Change of Main Rotor Collective and Cyclic Pitch

Simultaneous rotation of all the blades relative to the axial hinges in the same direction and through the same angle is termed collective pitch change. Increase of the collective pitch leads to increase of the main rotor thrust force. Sequential change of the blade pitch in azimuth is termed cyclic

pitch change. During cyclic pitch change, the pitch of each blade increases over a 180° azimuth range and decreases in the other half of the circle (Figure 104a).

Change of Main Rotor Collective and Cyclic Pitch

Figure 104. Main rotor cyclic pitch variation.

When the pitch is changed, the blade thrust changes and its moment about the horizontal hinge changes, which leads to flapping motions and tilt of the cone-of-revolution axis and deflection of the thrust force vector.

If the thrust force vector needs to be deflected in the direction of the 210° azimuth, the blade pitch must be minimal at this aximuth and maximal at the opposite azimuth — 30°. Then the pitch decreases from the 30° azimuth to the 210° azimuth and increases from the 210° azimuth to the 30° azimuth.

A similar pitch change is observed for each blade. Change of the collective and cyclic pitch is accomplished with the aid of a special system — the main rotor tilt control.