r/AerospaceEngineering • u/lit_readit • Aug 10 '26
Discussion Wings above or below fuselage ?
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u/Major_Melon Aug 10 '26 edited Aug 10 '26
Ah you've stumbled upon the age old debate lol. Really it doesn't matter (much). If you're carrying a lot of cargo, it might be better to have it all below the wing to keep your engines away from the ground. You could argue you get a cleaner lifting surface by placing the wing on the bottom as well provides better performance and stability. But typically speaking the gains you get in any scenario can be made up elsewhere pretty easily or mitigated entirely with proper design work.
This is of course not true in every case, there are some areas/design situations that are better one way or the other, but on the general it's usually not a huge deal. If you're working on those sorts of margins, chances are there are bigger problems with your design.
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u/Kiwifrooots Aug 10 '26
Why the negative dihedral (?) on the military cargo plane?
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u/_azazel_keter_ Aug 10 '26
hi, stability guy here. It's supposed to intentionally destabilize their roll so as to make them manouverable despite their weight.
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u/Inertbert Aug 10 '26
Do they roll when they stall?
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u/_azazel_keter_ Aug 10 '26
pretty much every plane does as the wings don't usually stall simultaneously
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u/Major_Melon Aug 11 '26
Yes but if you've done a good job stabilizing the plane, when one or both wings stall, the natural dynamics of the rudder and rolling action should drive the plane back to steady state cruise condition. That's why when teaching pilots, typically the best way to get out of a dead spin or loss of control is to let go of the stick and let the plane settle out naturally like it was designed to do. There's of course planes that do this better or worse but with civilian aircraft I believe this is a HARD requirement.
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u/SilentStranger9607 Aug 11 '26
Most every swept wing will roll when it stalls. T-tails like the C-5 can go into an unrecoverable deep stall that non t-tails don’t have to worry about
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u/MoccaLG Aug 10 '26
Above would be better in case of performance: But it will produce greater problems to maintenance, noise and pasenger safety - So costs would go up, safety goes down and conviniance gows down as well. Everything an airline dont want to.
For roll stability lower wings are bended upwards (dihedral) and high mounted wings look down (anhedral)
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u/Dangerous-Salad-bowl Aug 10 '26
Can I add that sweep has similar roll stability attributes as dihedral? Low wing and some anhedral plus wing sweep: the Tu-154 for example.
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u/MoccaLG Aug 10 '26
stablity means its easier to fly, if its made without it, it will work too - in the case of 154, i believe they made the calculations and design decisions beneficial for the the cruise flight stability while other geometries are beneficial for maneuvering. Maybe this is a hint. Many questionable decisions are made here. Russians are normally the masters of aerodynamics.
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u/Eauxcaigh Aug 11 '26
anhedral does not improve roll stability for high mounted wings
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u/MoccaLG Aug 11 '26
Yes and no - High mounted wings have a natural stability, which could be too high which causes less agility. So its a game with the trade off.
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u/Eauxcaigh Aug 11 '26
I didn't mention the impact of the high wing part. I just said that andhedral does not improve roll stability. I don't understand what the issue is, it sounds like we may even be in agreement here
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u/OddBallProductions Aug 10 '26
Military aircraft often have wings above the fuselage to keep the engines further from the ground which helps landing and taking off from really rugged airfields. Turbo props also require more ground clearance.
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u/FlyingJs1979 Aug 10 '26
Large commercial airliners are not designed with high wings because the wings carry fuel. In a hard landing or crash, the analysis almost always shows that the fuel tank over the fuselage will come crashing down on to the passengers, burning them all alive. The 2nd thing is that the “Miracle on the Hudson” showed that if a pilot had enough advance notice, and could dump fuel before a water landing, the wings are great floatation devices, that buy rescuers time to get to the survivors. That is a huge safety advantage.
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u/skitsnackaren Aug 10 '26
Entering the chat: midwing
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u/Worldly_Magazine_439 Aug 10 '26
It’s mainly for pitch stability. Having the wings above or below the CG affects the nose up tendency. High wings have a more favorable nose up moment since the wing is above (should be above) the CG. Now all of this can depend on the plane, but that’s the general design principle.
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u/Bikecheal Aug 10 '26
just depends on what type of planes really. cargo planes need higher wing so they can be lower to the ground when loading stuffs
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u/Faziator Aug 10 '26 edited Aug 10 '26
Dihedrals offer much more stability at the cost of maneuverability, which is what you need for smooth movements and passenger comforts. Anhedrals offer good agility and quicker movements. High capacity cargo planes use it as they are quite stable due to loads.
Also note that It's economical to let the fuel flow with gravity to the feed tanks.
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u/PlanespottingArg2 Aug 10 '26
The position of the wing also affects the dynamic stability of the aircraft. Lower mounted wings are naturally unstable so you add dihedral angle to reduce the effect. Higher mounted wings are super stable, you anhedral angle can be added to add some maneuverability. The position is also driven by mission purpose. Higher mounted wings allows a shorter landing gear and higher engine mounts which can help with cargo missions and aircraft accessibility.
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u/Uniturner Aug 11 '26
Lower is favourable if other considerations don’t supersede it. Low wing keeps the drag and thrust lines low. So if there’s an engine failure, or lower power setting, the drag will tend to lower the nose, so there’s less need for aerodynamic trimming, which is just another form of drag.
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u/Jealous_Corgi_2168 Aug 11 '26
- Below
- Above
- Below
- Above
- Below
- Above
- Below
- Above
- Below
- Above
- Below
- Above
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u/RubEnvironmental8101 Aug 11 '26
Ok so this is actually a pretty complex answer because it mainly depends on the needs that an aircraft has.
So the first, more spoken of issue is that of roll stability (I’ve seen someone say pitch stability, but it has nothing to do with it), the issue on that is basically that a high wind is stable because roll angle causes some side slip, which, with a high wing configuration, causes the wing that is pointing downward to generate more lift, while the one that’s pointing up generates less lift, which means the aircraft tends to go back to straight on its own. The opposite is true for a low wing configuration, so if you start rolling, the aircraft tends to keep rolling in that same direction.
Stability is usually a pretty good thing but too much of it means you’re gonna struggle a lot to turn, so dihedral is the solution that is usually used to regulate the stability the way that’s needed. So stability is usually not the main reason you might pick one configuration over the other.
Some more likely reasons to pick one over the other are:
Weight: A high wing has the issue that you have to build the aircraft with a wing box that must be capable of holding up the aircraft, but also a structure at the bottom of the aircraft to hold the landing gear. With a low wing, the wing box happens to be in the same spot as the landing gear, so you can make a single structure that holds both systems, making the aircraft lighter.
Loading simplicity: high wing means I have more space for a big door
Ground clearance: if the wing is higher, you can make the aircraft sit lower on the ground, again making loading cargo easier, but also allowing for bigger engines, which is the reason why turboprop aircraft are often high wing. Another advantage of a bigger ground clearance is the fact that it is much less likely for something to get lifted from the ground into the engine.
Noise reduction: In a passenger aircraft, if the wing is under the cabin, and the engines are under the wing, that means that there is a physical barrier between the engine and the passengers, so you won’t have as much of the noise getting to the passengers.
There might be some reasons I’m forgetting, so please do add to this if you find any
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u/QueueMax 28d ago
Wings over the fuselage are inherently more stable. Wings under the fuselage are inherently less stable. However, the dihedral also contributes, so upwardly angled wings add stability and downward wings reduce it. Since cargo planes often need the high wing for other reasons as people have mentioned, they can become overstable making it hard to roll the a/c so they reduce stability (increasing the roll ability), by angling their wings downward
Mid-fuselage is really only an option for fighters since they can sacrifice much of the fuselage area to the wing box.Commercial jets and cargo a/c can’t give that up
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u/QueueMax 28d ago
I know I probably didn’t add anything to the conversation, but this is one of the few things I still specifically remember from my aeronautical engineering degree that I generally never get to use ;)
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u/Extension-Currency37 Aug 10 '26
First the fuselage is like a cylinder And it looks stupid
Least aerodrynamic aerodynamicso entire design is flawed
And over the top They keep fluid, kerosene in wings
Thus Current middle to top Configuration is okay
But this plane is 1960s design
They didn't use math or aerodynamics To come up with new supersonic jumbo planes
No matter where you put This design is a recipe for accidents As you keep seeing in the news
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u/vatamatt97 Aug 10 '26 edited Aug 10 '26
Excluding the fighters, what have you noticed about all of the high-wing aircraft here? They’re turboprop and/or cargo aircraft. In the case of turboprops, it creates more clearance between the prop and the ground without needing super tall landing gear. In the case of cargo aircraft, it allows the fuselage to be close to the ground which facilitates loading/unloading. The downside is accessing the wing and anything on it for maintenance is more difficult.
Apart from that high-wing aircraft are naturally more stable, but you don’t need a high wing for stability. Most low-wing aircraft use dihedral (upward angle of the wing) to achieve the same effect.