Dave,
The N per rev vibration is a natural effect of the blade's motion around the head, and is intrinsic, as is the 1 per rev resonance of the blade in flapping. The rotor delivers a "root shear" to the head that sums at the head to the n per rev vibration in the static system.
These root shears are increased by stall, compressibility and blade dynamic responses. They have nothing to do with track and balance at all.
This vibration is intrinsic, and cannot be eliminated, only minimized or absorbed. The hinge offset increases the amount of root shear delivered by the blade, and the number of blades reduces the root shear delivered by each blade. Generally, the n per rev drops geometrically as the number of blades is increased, so that the natural n/rev vibration for 5 blades is a very small percentage of the magnitude of that for 2 blades.
see a brief explanation on page 12 of:
<a href="http://atrs.arc.nasa.gov/atrs/97/jacklin/977183/977183_jacklin.pdf" target="_blank">http://atrs.arc.nasa.gov/atrs/97/jacklin/977183/977183_jacklin.pdf</a>
In the rotating system, this vibration has two components, at N+1 and N-1 per rev. In other words, an absorber attached to a 4 bladed rotor head will see 3 per rev and 5 per rev vibrations, and the transmission will see the sum of these two as a 4 per rev.
Generally, the n/rev for a 2 or 3 bladed helicopter is high, and for more than 5 blades is very low.
PS Whoever rated me as listed in the column to the left - I am flattered, but also, isn't it a bit much? I'd settle for "Boyfriend of Liv Tyler" myself! <img src="wink.gif" border="0">
[ 14 January 2002: Message edited by: Nick Lappos ]</p>