On The Radar
When Alastair McIntosh first heard about the Lilium Jet and its array of 36 electric-powered ducted fans, he wasn’t convinced that the new eVTOL design would measure up to the lofty ambitions of the young German start-up company behind the project. At the time, he was managing director and head of engineering and technology with Rolls-Royce Germany, and during a 33-year career with that UK-based aircraft engines giant, he had worked on mighty turbofans such as the 118-inch-diameter Trent XWB, which powers the Airbus A350 widebody airliner. He had also had some involvement with Aurora Flight Science’s experimental hybrid-electric XV-24 Lightning Strike VTOL unmanned aerial vehicle, which featured two dozen ducted fans that bore a passing resemblance to the Lilium architecture.
McIntosh overcame his doubts in December 2020 when he threw caution to the wind and joined Lilium’s growing team of mostly younger engineers as the company’s chief technology officer. Among his colleagues is v-p product Patrick Nathen, who cofounded Lilium in 2015 with fellow graduates from the Technical University of Munich.
Since March 30, when the company announced its plans for an $830 million Nasdaq flotation through a merger with special purpose acquisition company Qell, the pair have been instrumental in trying to clear the somewhat opaque view that many outsiders have of Lilium’s intention to bring the seven-seat aircraft into commercial service in 2024. On social media, there is a small but vocal group of skeptics in the eVTOL-watching peanut gallery who question the engineering premise behind Lilium’s patented Ducted Electric Vectored Thrust concept and its ability to support operations between cities on routes of up to around 155 miles, by contrast with the company’s short-hop urban air mobility rivals.
In a conscious move to increase transparency, Lilium first published a paper entitled "Architectural Performance Assessment of an Electric Vertical Takeoff and Landing (eVTOL) Aircraft Based on a Ducted Vectored Thrust Concept," which was authored by Nathen (a Ph.D. in aeronautical engineering) and reviewed by five academics from Berlin’s Technical University, Stuttgart University’s aircraft design institute, and the University of Cambridge’s Whittle Laboratory. The 35-page report provides an extensive summary of the technical basis for Lilium’s design and how it has assessed the projected performance and technological risks associated with their aircraft. It is rich in detail and mathematical formulae.
For those who (like this writer) feel somewhat insecure about their grasp of STEM subject matter, McIntosh thoughtfully wrote a blog providing a 17-page summary of the key points. This gives his own candid assessment of the company’s thinking in a style that is more accessible, without being excessively dumbed down. Read the two together and you may feel smart enough to join the more esoteric online discussions about eVTOL technology.
One of the first takeaways from the publications is an understanding of how Lilium says it has been able to break the orthodox aerospace design tradeoffs between performance goals by “getting to first principle understanding of the physics and then systematically innovating and optimizing each subsystem in an integrated and coherent manner such that the resulting aircraft achieves all stated objectives.” In this regard, a key breakthrough has been achieving a much higher disc loading than eVTOL aircraft using rotors or propellers, while containing noise levels that would otherwise see communities slam doors in the face of this mode of air transportation.
According to the company, the Lilium Jet’s “footprint” is 10 to 15 times smaller than that of rival open-rotor eVTOL designs. By "footprint," it means the overall area of the propulsion surfaces required to lift the aircraft.
“For the public to accept these aircraft, we have to get to socially acceptable noise limits, and so we are targeting 60 dB noise level [at an observable 100-meter distance during hover], and we have already demonstrated [with a five-seat technology demonstrator] that we are very close to that,” McIntosh told FutureFlight in a Zoom call interview.
What does 60 dB mean? “It’s the noise of the conversation we are having right now,” chimed in Nathen. Traffic noise on a typical urban street comes in at around 70 dB, while a lawn mower is around 90 dB and older aircraft are at 110 dB. In addition to the relatively small disc-loading footprint, Lilium’s secret sauce is the acoustic liners that encase its 36 electric fans, which act like nacelles on an airliner’s turbofans.
While the science is sophisticated, Lilium, like its competitors in the crowded and overly caffeinated eVTOL gold rush, knows that it has to deal in the art of the possible. For now, the main limit on the possibilities is in the available battery technology.
The Lilium Jet will enter service with the ability to carry its pilot and up to six fare-paying passengers in a non-pressurized cabin on trips of up to 155 miles, at speeds of up to 175 mph and at altitudes no higher than 10,000 feet. To meet the limits for type certification under existing CS-23/Part 23 requirements in Europe and the U.S., the aircraft’s maximum takeoff weight is pegged at one pound below the 7,000-pound cutoff. Working with operating partners, the company intends to develop commercial service networks with the Lilium Jet, starting with markets such as Florida and Germany
But Nathen, McIntosh, and the 400-plus engineers who constitute more than two-thirds of the Lilium workforce can see beyond these limits. The company envisages that when battery performance improves, as optimists anticipate, a larger Lilium Jet will be able to carry up to 15 passengers on trips of up to around 310 miles. “Soon, we’re going to have more to say about batteries,” said Nathen, with a glint in his eye and bruises on his shins from where the company’s communications leader had digitally kicked him.
Before that, he and the team will start some wind-tunnel testing to further validate the engineering theory that has already been trialed in flight testing of the five-seat Lilium Jet technology demonstrator, which was subsequently destroyed by fire in February 2020 during maintenance work.
“Let’s be very clear,” McIntosh concludes in his blog. “From a technical perspective this path is the more difficult one, but at Lilium we are now confident that this approach will succeed and will ultimately pay off by offering new means of mobility to everyone.”
So does he miss being at Rolls-Royce where he could still be striving to deliver more incremental savings in the fossil-fuel consumption (and resulting carbon footprint) of old-school airliners? “It’s true that jumping from a blue-chip to a startup as a grey beard like me raised some eyebrows, but I was looking to do something that will make an immediate difference and I now know that we can make a big impact on how people travel,” he told FutureFlight.