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?