Vertiflite May/June 2021

Lilium says its ducted electric vectored thrust approach allows control and mitigation of the noise from its high disk- loading fans.

Flight test data to date vs. prediction: “Representation of the high lift concept allowing for attached flow on the wings, high controllability and steadily increasing efficiency throughout the transition phase.” The graph indicates that Lilium flew the 5-Seater to about 32% of its 186 mph (300 km/h) cruise speed, presumably 60 mph (97 km/h).

The duct also contains the noise, so it can only be projected forward or rearward, stopping it from radiating in all directions, as it does with open propellers.Within the duct itself, Lilium has applied acoustic liners, which absorb more of the noise energy and dampen those down or tune them out. A fan has to work quite hard in comparison to an open propeller for a given diameter, he said. However, it has the advantage that it’s a very efficient system because there is reduced leakage around the blade tips, producing an efficiency benefit of about 40%.

The ducts also help match the flow through the “electric engine” throughout its mission. A significant amount of air flow is required for takeoff, but much less is needed in cruise. To match the fan performance at cruise, Lilium changes the geometrical area at the back of the fans with a variable nozzle; as the angle of the ducted fan changes, there’s a simple mechanical link, which then closes down that area, optimizing the air flow both in hover and in cruise, McIntosh explained.

May / June 2021

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