Rotor blades don’t flap in the same sense that a bird’s wings flap; they rotate in a uniform circle. Flapping is only an illusion.
But when the rotor’s hub is bolted to the rotorhead and the axis of rotation of the rotor differs from the rotorhead’s axis of rotation, an accommodation is required. Flap/drag hinges are simply a universal joint that permits the rotor axis of rotation to differ from the rotorhead axis of rotation.
Floating hub rotors where the rotor’s hub is permitted to be aligned with the rotor tip plane axis don’t require flap/drag hinges.
It ain’t complicated.
But when the rotor’s hub is bolted to the rotorhead and the axis of rotation of the rotor differs from the rotorhead’s axis of rotation, an accommodation is required. Flap/drag hinges are simply a universal joint that permits the rotor axis of rotation to differ from the rotorhead axis of rotation.
Floating hub rotors where the rotor’s hub is permitted to be aligned with the rotor tip plane axis don’t require flap/drag hinges.
It ain’t complicated.