Flying cars could cut emissions and replace planes – but not soon enough

Solutions

When 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, thin, and have 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.

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% 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% 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% 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.

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 down time, 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%, but a 35mph headwind increases them by 25%. 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 everyones’ 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?

This article is republished from The Conversation under a Creative Commons license. It's written by Hugh Hunt, Reader in Engineering Dynamics and Vibration at the University of Cambridge. You can read the original article here

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  • The Sky has a Limit

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    Join us for the second of our Kinder Conversations - the Sky has a Limit.

    Following our investigation into the Future of Meat in February, we've turned our attention to the sky. We'll be talking about whether air travel can be more sustainable, and how.

    We're delighted to be hosting this event in collaboration with our friends at TQ, the Amsterdam tech hub where we're based. 

    📅 When Monday 17 June 18:30 - 20:30
    📭 Where TQ, Singel 542, Amsterdam
    🎯 Why should I fly Kinder? To hear about the latest research and technology making air travel more sustainable. To find out what you can do to reduce the impact your flights are having. To share a drink with like-minded travelers, and sample some of our vegan snacks (including beloved Professor Grunschnabel ice cream, as seen at the Future of Meat event)

    🎫 How can I get in? We’re offering two ticket levels: Economy (free) and Business Class (for the price of a donation to Cool Earth). Secure your seat now!

    More about The Sky has a Limit

    Here at Kinder, we believe that greener travel is one of the key ways in which we can tackle the climate crisis. Travelling green can mean a lot of things, but right now we’re concerned about the aviation industry.  

    If aviation were a country, it’d be a top 10 polluter - and C02 emissions from air travel are growing many times faster than any other form. We’re already in a very dangerous position, and although there are many potential solutions, we sometimes feel overwhelmed and uncertain about what to do about it.

    That’s where Kinder Conversations comes in.

    Kinder Conversations is a series of events which delve into the biggest issues facing the world. 

    At the Sky has a Limit, we’ll be bringing together representatives from research and technology, the aviation industry and the not-for-profit sector to talk about sustainable air travel. We’ll hear more about the problem, and a lot more about the solutions.

    Plus, there’ll be time to get a drink from TQ’s bar (buying a drink helps our friends from TQ support more events like this), try some vegan ice cream, and chat to fellow travellers about the steps you can take to travel greener.

    ✈️ Are you ready to #flykinder? Then secure your boarding pass here

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  • How flying cars could help solve the problem of air pollution

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    I don't have a driving license and when pressed about getting one by friends tired of chauffeuring me around I usually say I will only get one if I can drive something cool, like the Batmobile or a flying car. Unfortunately, I might have to honour that promise as it seems that flying cars are finally taking off (alas, no commercial Batmobiles in sight).

    Indeed, several promising startups around the world are working to deliver the "car of the future"  over the next few years. Like the Dutch company PAL-V that showed off a limited edition of its flying car at the Geneva Auto Show in Switzerland.

    The PAL-V is a hybrid between a car and a helicopter (or more precisely, a gyrocopter), able to reach a top speed of 160 km/h on the tarmac but also get airborne in just 5 minutes, hitting airspeeds of 180 km/h over a range of up to 500 km. But since buying a PAL-V will set you back around € 350,000 I might have to pass on this one. Moreover, flying this beauty requires not just a driving license but also (understandably) a license to fly, and that's just too much for me.

     

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    Called VTOLs (short for Vertical Take Off and Landing), these aircraft aim to become sort of an Uber of urban air travel, bringing customers to the opposite part of the city or even to a nearby city in a matter of few minutes.

    If you're at JFK airport in New York, for example, and have a meeting in Manhattan, instead of embarking on a 1-hour, Cosmopolis-style taxi ride, you could just hail a flying car and be downtown in 5 minutes.  

    Futuristic as it may sound, concrete plans to make it come true are underway. Earlier this month, German startup Lilium successfully completed the first test of its new five-seater Lilium Jet, an electric vehicle that, according to the company, will have a range of 300 km and a top speed of 300 km/h. 

    The reason electric flight is such an exciting area of research is not just because flying taxis will allow a handful of high rollers to drastically cut on their commuting time. Electric flying cars might be really good for the environment too.

    A recent study published by Nature highlighted that, in some cases, flying cars could eventually be greener than even electric road cars, cutting emissions while reducing traffic on increasingly busy roads.

    Moreover, developments in the field of flying cars could also boost the research on electric flight at large, including long haul electric flights, sort of the Holy Grail of aviation. And, as known, the civil aviation industry needs to find effective ways to lower its carbon emissions as soon as possible. 

    However, as explained by Hugh Hunt in an article on The Conversation that we republished here on Kinder World, "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." 

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  • Why the current state of aviation is one of the main threats to our planet

    Obstacles

    Let's get this one thing straight: most people prefer flying to other modes of transport, and we seem to do it more and more often. The airline industry is booming and 4.1 billion passengers have been transported last year. Almost every figure one looks at shows the impressive increase in flights over the last two decades.

    Alexandre de Juniac, head of the International Association for Flight Transport proclaims: “In 2000, the average citizen flew just once every 43 months. In 2017, the figure was once every 22 months. Flying has never been more accessible. And this is liberating people to explore more of our planet for work, leisure, and education. Aviation is the business of freedom."  

    However, this ‘business of freedom’ runs on fossil energy carriers as planes still almost exclusively fly on kerosene. Kerosene is a fuel produced by oil refining and carbon dioxide (CO2) is the major product of burning kerosene. The 2-5% of all global CO2 emissions the aviation industry emits is caused by its fuel consumption (and choice). And unlike other fuels like diesel or gasoline, airlines don't pay taxes on kerosene in most countries — making cheap air travel possible.

    In 2018 Europe’s biggest airline Ryanair became number 9 in the list of Europe’s biggest CO2 emitters and still claims to be the ‘greenest and cleanest airline’. Andrew Murphy – the aviation manager at the European Federation for Transport and Environment — argues that Ryanair the new coal when it comes to climate pollution. Ryanair CEO Michael O’Leary, on the other hand, dismisses such claims by saying the claims are ‘’complete and utter rubbish’’.

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    Compared to other modes of transport planes are the biggest CO2 emitters per travelled kilometre followed by cars, buses and finally trains which are the least polluting. The CO2 emissions, however, are only one half of the medal. The impact of flying on global warming is different than most other transport as it happens in the air high above the ground where the processes that cause or reduce global warming happen. These include CO2 and nitrogen oxide emission but also cloud formation, ozone and soot as well as methane reduction.

    The climate impact of the emitted greenhouse gases in the stratosphere are three times higher than on the ground. Flying also causes condensation trails and fog clouds in certain weather conditions. Such clouds can have a warming or a cooling effect on the climate. One way to improve the climate effect of flying would be planning better routes where warming clouds are avoided and the formation of cooling clouds is favoured — our current routes have an overall warming effect.

    So, hypothetically, some flights with clever flight-route planning might even reduce global warming. However, as we don't have time to hypothesise, we need to find and urgently implement other ways to bring down the impact of flying, like using better fuels or even better planes.

    This article was written by Eric Schuler for Kinder World. Schuler is a PhD candidate at the University of Amsterdam and works on new industrial sustainable chemistries to turn captured CO2 into useful things such as plastics or fuel. He's also a photojournalist with an interest in climate and land-use change.

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