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So, I was looking at an image of a Bell UH-1N, and noticed that the top if the rotor has a little stick or rodthis image

What is this rod for? I also see this rod on a RC helicopter.

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It is a stabilizer bar used on some two blade helicopters. The spinning weights want to stay in one plane and are used in conjunction with the cyclic to control changes in pitch and roll. Depending on how it is designed, it counters these changes making the helicopter more stable (less maneuverable).

enter image description here

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  • $\begingroup$ Means its less input needed on the pilots side? $\endgroup$
    – lpydawa
    Commented Sep 1, 2018 at 14:26
  • $\begingroup$ @lpydawa Means slower response, so less twitchy. $\endgroup$
    – Pilothead
    Commented Sep 1, 2018 at 18:46
  • $\begingroup$ is it to reduce the chance of pilot induced oscillation? $\endgroup$
    – lpydawa
    Commented Sep 1, 2018 at 22:55
  • $\begingroup$ @lpydawa PIO results from overcontrolling, so a slower response from the aircraft probably makes PIO worse. $\endgroup$
    – Pilothead
    Commented Sep 2, 2018 at 0:31
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    $\begingroup$ The bar is a gyro which wants to remain level. Its action on the rotor blade pitch angle mechanism causes the rotor itself to tend to seek a level plane of rotation, absent any input from the pilot's linkage, giving the helicopter's rotor a bit of stability it wouldn't normally have. You could call it a "self-leveling servo" device. So as Pilothead says, less twitchy, and a bit less tendency to wander off in any direction at any time. In the Huey Cobra, which is based on the same machine, the bar was done away with because responsiveness was a priority. $\endgroup$
    – John K
    Commented Sep 2, 2018 at 0:53

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