On April 20, 2017, a two-seat prototype of the "Lilium Jet," the first zero-emission electric airplane capable of Vertical Take-Off and Landing (VTOL), completed a series of rigorous flight tests in the skies above Germany. It is the only electric aircraft capable of both VTOL and jet-powered flight, using its wings for lift, similar to a conventional airplane.

This advanced capability consumes around 90 percent less energy than multi-rotor drone-style aircraft, like the eHang 184. This will enable the five-seat production version of the Lilium Jet to achieve a range of more than 200 miles with a maximum cruising speed of 200 miles/hour. In flight, the Jet's power consumption per mile will be comparable to a five-passenger electric car.

Take-off and landing for a Lilium Jet only requires a small open space or a landing pad on a building, alleviating traffic on congested roads.

Lilium is a startup created by a team out of the Technical University of Munich. In December 2016, it received 10 million euros in funding from London-based venture capital firm, Atomico. The plan is for a network of inexpensive landing pads, in-and-around congested cities. The business plan uses the mass produced five-seat vehicles to provide a ride-hailing service for people needing to get places where traffic is a problem. The Lilium website gives an example of a Manhattan to JFK taxi trip now involving 55 minutes and costing $56-to-73, depending on traffic. But, using the Lilium service, it will only take five minutes. Initially the cost would be $36, but eventually it should drop to $6.

This is a very big deal!

Imagine traveling from San Francisco's Marina to work in downtown San Jose-a drive that would normally occupy the better part of two hours-in only 15 minutes. What if you could save nearly four hours round-trip between Sao Paulo's city center and the suburbs in Campinas? Or, imagine reducing a 90-plus minute stop-and-go commute from Gurgaon to an office in central New Delhi to a mere six minutes. More importantly, what if the total cost was even cheaper than today's drive.

Every day, millions of hours are wasted on the road, worldwide. Last year, the average San Francisco resident spent 230 hours commuting between work and home-that's half a million hours of productivity lost every single day. In Los Angeles and Sydney, residents spend seven whole working weeks each year commuting, two of which are wasted unproductively stuck in gridlock. In many global megacities, the problem is even more severe: the average commute in Mumbai exceeds a staggering 90 minutes. For all of us, that's less time with family, less time at work growing our economies, more money spent on fuel-and a marked increase in our stress levels. A study in the American Journal of Preventative Medicine, for example, found that those who commute more than 10 miles were at increased risk of elevated blood pressure.

If Lilium was the only company working on this problem, this would just be another blip on our technology radar. But, Lilium is not just a "blip." It's part of a growing swarm that is about to engulf and radically transform transportation in the twenty-first century.

Since shortly after the invention of the airplane, visionaries have dreamed of flying rather than driving. In the 1920s, Henry Ford commissioned a team at the Ford Motor Company to create a tiny single-seat commuter plane called the "Ford Flivver." In the 1960s, "Saturday morning TV" featured the twenty-first century Jetson family in their own flying car. In the '70s, inventor Paul Moller began experimenting with small VTOL aircraft, which led to prototypes of his Skycar beginning in the 1980s.

But, until recently, safe, economically feasible Personal Air Vehicles (PAVs) seemed impossible.

Then, in 2015 a few forward-thinking professionals at NASA presented a preliminary plan for making airborne commuting a reality. That plan, highlighted in the November 2015 issue of Trends, was so exciting that Uber immediately embraced it, hired its principal author away from NASA, and launched an initiative called Uber Elevate.

Rather than create its own PAV, Uber's role is that of champion and promoter of a broader PAV business eco-system. As such, it sees itself making money by creating one of the first and largest "on-demand aviation services," resembling its on-demand ride service.

On-demand aviation has the potential to radically improve urban mobility; giving people back time lost in their daily commutes. And just as sky- scrapers allowed cities to use limited land more efficiently, urban air transportation will use three-dimensional airspace to alleviate transportation congestion on the ground.

A network of small, electric VTOL aircraft would enable rapid, reliable transportation between suburbs and cities and, ultimately, within cities. And according to Uber, this vision is achievable with- in the coming decade if the key actors in the VTOL ecosystem, including regulators, vehicle designers, communities, cities, and network operators, collaborate effectively.

NASA and the FAA recently spear- headed a series of On-Demand Mobility (ODM) workshops to bring the VTOL ecosystem together to identify barriers to launching an on-demand VTOL service. This includes emerging PAV vehicle manufacturers, federal agencies, private investors, professional societies, universities, and international aviation organizations.

They align quite well with the following 11 challenges identified in a landmark whitepaper published by Uber in October 2016:

1. The Certification Process. Before VTOLs can operate in any country, they will need to comply with regulations from aviation authorities. The US Federal Aviation Administration, (FAA), and European Aviation Safety Agency (EASA) are charged with assuring aviation safety; they regulate 50 percent and 30 percent of the world's aviation activity, respectively. VTOL aircraft are new from a certification standpoint, and progress with certification of new aircraft concepts has historically been very slow, though the process is changing in a way that could accelerate things significantly.
2. Battery Technology. Electric propulsion has many desirable characteristics that make it the preferred propulsion choice for the VTOL PAV aircraft contemplated for on-demand urban air transportation. For this purpose, batteries are the default energy source. That said, the amount of energy per unit weight provided by the battery, which ultimately determines the gross weight of the vehicle, of batteries today is insufficient for long-range commutes. Additionally, the speed at which the battery can be brought back to a nearly full charge, which determines operational idle time, is also currently too slow to support high-frequency ride-sharing operations. Furthermore, the number of charge/discharge cycles the cell can sustain before its capacity is less than 80 percent of the original and the cost per kilowatt-hour are also important to the economic viability of electric aircraft.
3. Vehicle Efficiency. Helicopters are the closest present-day proxy for the VTOLs dis- cussed considered here, but they are far too energy inefficient to be economically viable for large-scale operations. Helicopters are designed for highly flexible operations requiring vertical flight. With a more constrained use-case focused on ridesharing, a more mission-optimized vehicle is possible; for example utilizing distributed electric propulsion, or DEP, technology with many independent engines. Large efficiency improvements are possible because DEP enables fixed-wing VTOL aircraft that avoid the fundamental limitations of helicopter horizontal flight, because wings provide lift with far greater efficiency than rotors. Lilium is the first vehicle manufacturer to demonstrate a commercially viable aircraft featuring DEP. Test flights of others DEP designs are expected soon.
4. Vehicle Performance and Reliability. Saving time is a key aspect of the VTOL value proposition. In the ridesharing use case, it's critical to minimize the time elapsed between request and drop-off. This will be affected by cruise speed plus takeoff and landing time. It will also be influenced by system reliability, which Uber defines as the average time from request until pick-up. In this context, key challenges include PAVs designed for 150-to-200 mph cruise speeds and maximum one-minute takeoffs and landings. The PAV designs must also incorporate robustness in varied weather conditions, which would otherwise ground a large percentage of a fleet in an area at arbitrary times. Both the Lilium Jet and the Carter Copter have flying prototypes that could meet these challenges.
5. Air Traffic Control (ATC). Today, urban airspace is already technically open for business. And, with ATC systems exactly as they are, a VTOL service could be launched and even scaled to possibly hundreds of vehicles. Sao Paulo, for example, already has hundreds of helicopters flying through the sky every day. Under visual flight rules (VFR), pilots can fly independent of the ATC and when necessary, they can fly under instrument flight rules (IFR) leveraging existing ATC systems. However, a successful, optimized on-demand urban VTOL operation will require a significantly higher frequency and airspace density of vehicles operating over metropolitan areas simultaneously. In order to handle this exponential increase in complexity, new ATC systems will be needed. Uber's plan envisions low-altitude operations being managed through a "server request system" that can deconflict the global traffic, while allowing automated drones and VTOLs to self-separate, avoiding any potential local conflicts using rules much like existing VFR principles. There are promising initiatives underway, but this is one area that may ultimately bottleneck growth.
6. Cost and Affordability. While helicopters are the closest commercial proxy to the VTOLs contemplated by Uber, they are prohibitively expensive to operate as part of a large-scale transportation service. They are energy-inefficient and very expensive to maintain, and their high noise levels seriously limit use in urban areas. Because of these factors, demand for helicopters is modest. This translates into low manufacturing volumes; in fact, worldwide civil rotorcraft production is only approximately 1,000 units per year. And because it lacks critical economies of scale prices remain extremely high. The simpler, quieter, and more operationally efficient PAV designs will substitute digital control for mechanical complexity. This shift can kickstart a virtuous cycle of cost and price reduction as the industry follows an evolutionary pathway to a mass market based on affordable vehicles and operations.
7. As Uber sees it, VTOL PAV aircraft can and should be safer than driving a car on a fatalities-per-passenger-mile basis. Today, Federal Aviation Regulation Part 135 operations for commuter and on-demand flights, on average, have twice the fatality rate of privately operated cars. However, analysis shows that this rate can be lowered for VTOL PAV aircraft to 25 percent of the average Part 135 rate, or lower. That would make PAVs twice as safe as driving a car. DEP and flight automation are key pieces of the safety equation.
8. Aircraft Noise. For urban air transportation to thrive, the vehicles must be acceptable to communities, and vehicle noise plays a significant role. The objective is to achieve low enough noise levels that the vehicles can effectively blend into back- ground noise; UBER's ultimate target is for a VTOL PAV to be one-half as loud as a medium-sized truck passing a house. That said, a more sophisticated measure of "noise" is required in order to properly characterize the impact of vehicle sound on a community. Electric propulsion will be critical for this objective, as well: it enables ultra-quiet designs, both in terms of engine noise and propulsor thrust noise. One part of this solution may be implementing "noise-cancelling controllers" to synchronize multiple fans.
9. VTOL PAVs would represent a new mass-market form of urban transportation; as such, they should be ecologically responsible and sustainable. When considering helicopters as the starting point, there is a substantial opportunity to reduce emissions. Among the advantages of battery-based propulsion designs is that they have zero operational emissions. However, that leaves electricity generation, which today is still largely coal, natural gas and petroleum-based.
10. Vertiport/Vertistop Infrastructure in Cities. The greatest operational barrier to deploying a VTOL fleet in cities is a lack of sufficient locations to place landing pads. Even if VTOL PAVs were certified to fly today, cities simply don't have the necessary takeoff and landing sites for the vehicles to operate at fleet scale. A small number of cities already have multiple heliports and might have enough capacity to offer a limited initial VTOL service, provided these are in the right locations, readily accessible from street level, and have space available to add charging stations. But if VTOLs are going to achieve close to their full potential, infrastructure will need to be added.
11. Pilot Training. Training to become a commercial pilot under FAR Part 135 is a very time-intensive proposition, requiring 500 hours of pilot-in-command experience for VFR and 1200 hours for IFR. As on-demand VTOL service scales up, Uber expects the need for pilots to rapidly increase, and with existing training requirements, a shortage in qualified pilots would curtail growth significantly. Fortunately, pilot augmentation technology will significantly reduce pilot skill requirements, and this could lead to a commensurate reduction in training time.

Over the next eight to ten years, on-demand aviation has the opportunity to burst forth in much the same way automobiles did between 1910 and 1920 or the World Wide Web did in the '90s. The core technologies and the value proposition are in place; we just need to get out of the way and let it happen.

Given this trend, we offer the following forecasts for your consideration:

First, on-demand aviation will go a long way toward closing the infrastructure gap.

As discussed earlier, without a major infrastructure initiative, the United States is expected to suffer a $1.4 trillion infrastructure deficit by 2025. Even the proposed bi-partisan program will not fully close this gap. Fortunately, the development of infrastructure to support an urban VTOL network will likely have significant cost advantages over heavy-infrastructure approaches such as roads, rail, bridges, and tunnels. It has been proposed that the repurposed tops of parking garages, existing helipads, and even unused land surrounding highway interchanges could form the basis of an extensive, distributed network of VTOL hubs ("vertiports") with multiple takeoff and landing pads, as well as charging infrastructure or single-aircraft individual VTOL pads with minimal infrastructure, ("vertistops"). As costs for traditional infrastructure options continue to increase, the lower cost and increased flexibility provided by these new approaches may provide compelling options for cities and states around the world.

Second, by the late 2020s, the economics of manufacturing VTOL PAVs will become more akin to automobiles than aircraft.

Initially, of course, VTOL PAVs are likely to be quite expensive, but because the ridesharing model amortizes the vehicle cost efficiently over paid trips, that initial high cost should not end up being prohibitive. Once the ridesharing service commences, a positive feed- back loop will ensue that ultimately reduces costs and prices for all users. That is, as the total number of users increases, the utilization of the aircraft increases. Logically, this continues with the pooling of trips to achieve higher load factors, and this lower price will feed back to drive more demand. That will increases the volume of aircraft required, which in turn drives manufacturing costs down. Beyond this learning curve, savings will kick in lowering manufacturing costs even more.

Third, simultaneous adoption of self-driving cars will make the transitions to fully automated flight quite manageable.

During the start- up phase, on board pilots will be common and "remote control pilots" will be involved on other fights. However, as consumers and regulators become increasingly familiar and comfortable with self-driving cars, this technology will be implemented in PAVs.