Originally Posted by
The Sultan
It is my understanding that the “vertical hop” with Van Horn blades is asynchronous with main rotor 1/rev which makes it completely different from the “hop” with Bell blades. With Bell blades the “hop” at certain torque settings (ground and flight) is at 1/rev and is aggravated by a swashplate with freeplay when the sum of the forces on the swashplate oscillates through zero. With Bell blades this “feature” is never at a level that would be considered violent. The VH “hop” sounds more like it is sourced from a rotor mode coincident with an airframe/pylon mode.
Bell calls it collective bounce which I believe is the same term VH uses in its bulletin on the matter which goes into more detail. I just used the same terminology as the previous poster. I've always been instructed its main cause was pilot vs collective interaction that was magnified due to improper collective system friction which was addressed in the Bell OSN. There are also a number of other docs on the issue and its my understanding any 2-blade teetering system is susceptible to it regardless of blade type. As I noted, there was one L-model accident that was attributed to collective bounce back in the late 2000s with Bell blades.