Is the engine the same in both aircraft? My club used to have a Rallye
4-cilinder 180 hp with a 4-blade Hoffmann prop. I don't recall having
that fibre sheathing problem, but we did have problems with the
starter motor, and it was said the light weight of the prop was the
culprit, giving less inertial effect in comparison to the original,
heavier two-blade prop, and causing abrupt changes in angular
velocity. Perhaps the same phenomenon is causing excessive stress at
the propeller hub. If your Pawnee has a 6-cilinder engine, it's likely
to have more inertia...