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Topic: High Bipass Fan Nozzle
Username: Speedracer1407
Posted 2005-02-21 09:57:15 and read 2037 times.

Ok folks, here comes what might be a fairly rudimentary question, but one that I've been curious about and never bothered to ask.

Some modern turbofans, seem to collect all bipass and core exhaust into a single nozzle before it reaches the atmosphere, as on this 757:

photoid:780569

Others (seems like MOST others) rout bypass and core exhaust seperately all the way to the atmosphere, as on this 777:

photoid:777465


What are some of the factors that engineers consider when choosing one design over the other. I'm guessing noise may have something to do with it, since power rating is not consistent with the use of either design.

Also, on all wing-mounted engines, the pylon displaces bypass airflow at the 12 o-clock position. Obviuosly, this can't be avoided, and the overall thust of an engine as on the 777 couldn't be too bothered by a thin, aerodynamic obstruction, but does it reduce total thrust output at all? would engine #2 on a trijet like the MD-11 produce fractionally more thrust than the others? Thanks for any replies,

O

Topic: RE: High Bipass Fan Nozzle
Username: Speedracer1407
Posted 2005-02-21 10:22:40 and read 1991 times.

Doh! I screwed up the pictures...here they are in the order they should have appeared in my post.


View Large View Medium
Click here for bigger photo!

Photo © Simon Willson



And



View Large View Medium
Click here for bigger photo!

Photo © Miguel Cláudio

Topic: RE: High Bipass Fan Nozzle
Username: Dl757md
Posted 2005-02-21 10:28:06 and read 1994 times.


View Large View Medium
Click here for bigger photo!

Photo © Simon Willson




View Large View Medium
Click here for bigger photo!

Photo © Miguel Cláudio



There's the pictures you linked to.
Quoting Speedracer1407 (reply 0):
would engine #2 on a trijet like the MD-11 produce fractionally more thrust than the others?


No there is still a pylon on a tail mounted engine.

Dl757Md

Topic: RE: High Bipass Fan Nozzle
Username: Speedracer1407
Posted 2005-02-21 10:49:41 and read 1962 times.

Crap. my bad. It does indeed. I should have looked at some pics before i posted. Anyway....I'm still curious about the rest of my questions.

O

Topic: RE: High Bipass Fan Nozzle
Username: Air2gxs
Posted 2005-02-21 13:19:15 and read 1791 times.

As to design, I have no idea. But your hypothesis about tail mounted engines is incorrect. The engine is the same. Therefore the support structure for the engine doesn't change. Airflow through the core and the bypass is identical whether the engine is mounted on a wing or in the tail. Any performance variation comes from the length and/or shape of the inlet duct.

Topic: RE: High Bipass Fan Nozzle
Username: Widebodyphotog
Posted 2005-02-21 15:29:04 and read 1654 times.

The different types of mixing nozzles on HBPR turbofans is dictated by the method of noise attenuation. The cowl integrated mixer nozzle found on the Rolls-Royce 535 and 524 allows the bypass airflow to completely surround the hot high speed core exhaust flow on these relatively low, 4.3:1, BPR engines. It also serves to make the engines considerably heavier than their counterparts. By contrast, the higher BPR of for example, Trent 800 and GE90, allows for the use of segmented mixer nozzles because the relatively small core exhaust airflow does not make as high a contribution to noise. Basically it comes down to BPR. In general a BPR above 5.4-6 allows the use of lighter segmented mixer nozzles and engines below that will use cowl integrated mixer nozzles. There are also structural considerations depending on the type of installation, tail vs wing mounted, to consider as well but general the principal driving factor is BPR and noise attenuation.

-widebodyphotog


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