Aaaahhhh, the old Nper rev debate resurfaces.
Its remarkable that the helicopter owes it's very capabilities to rotor blades and how they perform - and yet they (the rotor blades) and their effects on the helicopter are proabaly the least understood thing about the aircraft by those within the industry.......truly remarkable.
Nick of course is correct particularly about the operation of the bifilar, but the explaination of the N per rev vibration problem can be expanded a deal further.
The principal of the bifilar has been used widely in automotive engines for some time - as Nick says. Those who remember the Astron engine in Mitsubishi cars of the '80's would remember the "revolutionary" silent shafts which were simply shafts with hollow compartments containing masses free to find their own equilibrium and tuned to roughly the operating regime of the crankshaft. The silent shafts were located close to the crank shaft in order that the countering (absorbing) effect of the free-to-move masses operated closer to the source of excitation. I'm sure there are many other models/manufacturers using the same principal.
But more on the N per rev vibration problem which appears to be grossly misunderstood throughout the helicopter community...
The N per rev always has 2 x components - a lateral & vertical component. The Bifilar (in Sikorsky's case) does take out (or tries to) ONLY 4/ rev in plane (ie predominantly 4/rev lateral) vibe. The vertical component is then absorbed or cancelled (or tried to be) by the cockpit(nose)/cabin absorbers and the interaction between them. If you ever get the chance, have a look at the cabin absorber/s working in flight or coming thru translational - very impressive little hummers. The Bifalars are not tunable by the operator - how they are delivered from the factory is how they shall remain - unless you are exceedingly brave or stupid.
On the other hand the nose/cabin absorbers (vertical n per rev) are tunable. They need to be as there are many variabls which can effect the magnitude and resonance of the vertical component of the N per rev vibration. Things such as loose/worn/frozen absorber bearings, loose pilot seat mounts, loose oleo busshings, loose cargo hook bearings, ESSS - note the fitment of the wing stub absorbers on all UH60L and above with the recommendation that they be retrofitted to models prior to the Lima if they were to be fitted with ESSS.
There is a huge mystique and mystery perceived around helicopter vibration - most of it borne from ignorance. To understand the Nper rev you need to know what causes it......
Try visiting
http://www.rwas.com.au/rtb.html
I have found it gives a good general insight into RTB and other helicopter vibration issues. While not extensive, it does expose a few myths and shows the basics very well. There is certainly scope for the education of the entire helicopter community in the ways of RTB and vibes in general. I can only hope the site above continues to expand its information
Hope you find as interesting as I.....
forever Shak'n