Re: Question relating to turbofan engines

From:         b18890@vaxb.phx1.aro.allied.com (Arthur Utay)
Organization: AlliedSignal Engines
Date:         22 Nov 94 00:42:59 
References:   1
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In article <airliners.1994.1706@ohare.Chicago.COM>, kannan91@iastate.edu writes:
> 
> I have reading about the construction of turbofan engines
> for commercial aviation and have run across two different
> approaches to building fans :
> 
> 		(a) Snubbered fan, and
>  		(b) Wide-chord fan.
> 

This discussion will be generic as much of the info is proprietary.

Basically, a good fan design should be all airfoil with minimal blockage.
Unfortunately, mechanical requirements, such as bird ingestion tolerance and
vibratory stiffness have historically required the addition of mid-span
snubbers, dampers, nubs, (different names from different companies).

Removing the damper not only improves flow through the fan, but it also lowers
the manufacturing costs associated with the damper protrusion which interferes
with a clean airfoil surface. Rolls-Royce did perfect a wide chord damperless
fan made of two titanium halves diffusion bodd together for the RB-211.
However, they were VERY expensive. 

With the advent of new technologies, the wide chord is back on track at all the
engines manufacturers. There are some risks however. They include:

* Fewer blades per rotor which means a heavier containment projectile which may
require a heavier containment system

* higher torsional strength requirements for bird ingestion

* design requirements to move vibratory signature of the blades AND rotor
assembly above any engine resonances

* longer blade chord requires larger, longer fan disk, larger bearings, stiffer
structure, etc. e.g. more weight

All these factors, with proper design, can be minimized, so there can be a
significant performance and cost benefit with wide chord blades

GE90 will be using composite wide chord blades; I believe other manufacturers
are staying with titanium for the present
-- 
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Arthur W. Utay, Senior Aerospace Specialist          | Your mileage may vary
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