Re: Center Engine performance (was: Delta Flt 1288)

From: (MikeM727)
Organization: America Online, Inc. (1-800-827-6364)
Date:         25 Sep 96 13:40:10 
References:   1
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Somebody wrote:

>>>> Let me re-state the question: "does the
>dgd> engine installed in the #2 position suffer a performance
>dgd> degradation due to the presence of the S-duct?"<<
>dgd> The only performance corrections that I know of for the center
>dgd> engine are if the 6th stage bleed system has been activated. That
>dgd> calls for a 0.05 EPR reduction for takeoff and GA power.

The max EPR is higher (from 0.02 to 0.04) on #2 than the pod engines.
However, this is not because of the S-duct, it it because of the bleed
configuration.  Normally the pod engines are the only ones bled for a/c -
pressurization.  Number 2 cannot be bled for A/C on takeoff.  If engine
anti-ice is selected, #2 is also bled, and the appropriate EPR correction
results in a "straight across" EPR setting. (Max EPR equal on all engines)
 BTW, engine anti-ice bleeds 6th, 8th and 13th stage bleed air from #2,
one cannot just select one stage of air to bleed.

>This is all very true.  However, from what I understand, because the
>#2 engine in B727 sees "dirty" air from the S-duct, it is more prone
>to stall/surge problems than engines #1 and #3.

I'm not sure what you mean by "dirty".  However, #2 is more prone to
compressor stalls when there is insufficient airflow into the duct.  This
often happens in crosswinds, when the wind blows across the S-duct, and
not straight down it.  The technique calls for pushing up #1 and #3 for
takeoff until some airspeed is generated to provide airflow down the
S-duct, at which point #2 is brought up for takeoff.

>Therefore, I think that the stall margin performance standard for the
>center engine is somewhat higher than those for the pod engines.  Note
>that this does NOT mean that the center engine is of a different type,
>just that after being tested in the test cell after overhaul, those
>engines which have the best demonstrated stall margins are designated
>for center engine duty.

Not that I'm aware of.  All engines are interchangeable and no special
considerations are given to which one goes where.  This would raise
operating expenses if operators had to maintain a stock of "center
engines" with higher demonstrated stall margins.

ATP/FE, Boeing 727
Props are for boats!