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I have heard of the idea of moving the cyclic forward and backwards in order to get an increase in rotor RPMs. Does anyone have any information on this as to when you would use it or airspeeds it would only be effective at?

Scenario is low altitude autorotation/unpowered flight:

About a 100 - 150 feet AGL, super slow (say 10 - 20 kts)
You lose an engine there.
The ideal thing would be to get as much airspeed as possible to reduce that sink rate as compared to a vertical descent / pull collective / do a hard run on.

However presuming you had to come straight down (trees, power lines or whatever) I have heard from more than one source that pushing the cyclic back and forth at quick segments will help build RPM's but I just couldn't figure out how.

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  • $\begingroup$ I can see you having a momentary change until the governor kicks in and corrects the rotor RPM, but you shouldn't deviate too much. $\endgroup$ – Ron Beyer May 1 '17 at 4:04
  • $\begingroup$ In an autorotation $\endgroup$ – Wes May 1 '17 at 10:49
  • $\begingroup$ Wes, do you perhaps mean pushing forward on the cyclic to pitch down for more airspeed, and—once a little airspeed has built up—pulling back to convert that airspeed to rotor RPM? That sounds plausible, but not something I know much about (@Simon, care to chime in?). What I just described sounds like a slower maneuver than what you seem to have described, Wes. $\endgroup$ – J Walters May 1 '17 at 13:54
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    $\begingroup$ Are you unhappy with the answer you got here? $\endgroup$ – KorvinStarmast May 1 '17 at 15:24
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    $\begingroup$ What sources? I cannot think of any advantage to doing this since every control input takes energy out of the system through losses. I can imagine an initial flare to preserve or re-gain RRPM followed by stick forward to gain airspeed all the way down to the final flare but backwards and forwards is not going to help. $\endgroup$ – Simon May 1 '17 at 15:44
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Cyclic pumping will likely degrade your performance

As explained here, it leads to rotor inefficiencies

As far as moving the cyclic around goes it destroys lift as the TRT vector is all over the place and not acting in one place or close too it and more power is required

What is this TRT he's talking about? Total Rotor Thrust

In a still air hover, TRT is, in broad terms, equal and opposite to weight. As the disc is tilted to commence a transition into forward flight, the TRT vector is tilted and must be increased so that its vertical component remains equal to weight if level flight is to be maintained.

With the above in mind, pumping the cyclic with the engines working simply means that you need more power from your engine to keep your Nr up to fly, as you are being inefficient.

If you have no power, pumping the cyclic is counter productive to your efforts at maintaining Nr since the back and forth creates sequential inefficiencies, and more drag but you have no engine to restore Nr... and in the situation you describe, your precious Nr is a critical resource to your walking away from the forced landing or crash.

Depending on what is happening, a slight turn or flare (aft cyclic, not foreward) may load the head and preserve Nr a bit so that it's still turning with enough inertia to help you as you near the ground and need that cushion from the collective as you hit the ground. Each bit of Nr you retain for that last pull is critical, in that situation.

It's a tough one, the low speed, not-quite-inside-the-Hv-diagram scenario that you presented.

Consult the manual for your model helicopter, and the H-V diagram. Know your limitations.

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  • $\begingroup$ I appreciate the feedback! You explained it very well to me! Thanks $\endgroup$ – Wes May 9 '17 at 19:53
  • $\begingroup$ @Wes glad to be of help. $\endgroup$ – KorvinStarmast May 9 '17 at 19:54

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