TimePilot
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Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 3:08 am

Yesterday I was out having a cigarette when a 737 flew overhead. I notice that on it, (and indeed other jets) the engines stick out forward from the wings.

Is this for effeciency? Asthetics? If the hung off the back would it interrupt air flow?
 
speedracer1407
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 3:39 am

Two reasons come to mine.

1. Ground clearance: Mounting the engines forward of the wings means they can also ride pretty high. Ever since big-fan engines arrived, they've been cantilevered way out in front of the wing. The 737 is a perfect example. Originally designed with low bypass, small diameter JT8-D mounted in the traditional (for the time) place directly under the wing, the MLG was nice and short. When the higher bypass, larger diameter CFM-56 was fitted to the -300s, they were mounted way the hell forward of the wing and high enough than the top of the nacelle was nearly level with the upper surface of the wing to allow adequate clearance.

2. Aerodynamics: I seem to recall reading on this forum that moving the engines forward of the wing had the initially unexpected effect of improving airflow under (and possibly over) the wing. I'm sure someone more knowledgeable than me can elaborate.

I guess it's worth noting that aesthetics would never influence something as important to aircraft design as engine placement.
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pilotpip
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 3:39 am

Mainly for aerodynamics. They also aren't truly "straight out", they're canted to allow the best airflow at their location. A good example of this is the DC-9/MD-80.
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TimePilot
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 4:26 am

Thanks for the answers!  Smile
 
tdscanuck
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 5:08 am



Quoting TimePilot (Thread starter):
Is this for effeciency? Asthetics? If the hung off the back would it interrupt air flow?

In addition to all of the above, it keeps most of the rotating bits from being in line with the fuel tank. You still find dry bays sometimes to protect particular areas of the wing, but there's no real way to meet the rotor burst requirements without taking a big fuel capacity hit if you park the engine right under the wing.

Tom.
 
TSS
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 6:12 am

Some early jet aircraft like the De Havilland Comet

had their (low-bypass) engines mounted inside the wing (I'm sure our own 2H4 can think of other examples of aircraft with engines mounted inside the wing, but the Comet is the only one I could remember).

[Edited 2009-11-17 22:13:30]
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DocLightning
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 7:04 am



Quoting Speedracer1407 (Reply 1):

2. Aerodynamics: I seem to recall reading on this forum that moving the engines forward of the wing had the initially unexpected effect of improving airflow under (and possibly over) the wing. I'm sure someone more knowledgeable than me can elaborate.

Look here. http://en.wikipedia.org/wiki/File:Patent_932410_Seite_5.gif

Now, these are models of aircraft that use the Whitcomb Area Rule of maximum efficiency for transsonic and high-subsonic aircraft. The basic idea is that the area of the cross-section of the aircraft should change gradually, even if locally it chances abruptly. So you can stick big pods out in front or in back of the wing to soften up the sharpness of the increase in cross-section starting at the wing root. This is also why the A380 has that odd shape to its wing box.

The Comet posted above was not very good about the Area Rule. The wing was completely smooth and the engines were embedded, which looks really pretty (and that's all engineers had to work with then; the true value of the Area Rule wasn't appreciated), but is actually really unaerodynamic.

The Convair 990, I believe, was the first to have this, because it was so fast:
http://en.wikipedia.org/wiki/File:Co...vair_990_on_ramp_EC92-05275-30.jpg

Today, those odd pods that you see have been replaced by massive engines slung well forward of the wing. They soften the sudden increase in the cross-section of the aircraft. And those huge flap fairings stick out behind the wing, way too large to house the machinery they contain, but they serve as antishock bodies, soften up the decrease in the cross-section of the wing.

And so modern airliners: http://en.wikipedia.org/wiki/File:A3...singapore_airlines_takeoff_arp.jpg
tend to have their antishock bodies appropriately placed. They're just disguised as functional parts of the aircraft!
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vc10
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 9:36 am



Quoting TSS (Reply 5):
had their (low-bypass) engines mounted inside

There was no by-pass low or otherwise on these engines, as they were just good old straight through turbo jets. Even the Comet 4 with RR Avons was a straight through jet

The British V bombers all had their engines buried in the wing

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oly720man
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 10:46 am

Engines in the wing


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The Tu104 was based on the Tu16 bomber so it wasn't originally designed as a pax plane.

Apparently there is some structural benefit in having the wings forward in that they act as a mass balance against flutter.

Quoting TimePilot (Thread starter):
If the hung off the back would it interrupt air flow?

If they hung off the back they'd have to be above the wing, otherwise the aircraft wouldn't be able to rotate to take off, unless the undercarriage was a lot longer. Engines above the wing reduce wing efficiency and the mounting structure needs to be stronger. They're also more likely to be prone to the effects of "bad air flow" generated by the wing, in a stall for example.

The VFW614 is the only example of a civil plane with engines above the wings.


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The Hondajet does have the engines on a pylon to the rear of the wing, but there's only one in existence.


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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 12:21 pm



Quoting Oly720man (Reply 8):
Apparently there is some structural benefit in having the wings forward in that they act as a mass balance against flutter.

Quite. Hanging the engines up front like that makes them counter the wing's tendency to twist.

Oly720man, thanks for posting the dreaded VFW-614. Only of my favorite ugly planes. Big grin
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 1:05 pm



Quoting Oly720man (Reply 8):
Apparently there is some structural benefit in having the wings forward in that they act as a mass balance against flutter.



Quoting Starlionblue (Reply 9):
Quite. Hanging the engines up front like that makes them counter the wing's tendency to twist.

Flutter and twist are two distinct phenomena. Flutter is periodic wing flex due to bending moment along the span. Twist is torsional deformation of the wing along its span under static (more or less) load.

Hanging the engines forward of the wing relieves twist moment, as Starlionblue pointed out. Oly, I'm not sure how an axial (forward) shift of engines relieves flutter. Can you elaborate or provide reference? Thanks.
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oly720man
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 1:24 pm



Quoting F14D4ever (Reply 10):
Can you elaborate or provide reference? Thanks.

Quoting from Torenbeek, Synthesis of subsonic Aircraft Design, p43.

Hopefully the link'll work. RH column about 1/2 way down, under c.

http://books.google.co.uk/books?id=N...nIY-cMpbAjfsO#v=onepage&q=&f=false
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oly720man
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 1:47 pm

To follow the flutter theme, here's a paper about a flutter analysis of the Hondajet

http://hondajet.honda.com/pdf/tech_papers/AIAA_2003_1942_Flutter.pdf
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F14D4ever
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RE: Why Do Jet Engines Stick Straight Out?

Wed Nov 18, 2009 3:16 pm

No go on the google books link, but the Hondajet paper was interesting.
Thanks.
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RE: Why Do Jet Engines Stick Straight Out?

Thu Nov 19, 2009 12:53 am

The Avro C102 jetliner was developed in canada in the late 40's but didn't go into production. It's engines were podded and mounted through the wing similar to the B-45, Tu16/Tu104.

The C102 was smaller than the Comet.
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RE: Why Do Jet Engines Stick Straight Out?

Thu Nov 19, 2009 6:26 am



Quoting Oly720man (Reply 8):
The VFW614 is the only example of a civil plane with engines above the wings.

Don't forget the Antonov 72 and 74.


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RE: Why Do Jet Engines Stick Straight Out?

Thu Nov 19, 2009 10:32 am



Quoting Viscount724 (Reply 15):
Don't forget the Antonov 72 and 74.

... or the words "pylon mounted" between "with" and "engines"

Yes, I'd forgotten the Antonov aircraft, and Beriev (though flying boats do need their engines high up).


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RE: Why Do Jet Engines Stick Straight Out?

Thu Nov 19, 2009 12:49 pm

Quoting Speedracer1407 (Reply 1):
Two reasons come to mine.

1. Ground clearance: Mounting the engines forward of the wings means they can also ride pretty high. Ever since big-fan engines arrived, they've been cantilevered way out in front of the wing. The 737 is a perfect example. Originally designed with low bypass, small diameter JT8-D mounted in the traditional (for the time) place directly under the wing, the MLG was nice and short. When the higher bypass, larger diameter CFM-56 was fitted to the -300s, they were mounted way the hell forward of the wing and high enough than the top of the nacelle was nearly level with the upper surface of the wing to allow adequate clearance.

2. Aerodynamics: I seem to recall reading on this forum that moving the engines forward of the wing had the initially unexpected effect of improving airflow under (and possibly over) the wing. I'm sure someone more knowledgeable than me can elaborate

Not to mention handling and balance benefits: when your wing is installed substantially on the center of the fuselage (as it should be to facilitate CG limits management and improve control harmony) forward engine installation provides more weight towards the nose of the airplane thereby contributing to better longitudinal stability. This facilitates handling/recovery in the stall.

Faro

[Edited 2009-11-19 05:21:17]
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keta
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RE: Why Do Jet Engines Stick Straight Out?

Thu Nov 19, 2009 3:45 pm



Quoting Faro (Reply 17):
forward engine installation provides more weight towards the nose of the airplane thereby contributing to better longitudinal stability. This facilitates handling/recovery in the stall.

Forward or aft the wing, it doesn't matter in terms of location of center of gravity. I mean, it's not that you have your fuselage and wing balanced, and then you put the engines thus moving the CoG; rather, you would think about where the engines are going and then move the wing so the center of lift and the center of gravity are where you want. I don't know if I'm being sufficiently clear...  boggled  What you're saying would mean that aircraft with tail-mounted engines would have worse stall behavior? Or am I missing something?
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RE: Why Do Jet Engines Stick Straight Out?

Thu Nov 19, 2009 5:55 pm



Quoting DocLightning (Reply 6):
The Convair 990, I believe, was the first to have this

Fascinating...however, didn't the 880 come out before the 990 did?
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RE: Why Do Jet Engines Stick Straight Out?

Thu Nov 19, 2009 10:38 pm



Quoting Western727 (Reply 19):

Fascinating...however, didn't the 880 come out before the 990 did?

Yes, but the 880 didn't have the antishock bodies on the wings.
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RE: Why Do Jet Engines Stick Straight Out?

Fri Nov 20, 2009 12:21 am



Quoting DocLightning (Reply 20):
Yes, but the 880 didn't have the antishock bodies on the wings.

Gotcha. Thanks.
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RE: Why Do Jet Engines Stick Straight Out?

Fri Nov 20, 2009 8:17 am



Quoting Vc10 (Reply 7):
Even the Comet 4 with RR Avons was a straight through jet

Upgraded to turbofans on the Nimrod  Smile
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RE: Why Do Jet Engines Stick Straight Out?

Sat Nov 21, 2009 11:58 pm



Quoting 9VSIO (Reply 22):
Upgraded to turbofans on the Nimrod

Yes, but they used the Spey, which only has a 1:1 bypass ratio. RR, however, describes it a "medium bypass engine".
 
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RE: Why Do Jet Engines Stick Straight Out?

Sun Nov 22, 2009 12:12 am



Quoting Avt007 (Reply 23):
Yes, but they used the Spey, which only has a 1:1 bypass ratio. RR, however, describes it a "medium bypass engine".

Not the latest Nimrod, the MRA4. Rolls-Royce BR710. Means the intakes are now a lot wider than the old Nimrod.
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avt007
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RE: Why Do Jet Engines Stick Straight Out?

Sun Nov 22, 2009 8:40 pm

They must be a lot quieter than the old Nimrods. That aircraft is the loudest I've ever heard at an airshow, including the Harrier and Concorde!
 
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RE: Why Do Jet Engines Stick Straight Out?

Sun Nov 22, 2009 11:14 pm



Quoting TSS (Reply 5):
had their (low-bypass) engines mounted inside the wing

But this had its own problems. How do you predict airflow into the engines mounted in the wing-root? Further more, the structure here would be quite heavy.

Planes such as the Boeing Dash-80 put the engines on pylons. The wing root didn't need to accommodate the engines (simpler), and the engines on pylons had a cleaner airflow. Quite a bit better.

Chris Orlebar's excellent book "The Concorde Story" has a good section that talks about aerodynamics and wing structure of early jet planes.
 
tdscanuck
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RE: Why Do Jet Engines Stick Straight Out?

Sun Nov 22, 2009 11:31 pm



Quoting Cpd (Reply 26):
How do you predict airflow into the engines mounted in the wing-root

Wind tunnel. Today you'd do it with CFD, but that probably wasn't an option then.

Quoting Cpd (Reply 26):
Further more, the structure here would be quite heavy

True, although that might be an advantage because the structure at the wing root is big and heavy anyway.

Tom.
 
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RE: Why Do Jet Engines Stick Straight Out?

Mon Nov 23, 2009 1:52 am

Mounting them forward also helps with an aircraft's CoG, not to mention the coupling between thrust/drag and lift/gravity in flight.
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cpd
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RE: Why Do Jet Engines Stick Straight Out?

Mon Nov 23, 2009 2:29 am



Quoting Tdscanuck (Reply 27):
Today you'd do it with CFD, but that probably wasn't an option then.

I think they figured back then it was a way of improving aerodynamics, but the B707/Dash-80 with heavily swept back wings was seriously speedy - proving that you don't need to bury the engines to get the same benefits.

Mounting the engines on pylons makes the whole structure simpler and lighter. While the swept back wings deal with the speed problem very well, and the engines should work better anyway - being in cleaner airflow.

Though some British designs like Sir Barnes Wallis' "Swallow" (one of the British SST proposals) also adopted engines on pylons, pivoting ones no less, right out at the extremities of the swing-wings. I would have thought the control problem would be quite complex to operate the pivot properly back then. Never mind what happens if one or more of the engines failed on one side!
 
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RE: Why Do Jet Engines Stick Straight Out?

Mon Nov 23, 2009 9:48 am

Here, they freed up space inside the wing for more fuel by mounting 2 of the 6 Allison J-35s on pylons, leaving 4 engines inside the wing:

The one and only Northrop YRB-49A
http://i49.tinypic.com/zmgmqa.jpg

http://i47.tinypic.com/x243dg.jpg


Sadly scrapped in 1953, but that's a different story....

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keta
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RE: Why Do Jet Engines Stick Straight Out?

Mon Nov 23, 2009 12:21 pm



Quoting Zkpilot (Reply 28):
Mounting them forward also helps with an aircraft's CoG

Again? (See reply 17.) I'll repeat myself, unless I'm missing something it has no effect.

Quoting Zkpilot (Reply 28):
not to mention the coupling between thrust/drag and lift/gravity in flight.

Could you elaborate this? I don't get what you mean.
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TSS
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RE: Why Do Jet Engines Stick Straight Out?

Mon Nov 23, 2009 2:37 pm



Quoting Keta (Reply 31):
ot to mention the coupling between thrust/drag and lift/gravity in flight.

Could you elaborate this? I don't get what you mean.

Perhaps he's referring to how with engines mounted in pods beneath low-mounted wings an increase in thrust tends to make the aircraft want to assume a more "nose up" attitude in flight?

I had never thought about this effect until a former BAe-146/Avro RJ pilot mentioned that those aircraft tend not to have that effect with increased thrust because the engines are more or less in-line with the fuselage, rather than below it.
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Zkpilot
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RE: Why Do Jet Engines Stick Straight Out?

Mon Nov 23, 2009 3:07 pm



Quoting Keta (Reply 31):

Quoting Zkpilot (Reply 28):
Mounting them forward also helps with an aircraft's CoG

Again? (See reply 17.) I'll repeat myself, unless I'm missing something it has no effect.

Sorry missed your post. Part of what I was talking about is that if you removed the engines from a lot of aircraft they will actually tip back onto their tail...

Quoting Keta (Reply 31):

Quoting Zkpilot (Reply 28):
not to mention the coupling between thrust/drag and lift/gravity in flight.

Could you elaborate this? I don't get what you mean.

This is the best diagram I could quickly find online for you:

Basically all the forces don't act straight through in a + shape... They are offset but the moments balance each other out. This is obviously a small aircraft diagram and it is different on a commercial jet but it gives you an idea of what I am referring to.
btw source for this is: http://pilotsweb.com/principle/forces.htm
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keta
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RE: Why Do Jet Engines Stick Straight Out?

Mon Nov 23, 2009 3:49 pm



Quoting Zkpilot (Reply 33):
They are offset but the moments balance each other out.

That's right, moments should be balanced. But even if the engines weren't mounted in front of the wings, you could equally balance the plane, I don't see any advantage regarding this in forward-hung engines.

The same happens with the CoG. Having the engines forward doesn't mean that you have "more weight towards the nose". If the engines are positioned closer to the tail, you'll just have to move the wing backwards until you have the lift and weight correctly balanced again.

Several advantages of forward-hung engines (which is what the thread starter was discussing) have arisen, namely improved flutter behavior or lighter wing structure. Neither CoG positioning nor moment balance would fall into this category.
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413X3
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RE: Why Do Jet Engines Stick Straight Out?

Mon Nov 23, 2009 4:34 pm

but wing design on tail mounted engines is different than engines mounted on the wings. So yes engines mounted at different places has a different effect on center of gravity. But this is dealt with by designing different wings to counteract a tail mounted engine
 
keta
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RE: Why Do Jet Engines Stick Straight Out?

Mon Nov 23, 2009 10:51 pm



Quoting 413X3 (Reply 35):
but wing design on tail mounted engines is different than engines mounted on the wings. So yes engines mounted at different places has a different effect on center of gravity. But this is dealt with by designing different wings to counteract a tail mounted engine

Wing design indeed is different and I never said the contrary. But the difference relies on structure and aerodynamics, not on the CoG being located in another place nor because rear engines "provide worse longitudinal stability".

I think some people are mixing up different issues. The thread is about the benefits of having the engines stick out forward from the wing, and there's no "better position of CoG" nor "better momentum balance" because of this.
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