Originally Posted by
IFMU
I used to be a Sikorsky/X2 guy. I'm probably not at liberty to say the particulars but I'll say you have your harmonics wrong with respect to what was creating vibes and what wasn't.
For a 4 bladed rotor system, 4P blade loads create 4P vertical shears at the hub. 3P and 5P blade loads get converted to 4P vibratory moments in the fixed system. Depending on where particular blade natural frequencies lay, you can somewhat minimize some of these responses at the expense of increasing others. I, personally, would rather deal with a pure 4P vertical shear from the hub than 4P pitch or roll moments in the fixed system. 2P vibes should be low but can exist if there are tracking or stiffness differences between the blades.
Pretty sure I have the above general statements correct since they would apply to any 4 bladed rotor (and ignoring higher harmonics). The X-2D, S-97 Raider, and SB>1 all have different hub and blade configurations. It’s not a given that the tuning and harmonic airloads across all three ships are identical, despite being 4 blades each. In fact, looking at the respective hub designs in patent art, I’m pretty sure they’re not identical. So the X-2D experience might not entirely translate to the larger ships.