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So the basics of helicopter control as I understand it, at least, in terms of roll and pitch, are as follows:

A helicopter is caused to move through space by means of tilting the main rotor disc, thereby changing the direction of the rotors lift vector to provide both a vertical lift and a horizontal thrust component in the direction that the rotor disc is tilted. This change in rotor attitude is affected by means of cyclically feathering the blades as they revolve around the hub, thereby creating an imbalance in lift over the whole of the rotor disc. This imbalance in lift results in the rotor disk, tilting 90° out of phase with the cyclically feathered blades due to gyroscopic precession.

To prevent continued tilting of the rotor disc, once the disc reaches the same attitude as the swash plate, the blades are no longer cyclically feathered, and the rotor continues to hold this attitude. The rotor is not capable of exceeding the limits of swash plate during flight operations, though semi rigid rotor designs are capable of striking the mast head during operation if an excess of rotor attitude from the horizontal is achieved.

Am I missing something here?

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  • $\begingroup$ So when a helicopter rotor spins, each blade also moves relative to the rotor hub? $\endgroup$
    – Someone
    Jul 5 at 5:43
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    $\begingroup$ As I understand it, yes. Either by cyclic input, coning, flapping, or dragging, each rotor blade is in a constant state of movement during forward flight. $\endgroup$
    – Romeo_4808N
    Jul 5 at 6:00
  • $\begingroup$ The answer to your questions depend heavily on the specific helicopter you want to know about. On one hand, the red bull aerobatic helicopter has rotorblades that are rigidly held at 90 degrees to the hub, while the huey basically hangs freely under the center of its rotor and can exert no torque on it. In general, there's no direct mapping from swashplate position to rotor position, but I would be totally unsuprised if there's some class of helicopter popular in germany or western canada or something, with that sort of mapping. $\endgroup$
    – QuadmasterXLII
    Jul 5 at 6:04
  • $\begingroup$ The blade tips always describe a perfect circle at any position of the cyclic, and they change their orbits only while the cyclic control is changing... $\endgroup$
    – xxavier
    Jul 5 at 8:13
  • $\begingroup$ @QuadmasterXLII But the so-called 'rigid rotors' are not really rigid... They have no flapping or lead-lag (and sometimes not even feathering) hinges, but the blades do really move in flapping, lead-lag and feathering, because the blade roots work as 'virtual' hinges due to their elasticity... So those rotor heads are not really rigid, but hingeless-articulated... $\endgroup$
    – xxavier
    Jul 5 at 9:09

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Just some finicky clarifications by my side 😉

thereby changing the direction of the rotors lift vector to provide both a vertical lift and a horizontal thrust component

Technically speaking helicopter rotors generate only a thrust which, as you correctly wrote, is tilted according to the manoeuvre which has to be flown.

This imbalance in lift results in the rotor disk tilting 90° out of phase with the cyclically feathered blades

The 90° is the delay between disk tilting and the feathering that causes it. The value in ° of this delay depends mainly on the distance of the flapping hinge from the rotor hub: for a teethering rotor the hinge is exactly by the rotor hub and the delay is 90°; for other rotor constructions the delay is smaller.

due to gyroscopic precession

Albeit similar, the delay is caused by the fact that the feathering of the blade is in resonance with the flapping movement: so you don't have to think about someone pushing the border of a spinning top rather to someone pushing a swing.

To prevent continued tilting of the rotor disc, once the disc reaches the same attitude as the swash plate, the blades are no longer cyclically feathered, and the rotor continues to hold this attitude.

Blades reach the attitude commanded by the swashplate (and in turn by the pilot) after <90° (i.e. ¼ of rotation, some 0.04s for a standard EMS helicopter) and they stay in that attitude as long as the pilot keep pushing on the commands. This is just like any other steering system: you rotate the steering wheel to the right and the car turns right. And just because it enters in a right curve doesn't mean that it stops turning: it just keeps on turning unless you don't rotate the steering wheel back in its central position.

The rotor is not capable of exceeding the limits of swash plate during flight operations

Not clear what you mean here.

semi rigid rotor designs are capable of striking the mast head during operation if an excess of rotor attitude from the horizontal is achieved.

I suppose you mean the must bumping problem. This is true for teetering rotors and it is due to the limited mechanical space between rotor head, blade roots and mast: at low or negative g manoeuvres the rotor head might tilt so much that it clashes against the mast. Something similar might happen also to non-teetering rotors but only at high negative g and actually with catastrophic outcome: in this case the blades are flapping downward so much that they can hit and cut through the tailboom.

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