Yea. The key assumption for the "put the armor where the bullets aren't" is that the initial bullets are spread evenly over the surface area (or, rather, that the bullets are spread evenly over the cross-section, which you then average over the typical orientation of the plane as it takes enemy fire). This turns out to be a good assumption because planes typically take highly dispersed enemy fire compared to the size of the plane.
On the other hand, if the enemy had extremely accurate guns and (say, for visibility reasons) always shot at and hit particular parts of the plane (say, the parts of the wings next to the necessarily shiny propellers) then the naive reasoning would be correct: the areas that were bullet-ridden on returning planes would be the ones that should be armored.
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Yea. The key assumption for the "put the armor where the bullets aren't" is that the initial bullets are spread evenly over the surface area (or, rather, that the bullets are spread evenly over the cross-section, which you then average over the typical orientation of the plane as it takes enemy fire). This turns out to be a good assumption because planes typically take highly dispersed enemy fire compared to the size of the plane.
On the other hand, if the enemy had extremely accurate guns and (say, for visibility reasons) always shot at and hit particular parts of the plane (say, the parts of the wings next to the necessarily shiny propellers) then the naive reasoning would be correct: the areas that were bullet-ridden on returning planes would be the ones that should be armored.