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DocLightning
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Engine Nacelle Shapes?

Sun Sep 28, 2008 1:06 pm

So I'm not sure if this has been covered, but something has been bugging me for a while.

On the 744, the GE and PW engines have a shape in which the exhaust stage from the core (or at least that's what I think it is) protrudes out the back of the nacelle. By contrast, the RR design features a nacelle that completely covers the entire engine.

Why? Why did RR choose such a different design from GE and PW? And why on the 777 did they not choose such a design?
-Doc Lightning-

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Blackbird
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Joined: Wed Oct 06, 1999 10:48 am

RE: Engine Nacelle Shapes?

Sun Sep 28, 2008 3:40 pm

I guess covering the whole engine would reduce exhaust velocity more, but better mix the hot and cold streams (hot stream = core, cold stream = fan-air) thus reducing noise a lot.

However, the shorter fan-duct allows a hotter, slightly faster exhaust stream which while a little louder at sea-level produces less of a loss in thrust at high-altitudes and better thrust efficiency.

The RB-211's probably have better pressure ratios and generally produce more thrust than the P&W and GE counterparts and as a result could probably afford to take the slight hit to reduce noise a bit more.


At least that's my guess.


Blackbird
 
CFMTurboFan
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RE: Engine Nacelle Shapes?

Tue Sep 30, 2008 4:17 am

Not sure why, but one of the main differences on the 744 engine nacelle is that the RR nacelle is engine mounted, not airframe mounted as the PW & GE engines. PW & GE have C ducts fitted which house the thrust reverser, and are therefore airframe mounted. RR engines have the Thrust reverser mounted on the rear of the fan case, which is removed as part of the engine during an engine change.

The rear of the bypass duct on the RB211 is called an INA (integrated nozzle assembly) this has to be removed in the engine shop prior to removing the thrust reverser.

In the event a RB211 trust reverser suffers damage on an aircraft, the whole engine needs to be removed from the aircraft in order to change the thrust reverser. I think, that C ducts can be changed on PW & GE powered aircraft without having to remove the engine. Maybe someone with more PW & GE experience can verify this.......
 
AA737-823
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RE: Engine Nacelle Shapes?

Tue Sep 30, 2008 8:58 am

There was a thread about this subject very recently- do a search and you MAY be able to find it... though I'm not hedging my bets.

Anyhow, There are tradeoffs, as you might imagine.
Someone will undoubtedly argue with me, and tell you that I'm wrong, but that doesn't typically bother me.

It's a tradeoff between weight and drag.
A full-length cowl offers less drag, but weighs more.
A shortened fan cowl weighs less (in some cases, a LOT less), but doesn't provide as pleasant a path for airflow around the engine.

The 777 was the first aircraft where... oh shoot, I can't remember the details here... memory getting fuzzy.
Previously, cowls were designed by Boeing, or the engine supplier, can't remember which. But with the 777, it swapped. I can't remember which way it swapped, but you get the idea.
That's according to "Modern Boeing Jetliners."
 
tdscanuck
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RE: Engine Nacelle Shapes?

Wed Oct 01, 2008 2:06 am



Quoting CFMTurboFan (Reply 2):
Not sure why, but one of the main differences on the 744 engine nacelle is that the RR nacelle is engine mounted, not airframe mounted as the PW & GE engines.

'cause the whole nacelle package for the RR on the 744 went to RR, including the cowl and reverser (they then farmed it out, to Goodrich I think). The nacelles on the GE and PW engines weren't done by the engine manucturers, hence the different packaging.

Quoting CFMTurboFan (Reply 2):
. I think, that C ducts can be changed on PW & GE powered aircraft without having to remove the engine. Maybe someone with more PW & GE experience can verify this.......

Confirmed. Also true of CFM. Most Boeing's work this way.

Quoting AA737-823 (Reply 3):
Previously, cowls were designed by Boeing, or the engine supplier, can't remember which. But with the 777, it swapped. I can't remember which way it swapped, but you get the idea.

737 cowls are Boeing, so I'm guessing it swapped to be the engine maker doing the cowl. Offhand, I'm not sure how the 787 was done.

Tom.
 
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Jetlagged
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RE: Engine Nacelle Shapes?

Wed Oct 01, 2008 3:06 pm



Quoting AA737-823 (Reply 3):
Previously, cowls were designed by Boeing, or the engine supplier, can't remember which. But with the 777, it swapped. I can't remember which way it swapped, but you get the idea.

Rolls Royce used to deliver fully cowled RB211s to the manufacturer. I'm fairly sure they designed them too, since as they used the mixed exhaust concept.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

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