Tapping the prop may have the same effect as railway wheel tappers, might be a way of detecting cracks.
Steam engines and old wind up record players use two bob weights spinning around a shaft that lifts a valve to govern the flywheel speed, in the case of steam engines.
As for the prop, use the analogy of wood screws.
Take off and climb takes a lot of effort, like trying to screw into hard wood, so you would use a fine thread screw, so that the effort required to turn travels a small distance.
Whereas screwing into chipboard takes little effort so the screw has a coarse thread, a bit like coarse prop setting in the cruise.