Re: 757 engine (fan?) noise during freewheel

Organization: AlliedSignal Engines
Date:         28 Sep 95 03:52:44 
References:   1
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In article <airliners.1995.1496@ohare.Chicago.COM>, (Henry Law) writes:
> 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?

I hope this response is not over kill, but that is problem when an engineer
tries to explain something.

The sound you hear is the fan blades rattling around hitting as the fan
slowly rotates.  The blade attachment (called a dovetail) is tapered;
narrow at top and wide at the bottom. The blade slides into a similarly
shaped slot in the fan disk.  The attachment is designed so the when the
fan is rotating at high speeds, centrifugal forces wedges the blades firmly
in the slots.  At very low speed (like when the fan is windmilling on the
ground), the blades are loose (for small engines, typically a few
thousandth of an inch).  As the rotor slowly spins, the blades on the
bottom are seated in the slot due to gravity, as they move to the top,
they fall out.  This makes noise.  Also, if the fan has mid span dampers
(those little ears on the blades), there is enough movement in the blades
that the adjacent blades can clank together.

So, you weren't far off the mark.  The blades aren't hitting the cowling,
just each other.

Mark Johnston
AlliedSignal Engines
Phx AZ