The remarks on the damage model are interesting. If you compare the fmd files of the SM81, Br20 and SM79, you'll see that The fuselage and nose of the Br20 are slightly more resistant than those of the SM 81, though the wings of the 81 are slightly more resistant of those of teh Br20. The difference in vulnerability btween these 2 planes are therefore much more related to the location of the damage collision boxes in the fuselage and wings (fuel tanks for instance) and I am far for sure that the Br20 was tougher than the SM81 (a very rough plane devised for operating in bad environment) in front of enemy fire. Remember that the Japanes, who used the plane, complained about its vulnerability and propension to take fire when shoot at (and this from users whose local produced planes had themselves a well deserved reputation of being "flying lighters"). So the debate remains open...
For the SM79 this is a quite different story - the original fmd of the SM79 is quite exceptional, probably the toughest plane in this game according to its fmd files parameters, much over the level of the Br20 and Sm81 parameters (almost 5-8 times over) - and it's right too, the construction design of this plane made it very difficult to shoot down - and when the plane appeared in this game, there were complaints that this aircraft was "almost impossible to shoot down" with small caliber weapons ( although this was conform to historical reality).