Connecting the rotating swash plate to the rotor shaft is an assembly known as the drive link. Because the swash plate needs to move up and down and pivot, the drive link has a joint that acts like a scissor – as such it is sometimes referred to as a scissors link. I have had several students ask me why it is needed.
The swash plate has a rotating and non-rotating side. The non-rotating side is on the bottom and is connected to the flight controls. The rotating side is on the top and is connected via pitch links to each rotor blade. The collective control moves the entire swash plate assembly up and down to change the pitch on each blade equally. The cyclic control tilts the swash plate, changing each blade’s pitch independently depending on its position around the rotor disk. This tilts the rotor disk in the desired direction.
Since the rotor mast runs from the transmission up through a sleeve that the swash plate moves around, there needs to be a method of turning the rotating part of the swash plate. This is the function of the drive link as it connects the mast directly to the swash plate. It is critical that this part be functioning correctly.
During preflight it should be examined closely as the failure of the drive link has caused several accidents. On the Bell 222 an improperly sized bolt that attached the drive link to the swash plate allowed play which caused the bolt to fail. As you can imagine without the drive link the blades will continue turning the swash plate through the pitch links. This stresses the pitch links in a manner they were not designed to handle and can result in a pitch link failure. In this case with the Bell 222 it caused an in-flight break up.
In 1988 the pilot of a Bell 47 spraying a field reported an extreme vibration followed by a loss of control and hard landing. Then in 1992 a CFI and student flying another Bell 47 also felt a sudden and severe vibration and managed to successfully autorotoate to a field. In both cases the center bolt connecting the drive link was missing and disconnected drive to the swash plate.