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A quick google search will show that a lot of Eurocopter models have their tail rotors contained within their tail booms, while a look at Bell or Sikorsky models (and I'm sure others: I'm using Bell and Sikorsky because they're major non-European manufacturers) shows out-of-boom tail rotors. I can see a couple of advantages to the Eurocopter design:

  • Less danger: the fan blades can be protected from harm and be further removed from people.
  • Less noise: I don't have empirical evidence for this, but drawing on knowledge of other ducted systems, this design probably produces less noise, at least in the directions perpendicular to airflow.
  • Pure symmetry: maybe I just like this from an aesthetic perspective, but unlike the unbalanced external tail rotor design, the in-boom design is perfectly balanced.

Is this design less efficient? Is it the space constraints placed on the rotor by the boom?

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The fenestron is a lot heavier than a free tail rotor. This is probably the main disadvantage; helicopters are more sensitive to extra mass than aircraft.

At the time Eurocopter was produced by joining Aerospatiale's and MBB's helicopter divisions in 1992, a follow-on to the successful Bo-105 was under development. That was the Bo-108. Immediately, development was stopped to implement a fenestron into the finished design in order to give all Eurocopter products a common, distinctive feature. This resulted in major structural changes and a heavier helicopter. The result is now known as the EC135.

From this Aerospaceweb article:

The only significant drawback to ducted fans like the Fenestron is that the shroud adds weight that offsets at least some of the improvements in performance.

As @Koyovis' answer points out, the low position of the lateral thrust vector is also a clear disadvantage. See here for more.

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  • $\begingroup$ I would also judge a fenestration to be a lot cleaner design in terms of aerodynamics than a conventional tail rotor. $\endgroup$
    – Romeo_4808N
    Commented Jul 7, 2017 at 15:39
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    $\begingroup$ @CarloFelicione: I understood the question to be about the disadvantage, not the benefits of the fenestron. $\endgroup$
    – Peter Kämpf
    Commented Jul 7, 2017 at 17:54
  • $\begingroup$ Major structural changes to a finished design. When designed in from the beginning, the fenestron does not necessarily lead to higher weight than a tail rotor. For one thing, there is a bit of structure removed: the hole where the fan is mounted. Fan-in-fin makes a lot of sense. $\endgroup$
    – Koyovis
    Commented Jul 7, 2017 at 20:27
  • $\begingroup$ @Koyovis: Oh, come on! All that bulging fairing around the tail rotor does add weight. A fin by itself could be much smaller. $\endgroup$
    – Peter Kämpf
    Commented Jul 8, 2017 at 11:35
  • $\begingroup$ Yes true. But a fenestron requires less power, smaller linkage/gearbox etc, which would provide weight offset in a helicopter designed from the ground up to incorporate one. $\endgroup$
    – Koyovis
    Commented Jul 8, 2017 at 13:23
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As you state, the fan-in-fin design has many advantages, the main ones being no blocking of the side thrust by the vertical fin, and shielding of the blade tips from impact with foreign objects.

There are a few design considerations that could be less advantageous:

  • The fan-in-fin can only be placed at the end of the boom, while a tail rotor can be mounted anywhere on the vertical fin. This can be advantageous in both roll trim (see picture below), in sideways flight and in Dutch roll tendencies. This was found for the AH-64 Apache but probably much less of an issue for a civil helicopter. Source: Raymond Prouty, Helicopter Performance, Stability & Control.
  • The fan-in-fin shroud must be designed carefully so that flow separation at the inlet lip does not occur.
  • The fan-in-fin has more blades than a tail rotor, and produces sound that is higher in frequency than that of a tail rotor. The higher frequency sound is easier for humans to detect, but carries not as far as the lower frequency sound. Experiments have been done with sound phase modulation through unevenly spaced blades. Source: J. Gordon Leishman, Principles of Helicopter Aerodynamics.

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