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I saw these things swinging in a video of a rotor from the blade point of view. I don't know what are they for but I am quite curious. And why some helicopters have it and some don't?

 blades of a helicopter standing on land on a helipad
Source: alamy

Helicopter Main Rotor Blade In Flight Slow Motion
Source: Helicopter Main Rotor Blade In Flight Slow Motion -- YouTube

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    $\begingroup$ Reminder: please no answers (or attempts) in the comments $\endgroup$ – Federico Jun 18 at 7:50
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The devices are pendulum (or pendular) absorbers:

Vibration suppression of helicopter blades by pendulum absorbers - Imao Nagasaka.

The pendulum absorbers are used for suppressing the vibrations in helicopter blades. Compared to fixed weight attached to the blade, the pendulum configuration allows for shifting of the damping range depending on rotor angular speed.

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    $\begingroup$ Re: depending on rotor angular speed -- isn't a helicopter's rotor's RPM fixed, and the variable being the torque / blade pitch. Is it perhaps depending on load? I'm not claiming to know, just wondering. $\endgroup$ – ymb1 Jun 18 at 11:25
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    $\begingroup$ @ymb1 My educated guess would be that you need damping during the start-up sequence, too, to prevent harmful oscillations when the rotor RPM and blade eigenfrequency (which changes due to centrifugal stiffening) coinciding. $\endgroup$ – Sanchises Jun 18 at 11:46
  • $\begingroup$ Good point @ymb1! I'm guessing Sanchises has it right. $\endgroup$ – Jpe61 Jun 18 at 17:07
  • $\begingroup$ The hinged arrangemet may also be better, since the mass is connected to the rotor by a shaft, thus affecting which frequency modes it dampens (not sure about terminology here). A simple fixed mass would have mechanically very different characteristics. $\endgroup$ – Jpe61 Jun 18 at 17:11
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    $\begingroup$ @Jpe61 Yes. The hinge decouples the mass's moment of inertia in the axis going across the blade - it's as if the mass had zero moment-of-inertia in that axis. I'm not sure whether this effect is necessary/useful, but it sure is there. $\endgroup$ – Kuba hasn't forgotten Monica Jun 18 at 19:20

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