lotsofactors

> There are lot of bombers with high wing (B52, almost anything several engines from WWII).

Most bombers dispense ordnance from their undersides, ideally near the center of gravity; limiting the structural members needed in those parts simplifies things.

> About half of all propeller driven passenger planes have high wing.

These are designed not to require ground vehicles for loading and unloading. Propellor ground clearance is also an issue for the larger types.

> Unlike the An-124, the An-225 was not intended for tactical airlifting and is not designed for short-field operation > It still has high wing.

The An-225 carries cargoes of unusual bulk to unusual places. Being able to get these cargoes on and off the aircraft without some sort of lift has advantages.

Low-wing freighters, which are all variants of passenger types these days, almost always carry palletized cargo of standard dimensional units (See https://en.wikipedia.org/wiki/Unit_load_device) - this allows for standardized ground handling equipment for most loads.

> And U2 has high wing. It's probably the plane with most carefully calculated weight ever. > If it was inherently heavier, this would make no sense.

The U2 is not a high-wing aircraft, it is a mid-wing aircraft with its spar roughly running through its center of gravity. This is actually the most efficient and aerodynamically cleanest configuration, and many high-performance aircraft use this arrangement. The main problem with it for passenger or cargo use is that the wing structure ends up interfering with the middle of the fuselage volume near the center of gravity, which tends to be valuable space in a transport aircraft.

Mostly, the high-wing and landing gear configuration on many cargo aircraft designs is intended to facilitate a low loading floor and the ability to load and unload the aircraft on rough fields without special ground equipment (e.g. by means of a ramp integrated with the the rear doors of the aircraft like a C-130 or with a deployable forward ramp and kneeling front gear like the An-124.) Another benefit is providing extra ground clearance for the engines to reduce foreign object ingestion.

Everything else flows from this. The wing spar and structural supports needed in a cantilever monoplane (i.e. not a biplane with a truss structure) take up a fair amount of room. By moving these structural elements to the top of the aircraft, it's possible to get a low floor and still hang engines from pylons beneath the wings. The wings themselves will be angled downward from root to tip (known as anhedral) to partly counteract the pendulum stability caused by having a center of gravity so far below the center of pressure.

Passenger aircraft do not usually use this configuration because it is heavier - the sides of the fuselage must be built stronger to support the load, compared to a low-wing configuration. Weight costs performance costs fuel costs money, so you don't build a large aircraft like this unless it has to operate at fields without ground equipment.