Re: Geared turbofans

Date:         21 Mar 98 16:31:29 
From:         euclid1@juno.com (Ptolomy)
Organization: Netcom
References:   1
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"Campfire" <dcampbell@navnet.net> wrote:
>This is my first posting to this apparently nearly deceased newsgroup(is
>ANYONE out there? :)
>My curiosity was piqued when I read about PW developing a new geared
>turbofan. PW put out a news release stating it was the last great
>improvement to be made with current technology(paraphrasing here, so I hope
>I got it right). Does anyone know what advantages PW expect out of this?
>I'm assuming they'll be running a bigger fan and will need to slow it down
>from the core speed, and that the bigger size is because this engine would
>be for the proposed superjumbo jets. Anyone have any info? Comments? What
>about some of their competitor's comments that this is old technology with
>no inherent advantages?

Dave,
    The basic idea behind the geared fan is maximum efficiency. (of
course)  Anyway,  in most t-fans,  the fan is part of the low pressure
spool - there being the fan, several stages of low press. compressor
and the low pressure turbine all physically linked by a single shaft,
and is separate from the core - or high pressure spool.  The problem
is that all of the lp components rotate at the same rpm, but have
different aerodynamic efficiency ranges; therefore, this set up is a
compromise - reducing overall efficiency.  The fans best eff. is
slower than that of the lpc and the lpt.  By introducing a geared fan
you can slow the fan while increasing the rpm of the lpc and the lpt,
raising overall efficiency.  This will allow higher temps to be
maintained, and better power extracion by the turbines, having the
direct benefit of increased fuel efficiency and ultimately range.
The geared fan is not a new concept, and it is applied effectively in
some small t-fan engines.  P&W's proposal takes the idea a bit further
and applies it to larger engines. It's biggest drawbacks are increased
weight and maintenance costs.  One of the reasons the geared fan idea
has not takenoff in the past has been durability problems with the fan
drive gearing.  Increased efficiency must go hand in hand with
durability and relatively low maintenance costs if one wishes to sell
engines.  Hope this helps.
Regards, JCD.