What is the function of the scissors link in helicopter flight controls?
the scissor link ensures the upper swashplate rotates with the rotor blades (and the lower with the body) while ensuring enough movement for the plate to control the craft (tilting and up and down).


(http://en.wikipedia.org/wiki/File:Robinson-Rotorhead.jpg)


  To keep the stationary swashplate from rotating with the rotor, a scissor mechanism is added. The scissor connects the swashplate to the fuselage. The scissor consists of 2 parts which are connected by a hinge. One end is connected to the helicopter using a hinge, while the other is connected using a ball joint. This means that the swashplate can move freely up and down, and can freely tilt in any direction, but can't rotate around the mast.[source]


this is needed because the control linkage is typically attached with balljoints which would otherwise allow for too much relative movement.