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Let's say a helicopter wants to move forward. To do so, cyclic pitch is applied to the blades 90 degrees off which applies a torque on the blades to tilt the craft forward. Once the helicopter is tilted there is now lift upwards and lift forward. But why doesn't the helicopter just keep tilting forward, doing flips?

My first thought was that the pilot only applies the torque until they are satisfied with the tilt angle then returns to normal control. But this can't be the case since in many videos I've seen the swash plate remains tilted throughout the entirety of the movement. I can't think of another way that the helicopter could remain at the desired forward angle and not just flip repeatedly.

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Very good observation - helicopters actually do want to flip over, in the hover. While hovering, the helicopter is in an unstable equilibrium and the pilot will have to make constant cyclic inputs to prevent the heli from flipping indeed.

Your observation of constant forward cyclic deflection is when flying forward, and this reveals the stabilising factor: airspeed.

  • The incoming frontal horizontal airflow component wants to blow the rotor backwards, which would actually happen with a centred cyclic. Maintaining forward cyclic prevents this from happening.
  • But the required forward cyclic input is very small, still requiring close attention from the pilot. In order to create an equilibrium state requiring a larger forward cyclic deflection, helicopters have stabilisers at the tail. With a nose-down tilted fuselage, forward velocity creates a nose-up moment.
  • The helicopter fuselage follows the tilt of the rotor plane, moving the CoG backwards, as depicted below. This creates a stabilising moment nose-up.

enter image description here

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  • $\begingroup$ The two torques on your picture (lift+weight force pair and thrust+drag force pair) cancel out when the rotor is not tilted relative to the craft. The real reason is that forward speed increases lift on the advancing side, which creates the same effect as aft cyclic. And that effect isn't subtle. It also changes the behavior from slightly unstable at hover to slightly stable at high speed. $\endgroup$
    – Jan Hudec
    Jun 3, 2021 at 9:05
  • $\begingroup$ @JanHudec 1. rotor blowback, as mentioned in the first bullet point. And indeed, the mechanism of rotor blowback is as you describe, causing blade flapping. 2. Horizontal stabiliser. 3. The fuselage is tilted nose-down at forward velocity and the two torques do not cancel out, as mentioned in this answer $\endgroup$
    – Koyovis
    Jun 3, 2021 at 9:21
  • $\begingroup$ The first point would then deserve explaining. From the term “blowback” I don't understand the mechanism it is supposed to describe. $\endgroup$
    – Jan Hudec
    Jun 3, 2021 at 9:23
  • $\begingroup$ Bullet point 3 appears to be the pendulum/rocket fallacy. The aerodynamic force is still perpendicular to the rotor, thus in line with the CoG. There are videos on YouTube demonstrating that a tilted rotor stays tilted. $\endgroup$
    – TomMcW
    Jun 4, 2021 at 17:34

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