Origin of the MP/RPM square rule
Is there a reasonable rationale behind the 'square rule', that the correct combination of manifold pressure and rpm for variable-speed props is that their ratio, in (inches-of-mercury x minutes per rev) is ~1/100 ? Let me pre-empt helpful comments that it ain't necessarily exactly 1/100 - it's bound to be aircraft specific and a happy accident of how the units are defined. But there seems to be much less variation than there usually is in rules of thumb, which often suggests a simple truth behind it all.
I'm also curious because, for the geared-down Rotax 912 at least, it seems to be prop rpm that fit the rule, rather than engine rpm.
Apologies for raising a rather esoteric question on a normally very down-to-earth forum, and for being an ignorant newcomer to variable-speed props.
Windrusher