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Fully Ducted Turbofans  
User currently offlineBoeingnut From , joined Dec 1969, posts, RR:
Posted (13 years 7 months 4 days 21 hours ago) and read 6512 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

31 replies: All unread, showing first 25:
 
User currently offlineTrent_800 From United Kingdom, joined Jan 2002, 136 posts, RR: 0
Reply 1, posted (13 years 7 months 2 days 22 hours ago) and read 6417 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 (13 years 6 months 4 weeks 10 hours ago) and read 6370 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 (13 years 6 months 3 weeks 6 days 9 hours ago) and read 6318 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, 525 posts, RR: 1
Reply 4, posted (13 years 6 months 3 weeks 6 days 7 hours ago) and read 6313 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 (13 years 6 months 3 weeks 6 days 6 hours ago) and read 6307 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, 525 posts, RR: 1
Reply 6, posted (13 years 6 months 3 weeks 6 days 5 hours ago) and read 6297 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 (13 years 6 months 3 weeks 5 days 22 hours ago) and read 6283 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 (13 years 6 months 3 weeks 5 days 16 hours ago) and read 6277 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 (13 years 6 months 3 weeks 5 days 14 hours ago) and read 6267 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, 525 posts, RR: 1
Reply 10, posted (13 years 6 months 3 weeks 5 days 13 hours ago) and read 6255 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 (13 years 6 months 3 weeks 5 days 12 hours ago) and read 6240 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 (13 years 6 months 3 weeks 1 day 5 hours ago) and read 6203 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.

User currently offlineLH707330 From United States of America, joined Jun 2012, 1192 posts, RR: 1
Reply 13, posted (2 months 1 week 1 day 23 hours ago) and read 3508 times:

The glory days for the mixed exhaust based on the aero and weight tradeoff was in the 1985-1995 timeframe, with the RB211-524G/H, -535E, V2500, CFM56-5C, and T700. With a BPR around 5-6, it seems the propulsive advantages outweighed the weight penalties, or were at least similar, which is why we see a diversity of designs from then. Based on the A340 design, I think that the mixed exhaust is relatively better on longer trips, where the burn benefit becomes more important relative to the added weight. The same seems to hold true on the A320, where the 2-stage HPT V2500 is more efficient, and also has a more long-range optimized exhaust.

With respect to how these designs compare with their competitors, I've got the following:

RB211-535E: worse burn but better on-wing time than the clean-sheet PW2000.
RB211-524G/H: heavier and worse burn than GE CF6-80C2 on the 744 and 763, didn't sell well as a result
V2500: better than CFM on burn, but higher mx costs due to 2-stage HPT. Better on longer ranges and A321
CFM56-5C: only game in town after Superfan cx, optimized for long-range (hence the mixer)
T700: most popular A330 engine, but I'm not sure why. Can someone chime in?

To me, the A340 application make the most sense for the design, where the engine and nacelle are optimized for longer hauls. The 320 makes sense as the longer-range preferred option, with the mixer helping the IAE engine's SFC advantage over the CFM56-5B though weighing more. This may help explain why B6 went IAE and why A321s are IAE tilted.

Now the 744 and 757 applications don't make as much sense to me, because the nacelle mixer is long-haul biased where the engines have a disadvantage relative to their GE and PW brethren. A possible theory is that because the triple-spool engines are shorter, the weight penalty of the mixer isn't as high, and that tips the scale in favor of the mixer. That may also explain why the T700 on the 330 gets the mixer but PW and GE on the A330 did not.

From the 777 onward, almost all engine installations have converged on a design with unmixed exhausts and cones sticking out the core duct. This makes it tougher to tell engine types apart, like on the A380 or 787. As bypass ratios and engine diameters went up, the increased weight penalty of the longer duct must have outweighed the propulsive efficiency gain of the mixed designs.


User currently offline77west From New Zealand, joined Jun 2009, 194 posts, RR: 0
Reply 14, posted (2 months 1 week 1 day 23 hours ago) and read 3482 times:

Quoting LH707330 (Reply 13):
Quoting LH707330 (Reply 13):
The glory days for the mixed exhaust

You realize this thread is 13 years old?

Interesting nonetheless.



77West - B190 - Q300 - ATR72 - DC9-30 - MD80 - B733 - A320 - B738 - A300-B4 - B773 - B77W
User currently offlineLH707330 From United States of America, joined Jun 2012, 1192 posts, RR: 1
Reply 15, posted (2 months 1 week 1 day 21 hours ago) and read 3441 times:

Quoting 77west (Reply 14):
You realize this thread is 13 years old?

Interesting nonetheless.

Yep, I can read the time stamps  . There was another thread where the mixers were brought up, so instead of taking that one OT I googled and found this one.
BA And The RB211 (by 29erUSA187 May 23 2015 in Civil Aviation)


User currently offlineChaosTheory From United Kingdom, joined Mar 2013, 579 posts, RR: 0
Reply 16, posted (2 months 1 week 1 day 1 hour ago) and read 3000 times:

Quoting LH707330 (Reply 13):
V2500: better than CFM on burn, but higher mx costs due to 2-stage HPT. Better on longer ranges and A321
Quoting LH707330 (Reply 13):
The 320 makes sense as the longer-range preferred option, with the mixer helping the IAE engine's SFC advantage over the CFM56-5B though weighing more. This may help explain why B6 went IAE and why A321s are IAE tilted.

The problem that faces higher thrust variants of the V2500 and CFM56 (A321 engines) is EGT margin degradation which is the primary reason for engine removal.

As such, both CFM and IAE have developed hardware updates in the form of TechInsertion and SelectOne packages to address these concerns. The SelectOne which became available in 2011 is estimated to have an additional 10-15 degrees of EGT margin over the older build standard. Given that the V2500-30/33 enjoys slightly better performance retention following core restoration, it is not surprising it is the engine of choice on the A321.

At the lower thrust ratings which equip the A318-A320 up to 27k, the CFM56 enjoys slightly better performance retention, and when new a CFM56 (27k) will have about 115C EGT margin compared to a SelectOne V2527 with an EGT margin of 100C.

Smarter airlines will often remove and derate higher thrust CFM56 engines following core restoration to their A319/A320 fleets so that the next performance restoration doesn't occur before the LLPs become due for replacement.

Quoting LH707330 (Reply 13):
T700: most popular A330 engine, but I'm not sure why. Can someone chime in?

Phenomenal hot/high performance coupled with excellent performance retention. It's flat rated to 35C+ compared to 30C for the PW4000/GE CF6-80.

The overall maintenance costs for the T700 are also slightly lower. A little strange and counterintuitive considering the LLPs on the T700 require replacing at 10k flight cycles versus 14-15k of the PW/GE offerings. (LLPs are not usually covered by RR TotalCare power-by-hour contracts)

[Edited 2015-05-26 17:27:13]

User currently offlineLH707330 From United States of America, joined Jun 2012, 1192 posts, RR: 1
Reply 17, posted (2 months 1 week 21 hours ago) and read 2920 times:

Quoting ChaosTheory (Reply 16):
Quoting LH707330 (Reply 13):
T700: most popular A330 engine, but I'm not sure why. Can someone chime in?

Phenomenal hot/high performance coupled with excellent performance retention. It's flat rated to 35C+ compared to 30C for the PW4000/GE CF6-80.

The overall maintenance costs for the T700 are also slightly lower. A little strange and counterintuitive considering the LLPs on the T700 require replacing at 10k flight cycles versus 14-15k of the PW/GE offerings. (LLPs are not usually covered by RR TotalCare power-by-hour contracts)

Thanks for that explanation. Do you know how the three compare on burn? I've heard that RR and PW are similar, with GE 3rd place.


User currently offlineChaosTheory From United Kingdom, joined Mar 2013, 579 posts, RR: 0
Reply 18, posted (2 months 1 week 10 hours ago) and read 2736 times:

Quoting LH707330 (Reply 17):
Do you know how the three compare on burn? I've heard that RR and PW are similar, with GE 3rd place.

When new, the CF6-80 has the lowest fuel burn of the three. The difference is negligible though (less than 0.5%) and on a 10hr+ sector that translates to at most 50USG. However, over the life of the engine, the T700 has the lowest fuel burn followed by the PW4000 and the CF6-80 last..


User currently offlineOKCFlyer From United States of America, joined May 2015, 102 posts, RR: 0
Reply 19, posted (2 months 1 week ago) and read 2531 times:

How about on the 772 platform, GE, RR, PW? New and over-life?

User currently offlineViscount724 From Switzerland, joined Oct 2006, 27369 posts, RR: 22
Reply 20, posted (2 months 6 days 23 hours ago) and read 2511 times:

Quoting LH707330 (Reply 15):
Quoting 77west (Reply 14):
You realize this thread is 13 years old?

Interesting nonetheless.

Yep, I can read the time stamps . There was another thread where the mixers were brought up, so instead of taking that one OT I googled and found this one.

It would be courteous to other members when old threads are resurrected years later to mention that in the first new post. I started reading through this entire thread thinking it was new before noticing that it was 13 years old and all the user IDs from the original thread no longer exist.


User currently offlineChaosTheory From United Kingdom, joined Mar 2013, 579 posts, RR: 0
Reply 21, posted (2 months 6 days 23 hours ago) and read 2511 times:

Quoting OKCFlyer (Reply 19):
How about on the 772 platform, GE, RR, PW? New and over-life?

The GE90 and PW4090 have far lower fuel burn than the RR T800. On a long sector like LHR-SIN, a T800 powered 772 will burn 500USG more than the PW/GE options. PW/GE will maintain that advantage over the life of the engines

Despite the much higher fuel burn, the T800 remains competitive where maintenance costs are concerned. I think RR has the lowest maintenance costs out of the three.


User currently offlineAA777223 From United States of America, joined Feb 2006, 1298 posts, RR: 6
Reply 22, posted (2 months 6 days 10 hours ago) and read 2372 times:

That's very interesting. I was always under the impression that it was the PW that had the high fuel burn further exacerbated by the fiasco with the PW4098 attempt.

I thought the GE90 also had a tendency to be less efficient in its lower thrust forms, due to its massive size and weight. Here, all along, I was thinking the RR was the most efficient engine on the 77E, and it turns out it was the opposite. The things you learn... I'm surprised the RR was such a hot seller on the 77E (largest market share?).

Thanks for the info!



Sic 'em bears
User currently offlineLH707330 From United States of America, joined Jun 2012, 1192 posts, RR: 1
Reply 23, posted (2 months 6 days 5 hours ago) and read 2291 times:

Quoting ChaosTheory (Reply 21):
Quoting OKCFlyer (Reply 19):
How about on the 772 platform, GE, RR, PW? New and over-life?

The GE90 and PW4090 have far lower fuel burn than the RR T800. On a long sector like LHR-SIN, a T800 powered 772 will burn 500USG more than the PW/GE options. PW/GE will maintain that advantage over the life of the engines

Despite the much higher fuel burn, the T800 remains competitive where maintenance costs are concerned. I think RR has the lowest maintenance costs out of the three.
Quoting AA777223 (Reply 22):
That's very interesting. I was always under the impression that it was the PW that had the high fuel burn further exacerbated by the fiasco with the PW4098 attempt.

I thought the GE90 also had a tendency to be less efficient in its lower thrust forms, due to its massive size and weight. Here, all along, I was thinking the RR was the most efficient engine on the 77E, and it turns out it was the opposite. The things you learn... I'm surprised the RR was such a hot seller on the 77E (largest market share?).

Thanks for the info!

The T800 weighed a few tons less than the GE90 and PW4090 because it was a smaller and shorter engine that also had better climb burn (three-spool), so it was better on


User currently offlineOKCFlyer From United States of America, joined May 2015, 102 posts, RR: 0
Reply 24, posted (2 months 6 days ago) and read 2187 times:

I thought the 4090 was fairly light (relatively) due to its smaller fan size and frankly smaller overall size due to being the least capable of them all.

Strange to me short haul flying is considered important on a 14-hour capable frame. Seems like a large portion of a 772ER's time is spent at cruising altitude. Not short hoping??


25 ChaosTheory : Whilst a T800 777 does weigh 3t less than a GE90 777, it's only on very shorts sectors ( Seating 301 pax on a 3000nm sector, a 777-200ER will burn: G
26 Post contains images ChaosTheory : The figures I give in #25 are taken from a Boeing document '777 Performance Summary'. It was available on the Boeing website prior to the new "improve
27 OKCFlyer : Dang. Thanks for the post. The GE really kicks butt in fuel burn. It must have been a real maintenance nightmare considerin it wasn't well received. A
28 ChaosTheory : It isn't/wasn't a maintenance nightmare. Airlines take a holistic approach, and in the case of the 777, the overall package (fuel burn,maintenance, s
29 LH707330 : They got a good deal on the first batch when they had teething issues. GE spent 2 billion on R&D, so presumably the capex was higher on the GE90s
30 OKCFlyer : Ahhh! Makes sense. Thanks for the info. My bad. Continue normal discussion![Edited 2015-05-29 03:26:39]
31 LH707330 : We all rabbit holed, that's just how it goes. If you've not yet read Boeing versus Airbus by John Newhouse, I highly recommend it. Lots of behind-the
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