It isn't an airframe issue, it is an engine-prop combination issue. It
is a high frequency (about 220 Hz or a little below middle C on a
piano) torsional resonance involving the crankshaft and the propellor.


The mode of damaging vibration is such that a rotating observer sitting
on the also-rotating spinner would see the back of the crankshaft
rotationally deflecting to-and-fro while also seeing the propeller tips
going fro-and-two. The spinning observer gets a smooth ride.

Props are highl;y stressed. A spinning propeller (wood or metal) is
almost 1/8 of an inch larger in diameter than when standing still. The
prop blade leading and trailing edges accumulate the most fatigue
damage from this 220 Hz bending mode.

OK - the mode shape definition is a little honky but that is the best
way I know to describe it.