Earlier I posted my gallery of CES gadgets, and included a photo of the eHang 184 from China, a "personal drone" able, in theory, to carry a person up to 100kg.

Whether the eHang is real or not, some version of the personal automated flying vehicle is coming, and it's not that far away. When I talk about robocars, I am often asked "what about flying cars?" and there will indeed be competition between them. There are a variety of factors that will affect that competition, and many other social effects not yet much discussed.

The VTOL Multirotor

There are two visions of the flying car. The most common is VTOL -- vertical takeoff and landing -- something that may have no wheels at all because it's more a helicopter than a car or airplane. The recent revolution in automation and stability for multirotor helicopters -- better known as drones -- is making people wonder when we'll get one able to carry a person. Multirotors almost exclusively use electric motors because you must adjust speed very quickly to get stability and control. You also want the redundancy of multiple motors and power systems, so you can lose a rotor or a battery and still fly.

This creates a problem because electric batteries are heavy. It takes a lot of power to fly this way. Carrying more batteries means more weight -- and thus more power needed to carry the batteries. There are diminishing returns, and you can't get much speed, power or range before the batteries are dead. OK in a 3 kilo drone, not OK in a 150 kilo one.

Lots of people are experimenting with combining multirotor for takeoff and landing, and traditional "fixed wing" (standard airplane) designs to travel any distance. This is a great deal more efficient, but even so, still a challenge to do with batteries for long distance flight. Other ideas including using liquid fuels some way. Those include just using a regular liquid fuel motor to run a generator (not very efficient) or combining direct drive of a master propeller with fine-control electric drive of smaller propellers for the dynamic control needed.

Another interesting option is the autogyro, which looks like a helicopter but needs a small runway for takeoff.

The traditional aircraft

Some "flying car" efforts have made airplanes whose wings fold up so they can drive on the road. These have never "taken off" -- they usually end up a compromise that is not a very good car or a very good plane. They need airports but you can keep driving from the airport. They are not, for now, autonomous.

Some want to fly most of their miles, and drive just short distances. Some other designs are mostly for driving, but have an ability to "short hop" via parasailing or autogyro flying when desired.

Robocars offer an interesting alternative. You can build a system where a robocar takes you from home to the best local short airstrip, taking you right out to an autonomous aircraft that is sitting waiting. You transfer, and it immediately takes off and flies you to another short airstrip, where another robcar awaits you. This allows you to travel in a car that's a car and a plane that's a plane, with no compromise.

The big challenges

Automating the intense level of safety and equipment reliability

In general, planes today are not fast modes of travel for their pilots. A typical small aircraft owner going out to fly has to drive to an airport that's not very convenient, park and get their plane. (If they planned ahead, the hangar crew has taken their plane out and done the basics on it.) Even with the prep, there is a fairly long pre-flight check to do, assuring everything is just so, checking fuel levels with your eyes as well as instruments and more. Then you go through a dance with the control tower, taxi around (possibly in line behind others) and eventually get to take off and start climbing. Only then are you on your way. At the other end, you do it all in reverse, tie down and hangar your plane, and find your way to a rental car or ground transportation. For trips of under 100 miles, it's not usually worth it.

Autonomous flying cars require more than just well built and superbly safe flying systems. (Flying itself is actually a pretty easy robotics problem.) It's all the other stuff that will be the challenge. Because failures of equipment while up in the air can be so dangerous, vehicles must be maintained and checked to a level that is orders of magnitude greater than what we do with cars. If your car engine conks out, you pull off to the side of the road. If your brakes go out, it's bad, but you apply the emergency brake and call a tow truck.

We'll demand fail-safe operation for all parts of the flying car. It will have to be able to lose any major component and get you down safely.

Noise

Problem number one for VTOL is noise. Helicopters are not anywhere near silent. You might crave one for yourself, but no way you'll accept your neighbours constantly flying helicopters in and out of their backyard, next to yours, at all hours. Not compared to the silence of the electric car.

Even if we have VTOL cars, we might still limit their operations (especially at night) to special landing yards. Your robotaxi could get you to the landing yard so it's not as much of a burden, but using your own yard (unless you have a large estate, or live in a high-rise building with heliport on top) is going to be difficult.

Energy & Range

Right now, multirotor aircraft use a lot of energy to fly. Ground cars can be much more efficient. Society as a whole is seeking to greatly improve the efficiency of our transportation, not make it worse. Unless we make the flying car super efficient, it will be relegated only to specialty use, where the ground car just won't work.

Fixed wing aircraft can be more efficient. Jets are very wasteful but lower speed aircraft can be efficient after takeoff.

The energy problem means short range for battery powered vehicles. However, combining robotaxis with autonomous short-range flying vehicles may mean that short range can be OK. A robotaxi takes you to a vehicle. It flies you 10-15 miles -- over congestion, bodies of water, mountains etc. -- to where you swap to another vehicle, or to a robotaxi if close to your destination. Battery swap allows quick re-use of the vehicles.

Another likely solution is electric multirotors for VTOL and conventional liquid fuel direct driven turbines or props for fixed wing flight. To be more green, the liquid fuel would be synthetic and created using renewable energy. Today that is costly but the cost could come down.

Crowded skies

If personal flight becomes very popular, we would face the prospect of a sky seriously crowded with the vehicles in urban spaces. Computer systems could probably handle management of the traffic, since in 3 dimensions you get extra room, though you want much longer headways than cars use. In addition to being a visible blight and a noise source, there will be some safety concerns. Even a tiny number of these vehicles falling out of the sky and hitting things (or people) on the ground will cause more concern than cars do, even though they depart the road and hit people. This would be added to the large traffic in cargo drones.

The traffic management is non-trivial, but I believe it can be solved. There are still issues even after it's solved.

Regulation

At first, these things will require pilot's licences -- and controls for pilots. In time, though, they will reach the safety level where use by the public is considered, though there may be questions for some time about their use in difficult weather (high winds, icing, storms.) The problem about weather restrictions is that people don't want to depend on something that has to shut down on certain days without warning. "Can't drive, too much crosswind" is something very rarely said.

Tourism

One of the places we might see radical change quickly is in tourism. If it's cheap and easy, tourists will want to see everything from a flying car, especially one that can hover. Every amazing view, every scene, every architectural wonder, every city, will probably be best viewed from the air, or certainly desirable to view from the air as well as the ground. Every hiking trail you've not taken to some interesting sight will become a potential place people would like to go in their flying car.

Outside the cities, the problems of the flying cars are less present. The flights will be short and slow. You can travel to special locations for takeoff and landing, and make noise there. The territory will be rural or parkland in many cases, with more modest crowds and nobody to fall on in the event of rare safety failures.

I predict this could be so popular that it would have to be restricted. The crowds of tourists in buses at many locations are already overwhelming them. Now offer everybody something like the eHang. You might forbid this in national parks, but legally it would be difficult to forbid it for the disabled. The disabled can't get up those trails, can't get access to those amazing viewpoints. And by the disabled, I mean all the elderly, who can claim that their physical condition prevents their access to a site. The issue, of course, is that a large fraction of tourists are the elderly, armed with free time and extra money. Tourism in flying cars would be a dream come true.

There are places in the world (such as above Mauna Loa's active volcano) that already have so much traffic from helicopter tours (at $200/person or more) that they need to have controlled airspace and specific flight patterns. Autonomous flying vehicles will be able to handle that concept, but still, the volume of traffic will be huge when anybody can take the ride for $10.

Public transport

Since we can't make a multirotor for a single person, talking about group vehicles is even more premature, but we already have lots of public transit aviation today. Right now it's done at airports, and never used for short distances because you spend far more time going through the airport than in the air. As I describe in A Robocar Airport it's possible to make a much more efficient airport even for traditional planes. It would be great to go further and imagine the "flying bus" -- an automated vehicle for a small group which is less like an airplane and more like one of the vans I describe in The Future of Transit. There, travel is coordinated and 10-20 single person robocars would converge within a minute of one another next to the autonomous flying bus. They would quickly get in -- no security for something this small and fast -- and within one minute be taking off down the runway.

Such a service might be better than things like high speed rail for travel in big cities. Because it can go from any airport to any airport -- or with VTOL from any landing yard to any landing yard -- such vehicles would offer superior travel times, free from congestion. If a flying bus service took you from Silicon Valley to San Francisco's ferry terminal in 15 minutes at a decent price, it would be quite popular and displace car traffic.

Non-urban

Mostly I've examined urban use (other than tourism) but in the comments the question of rural use is put. There, the issues of noise are mostly gone, and suddenly all sorts of properties are accessible that were not before. Travel can also be much faster, though usually at a cost in energy. The skies won't be as crowded.

Specialty uses

If we don't let everybody fly all the time, who will be the special cases we let fly? Will it simply be the rich who pay a high fee for the opportunity? (The fee can't be so high as to match the cost of a helicopter today.)

• The flying ambulance is an obvious win -- though we're not at the level of building electric multirotors that could fly something that heavy. The lack of emergency vehicles on the regular roads will also improve traffic for others.
• Some delivery will go by drone, though perhaps only the light and urgent packages.
• There could be a lottery or other allocation, letting people fly some days, but not most.
• Government officials will certainly want to claim they have the importance to justify this. In some cases (like VIPs so big they close roads for their motorcades) this is a win for all.
• The police will clearly do this, as will some portion of fire crews (those not carrying heavy gear.) Anybody who uses helicopters today.
• People who live or work in remote country locations who can make what noise they want at their home, and mostly fly over uninhabited country.
• People populating mountainsides in crowded cities, though possibly only to transfer to a robotaxi in the flatlands.
• People living on islands in seaside cities, though possibly only to transfer to a robotaxi on shore.
• Flying carpooling (above and beyond the transit described below.) This requires multi-person flying cars.

Many cities are along the water, even rivers. This could mean the water could be a corridor for flying car use, without the constraints that use over homes implies. In the San Francisco Bay Area, a robotaxi might get you close to the water for quick transfer to flying taxi to any other point on the shore, and robotaxi inland. New York might see the same.

If we don't allow anybody who can afford it to fly, who else might get the privilege? Answer in the comments.