May 6, 2013 by Tim McAdams

Dissymmetry of lift occurs when a rotor system is flown edge-wise through the air. With helicopters, many times these discussions center on the main rotor system. However, this aerodynamic condition also affects the tail rotor.

Just like the main rotor, a tail rotor will equalize lift by flapping. However, most tail rotor flapping takes advantage of the Delta-3 effect. Also known as pitch-flap coupling or K-Link (French term). This effect is achieved by having the pitch horn on a different plane than the flapping hinge, which mechanically changes the pitch angle of the blade as it flaps. The amount of the delta-3 offset is measured in degrees and determined by design engineers after considering many factors. This offset can be also be accomplished by using a Delta-3 hinge (setting the hinge at an angle to the chord of the blade). In either case, when the advancing blade (the blade that experiences a higher relative wind) starts to flap the offset lengthens the distance between the blade’s pitch horn and the pitch link’s attach point. This forces the pitch link to pull the blade’s pitch horn closer, thereby reducing its pitch angle. On the retreating side, the distance is shortened and the pitch link forces the pitch horn further away, increasing the blade’s pitch angle. This effect minimizes flapping in order to control dissymmetry of lift on the tail rotor.

This can be demonstrated by moving a tail rotor blade with a delta-3 hinge through its flapping range and observing the pitch angle changes as you manually flap the blade.


AS350 tail rotor

AS350 tail rotor

  • Ehud Gavron

    Great article about an often-overlooked and perhaps poorly understood yet simple and elegant mechanism.

    A long time ago I read a fascinating article on the use of Delta-3 in the R-22 MR swashplate. Here is the link:

    Ehud Gavron
    Tucson AZ US

  • Alan Barnes

    “Dissymmetry of lift occurs when a rotor system is flown edge-wise through the air.”

    It’s the movement of the air through the rotor system, not necessarily movement of the the rotor system through the air. The helicopter could be stationary in a hover, or even still on the ground, for dissymmetry of lift to occur in the rotor system. It’s quite rare that that it’s NOT occurring.

    Yeah, technicalities. But sometimes those technicalities are important.

  • Tim McAdams


    I agree, thanks for pointing it out.


  • Gregg Gause

    Alan, while what you say is correct in principle, what you are talking about is called Translational Lift, which is a different subject. And yes, it effects rotor performance anytime air is moving across either rotor, for any reason.

    The OPs explanation was the clearest , easiest to understand explanation I have ever heard. Thank you.