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Fully Ducted Turbofans  
User currently offlineBoeingnut From , joined Dec 1969, posts, RR:
Posted (12 years 7 months 2 days 18 hours ago) and read 2546 times:

Okey, I'm just a civilian, not in the industry, so this is probably an easy question for you techies out there.

Is there any advantage to having a fully ducted fan (like the RB211 on 744 or CFM56 on 343) over the non fully ducted version?

And, why the switch from "normal" RB211 as on the L-1011 to the fully ducted version on the 744?

I know this should probably go into general aviation, but I know if it went there all the responses I would get would be along the lines of "because RR rules!"

Thanks in advance guys

-Boeingnut

12 replies: All unread, jump to last
 
User currently offlineTrent_800 From United Kingdom, joined Jan 2002, 136 posts, RR: 0
Reply 1, posted (12 years 7 months 19 hours ago) and read 2445 times:

Hi Boeingnut,
Im no expert on the matter but i do have a theory...
On the "non fully ducted" or Half ducted type of turbofan engines the bypass air is allowed to "spread" out after leaving the duct, so not all of it is mixing with the central exhaust gasses. If the duct is extended right to the back of the engine then the Bypass air is mixed with and surrounds the exhause gasses, Thus "masking" the noise a little. So the engine runs quieter. (Im not sure but i think thats how hushkits work on old low-bypass engines. like on the 732)
Im Open to correction but i think that is the reason for it.

Trent_800


User currently offlineBWIrwy4 From United States of America, joined Nov 2001, 940 posts, RR: 1
Reply 2, posted (12 years 6 months 3 weeks 5 days 7 hours ago) and read 2401 times:

That makes sense to me. The RB211s (757) flying over my house are always much quieter than the CFM56s (737), which are smaller engines.

User currently offlineTrent_800 From United Kingdom, joined Jan 2002, 136 posts, RR: 0
Reply 3, posted (12 years 6 months 3 weeks 4 days 6 hours ago) and read 2349 times:

I think that is to do with bypass ratios. For example say two companys make a high-bypass gas turbine engine rated at 40,000lbs thrust each. company 1 use a bypass ratio of 4-1 and company 2 use a bypass-ratio of 7-1 then company 2s engine would be quieter as there is more cold bypass air to surround and mask the hot exhaust gasses.

User currently offlineRmm From Australia, joined Feb 2001, 524 posts, RR: 1
Reply 4, posted (12 years 6 months 3 weeks 4 days 4 hours ago) and read 2344 times:

But then why is the CFM56-5 fully ducted on an A340 and not on an A320? The Trent is fully ducted on an A330 but not on the 777.

Rmm


User currently offlineMetwrench From United States of America, joined Aug 2001, 750 posts, RR: 2
Reply 5, posted (12 years 6 months 3 weeks 4 days 3 hours ago) and read 2337 times:

Sorry, I don't get it. A ducted fan is enclosed by some sort of shroud/duct, right? Let's get beyond the split spool, N1, N2 engine speed etc.

Except for some experimental, Unducted fan applications, which I believe were "pushers", an unducted fan is basically a fancy term for "turbo prop".

But you can't use the word propeller these days, because they are dangerous. Just ask the "Press".


User currently offlineRmm From Australia, joined Feb 2001, 524 posts, RR: 1
Reply 6, posted (12 years 6 months 3 weeks 4 days 2 hours ago) and read 2328 times:

Metwrench,

This is what we are talking about. Same engine, but different nacelle design. This is the CFM56-5 on an A320 and an A340.

Click for large version
Click here for full size photo!

Photo © Glenn Alderton



Click for large version
Click here for full size photo!

Photo © Jottier Jochem



Rmm


User currently offlineTrent_800 From United Kingdom, joined Jan 2002, 136 posts, RR: 0
Reply 7, posted (12 years 6 months 3 weeks 3 days 19 hours ago) and read 2317 times:

I was thinkng along the lines of these type of ducting

Click for large version
Click here for full size photo!

Photo © Jason Taperell


Where the Bypass duct is very short in comparrison to the length of the engine.


User currently offlineBoeingnut From , joined Dec 1969, posts, RR:
Reply 8, posted (12 years 6 months 3 weeks 3 days 12 hours ago) and read 2311 times:

RMM has it right, that was exactly what I was thinking about!

Fully ducted RB211

Click for large version
Click here for full size photo!

Photo © Jason Taperell



"normal" RB211

Click for large version
Click here for full size photo!

Photo © Torsten Maiwald



So does anyone have any solid reasons for this, or just a "I think its this from what I hear going over my house" type of thing. I'd like to hear from some engineers or techs if any are out there.

Thanks for all your replies, guys, too! But I'm still unclear as to WHY some engines have the full length nacelle and others of the same engine family on different airframes don't


User currently offlineJsuen From United States of America, joined Aug 2001, 211 posts, RR: 0
Reply 9, posted (12 years 6 months 3 weeks 3 days 11 hours ago) and read 2301 times:

http://yarchive.net/air/airliners/engine_maker_overview.html says:


Other visible RB211 differences are: 1) the engine rotates in the "wrong" direction (clockwise, forward looking aft), 2) the newer RB211s (-535E4 and
-524G/H) have "integrated exhaust nozzles", i.e., the bypass flow is mixed with the hot jet flow from the core before it is exhausted. The most visible characteristic of integrated exhaust nozzles is that the cowling run through the whole engine length. The advantages of mixing the bypass and core flows are improved thrust, reduced specific fuel consumption and reduced noise; but there will be more drag, the engines will be heavier and more items are needed to be maintained. Other high-bypass turbofan engines that use the same feature include the V2500 and the CFM56-5C on the A340. However, the Trent 800 does not seem to have the integrated nozzle.


Also, may I add that the integrated nozzles also enhance reverse thrust performance.


User currently offlineRmm From Australia, joined Feb 2001, 524 posts, RR: 1
Reply 10, posted (12 years 6 months 3 weeks 3 days 10 hours ago) and read 2289 times:

I can't see how the integrated nozzle improves reverse thrust. If your thinking along the the lines that they reverse both hot and cold stream thrust they don't. The
ones I've looked at have the blocker doors ahead of the inner exhaust nozzle.

While were on the subject the fully ducted design goes back along way. Take a look at the JT3D's fitted to a 707 and the same engine fitted to a DC8.

Rmm



User currently offlineJsuen From United States of America, joined Aug 2001, 211 posts, RR: 0
Reply 11, posted (12 years 6 months 3 weeks 3 days 8 hours ago) and read 2274 times:

What about bucket-type thrust reversers? The BR715 is a good example.


Click for large version
Click here for full size photo!

Photo © Marc Sauthier



Click for large version
Click here for full size photo!

Photo © Aric Thalman



Here's something else interesting: saw-tooth edges on the nacelle: http://www.rolls-royce.com/perspective/tech/default.htm#


User currently offlineBabyJumbo-SP From Hong Kong, joined May 2001, 28 posts, RR: 0
Reply 12, posted (12 years 6 months 2 weeks 6 days 2 hours ago) and read 2236 times:

I don't know exactly why some engines are fully ducted and some aren't, but I would like to clarify that the RB211-22B used on Tristar is over 10 years older design than the RB211-524G/H used on 744, so the difference in duct design concept is understandable. About the difference between Trent 700 on A330 and Trent 800 on 777, I think the aircraft manufacturers also contributed a lot on the duct design, so the concept used may seem different.

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