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Engines On The Tail Like MD 11  
User currently offlineEK156 From United Arab Emirates, joined May 2005, 765 posts, RR: 2
Posted (8 years 11 months 3 days 2 hours ago) and read 5397 times:

What major difference does a tail engine do, like the ones on the MD 11?

I always liked the look of these engines on the tail. But why not just have them under the wing like airbus and boeing?

Can you anyone help?

18 replies: All unread, jump to last
 
User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 1, posted (8 years 11 months 3 days 2 hours ago) and read 5365 times:

Because the manufacturer wanted a 3 engine aircraft and centerline thrust is the best way to do that. Where would you put the 3rd engine if not the tail?

User currently offlineDtwclipper From , joined Dec 1969, posts, RR:
Reply 2, posted (8 years 11 months 3 days 2 hours ago) and read 5362 times:

Take a look at the history of the A/C!



"The initial specifications were set by American Airlines, who required the new aircraft to be a large capacity, short-to-medium range twin-engined jetliner capable of taking off from smaller airports, with New York's La Guardia being mentioned specifically. The initial design had seating for 250, a maximum gross weight of 300,000 pounds (136,000 kg), and a range of 1,850 nautical miles (2,990 km) with two 50,000-pound-thrust (223 kN) turbofan engines. However, Lockheed was eager to establish a more broader basis for its overall design. Eastern Airlines favored a trijet configuration, as much of their longest routes were overwater, and it was still an era before ETOPS (Extended Twin-engined OPerationS). Trans World Airlines also favored a trijet as they intended the new aircraft to fly U.S.Midwest-to-West Coast segments where the Rocky Mountains were a concern. After discussions with several airlines went on, most of them started to favor a three-engined configuration, although adding a requirement of having a transcontinental range for greater flexibility.

Appearances of the L-1011 and the DC-10 became very much alike, as they were built to meet the same requirements: a 300-seat airliner with a Mach 0.8 cruise and operations from a 9,000 ft (2,700 m) runway on a hot day. However, the most difficult specification to meet was no doubt the capability to operate within the restrictions of La Guardia. Performance parameters were strongly influenced by the airport's geometric and dimensional restraints as well as runway length and a limited maximum gross weight of 270,000 pounds (122,600 kg), although this limit has since been extended. However, though these two are similar, they are not at all identical. When distinguishing the two, the most noticeable difference is the position of the aft engine, or otherwise called the No.2 engine. McDonnell Douglas opted for a straight-duct configuration with the engine mounted on the tail in order to optimize engine performance. On the other hand, Lockheed allocated the third engine in the rear fuselage with the air intake on top of the fuselage. This arrangement is commonly known as the S-duct. This is more similar to that of the Boeing 727 than the DC-10. A series of experiments showed efficiency loss of the engine to be negligible and also showed significant gains in directional stability and maintainability of the engine. In addition, the aft fuselage placement resulted in an improved aerodynamic aft-fuselage configuration, which in turn allowed the fuselage to have a wider aft cabin layout, meaning better comfort for the passenger."

http://flytristar.tripod.com/page/history.html


BTW....there is a thing callled "GOOGLE" and you will find a lot of information there!


User currently offlineHaveBlue From United States of America, joined Jan 2004, 2090 posts, RR: 1
Reply 3, posted (8 years 11 months 2 days 9 hours ago) and read 4901 times:

Excellent description Dtwclipper. However, I take exception to everyone quoting the 'Google' and 'Search' in reply to threads. If everyone Googled and Searched their answers, there would be little to nothing going on here at the forums. I for one am glad to see questions answered that I might not even have thought to ask, and to see different peoples perspectives and wisdoms, be that from an airliner pilot, CFI, ATC'er, or enthusiast.

Just a thought.



Here Here for Severe Clear!
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16908 posts, RR: 67
Reply 4, posted (8 years 11 months 2 days 9 hours ago) and read 4898 times:

Quoting EK156 (Thread starter):
But why not just have them under the wing like airbus and boeing?

Ironically, the tail mounted engine in many ways became the undoing of MD. The MD-11 duct was very expensive to develop, and planned stretches (sometimes known as the MD-12 or MD-XX) came to naught in large part due to the expense of the duct.


Trijets are cool but putting an engine backthere is not an optimal solution on a large jet unless you have to meet very specific performance and dimension targets, like the LGA thing. Nowadays, engines are powerful enough that twins (330, 767, 777) can manage the same job. Any plane large enough to need more than two engines (in theory, three engines could power something in the 744 weight class) would make a middle engine placement far too expensive and complex, so it's easier just to make a quad.

Although Boeing did toy with this:
http://www.rosboch.net/various/B747-300_Concept_with_three_engines.jpg



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineStealthZ From Australia, joined Feb 2005, 5609 posts, RR: 45
Reply 5, posted (8 years 11 months 2 days 3 hours ago) and read 4819 times:
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The MD-11 duct was very expensive to develop...

What was so complex about the DC-10/MD-11 duct, seems like a fairly straight forward concept apart from carrying the tail structure around it?

Chris



If your camera sends text messages, that could explain why your photos are rubbish!
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16908 posts, RR: 67
Reply 6, posted (8 years 11 months 1 day 23 hours ago) and read 4756 times:

Quoting StealthZ (Reply 5):
What was so complex about the DC-10/MD-11 duct, seems like a fairly straight forward concept apart from carrying the tail structure around it?

As far as I can figure, the expensive bit was twofold, but I'm sure someone has better info:
- The darned engine had to be cantilevered up there with the so called "banjo fitting", a rather expensive thing to design and build. Essentially you're putting this enormous heavy blob away from the fuse or the wings, and you have to transfer the forces somehow.
- The intake was aerodynamically tricky, especially in a crosswind.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6289 posts, RR: 54
Reply 7, posted (8 years 11 months 1 day 15 hours ago) and read 4710 times:

Quoting Starlionblue (Reply 4):
The MD-11 duct was very expensive to develop...

Dear Stallionblue, some thirty years ago I read the opposite, that Douglas chose the fin mounted #2 engine in order to save development money, and development time - to get ahead of the competing L-1011.

(We are of course talking about development of the DC-10. The MD-11 must be considered sort of "DC-10NG" just like the DC-3 is a DC-2NG).

It was at that time well known that early B727's frequently suffered #2 compressor stalls during rotation when the high angle of attack created turbulence on top of the fuselage. And the new high bypass ratio engines were supposed to be even more prone to compressor stall than the old JT8D. (Later 727's had the S-duct changed so compressor stalls became much less frequent - most noticeable difference is a more oval intake).

Lifting the engine out of that turbulence freed Douglas from lenghty wind tunnel tests to get the internal shape of the S-duct right.

They knew that the high engine would mean a structural weight penalty, but also a slight engine efficiency gain due to the absence of the S-duct.

I don't see why the high mounting should be any more expensive. It is after all only an ordinary wing mounted engine pylon mounted upside down. And then structurally dimensioned to carry a vertical fin on top.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineMD11Engineer From Germany, joined Oct 2003, 13794 posts, RR: 63
Reply 8, posted (8 years 11 months 1 day 14 hours ago) and read 4663 times:

Quoting Prebennorholm (Reply 7):
don't see why the high mounting should be any more expensive. It is after all only an ordinary wing mounted engine pylon mounted upside down. And then structurally dimensioned to carry a vertical fin on top.

The No. 2 engine is not mounted upside down. In fact it is hanging from a pylon jutting out after from the rear banjo fitting (there are 4 of them).

Jan


User currently offlineFlybyguy From United States of America, joined Jun 2004, 1798 posts, RR: 1
Reply 9, posted (8 years 11 months 1 day 11 hours ago) and read 4601 times:

Not to hijack this thread... but I think this question is relevant without having to create a new thread on the topic...

When pilots inspect a tri-jet during their walkaround is there any special procedure for visual inspection to ensure that the #2 engine has not encountered FOD ingestion such as a bird strike or any other potentially detrimental damage.



"Are you a pretender... or a thoroughbred?!" - Professor Matt Miller
User currently offlineGQfluffy From , joined Dec 1969, posts, RR:
Reply 10, posted (8 years 11 months 1 day 7 hours ago) and read 4556 times:

Quoting Prebennorholm (Reply 7):
Later 727's had the S-duct changed so compressor stalls became much less frequent - most noticeable difference is a more oval intake

The 727-200s and -200 ADV had circular intakes. The 27-100 had the oval intake.

fluffy


User currently offlineStarlionblue From Greenland, joined Feb 2004, 16908 posts, RR: 67
Reply 11, posted (8 years 11 months 1 day 7 hours ago) and read 4553 times:

Quoting Prebennorholm (Reply 7):
Quoting Starlionblue (Reply 4):
The MD-11 duct was very expensive to develop...

Dear Stallionblue, some thirty years ago I read the opposite, that Douglas chose the fin mounted #2 engine in order to save development money, and development time - to get ahead of the competing L-1011.

Yes well I will agree it's probably cheaper than the Tristar mounting in many ways. However it's still an expensive way of doing things compare to a quad or a twin. The tail gets complicated if you put an engine there, regardless of fin or fuse mounting.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineEilennaei From , joined Dec 1969, posts, RR:
Reply 12, posted (8 years 11 months 1 day 6 hours ago) and read 4544 times:

Makes me wonder if the DC-10-40 and MD-11 have the common construction of the #2 installation? The intake's shape is different from the earlier DC-10 models, deflecting inwards in the middle. Something related to a redesign of the intake adapter ("bellmouth")?

See discussion on p. 15 of
http://216.239.59.104/search?q=cache...ir_Cargo.pdf+dc-10+bellmouth&hl=sv

From what I can judge, servicing the tail engine seems to be arranged just like the other engines once you have got up there. For the parting of the engine the inner sections of the elevators seem to have a "resting" facility to make space for the engine and for the temporary cable runs, which will be long.

The design is a "straight-through" one. In terms of aerodynamic efficiency this must be close to the optimum solution.

-Eilennaei

[Edited 2005-05-24 09:28:31]

[Edited 2005-05-24 09:30:30]

User currently offlineEK156 From United Arab Emirates, joined May 2005, 765 posts, RR: 2
Reply 13, posted (8 years 10 months 4 weeks 1 day 8 hours ago) and read 4351 times:

Quoting Flybyguy (Reply 9):
Not to hijack this thread... but I think this question is relevant without having to create a new thread on the topic...

When pilots inspect a tri-jet during their walkaround is there any special procedure for visual inspection to ensure that the #2 engine has not encountered FOD ingestion such as a bird strike or any other potentially detrimental damage.

You are soooo right Flybyguy... How can a Pilot do his checks on a Triengine plane when one of the engines is all the way up on the tail??? Do they get him a big ladder??? Curious to know!!!!


User currently offlineHAWK21M From India, joined Jan 2001, 31573 posts, RR: 57
Reply 14, posted (8 years 10 months 4 weeks 1 day 8 hours ago) and read 4348 times:

Quoting EK156 (Reply 13):
Quoting Flybyguy (Reply 9):
Not to hijack this thread... but I think this question is relevant without having to create a new thread on the topic...

When pilots inspect a tri-jet during their walkaround is there any special procedure for visual inspection to ensure that the #2 engine has not encountered FOD ingestion such as a bird strike or any other potentially detrimental damage.

You are soooo right Flybyguy... How can a Pilot do his checks on a Triengine plane when one of the engines is all the way up on the tail??? Do they get him a big ladder??? Curious to know!!!!

I suppose the Mx would have done this Already.But a very good question no doubt.
Anyone with Trijet Experience.
regds
MEL



Think of the brighter side!
User currently offlineFDXMECH From United States of America, joined Mar 2000, 3251 posts, RR: 35
Reply 15, posted (8 years 10 months 4 weeks 1 day ago) and read 4319 times:

Quoting EK156 (Reply 13):
You are soooo right Flybyguy... How can a Pilot do his checks on a Triengine plane when one of the engines is all the way up on the tail??? Do they get him a big ladder??? Curious to know!!!!

No. #2 engine intake is usually inspected using a cherrypicker and walking down the intake to inspect the blades. And even that is only done on "A" checks and higher (depending on carrier).

Of course during deicing operations, the engine is inspected after each spraying.



You're only as good as your last departure.
User currently offlineFDXMECH From United States of America, joined Mar 2000, 3251 posts, RR: 35
Reply 16, posted (8 years 10 months 4 weeks 23 hours ago) and read 4311 times:

Quoting EK156 (Reply 13):
How can a Pilot do his checks on a Triengine plane when one of the engines is all the way up on the tail??? Do they get him a big ladder??? Curious to know!!!!

But on the other hand a pilot doesn't check the #2 engine of a 727 either. Or (depending on carrier) can't look into the intake of 1 or 3 as well. Yes, I know about the aft entry doors of the 727.



You're only as good as your last departure.
User currently offlineAirxLiban From Lebanon, joined Oct 2003, 4504 posts, RR: 54
Reply 17, posted (8 years 10 months 4 weeks 11 hours ago) and read 4214 times:

Hmm, a cutout drawing of the L-1011 would be nice. I tried our friend Google but wasn't sure what to call it. Anyone know where we can find one?


PARIS, FRANCE...THE BEIRUT OF EUROPE.
User currently offlineHAWK21M From India, joined Jan 2001, 31573 posts, RR: 57
Reply 18, posted (8 years 10 months 4 weeks 10 hours ago) and read 4205 times:

Quoting AirxLiban (Reply 17):
Hmm, a cutout drawing of the L-1011 would be nice. I tried our friend Google but wasn't sure what to call it. Anyone know where we can find one?

Flight International has a lot of Cutaway drawings.
regds
MEL



Think of the brighter side!
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