I have a large cargo aircraft concept and I am debating between a high or low wing. It seems like both are viable, but intuitively it seems structurally harder to support a high wing. Alternatively though I like the idea of a high wing to raise the engines and provide better field landing abilities. Can anyone comment on the weight penalties associated with one concept or the other? Is there a structural advantage to one configuration?
@Ron Beyer has pointed you in the right direction. Study existing designs going all the way back to the Gigant.
The wing is by far the strongest part of the aircraft as it must carry leveraged aerodynamic loads and the weight of the aircraft while flying. A low wing may be slightly stronger to positive G loads as the fuselage forms an arch over the middle of the wing. However, for cargo transport the high wing offers the advantage of a lower set, roll through cargo bay and more stability in flight.
As far as landing gear, the low wing offers placement of gear on wings for wider track. High wings mount theirs in the fuselage, but the high wing offers the opportunity for greater ground clearance of obstacles on "unfinished" landing strips.
As you can see both are made and both are in service worldwide. Commercial freight would favor the 747 type, military the C-17. And let's not forget turboprops for fuel efficiency!
Both high and low wings will fulfill the requirements for cargo hauling, what about both on one plane to get wingspan back under 150 feet!.
They usually have high wings for practical reasons, it's a lot easier to load the cargo, than to a a low winged aircraft where the fuselage is higher to give enough clearance.
In terms of structural weight, there is probably little difference. I checked Howe's conceptual design book and high or low wing doesn't seem to affect the structural mass.
Packaging the landing gear and its difference in length can change the mass, also if any aerodynamic fairings are required it can increase drag and result in fuel penalty.
As already pointed out, the best thing you can do at this stage is to study existing designs. There are plenty of them.
You will notice that nearly all aircraft that have a ramp and are designed to carry self-loading cargo (like vehicles) are high-wing design. (Most of them are military). So, there must be a good reason for it, even if it has some penalties. And the reason is simple: the ramp slope can't be too steep, and you don't want the ramp to be too long; thus you want the cargo deck to be as low as possible. This is only achievable with a high-wing design, with its low ground clearance.
By contrast, most commercial cargo aircraft are loaded at well-equipped airports, mostly with containers, and don't need a ramp. They typically are converted passenger aircraft, which are predominantly low-wing.
There are 'objective' advantages and disadvantages to each design. Briefly, the high-wing design is better aerodynamically (has less drag, other things being equal), but presents more structural challenges. Smaller aircraft, e.g. Dash-8, can afford to run the main landing gear up to the wing, where you really want to mount them. But otherwise you need serious strengthening of the fuselage between the wing and the landing gear mount. Presence of a ramp, and thus a big cut-out in the fuselage, is another headache.
Nevertheless, the respective advantages and problems are not fundamental, and practical considerations outweigh them, as you can see from the study of existing examples.