Question

why does gyroscopic percession occur in helicopters

Answer 1

Gyroscopic precession in a helicopter refers mainly to the position of control inputs that account for it. Simply (actually over-simply), the helicopter moves in any direction by tilting the rotor disk (the plane of rotation). This is accomplished by applying cyclic changes to the angle-of-attack of each of the rotor blades as they rotate through their 360 degree arc. In order to do this, the control input must be applied at a point 90 degrees forward of intended reaction point in that arc.