Originally Posted by
The Sultan
IFMU
Some of the highest vibration in helicopters occurs in the rotor transverse flow range which is the 20 to 40 knot range. These vibration levels often exceed cruise flight levels and do seem to be the speeds the SB-1 is still trying to achieve.
Additionally, as you know, the X-2 relied on an active vibration suppression system to make the vibration in the cockpit tolerable. Unlike rotor or transmission mounted devices this system has only an affect on the area in proximity of the device and can significantly increase the vibration in other areas of the fuselage. A cockpit mounted AVS could easily double tail vibration over a not having one.
The translational airspeed region was not as challenging as high speed flight in the X2. In fact for most of the lower speed region the X2 vibes could be managed without AVC using only classical methods to balance the rotors.
When I was a younger man I also thought AVC would make vibes better in some areas but worse in others. Makes sense from what we learned in freshman level physics. On an S92 with cabin mounted force generators (FGs) surveys showed that vibe levels pretty much decreased everywhere though pilot and passenger were the focus. So reality and theory diverge here.
The X2 FGs were not cabin mounted but were on the structure as close to the dynamic system as possible. This reduced the vibes before it got to the cabin. The result was not merely tolerable, but the IPS at 250kts was lower than a UH60 at cruise. That is exceptional!