Why do big commercial airplanes have wings on the bottom and big (US) military airplanes have their wings on top?
In: Engineering
Low wings leave the engines and fuel tanks closer to the ground which aids in maintenance, and keeps them away from the passenger area which marginally increases safety.
High wings keeps the engines and fuel tanks away from the ground which can keep them safer from debris, which is important for a military aircraft that might be operating in areas with poor infrastructure. It also allows the fuselage to be closer to the ground which can make loading cargo easier when there isn’t the kind of aid that a commercial airport can offer.
Higher wings lift better.
For short, or relatively short, runways this is a huge boon.
Military aircraft are all about weight capacity, not fuel efficiency/cost reduction like a civilian aircraft.
Where do you want the wing spar to cut across the fuselage? For passenger aircraft, preferable to have passengers in top half of fuselage (wider base for seats than for headroom), so wing cross fuselage at the bottom.
For military transports, easier to have wing cross at top of fuselage because wing spar less obtrusive for load and able to have overall plane with lower clearance to ground.
For military bombers, wing crossing top of fuselage means more space for bomb bays.
A high wing design allows for an aircraft to operate from airfields with minimal ground facilities as the fuselage can have low ground clearance so things like internal steps and loading ramps can be most easily used. A similar low ground clearance is possible with a rear mounted engine configuration. Regional jets and turboprops for civil uses at airports with minimal facilities often have either a high wing or rear mounted engines for this reason. These types of aircraft typically need a T tail configuration which is structurally inefficient needing more weight.
High wings allow the fuselage to sit closer to the ground, which aids in ease of loading. You can drive a tank into a C-17, but it would be hard to do it in a Dreamlifter.
The secondary benefit is the engines sit higher, lowering the risk of debris ingestion during take-off. There is a danger during landing, but if the engine fails after you hit the ground it’s not as dangerous.
For low wing aircraft, placing the engines close to the ground for ease of maintenance is important. Also, usually placing the wing structure (spar) through the bottom of the fuselage avoids having to strengthen a high wing design. The landing gear can also be placed on the wings in a low wing design.
There are some commenters who speak to the differences in flight characteristics, but I don’t really think that’s a major consideration when deciding, in these particular classes of aircraft, where to place the wings. There are tradeoffs when it comes to rolling stability and whatnot but I don’t think those are the main design objectives.
Military aircraft are built to handle rough conditions. Most military planes are purpose built for use on dirt and gravel airstrips. Which means engines that are low to the ground would suck up dirt & stones and implode on take off. Which is a bad thing.
Comercial airplanes are built for use only on well maintained runways. In fact, a comercial airplane can only land at airports that are certified for that aircraft’s type.
Military planes don’t have the luxury of waiting for the transportation regulators to say it safe to fly in or out of an airport. In combat conditions, a perfect airstrip is a luxury. Military aircraft don’t have time to wait for a smooth surface when they are being shot at.
Note the difference, airstrips vs runways, the choice of words was intentional.
Search Anhedral (wing tips lower than wing to body position) and Dihedral (wing tips higher than wing to body position) .. Anhedral is used as a principle for Military aircraft for increased manoeuvrability due to reduced stability(specifically ‘roll stability ). (Very simple description. When you read further into it you will come across terms like Dihedral effect,spiral mode,etc.)
Military cargo aircraft need ramps and clam shell doors close to the ground, a long wide fuselage with no obstructions and crucially for the military, to operate independently of ground handling equipment. The wings therefore have to go on top to have room for the engines. This layout has been used since immediate post WW2 on relatively small transports such as the Bristol Freighter(1946) right up to the massive Lockheed C5 Galaxy (1968) and Antonov 225 (1980s).
It’s not so much a civilian/military thing as a passenger/cargo thing.
Large military planes not used for cargo such as the KC135 refuelling plane or the E3 radar plane have their wings on the bottom. Similarly civilian cargo planes will have their wings on the top.
I have skimmed, but hadn’t seen anyone touch specifically on clearance from ground vehicles. Military bases tend to make heavy use of ground transport for loading/unloading people and equipment (contrast to a commercial terminal with jetways and a structured cargo loading network), so extra clearance under the engines and especially the wingtips (which are also well above the level of the driver in most vehicles) should help to prevent collisions.
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