# What would be the potential thrust of an “electric jet” like Lilium's?

Lilium claims to fly with "electric jets". Throwing out the debate over whether a non-combustion compression of a stream of air is a jet or only a ducted fan, what is the potential thrust each of these could be getting?

Edit: since it's not compressing, what potential lift could the Lilium prototype engines provide, and are there examples of electric engines today that provide compression in a significant way?

Ignoring the stability issue of Lilium as well. Just curious about the ducted fan design they have. Also ignore the problems with it being electric and battery powered please :)

• Full marks for the marketing guys at Lilium. What a great scheme, to call it an electric jet instead of a garden variety household fan. I reckon the range of the Lilium is only limited by the length of the extension cord. – Koyovis May 28 '17 at 14:56
• There is no way that design creates any kind of meaningful compression, ignoring the fact that the rapid expansion by burning fuel is the main reason for compressing the air in the first place... – Ron Beyer May 28 '17 at 16:28
• Related, if not duplicate: Could an electric engine provide the same performance as jet engines on current aircraft? – mins May 28 '17 at 16:56
• Also, not a duplicate mainly because I'm curious what thrust these specific engines could currently be providing. – oeste May 28 '17 at 17:10

They look like 15 cm fans, thrust of which is competently ROM-ed in this answer.: about 40 to 60 Newton.

Note that the $C_T$ in the equation depends on fan solidity, the higher the number of blades and the wider they are, the more thrust they produce, but also the higher power required. For a given fan power (and battery life) it makes more sense to increase fan diameter and decrease fan solidity.

• The linked answer assumes 27,000 RPM and Mach 0.65 tip speed. That is already pushing it. +1, also for the comment under the question. – Peter Kämpf May 29 '20 at 11:33

Lilium's 15 cm thrusters remain THE big mystery in the eVTOL community. Nonetheless, Lilium was successful in attracting another 'few' tens of millions of VC recently. There's a reason why helicopters use wide-diameter rotors. Has to do with the ability to lift, torque and energy efficiency of course. Interesting comparison:

• This doesn't answer any of the questions asked. – FreeMan May 29 '20 at 10:58
• Still, it shows two things: That Lilium's claims are highly dubious and that their investors are extremely gullible. – Peter Kämpf May 29 '20 at 11:36
• The helicopter comparison is perfect. The Lilium may gain some efficiency by operating as a "blown wing". There was some research done on "channel wings" years ago (this would be a "straightened channel wing"). – Robert DiGiovanni May 30 '20 at 7:32

There is a difference between a fan and a compressor: the one provides thrust, the other compression. In a turbofan engine, there are separate sets of blades to do each job.

The Lilium's ducted fan is an example of a wider class of ducted propulsor. They all work the same way, to accelerate air in one direction so that the reaction force pushes the fan or propeller in the other.

If the blade disc were used to provide compression, the device would not be a propulsor but a compressor and would not exit to the free air stream.

Blade aerodynamics being equal, the maximum potential thrust is determined by intake diameter, which is obviously rather small: a single device would power a 3 m (10 ft) span model plane quite nicely.

Stacking up crazy numbers of small fans obviously increases total thrust in proportion, but it is less efficient than a couple of big ones. Integration with the airframe aerodynamics, especially the wing, is critical. It was studied intensively about half a century ago, such as for the Vought ADAM V/STOL jetliner project (illustrated article here). One can at least say that the Lilium configuration is not optimal (there was a question about exactly that a couple of months ago, if anybody can find it).

• The main difference between blower fan and compressor is the amount of blockage in the exhaust. If there is a turbine in the way, compression occurs. If the nozzle is suitably shaped and free of obstructions, air gets just accelerated. – Peter Kämpf May 29 '20 at 11:38
• Indeed. However the blade design parameters change accordingly and in the wider context beyond aero engines, the distinction is well established (although the term "blower" is imprecise as it is also often used for a high-throughput, low-pressure compressor). – Guy Inchbald May 29 '20 at 12:23