Do some e-vtol aircraft, like the Lilium need shrouds and stators to maximize thrust from stacked electric motors and props? Liliumjet

The aircraft is actually called the Lilium Jet, although it does not heat the air passing through the props by burning a fuel like in traditional jet engines. Not sure if that really qualifies it to be called a jet.

I understand that the Lilium jet, NASA's X57 and the Opener Blackfly use a concept called distributed propulsion, to accelerate air over a wing.

Eg. If I want to fly the Blackfly at 28mph with a single prop, according to my calculations, I would need a wing with about 180 sq ft ( the Lazair had 140 sq ft, or a wing loading of about 3 lbs/sq ft). Using distributed propulsion, the wing is seeing about 2.5 times that speed, so it only needs 40 sq ft or about 10 lbs/sq ft of wing loading. The smaller the wing area, the less drag at cruise speeds, so the smaller the wing area the better.

What if I stack 2 motors and props axially?

The exit velocity of the first prop (about 70 mph, or about 230% increase), becomes the inlet velocity of the next prop, which in turn increases the exit velocity (say only 130% increase, the higher the inlet velocity, the lower the increase), say 90mph. Now I can size my wings for a minimum velocity of 90mph, or maybe 20lbs/sq wing loading, even though my aircraft is still only flying at 28mph.

Does this work?

Would it work or work exponentially better with a shroud?

What if I put a stator ( eg, a "fixed 8 bladed prop" with blades arranged opposite to the rotation of the previous prop) in-between the props inside a shroud to slow down the velocity of the inlet velocity of the next prop and increase the pressure. Now the exit velocity increase of the 2nd prop would be much higher (also around 200%??). Now I can add an expansion nozzle at the end of the shroud, to convert that increase in pressure to an increase in velocity. Now I could resize my wings for maybe 200mph minimum speed, or about 60-70lbs/sq ft wing loading, even though my aircraft is still only flying at 28mph.

Essentially you're building a jet engine, but without any heating.

Does this work, at least conceptually, even though you're not heating any air with fuel?

NASA's X-57 only has a single row of folding props, so I imagine that this is why the shrouds are not needed.


  • $\begingroup$ We have a number of questions already about the use of shrouded/ducted fans, e.g. here, here, here (and others linked from them). Do any of them help? $\endgroup$
    – Pondlife
    Commented Aug 30, 2021 at 15:42
  • $\begingroup$ Not really. I've adjusted the question slightly. I'm really asking if stacked props need shrouds. $\endgroup$
    – Fred
    Commented Aug 30, 2021 at 19:36
  • $\begingroup$ The Lilium Jet uses ducted fans, but turbofan engines are essentially the same with a different power source. $\endgroup$ Commented Aug 31, 2021 at 2:34

1 Answer 1


The main problem with a setup like this is it can’t really glide. You would never be able to land conventionally as you would have to increase thrust to get the lift needed to land. This isn’t a problem when taking off but when landing you need to be slowing down not speeding up.

Also the benefit of smaller wings would be quickly outweighed by the very low propulsive efficiency of the fans. Compressing air and expelling it via a convergent-divergent nozzle is very inefficient for sub sonic flight. It would likely take 3-4x more power for the same thrust.

To answer your question in the comment there are many examples of counter-rotating props without shrouds.


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