I recall from my days on Lynx (AH1) the umpteen problems with regard to airframe vibrations, however age, and probably the vibration, has dulled my brain and I am now seeking an explanation on the difference between 1R and 4R vibrations.
I confess, I was always impressed when guys in the crewroom confidently suggested that airframe xyz had a 4R vibration, or a 1R vibration, and always wondered exactly how these different vibrations manifested themselves through the airframe/controls/backside.
maybe the british military never ran to coffee-in-flight, that's a good way to read the vibes. One of our machines I remember had so many coffee stains that worked like glue on the centre console that it was impossible for the venerable mug to be displaced in any type of gyration. used to be quite amusing though to tool up alongside the usual driver and watch his helmet bobbing around whilst the eyes stayed steely still. his coffee though as thick as, had been labled by the boss as 'UBoat fuel".
There are different sources of the vibrations transmitted to an airframe. Some come from the main rotor, some from the gearbox, etc. The 1P, 2P, etc. main rotor vibrations are low frequency but are the ones you feel "in the seat of your pants". The vibrations produced by the gearbox are of much higher frequencies and can be quite loud. You don't really feel these vibrations "in the seat of your pants", but the high noise levels can be just as bad because they are mentally fatiguing.
The in-plane rotor modes (lead/lag) can be mitigated pretty well with hydraulic dampers or bifilars. The out-of-plane rotor modes (flap) are now being addressed with active vibration control systems or with individual blade control. However, the gearbox vibration problem still needs to be worked on.