Originally Posted by
albatross
Sorry Clutter. What is a stress riser? Are you referring to bending forces being applied to the mast by the rotor head and blades.
Isn’t it the blades and head flapping excessively, especially in a low or -G situation causing. the bump stops on the head to impact the mast causing it to deform, lose strength and break.
Hence occurrences of mast bumping in nap or the earth flying when the aircraft comes over a ridge line and the pilot abruptly moves the cyclic forward and perhaps even decreases collective pitch to follow the terrain. Low or even negative G, low power, rotor system flapping…the holes in the swiss cheese line up quickly.
I vaguely remember being told the G limits were -0.5 G to +2G and to especially avoid low or -G to avoid bumping.
An amazing amount of non-helicopter folks think the mast is a solid steel rod not a cylinder.
When bumping occurs, the main rotor shaft will be scratched or gouged. This is the weak point where the failure actually starts. Think about the tear-off receipts on your bills. The blades, head and rotors do not initially separate cleanly, they tilt over. This is when the rotors contact either the cabin or the tail. (The main rotor shaft has an impressive amount of twist when under load. This is why a small amount of damage can initiate a failure.)