For Caprera.
"The fuselage of the SM.79 was made of a welded tubular steel frame and covered with duralumin in the forward section, duralumin and plywood on the upper fuselage surface, and fabric on all other surfaces"
That means that, if a shot didn't hit the steel frame, it won't cause excessive damage because:
- The tubular steel frame with fabric means that the surface isn't "working" to support the stress, and so, the facbric it's there just to give lift or for aerodinamic reason, not for structural integrity.
That's because the fabric, that resist to elongation, can't resist to compression.
So, a hole in the fabric it's just a loss in lift ( or an increase in drag, if it's on the fuselage ), NOT in structural integrity.
- The tubular steel frame with duraluminium it's more vulnerable, because the duraluminium is a working part of the fuselage ( semi monocoque fuselage, in these case ), but, again, it's the steel frame that support the "real" stress, the duraluminium just help ( even tought these help it's extremely important ).
- Same thing for the duraluminium and plywood, as before, but the plywood it's obviously less resistent to stress.
Simply, if the "cover" of the wing it's made of rigid material, it can withstand a lot of stress, because the spar, the skin and the ribs ( not sure abput these traslation, in tialian it's "centine" ) work togedet to create a closed box.
And a closed box can withstand a lot of stress more than an open box, or weight a lot less with a similar resistence.
"The wings were of all-wood construction, with the trailing edge flaps and leading edge slats (Handley-Page type) to offset its relatively small size. The internal structure was made of three spars, linked with cantilevers and a skin of plywood."
In addition, the wing of SM79 has three main spar, while normally aircraft has only two main spar, or even only one.
Having three main spar means that there isn't only one "wing box", but two, with the leading edge and the trailing edge as separate structure that can withstand torque.
That means that the wing of SM79 was extrmely strong for different reason:
- Because of the three spar arrangement.
- Because of the working skin of plywood ( the less resistent it's the skin, and the less important it is in structural integrity if it's lost).
- Because of the eavy armour carried for the engine and the fuel tanks.
So you have to hit ( and hit hard ) at least two main spar to create some serious damage, and not only the skin ( qute like the ww1 plane ... ).
These kind of structure with three main spar, was used only in italy, and only by the "Marchetti" industries.
Again, i'm sorry for my english. If something it's not clear, feel free to ask for explanation.
