From: David Lednicer <dave@amiwest.com>
Newsgroups: rec.aviation.military
Subject: Re: Shrouded "Fenestron"???
Date: Tue, 13 Jan 1998 09:37:58 -0800

Bob and John McKellar wrote:
> I question this part!

        It all depends on which fenestron you hear and how you measure
noise.

        First, noise is measured by finding the amplitudes at a discrete
number of frequencies and then coming up with a weighted average.  The
weighting is done to try and mimick what the human ear hears.  However,
this system can be fooled.  The early fenestrons have a high amplitude
pure tone.  This means a lot of noise occurs at one frequency and
multiples of this frequency.  In the weighting, this dissapears to a
degree and the fenestron is measured as being quiet.  This pure tone
comes from the fan blades being at equal azimuthal locations, so the
blade passage frequency is the primary tone frequency.

        Newer fenestrons have the blades unequally spaced, like an auto
cooling fan.  This way, the blade passages are not at equal time
intervals and the noise is spread over a lot of frequencies, and none
are have the same peak amplitude as before.  The EC120, EC135 and new
generation Dauphin have such fans.  If my new design ever gets flying,
it too will have this feature.

        The worst noise from almost any helicopter is tail rotor noise.
Rotors don't like being dragged through the air sideways.  The tail
rotor
on the Sikorsky S-76 is particularly bad.  From my research, I find that
even first generation fenestrons produce less noise than equivalent tail
rotors.

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David Lednicer             | "Applied Computational Fluid Dynamics"
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