Extra300
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Engines Under Wing Or On Fuselage

Thu Oct 18, 2012 3:02 pm

Hi, I have a question about airliner design.

Why is the engines under the wings design so common? All current Boeings and Airbuses have that design, even CSeries, Embraer 170, Sukhoi SSJ100 and so on. I can only think of the MD80/717 and CRJ that has fuselage mounted engines. For some reason the design with engines under the wing must be superior?

When I try to think this through myself I come up with this

Pros with fuselage mounted engines
-Shorter and lighter landing gear for a given ground clearance
-Shorter landing gear also gives smaller landing gear bays
-The wing cuts through 'clean air' without disturbance from the engines


Pros for engines under the wing design:
-Shorter fuel lines from tank to engine
-Any ice from the wing cannot in any way hit the engines
-Less noise in the back of the cabin

Please help me, there must be more to it! What are the pros and cons with the different designs? And why is the engines under hte wing so dominating?


(Moderator please move this thread to 'technical' if it is more suitable)
 
flyingturtle
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RE: Engines Under Wing Or On Fuselage

Thu Oct 18, 2012 3:22 pm

Hello!


This question pops up from time to time. So it is kind of easy to answer.  
Quoting Extra300 (Thread starter):
Why is the engines under the wings design so common? All current Boeings and Airbuses have that design, even CSeries, Embraer 170, Sukhoi SSJ100 and so on. I can only think of the MD80/717 and CRJ that has fuselage mounted engines. For some reason the design with engines under the wing must be superior?

It really depends... there are old aircraft with either design, and there are newer ones with either design.

Quoting Extra300 (Thread starter):

Pros for engines under the wing design:
-Shorter fuel lines from tank to engine
-Any ice from the wing cannot in any way hit the engines
-Less noise in the back of the cabin

These reasons are right. Additional ones:
- The engine creates a "counterweight" to the lift that is generated by the wings. So, the wing structure can be lighter with wing-mounted engines (although it sounds paradoxical).
- easier maintenance, as the engine is near the ground. (There are time-lapse videos of engine changes on YouTube - changing an engine takes less than three hours.)
- the problem of uncontained engine failures. In a rear-mounted engine, engine parts could slice through the aircraft, severing hydraulic lines (United Airlines 232 in Sioux City for example). That's much less of a problem in wing-mounted engines.

Quoting Extra300 (Thread starter):

Pros with fuselage mounted engines
-Shorter and lighter landing gear for a given ground clearance
-Shorter landing gear also gives smaller landing gear bays
-The wing cuts through 'clean air' without disturbance from the engines

These reasons are also right... additional ones:
- the shutdown of one engine is less of a problem (asymmetric thrust)
- the wing contains less wirings and hydraulic lines

It really depends if you want to build a big aircraft (which comes with longer landing gear struts anyway as the A/C is getting longer, enabling wing-mounted engines) or a small one.



David
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xlc
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RE: Engines Under Wing Or On Fuselage

Thu Oct 18, 2012 3:29 pm

Under the wing is closer to the ground and easier for the mechanics. A forklift is easier (and cheaper) to operate than a crane. Under wing engines are (sometimes) visible from the cockpit. You don't have a seating area close to the noise and vibration of the engines.

But, tail mounted engines are really cool looking. Especially on the old L-1011.  
 
tdscanuck
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RE: Engines Under Wing Or On Fuselage

Thu Oct 18, 2012 3:44 pm

Quoting Extra300 (Thread starter):
Why is the engines under the wings design so common?

Basically, because at the end of the day, it's cheaper. The roll-up of all the factors (maintenance, weight, performance, etc.) come out to under-wing engines having lower total cost-of-ownership.

Quoting Extra300 (Thread starter):
Pros for engines under the wing design:
-Shorter fuel lines from tank to engine

Although this is true, it's not as big a factor as you might think; on aircraft with under-wing engines the APU is almost always in the tail so you have to run a fuel line back there anyway. It may be a smaller diameter line (lighter) but this isn't a huge consideration.

Tom.
 
Extra300
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RE: Engines Under Wing Or On Fuselage

Thu Oct 18, 2012 4:32 pm

Quoting xlc (Reply 2):
But, tail mounted engines are really cool looking. Especially on the old L-1011.

I totally agree!  


Thanks for your answers, this was escecially interesting and a bit surprising

Quoting flyingturtle (Reply 1):
The engine creates a "counterweight" to the lift that is generated by the wings. So, the wing structure can be lighter with wing-mounted engines (although it sounds paradoxical).

 
 
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RE: Engines Under Wing Or On Fuselage

Thu Oct 18, 2012 8:31 pm

Quoting Extra300 (Reply 4):
Thanks for your answers, this was escecially interesting and a bit surprising

Quoting flyingturtle (Reply 1):
The engine creates a "counterweight" to the lift that is generated by the wings. So, the wing structure can be lighter with wing-mounted engines (although it sounds paradoxical).

If you think of the entire wing (from wingtip to wingtip) as a long flexible metal ruler held up by lift (two bricks under the inner third of the wings representing the average lift generation) and weighed down by the fuselage (one brick in the middle and on top) you can see how adding weight outboard decreases bending moment in the middle.

This is also the reason that center tank fuel is used first and wing tank fuel last. The wing tank fuel decreases wing bending while the center tank fuel increases it.
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RE: Engines Under Wing Or On Fuselage

Thu Oct 18, 2012 9:01 pm

Quoting flyingturtle (Reply 1):
It really depends if you want to build a big aircraft (which comes with longer landing gear struts anyway as the A/C is getting longer, enabling wing-mounted engines) or a small one.

Yes, as for the 'small one', conversely, almost all business jets have rear mounted engines.


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RE: Engines Under Wing Or On Fuselage

Thu Oct 18, 2012 11:05 pm

Rear engine, T-tail designs don't permit as much of the overall length of the aircraft to be used for passengers.

For example, the overall length of the 727-200 is a few inches longer than the longest 707 (the -320/-420).

The shortest DC-9-10 is also longer overall than the 737-200 due to the rear-engine,T-tail design, but the the passenger cabin is several seat rows shorter.
 
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RE: Engines Under Wing Or On Fuselage

Fri Oct 19, 2012 7:00 am

Quoting Viscount724 (Reply 7):

Gee, I haven't thought of that.  


David
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Extra300
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RE: Engines Under Wing Or On Fuselage

Fri Oct 19, 2012 8:01 am

Quoting Viscount724 (Reply 7):
Rear engine, T-tail designs don't permit as much of the overall length of the aircraft to be used for passengers.

¨
Speaking about T-tail. Does the T-tail have any aerodynamic advantages or disadvantages?
 
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RE: Engines Under Wing Or On Fuselage

Fri Oct 19, 2012 8:28 am

Quoting Extra300 (Reply 9):
Speaking about T-tail. Does the T-tail have any aerodynamic advantages or disadvantages?

One advantage of the T-tail is that the horizontal stab works in unmolested air. A far bigger one is with military transports. It gives more ground clearance if you maneuver with trucks around the loading ramp.

The disadvantage of the T-tail is the possibility of deep stall:



It doesn't affect ⊥-tailed aircraft as much.

David

[Edited 2012-10-19 01:34:06]
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flipdewaf
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RE: Engines Under Wing Or On Fuselage

Fri Oct 19, 2012 11:47 am

Fuselage mounted engines also allow the aircraft to have shorter landign gear and move the fuselage closer to the ground, this means you can have much easier access without specialist equipment. This is one of the main reasons why most biz jets have fuse mounted engines.

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RE: Engines Under Wing Or On Fuselage

Fri Oct 19, 2012 12:33 pm

Quoting flipdewaf (Reply 11):
This is one of the main reasons why most biz jets have fuse mounted engines.

It's rather the other way round.

It's not "Hey, we want to build an aircraft with easy access to the fuselage, in order to remove and load baggage without equipment." This inevitably leads to "Oops, now we have an A/C we can only market as a business plane."

The requirement is: What should the A/C able to do? Fine, if you envision a business jet from the beginning. Or a 300-seat airliner. Form follows function.


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DocLightning
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RE: Engines Under Wing Or On Fuselage

Sat Oct 20, 2012 8:47 pm

There is also an aerodynamic advantage to wing-mounted engines.

The Whitcomb Area rule states that one should avoid sudden increases or decreases in the total cross-section of the aircraft. http://en.wikipedia.org/wiki/Area_rule

When the engines are positioned just in front of the wing, this softens the sudden increase in cross-section encountered right at the wing root. This can reduce wave drag in high-subsonic aircraft like airliners. If you look carefully at a modern airliner's wing you will notice that the engines are teardrop-shaped bodies slung below and in front of the wing. You will notice that most of the nacelle is in front of the leading edge:



You will also notice that the flap track fairings are often a lot bigger than they strictly need to be for mechanical purposes alone:


The engines and flap track fairings double as anti-shock bodies that soften up the sudden changes in cross section that occur at the wing root and trailing edge. While I often read posts extolling the virtues of the "clean wing," it actually turns out that the addition of lumps and bulges can actually improve the aerodynamics dramatically over those of a "clean wing."
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RE: Engines Under Wing Or On Fuselage

Sun Oct 21, 2012 3:44 am

Quoting DocLightning (Reply 13):

You will also notice that the flap track fairings are often a lot bigger than they strictly need to be for mechanical purposes alone:

I'd wondered about that.
 
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RE: Engines Under Wing Or On Fuselage

Sun Oct 21, 2012 6:59 am

Quoting SSTeve (Reply 14):
Quoting DocLightning (Reply 13):

You will also notice that the flap track fairings are often a lot bigger than they strictly need to be for mechanical purposes alone:

I'd wondered about that.

It is probably so that the external (to the wing shape) flap mechanism has become more accepted because some of the drag it creates does get compensated by a slight area rule improvement. In the time before transonic speeds one found more frames which kept also rather elaborate flap mechanism inside the wing, now it is the norm to put them in trailing gondolas. The frame that you gave as an example, the 788, is a bad example however, seldom has the flap gondolas been so slim and short as on the 787:



It is more pronounced on the Airbus DA like the 330 wing you had in your post.

[Edited 2012-10-21 00:01:22]
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RE: Engines Under Wing Or On Fuselage

Sun Oct 21, 2012 4:55 pm

Quoting ferpe (Reply 15):
It is probably so that the external (to the wing shape) flap mechanism has become more accepted because some of the drag it creates does get compensated by a slight area rule improvement. In the time before transonic speeds one found more frames which kept also rather elaborate flap mechanism inside the wing, now it is the norm to put them in trailing gondolas. The frame that you gave as an example, the 788, is a bad example however, seldom has the flap gondolas been so slim and short as on the 787:

They were slimmer and shorter on the MD-11, mere fins, really. MD actually revised them to be bigger and bulkier so as to reduce the fuel burn.

ON the CV 990, antishock bodies were installed to reduce drag, but they did not serve any other purpose.

http://3.bp.blogspot.com/-ddK0AvFH-rc/UARf8_gIHzI/AAAAAAAABFU/ZLG-_C6feJs/s1600/AA+990_aerial.jpg

On modern airliners, the flap track canoes serve that purpose. In addition, on the newest aircraft, like the 787, the supercritical wing is probably shaped in such a way that the antishock bodies do not need to be as large and so the flap track canoes are smaller.
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RE: Engines Under Wing Or On Fuselage

Sun Oct 21, 2012 7:05 pm

Quoting DocLightning (Reply 16):
On modern airliners, the flap track canoes serve that purpose. In addition, on the newest aircraft, like the 787, the supercritical wing is probably shaped in such a way that the antishock bodies do not need to be as large and so the flap track canoes are smaller.

Modern airliners have good enough wing designs (mostly thanks to CFD) that they don't need antishock bodies. The flap fairings are the way they are because you can only close so tightly before you get separation and drag. The 787, in particular, has tiny flap fairings because it doesn't have flap tracks (thank you MD!), just offset hinges, so there's far less mechanics to contain within the fairing.

Tom.
 
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RE: Engines Under Wing Or On Fuselage

Sun Oct 21, 2012 10:20 pm

Quoting tdscanuck (Reply 17):
Modern airliners have good enough wing designs (mostly thanks to CFD) that they don't need antishock bodies.

That's true from about the 777/A330/A340 and onward, AFAIK.

But there is a very good reason why MD enlarged their flap track fairings in the MD-11 and it is precisely because they reduced drag. Otherwise, the 787 could have the same tiny fins that the MD-11 had. In fact, if you look closely, just at the part where the underside of the wing begins to taper back from its maximum thickness, the track fairings start. They are still wider and longer than they need to be for strictly mechanical purposes.

So I submit that the area rule is being taken very seriously here. After all, they wanted this aircraft to be as efficient as possible.
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RE: Engines Under Wing Or On Fuselage

Sun Oct 21, 2012 11:55 pm

Quoting DocLightning (Reply 16):
ON the CV 990, antishock bodies were installed to reduce drag, but they did not serve any other purpose.

They did serve another purpose. They were fuel tanks. See Note 2 at the bottom of this document related to a patent lawsuit in 1970/71 involving the Convair 990 antishock bodies.
http://openjurist.org/443/f2d/630/ge...-dynamics-corporation-v-t-whitcomb
 
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RE: Engines Under Wing Or On Fuselage

Mon Oct 22, 2012 4:25 am

Quoting Viscount724 (Reply 19):
They did serve another purpose. They were fuel tanks.

Were they also installed on the 880?

They are pretty big fuel tanks. I wonder how much the wing needed to be modified for the weight and inertial moment.
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ferpe
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RE: Engines Under Wing Or On Fuselage

Mon Oct 22, 2012 8:10 pm

Quoting DocLightning (Reply 20):

Were they also installed on the 880?

Nope, and Wiki says Convair ran into trouble using them as fuel tanks on the 990 due to vibration, the inner pair ultimately served as fuel dump hoses, the outer only as area rule bodies.
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RE: Engines Under Wing Or On Fuselage

Mon Oct 22, 2012 9:50 pm

Quoting ferpe (Reply 21):
Nope, and Wiki says Convair ran into trouble using them as fuel tanks on the 990 due to vibration, the inner pair ultimately served as fuel dump hoses, the outer only as area rule bodies.

That's what I thought. I thought that there was some investingation into using them as tanks, but they wound up being mostly hollow.
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RE: Engines Under Wing Or On Fuselage

Tue Oct 23, 2012 6:48 pm

The other tradeoff is that to install engines next to the fuselage, you need to significantly beef up the fuselage to withstand sonic fatigue forces on the aluminum skin.
 
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RE: Engines Under Wing Or On Fuselage

Tue Oct 23, 2012 6:56 pm

Quoting pygmalion (Reply 23):
The other tradeoff is that to install engines next to the fuselage, you need to significantly beef up the fuselage to withstand sonic fatigue forces on the aluminum skin.

But that's true whether they are wing-mounted or rear-mounted. Wing-mounted engines are usually mounted pretty far inboard.
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RE: Engines Under Wing Or On Fuselage

Tue Oct 23, 2012 10:57 pm

Quoting DocLightning (Reply 24):
But that's true whether they are wing-mounted or rear-mounted. Wing-mounted engines are usually mounted pretty far inboard.

Not in comparison:


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Sure, sonic effects need to be dealt with in both cases, but they're substantially more severe in the fuselage mounted case.
 
ferpe
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RE: Engines Under Wing Or On Fuselage

Wed Oct 24, 2012 7:06 pm

Quoting pygmalion (Reply 23):
The other tradeoff is that to install engines next to the fuselage, you need to significantly beef up the fuselage to withstand sonic fatigue forces on the aluminum skin.

I would venture the sonic fatigue comes from the exhaust stream and not from the intake? In such case the underside of the wing is as close as the aft body, guess the underside of the wing has thicker skins for other reasons however .
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RE: Engines Under Wing Or On Fuselage

Thu Oct 25, 2012 7:41 am

There is an aircraft with wings mounted on top of the wings. I'm sure it was Russian built But don't know the make.
I wonder what advantage was thought of when doing that.
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RE: Engines Under Wing Or On Fuselage

Thu Oct 25, 2012 8:04 am

Quoting jumbojim747 (Reply 27):
I'm sure it was Russian built But don't know the make. I wonder what advantage was thought of when doing that.

Antonov 72 and 74, designed for rough terrain operation. You want to have the engines AWAY from all the dirt and stuff flying around.

Beriev Be-200, of course, is a seaplane. Again, you want the engines away from the water.

And then there is the German airliner.. low-wing, monoplane, with engines mounted above the wing. Huh.
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RE: Engines Under Wing Or On Fuselage

Thu Oct 25, 2012 10:07 am

Thanks fabo.
I wonder how much stronger the wings would need to be to carry the load above and not under.
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RE: Engines Under Wing Or On Fuselage

Thu Oct 25, 2012 11:02 am

Quoting jumbojim747 (Reply 27):
aircraft with wings mounted on top of the wings.

You mean a like a biplane? :P

Quoting jumbojim747 (Reply 29):
I wonder how much stronger the wings would need to be to carry the load above and not under.

Ask the Hondajet chaps!
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RE: Engines Under Wing Or On Fuselage

Thu Oct 25, 2012 8:53 pm

Quoting jumbojim747 (Reply 29):
I wonder how much stronger the wings would need to be to carry the load above and not under.

The weight of the engines is not the issue in itself. That is, the engines are pretty light compared to the fuselage, fuel and payload. Forward slung engines under the wing provide a lot of twisting relief, meaning the wing can be made lighter. If you mount the engines on top, you miss out on this.
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rwessel
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RE: Engines Under Wing Or On Fuselage

Fri Oct 26, 2012 12:14 am

Quoting Starlionblue (Reply 31):
The weight of the engines is not the issue in itself. That is, the engines are pretty light compared to the fuselage, fuel and payload. Forward slung engines under the wing provide a lot of twisting relief, meaning the wing can be made lighter. If you mount the engines on top, you miss out on this.

Could you elaborate on this? I'm having trouble seeing which (potential) torques on the wing structure couldn't equally be balanced by an engine on top of the wing, either forwards or aft, as appropriate. Or is a combination of more than one?
 
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RE: Engines Under Wing Or On Fuselage

Fri Oct 26, 2012 12:40 am

Quoting rwessel (Reply 32):
Quoting Starlionblue (Reply 31):
The weight of the engines is not the issue in itself. That is, the engines are pretty light compared to the fuselage, fuel and payload. Forward slung engines under the wing provide a lot of twisting relief, meaning the wing can be made lighter. If you mount the engines on top, you miss out on this.

Could you elaborate on this? I'm having trouble seeing which (potential) torques on the wing structure couldn't equally be balanced by an engine on top of the wing, either forwards or aft, as appropriate. Or is a combination of more than one?

The wing wants to twist, with the leading edge wanting to go up and over the top of the wing. So if you sling the engines out in front like on your typical 767 or whatever, you essentially throw out this big weight that counteracts the twisting motion. If you put the engine on top and forward, of course you would sorta get the same effect, but both the dreaded VFW-614 and the HondaJet have the engine further back.

Or I could be wrong. Big grin

[Edited 2012-10-25 17:42:27]
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DocLightning
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RE: Engines Under Wing Or On Fuselage

Fri Oct 26, 2012 8:14 pm

Quoting jumbojim747 (Reply 29):

Thanks fabo.
I wonder how much stronger the wings would need to be to carry the load above and not under.

Wouldn't make much difference. If you draw it as a free-body diagram, what matters is the downward force, not where the force comes from. You could even mount forward-slung engines above the wing. It just makes it into a maintenance access nightmare and blocks the view for passengers (the latter is less important, of course). It also makes engine replacement that much more complicated.
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RE: Engines Under Wing Or On Fuselage

Sat Oct 27, 2012 1:04 am

Quoting jumbojim747 (Reply 27):

A Google search reveals that these top mounted engines utilize the so-called Coandă effect that generates high lift and can be thus used for STOL.

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