As previously discussed, the flapping behaviour of a rotor blade is governed principally by the balance between the aerodynamic lift and the centrifugal force moments about the flapping hinge. These are both dependent on the square of the rotor speed and so, under atmospherically still conditions, the balance is preserved at any rotor speed. However, when operating in high wind conditions, such as on a ship, the balance can be upset. During any helicopter sortie, the rotor must be spun up to speed from rest (engagement) and slowed to a halt (disengagement). At the low-speed ends of these sequences the centrifugal moment is of a small magnitude but the aerodynamic moment can be enhanced by the adverse wind conditions and the blade can experience excessive flapping angles. This is known as blade sailing or, because of its potential to inflict damage to the upper tail boom, tunnel strike.
4.1.1 Lagging Motion
As already described, the helicopter rotor must attain a trimmed condition in forward flight. The disparity between the advancing and retreating sides of the disc is handled by the inclusion of flapping hinges on the rotor hub. Flapping motion introduces a phenomenon associated with a rotating system – that is, the rotor hub. This is the Coriolis acceleration.