Re: Boeing naming convention for 777

Date:         04 Sep 99 11:38:41 
From:         "robert wright" <kdol97@home.com>
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>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.

    I believe it is sort of a combination.  As I understand it, the PW4074
and 4077 are the same, but the FADEC for the 4074 limits the fuel flow and
there are probably slightly different stator vane schedules to limit
airflow, as well.  I'm not sure how different the 4084 and 4090 are, but the
4098 is a very different machine.  It has a new compressor with extra
stages, for one thing, and the burner exit area is different as well, and in
fact is different from what was originally planned, due to certification
problems!

>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

    I don't think this is correct.  As far as I know, the 4098 has the
highest turbine inlet temperature of any commercial engine in service.  I
don't know if I can say what it is, but it is a lot higher than 2300 F.
(Like, more than 1000 degrees F hotter)

>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

    I think if there is a small market for the smaller engines it may be
worth it to the manufacturer to risk losing a few sales by having lower
efficiency and thereby streamline the production system.  (Fewest numbers of
different configurations = fewest number of components to make and fewest
number of possible mistakes.)  I know several operators use higher-thrust
engines derated to increase component life.  I am not familiar with how
ETOPS regulations take this into account.

>Anybody know for sure how P&W does it?

    That's my take.  If I get a free moment this week I'll try to walk
downstairs and find the PW4000 guys....

RSW