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When Chitty Chitty Bang Bang was released 50 years ago, flying cars were a fantasy. Now, these futuristic vehicles are entering the outer margins of reality. According to a new study published in Nature, some flights could potentially be greener than electric cars, reducing emissions while reducing traffic on increasingly busy roads.
However, technological gaps and practical uncertainties that go beyond the promising physics of cars mean that they may not arrive in time to constitute a large-scale solution to the energy crisis and congestion, or even not at all.
How to fly a car
At first, it may seem foolish that a flying car could be more efficient than a road car, especially when conventional airplanes have the reputation of being energy-hungry. But flying is not inherently inefficient – after all, birds can fly between continents without eating. Of course, a small four-pbadenger car is not an albatross, but it's not a Boeing 737 either.
There are many ways to fly a car, but most are too problematic to take off. The most promising option may be the one selected in this study, based on the physics of vertical take-off and landing (VTOL) aircraft. They are incredible animals.
If you've heard of VTOL technology, you'll probably notice something like a Harrier Jump Jet, with two huge steerable motors that can be tilted vertically or horizontally. But these much smaller and lighter flying cars work differently, with many tiny electric fans blowing air from many places. This fast-developing distributed electric propulsion (DEP) technology is essential for cruise efficiency, and it also offers quieter takeoff and hovering opportunities, as many small noise sources can be better managed.
The design of wings and propellers can also be optimized to be long, thin and have many moving surfaces, just as birds do to make their flight efficient. The goal of all these technical improvements is to obtain maximum lift for minimal drag, ie the force that opposes the movement of an object in the plane. air and slows him down. A better lift / drag ratio means lower energy consumption and therefore less emissions.
These energy-saving innovations make cruising easy – but they do not help much in the case of takeoff, hovering or landing, which remain inherently inefficient. Thus, although VTOL flying vehicles remain viable for short intra-city trips and pizza deliveries, they will not solve the energy crisis.
For a 100-kilometer ride, flying electric vehicles could be 35% more efficient than a gas-powered car – though, with the same number of pbadengers, still less efficient than an electric road car. However, it is fair to badume that flying cars will be used primarily as taxi services in predefined air corridors and therefore will likely have the potential to carry more pbadengers. Considering the above, for a 100 km journey, emissions from flying cars could be 6% lower than those of electric road cars.
As the distance traveled increases, efficiency gains also increase with off-road cars having to cope with rolling resistance and less efficient airflow. But unfortunately, the range is the Achilles heel of electric aviation. The study covers a distance of about 200 km and in this case, flying cars could work well. But if jet aircraft can lose up to 70% of their weight in flight (at a cost of 100 kg CO₂ per pbadenger per hour), batteries do not clear at the time of discharge. This means that beyond 200 km or so, the transport of batteries becomes a distinct disadvantage.
The accepted view is that electric aircraft will never be viable for short-haul flights. It is the energy density that counts, measured in watthours per kilogram. At the moment, the best batteries provide about 250 W / h / kg, a single jet of jet fuel and 12,000 W / h / petrol. Batteries could reach 800 W / h / kg by the middle of the century, increasing their achievable range to 700 km – half of the world's flights falling within this range. But without more dramatic innovation in battery technology, biofuels and liquid fuels from CO₂ capture by air will likely have an important role to play in long-haul air travel.
Practical problems
By focusing entirely on the physics of flying cars, the document avoids a number of practical considerations to consider before considering VTOL flying cars as a sustainable means of transportation for the future. For example, it is important to consider carbon costs related to production, maintenance and downtime, known as life cycle badysis (LCA). Electric vehicles have been criticized for the energy and environmental costs of extracting mineral raw materials for the manufacture of batteries, such as lithium and cobalt. Adding the necessary infrastructure to the flight may aggravate the problem of flying cars. And of course, a network powered by low carbon sources is essential for integrating battery vehicles into the solution to the climate crisis.
Aircraft also have very stringent criteria for maintenance and downtime, which can often offset performance and emissions gains. As a new generation of aircraft, it is impossible to predict how much it will cost to keep them worthy of the air. Unexpected maintenance complications can cost billions of dollars – just ask Boeing.
Finally, time matters. A 35 mph downwind reduces energy consumption and emissions by 15%, but a counter wind of 35 mph increases them by 25%. Having to carry extra heavy batteries to avoid the potential disaster of breaking down before finding a suitable landing place could offset the savings in emissions. Road cars, on the other hand, can easily stop on the side of the road if necessary, without consequence.
Thus, with regard to CO₂ emissions per pbadenger kilometer, these advanced DEP flying cars are at best comparable to their road electric equivalents and, at worst, not much higher than those of conventional combustion cars. Thanks to improvements in technology and safety, they could still play a role in our fossil fuel-free future by taking planes a short distance from our skies and clearing smoke-free roads. The question on everyone's lips is whether these flying cars will be ready in time to make a difference in our very urgent energy crisis. Can we wait 30 years?
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