What is Urban Air Mobility?

Geoff Dymott

14/06/2023

Urban Air Mobility, or UAM, is the term given to a new form of personal urban transportation set to change the way we travel around our cities. It’s roots lie in the various flying car projects that have appeared over the last 70 years, but due to their impracticalities, have remained a novelty.

However, like many recent technological advances, the game-changer is the lithium-ion battery. The energy capacity and range of these modern day batteries could allow UAM vehicles to be certified to fly in our skies as early as 2024 [1].

The benefits of urban air mobility
eVTOL vehicles
Multi-rotor wingless eVTOLs
Lift and cruise eVTOLs
Vectored thrust eVTOLs
Urban air mobility certification
Developing an eVTOL powertrain
Minimising eVTOL noise
Vertiports
Improving eVTOL design with CFD

The benefits urban air mobility can bring to cities

One of the main drivers for UAM is to reduce traffic, congestion and pollution in major cities, which is continuing to remain a problem. New York and London are two of the busiest cities in the world. Recent traffic surveys show that average car speeds were 7.1mph (11.4kmh) in central London [2] and 4.9mph (7.9kmh) in central Manhattan [3].

Contrast that to predictions that a UAM flight from Battersea to London Heathrow could take only 12 minutes. While travelling from Lower Manhattan to JFK could be a quick 8 minute flight. So you can start to appreciate the potential for urban air mobility and how it could revolutionise city travel.

eVTOL vehicles

The idea is for UAM to be part of a network of zero-carbon public transport that ultimately replaces cars on the road. The key to this is the development of Electric Vertical Take-Off and Landing (eVTOL) vehicles which combine horizontal and vertical flight using a mixture of propellors and wings. This allows eVTOL’s to take off and land vertically, avoiding the need for large runways, as well as horizontal flight.

As engineers continue to design and optimise this unique type of aircraft, new eVTOL concepts are coming to market every year. Although, all are based on the principle of transporting passengers across cities and landing and taking off on dedicated hubs known as vertiports.

Perspective view of the CityAirbus in flight with labels showing the main design features — The CityAirbus concept shows a typical configuration and specifications for a UAM vehicle. CREDIT: Airbus

Multi-rotor wingless eVTOLs

The most basic type of eVTOL are multi-rotors, which are wingless vehicles that use horizontal rotors for lift and propulsion, similar to a large multicopter drone. An example is the Volocity from Volocopter which has 18 rotors, powered by individual motors. It’s 9 battery packs help it to achieve a top speed of 110kmh (68mph) and a range of 35km (22 miles) [4]. It is capable of carrying two passengers plus luggage. The arrangement of the rotors aids controllability and keeps them out of the way of the passengers.

A white landed Volocity multi-rotor eVTOL landed with two passengers — The Volocity features 18 smaller rotors mounted to structure that has a diameter of 9.3m [4]. CREDIT: Volocopter

Lift and cruise eVTOLs

Multicopters are simple but require high power to remain in the air during forward flight as lift is only generated via propellors. Lithium-ion batteries have moved the boundaries on what is currently possible, but battery range is still a limitation. One approach to minimise this is to design eVTOLs with lift producing wings and propellers. This way, the vehicle can travel using a combination of battery power and lift, although a further set of rotors is required for vertical take-off and landing.

A white Voloregion multicopter in flight — The lack of a requirement for fuel storage, a high proportion of composite materials and the distributed nature of the motor/rotor units has led to a wide range of unconventional layouts such as Volocopter’s Voloregion. CREDIT: Volocopter

Vectored thrust eVTOLs

Winged aircraft solve many of the multicopter’s problems, but create some new ones too. The horizontal VTOL rotors are not efficient in level flight, creating a drag and weight penalty. However, this can be addressed by vectored thrust and tiltrotor vehicles. This is where the same rotors can be used to generate lift during vertical flight and then rotate to provide forward thrust during horizontal flight.

Top view of a white landed Lilium jet — The Lilium Jet uses pivoting ducted fans on the trailing edges of the wings to generate lift and thrust. CREDIT: Lilium

Urban air mobility certification

Before an eVTOL can take to the skies, it has to be fully certified to ensure it has been designed, manufactured and modified for safety throughout its life cycle. However, with UAM such a new concept, few regulations exist. Consequently, manufacturers and regulators such as the Federal Aviation Authority (FAA) in the USA, the European Union Aviation Safety Agency (EASA) in Europe and the Civil Aviation Authority (CAA) in the UK are working together to shape these regulations.

Discover how CFD has become a key tool in the eVTOL certification process

Graphic showing several eVTOl aircraft flying over a busy city — With eVTOLs set to operate above busy cities, safety is of utmost importance for regulators. CREDIT: NASA/Lillian Gipson

Joby Aviation were the first manufacturer to complete stage 4 G-1 certification from the FAA and airworthiness approval from the United States Airforce for its eVTOL aircraft. This concept uses pivoting rotors and conventional wings, much like the CV-22 Osprey tilt-rotor military cargo plane.

A white Joby Aviation eVTOL in flight — G-1 certification covers airworthiness and environmental standards for eVTOL aircraft and Joby Aviation were the first manufacturer to reach stage 4 . CREDIT: Joby Aviation

Developing an eVTOL powertrain

As UAM has moved from concept to reality, engineers have had to develop powertrains that satisfy the stringent power and weight criteria for this unique type of aircraft. Typically eVTOL powertrains consist of high voltage batteries that provide energy to electric motors which power the rotors.

However, vertical flight requires a substantial amount of energy and therefore a high energy dense battery. Some companies are aiming for a gravimetric battery energy density of around 450Wh/kg. For comparison, a typical EV battery has an energy density of approximately 250-450Wh/kg [3].

Close up shot of a MAGicALL motor attached to a rotor — A combined motor and controller for an eVTOL from MAGicALL which achieves torque densities of 50nM/kg and efficiencies exceeding 95%. CREDIT: MAGicALL

Unfortunately, energy dense batteries come with a weight penalty and this trade off between performance and payload is a constant headache for engineers. Consequently, advances in battery technology that enable higher power to weight ratio battery packs will bring the biggest performance gains to eVTOL aircraft.

Another challenge surrounding eVTOL powertrains is fast charging. Developing an eVTOL with the range and payload required for an air-taxi service is not practical without the ability to recharge quickly. The range of the battery is also of particular interest to the regulators and requires careful certification to ensure that commercial aircraft have sufficient energy reserves to divert to alternate landing sites if necessary.

Minimising eVTOL noise

The rotors are the final piece of the eVTOL powerplant puzzle and are key to payload, efficiency and noise. Noise might not be an obvious problem, but it is crucial to the success of urban air mobility. Previous forms of transport such as the Concorde failed to progress due to take-off noise and sonic boom problems over land. For eVTOLs, minimising noise is even more crucial as they will primarily operate over densely populated areas.

A graphic showing the different noise levels of eVTOl aircraft compared to speech, road noise, truck noise and a helicopter — eVTOLs are being developed to be quieter than busy road traffic to avoid noice complaints in cities. CREDIT: Volocopter

One of the key ways to reduce noise is to optimise the design of the rotors. As rotors rotate, the interaction between the blade vortex and the wake creates substantial noise. This is why helicopters, with their horizontal rotor layout are so loud when they fly overhead.

Simulation showing the wind noise of helicopter blades interacting with the wake — The distinct noise of a helicopter comes from the interaction of the blade vortex and the wake. CREDIT: University of Stuttgart

To improve the efficiency of the blades moving through air which will ultimately reduce the amount of wasted energy dissipated as noise, engineers aim to reduce disk loading. This is the ratio of weight per unit rotor area and is calculated by the below formula:

Where: DL = disk loading, W = total aircraft weight (kg), n = number of rotors, R = radius of the rotors

By reducing this value using multiple rotors along with optimising tip velocity, blade pitch, rotational speed and motor torque, the efficiency of the rotor can be improved and noise minimised. Furthermore, the use of ducted fans in eVTOL design eliminate the trailing rotor wake which not only increases efficiency and reduces noise, but also improves safety as there are no exposed rotors in close proximity to passengers.

Diagram showing how the Lilium ducted fans work — The Lilium Jet uses true ducted fans to generate vectored thrust for lift and propulsion. CREDIT: Lilium

Vertiports

The co-operative nature of UAM development means that regulators, designers and infrastructure providers are all working together. This is particularly apparent in the design of vertiports, which have been defined by EASA as ‘an area of land, water or structure used or intended to be used for the landing and take-off of VTOL aircraft’.

These dedicated areas will provide the infrastructure to enable passengers and goods to be transported safely via eVTOLs. They will need to be easily accessible with good links to conventional travel networks and so will most likely be located at either existing airports or on rooftops within cities.

Render showing the layout of a vertiport in a city — Vertiports can be clean sheet designs, working in collaboration with aircraft designers. CREDIT: Architectural Digest

Improving eVTOL design with CFD

Many of the UAM projects currently in existence are paper exercises from tech start-ups looking for investment. With few physical aircraft, simulation is playing a vital role in the development of eVTOL design. This can range from simple spreadsheets to determine power and payload requirements to full CFD simulations of the entire aircraft.

Find out how AirShaper helped Sensefly optimise VTOL drone aerodynamics

A model in AirShaper CFD software of a drone with propellors — Adding propellor or rotor aerodynamics to a CFD simulation adds accuracy. CREDIT: AirShaper

One of the most difficult aspects to design on tiltrotor or vectored thrust eVTOLs is the transition from hover to level flight. Computing power and CFD are not quite capable yet of simulating these relatively long transition events. Instead, multiple cases can be solved to cover all possible configurations. For those that have physical designs, wind tunnels can be used to test models, perhaps with active parts to truly study the moving elements.

Close up shot of miniature wool tufts on a wind tunnel model of an eVTOL — Wool tufts are still used to establish flow quality in wind tunnels. CREDIT: Flight Global

Time will tell whether the UAM concept and its eVTOLs are the 21st century flying cars or a true mobility solution revolution. But the race for the first eVTOL in the sky is accelerating the development of both eVTOL technology and aerodynamics. So keep your eyes on the skies because UAM eVTOLs could be flying sooner than you think.