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.
Sultan: when you say "asynchronous with main rotor 1/rev" are you saying the "hop" is at 0.5/rev or perhaps instead 2/rev? I am no aviation engineer, but I would associate a 1/rev with a mass imbalance in main rotor, or inconsistent incidence settings on (both) blades. The 2/rev corresponds to blade passing frequency of a 2-blade main rotor and can easily be explained by unsteady aerodynamics of the rotor in forward flight, even with perfectly balanced and set-up rotor. The 2-rev is what the Nodamatic suspension on the Long Ranger (and a few other Bell types) is intended to smooth out. A 0.5/rev might be possible too. The equivalent on ships operating in a seaway is called "parametric rolling" (sometimes also called asynchronous rolling) as opposed to "synchronous rolling" and sometimes leads to loss of containers when very large but relatively slow roll motions build up on containerships.
I still have an uneasy feeling that blades with different mass and stiffness characteristics than OEM Bell blades might be adversely interacting with the Nodamatic system in a way that hasn't been fully understood by VH, Bell or regulators? Furthermore, I feel it might be possible any "hop" may not be doing much good to the securing arrangement for the inertia masses in the blade tips. So the need for the tap testing (and VH has a more recent reminder of the importance of this on their website) and the incidence of "hopping" could be linked. Speculation?... sure, I accept it is. The sooner NTSB has findings on New York accident and TSB has findings on latest Long Ranger loss, the better.