Lilium Jet technology:
Frequently Asked Questions
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We intend this FAQ to address questions around our transformative Lilium Jet technology. For more questions, please reach out to Lilium Investor Relations at email@example.com. Throughout this FAQ, reference is made to certain historical posts and information that describe illustrative potential applications of our technologies. For the most up to date information regarding the Lilium Jet, please consult our most recent public disclosures, available at ir.lilium.com.
At Lilium, we are pleased to see that a robust discussion is emerging around key performance attributes of various eVTOL concepts. This exchange was overdue, and we hope that the whole eVTOL community continues to participate in fruitful debate among scientists, engineers, and researchers from various fields and backgrounds.
Compared to other eVTOL concepts, the Lilium Jet has a novel architecture, characterized by its ducted fans, or Ducted Electric Vectored Thrust (DEVT). This architecture is designed to give the Lilium Jet some notable advantages, which we believe will allow us to fulfill our vision of sustainable and affordable regional air mobility and the decarbonization of regional transport.
Because of its novelty, some commentators have struggled with properly assessing this innovative aircraft architecture and the planned performance characteristics of the Lilium Jet. These misconceptions often lead to errors when third parties estimate and comment on our aircraft’s performance potential. We would like to, where possible, help clarify some of these misconceptions.
Two core misconceptions in various third-party assessments of our technology:
For more detailed information on a potential illustrative example of our architecture, please refer to the Technical Paper (co-authored in 2021 with 5 independent aviation experts) and Technology Blog by our Chief Technology Officer, Alastair McIntosh.
Cell performance and pack design are fundamental to our overall aircraft performance and thus the choice of cells and pack design must be carefully matched to the aircraft architecture in question.
Often, the characteristics of generic or outdated cell technologies are assumed in third-party assessments, with some, for example, applying inapposite automotive cell standards. Reference to these standards ignores current cell technologies and cell innovations, which Lilium, as well as other eVTOL OEMs and high-performance automotive companies, are currently relying on or expect to be relying on to operate their aircraft.
For example, a common misconception is that no cell can fulfill the required high specific power needed for the Lilium Jet’s hover phase, combined with the high energy density that is required to achieve our physical range targets. However, several new cell technologies, such as Silicon-Anode Lithium-Ion cells, provide significantly better power-density performance than conventional EV standard cells.
Silicon-Anode Lithium pouch cells are the technology that we anticipate using in the Lilium Jet and on which we are collaborating with our previously announced battery supplier, CUSTOMCELLS. A few additional notes:
- Together with CUSTOMCELLS, we are preparing new production cells that we anticipate will be able to achieve a power density of ~2.5kW/kg required for the Lilium Jet’s hover phase and an energy density of approximately ~330 Wh/kg - on a cell level for the physical range targets demanded by mission profiles.
- The cells we plan to use need to achieve aerospace safety levels according to our system architecture and the requirements of the certifying authorities (EASA and FAA).
- The cells we plan to use need to deliver the charge time and cycle-lifetime for our business case.
Additional misconceptions we see in third-party reports relate to battery pack design. Energy system architectures vary significantly depending on the cooling system, cabling, redundancy, and other factors like the integration of the pack into the vehicle structure. This leads to a wide set of design pathways and cell-to-pack ratios that can differ significantly from vehicle to vehicle. Third-party assessments often use a ‘one-size-fits-all' cell-to-pack ratio that is usually based on known automotive standards, which can lead to incorrect conclusions in the assessment. We believe the only truly accurate way to compare specific battery performance values across the variety of eVTOLs is to compare aircraft performance on a cell level, not on a total pack level. This is because the design of packs can vary greatly depending on the aircraft architecture and mission profile.
It is worth mentioning that a much higher cell-to-pack mass ratio is desired (i.e., as close to 1 as possible) for eVTOL aircraft. At Lilium we go the extra mile and strive to keep that ratio very high. As an example, automotive solutions are mostly built on aluminum packs with less weight consideration, while we are focusing on, among other things, carbon composite designs that we believe can provide a superior price/performance ratio and reach a pack-weight reduction of more than 50%.
Aerodynamic assessment of an aircraft architecture with distributed ducted fans.
Essential physics about ducted fan aerodynamics are occasionally misunderstood and lead to under estimating efficiencies in the hover phase of flight for an architecture like the Lilium Jet’s. These misunderstandings can lead to overestimation of the power requirements for our jet design, and further miscalculations of physical range capabilities or hover power consumption.
To get a more thorough and transparent technical explanation of the Lilium Jet (as an illustrative distributed ducted fan architecture) we published a scientific paper, co-authored by our co-founder Patrick Nathen and five aviation industry experts from the University of Cambridge, the University of Stuttgart and TU Berlin. The paper highlights our illustrative ducted fan performance, external aerodynamic performance, and architectural considerations. In the paper, the authors assessed the following:
- A representative mission profile for a distributed ducted fan architecture, followed by a structured step-by-step performance analysis based on aviation standards.
- The high-power demand during hover flight due to high disc-loading and, by contrast, the high aerodynamic efficiency in cruise flight, leading to a ratio of power demand between these two flight phases of approximately 10 to 1, meaning cruise flight would use one tenth the power required for hover.
- A variable nozzle that allows moving the cruise and hover operating point closer together. This allows high aerodynamic efficiencies of minimum >85% in hover and >80% in cruise.
- When comparing aerodynamic performance of ducted fans to open rotors, the underlying methods and references need to be reviewed carefully and put into context. Data for open propellers from available literature require careful review with respect to operational points, tip speeds, Reynolds number, disc loading (etc.) when applied to ducted fans, so direct comparisons may be misleading.
- It is accepted in the aerospace expert community that ducted fans are aerodynamically more efficient than open propellers. For example, at the same static thrust, an improvement of approximately 40% in aerodynamic efficiency can be found in a ducted fan. The respective formula and aerodynamics are applied across the turbo-fan industry.
In the Lilium Jet we are using electric ducted fans for propulsion. The advantage of the ducted fans is that they are much smaller than open propellers to lift the same weight of an aircraft. Or in technical terms - they can operate in high disc loads. The consequence of this is that VTOL aircraft using ducted fans need less ground footprint for a given weight and passenger (PAX) capacity of the aircraft. This in turn creates the potential to scale the aircraft to higher PAX and take-off weight for a given size of landing infrastructure.
Ducted fans also allow for the application of acoustic liners in the ducts to reduce noise signature.
Safety is increased since the engine duct can contain blade loss events or ice shedding. Additionally, we believe vibration levels and ride comfort are better with ducted fans than open propellers.
In our DEVT concept we embedded the engines into the wings and provided them with actuators to pivot the engines and thus their thrust vector. The pivoting mechanism enables the Lilium Jet to do vertical take-off and landing by pointing the engines downwards to lift the aircraft. The thrust vectoring helps achieve high maneuverability in low-speed flight. As we can also use the thrust vectoring to control the aircraft in high-speed flight, we can avoid the complexity and weight of an aerodynamic control system.
Embedding the engines into the wings reduces overall surface area of the aircraft which leads to reduced aircraft drag in cruise flight and reduced structural weight.
For more detailed information on an illustrative example of our architecture, please refer to the Technical Paper (co-authored in 2021 with 5 independent aviation experts) and Technology Blog by our Chief Technology Officer, Alastair McIntosh.
The Lilium Jet is a Canard configuration. In cruise flight the engine nacelle surfaces act similar to aerodynamic control surfaces of a conventional aircraft. This means the engines on the front wing (Canard) act as elevators and the outer engines on the main wing act as ailerons. Directional stability is provided by the winglets as these are sitting far aft in a canard aircraft and directional control is provided by differential thrust of the engines.
Unlike helicopters or Multicopters, the Lilium Jet is an airplane using its wings to lift the aircraft in cruise flight. Wings need several times less power to keep the vehicle in the air compared to powered lift from engines/propellers.
Embedding the engines into the wings further reduces surface area and drag of the airplane.
The Lilium Jet’s electric engines themselves are designed to be highly efficient in cruise flight as they will be equipped with variable nozzles to help ensure the engines always operate in their most efficient operating point. A major design challenge is designing a fan which will operate efficiently at both cruise and hover. The Lilium Jet is designed to solve this problem using a variable area nozzle at the exit of the duct. Changing the area of the exit nozzle moves the operating point of the fan, allowing it to be moved to its most efficient operating point in hover as well as in cruise. This solution is similar to concepts found in civil aviation, which use variable area nozzles to optimize take-off and cruise.
Cruise: By closing the variable nozzle in the rear of the flap during cruise flight, we can reconstruct a significant portion of the aerodynamic hover efficiency.
Hover: While the nozzle is open in hover flight, we achieve a maximum of aerodynamic efficiency. The design of the fan still allows for control margins and low noise in hover flight.
We have been able to validate fan efficiency predictions in different flight phases through dedicated test rigs and wind tunnel campaigns.
As part of a rigorous Preliminary Design Review (PDR), a key milestone in the design and certification of an aircraft, we reduced the number of engines to 30 from the previous expectation of 36. With fewer engines, we expect to reduce the total system weight, part counts and the respective complexity of the aircraft. As we change the ratio of the engines being distributed between the canard and main wing from 1:2 for the 36-engine version to 2:3 for the 30-engine version, we intend to improve aerodynamic balance and the total expected performance margins, which may lower certification risks.
We recently began flight testing in Spain and aim to accomplish full transition to wing borne flight as well as high-speed flight starting in Q2 2022, following a ramp-up of the flight campaign which will further extend the flight envelope. Please check our News Releases page for updates.
The purpose of a technology demonstrator is not to show achievement against the final range target of the certification aircraft – it is to validate the flight physics and control laws of our architecture. It is common practice to calculate with high accuracy the final range based on known power consumption in the various phases of flight and the known energy curves of our target batteries.
Since the architecture of our demonstrator is very similar to that of our anticipated certification aircraft, overall performance metrics can be transferred with high accuracy, from demonstrator to certification (conforming) aircraft. By tracking the power demand per flight phase of the demonstrator, we can make predictions of the certification aircraft’s performance due to similarity of the architecture and the proven knowledge we have about the final battery system.
In order to avoid confusion and erroneous range performance predictions of Ducted Electric Vectored Thrust (DEVT) concepts, we recommend reviewing the calculations provided in our Technical Paper that allow an in-depth assessment of the illustrative target range.
All our range figures expressly state they represent physical range and do not account for reserves, which will apply to eVTOL aircraft and are still under review by the regulators.
Demonstrator aircraft are built and flown to demonstrate new technologies or novel configurations. These aircraft do not comply with any certification standards and are not generally built to be in compliance with published aerospace quality standards. These aircraft are often remotely piloted. The advantage of demonstrators is the ability to test new technologies without the immediate burden of fulfilling full conformance management in the supply chain and assembly.
A conforming aircraft is built in an aerospace quality management system to ensure conformity of the aircraft as built to the design on the drawings. In addition, conforming aircraft require the whole supply chain must have implemented process controls, traceability and configuration management against all components, processes and materials used. These requirements demand high effort and investment and due to this producing conforming aircraft is usually only done utilizing aircraft designs which fulfill certification requirements.
To claim credibility of a test for certification purposes the component subject to the test as well as the test bench must be conforming and controlled to demonstrate compliance against requirements. Hence only organizations which have implemented aerospace quality management across all design, production and testing can produce components and complete tests which can be used for certification purposes. For this reason, at Lilium we have invested heavily during the last two years to establish an aerospace quality management system which will allow us to produce conforming parts, materials and tests. This is a big step that needs to be fulfilled before a conforming aircraft can be built.
In terms of flights hours, it is important to note:
- Tests or flight hours on a demonstrator aircraft do not contribute to certification. Lilium is currently using its demonstrators to analyze technology and subsystem performance.
- Progress towards certification can only be done on a conforming aircraft designed and built to known certification requirements and performed in a final test campaign. Lilium is completing the design confirmation of its production / conforming aircraft and will then proceed to release drawings / data to the aerospace supply chain to build that aircraft in 2023, before the final test campaign.
We believe that our aerospace team is one of the most capable in the eVTOL sector. Collectively, they have held instrumental roles in the delivery of the Airbus A350 XWB, Airbus A380, Airbus A320, the Gulfstream G-650 jet engine, the Eurofighter Typhoon and the Harrier jet, among others. They are supported by approximately 450 aerospace engineers and a business team with a strong track record in building successful companies in Silicon Valley and Europe. In addition to our Co-Founder and Chief Executive Officer, Daniel Wiegand, our Board includes our Chairman, Dr. Thomas Enders, as well as Henri Courpron, Barry Engle, David Neeleman, Margaret M. Smyth, Gabrielle Toledano, David Wallerstein and Niklas Zennström.