I was up at the Sikorsky factory back in the 70’s on a visit pertaining to the CH-53D. While there, they gave me the Grand Tour of the ABC.
It had THREE swashplates! One for each rotorhead and a third on the BOTTOM of the transmission to vary the relationship between the two main swashplates to give the helo YAW control!
Wild!
My Father worked in propulsion engineering. He worked on the Blackhawk, the Black Hawk, and he put the third engine in the CH-53. Kind of wish I had made my career there too.