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Rear Mounted Engines  
User currently offlineDash8pilot From Canada, joined Mar 2005, 86 posts, RR: 0
Posted (6 years 9 months 1 week 6 days 6 hours ago) and read 18739 times:

Can anyone give a few reasons as to why some aircraft have rear mounted engines (MD82, CRJ ect) rather than under wing engines.? What are the advantages of having engines placed in the back??

43 replies: All unread, showing first 25:
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17025 posts, RR: 67
Reply 1, posted (6 years 9 months 1 week 6 days 5 hours ago) and read 18861 times:

Advantages of tail mounted engines:
- Plane can sit lower, so gear will be shorter (=lighter). This simplifies loading/unloading.
- Simpler wing design.
- Lower risk of FOD.
- Lack of noise in the front.
- Smaller fin and rudder due to less yawing moment in engine out situations.

Disadvantages:
- Plumbing needs to be taken back to the tail instead of just feeding fuel from a wet wing to an engine.
- Strengthening of rear fuselage adds weight.
- Noise in the back.
- Since tail engines typically lead to a cruciform or a t-tail, there are deep stall concerns.
- Wing mounted engines counteract wing bending.
- Wing mounted engines counteract wing twist if mounted ahead of wing as is the norm today.
- Very high bypass engines with large fans are impractical to mount in the tail.
- While it has been done, more than one engine per side has issues, especially with large fans. So in practice you're limited to two engines.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineTupolevTu154 From Germany, joined exactly 10 years ago today! , 2181 posts, RR: 28
Reply 2, posted (6 years 9 months 1 week 6 days 5 hours ago) and read 18785 times:

Tail Mounted Engines (by MPDPilot Nov 1 2006 in Tech Ops)

Position Of Engines (by F.pier Jan 6 2004 in Tech Ops)

No Desire For Tail Mounted Engines? (by Cancidas Nov 20 2003 in Civil Aviation)

Engine Placement Question (by Traveler_7 Mar 15 2002 in Civil Aviation)

 Wink

Tom Big grin



Atheists - Winning since 33 A.D.
User currently offlineAlexEU From Serbia, joined Oct 2007, 1817 posts, RR: 2
Reply 3, posted (6 years 9 months 1 week 6 days 5 hours ago) and read 18764 times:

What is the future of usage tail mounted engines ? I think that they will still be used by regional jets, although i am not sure about designing of an aircraft such as DC-9, B727, MD80 etc.

Why are rear mounted piston or turboprop planes very rare? I can only think of 2 aircraft types which have tail props.


User currently offlinePilotboi From United States of America, joined Sep 2007, 2366 posts, RR: 9
Reply 4, posted (6 years 9 months 1 week 6 days 5 hours ago) and read 18737 times:



Quoting AlexEU (Reply 3):
Why are rear mounted piston or turboprop planes very rare?

For the same reason high-bypass turbofans aren't rear mounted - too large of a diameter.


User currently offlineBok269 From United States of America, joined May 2007, 2105 posts, RR: 0
Reply 5, posted (6 years 9 months 1 week 5 days 18 hours ago) and read 18606 times:



Quoting AlexEU (Reply 3):
Why are rear mounted piston or turboprop planes very rare?

To have a prop, you need a large area bigger than the engine itself in which to spin. To place a prop on the fuslage would require you to place it on supports that stick a considerable distance out from the Fuselage. All of that is more weight and needless engineering into the equation. The reason the P-180 and the starship can have a rear mounted prop is that it is rear facing, and doesn't require a support to stick out a significant distance from the fuselage.

Just an educated guess. Anyone feel free to correct



"Reality is wrong, dreams are for real." -Tupac
User currently offlineViscount724 From Switzerland, joined Oct 2006, 25154 posts, RR: 22
Reply 6, posted (6 years 9 months 1 week 5 days 16 hours ago) and read 18573 times:



Quoting Starlionblue (Reply 1):
Disadvantages:
- Strengthening of rear fuselage adds weight.
- Since tail engines typically lead to a cruciform or a t-tail

Rear engines and T-tail also result in a significantly longer aircraft in relation to the length of the passenger cabin. Even the overall length of the shortest DC-9-10 is about 4 feet longer than the 737-200 although the passenger cabin is several feet shorter. The 727-200 is also a few inches longer than the 707-320.

I believe rear engines are also more prone to ingest ice that may accumulate on the wings. If not mistaken there have been a few DC-9/MD-80 engine failure incidents caused by ice ingestion.


User currently offlineTrijetsRMissed From United States of America, joined Oct 2006, 2350 posts, RR: 7
Reply 7, posted (6 years 9 months 1 week 5 days 16 hours ago) and read 18566 times:

A more aerodynamic wing is the greatest benefit to having the rear mounted engine, t-tail design.

Quoting AlexEU (Reply 3):
What is the future of usage tail mounted engines ?

Out the window along with the tri-jet, once McDonnell Douglas was taken over. I'm afraid we will mostly see them on only CRJ/ERJ's and business jets in the future.



There's nothing quite like a tri-jet.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17025 posts, RR: 67
Reply 8, posted (6 years 9 months 1 week 5 days 13 hours ago) and read 18540 times:



Quoting Viscount724 (Reply 6):
believe rear engines are also more prone to ingest ice that may accumulate on the wings. If not mistaken there have been a few DC-9/MD-80 engine failure incidents caused by ice ingestion.

Quite. And a few 727 blue ice issues.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineStil From Spain, joined Apr 2006, 345 posts, RR: 6
Reply 9, posted (6 years 9 months 1 week 2 days 1 hour ago) and read 18264 times:

I once heard T-tail concept is copyrighted and all the T-tail aircrafts has to pay royalties to the owner of the Caravelle design (supposed to be the first T-tail aircraft)...
Is it true?

Stil



....... Gueropppa! ......
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 10, posted (6 years 9 months 1 week 1 day 21 hours ago) and read 18216 times:



Quoting Stil (Reply 9):
I once heard T-tail concept is copyrighted and all the T-tail aircrafts has to pay royalties to the owner of the Caravelle design (supposed to be the first T-tail aircraft)...
Is it true?

Not true, and Lockheed did not have to pay Boeing for the S-Duct on the L-1011. Additionally you "copyright" written or artist works. Mechanical designs are "patented".


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17025 posts, RR: 67
Reply 11, posted (6 years 9 months 1 week 1 day 20 hours ago) and read 18193 times:



Quoting Stil (Reply 9):
I once heard T-tail concept is copyrighted and all the T-tail aircrafts has to pay royalties to the owner of the Caravelle design (supposed to be the first T-tail aircraft)...

Besides, Caravelle does not have a T-tail. It has a cruciform tail. You can of course see the genetic material reused in the Falcon series.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 12, posted (6 years 9 months 1 week 1 day 18 hours ago) and read 18191 times:

According to Jeppesen:

The main advantage to Rear mounted engines is the huge reduction in the yawing moment in the event of an engine failure.

The main advantage to a T-tail is better aerodynamic efficiency by keeping the horizontal stab and elevators out of the turbulence created by the wing.

Rear mounted engines tend to pick up less random FOD than wing mounted, however they have been known to ingest water sprayed up by the landing gear during very wet conditions (causing a flameout). This is why the MD-80,MD-90,717 and some DC-9s had chines on the landing gear to divert the water spray.

Rear mounted engines also eliminate the problem of wing and pylon corrosion due to exhaust gas.



Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 13, posted (6 years 9 months 1 week 1 day 18 hours ago) and read 18180 times:
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Quoting Pilotboi (Reply 4):
For the same reason high-bypass turbofans aren't rear mounted - too large of a diameter.

You know, I'm not sure this is a limiting factor. I'll be happy to be proven wrong, but smaller aircraft have proportionally large fans mounted at the rear:


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What technical limitations prevent larger-diameter fuselages (ie: 737, A320) from having proportionally larger engines mounted back there, too?

2H4



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User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 14, posted (6 years 9 months 1 week 1 day 18 hours ago) and read 18172 times:



Quoting 2H4 (Reply 13):
Quoting Pilotboi (Reply 4):
For the same reason high-bypass turbofans aren't rear mounted - too large of a diameter.

You know, I'm not sure this is a limiting factor. I'll be happy to be proven wrong, but smaller aircraft have proportionally large fans mounted at the rear:

It's not just RJs and Biz Jets either, the IAE V2500s on the MD-90 and the RR BR715 on the 717 are both high bypass turbofans. However you would reach an upper limit i.e. I highly doubt you'll ever see a rear mounted GE-90, the structural and aerodynamic issues are just too great



Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offlineMark5388916 From United States of America, joined Aug 2007, 377 posts, RR: 0
Reply 15, posted (6 years 9 months 1 week 1 day 18 hours ago) and read 18171 times:

I would think that landing gear length would be a big one. Of course there has been the 757 and the like that just have huge landing gear.

Mark



I Love ONT and SNA, the good So Cal Airports! URL Removed as required by mod
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 16, posted (6 years 9 months 1 week 1 day 18 hours ago) and read 18165 times:
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Quoting MQTmxguy (Reply 14):
I highly doubt you'll ever see a rear mounted GE-90, the structural and aerodynamic issues are just too great

Yes, I see your point. I wonder, however, if present and future advances in airframe/composite technology might overcome the structural issues.

What, specifically, are the aerodynamic issues?

2H4



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User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 17, posted (6 years 9 months 1 week 1 day 18 hours ago) and read 18161 times:

One problem with giant rear engines is CG issues. The heavier the engines, the more weight you need ahead of the center of lift. Now this normally means making the fuselage longer ahead of the wings. Now you have a really long fuse (a la MD-90) which is a logistical issue on the ground and maintaining your CG gets trickier because you get a much bigger moment-arm up front for much less weight.

edited for spelling

[Edited 2007-11-23 19:50:03]


Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 18, posted (6 years 9 months 1 week 1 day 18 hours ago) and read 18155 times:



Quoting 2H4 (Reply 16):
What, specifically, are the aerodynamic issues?

The way it was explained to me is that it has to do with loss in volumetric efficiency in the big turbofans due to wing turbulance, your not getting the same kind of non-turbulant ram air as in a wing mount.



Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 19, posted (6 years 9 months 1 week 1 day 18 hours ago) and read 18146 times:
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Quoting MQTmxguy (Reply 18):

Interesting. Thanks for the info.

2H4



Intentionally Left Blank
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17025 posts, RR: 67
Reply 20, posted (6 years 9 months 1 week 1 day 14 hours ago) and read 18114 times:



Quoting MQTmxguy (Reply 12):
This is why the MD-80,MD-90,717 and some DC-9s had chines on the landing gear to divert the water spray.

727s have chines on the tires. The McDonnell stuff has spray guards.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 21, posted (6 years 9 months 1 week 1 day 13 hours ago) and read 18106 times:



Quoting Starlionblue (Reply 20):
727s have chines on the tires. The McDonnell stuff has spray guards.

Yep you are correct, my bad.



Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17025 posts, RR: 67
Reply 22, posted (6 years 9 months 1 week 1 day 12 hours ago) and read 18093 times:



Quoting MQTmxguy (Reply 21):

Quoting Starlionblue (Reply 20):
727s have chines on the tires. The McDonnell stuff has spray guards.

Yep you are correct, my bad.

Don't feel bad. Having something like that at my fingertips without being a pilot or mechanic just makes me "the man who hath no life". Big grin



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 23, posted (6 years 9 months 1 week 21 hours ago) and read 18032 times:



Quoting Starlionblue (Reply 22):
Don't feel bad. Having something like that at my fingertips without being a pilot or mechanic just makes me "the man who hath no life".

hahaha, well at least your good it, I AM a mechanic and you caught me.

And those that may not know the difference, tire chines are a built in, outward curved flange on the tire that directs the spray away. Spray gaurds are like a plane or a rigid flap extending either outward and aft from the hub of the NLG, or aft and outward from the center of the MLG trucks. Something like an airplane mudflap.



Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offlinePMN1 From United Kingdom, joined Jun 2007, 78 posts, RR: 0
Reply 24, posted (6 years 6 months 2 weeks 1 day 3 hours ago) and read 17566 times:

From BAC Three-Eleven: The British Airbus that should have been by Graziano Freschi

BAC’s engineers listed several ‘significant’ advantages of the rear-mounted layout, including superior cross-wind landing capability; better directional control with asymmetrical thrust; negligible trim changes with power adjustment; power-plants being less prone to bird and debris ingestion during take-off and landing; greater safety in wheels-up landing; the aircraft’s lower height allowing a high degree of compatibility with existing hangers and current new generation ground equipment; quieter cabin.

However, aerospace engineers know that the rear-mounted engine configuration also gave rise to some disadvantages, which BAC took care not to highlight in its marketing literature. The principal disadvantages include:

Lack of wing bending relief, this being one of the main benefits for an aircraft when the engines are mounted under the wing. The lack of bending relief usually requires the wing to be strengthened structurally, resulting in a higher weight penalty.

The presence of the engines at the rear of the fuselage also requires the tail structure of the aircraft (the area aft of the rear pressure bulkhead, and which includes the tailcone, fin rudder, tailplane and elevators) to be strengthened, and this again incurs an extra weight penalty.

Rear-engined aircraft present centre of gravity issues, which are addressed with the greater length of the fuselage (and any significant fuselage stretches) ahead of the wing.

T0tailed aircraft have a tendency to enter a ‘deep stall’ under certain conditions (when the wing reaches very high angles of attack and the resulting turbulent air nullifies the efficiency of the tailplane and elevators). The ‘deep stall’ phenomenon emerged in accidents during early fight testing of the Boeing 727, HS Trident and BAC1-11, with tragically fatal consequences for the test crews involved.

On large aircraft like the Three-eleven, the rear-engine configuration makes engine inspection and maintenance more difficult than in underwing configuration, as the engines are higher up from ground level.


25 PMN1 : Was there a British engine around at the time that would have allowed the DH121 (original spec) to have two underwing engines?
26 BoeingOnFinal : Are there maintenance issues? I just read that there are disadvantages with maintenance on aircrafts like the DC-10/MD-11 etc. because of the third, h
27 PMN1 : How about on the wing but without using a pylon so it would be like the DH119 and 120 or Concorde?
28 2H4 : ...Like the 737-200? 2H4
29 Starlionblue : That means another trade-off. The pylon puts the engine in a position where it is less disturbed by the wing's airflow and counteracts wing twisting
30 Ex52tech : You are correct. You can change both wing engines, one at a time, in the time it takes to change a #2 on a DC10/MD11. Espically if you are using the
31 MD-90 : I think aesthetics should be included as well. Some of the larger bizjets could've been designed to accommodate underwing engines but they look bette
32 Starlionblue : True. However the companies involved also have experience with one and not the other, which may have counted.
33 Mandala499 : I'm surprised no one has mentioned "rudder blanking" when reverse thrust are used. The proximity of the engines to the rudder means that when in reve
34 BoeingOnFinal : Ex52tech, thank you for an informative post! So what are your opinion on high mounted wings on smaller regionals and business jets from a mechanics st
35 Ex52tech : The smaller jets are not such a problem, even with a shoulder mounted wing, because the airplane is not that big around, but the smaller engines shou
36 BoeingOnFinal : Thanks for your input Ex52tech, it is greatly appreciated!
37 PMN1 : When the designers of the DC-10 and Tristar were looking at engine placement, did they consider putting all three engines at the rear? In the same way
38 Post contains links and images 2H4 : Boeing considered such a configuration for the 747: 2H4
39 Mark5388916 : Another pick from 2H4's obscure photo gallery! Mark
40 Post contains images Starlionblue : Tsk. I had that one too.
41 Post contains images 2H4 : Oh, just wait until the next round of Identify This, my friend.... 2H4
42 NorthStarDC4M : I've seen it before as well, one of the pre-SP proposed short 747 designs for NWA and Pan Am.
43 SEPilot : With efficiency becoming the main objective of all commercial aircraft, I doubt very much that we will ever see tail mounted engines on a traditional
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