Re: Question relating to turbofan engines

From:         chuanga@iia.org (Andrew Chuang)
Organization: International Internet Association.
Date:         22 Nov 94 00:42:59 
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In article <airliners.1994.1706@ohare.Chicago.COM>,
 <kannan91@iastate.edu> wrote:
>
>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.
>
>Would it be possible for someone to enlighten me a little
>more on this subject (in terms of the differences in 
>performance) and/or point me in the right direction.

There was an excellent article in Flight International titled
"Wide Chord Fan Club".  I don't have the date of the article.  I
believe it was in their 1990 Commercial Engines Directory issue,
which is usually published during mid-year.  If it is not, then
check out the 1991 Directory issue.

In a few words, snubbered fan (or shrouded fan) blades are narrow
and long (high aspect ratio).  The part-span shrouds are there to
prevent vibrations.  They are usually made of solid titanium.  On
the other hand, the wide-chord fan (WCF) blades have much lower aspect
ratio.  Since, there is no blockage in the passage, the fan will
pump more air, and the fan efficiency will be higher.  Rolls-Royce
was definitely the pioneer, and they paid the price.  The original
design for the RB211-22B (the L1011 powerplant) used composite
materials for the WCF.  During their development, they encountered
many difficulties, and that was a major reason for R-R's bankruptcy.
Eventually, they went back to the conventional fan.  (Arguably, R-R's
bankruptcy resulted in the eventual demise of the L1011.)  The first
in-service WCF was the RB211-535E4 on the B757.  This time, the blades
are made of honeycomb-filled titanium (or hollowed titanium).  The reason
for using light-weight materials is because the WCF are much larger than
the conventional blade.  If same material is used, then the weight
penalty will erase any of the aerodynamic gain.

Other in-production or "near-production" large engines that use WCF
blades are the RB211-524G/H, V2500, PW4084, GE90, Trent 700/800, and
CFM56-7.  The GE90 is the only one in the pack that uses composite WCF.

-- 
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         |  H Andrew Chuang    chuanga@iia.org  |
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