Which mechanism is used in helicopters for lift adjustment during flight?

The mechanism used in helicopters for lift adjustment during flight is the collective pitch control. Collective pitch control allows the pilot to simultaneously change the pitch angle of all main rotor blades, which directly affects the amount of lift generated. By increasing the pitch of the rotor blades, the helicopter can gain altitude, while decreasing the pitch will reduce lift and cause the helicopter to descend. This control is essential for managing the helicopter's vertical movement and overall flight performance.

On the other hand, the option regarding flapping of main rotor blades refers to the natural hinging movement of the blades as they rotate, which helps to equalize lift and manage centrifugal forces, but it is not a direct control mechanism used by pilots for intentional lift adjustment. Tail rotor adjustment primarily affects the helicopter's yaw, helping to counteract torque from the main rotor, rather than its lift. Lastly, landing gear configuration pertains to the structure used for takeoff and landing, with no impact on lift during flight. Therefore, collective pitch control is the correct and crucial mechanism for lift adjustment in helicopters during flight.