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cancidas
Topic Author
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Joined: Thu Jul 03, 2003 7:34 am

No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 12:02 pm

is there something undesirable for aircraft manufacturers today when it comes to mounting the engines on the tail of the aircraft? the B717 is the only new built aircraft that has them. no others exist. what's the reason for this? regional aircraft do have them but it is my understanding that due to thier small size there is no way the engines could be placed under the wings. way back when there was the VC-10, IL-62, B727, TU-154, DC-9, etc.
"...cannot the kingdom of salvation take me home."
 
futureualpilot
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RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 12:07 pm

My guess is the added equipment it takes to pump the fuel up to the engines from the wing/center tanks, where as wing-mounted engines dont require the needed equip.

Just my .02!
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planemaker
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RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 3:24 pm

The main reason why there are no "new" rear engined jets is mainly due to weight and balance and structural issues. When you hang the engines under the wings it just makes it a lot easier.

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RNOcommctr
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RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 3:30 pm

Planemaker, what you write makes sense. So what was the original rationale for putting engines in the back, e.g. 727 and DC-9?
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challengerdan
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RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 3:31 pm

I think it has more to do with the weight than with the equipment. The under-wing slung engines still need pumps and stuff so that they are constantly fed with a positive pressure. By having the engines under the wing, it makes the weight more concentrated towards the CG of the aircraft. When talking about bigger aircrafts, you're also putting more powerful and heavier engines. Have you ever noticed how far back the wing is mounted on a tail-engined aircraft?
It is probaly easier, on an engineering point of view, at some point, to put the engines under the wing to reduce fuselage bending moment problems. Also, having the heavy load of the engines under the wing helps cancel out a part of the wing flexing moment caused by the creation of lift, especially at wing root.

now, i'm not an engineer, i'm just an aircraft mechanic. This is just some sort of educated guess. If anyone can come up with more specific reasons, they would be welcome  Smile
if your flight goes MX in YUL, I might be called to fix it!
 
Trident
Posts: 477
Joined: Sun Feb 27, 2000 4:49 am

RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 4:49 pm

The idea of placing the engines under the wings (as pioneered on the Boeing B-47 Stratojet) was to reduce the bending moment of the wings. If the engines had been placed anywhere else the wings would have to be made stronger to counteract this bending, thereby making the aircraft heavier.

The popularity of rear mounted engines came about due to factors which are not that important these days:

i) cabin noise - early jets were very noisy so placing them at the rear kept cabin noise levels down, except for the unfortunates sitting in the rear.

ii) wing efficiency - even though the wing might be heavier, it was calculated that a "clean" wing could be made more efficient than one cluttered up with pylons and engine pods. Wing design has moved on a lot since the late 50s and early 60s so this factor is not as imporatnt as it once was.

iii) fashion - like everything else carried on ny humans, fashion and style play a part in what we make.This is probably less of an issue today as most aircraft are designed using CAD systems with less human aesthetic involvement. Although I have to admit that the new Boeing 7E7 looks quite stylish.

Rear engines are still popular on biz jets and regional jets for the simple reason that, as the planes are relatively snall, there is simply no room inder the wings to place podded engines.
 
liamksa
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RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 5:27 pm

Wing flutter is also a major factor in engine location. It's one of the reasons engines are mounted so far forward below the wing.
 
POSITIVE RATE
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RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 5:30 pm

There hasn't been any in the past few years, but in the last 10 years or so we have had:
717
MD90
F70
F100
CRJ
EMB-135/145


So rear-mounted engines haven't died out entirely, especially as far as regional jets go.
 
challengerdan
Posts: 145
Joined: Sat Sep 27, 2003 3:17 pm

RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 5:53 pm

Liamksa:
Could you elaborate about wing flutter? This is something i've never heard before
if your flight goes MX in YUL, I might be called to fix it!
 
geg2rap
Posts: 739
Joined: Sun Sep 21, 2003 11:02 am

RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 6:33 pm

Rear engines with t tails are more prone to deep stalls, according to Boeing
 
cancidas
Topic Author
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Joined: Thu Jul 03, 2003 7:34 am

RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 7:22 pm

challengerdan, if you were to look out the window on an aircraft with a supercritical wing, say B737-800, in turbulent air the wing will be "bouncing." that phenomenon is known as wing flutter. today's lighter wings are more suceptable to it.
"...cannot the kingdom of salvation take me home."
 
traveler_7
Posts: 448
Joined: Tue May 23, 2000 12:45 am

RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 7:45 pm


Are there any significant advantages of rear-mounted engines over wing-mounted?

I mean advantages significant enough to build, in some future, a new rare-engined aircraft with a size like A320 or 737-800?

Positive rate
As I remember FD 428 & 528 were supposed to have wing mounted engines.
It looks like regional jets also "infected " by the idea to place engines under wings?


 
MD-90
Posts: 7836
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RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 9:33 pm

Before I bailed out of aerospace engineering (I'm just not that fond of math), we learned that generally, the best place for engines to go is on the wing. That's pretty much a universal truth, as far as turbines go.

Weight and balance is an issue. Boeing had some slight problems while designing the 727, because as fuel burned off the cog shifted forwards. The tail area has to be built up if you have engines back there and thrust lines (especially the DC-10 and MD-11. L-1011 wasn't so bad). The usual relief from lift stresses out on the wings (and trust me, it can be a big, big factor) outweighs the aerodynamic penalties (which are pretty much slight anyway). And of course, the danger of a deep stall is avoided (although if we ever have canards on Boeings we might have to revisit that).

Nowadays, Traveler_7, you're not going to find any rear-engined commercial aircraft bigger than an RJ. There's just no reason to do it. Don't expect someone to design a new MD-80 lookalike. But for smaller planes (anything smaller than the Embraer 170), it's just too clunky to have landing gear tall enough to allow clearence for the engines. Hence the reason why all Cessna Citations, Learjets, Gulfstreams, DC-9s, etc are rear-engined.

And furthermore, Cancidas, wing flutter and the wing bending you've observed in flight are two different things altogether. The softness of a wing (how much it tends to bend, to absorb the affects of turbulence inflight) is not related to its flutter characteristics. New materials and much better computer calculations allow for thinner margins. Wings don't have to be so stiff (which means heavy) anymore, because we can model failures and know just how much strength is really needed.
 
POSITIVE RATE
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Joined: Sun Sep 16, 2001 11:31 am

RE: No Desire For Tail Mounted Engines?

Thu Nov 20, 2003 11:25 pm

There are still a few advantages to tail mounted engines:
Less yaw moment in an engine failure
general quieter cabin
less FOD to engines
clean wing/less parasite drag
 
jgardiner
Posts: 112
Joined: Tue Mar 27, 2001 3:41 am

RE: No Desire For Tail Mounted Engines?

Fri Nov 21, 2003 2:08 am

Don't forget that tail mounted engines also limit the amount by which you can stretch the fuselage.


 
desertjets
Posts: 7693
Joined: Fri Feb 18, 2000 3:12 pm

RE: No Desire For Tail Mounted Engines?

Fri Nov 21, 2003 2:17 am

Correct me if I am wrong, and I may be WAY OFF base on this one.

But isn't part of the rationelle for using tail mounted engines, especially in planes like the DC-9, was that the cargo doors and most service points were lower to the ground, allowing baggage to be loaded without the use of a bag belt? Which would have been an important concern at some of the smaller outposts that the DC-9 was intended to serve.
Stop drop and roll will not save you in hell. --- seen on a church marque in rural Virginia
 
MD-90
Posts: 7836
Joined: Mon Jan 17, 2000 12:45 pm

RE: No Desire For Tail Mounted Engines?

Fri Nov 21, 2003 5:06 am

I'm sure that was a factor, DesertJets. Imagine how tall the landing gear would've had to have been to accomodate JT8-Ds under the wings?

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