Re: 757 engine (fan?) noise during freewheel

From:         bradg@io.org (Brad Gillies)
Organization: Leading Edge Aviation Services
Date:         28 Sep 95 03:52:45 
References:   1
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In article <airliners.1995.1496@ohare.Chicago.COM>,
   hjl@thelaws.demon.co.uk (Henry Law) wrote:
>Frequently when boarding a BA 757 (RR engines) by steps rather than
>through an air bridge I hear a distinct metallic clanking from the
>engine closest to me, which is nearly always still freewheeling from the
>previous sector.  It's regular, but with gaps - rather like the sort of
>noise you'd make by rattling something metal along some iron railings.
>I've heard it in these circumstances for years so it's obviously not
>what it sounds like (which is the fan blades hitting the cowling! :-)
>but I've always wondered what it was.  Can one of our 757 jockeys
>enlighten me?

The noise you hear is the sound of the "Part span shrouds" banging together
as the fan slowly rotates the "Clappers" hit together and make the noise.
The shrouds are there to cause rigidity of the fan when under load
(running) They maintain balance and spacing of the blades. On newer
generation engines the fan has rigid blades which do not require these
shrouds so there is no noise when the fan "freewheels"