During which phase of flight is the “flapping” motion of rotor blades most critical?

The flapping motion of rotor blades is indeed most critical during all phases of flight. This flapping is an adaptive movement that allows the rotor blades to manage changing aerodynamic forces and maintain lift efficiency while mitigating the effects of weight distribution, particularly in helicopters which are subject to variations in load and asymmetric flight conditions.

During takeoff, the rotor blades need to adjust their pitch to provide the necessary lift as the helicopter transitions from a static to a dynamic state. Any discrepancies in flapping behavior can lead to insufficient lift or control issues, potentially leading to an unsafe situation.

In the landing phase, similar dynamics occur where the rotor blades must adjust to maintain appropriate lift as the helicopter descends and prepares to touch down. An improper flapping motion could result in unstable landings, thereby increasing the risk of accidents.

In hovering, the rotor system must be particularly sensitive as the blades experience differing airflow conditions. The flapping motion ensures that each blade can cope with changes in lift requirements, dictated by variations in weight distribution and wind effects.

Therefore, throughout all phases of flight—takeoff, landing, and hovering—the rotor blades' ability to flap effectively is critical for maintaining stability, control, and safety of the aircraft.