Collective Pitch Change in Forward Flight

Another longitudinal flapping effect is that caused by an increase in collective pitch in forward flight. Both the advancing and retreating blades receive the same change in angle of attack; but the advancing blade, being at a higher dynamic pressure, develops more additional lift than the retreating blade. Both blades flap up a quarter of a revolution later, but the advancing blade flaps up more than the

retreating blade, resulting in both an increase in coning and a net rearward tilt of the tip path plane. This type of flapping is sometimes noticed by the pilot when he decreases collective pitch when entering autorotation. In this case, of course, it is a nose-down tilt of the tip path plane that results. It is also partially responsible for the trend of cyclic stick position with speed. From hover to approximately the speed for minimum power, the collective pitch required for trim decreases, thus causing the tip path plane to want to tilt forward. An aft cyclic stick motion is required to keep the helicopter in trim. This effect may be larger that the aft flapping caused by the increase in forward speed; and, as a consequence, the trimmed stick position may initially move aft or have an unstable gradient. At high speeds, where collective pitch is increasing, the gradient will almost always be stable.