Flying taxis still seem very much like something pulled right out of science fiction, but when transport heavyweights like Boeing, Airbus and Uber start pumping millions of dollars into their development, it might be time to take all of this a bit more seriously.

Beyond the cool factor of personal flight, electric flying taxis would have a profound impact when it comes to society, the economy and the environment. By reshaping how people move around cities they have the potential to disrupt conventional transport systems like highways, trains and buses, put a dent in pollution around urban centers and make for much faster commutes, therefore making society more efficient and productive as a whole.

The examples we look at here all have their own unique designs, and are at different points in their development, but they all promise to essentially do the same thing, which is move passengers through the air from point A to point B at a push of the button. Thanks to electric propulsion and autonomous navigation systems, they would have no operating emissions and no pilot and would generate minimal noise.

If these kinds of aircraft were to become commonplace, it would be a fundamental shift in how cities function. Although plenty of skepticism still abounds, somebody who needs no convincing of either their potential or impending arrival is Vikas Prakash, a professor of mechanical and aerospace engineering at Case Western Reserve University.

"I don't have any doubt," he says. "In a few years, you will be able to call an air taxi from Uber or someone else to travel maybe 100 miles in a vehicle with two other people. I'm very excited about this."

Prakash recently received a US$1.3 million grant from NASA to develop advanced batteries capable of powering such electric air vehicles. We put a few questions to him about how, when and why we might see these things in action.

Why do you think we are on the precipice of a flying taxi revolution?

"Driven by concerns about climate change, governments and companies worldwide are making plans for a post-oil era. While there have been efforts to limit carbon emissions in the aviation industry by using alternative fuels, as with the car industry, electrical propulsion seems to be the way forward for air travel.

"However, electric air-vehicles have more challenges than their land-based counterparts in terms of their onboard space and weight limits, impacting performance. Today's batteries pack much less energy per unit weight and volume when compared to jet fuel. Accordingly, the required battery packs are simply too heavy to maintain efficient flight capabilities. These energy limitations become especially acute in smaller crafts.

"State-of-the-art electric motors partly compensate this disadvantage by being more efficient in converting energy into power, and major industry players, research organizations and entrepreneurs are working on several possible paths to make commercial electric flying a reality."

Can you explain some of the recent technological advances that have made flying taxis viable?

"For air-taxis to serve as on-demand urban transit, they need to be safe, quiet, clean, and efficient. All electric air vehicles, which utilize battery propulsion over jet propulsion, are expected to have zero operational emissions, and be quiet enough to operate in cities without disturbing the neighbors. At flying altitude, noise from advanced electric vehicles is expected to be barely audible.

"Even during take-off and landing, the noise will be comparable to existing background noise. We also believe, successful air taxi design will not use the rotary-wing design of today's helicopters. Instead, it will be the vertical take-off and landing capabilities (VTOL) combined with distributed energy propulsion (DEP) that will make it possible for us to fly an air taxi from building to building.

"While rotary wing helicopters are the closest current-day proxy for the VTOLs, they are far too noisy, energy inefficient, and pollute too much to be economically viable for large-scale operations. Rather, we envision instead many smaller, electric motors distributed along the fuselage, as motor efficiency in this case does not benefit from a size increase, unlike todays large jet engines, positioned under the wings.

"Further, significant efficiency improvements are possible with DEP, since it enables fixed-wing VTOL aircraft to avoid the fundamental limitations of helicopter edgewise rotor flight during cruise, and provides lift with far greater efficiency than rotors. Also, these VTOL designs will also be markedly safer since VTOLs, unlike helicopters, do not need to be dependent on any single part to stay airborne and will ultimately use autonomy technology to significantly reduce operator error."

And which technological hurdles still stand in the way?

"Some of the biggest challenges in establishing a viable air-taxi industry are related to: (1) battery technology, including their energy and power densities, charge rate, and cycle life; (2) Successful development and FAA certification of VTOL and distributed energy propulsion (DEP) technology which is directly related to vehicle efficiency; (3) vehicle performance and reliability in varied weather conditions; (4) vehicle cost and affordability; (5) safety related to vehicle partial-autonomy navigating congested skies; (6) aircraft noise and air pollution, especially over populated areas; (7) landing and takeoff infrastructure including landing pads at key city locations to deploy a VTOL fleet."

What will the range of the first flying taxis be?

"An air taxi could be defined as a flying vehicle with a range of 50-120 miles (80 to 193 km), carrying two to four passengers and cruising at an altitude of 3,000 to 5,000 ft (914 to 1,524 m). In the near term, based on the current battery technology, the most-common commute might be a 50-mile (80-km) round trip with two short vertical takeoffs and a 30-minute energy reserve on a single battery charge."

How soon do you think we might see flying taxis in action?

"Urban airspace is open for business today, and with air-traffic control (ATC) systems exactly as they are, a VTOL service could be launched and even scaled to possibly hundreds of vehicles. However, a successful, optimized on-demand urban VTOL operation will necessitate a significantly higher frequency and airspace density of vehicles operating over metropolitan areas simultaneously.

The current air traffic control will have to evolve and new ATC systems will be needed to handle these extra crafts, especially if a city were to add multiple hubs and potentially hundreds of air taxis. It will be the government policy in the end that will dictate when we will see flying taxi fleets."

The contenders

The list of air taxi concepts presented here is by no means exhaustive, with plenty of projects from startups and aviation incumbents that promise to shake up the way folks move through urban centers. But these particular examples have caught our attention over the past couple of years for different reasons, be it through manned test flights, powerful partnerships or huge investments from notable parties.

UberAir

Uber first revealed plans for its flying taxi service in a 97-page white paper in 2016. It plans to use sets of small electric rotors to power aircraft with two or four seats. These would take off vertically but then convert to some sort of horizontal flight cruise mode with tilting wings or rotors, saving on energy in the process as they travel along fixed routes between "Skyports."

In November last year, Uber formed an agreement with NASA to develop an unmanned traffic management system to deal with all the expected air traffic. It also announced plans to start testing flying taxis in Dubai and Dallas in 2020, and has recently added Los Angeles to the list. It is referring to the service as UberAir for now.

Airbus Vahana

Airbus launched its Vahana project around two years ago, and is making the sort of progress you might expect from some of aviation's more experienced heads. A full-scale version of the electric, autonomous VTOL aircraft completed its first ever test flight earlier in February, lifting into the air for 53 seconds and reaching an altitude of five meters (16 ft). With its first vertical takeoff and landing proving a success, the team is now planning tests where it transitions from vertical to horizontal flight.

Workhorse Surefly

We first caught wind of the Workhorse Surefly air taxi during the Paris Air Show last June, and last month it received an Experimental Airworthiness Certificate from the Federal Aviation Administration (FAA) to kick off test flights last month at CES. Workhorse also has a partnership with global shipping giant UPS, and the engines used in the Surefly are the same ones used in BMW's i3 and C600 super scooter. It hopes to receive full FAA approval in late 2019.

Volocopter

Of all the flying taxi concepts doing the rounds, the 18-rotor electric Volocopter is the one that seems to garner the most attention. In its current iteration, it can fly for 30 minutes at a time and hit a top speed of 100 km/h (62 mph). Since emerging as a pretty out-there project back in 2013, it has completed manned test flights in Germany, unmanned flights over Dubai and, last month at CES in Las Vegas, a short onstage test flight in front of a live audience. That marked its first flight in the US, and the city of Dubai is trialing it over the next five years as part of a service called the Autonomous Air Taxi.

Autonomous Passenger Drone

Just announced in September, the Autonomous Passenger Drone is built form carbon fiber composites and uses 16 electric rotors to get airborne. It can be flown manually with a joystick if need be and features two seats, one behind the other. Its makers released videos accompanying the announcement that show the craft already completing manned test flights. So, although it certainly wasn't the first flying taxi on the scene, it is certainly seems further along in its development than some competitors.

Ehang 184

Ehang burst onto the scene at CES in 2016 with a prototype of its 184 flying taxi, but has been relatively quiet since then. Like the Workhorse Surefly, it has received an Experimental Airworthiness Certificate from the FAA, but has done little to update us on its progress in the meantime.

It broke its silence this month by revealing footage showing people riding aboard the Ehang 184 for the first time. These test flights purport to show the aircraft being put through its paces in a force seven typhoon, navigating heavy fog, climbing to an altitude of 300 m (1,000 ft) and completing a long-range test flight of 8.8 km (5.5 mi). Like the Volocopter, the Ehang 184 is set to be trialed in Dubai over the coming years.

Joby

Joby Aviation's multirotor convertible aircraft might be the most eye-catching of the concepts outlined here, simply because it's the biggest departure from the conventional rotary-wing helicopter. The custom-designed tilt system sees the rotors spin horizontally during takeoff and landing, and then turn 90 degrees for low-energy forward flight just like a fixed-wing aircraft.

We have paid a visit to Joby Aviation's headquarters and left with the firm belief that this thing is absolutely happening. Intel and Toyota are also convinced, who along with other investors just handed the company $100 million to continue development of the aircraft.

Lilium

Lilium Aviation completed its first unmanned test flights of a two-seater version of its electric VTOL jet in early 2017, and promptly turned its attention to a larger five-seat production version. It says this will have a massive range of over 300 km (186 mi) and a top speed of 300 km/h (186 mph). In September last year, it raised US$90 million in fresh funding to forge ahead with its work, and is targeting 2019 for its first manned flights.

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