Bill Tindall
BTW thanks for the accurate formula. The lower effective angle would increase the levering force on the fibers leading to more tear-out, if that was all that was going on. Hence, the favorable action of skewing the blade must have more to do with how force is distributed to the bundle of wood fibers ahead of the blade tip.

There are levering forces, as Kato and Kawai call them, that split the fibers apart ahead of the blade tip. Wood is not isotropic, which is to say that its properties to resist splitting vary by the direction the force is applied. It is desirable to minimize these forces overall, and/or direct them in a direction where the wood is stronger, which is across the grain.

There are shearing forces that sever the wood fibers and again the resistance of the wood structure to shear(slice) is direction dependent. For ease of pushing the plane these forces should be minimized.

It must turn out that the increase in splitting resistance as the blade's force is apportioned between the favorable and unfavorable grain orientations gets better as the blade is skewed even though the overall levering force becomes unfavorable. Once again practice likely preceded explanation by centuries.

I am imagining that some mechanical engineer has proposed some equation relating all these forces and solved it to determine the minimum levering force in the unfavorable with-grain orientation.