To some extent, it depends on the choice of engine mounting position. I think a rear engine mount is the heaviest at most Pax aircraft sizes (a la MD-80, not DC-10). This is size dependant - smaller you get, less the problem is.
The fuselage floor must, in some manner, be supported by the wing. In a low & mid wing, the job of the fuselage structure is basically not to collapse under bending & its own weight during manoeuvring and to cope with aero loads & pressure differences.
Structurally (big simplification coming) a low or mid wing airplane is two box sections assembled in a cross, bolted to each other, with a stiffened balloon or tent for the fuselage.
A high wing has the same two primary members, but also needs a support frame at least 5 feet high between them.
As a result, in a high wing, the fuselage bending is exactly the same as any other configuration with the same engine loc'n relative to the wing - so the skin thickness is at least the same as the low. But you also need to thicken the centre section to transmit floor loads up to the wing.
In the event that you fuselage mount the main gear, although you can save undercarriage length - and the associated weight too - you need an additional lump of structure approximately like the wing centre box sitting between the gear, which I think more than cancels it out. Or you need to further thicken the centre section skins.
You only do that if you want to absorb high sink rates (easier with legs that are shorter once loaded) and/or house the tyres in the belly because the engine nacelles are too small or unusable. The tyre size is only a feature of the desired bearing ratio/ground pressure.
I'm afraid I don't have the numbers to hand - I can dig something open source up over the weekend comparing high/low & underwing engine/tail mounted.
Last edited by RugGun; 26th Sep 2008 at 14:21.
Reason: grammar!