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Old 12-20-2020, 03:08 PM   #1 (permalink)
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Default Fast tail shakes during circuits (KST215 compatibility issue?)

I'm having similar issues on both my helis. Both are running KST215's for the tail servo (and obviously fc6250's). Blade 360 cfx and 270 Fusion. Mechanically everything is smooth and limits are set properly. Setup at 333 hz in forward programming (KST215 working frequency:1520us/333Hz, 800us-2200us).

Hovers and slow flying are fine, but faster circuits bring out the rapid tail shakes. Especially after I go full collective to pick up speed and fly in a straight line (It doesn't happen at full collective, but afterwards when I'm near neutral going fast forward). You can also audibly here the tail bouncing. This is in stunt 1 and stunt 2 (worse in stunt 2 with the higher head speed).

After reading this thread, I thought my tailrotor P gain was set too high. But I brought it down as low as 55 and still would get the shakes, but the tail would also blow out easy when doing consecutive flips. Now I'm back up towards 90 on P (no blowouts on flips) and a little less on I. I tried messing with D gains between 10 and 28, but that didn't seem to eliminate it either.

Any suggestions? Is my D gain still too high with the faster tail servo?
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Last edited by HoverMe; 12-21-2020 at 09:26 AM.. Reason: added details
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Old 12-20-2020, 04:41 PM   #2 (permalink)
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To add a little more background. The 360 cfx was locked in perfectly before putting in the KST215. The F270 was bought used and came with a bad tail servo. I had temporarily put in a stock spektrum cyclic servo (H3055) until the KST215 showed up in the mail. It didn’t have the fast shakes on circuits either. So I’m thinking it’s something to do with the KST’s, but not sure what to change.
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Old 12-20-2020, 05:48 PM   #3 (permalink)
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You could try running the refresh rate at 200mHz and see if that calms it down. It certainly seems that it is a problem with the KST servo and the system as it didn't happen with the other servo in your 360. It won't hurt anything to try it. Take care.

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Old 12-21-2020, 09:13 AM   #4 (permalink)
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Thanks Keyrigger. I'll try that today.

I updated the title of this thread because the more I think about it, the more I'm convinced it's something to do with the kst215 and the fc6250hx.
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Old 12-21-2020, 10:19 AM   #5 (permalink)
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My 4th KST215 v3 servo has been my bench test servo for a few years now, just was not a good tail servo and I did not feel like buying 2 more to use it on the swashplate.

MKS DS93i on the other hand makes a very good tail servo.
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Old 12-21-2020, 11:55 AM   #6 (permalink)
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The KST DS215 is not intended to be a tail servo. It can work in some situations but compared to dedicated tail servos it is suboptimal. I'd imagine that is at least part the source of your tail wag.
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Old 12-21-2020, 05:09 PM   #7 (permalink)
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Thanks all for the feedback. I put the stock Spektrum H3060 back in and the shakes are gone. I'll just save the KST as a back up for my cyclics (which are all kst).
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Old 12-23-2020, 09:26 AM   #8 (permalink)
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I run 4 KST ds215mg in combination with a FC6250HX on my Fusion 360 without any problems. Tail even seems to respond better than the H3065. When I get home I ll check what PIDs I have and send them to you.

Is your tail set up mechanically properly? I learned from CoeShow that in neutral (right after initialization) the tail rotor should slightly make the heli turn right with the nose as to already compensate for the left movement the frame gets as a result of the main bladesí movement. If thatís not set up properly the fbl/gyro will constantly have to Ďover compensateí the movement because the mechanical setup is faulty by default. This will lead to the wagging.

Hereís one of his videos:
How to Set your Tail Rotor Perfect on a R/C Helicopter and Why this Method Works. (8 min 52 sec)


I have my tail blades set to 3-5 degrees offset for a right yaw. Maybe your H3060 wonít show because itís a faster servo and is able to respond quicker to this mechanical effect.


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Old 12-23-2020, 10:08 AM   #9 (permalink)
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Thanks Fusion! Any idea how to turn off heading hold on the fc6250hx? Definitely worth checking. I've watched a lot of CoeShow videos, but hadn't seen that one so thanks for passing along!

With the H3060 servo back on, now I noticed when flying in reverse doing an aggressive loop the tail will sometimes blow out sideways. If I'm easier on it no problems. Or aggressive loops going forward is not a problem. No shakes. I think I need to up my P gain more.
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Old 12-23-2020, 10:39 AM   #10 (permalink)
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I didnít turn off heading hold, I just offset the tail by 3-5 degrees.


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Old 12-23-2020, 11:54 AM   #11 (permalink)
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I have replaced my H3050 cyclic servos in my 6S Trio 360cfx with KST DS215MG. FBL is AR7210BX. They work fine there, but can not meet the H3050 performance at 5,5V. I would never think about using KST DS215 for tail, way to slow compared to a H3060 servo.

The KST DS215 can barely work as smooth at 200Hz in my heli as the H3050 did at 165Hz.
For me, KST DS215 is fine for cyclic. Not for tail.
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