Extra300 From Sweden, joined Sep 2011, 66 posts, RR: 0 Posted (7 months 3 days 9 hours ago) and read 4302 times:
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 From Switzerland, joined Oct 2011, 1268 posts, RR: 2 Reply 1, posted (7 months 3 days 9 hours ago) and read 4269 times:
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
Even a letdown, if it is thoroughly and final, is a step forward.
xlc From United States of America, joined Apr 2012, 22 posts, RR: 0 Reply 2, posted (7 months 3 days 9 hours ago) and read 4258 times:
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.
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.
Extra300 From Sweden, joined Sep 2011, 66 posts, RR: 0 Reply 4, posted (7 months 3 days 8 hours ago) and read 4216 times:
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).
Starlionblue From Hong Kong, joined Feb 2004, 15869 posts, RR: 66 Reply 5, posted (7 months 3 days 4 hours ago) and read 4102 times:
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.
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
bond007 From United States of America, joined Mar 2005, 5098 posts, RR: 9 Reply 6, posted (7 months 3 days 3 hours ago) and read 4082 times:
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.
Jimbo
I'd rather be on the ground wishing I was in the air, than in the air wishing I was on the ground!
Viscount724 From Switzerland, joined Oct 2006, 21464 posts, RR: 24 Reply 7, posted (7 months 3 days 1 hour ago) and read 4039 times:
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.
flyingturtle From Switzerland, joined Oct 2011, 1268 posts, RR: 2 Reply 10, posted (7 months 2 days 16 hours ago) and read 3908 times:
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]
Even a letdown, if it is thoroughly and final, is a step forward.
flipdewaf From United Kingdom, joined Jul 2006, 1522 posts, RR: 1 Reply 11, posted (7 months 2 days 12 hours ago) and read 3844 times:
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.
flyingturtle From Switzerland, joined Oct 2011, 1268 posts, RR: 2 Reply 12, posted (7 months 2 days 12 hours ago) and read 3823 times:
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.
David
Even a letdown, if it is thoroughly and final, is a step forward.
DocLightning From United States of America, joined Nov 2005, 16811 posts, RR: 57 Reply 13, posted (7 months 1 day 3 hours ago) and read 3489 times:
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."
SSTeve From United States of America, joined Dec 2011, 432 posts, RR: 0 Reply 14, posted (7 months 20 hours ago) and read 3380 times:
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:
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.
DocLightning From United States of America, joined Nov 2005, 16811 posts, RR: 57 Reply 16, posted (7 months 7 hours ago) and read 3251 times:
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.
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.
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 17, posted (7 months 5 hours ago) and read 3206 times:
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.
DocLightning From United States of America, joined Nov 2005, 16811 posts, RR: 57 Reply 18, posted (7 months 2 hours ago) and read 3159 times:
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.
DocLightning From United States of America, joined Nov 2005, 16811 posts, RR: 57 Reply 20, posted (6 months 4 weeks 1 day 20 hours ago) and read 3093 times:
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.
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.
DocLightning From United States of America, joined Nov 2005, 16811 posts, RR: 57 Reply 22, posted (6 months 4 weeks 1 day 2 hours ago) and read 2900 times:
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.
pygmalion From United States of America, joined Jun 2006, 931 posts, RR: 38 Reply 23, posted (6 months 4 weeks 5 hours ago) and read 2736 times:
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.
DocLightning From United States of America, joined Nov 2005, 16811 posts, RR: 57 Reply 24, posted (6 months 4 weeks 5 hours ago) and read 2734 times:
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.
rwessel From United States of America, joined Jan 2007, 1989 posts, RR: 2 Reply 25, posted (6 months 4 weeks 1 hour ago) and read 2780 times:
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.
ferpe From France, joined Nov 2010, 1771 posts, RR: 57 Reply 26, posted (6 months 3 weeks 6 days 5 hours ago) and read 2640 times:
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 .
jumbojim747 From Australia, joined Oct 2004, 2462 posts, RR: 50 Reply 27, posted (6 months 3 weeks 5 days 17 hours ago) and read 2594 times:
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.
Starlionblue From Hong Kong, joined Feb 2004, 15869 posts, RR: 66 Reply 31, posted (6 months 3 weeks 5 days 3 hours ago) and read 2457 times:
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.
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
rwessel From United States of America, joined Jan 2007, 1989 posts, RR: 2 Reply 32, posted (6 months 3 weeks 5 days ago) and read 2419 times:
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?
Starlionblue From Hong Kong, joined Feb 2004, 15869 posts, RR: 66 Reply 33, posted (6 months 3 weeks 5 days ago) and read 2420 times:
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.
[Edited 2012-10-25 17:42:27]
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
DocLightning From United States of America, joined Nov 2005, 16811 posts, RR: 57 Reply 34, posted (6 months 3 weeks 4 days 4 hours ago) and read 2310 times:
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.
voiceofgoa From United States of America, joined Feb 2011, 16 posts, RR: 0 Reply 35, posted (6 months 3 weeks 3 days 23 hours ago) and read 2259 times:
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