Originally Posted by
Commando Cody
What Rugen is describing is pretty much what was always planned for FVL-M. His statement about, " We know a lot about that lift-offset compound already..." doesn't make a lot of sense given X2's track record (or lack thereof) so far.
What the Army may well know is that a flatwise rigid rotor will have very significant levels of vibratory hub moments and vertical shears unless the number of blades +/- 1 is higher than any significant harmonic air loads in the blades. With 3 blades, the XH-59A had blade 2P, 3P, and 4P translating into vibe hub loads and blade 2P is always quite high leading to epic levels of vibration. A four bladed rotor avoids that 2P generating vibe hub loads, but 3P, 4P, 5P are still reasonably large and it's nearly impossible to not have a flatwise blade mode in that region, amplifying one or more of those air loads. The 4 bladed X-2 rotors will have less-than-epic, but still incredibly high levels of vibratory hub loads.
The aerodynamic benefits of the ABC rotor are significant but they come at considerable cost.