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I have read that it increases thrust and reduces noise. Is there a downside to this?

E-Fan aircraft in flight (Image source: Wikipedia)

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An answer to your question can be found here.

Advantages of ducted fans would be:

  • reduced noise
  • Better containment of broken blades
  • protection of ground personnel when the engine is running
  • Smaller size than a comparable propeller
  • They look like a fancy jet engine. This is probably the main motivation of Airbus for their use on the E-Fan.

For almost all aircraft designers, their drawbacks have weighed more heavily:

  • Less efficiency, because a smaller mass of air is used for propulsion
  • Additional friction drag and weight from the shroud

The highest propulsion efficiency is possible with a large, slow-spinning prop. Once flight speeds approach the speed of sound, the propeller has to become smaller to avoid supersonic propeller tips, and then the shrouded design becomes more attractive. Electric airplanes so far are not fast enough to profit from this effect. I would be very skeptical of the increased thrust claim - both efficiency and thrust of the E-Fan ducted fans are lower than what a well-designed propeller can offer.

The addition of the shroud only increases thrust when compared to the naked fan, but the fan is a poor replacement for a decent propeller at low speed.

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    $\begingroup$ Note that there is a difference between a gasoline engine and an electric motor - a motor produces more torque but is less effective at producing lots of "power" in the high-revs-high-horsepower sense of a "regular" engine. A ducted fan may be more effective at pushing a smaller volume of air "harder" than an engine, almost like a mini turbine with much higher angles of attack per blade than an equivalent propeller. This would be particularly noticeable at lower air/propeller RPM. I don't disagree with your answer, I just think we need to be careful not to directly equate a motor and an engine $\endgroup$ – Jon Story Dec 1 '14 at 11:36
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    $\begingroup$ @JonStory: No, the motor is not important here. What counts is the propulsion efficiency, and this is determined by the fraction $\frac{v_{\infty}}{v_{\infty} + \Delta V}$. For the same thrust, a smaller diameter will require a higher $\Delta v$, which results in lower efficiency. $\endgroup$ – Peter Kämpf Dec 1 '14 at 13:37
  • $\begingroup$ I'm not talking about the efficiency of the propulsion at different velocity, I'm talking about the fact that a motor may be more efficient at a different velocity/power output to a motor. A large, slow propeller may be more effective, but it may be easier for an electric motor to turn a smaller fan faster: eg due to lack of gearing required, and the differing power delivery. That doesn't make a ducted fan better than a propeller, but it could reduce some of the disadvantages usually faced. Leaving more sway for the "It looks cool and is more marketable" factor. $\endgroup$ – Jon Story Dec 1 '14 at 15:45
  • $\begingroup$ @JonStory: OK, that way we can agree. I, however, would prefer if the E-Fan makes the best use of the little energy its batteries provide, and this would be by means of a propeller. $\endgroup$ – Peter Kämpf Dec 1 '14 at 15:56
  • $\begingroup$ @JonStory: I don't think it has anything to do with the motor but rather with the noise, controllability (lower P-factor) and the fact that they probably do plan to develop it to aircraft flying at jet speeds. Electric motors have no trouble with low rpm. $\endgroup$ – Jan Hudec Dec 2 '14 at 18:22
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R W Hovey indicates Ducted Fans are more efficient below 100-110 mph, with 80 % propulsion efficiency (percent of delivered mechanical power that is converted to thrust), above this, a free propeller is better, 85 % efficiency from 200 to 300 mph, up to 400 mph, when it drops to 60 %, and is catched by turbofans, 64 % efficiency at 450 mph; above 500 mph, turbojet leads, but with a low 61 % efficiency, figures for jets start in 40 % at 300 mph.

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