> BugBear calculated the correct ( or original )  radius of a scrub plane
> blade to be 2 1/2 inches.
>
It seems to me that the optimum curve would have an exposed depth equal to
your depth of cut (thickness of shaving), which would depend on the wood
species, grain nastiness, and your strength.  The other dimension would be
the width of the exposed iron (a chord?) which should be a bit less than the
blade width on each side, so that the shaving tapers on the edges to nothing
and you don't get tearout at the edges.

I've found myself wanting a shallower radius than the previous owners of
scrub planes.

Cheers,
Croxton, enjoying the heck out of his new/old 40-1/2.