dash8pilot
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Rear Mounted Engines

Mon Nov 19, 2007 3:50 pm

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??
 
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Starlionblue
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RE: Rear Mounted Engines

Mon Nov 19, 2007 3:59 pm

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." - John Ringo
 
TupolevTu154
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RE: Rear Mounted Engines

Mon Nov 19, 2007 4:04 pm

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
 
ALexeu
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RE: Rear Mounted Engines

Mon Nov 19, 2007 4:05 pm

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.
 
pilotboi
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RE: Rear Mounted Engines

Mon Nov 19, 2007 4:27 pm



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.
 
bok269
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RE: Rear Mounted Engines

Tue Nov 20, 2007 3:40 am



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
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Viscount724
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RE: Rear Mounted Engines

Tue Nov 20, 2007 5:41 am



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.
 
TrijetsRMissed
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RE: Rear Mounted Engines

Tue Nov 20, 2007 5:46 am

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 trijet.
 
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Starlionblue
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RE: Rear Mounted Engines

Tue Nov 20, 2007 8:19 am



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." - John Ringo
 
Stil
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RE: Rear Mounted Engines

Fri Nov 23, 2007 8:02 pm

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
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474218
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RE: Rear Mounted Engines

Sat Nov 24, 2007 12:15 am



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".
 
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Starlionblue
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RE: Rear Mounted Engines

Sat Nov 24, 2007 1:30 am



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." - John Ringo
 
MQTmxguy
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RE: Rear Mounted Engines

Sat Nov 24, 2007 2:58 am

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
 
2H4
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RE: Rear Mounted Engines

Sat Nov 24, 2007 3:03 am



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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MQTmxguy
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RE: Rear Mounted Engines

Sat Nov 24, 2007 3:23 am



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
 
mark5388916
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RE: Rear Mounted Engines

Sat Nov 24, 2007 3:25 am

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
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2H4
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RE: Rear Mounted Engines

Sat Nov 24, 2007 3:28 am



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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MQTmxguy
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RE: Rear Mounted Engines

Sat Nov 24, 2007 3:36 am

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
 
MQTmxguy
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RE: Rear Mounted Engines

Sat Nov 24, 2007 3:45 am



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
 
2H4
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RE: Rear Mounted Engines

Sat Nov 24, 2007 3:48 am



Quoting MQTmxguy (Reply 18):

Interesting. Thanks for the info.

2H4
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Starlionblue
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RE: Rear Mounted Engines

Sat Nov 24, 2007 7:37 am



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." - John Ringo
 
MQTmxguy
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RE: Rear Mounted Engines

Sat Nov 24, 2007 8:11 am



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
 
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Starlionblue
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RE: Rear Mounted Engines

Sat Nov 24, 2007 9:41 am



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." - John Ringo
 
MQTmxguy
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RE: Rear Mounted Engines

Sun Nov 25, 2007 12:08 am



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
 
PMN1
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RE: Rear Mounted Engines

Fri Feb 15, 2008 6:07 pm

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.
 
PMN1
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RE: Rear Mounted Engines

Fri Feb 15, 2008 6:10 pm

Was there a British engine around at the time that would have allowed the DH121 (original spec) to have two underwing engines?
 
BoeingOnFinal
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RE: Rear Mounted Engines

Fri Feb 15, 2008 6:35 pm

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, high-mounted engine. Is that directly related to accessibility, or are the issues I was reading about type specific?

Of course there are a huge difference in height between a G550 engine and an MD-11 tail mounted engine. And I cannot imagine this being a deciding factor in choosing the design, as landing gear weight and operational advantages would probably far outweigh the maintenance cost. But it would be nice to get a mechanics view on this topic.
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PMN1
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RE: Rear Mounted Engines

Fri Feb 15, 2008 6:52 pm

How about on the wing but without using a pylon so it would be like the DH119 and 120 or Concorde?
 
2H4
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RE: Rear Mounted Engines

Fri Feb 15, 2008 7:01 pm



Quoting PMN1 (Reply 27):
How about on the wing but without using a pylon

...Like the 737-200?

2H4
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Starlionblue
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RE: Rear Mounted Engines

Sat Feb 16, 2008 12:11 am



Quoting PMN1 (Reply 27):
How about on the wing but without using a pylon so it would be like the DH119 and 120 or Concorde?

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 moment. Also it makes changing the engine easier. Removing the pylon would solve some problems and introduce others.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
ex52tech
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RE: Rear Mounted Engines

Sat Feb 16, 2008 3:06 am



Quoting BoeingOnFinal (Reply 26):
Is that directly related to accessibility, or are the issues I was reading about type specific?

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 skyclimbers to change that engine. I had a skyclimber cable fray, twist up and bind in the feed shield on the rear winch of a skyclimber when we were (luckly) raising the empty cradle into position to remove the bad engine. We could not raise or lower the back of the cradle with the winch. We ended up lowering the rear of the cradle with a forklift, while using the two front skyclimbers as advertised. Imagine if the engine had been on that cradle when that happened, the airplane was on jacks at the time, and the main gear was out. The skyclimbers had just come back from a rebuild, and calibration.

NWA adapted a set of engine rails to a rather large basket that could be lifted into position by a big forklift, it made changing the #2 much easier. You had a means to raise and lower the engine and have a built in work stand all in one. Not to mention it was faster and safer than the skyclimbers.

But just working on the #2, even the simplest tasks take much longer due to the accesability to that engine. I once changed the main gearbox on a CF-6 from the patio (work platform) on a road trip, it would have been faster to change the engine, if we had actually had a spare.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
MD-90
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RE: Rear Mounted Engines

Sat Feb 16, 2008 4:59 am



Quoting Starlionblue (Reply 1):
Advantages of tail mounted engines:

I think aesthetics should be included as well. Some of the larger bizjets could've been designed to accommodate underwing engines but they look better with tail-mounted engines (such as the GLEX and G-550).
 
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Starlionblue
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RE: Rear Mounted Engines

Sat Feb 16, 2008 6:28 am



Quoting MD-90 (Reply 31):
I think aesthetics should be included as well. Some of the larger bizjets could've been designed to accommodate underwing engines but they look better with tail-mounted engines (such as the GLEX and G-550).

True. However the companies involved also have experience with one and not the other, which may have counted.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
mandala499
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RE: Rear Mounted Engines

Sat Feb 16, 2008 8:31 am



Quoting PMN1 (Reply 24):
including superior cross-wind landing capability

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 reverse, the rudder is largely in turbulent air and can severely reduce it's effectiveness. This can be a problem when trying to keep the aircraft on the centerline at high speeds...

This was mentioned a lot by former F28 (no reversers there!) pilots who moved to MD80 when Lion started to introduce them in Indonesia. A few runway excursions happened when landing long on wet runways and they bung on the full reversers in cross wind, and the nose begin to move into the wind, and full opposite rudder didn't help much... I'm told that they were told of a technique used in Scandinavia, that is to reduce the reverse power when the rudder begins to blank out at the higher speeds. Unfortunately, several crew were "overenthusiastic" when using this technique and ended up "recycling" the reversers (ie: stowing them and redeploying)... this lead to slower and uneven bucket deployment causing a different set of problems. And there have been cases where recycling ended up in the throttles entering positive thrust position, stowing the spoilers... hence the "high" number of runway overruns by Lion in 2003 - 2005... culminating in the fatal SOC accident (reportedly witnesses said spoilers deployed and then stowed and never redeployed, and nosewheel never touched the ground).

Just my half a cent's worth...

Mandala499
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BoeingOnFinal
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RE: Rear Mounted Engines

Sat Feb 16, 2008 11:40 am

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 standpoint? It would seem to me that a lot of the problems you are mentioning would not apply to aircrafts with less height and engines not mounted below the vertical stab. Do you think it would be just as problematic for a Falcon 900?
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ex52tech
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RE: Rear Mounted Engines

Sun Feb 17, 2008 4:07 am



Quoting BoeingOnFinal (Reply 34):
So what are your opinion on high mounted wings on smaller regionals and business jets from a mechanics standpoint? It would seem to me that a lot of the problems you are mentioning would not apply to aircrafts with less height and engines not mounted below the vertical stab.

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 should be as high off the ground as possible. A piece of FOD that wouldn't hurt a JT8, could do some serious damage to a smaller high bypass engine.
An MD80 or 727 engine is not that difficult to work on, because the engines themselves are not that big in diameter. So the smaller diameter engines even though they are mounted fairly high off of the ground, are not really that much trouble. You can place a stand next to the engine, and reach most components on the engine.

But with any 747 for instance. Even though it has a low mounted wing, the diameter of the fuselage, the diameter of the engines, and wing dihedral, work against you. The outboard engines are not easy to work on because you need a stand to get to them. If the engine is a JT9-7Q, and you have a fuel pump/fuel control, or an EVBC to replace, you just want to go home......ah.......anyway...... If you have to work on the entire engine then you will have to move the stand up and down, and in most cases you will have to remove the safety railings in order to get the stand where you need it. Not to get to anecdotal, but, I knew a mechanic that was installing pylon panels on an outboard pylon, on a "Whale", he slipped, slid off of the closed engine cowlings, and broke both ankles. The engine was a JT9, so the cowlings had a liberal amount of oil coating them, which aided in his acceleration toward the ground.

The DC10, MD11, 757/767, and most Airbus wing engines are close enough to the ground that what you can't reach from the ground, in most cases, a 6' ladder will allow you to get to what you need. In heavy maint. check if we had all the engines off of a DC10, we would try and put the best of the three in the #2 hole, the theory being, IT MADE SENSE. So you can imagine what management thought of the idea.

It has always amazed me how aircraft design is such a compromise. I do agree that tail mounted engines on larger airliners is a thing of the past. Weight, diameter, and thrust output of the newer more efficent engines has relegated them to be placed on the wings.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
BoeingOnFinal
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RE: Rear Mounted Engines

Sun Feb 17, 2008 11:20 am

Thanks for your input Ex52tech, it is greatly appreciated!
norwegianpilot.blogspot.com
 
PMN1
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RE: Rear Mounted Engines

Tue Mar 04, 2008 4:25 pm

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, did the designers of the 727 and the Trident consider 2 wing and 1 tail mounted engine?
 
2H4
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RE: Rear Mounted Engines

Tue Mar 04, 2008 7:29 pm



Quoting PMN1 (Reply 37):
In the same way, did the designers of the 727 and the Trident consider 2 wing and 1 tail mounted engine?

Boeing considered such a configuration for the 747:

http://i32.tinypic.com/1z4amig.jpg


2H4
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mark5388916
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RE: Rear Mounted Engines

Thu Mar 06, 2008 6:22 am



Quoting 2H4 (Reply 38):

Another pick from 2H4's obscure photo gallery!

Mark
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Starlionblue
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RE: Rear Mounted Engines

Thu Mar 06, 2008 8:59 am



Quoting Mark5388916 (Reply 39):
Another pick from 2H4's obscure photo gallery!

Tsk. I had that one too.  Wink
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
2H4
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RE: Rear Mounted Engines

Thu Mar 06, 2008 3:27 pm



Quoting Starlionblue (Reply 40):
Tsk. I had that one too.

Oh, just wait until the next round of Identify This, my friend....  biggrin 

2H4
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northstardc4m
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RE: Rear Mounted Engines

Thu Mar 06, 2008 3:48 pm



Quoting Starlionblue (Reply 40):

Quoting Mark5388916 (Reply 39):
Another pick from 2H4's obscure photo gallery!

Tsk. I had that one too. Wink

I've seen it before as well, one of the pre-SP proposed short 747 designs for NWA and Pan Am.
Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety.
 
SEPilot
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RE: Rear Mounted Engines

Thu Mar 06, 2008 5:15 pm

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 tube-with-wings airliner again. The structural issues alone are enough to make it uneconomic, and with the modern large diameter turbofans it is even worse. Historical note: the original 737 design was supposed to have tail mounted engines (following the 727, this was to be expected). The reason was that they wanted the height to be as low as possible, speeding loading and unloading (remember, this was before jet bridgeways) enabling fast turnaround. Joe Sutter (of 747 fame) was one of the engineers working on it, and he figured out that by mounting the engines behind the main spar with a cowling that extended in front of it they had very little aerodynamic penalty over the strut mounting originally developed for the B-47, and the structural weight and space required was enough less that they could add 2 additional rows of seats and keep the same power requirements. With engines being so much larger in diameter the tradeoff is even more advantageous for wing mounted engines.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler

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