When it comes to military transport planes, I’ve noticed that, apart from the ones that are obviously commercial plane models that have been converted for miltary use, military transports usually seem to have top-mounted wings, while most civil transport planes have bottom-mounted wings.
Actually you could also include large bomber aircraft such as the B-52, as well as troop and freight transporters in the high-wing group. But why the trend toward high mounted wings in the military? I can see that there would be some practical advantage, in that with high mounted wings you can have the fusilage very low to the ground, which makes loading and unloading easier. You obviously can’t do that with low-mounted wings, because if the engines are on the wings, that means the “rolling clearance” of the plane has to be high enough to accommodate jet engines that hang down, or propellers.
So why the difference between military and commercial? And what are the structural considerations surrounding the placement of the wings?
Very few civilian aircraft transport trucks or tanks. The military on the other hand transports a lot of both, plus Humvees, and trailers. They will drive a fully loaded truck onto the transport, and then off at the other end.
Come to think of it, I drive under the runways at LAX almost every day to and from work, and lately I’ve been seeing a new kind airliner that does have top-mounted wings, and is propeller or turboprop driven. It looks quite large for a propjet, perhaps almost as large as a Boeing 737. I couldn’t catch the name of the airline though, or I"d be able to go on their site and find out the type of aircraft.
There’s another factor in favor of high-wing military transports, and that’s ground obstacle clearance.
Civil transports can count on landing on nice runways with well-groomed spaces on either side, usually short-trimmed grass but basically very, very flat. Military planes, however, may be called upon to fly into relatively rough fields that may have weeds, shrubbery, and other potential obstacles to the sides. A higher wing gives better clearance over such things. For the same reason, bush planes transporting people and goods into and out of remote areas also tend towards high wings. It can be the difference between mowing/clearing a strip 50 feet wide vs. 500 feet wide - the latter being more effort to create and maintain.
i’d also imagine a high mounted wing assembly allows the aircraft to store more cargo (especially when dealing with the transport aircraft like the C-5/C-130/etc…)
Don’t forget that on mid-wing planes, the wings aren’t just bolted onto the outside of the fuselage. They usually pass through the plane and connect in the middle. On airliners this is between the passenger compartment and the cargo bay.
But passing the wing support structure through the middle of a cargo plane’s fuselage would be like supporting the roof of a hangar with columns: somewhat counterproductive. Since you want the maximum clear span volume in your cargo bay, you hang the fuselage under the wing instead.
(I think this may be what MacTech was suggesting.)
How about if we reverse the question, is there any good reason that the overwhelming majority of jet airliners have low mounted wings? Are there any drawbacks to a shoulder wing design?
According to a USAF maintenance officer I met, the top-wing design is a pain in the ass to maintain and repair. He had worked on C-5s as well as KC-135s, and said it was a world of difference. The high-wing design requires depot tear-downs more often, too – so why do it this way at all? Because the military gets their maintenance personnel “for free” - the operational unit is allocated personnel, and no matter how many hours they work, they get paid the same, and the planes are expected to fly. Commercial airlines use union labor, at hourly rates, and are responsible to their shareholders for making a profit.
Another reason to use the high-wing design is that you can fit much larger engines on the wing, which (generally) boosts your ability to carry more mass.
Actually, this is the real question and it is more an example of unintentional evolution than anything else.
Original prop planes tended to be low wing for the reasons cited above regarding maintenance. Since the earliest planes were tail-draggers, anyway, their cargo doors were not that high off the ground and there was no serious problem.
When the first generation of tricycle gear planes came out, they were still using low wings from “habit” as much as anything else. However, when airports began being built to especially accommodate aircraft with telescoping, wheeled, covered walkways to the planes (rather than simply rolling stairways up to the exits), the planes tended to be higher, so the airport walkways were built on the higher level. Of course, that meant that the next generation of commercial aircraft needed to be built higher off the ground so that the ramps would not have to be dropped. Thus a cycle was born.
Meanwhile, the military decided that rolling trucks up a 45° incline to get them into a Globemaster was a pain in the ass and noticed that they could add a lot more heavy-duty landing gear to support the plane if they did not have to stand them up on stilts, so they began ordering planes with high wings and fuselages that squatted on the ground. Since the military aircraft tend to not load up at commercial airports, there is no conflict with their designs in that regard.
IIRC, the lower the wing mount, the more maneuverable the plane is, and the higher the wing mount, the more stable. If you look at a lot of WWII fighter planes, you’ll see a lot of low-wing mounts, and in a few cases (such as the F4U Corsair), wings that mount low with an inverse-gull shape, putting the wings themselves even farther below the fuselage than the wing mount.
Yup. Airplanes rotate in all three axes about the center of gravity. An airplane with high mounted wings experiences what is known as pendulum effect. Much like a pendulum hanging below a pivot, the center of gravity of the airplane hangs below the center of lift of the wings. This increases stability in both roll and, to a far lesser extent, pitch.
As you correctly stated, stability is a tradeoff for maneuverability. You’ll notice that high-winged airplanes have the wings canted downward (the tip is closer to the ground than the root). This is called anhedral, and it lowers the center of lift, reducing roll stability and increasing maneuverability. Conversely, you’ll notice that most low wing aircraft have the tips of their wings higher than the root. This dihedral raises the center of lift closer to the height of the center of gravity, increasing stability.