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Why Do Jet Engines Stick Straight Out?  
User currently offlineTimePilot From Switzerland, joined Sep 2005, 296 posts, RR: 0
Posted (4 years 10 months 1 week 2 days 18 hours ago) and read 10536 times:

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?

36 replies: All unread, showing first 25:
 
User currently offlineSpeedracer1407 From United States of America, joined Dec 2004, 333 posts, RR: 0
Reply 1, posted (4 years 10 months 1 week 2 days 17 hours ago) and read 10520 times:

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.



Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 10
Reply 2, posted (4 years 10 months 1 week 2 days 17 hours ago) and read 10518 times:

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.


DMI
User currently offlineTimePilot From Switzerland, joined Sep 2005, 296 posts, RR: 0
Reply 3, posted (4 years 10 months 1 week 2 days 17 hours ago) and read 10477 times:

Thanks for the answers!  Smile

User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 4, posted (4 years 10 months 1 week 2 days 16 hours ago) and read 10458 times:



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.


User currently offlineTSS From United States of America, joined Dec 2006, 3068 posts, RR: 5
Reply 5, posted (4 years 10 months 1 week 2 days 15 hours ago) and read 10426 times:

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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User currently offlineDocLightning From United States of America, joined Nov 2005, 19727 posts, RR: 58
Reply 6, posted (4 years 10 months 1 week 2 days 14 hours ago) and read 10395 times:



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!


User currently offlineVc10 From United Kingdom, joined Nov 2001, 1411 posts, RR: 15
Reply 7, posted (4 years 10 months 1 week 2 days 12 hours ago) and read 10330 times:



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

littlevc10


User currently offlineOly720man From United Kingdom, joined May 2004, 6738 posts, RR: 11
Reply 8, posted (4 years 10 months 1 week 2 days 10 hours ago) and read 10306 times:

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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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17044 posts, RR: 66
Reply 9, posted (4 years 10 months 1 week 2 days 9 hours ago) and read 10262 times:



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



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineF14D4ever From United States of America, joined May 2005, 319 posts, RR: 4
Reply 10, posted (4 years 10 months 1 week 2 days 8 hours ago) and read 10230 times:



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.



"He is risen, as He said."
User currently offlineOly720man From United Kingdom, joined May 2004, 6738 posts, RR: 11
Reply 11, posted (4 years 10 months 1 week 2 days 8 hours ago) and read 10223 times:



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



wheat and dairy can screw up your brain
User currently offlineOly720man From United Kingdom, joined May 2004, 6738 posts, RR: 11
Reply 12, posted (4 years 10 months 1 week 2 days 7 hours ago) and read 10209 times:

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



wheat and dairy can screw up your brain
User currently offlineF14D4ever From United States of America, joined May 2005, 319 posts, RR: 4
Reply 13, posted (4 years 10 months 1 week 2 days 6 hours ago) and read 10166 times:

No go on the google books link, but the Hondajet paper was interesting.
Thanks.



"He is risen, as He said."
User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 14, posted (4 years 10 months 1 week 1 day 20 hours ago) and read 9870 times:

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.



"Saddest thing I ever witnessed....an airplane being scrapped"
User currently offlineViscount724 From Switzerland, joined Oct 2006, 25356 posts, RR: 22
Reply 15, posted (4 years 10 months 1 week 1 day 15 hours ago) and read 9763 times:



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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User currently offlineOly720man From United Kingdom, joined May 2004, 6738 posts, RR: 11
Reply 16, posted (4 years 10 months 1 week 1 day 11 hours ago) and read 9713 times:



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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wheat and dairy can screw up your brain
User currently offlineFaro From Egypt, joined Aug 2007, 1548 posts, RR: 0
Reply 17, posted (4 years 10 months 1 week 1 day 8 hours ago) and read 9670 times:

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]


The chalice not my son
User currently offlineKeta From Germany, joined Mar 2005, 448 posts, RR: 0
Reply 18, posted (4 years 10 months 1 week 1 day 5 hours ago) and read 9623 times:



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?



Where there's a will, there's a way
User currently offlineWestern727 From United States of America, joined Jan 2007, 751 posts, RR: 4
Reply 19, posted (4 years 10 months 1 week 1 day 3 hours ago) and read 9570 times:



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?



Jack @ AUS
User currently offlineDocLightning From United States of America, joined Nov 2005, 19727 posts, RR: 58
Reply 20, posted (4 years 10 months 1 week 22 hours ago) and read 9493 times:



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.


User currently offlineWestern727 From United States of America, joined Jan 2007, 751 posts, RR: 4
Reply 21, posted (4 years 10 months 1 week 21 hours ago) and read 9467 times:



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

Gotcha. Thanks.



Jack @ AUS
User currently offline9VSIO From United Kingdom, joined Dec 2006, 717 posts, RR: 2
Reply 22, posted (4 years 10 months 1 week 13 hours ago) and read 9383 times:
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Quoting Vc10 (Reply 7):
Even the Comet 4 with RR Avons was a straight through jet

Upgraded to turbofans on the Nimrod  Smile



Me: (Lining up on final) I shall now select an aiming point. || Instructor: Well, I hope it's the runway...
User currently offlineAvt007 From Canada, joined Jul 2000, 2132 posts, RR: 5
Reply 23, posted (4 years 10 months 5 days 21 hours ago) and read 9111 times:



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".


User currently offline9VSIO From United Kingdom, joined Dec 2006, 717 posts, RR: 2
Reply 24, posted (4 years 10 months 5 days 21 hours ago) and read 9109 times:
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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.



Me: (Lining up on final) I shall now select an aiming point. || Instructor: Well, I hope it's the runway...
25 Avt007 : 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!
26 Cpd : 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
27 Tdscanuck : Wind tunnel. Today you'd do it with CFD, but that probably wasn't an option then. True, although that might be an advantage because the structure at
28 Zkpilot : Mounting them forward also helps with an aircraft's CoG, not to mention the coupling between thrust/drag and lift/gravity in flight.
29 Cpd : 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 - prov
30 Post contains links and images Scooter01 : 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
31 Keta : Again? (See reply 17.) I'll repeat myself, unless I'm missing something it has no effect. Could you elaborate this? I don't get what you mean.
32 TSS : 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
33 Post contains links and images Zkpilot : 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 t
34 Keta : 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
35 413X3 : 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 eff
36 Keta : 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
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