Nick has summed up very neatly a very complex area in rotorcraft dynamics the theory understanding and modelling of which is the preserve of very well educated men. Cool Link Nick
If I may add my pen'th to the bun fight.
Nper rev has very little or nothing to do with 1 per rev, it is however very much more noticeable if the 1 per rev is very low as on a well tracked and balanced machine, on a 2 bladed machine if the 1 per rev is too low the overall ride quality can be very poor. There is a trade off to be had here to ensure the perceived vibration is acceptable to the crew. Bell pioneered Nodamatic ride systems to improve Nper rev on their heavy two bladed machines viz the 214,222 etc.
Nper rev on the Lynx helicopter is very pronounced, it has a 4 bladed semi rigid rotor head and Main gearbox that is bolted directly, undamped, to the airframe. These are all design features that confer the huge control power that make the aircraft very good for what it was primarily designed for, deck landing (and staying on) the back of small ships in rough seas. These features come with a price in vibration terms.
Nper rev on the Lynx is controlled by scheduled maintenance inspections and actions as a result of in service vibration survey operations. Key contributors to the levels of Nper rev are the mechanical condition of the mountings of any of the heavy components on the airframe e.g. engine mount bearings, horizontal stabiliser (which lives in the downwash environment), cabin door latches, Instrument panel, and avionic box AVM, rescue hoist mounts etc. Any wear or slop in these and the Nper rev cocktail gets bigger.
The most significant driver to the levels of Nper rev on the Lynx is the Main gearbox top bearing pre-load figure. This if you like controls the free play between the rotor system and the airframe, too low a pre-load will allow excessive relative motion between the two systems and will contribute hugely to the Nper rev generated by the aircraft.
The Composite rotor blades with Berp tips also increased the Nper rev of the Lynx to noticeably higher levels than that experienced with the conventional shaped metal blades fitted to the earlier marks. The Horizontal stabiliser was cropped span wise and elastomeric engine mounts were introduced to compensate for this increase in vibration. The increased performance offered by the Berp blades also comes with a price in vibration terms.
Blade pass downwash acting on the upper surface of the fuselage is also a contributor to the Nper rev values on a helicopter; the rotor mast was reputedly lengthened on Blackhawk/Seahawk to reduce its Nper rev figures. Maybe Nick has the inside scoop on this?
Wunper
P.S this is not a dig at the Lynx! it is a very fine helicopter but serves the explanations for this thread well
PPS this is my first technical post so I will just put the tin hat on and zip up the Kevlar jacket,,,,,,, <img src="cool.gif" border="0"> <img src="cool.gif" border="0">