Urban air mobility and flying cars: Overview, examples, prospects, drawbacks, and solutions

3.1 UAM

As the name implies, UAM is a transportation technology that targets urban areas (cities and attached suburban zones) using routes in air. This implies a low-altitude flight [9] because using high altitudes means that time and vehicle energy are lost in large initial ascending and final descending phases, whereas the main goal is horizontal translation during the cruise phase. The following definition is proposed for UAM:

• UAM is a transportation system, which uses low-altitude aircraft that can be either human piloted or autonomous to move passengers or cargo within cities or between cities or suburban zones using aerial routes.

There are other terms related to UAM, such as advanced air mobility (AAM), which is an extension of UAM [10], either geographically or functionally beyond urban transportation, such as serving rural areas and being used in tourism (sightseeing). The term autonomous aerial vehicle (AAV) may also overlap with the aircraft used in UAM. It emphasizes the situation when the flying vehicle does not have a human pilot on board; however, it can still have passengers [11]. The term unmanned aerial vehicle (UAV) or drone is a special case of AAV, where the flying vehicle neither has a pilot nor has passengers. UAVs may also be referred to as unmanned aerial systems (UASs), remotely operated aircraft (ROA), or remotely piloted aircraft (RPA) [12]. The term personal aerial vehicle (PAV) overlaps with UAM. PAV may concern the case where the small aircraft is carrying passengers [13] or when it is a private property, owned by a person. The term electric vertical takeoff and landing (eVTOL) is also an important term in UAM because UAM aircraft commonly (but not always) follow this type of flight, being electrically powered by batteries and electric motors, and able to take off and land vertically without the need for a long runway [14]. Using a helicopter as a UAM aircraft excludes it from the eVTOL category because it is not electrically powered; however, it still has a vertical takeoff and landing capability (VTOL) [15].

Al Haddad et al. [16] realized the lack of understanding of users’ perceptions of UAM. They conducted a study with a stated-preference survey for assessing this perception. They found that aspects such as affinity to automation, safety and trust, and social attitude are important.

Straubinger et al. [17] described UAM as a novel mobility concept, enabling the use of passenger drones for on-demand transport in an urban environment. They investigated the distribution of UAM impacts while considering the spatial dimension (suburbs versus cities) and the low-skilled and high-skilled households, by comparing certain socioeconomic variables with and without UAM. Their study covers US-based cities and Europe-based cities. It utilized a numerical simulation with a developed model. Some of their model parameters (such as the working hours per day and the gasoline price) were based on reliable sources. A simulation benchmarking compared model results with empirical values from different sources. As an example for the provided results, wages are expected to nearly remain unchanged after introducing UAM, and this applies to both types of cities (US based and Europe based), both classes of workers (high skilled and low skilled), and both spatial settings (cities and suburbs).

Pukhova et al. [18] considered UAM as synonymous with “flying taxis,” which is a reasonable view given that they represent a key potential application of UAM. They developed an agent-based travel demand model for simulating UAM demand around Munich in Germany. Their results suggest that adopting the flying taxis mode of transportation in a metropolitan region with the existing road network and public transit services may not lead to any remarkable time saving. Their model also shows that switching to UAM for transport does not lead to reduction in kilometers traveled by car, which even increase by 0.3% after including access and trips to and from the vertiports (special airports for vertical takeoff and landing, to be used for UAM aircraft). Their study is important in demonstrating the competition expected between UAM and the existing conventional modes of transit.

Biehle [19] conducted a comprehensive review study about the impact of passenger urban air mobility (pUAM) on the existing urban transportation systems, focusing on Europe. The study addressed social and economic perspectives, through analyzing the expected acceptance of the public to this aerial transportation system (considering safety and noise), its affordability (compared to the existing classical modes of transportation), its inclusivity (suitability to mobility-impaired groups such as pregnant women), its accessibility (being effectively connected to all districts in an urban area), and the level of customer satisfaction (with potential flight issues such as response to a gust wind). Interestingly, the study showed an upsurge in the predicted number of electric aircraft for passengers’ transportation in Europe by 2050, jumping from 10,000 in 2016 to 160,000 in 2020. Due to the large number of passenger drones to be in the urban airspace, specialized support services should also be running in parallel to ensure safety and a favorable environment. In general, this concept or framework is called unmanned air traffic management or unmanned aircraft system traffic management [20,21,22]. In Europe, the regional term is U-space [23].

3.2 FCs

An FC can operate in two modes: (1) a traditional road car and (2) a small aircraft. It can be also called a roadable aircraft. The following definition is suggested for a flying car:

• A flying car is a convertible powered wheeled vehicle that can drive on public roads, be parked in normal parking zones, and uses normal car fuels (if it has an internal combustion engine), while also having the ability to operate as a small aircraft.

A key difference between UAM aircraft and FCs is the use of roads as normal routes for ground transportation (excluding transient short movements like preparing for takeoff). In other words, the presence of a road-driving mode of operation makes the aerial vehicle a flying car. Conversely, UAM vehicles are aircraft, and they are intended to transport through flying only, not through traveling on the ground. The provided distinction between UAM aircraft and FCs is not absolutely agreed upon. FCs may be viewed as a special type of UAM vehicle [24], which can contribute to fulfilling the broad UAM mission. This view is respected and has a logical justification, despite not being followed in the present work.

The idea of a transportation machine that combines two modes of transportation (ground transportation as a car and aerial transportation as an aircraft) goes back about one century, as attributed to the American aviator and noted motorcycle builder and racer Glenn Curtiss. He made a prototype of what was called “autoplane,” with three detachable wings, and it was tested in 1917. It was able to make a few short hops, but did not perform a successful flight [25]. The realization of the concept was forecasted long time ago in 1940 by the founder of the American Ford Motor Company, Henry Ford [26]. The earliest dedicated project for building an FC was by the American company Moller International for a model called Moller M400 Skycar, and the project started in 1983. After many years, an experimental car was eventually built, with its first flight being in 2001. After several flights in restricted environments, it was not found successful for deployment in the market and lacked the approval of flying operation from the United States Federal Aviation Administration [27].

Postorino and Sarné [28] reported that ‘flying vehicles’ (as a more general term than FCs) can reduce travel time compared to conventional ground-only transportation, but this benefit can vary largely depending on the average transportation distance and the number of traveling vehicles between origin/destination locations of a trip. They performed an analysis using simulations of an agent-based model and obtained time savings that ranged from 18 to 71% while considering 12 different scenarios.

Pan and Alouini [29] indicated that FCs offer an opportunity to exploit the near-ground space for transportation. They listed advantages of the FC transportation system (FCTS), which include the possibility of flexible and fast door-to-door transportation. They classified FCs based on four design aspects, which are as follows: (1) take-off and/or landing direction (horizontal or vertical), (2) piloting modes (human piloted, self-piloted, and hybrid), (3) operation modes (airplane, helicopter, and car), and (4) power types (electric, fuel, and hybrid). These design aspects affect the flexibility, comfort, stability, complexity, and environment friendliness of FCs. They also discussed different design issues related to FCTS and challenges that may appear during developing and running FCTS, including safety, commercial performance, and ethical perspectives. For example, they emphasized the importance of the ability of FCs to withstand extreme weather conditions such as thunderstorms and heavy winds, taking this into consideration during the design process.

4.1 Common challenges

Challenges commonly affecting UAM and FCs include:

• Regulation (certification of vehicle by the local authority, either civil or military)

• Pilot training and certification (if there is a pilot aboard)

• Safety standards

• Social resistance

It is possible that a country does not yet have a set of rules that fits the two emerging technologies, which means a new set of rules should be established or existing ones should be adapted. An uncontrolled failure in a flying object within a city can lead to serious damage to properties or personnel. High standards of safety are needed, with redundancy in the powering source and ballistic parachute system (BPS), which forcefully ejects the parachute canopy, thereby resulting in a rapid deployment [30]. Social resistance may stem from concerns about safety, and also the potential breach of privacy by flying at low altitudes over fenced homes with open spaces.

4.2 UAM

It is here assumed that UAM vehicles are not mainly privately owned, but aimed for commercial operation by an operator company. With this assumption, two more challenges for UAM are added as follows:

• Approval of operators (if intended as commercial businesses)

• Public infrastructure (designated landing locations)

4.3 FCs

Compared to an average car, FCs are expected to be expensive, given the more-complex design. This poses a constraint, at least initially before mass production is realized, leading to limiting the technology to wealthy owners. So, the following challenge is added for FCs:

• Profitable sales volume

6.1 EHang 216 by EHang

EHang Holdings Limited is a Chinese company that is world’s leader in AAVs. It is headquartered in the port city of Guangzhou [33], the capital of Guangdong province, in southeast China. The company’s key model is EHang 216, which is an eVTOL aircraft having two seats.

On Tuesday, June 24, 2020, EHang announced that EHang 216 completed its first official flight in South Korea in three locations: Seoul, Daegu, and Jeju Island [34]. In the following year, on Friday, June 4, 2021; EHang announced that EHang 216 completed a successful autonomous flight test in Japan, after obtaining a trial flight permit issued by the Japanese Ministry of Land, Infrastructure, Transport and Tourism [35]. EHang 216 reached a mature stage of development and has entered the commercial sales phase with 3 units sold in 2018, 60 units in 2019, and 70 units in 2020 [36,37]. EHang launched other models such as EHang 116 (single seat) and EHang 184 (single seat); however, EHang 216 (two seats) is the dominant model as of 2021, with a unit cost of about $330,000.

Figure 1 illustrates the exterior view of EHang 216. There are eight arms extending from the bottom of the cockpit, and each arm carries two propellers at its outer end. The eight propeller pairs are thus distributed around the cockpit. Figure 2 shows an EHang 216 unit during a flight test.

Figure 1

An illustration of the EHang 216 UAM aircraft (image credit: EHang Holdings Limited, used with permission).

Figure 2

A real view of the EHang 216 UAM aircraft during a flight test (image credit: EHang Holdings Limited, used with permission).

Table 1 lists some characteristics of the EHang 216 UAM aircraft.

6.2 VoloCity by Volocopter

The VoloCity UAM model by the German company Volocopter GmbH is an eVTOL aircraft, aimed to operate as a two-seat air taxi [38]. Volocopter GmbH was founded in 2011. It has offices in Bruchsal and Munich in Germany, as well as in Singapore. The company attracted multiple investors like the German automotive manufacturer Mercedes-Benz Group AG (formerly Daimler AG), Zhejiang Geely Holding Group (Chinese automotive company, the parent of Volvo Cars), and Intel Capital (a division of Intel Corporation that invests in innovative new businesses). VoloCity has flown in the center of Singapore. The aircraft has two seats. Commercial operation of VoloCity is expected to start with piloted flight (one pilot, one passenger) before shifting to autonomous flight (no pilots, two passengers).

VoloCity adopts redundancy in the battery and the propellers. Losing one or two out of the nine battery packs is tolerable. Similarly, if 1 or 2 out of the 18 propellers fail, a flight can continue safely.

Figure 3 shows a real view of an experimental unit of the VoloCity aircraft. Figure 4 shows the close-up of its cockpit. Figure 5 is an imagined view of that aircraft during flight.

Figure 3

A real model of Volocopter’s VoloCity air taxi, aimed for commercial UAM services (image credit: Volocopter GmbH, used with permission).

Figure 4

A real view of the cockpit of Volocopter’s VoloCity (image credit: Volocopter GmbH, License(s): Zur freien Verwendung – for free use, used with permission).

Figure 5

A rendering for Volocopter’s VoloCity while flying over Singapore (image credit: Volocopter GmbH, License(s): Zur freien Verwendung – for free use, used with permission).

Table 2 lists some characteristics of the VoloCity UAM aircraft.

7.1 PAL-V Liberty by PAL-V

In 2008, the Dutch company PAL-V International B.V. was founded. PAL-V stands for “Personal Air and Land Vehicle” [43]. The company is located in the town of Raamsdonksveer in the Netherlands, with representatives in all the six inhabited continents (as of September 2022).

With the country of registration being the Sultanate of Oman, the company offers two models: PAL-V Liberty ME Sport Edition and PAL-V Saqer Edition. As of October 2021 (and again in September 2022), the published expected price by the company for PAL-V Liberty ME Sport Edition was $649,000, while PAL-V Saqer Edition has a higher expected price of $799,000. The Saqer Edition is made specifically for the Middle East and North Africa (MENA) region. For the Netherlands or Germany as the country of registration, the company offers PAL-V Liberty Sport Edition with an expected price of €299,000, and PAL-V Liberty Pioneer Edition with an expected price of €499,000. The PAL-V Liberty Sport model is the standard edition of the PAL-V Liberty flying car, while the PAL-V Liberty Pioneer offers a level of personalization, with a limited number of vehicles to be made under this edition.

Figure 6 shows a view of the PAL-V Liberty Sport model with the rotor (at the top) and the propeller (at the back) being extended, while Figure 7 shows the vehicle in the contracted configuration (folded rotor and propeller). Figure 8 shows an interior view of the vehicle.

Figure 6

PAL-V Liberty Sport from outside with extended rotor and propeller (image credit: PAL-V International B.V., used with permission).

Figure 7

PAL-V Liberty Sport from outside with folded rotor and propeller (image credit: PAL-V International B.V., used with permission).

Figure 8

PAL-V Liberty Sport from the inside (image credit: PAL-V International B.V., used with permission).

PAL-V Liberty (regardless of the edition) has three wheels and two seats. It does not have a wing. It adopts a gyroplane (gyrocopter) concept for the flight mode, which means it combines a helicopter rotor and an airplane propeller. After the airplane is lifted in the air, the rotor is not powered. It rotates as a reaction to the airflow through it.

Table 3 lists some characteristics of the PAL-V liberty sport edition.

7.2 ASKA™ by NFT Inc.

ASKA™ eVTOL drive-and-fly vehicle for consumers is an envisioned product from the American company NFT Inc., which is doing business as ASKA. The company is located in the city of Los Altos, in the San Francisco Bay Area, in California [44].

The flying car is designed with four wheels and four seats. Two propellers are attached to the wing, while the four remaining propellers are attached to four retractable arms. The initial version (signature model) product has a price of $789,000 (the same price was announced in October 2021 and in September 2022). Pre-orders were made possible through a deposit of $5,000, which also enables a share equity in NFT Inc. (if certain legal eligibility conditions are satisfied). This deposit is fully refundable after a year. A full-scale prototype was made (without flight testing). Full-scale flight testing is expected by early 2023, while a small-scale pilotless prototype was already used in flight testing. The product delivery is anticipated by 2026. It is worth mentioning that the word Aska or Asuka is synonymous with (flying bird) in Japanese when uttered [45].

Figure 9 shows how the flying car ASKA™ may look like, with the wing and propeller-carrying arms extended. Figure 10 shows how it may look when the wing and propeller-carrying arms are folded. Figure 11 illustrates the orientation of the wing during landing, where the wing tilts to make its two attached propellers in a vertical orientation, generating an upward lift force rather than a forward thrust force. On the other hand, the other four propellers not attached to the wing are always in a vertical orientation when operating, thereby contributing to the lift force to counteract the weight.

Figure 9

An imagined view of the ASKA flying car with the wing and propeller-carrying arms extended (image credit: NFT Inc., used with permission).

Figure 10

An imagined view of the ASKA™ flying car with the wing and propeller-carrying arms folded (image credit: NFT Inc., used with permission).

Figure 11

An imagined view of the ASKA™ flying car during landing (image credit: NFT Inc., used with permission).

The proposed design for this flying car featured the idea of in-wheel motor, with an electric motor attached directly to the wheel itself. This frees some of the interior space to accommodate four seats and also improves the driving experience.

Table 4 lists some characteristics of the designed ASKA™ flying car.

8.1 eVAT model

Based on an insight article dated May 2019 [46] by a team at Deloitte Development LLC, a member company of the London-based international network of professional services Deloitte; people or cargo transportation through UAM at a large scale seems to dominantly utilize eVTOL aircraft, with hybrid-electric vertical takeoff and landing aircraft also considered candidates among the service fleet. In the envisioned UAM system, the MaaS business model was assumed. Instead of privately owned aircraft, these UAM aircraft operate as flying taxis with centralized management under a commercial operator. This section focuses on this business model of UAM, with eVTOL flying taxis (or electric vertically-flying air taxis (eVAT)). The term “air taxi” here is suitable particularly for passenger-oriented UAM. However, cargo shipping can be included within the offered MaaS model. The term “air taxi” here is still used, bearing in mind that it may actually be referring to an aircraft doing air delivery of a shopped item, a document, or a parcel.

The use of fully electric aircraft is advantageous from an environmental perspective due to being zero-carbon-ready (ZCR) [47], which means that there are no emissions of carbon dioxide released directly from them during their operation, since they have electric motors rather than internal combustion engines. However, reaching a zero-carbon status (a better environment-friendly status beyond ZCR) demands that the electricity used for charging the on-board batteries comes from a clean energy source that does not release polluting carbon dioxide (or other greenhouse gases), such as solar energy or wind energy. As a benchmarking value, the average carbon-dioxide emissions from new passenger cars in Europe was 122.3 g CO₂/km in 2019 [48]. According to a released document in 2018 [49], the US Environmental Protection Agency reported an emission rate of 404 g CO₂/mi for the average vehicle in the United States, which is equivalent to 251 g CO₂/km. Thus, an eVTOL UAM system can prevent the release of a large amount of CO₂ to the atmosphere if that UAM system is successful in replacing trips made by engine-powered private cars with trips made by zero-carbon (ZC) UAM aircraft.

In a subsequent study [50] from Deloitte Development LLC, more than 35,000 persons of driving age in 20 countries were surveyed during October 2019 (using an online questionnaire, translated into the local language) to give their opinion about various transportation issues, including UAM. In that study, the perception of potential customers about the eVTOL-MaaS (air taxis) system was assessed in terms of (1) utility and (2) safety. Also, changes in this perception with respect to a similar older survey in the previous year of 2018 were presented. The perception was found to be dependent on the age and country of the respondents. Overall, about half of the respondents (49% in 2019 versus 48% in 2018) agreed that “Passenger drones/air taxis would be a viable solution to roadway congestion.” Thus, they feel that the eVTOL-MaaS system is useful. About one-third of the respondents (32% in 2019 versus 35% in 2018) disagreed that “Passenger drones/air taxis will not be safe,” which addresses the safety feature of these air taxis. Respondents in China showed a noticeably higher rating for utility (65% in 2019 versus 73% in 2018) than the average rating, and younger respondents showed also a higher utility rating than older ones.

8.2 Ground infrastructure

According to the Deloitte’s insight article, the largest anticipated barrier for the air taxis system (eVTOL-MaaS or eVAT) was designing and building the necessary ground infrastructure. The term “vertiport” was generalized into “vertiplaces,” which is the area having landing/takeoff facilities that includes the “vertiports” as well two other types: “vertihubs” and “vertistations.” All three types of vertiplaces provide landing and takeoff pads for the eVTOL taxis. However, they differ in the size and the targeted urban environment. Vertiports are aimed to be in the middle of the city, used for both passenger and cargo transportation. They can be placed at the roof of a building. UAM operators should place vertiports at or near primary destinations, such as central business districts, shopping malls, and ground transportation stations (such as those for trains and subways). Integration with the existing ground transportation is important. Vertiports are expected to be large enough to serve multiple eVTOL aircraft at the same time. They also can have fast charging (or refueling) systems, security checkpoints, and facilities to perform minor repair work. Vertistations are smaller versions of vertiports, with the capacity to accommodate only two or even one eVTOL aircraft. This makes them easier and cheaper to build than vertiports. They also do not need to provide any charging/refueling systems. Vertistations help in expanding the UAM service to suburban areas. Opposite to vertistations, vertihubs are the larger versions of vertiports. They are viewed as dedicated UAM airports. Their suitable location is the periphery of urban or suburban zones. There should be at least one vertihub in each city covered by the UAM service. Vertihubs are the largest ground structure among all UAM components. In addition to landing and takeoff pads, they should have maintenance and repair facilities, as well as a parking area.

8.3 Prospects

The declared period for expected deployment of UAM service in more than one place in the world was around 2025 (2024–2025 for European Union cities, 2024 for the American city Miami, and 2025 for South Korea) [51]. The launch of this service commercially and sustainably depends largely on the readiness of infrastructure, support from regulatory bodies, certification of aircraft and pilots, defining safety procedures at vertiplaces, establishing adequate unmanned air traffic management services, and integration with the existing ground transportation. There is a large disparity among different countries in their capacity with regard to these items, with China, the United States, Germany, and South Korea being in a leading position [52,53]. Thus, the year 2025 is the estimated year to start commercial UAM trips using aircraft at one or more of these leading candidate countries. It is likely that the eVTOL-MaaS UAM system starts with piloted trips, before adding the more complex autonomous ones at a later stage after gaining confidence about the service, from the side of the corporate operators as well as the side of society members.

8.4 Drawbacks and solutions

This section presents some potential drawbacks for the air taxis (eVTOL-MaaS or eVAT) and suggested solutions. These drawbacks are generic and are not specific to a particular location. Thus, some of them may not be applicable in certain urban communities.