eVTOL taxis: The tight-rope walk of trade-offs to reach commercial success

This is my 4th post covering the exciting world of eVTOL Urban Mobility and its emergence as a viable transport option. Here I cover various trade-offs that must be considered by the 170 or so companies world wide that are currently developing such vehicles…

Evening clouds above

One of the most exciting elements of the emerging eVTOL taxi industry is just how diverse the designs and concepts are showing up to be. The use cases that they are intending to serve have up until very recently been technically unfeasible, and with no successful baselines to refer back to, creativity and experimentation rule the day. You would expect that over time designs will become more and more homogeneous, as has happened with the aerospace and automotive industries. However, until then, we have many years of varying designs to look forward to as many organisations seek to find the magic formula to ensure that they are more effective than their competitors.


Unfortunately, as with everything in life, an optimal design will come from effectively balancing between competing requirements, you can’t have everything. Below I explore some of the more pertinent of these necessary trade-offs.

Take-Off Efficiency Vs Horizontal Flight Efficiency

On the one extreme you have a design similar to a helicopter, with vertical facing rotors, this concept is very efficient at providing vertical lift, and hovering in one position. However, when it comes to flying in a horizontal direction, this design is very inefficient.

On the other extreme you have a fixed wing design, similar to a Harrier Jump Jet, this is very maneuverable and efficient when in forward flight, but takes a lot of skill and energy to enable vertical take-off. Not to mention produces a lot of noise during this process, as shown in the video below:

Although developments in electrical propulsion systems mean that more advanced concepts can be used to power this next generation of vehicles, thus leading to quieter and more energy efficient solutions. The vertical vs horizontal trade-off is unavoidable. Inherently, solutions optimised for vertical lift will be more suited to shorter journeys and more frequent landings, but will tend to be more noisy and slower in horizontal flight. Those designs optimised for horizontal fight will tend to have noisier and more energy intensive take-offs. According to Uber’s elevate white paper: Uber Elevate, an eVTOL vehicle can expect to use the same amount of energy to take-off and land as it would need to travel horizontally for 50 miles.

rotor fixed wing

Performance & Efficiency VS Ease of Maintenance and Certification

One way of countering the vertical vs horizontal trade-off is to design a vehicle with propulsion systems that rotate into optimal positions for both modes of flight (see example above). Several eVTOL developers have opted for variations of this solution. The draw back of this approach is that with significantly more moving parts and mechanisms to control, there are a lot more things that can go wrong. This will mean increased frequency of scheduled maintenance as well as the need for more replacement parts. This also means that the vehicles will be more complex and expensive to pass through certification. Estimates can be taken from the aerospace industry, but judging by the variations in designs it is not yet obvious where the sweet spot is in terms of efficiency of flight operations vs cost of maintenance and certification.


Battery Service Life vs Battery Weight & Logistics

Lithium Ion Batteries are happiest when 50% charged, and least happiest when 100% or 0% charged. In general the more that they are subjected to 100% or 0% charge the quicker they will degrade and need to be replaced. Volocopter estimates that the battery will consist of up to 75% an eVTOL vehicle’s operating costs. Most appliances (including Tesla’s vehicles) get around this by not allowing you to charge up to 100%, or run down to 0%, but this means your battery capacity needs to be bigger than what is actually accessible. As batteries tend to be heavy, this becomes an issue in vehicles that need to escape the Earth’s gravity. Thus battery size and capacity needs to be carefully matched against likely journey times.

There is an added complication in that fast charging these batteries also causes them to degrade more quickly. An option can be to simply swap out batteries at service stations, and then charge the spares at a slower rate. This means that more batteries need to be held in inventory, complex mechanisms and fastenings need to be utilised to enable them to be switched, and inspection and certification requirements will become more complex.

overloaded bus.jpg

Carrying Capacity Vs Noise Pollution

This is a particularly tricky trade off. As seen with traditional commercial airliners, the most economic and cost effective method of transporting people in the air is to build a fairly large aircraft. This improves the mass efficiency, as the increase in the weight of the aircraft is less than the increase in weight carrying capability of passengers and their luggage. Unfortunately however, the power required to create lift increases at a greater rate than the increase in passenger weight. This isn’t so much of a problem when you can take a long take-off, utilising the lift efficiency of a fixed wing, but when you are relying on rotors or turbines to create lift, this large increase in power requirements means noise increases very quickly. This problem is compounded when utilising electric power, as it means more battery power (and therefore battery mass) is also required. Thus there is a direct conflict between carrying capability and the economic benefits of  carrying more passengers in one vehicle, vs the power and noise that is necessary for take-off and landing. Once again, the use case of the vehicle will need to be carefully considered to ensure that an optimal balance is achieved.


Population Density & Demand Vs Certification & Regulation Requirements

Basic logic dictates that demand for transport services that will drastically reduce travel times will be in higher demand in areas where there are higher population densities, where people tend to be more time poor, and where there is congestion problems with incumbent substitute travel options. However, as specified by Volocopter in their recent white paper Volocopter White Paper certification requirements are 100 times more stringent over cities than they are over rural areas. This is without considering that before too long there could be traffic jams in the sky, and thus there will likely be tariffs to cover the cost of directing and managing air traffic in densely populated areas. Another factor to consider is that in densely populated areas you are never far away from people, great for potential customers, but not so great when it comes to noise pollution and regulations that come with it.

Considering this, it is understandable to see small businesses such as Esprit Aeronautics covered in my previous blog artical: eVTOL, interview: Simon Scott. Founder and Owner of Esprit Aeronautics choose to operate away from cities in under-served rural areas. However, the main players in this sector are developing much more expensive vehicles that will need to be utilised more productively than serving rural areas will likely allow. Thus a strategy to overcome the barriers of operating above cities will be necessary.


Market Segment vs Public Acceptance

The above mentioned Uber Elevate paper estimates that initial flight costs will be approximately $150 for a 50 mile journey. In energy and maintenance terms, it is likely that half of that cost will be just for take-off and landing. Thus there will be an initial $75 levy needed to break even before the journey duration is even considered. Over time Uber expect the cost to fall significantly to just $25 for a 50 mile journey, but in the near time it is clear that the service offering will need to be pitched at wealthy clients willing to choose a more expensive alternative to existing services in order to save journey time.

The problem here is that geographical locations where high net-worth individuals live tend to also to be areas with the least noise pollution, while also being the most aesthetically pleasing. Thus it will be harder to gain acceptance for services that bring easily noticeable noise pollution as well as potentially become a blot on an otherwise beautiful landscape. Further to this, if only 5% of the population can afford to use such a service, then the other 95% of the population are likely to be unhappy to be disturbed to enable the ‘privileged few’ to live a life of increased convenience. In nations where  introducing legislation tends to be a democratic process, introducing eVTOL services under the above circumstances may prove to be a challenge.


First Mover Advantage Vs The Curse of the First Mover

First mover advantage will enable an organisation to work with certification agencies, which will in turn mean that legislation will naturally lean to cover their specific products. It will give you a head start in building a portfolio of qualified materials, processes and operating criteria, so you can tailor these to your business’s strengths. Pilots will be trained on your products and get used to flying them. You will get first option on licencing of areas (assuming air traffic control will be restricted by licencing areas and capping amount of traffic). You will also get a chance to establish your brand while there is less competition.

However, especially considering all of the uncertainties and trade-offs above, committing to capital investments when the industry is young means you are also committed to a strategy that carries a higher risk of failure than for a more mature market. Organisations that come late to the party can gain valuable insight from the first movers’ lessons learned, they can wait to see what concepts and business models become the most successful, and then simply target to beat these early leaders in order to gain market share. The first movers may find it hard to react to these late entrants as they have already spent their capital to develop a market which will likely be unprofitable during its early years.

The same can be said for infrastructure, early investment gives you a likely cost advantage as competition is low, and a head start in setting up key hubs. However, there is risk that as the sector develops, customer’s needs will also develop, and the original locations and architectural designs become quickly overlooked by newer options.

This is not to say that the early movers are destined to fail, but history doesn’t favour their chances. They will need to maintain agility to ensure that they adapt with the fast moving market and it’s regulations.


In my next article I will look at a couple of the first movers in this emergent market, assess the strategic decisions they have made with regards to the above trade-offs, and evaluate what I feel are their strengths and weaknesses…


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