Why do big commercial airplanes have wings on the bottom and big (US) military airplanes have their wings on top?
In: Engineering
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They aren’t always split that way – while you often have custom/unique designs for military aircraft (bombers, fighters, large transport aircraft), you have just as many which use civilian/commerical designs. Tankers are a good example, with the KC-46 and KC-135 refueling aircraft built using the Boeing 767 and 707, respectively (and both having low/middle mounted wings) but there are many other examples. And on the civilian side the popular Cessna 172 has a high mounted wing (or the T-41, which is the military version of the 172)
Wing position is a design choice to help with aerodynamic stability. It is just one of many factors that contribute to stability and it relates to the overall ease of flying and maneuvering the plane. It’s not as cut and dry as commercial is below, military is above since there are plenty of other factors that contribute to stability.
Other factors are the center of gravity, sweep angle (perpendicular to the fuselage or angled backwards typically), engine placement and thrust, tail design, fuselage design, whether you’ll have external stores (military only), and a few others.
So as you can see it’s not quite as simple as just the wing location – what’s most important is the overall drag, lift, and efficiency required for the mission the plane is performing. Commercial aircraft have a fairly predetermined weight and typically try to maximize fuel efficiency and comfort. The military world of planes is all mission dependant, but the plane still needs to be relatively stable too.
I’m an engineer that works with aircraft so if you have any questions feel free to ask. It’s definitely a complicated topic.
It’s multi-reason, and there’s a really thorough explanation at https://www.quora.com/What-are-the-pros-and-cons-of-low-wing-aircraft-as-opposed-to-high-wing-design
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