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According to this article https://www.wired.com/story/lilium-electric-aircraft-prototype-first-flight-jet/ https://www.wired.com/story/lilium-electric-aircraft-

According to this article https://www.wired.com/story/lilium-electric-aircraft-prototype-first-flight-jet/ https://www.wired.com/story/lilium-electric-aircraft-prototy... > The “Lilium Jet” uses 36 electric-powered ducted fans. Inside each, a small rotor ingests air from the front and pushes it out of the rear at higher speeds. They’re not technically jet engines (so the aircraft isn’t a “jet”). The lack of spinning blades improves efficiency, reduces noise, and eliminates the risk of turning passing birds into chop suey.

mannykannot 8y ago
That article is not very accurate: > The lack of spinning blades improves efficiency,... A ducted fan is, in fact, a bunch of blades spinning in a duct. A well-designed ducted fan can be more efficient than a propeller of the same diameter. That efficiency is also more dependent on airspeed, as any given duct shape must be designed for a particular flow [1]. (it looks as though this design may have a variable nozzle, which could broaden the efficiency hump over a wider speed range.) >...reduces noise... To some extent, though the ducted fans on airliners still demonstrate a fair amount of noise from the fan itself. The video continues the practice of not giving a clear indication of how much noise is being generated. >...eliminates the risk of turning passing birds into chop suey. As there are rotating blades, this does not follow. It is not going to be healthy for either bird or fan when they collide. At least this airplane has a lot of fans. [1] http://massflow.archivale.com/ductbook.htm http://massflow.archivale.com/ductbook.htm
- larkeith 8y ago
  > eliminates the risk of turning passing birds into chop suey. To be fair, it's a lot easier to put a debris (including birds) filter in front of a ducted fan than a prop.
  - mannykannot 8y ago
    There are efficiency issues, and it is not often done. It is hard to tell, but it does not seem to have been done here.
- aidenn0 8y ago
  They say "20% as loud as a helicopter" and I found a Bell J2-A at 100 Ft is 100dB, so maybe 93ish dB at 100 feet? That's about as loud as a motorcycle at 25 feet, so significantly louder still than any road traffic.

mannykannot 8y ago

That article is not very accurate: > The lack of spinning blades improves efficiency,... A ducted fan is, in fact, a bunch of blades spinning in a duct. A well-designed ducted fan can be more efficient than a propeller of the same diameter. That efficiency is also more dependent on airspeed, as any given duct shape must be designed for a particular flow [1]. (it looks as though this design may have a variable nozzle, which could broaden the efficiency hump over a wider speed range.) >...reduces noise... To some extent, though the ducted fans on airliners still demonstrate a fair amount of noise from the fan itself. The video continues the practice of not giving a clear indication of how much noise is being generated. >...eliminates the risk of turning passing birds into chop suey. As there are rotating blades, this does not follow. It is not going to be healthy for either bird or fan when they collide. At least this airplane has a lot of fans. [1] http://massflow.archivale.com/ductbook.htm http://massflow.archivale.com/ductbook.htm

larkeith 8y ago
> eliminates the risk of turning passing birds into chop suey. To be fair, it's a lot easier to put a debris (including birds) filter in front of a ducted fan than a prop.
- mannykannot 8y ago
  There are efficiency issues, and it is not often done. It is hard to tell, but it does not seem to have been done here.
aidenn0 8y ago
They say "20% as loud as a helicopter" and I found a Bell J2-A at 100 Ft is 100dB, so maybe 93ish dB at 100 feet? That's about as loud as a motorcycle at 25 feet, so significantly louder still than any road traffic.