Introduction

Urban Air Mobility (UAM) is on the cusp of a renaissance. For long, UAM was the domain of the ubiquitous helicopter. However helicopters had their limitations on the noise & emissions fronts in addition to being expensive and therefore rarified to the rich and famous.

Much like Tesla revolutionized the EV and the automotive landscape in general, eVTOLs are set to do the same for UAM. Like helicopters they have Vertical Take Off & Landing (VTOL) ability , however they bring advantages such as efficiency, zero emissions & mitigated noise pollution in their attempt to democratize UAM.

Tesla’s biggest developments lie in the integrating and increasing battery density and management systems. When Tesla launched the Roadster in 2008 the battery density was approximately 120 Wh/kg and today we are getting past 300 Wh/kg (marginally enough for eVTOLs). Similarly when we compare the power output improvements of the Roadster with the Model S (Plaid) we have numbers of 248 hp vs 1020hp (trimotor) in addition to thermal management of all onboard systems. eVTOLs face similar challenges with mass being all important and zero tolerance for failure.

We speak of four aspects of eVTOLs, the batteries, the motors, the propellers & lastly the design.

The Batteries

Range anxiety has always plagued EVs and Tesla was the first to create infrastructure such as superchargers across the USA, however before infrastructure comes battery technology.

The 2019 Nobel Prize for Chemistry went to Stanley Whittingham, John Goodenough & Akira Yoshino. Whittingham established the concept of intercalation, simply put, discovered how to make a rechargeable battery in the 1970s. Goodenough discovered the lithium cobalt oxide cathode that doubled existing battery power. Yoshino developed the first commercially safe lithium ion battery by using petroleum coke as the anode eliminating the use of pure lithium metal and making the battery safe for general use. The three of them are responsible for the power most of our devices use today and it took over 35 years of work to get the Nobel.

Tesla created the framework to harness battery power safely by using standard cylindrical cells in their Roadster. They created a sophisticated Battery Management System (BMS) to manage individual cells in battery packs to optimize performance across the temperature range and manage charge distribution. Tesla created a liquid cooling system for battery packs to maintain optimal operating battery temperature. They also pioneered the process of integrating a large number of batteries into the structure of their vehicles resulting in efficiencies across both performance & manufacture. Such integration also drove structural integrity. Reading this we realize it’s not surprising a significant number of battery engineers at eVTOL start ups cut their teeth while at Tesla.

To understand thermal management & put things in perspective, imagine having a Teams interview on your smartphone in a hot car with the AC running. Within a few minutes the battery pack in the phone heats and at some point it shuts the phone down (happened to me!). After cooling the phone in front of the AC vent your interview continues without video or speakers on. Now imagine 4680 cells working together (Tesla’s Roadster had this many) and the heat they generate. How do you manage to keep them cool and working optimally? This is where the BMS comes in along with the liquid cooling. BMS monitors each cell in the battery pack for voltages, temperatures and current flow. The system enforces safety limits on all of the above in addition to charge limits, heat & short circuits. All these need to be actively managed to avoid thermal runways (more on this later). Furthermore the BMS balances the loads on individual cells to ensure all cells have a balanced state of charge (SOC). The brain of the BMS is an algorithm that employs AI & machine learning tools to figure out what works best for each individual battery pack. The brain/algorithm further communicates with the EV’s systems as a holistic unit.

The heating/cooling system runs antifreeze through Tesla patented cooling channels that have contact with each and every cell in the battery pack. The system is further integrated with the car’s HVAC system, and as needed the antifreeze is pumped through a heat exchanger for heating/cooling  as needed. The cooling system is integrated to the BMS to be actively managed. 

A thermal runway is an instance of a rapid and uncontrollable chain reaction within a cell or battery pack where temperature increases further accelerates the chain reaction. Thermal runways are triggered by an internal short circuit or external damage (ex: Richard Hammond’s 2017 Rimac, Concept One crash in Switzerland. The battery pack burned for five days after as per James May, more recently the AI171 crash is a thermal runway possibility, although no conclusions have been published. The pointers are there). Thermal runways need to be actively managed through prevention and if not, through mitigation. The same can be managed through BMS as mentioned earlier and mitigated through cell design such as using small cylindrical cells of limited number, fire retardant barriers between cells and modules such as mica. The battery packs themselves are designed to have venting systems that vent hot gases away from the passenger cabin and the rest of the battery pack to prevent spreading. Lastly the battery pack itself is created in a manner to contain such events in the safest manner possible.

The entire battery structure/BMS/cooling system is replicated in eVTOLs albeit with customization as per specific  individual needs.

Note: While battery power density might be ≥300Wh/kg eVTOLs unlike EVs have Depth of Charge (DoD) limits. In layman’s terms if a battery pack is at 300Wh/kg the eVTOL can use not more than 80% of the battery capacity. The first reason is safety in case the eVTOL needs to divert or the battery pack itself has a voltage crash. In actual terms this translates to not more than 240 Wh/kg of battery density available at any given time. The second is battery pack preservation as shallow DoD preserves packs. Lastly the FAA mandates these reserves. It needs to be mentioned that eVTOLs have very high power demands at takeoff/hover & landing. Typically Take off can demand approx 25% DoD, Cruise anywhere between 10-30% and landing a further 10%. Such a flight profile will use approx 65% DoD and keep the eVTOL within limits. 

The Motors

Wiki says the electric motor is a machine that converts electrical energy to mechanical energy through the interaction of the motor’s magnets with the electric current in a wire winding. The interaction produces torque which is applied to the motor’s shaft.

While electric motors have existed for about two centuries, we are more concerned with the developments of the last 25 years. In eVTOLs where power to weight ratio is all important, the role of electric motors takes on added importance when balancing weight, efficiency & power density all at the same time.

The power density of motors over the last 25 years has improved by a factor of 10! The trend points to power density doubling every 8 years from 2kW/kg in 2000 to 15kW/kg(In Oct’25 YASA created a motor that weights in 12.7kg and generates an insane 750kw! That is over 59.1 kw/kg). So contributes to the power density increases and where will we be in 5 years?

Materials are the first contributor to improved power densities in motors.

High performance rare earth magnets that contain materials such as neodymium has pushed up magnetic flux density inside motors which in turn produces stronger magnetic fields that produce higher torque. Energy loss is mitigated inside the motor core through use of thinner steel laminations, high efficiency and low loss materials and soft magnetic composites (SMCs) that permit complex 3D shapes for better efficiency. Lastly copper wire winding techniques themselves are being looked at to mitigate power loss and improve motor efficiency. One such example is ‘hairpin winding’, a technique that used solid rectangular conductors instead of wires to maximize conductor surface area.

The second contributor is motor design.

Motor design innovation is looking at pushing up motor speed of operation using fresh designs and parts that can handle higher centrifugal forces inside the motor. Motor architecture is looking axial flux architecture where the magnetic flux path is parallel to the axis of rotation instead of radial to it. Yokeless axial motors are motors where the stator (the motor winding) has teeth without a black iron or yoke connecting them.  

The third contributor is thermal management.

Thermal management inside the motors is critical to their performance. Liquid cooling using water or oil filled jackets are wrapped around or sprayed onto the stator. Advanced materials at the jacket / motor contact points improve thermal transfer with minimal resistance. 

The fourth contributor is systems integration.

Integration of electronics such as the inverter into a single unit reducing weight and improving efficiency. Wide gap semiconductors such as gallium nitride (GaN) and silicon carbide (SiC) significantly allow for a larger bandgap over traditional silicon, giving them the ability to handle higher voltages, temperatures while switching at higher frequencies. Algorithms such as advanced Direct Torque Control (DTC) permit precise and faster motor control.

All eVTOLs use Distributed Electric Propulsion (DEP) where they have between 6-36 motors & propellers to hedge against failure. The motors are fixed while the props have variable pitch or tilt. The motor / prop assembly is direct drive to improve efficiency (vs a gearbox). eVTOLs have the ability to land safely with just 70% motors operational as a result of this.

eVTOLs use a combination of axial and radial motors depending on application.

The Propellers

Now that we have spoken of the batteries & motors the third aspect to powering an eVTOL is propellers. They need to be efficient across the mission profile, safe and quiet.

Disk loading is the total thrust divided by the total swept area of all propellers and is measured in N/m2 . Low disk loading (50-200 N/m2) translates to large slow turning props that are quiet and have an efficient hover. High disk loading (500-1000 N/m2) translates to small, large fast turning props that are compact,loud and have a powerful takeoff and are inefficient at hover. In the urban space since noise pollution plays a role large props with low disk loading win as sound generally stays below 65dB while lifting efficiency is maintained.

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Propellers have four kinds of setups.

Fixed-Pitch Open Rotor: Blades are locked at one angle, while the entire prop assembly tilts (Joby Aviation) or separate lift (fixed) and cruise (tilt) props (Archer Aviation). Typically such engines use between 3-5 props (Archer started its rear props with 2 blades each but has now switched to four bladed fans as they produced a vibration when they moved edgewise forward during cruise).  The props themselves are extremely lightweight made of carbon composites with 3D printed titanium leading edges and sleeves. Since the props have no moving parts they are simple , light & cheap while being low on noise. Such props are optimized for one speed such as hover (, Beta A250) and cruise (Archer Midnight)

With a project marketsize of $1 tn by 2040 (as per Morgan Stanley who downgraded the numbers from $1.5 tn due to development delays in their report  eVTOL/Urban Air Mobility TAM Update: A Slow Take-Off, But Sky’s the Limit , skeptics are much more conservative at approx $100-300 Bn range)the eVTOL industry is maturing rapidly. With high costs of development & certification most of the companies and products mentioned above have high burn rates and staying in the game is important. This is where partnerships & consolidation come in. Some examples below

Variable-Pitch Open Rotor:  Blades rotate on a hub much like a helicopter. The blades continue to be made of extremely light weight carbon compete with 3D printed titanium hinges. For VTOL the propeller pitch is set to high (steep angle relation to axis of rotation) and for cruise the props are set to low pitch (relatively shallow angle relative to axis of rotation). Such a setup can be used for all mission profiles. Joby S4 & Vertical Aeropace VX4 has such tilting props on the front having the best of both hover and cruise as needed.

Electric Ducted Fan (EDF): On the face of it an EDF looks very similar to a jet engine, they are even used on RC jet planes to mimic the jet engine effect, however they are very different. EDFs have a very small diameter compared to the other fans we have spoken of (Liliums fans were 0.8m diameter). They at very high speeds of over 6,000 rpm and generate over 2,000N of thrust per far. The disk loading is approx 800 N/m2 . The ducts block blade noise and to <55dB at height. Ducts do induce drag in cruise and are heavy in flight.

Coaxial Contra Rotating Rotors: Such props have two props on the same axis rotating in opposite directions.Such prop setups cancel torque within the assembly itself vs needing to have a similar assembly on the opposite side for torque cancellation. The EHang 216/VT-30 use such a setup. The blades themselves are made of carbon composite and have no gearbox. The setup is each rotor assembly has 2 sets of 4 bladeded props. They generate approx 1,000N thrust per assembly and disk loading is at 300 N/m2 . Such a setup is great for urban mobility, however range is limited due to high power needs.

The Design

Joby Aviation S4: A hybrid helicopter/aircraft design. It has six variable pitch tilting props with four on the wing and two on the V tail. Both the wingtips have and V tail tips have props to counter wingtip turbulence.The props have a diameter of 1.8m and have 5 blades each. The props on the S4 have scimitar-like curved tips for the purpose of aerodynamic efficiency and noise reduction. The S4 has a capacity of 1pilot + 4passengers .The wingspan & length are 39 feet length &  21 feet respectively with a height of 12 feet. The four battery packs are integrated into the carbon composite wings and have built in redundancy. Between the batteries , motors & props the S4 has MTOW of 4,800 pounds, a maximum cruise speed of 200 mph, range of 150 miles and a max altitude of 15,000 feet. The cabin sound is at 62 dB and the S4 is about 70% through its FAA certification process.

Archer Aviation Midnight: The Midnight is once again a hybrid helicopter/aircraft design. It has a total of 12 props with 4 blades each. While all the props are loaded on the wings and have a diameter of 3m each with six on the leading edges of the wings are tilting and the six on the trailing edges are only for VTOL and fixed. The Midnight has a capacity of 1 pilot + 4 passengers. The wingspan & length are 48 & 41 feet respectively and the height is 12 feet. The batteries, motors & props give the Midnight MTOW is 6,500 pounds, a maximum cruise speed of 150mph with a max range of 100 miles, however the midnight is optimized for 20 mile trips. The six independent battery packs are in the wings . Cabin noise levels are around 45dB and the Midnight is approx 15through the FAA certification process as well.

Lillium Jet: Defunct Lillium Jet is in the lift due its stand out thrust vectoring EDF design. When their Intellectual Property went up for auction there was a bidding war between Joby & Archer and Archer won Lillium’s 300 patent portfolio for a total of $21million. The ducted EDFs had high thrust density and the all moving ducted fans were much wanted. The 7 seater Lillium Jet had 36 EDFs with a 24 fans of 0.45m each on the wings and 12 on the forward canards. The Lillium had 12 battery packs distributed through the wings and canards with high redundancy. The MTOW was to 7,055 pounds a range of 155 miles and a cruise speed of 175mph. The cabin noise was expected to be around 60dB. Lillium spent over $1.6 bn on developing the moonshot but did really proceed beyond a subscale demonstrator which again had a different set of fans with chevrons on them. When Archer won the bit Adam Goldstein their CEO said “Lilium’s pioneering work advanced the frontier of eVTOL design and technology, and we’re excited to bring their cutting-edge technologies into the fold at Archer as we advance our product roadmap”. So today Lillium can be considered a part of Archer Aviation.

Beta Technologies Alia A250: The A250 continues the helicopter/aircraft hybrid design. It had four fixed VTOL props + 1 pusher prop for cruise.The A250 has a lot of commonality with its sist eCTOL aircraft the CX300.The VTOL props have a 2.5m diameter, and 1.8m for the pusher prop. The A250 has two configs the first is 2 pilots + 1400 pounds of cargo or 5 pax. The battery packs are located in the fuselage floor for stability. The length & wingspan are 45 & 50 feet respectively with a height of 12 feet.The MTOW is 7000 pounds with a range of 250 miles and cruise speed of 170mph. The A250 is in advanced stages of certification as well.

Volocopter Volocity 2X: Velocity is more like a helicopter with distributed propulsion. The Volocity 2X has 18 props distributed in two concentric circles above the fuselage. The diameter of each rotor is 1.8m The diameter of the outer rotor ring is 36 feet and the height of the Volocity 2X is 8.2 feet. The aircraft is designed for 1 pilot + 1 passenger with a MTOW of 1,984 pounds. The cruise speed is 68mph and range is 22 miles. Volocopter went bankrupt in early 2025 and Chinese company Wanfeng acquired their assets for $12 mn. Wanfeng is continuing the Volocity 2X’s certification through its Austrian subsidiary Diamond Aircraft.

Ehang EH-216: The EH-216 is the only eVTOL to commence commercial operations in specific locations in China for purposes of tourism and sightseeing. The EH-216 has 16 coaxial rotors of approx 1m each which spread out radially under the passenger cabin. The EH-216 is fully autonomous and can carry two passengers. The MTOW is 1,367 pounds with a range of 22 miles and a cruise speed of 81 mph. The aircraft has 12 battery packs integrated into the fuselage floor and rotor arms. The cabin noise is at 70dB and while flying in China FAA certification is in process.

Partnerships & Consolidation

• Toyota has invested $894mn in Joby and is a deep manufacturing partner
• Joby has acquired H2Fly for its hydrogen tech & Blade air mobility’s passenger ops across 12 US vertiports
• Integrated to Uber Elevate to tap into Uber’s UAM market
• Stellantis has invested over $300 mn in Archer aviation & United over $10mn in Archer
• We have already spoken of Lillum & Volocopter
• Beta Technologies has a $46mn contract with USAF & UPS

Looking at these partnerships we see a clear blurring of lines in the manner people travel and the potential of this fledgeling industry. There are many more that have not been mentioned or are being incubated.

Innovation is here to stay….

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