When a rotorcraft pilot moves pushes on the cyclic control stick, the swashplate tilts so that angle of attack of the retreating blades increase and of the advancing blades decrease, which creates forward pitching moment on the rotor (shifted 90° due to gyroscopic effect).

Now the first reaction is that the blades flap up aft and down forward, tilting the plane of rotation forward. But which is then the dominant effect that makes the helicopter body to follow the pitch?

  • Is is the blades pulling on the hub due to centrifugal force trying to align it with their own plane of rotation,
  • or is it that the lift vector tilts with the rotor so its action line no longer passes through the centre of gravity?
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    $\begingroup$ A good way to think of it is with a teetering system, you fly the rotor disc here and there and the machine itself is just slung under it like a ball on a string that follows wherever the forces take it. Articulating rotor is a bit like the body is connected to the rotor disc by a flexible rod instead of a string, it still tilts and flies around as you command it, but it also imparts some of its tilt to the body. Articulating masts and the transmission mounts have to take bending as well as lift loads, whereas teetering sees no bending loads, unless you get into mast bumping, and you're dead. $\endgroup$ – John K May 5 at 1:13
  • $\begingroup$ Related - - aviation.stackexchange.com/questions/38859/… --even with a teetering system, the helicopter does not just dangle freely from the rotor disk -- see last paragraph for detailed explanation. $\endgroup$ – quiet flyer May 6 at 12:20
  • $\begingroup$ @quietflyer, that answer was (the answer is deleted now) already linked below. I already read it, and it definitely does not say that. What it does say is that when the body tilts, it will make the rotor tilt too. That is clearly important to prevent the rotor hitting the body, but because the moment is between the rotor and air, it does not create any moment on the body. The body does actually “dangle freely” (on teetering rotor), though that's not really a good term, because it evokes a fixed mount, which the rotor definitely isn't. $\endgroup$ – Jan Hudec May 6 at 15:25
  • $\begingroup$ @JanHudec -- so you disagree with the following? (Highly condensed from the link above) -- (With hinged rotors) "As soon as the rotor plane changes as a result of cyclic stick input, the resulting rotor hinge moment will change fuselage angle accordingly, providing the desired feedback cue." "With teetering rotors, there is an indirect aerodynamic coupling between fuselage and rotor shaft. This provides the same cues as the hinge offset solution but at a slower rate." $\endgroup$ – quiet flyer May 6 at 18:00
  • $\begingroup$ @quietflyer, yes, I disagree with that. The aerodynamic coupling in case of teetering rotor (it exists for every rotor anyway) is from body to rotor only, because the actual force is between rotor and air and the body position only affects cyclic. Of course the effect of rotor tilt on the body is there, but the discussion is now what effects there are besides that. And it does have practical implication too: teetering rotor loses all control effectiveness when unloaded, hinged or flexible maintains some. $\endgroup$ – Jan Hudec May 7 at 5:16

The impact of these effects depend on the rotor design. E.g. a teetering rotor only includes the second effect you mentioned (tilted thrust vector), the 1st effect is absent. The first effect you mentioned increases as the "flap hinge" is moved away from the hub, giving more control force utilizing CF. You may be interested to know there's typically a 3rd effect - if the blades are restrained from flapping freely (in the simplest case a spring) that restraint creates a pitch moment about the hub which also pitches the aircraft. Each of these is discussed here.

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  • $\begingroup$ @Koyovis, I am probably missing something, but that explains why the rotor wants to return to aligned with the fuselage, not why it pulls the fuselage to where it's turned. $\endgroup$ – Jan Hudec May 5 at 12:08
  • $\begingroup$ @JanHudec The rotor disk and the fuselage want to align relative to each other. The pilot commands the rotor disk angle to speed up, this is the offset in the coupling. Yes the fuselage exerts a moment on the rotor as well, but the pilot has control over the desired disk deflection. $\endgroup$ – Koyovis May 5 at 13:39
  • $\begingroup$ @Koyovis, so there is absolutely no disagreement then. For teetering rotor, the angling of the lift is the only effect that applies moment to the fuselage exactly as this answer states (and then the relative angle subtracts from the cyclic command, which prevents the rotor tilt from running away, which is interesting, but orthogonal to the question). $\endgroup$ – Jan Hudec May 5 at 13:47

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