Re: Boeing naming convention for 777

Date:         27 Aug 99 03:07:56 
From:         "M. Jones" <rmjones@cyberhighway.net>
References:   1 2 3
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Karl Swartz wrote:
>I was specifically thinking of the three different models of smaller
>PW4000s used by United:
>
>   767-300	PW4052
>   747-400	PW4056
>   767-300(ER)	PW4060
>
>They're all physically the same.  I had the impression that UA could
>actually swap one for another and twiddle a few bits to give it the
>right thrust level, presumably including some extra payments to Pratt
>and lower life of some components.  That way, if an AOG 767 could get
>a 747-400 engine if that was the only spare available, or vice versa.

Maybe, maybe not.  This is speculation on my part.  I'm not sure how P&W
does things, but I do know how other GT manufacturers handle this
situation.  You MAY be able to de rate a higher thrust version for use
where lower is required if you were willing to give up some efficiency,
but I'd be somewhat surprised if you could go the other way.

For max efficiency, a turbine engine likes to be at as high a firing
temp as the current state of materials allows.  For air-cooling, that
currently means about 2300 deg. F. (1260 deg. C.), and that is pretty
much where all the manufacturers are these days.  This carries through
to part-load operation as well.  In cruise, a high firing temperature is
still desirable from an efficiency standpoint.  It is inefficient to
have an engine with too much thrust for the desired mission;  at cruise
you are backing it down too far for maximum efficiency.  This goal of
matching desired thrust to the mission is very often accomplished within
a given engine family by using turbine (referring to the specific
component, and not the engine as a whole) inlet nozzles of specific flow
area.  Smaller flow-area nozzles decrease the maximum possible mass flow
through the core, but do allow you to fire appropriately in cruise for
max efficiency.  Changing these out would typically be a multi-hour
field operation.

Compressor guide vanes (variable-geometry stator elements which tailor
flow to a specific compressor stage in order to improve compressor
efficiency and surge margin at part load) are controlled as a matter of
course by the engine controls and can be varied to provide some of the
same effect as changing turbine nozzles, but only over a relatively
narrow range, and only to lower thrust.  (i.e. they would normally be
scheduled to provide max thrust, but they could be closed somewhat
compared to optimum to reduce thrust for a given firing temp.)

So two engines of the same family (same core) might have very different
thrust ratings because of internal differences.

Anybody know for sure how P&W does it?

Mike Jones