Performance of Air Taxis – Justified Criticism or Greater Potential?
January 18, 2024 | Simon Eck, Felix Meckenstock
The idea of simply flying to work or any destination in the future instead of taking a slow journey by car or bus in congested traffic is on many people’s minds. At the same time, the demand for sustainable and environmentally friendly means of transportation continues to grow. For these reasons, a number of companies around the world have set themselves the task of developing an electrically powered flying vehicle. These vertical take-off and landing eVTOLs (electric Vertical Take-Off and Landing) are about to be certified, according to manufacturers, and should be ready for the market in the next one to two years. However, there is already criticism and the question of whether the aircraft can actually deliver the desired and announced performance and thus provide added value compared to other means of transportation.
For this reason, we took a detailed look at the 16 models currently under development in order to obtain an estimate of their future performance. Using a self-developed calculation tool and publicly available or estimated data on the architecture of the aircraft, analyses were conducted in three dimensions. The main focus is on the differentiation between the three basic concepts of multicopter, lift & cruise and vectored thrust.
Dimension 1: Flight and hover times
The hover time is relevant for the safety of eVTOLs, as the aircraft must be able to hover for a sufficiently long time in the event that preparations have to be made during the take-off or landing phase or other incidents occur that force the aircraft to hover. Multicopters impress with their long hover times. Their architecture enables hovering and vertical flights in particular, making them best suited for flight missions with a large proportion of steep flights. Lift & Cruise and Vectored Thrust have good hover times of between 10 and 20 minutes. Only the Lilium Jet has a hover time of just over 3 minutes due to its special architecture, which could endanger some flight missions.
In terms of travel times, Vectored Thrust convinces with its high travel speed. For a 30 km flight, a travel time of 9-10 minutes was calculated in the calculation model. Lift & Cruise travel slightly slower, resulting in an average travel time of 11-16 minutes for the same 30 km flight. Multicopters fly significantly slower, resulting in a calculated travel time of 19-22 minutes. Nevertheless, they are still faster than conventional means of transport. For cars and public transport, times of 45 (car) and 85 (PT) minutes on average were calculated for the pure distance over 6 major cities worldwide.
Dimension 2: Range
The achievable range is of decisive importance for the operator of an air taxi operation, as this influences which flight missions can be flown. Vectored Thrust achieve the greatest range of 200 to 280 km. Once again, the Lilium Jet is an exception here, as it consumes a significant amount of energy during the take-off phase. A range of 150 to 180 km was calculated for Lift & Cruise, which means that, as with Vectored Thrust, a large number of different flight missions such as transfers between several cities can be flown. Multicopters, on the other hand, have a maximum range of 40 km, making them only suitable for flights within a city or region.
When designing an eVTOL, the aim is not only to fly a long range but also to conserve resources. It is therefore also desirable to use as little battery mass as possible for a good range. In order to classify which aircraft achieves the best use of the available battery, we divide the passenger-km in the maximum range by the battery weight. The figure represents how far a person can fly with one kg of battery mass and is an indication of the efficiency of the eVTOL. The evaluation shows that the combination of primarily low weight and secondly good flight characteristics leads to satisfactory results. Here, too, the current multicopter models are at the bottom of the rankings, as they have as already mentioned a short range and above all a small number of passengers. They can carry one or, in autonomous flight, a maximum of two people, while the other models can usually transport 4 to 6 people. When fully occupied, the other models are therefore significantly more efficient.
As the specific energy density of the batteries increases, the range also increases due to the greater amount of energy. It can be seen that the range increases over-proportionally. With a tripling of the specific energy density (at pack level), an average increase in range of 3.2 times could be calculated. The Lilium Jet has the greatest potential here. The longer the journey, the smaller the percentage of the take-off and landing phase. Once the Lilium Jet is in the air, it can achieve long ranges thanks to its good flight characteristics. Therefore, an improvement in battery technology has a significant impact on the range and will lead to many more possible applications.
Dimension 3: Energy consumption
In order to create a sustainable and environmentally friendly means of transportation and to be competitive with alternatives, energy consumption must be kept low. For this reason, the energy consumption was calculated for each eVTOL for different distances. It is noticeable that for each eVTOL the average consumption per passenger-km is higher for shorter flights than for longer ones. This is due to the fact that the phases of vertical take-off and landing, during which consumption is particularly high, account for an increasingly smaller proportion of the total flight the longer the flight lasts.
For a comparison between the eVTOLs, but also to compare consumption with other means of transportation, the case of a 30 km flight is examined once more. First of all, it should be noted that all eVTOLs have better energy consumption than helicopters when fully occupied. They are therefore better alternatives from a purely ecological point of view and could take over flights from helicopters. It can also be seen that multicopters and particularly heavy eVTOLs need a lot of energy. Otherwise, it is surprising that most eVTOLs are competitive in terms of consumption. They have a lower level of consumption than conventional cars with a single occupant or, in some cases, even compared to buses. Of course, it has to be said that the electrified versions of buses and cars still have advantages over eVTOLs, especially when they are fully occupied. However, if a person traveling alone uses a fully occupied air taxi instead of a fossil-fueled cab, the energy consumption can be significantly lower.
After considering the three dimensions, there are several conclusions to be drawn.
- Vectored Thrust models are powerful eVTOLs with the longest range and the potential for low consumption as long as the weight is kept low. They can be a good addition to existing offerings, especially for long distances.
- Lift & Cruise models are characterized as versatile all-rounders. They can be used at both short and medium distances and have the lowest consumption while keeping the weight low.
- Multicopters appear to be facing challenges that could endanger their long-term competitiveness due to their inadequate performance and significantly higher energy consumption. However, they could be suitable for flights with a high vertical and hovering component.
Choosing the right basic concept is therefore of crucial importance when planning the development of an air taxi operation.
The further development of battery performance plays a decisive role and offers the greatest potential for increasing the maximum range. Overall, the weight of the eVTOL should be low in relation to the number of passengers to be transported and the flight characteristics should be high in order to achieve low consumption and a long range.
Compared to other means of transportation, eVTOLs can impress with their ability to cover distances quickly and their in some cases comparable energy consumption, making them a competitive option.
With the right model and concept, the dream of an air cab is still possible, at least in terms of performance. It remains exciting to see how certification will proceed and how battery technology will develop.
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