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Rigid rotor system
Semi Rigid rotor (Seesaw)•Semi rigid- allows for two different movements,, flapping land feathering. Normally comprised of two blades, which are rigidly attached to the hub. •The hub is then attached to the rotor mast by a trunnion bearing or teetering hinge. This allows the blades to see-say or flap together.•There is a feathering hinge also.
Semi-rigid rotor•The blades are attached rigidly to the hub but the hub itself is free to tilt in any direction about the tip of the mast.•Usually a two blade system.•No lag or lead movement, but the blades can still flap, or rock up and down to compensate for the dissymmetry of lift. The rotor is free to tilt-called teetering hinge.•Has more complicated joint between the drive shaft and rotor hub.•The design of the cyclic-pitch-control system is such that it decreases the angle of attack on the advancing blade and increases the angle on the retreating blade.•Total effect is to make the lift more equal
Semi rigid rotorTeeter hinge, allowing one blade to rise vertically while the other falls vertically. This motion occurs whenever translational relative wind is present, or in response to a cyclic control input.
Rigid rotor system•Rigid-is mechanically simple, but structurally complex because the loads are absorbed in the blades by bending rather than through hinges.•The blades absorb all of the hunt and flapping.•The blades can’t flap or lead/lag•The blades can be changed by feathering, but no other movement is allowed.
Articulated rotor•Articulated rotor. Hinges at the root of each blade permit the blades to move up and down.•The vertical mounted drag hinge permits each blade to move back and forth slightly in the horizontal plane.•The terms dragging, hunting and lead-lag are also used to describe this movement.•Articulated blade is designed to leave the blades as free as possible to flap up and down or move in the horizontal plane.•All the aerodynamic forces are balanced at the 25% chord. •Articulated rotor is widely used.
Blade Stall•There is a limit beyond which a helicopter cannot fly.•As forward speed increases, the R.W. velocity on the retreating blade deceases and in order to maintain its share of the lift, it must continue to increase its AOA.•Additionally the load is shifting towards the tips.•At a certain R.W. velocity the retreating blade will stall.•The stall begins at the tip and works it way inward as the forward speed increases. First evidenced by a vibration.•In a severe stall the helicopter will pitch nose up, rolls left, and because of gyroscopic precession, the effect will take place near the tail rotor.
Ground Effect•When hovering near the ground, the downward stream of air strikes the ground and does not escape.