport Secretary Paul Channon (right) surrounded by police and firemen as they visit the scene of the King's Cross Station fire Getty 100 years of the Ministry of Transport 1994 The Queen unveils a plaque to celebrate the official opening of the eurostar passenger train terminal at waterloo international station, london PA 100 years of the Ministry of Transport 2000 Prime Minister Tony Blair and Labour London Mayoral candidate Frank Dobson on the Underground at London Bridge Station following a long-awaited joint appearance, where they attacked Ken Livingstone's plans for transport PA 100 years of the Ministry of Transport 2000 Deputy Prime Minister John Prescott at the completion of a key section of the Channel Tunnel near Aylesford in Kent. He made light of Tony Blair's Women's Institute experience while addressed workers at the tunnel PA 100 years of the Ministry of Transport 2002 Transport Secretary, Stephen Byers, answering transport questions at the House of Commons, London PA 100 years of the Ministry of Transport 2004 Transport Secretary Stephen Byers tells journalists, at a news conference in Downing Street, that he is resigning. The minister, who was widely criticised for the running of his department, said he was leaving because it was the right thing to do PA 100 years of the Ministry of Transport 2005 The Secretary of State for Transport Alistair Darling is given a tour of the latest railway integrated control centre (ICC) as he opens the Anglia ICC PA 100 years of the Ministry of Transport 2005 Britain's Prime Minister Tony Blair chats with the media aboard a train from Bristol to London after visiting a science and natural history educational centre PA 100 years of the Ministry of Transport 2007 Tony Blair, who is regarded by drivers as one of the last people they would want as a car passenger, according to a poll out in 2007 PA 100 years of the Ministry of Transport 2008 Transport Minister Ruth Kelly with Prime Minister Gordon Brown at the Labour Party conference, in Manchester PA 100 years of the Ministry of Transport 2008 Transport Secretary Ruth Kelly near the A3 at Hindhead to witness the start of excavation work on the longest road tunnel under land in the UK PA 100 years of the Ministry of Transport 2012 Transport Secretary Justine Greening during the official opening of the new terminal building at London Southend Airport in Essex PA 100 years of the Ministry of Transport Transport secretary Chris Grayling is facing two parliamentary inquisitions after the Brexit ferry fiasco cost the taxpayer at least £33m. The transport secretary was widely criticised after agreeing to pay the sum to Channel Tunnel operator Eurotunnel to stop it pursuing a High Court battle against the government. Eurotunnel had taken legal action over what it said was a “secretive and flawed procurement process” in relation to three government ferry contracts awarded for freight services in the event of a no-deal Brexit – one of them to a firm that did not own any ferries PA

Problems in practice

In focusing entirely on the physics of flying cars, the paper steers clear of a number of practicalities that must be considered before we embrace Vtol flying cars as a sustainable form of transport for the future. For example, it is important to consider the carbon costs of production, maintenance and downtime, known as life-cycle analysis (LCA). Electric vehicles have been criticised for both the energy and environmental costs of mining primary materials for batteries, such as lithium and cobalt. Added infrastructure required for flight may worsen the problem for flying cars. And of course, a grid powered by low-carbon sources is essential to make battery-powered vehicles part of the solution to our climate crisis.

Aircraft also have highly stringent criteria for maintenance and downtime, which can often offset gains in performance and emissions. As an entirely new breed of planes, it’s impossible to predict how much it might cost to keep them air-worthy. Unforeseen maintenance complications can cost billions – just ask Boeing.

Finally, weather matters. A tailwind of 35mph reduces power use and emissions by 15 per cent, but a 35mph headwind increases them by 25 per cent. Having to carry heavy extra batteries to avoid the potential catastrophe of running out of charge before encountering a suitable landing place could offset emissions savings. Road cars, by contrast, can easily pull over to the side of the road when needed, without consequence.

So when it comes down to CO2 emissions per passenger kilometre, at present these advanced DEP flying cars are at best comparable to their road-going electric equivalents, and, at worst, little better than conventional combustion cars. With technology and safety improvements, they could yet play a part in our fossil-fuel-free future, taking short-haul planes out of our skies and freeing up fume-filled roads. The question on everyone’s lips is whether these flying cars will be ready in time to make a jot of difference to our very pressing energy crisis. Can we wait 30 years?

Hugh Hunt is a reader in engineering dynamics and vibration at the University of Cambridge. This article first appeared on The Conversation