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What do we mean by rotor damping (in terms of helicopters)?

How is it affected by air density?

How does helicopter stability depend on rotor damping?

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I will try to answer these questions in terms of structural dynamics. Please correct me if I say dumb things on other parts of my answer.

You ask about damping. Damping must be considered when dealing with structural dynamics, or vibratory phenomena. In a helicopter rotor, multiple phenomena can lead to vibrations. For instance :

  • Differences in blade properties (mass mostly, but also stiffness)
  • Rotor imbalance due to machining uncertainties
  • Bearings
  • Engine, which may produce torque oscillations of some sort
  • Gearbox, which also may produced oscillations (look for gear mesh vibations)
  • Helicopter operations (if you have quick flight trajectory modifications,this may lead to increased or reduced lift needs, which induced torque oscillations in the drive line)

So long story short, multiple phenomena can excite the rotor. As any structure, a helicopter has natural vibration patterns called natural modeshapes, associated with natural frequencies. However, if you excite a component enough on its natural frequency, this may result in a fatigue failure, hence the utmost importance of damping.

As far as I know, the most visible technology used to increase damping on a helicopter rotor are the blade root dampers. Multiple technologies exist, however they seem to damp the same phenomenon : blade edgewise motions or so-called lead-lag motions or hunting motions.

If you want to have a deeper understanding of these issues, have a look at this article for instance. This page may aso be helpful

The first paper adresses semi-active dampers to control the blade egewise bending motions in order to reduce the constraints on the hub. Basically, if the blade was mechanically fixed to the hub, large contraints could occur due to vibration loads, leading to for instance fatigue failure. In the configuration presented in the paper and reproduced here, enter image description here the liaison between the hub and the blad is a cylindrical liaison, allowing in-plane rotational motion. The (stiffness-)damper system is then used to absorb the kinetic energy of the blade in order to limit the loads on the hub, which allows to reduce its overall wheight.

The second paper provides some more general concepts on the technologies involved in helicopter rotors. In particular this here enter image description here is a good schematic of a full articulated rotor. Other rotor technologies exist : semi-rigid or rigid rotors but they use a different strategy for vibration reduction and blade-hub liaison.

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  • $\begingroup$ Could you include a few relevant thoughts from the two articles you linked, just in case the links die? That would help tremendously (and is expected). $\endgroup$ – FreeMan Oct 19 at 18:27
  • $\begingroup$ @FreeMan, will do ! I am still new to this community, so thanks for the advice $\endgroup$ – BambOo Oct 20 at 10:06

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