Dave Jackson asked (Nick added paragraph letter headings):
A) Could this vibration (vibration during translation) be (partially) due to the horizontal stabilizer and its change of location in respect to the rotor's downwash?
B) Could it also be that the forward tilt of the rotor, has resulted in the axis of the tip path plane to not be concentric with the axis of the mast and this results in a oscillating dynamic load, since the hub acts as a knuckle joint, not a constant-velocity joint?
. .Nick sez:. .A) I believe that translational vibration is mostly due to aerodynamic duisturbence of the blade by the previous blade wake, and is especially a problem on some helos during approach when the descent allows more BVI (blade vortex interference). Ever fly a BO-105? The panel is shock mounted to prevent atomic dissassembly of the gages, I think.
OTOH, I do believe that stabilizer vibration can affect n/rev anywhere in the envelope, as at least one Sikorsky model has n/rev that is strongly influenced by main wake on the tail at mid cruise speed.
B) I don't believe this has much affect, Dave. I have flown many machines for vibration tuning where we adjust the CG (and therefore the steady flapping angle) with no appreciable change in the n/rev. In fact, flapping is highest at forward CG, and for many Sikorsky models, forward CG is the smoothest.