By Eric Adams

Eric Adams is a longtime transportation and technology journalist and analyst, a regular contributor to Wired, Popular Science, Gear Patrol, Forbes, and The Drive, and a professional photographer. Follow him on Instagram and Twitter at @EricAdams321

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Lilium doubles down on distributed propulsion

When Lilium debuted in 2017, the aircraft made news thanks to its unconventional configuration and the fact that videos showed the subscale prototype actually flying. True, it was mostly hovering and never left airport property, but still — the thing flew. In the eVTOL world, short distances still go an awfully long way.

Flash-forward all the way to 2019 — itself an oceanic span of time in eVTOL years. The Munich-based startup, which has barely been heard from since those initial videos, resurfaced in early May with a full-scale prototype. The company showed it off several weeks after its first low-altitude hover test.

Lilium founders pose with Lilium Jet
Lilium’s founders pose with their newly unveiled full-scale prototype. Lilium Photo

The new Lilium Jet uses the same distributed propulsion strategy as its predecessor, with 36 small electric ducted fans embedded in the wing and forward canard, the same lifting-body fuselage to supplement aerodynamics in forward flight, and the same generous canopy draped over the passenger compartment. In fact, the most startling thing about the new aircraft is how similar it is to its predecessor, given the lack of really expansive demonstrations of such designs in actual flight — videos of the first prototype gingerly maneuvering across the runway notwithstanding.

In response to queries about the viability of the configuration, which combines propulsion and control into a single system — something many aerospace engineers aren’t fans of — the company indicated that it remains confident in the strategy. “By having 36 independently controlled engines we think we get much more control than conventional aircraft,” the company said in an email. “While the aircraft doesn’t feature some of the commonly expected aircraft features such as a tail or rudder, we’re able to provide all of that control using the engines. Using thrust vectoring, we are capable of making very small adjustments to the direction of travel in split seconds. The engines and actuators are independent, meaning in the likely event that one should fail, there are many, many more to pick up the workload.”

Front view of Lilium Jet
A view of the Lilium Jet from the front, providing a closer look at some of its 36 ducted fans. Lilium Photo

Nevertheless, skepticism remains. During a panel discussion at the Vertical Flight Society’s annual forum and technology display a few weeks after the new aircraft debuted, Mark Moore, Uber’s aviation engineering director, concluded that the Lilium Jet’s disc loading in a hover is much too high. (Lower disc loading indicates more efficient propulsion systems.) The company’s research has established that it needs to be less than 12 pounds per square foot for open rotors and less than 25 pounds per square foot for ducted rotors — figures that are far exceeded by the Lilium Jet.

Responding to Moore’s comment, a Lilium spokesperson told eVTOL.com, “Whenever you develop a new product there is always a lot of progress that isn’t immediately visible to the outside world. We’re confident that the progress we’re making ‘behind the scenes’ will enable us to deliver our stated goals and we look forward to proving that in due course.”

Lilium air taxi app
Lilium is aiming to launch its own air taxi service by 2025. Lilium Image

Lilium has nevertheless indicated that in its final configuration, the aircraft will be able to achieve 186 miles per hour (300 kilometers per hour) top speeds for an hour on a single charge — via today’s battery technology — and that during forward cruise it only requires 10 percent of available thrust to maintain altitude. Though the previous version of the aircraft was built from standard prototyping materials, the new version uses fully aerospace-grade composite materials, and the propulsion system is built entirely in-house rather than being assembled through off-the-shelf components as the subscale prototype was.

The high disc loading suggests that the aircraft could be very noisy, but Lilium says the ducting compensates for that, in addition to making the fans more efficient and safer than open rotors. Also on the safety front, the aircraft has a triple-redundant flight control computer, the ability to glide — though its precise ratio hasn’t been disclosed — and a whole-aircraft ballistic parachute.

Lilium currently expects to be ready for widespread commercial air taxi operation — via its own service — by 2025, but will initiate trials in urban locations well before that time.

This story has been updated with additional comment from Lilium.

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