I understand the following -

  1. Due to dissymmetry of lift, flapping up and down occurs.
  2. Since angular momentum has to be conserved, the speed of blade increases/decreases during flapping up/down motion.
  3. To compensate for this blade leads front by a certain degree in the rotor plane, in advancing half and lags by a certain degree in retreating half.

My question is - what does the "compensation means"? Even after rotating the blade front by 1-2 degrees (leading), the speed of blade will remain the same after it is reached its maximum lead angle. So, what happens after that?

Is it the case that lead/lag helps in absorbing the load/vibrations by allowing some degree of freedom for blade to move by few angles?

  • $\begingroup$ None of these is true. The advancing/receding blade moves faster/slower relative to air so they need to have more/less angle of attack to generate balanced amount of lift. Flapping doesn't happen in the up/down or forward/backward dimension, but in the axial dimension of the blade. $\endgroup$ Commented May 26, 2020 at 16:13
  • 1
    $\begingroup$ @user3528438: Yes, I understand the blade flaps up and down, thereby changing the angle of attack. What do you mean by "axial dimension of the blade"? I understand flapping happens perpendicular to the hub-plane. Where am I wrong? $\endgroup$
    – Raj Arjit
    Commented May 26, 2020 at 16:17
  • $\begingroup$ No flapping is up and down vertically on the flapping hinge axis, which is horizontal and perpendicular to the blade axis. On a two blade system the flapping hinge is the teetering hinge in the center of the hub. The axial motion is blade incidence as set by the swash plate. The blade flapping up and down is simply the blade following the path determined by the incidence set by the swash plate as it moves, with determines the overall rotor disc plane. With an articulated rotor, the blade flapping range of motion is the rotor's tilt range of motion. $\endgroup$
    – John K
    Commented May 26, 2020 at 17:37

1 Answer 1


Blade flapping is simply the action of the tilting of the rotor disc; the blade's tip is following a particular path attached to a hub that is rigidly fixed on its vertical axis. The blade flaps up and down because its incidence/pitch is being continuously varied (or not if the stick is centered) by the swash plate. The limits of the rotor disc's tilt range is, beyond the swash plate limits, ultimately the blade flapping travel limits. Articulating rotors can handle this, but on a two blade teetering rotor, achieving those limits is called "Mast Bumping" and is a fatal condition.

For lead lag, the issue is you need to allow for that motion on an articulating rotor helicopter because Coriolis force makes a blade want to speed up as it flaps up relative to the other blades (actually, moving off perpendicular to the mast's axis). When it flaps up, its center of mass moves closer to the hub than a blade on the other side that is closer to perpendicular to the mast, which makes it want to speed up, like a figure skater pulling arms in while spinning. The lead/lag hinge provides the necessary compliance to accommodate the speeding up and slowing down tendency of the blade as its center of mass moves closer and farther from the mast as it rotates while moving up and down, so it doesn't force the rotor hub to absorb the forces. The lead/lag damper is there to resist sudden movements of this lead/lag action that can be caused by sharp ground contact, which can lead to ground resonance.

On a teetering rotor system, you don't need lead lag hinges because you have the geometry of the teetering hinge, which is the blade flapping hinge for both blades on a single flapping axis. The up flapping blade moves away from the hub because the hinge is above the blade axis (called an under slung hub) and this keeps the CG from moving closer to the hub significantly, eliminating most of the Coriolis forces, or reducing them to a level can be absorbed by the rotor head and blades.

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  • $\begingroup$ Do you mean 'Conservation of Angular Momentum' rather than 'Coriolis Effect'? $\endgroup$ Commented Jun 6, 2020 at 15:20
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    $\begingroup$ Same thing. Conservation of momentum in an object rotating about an axis. Also called "Coriolis Force". en.wikipedia.org/wiki/Coriolis_force That said, "Coriolis Effect" tends to be associated with meteorology so I changed it to Coriolis Force, so thanks for the comment. $\endgroup$
    – John K
    Commented Jun 6, 2020 at 16:30

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