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How are Fenestrons (or fantails) able to reduce the tip vortex losses in a helicopter?

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    $\begingroup$ it's basically a ducted fan: aviation.stackexchange.com/questions/13995 $\endgroup$
    – Federico
    Jun 10, 2015 at 10:53
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    $\begingroup$ A link to some background information would be really helpful here: I have no idea who or what Fenestron is $\endgroup$
    – Pondlife
    Jun 10, 2015 at 11:06
  • $\begingroup$ Which tips, tail or main rotor? If main rotor, they don't. If tail rotor, the duplicate covers this. $\endgroup$
    – Simon
    Jun 10, 2015 at 16:50

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There are basically two effects here:

  • One is the ducted fan effect. Ducted fans are more efficient than open fans, specifically creating smaller vortices at the blade tips.

  • The other is the fact that the tail rotor of a helicopter is not situated in an area with almost linear airflow along the rotor axis. It is also submitted to:

    • A steady flow due to helicopter forward motion. This is coming from the side of the rotor, so affects opposite rotor blades differently.
    • A buffeting flow from the h/c main rotor. This is an unsteady flow, also coming laterally from the point of view of the tail rotor (actually, the top).

Fenestron tail-rotor ducts can be considered a fairing that reduces airflow across the rotor. The end result is that the rotor blades are in a much smoother flow that helps reduce unwanted turbulence. This is one of the reasons such rotors are quieter than regular open tail-rotors (the other being the fairing material absorbing some of the noise).

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