Twitchy Tail in Fusion 50

[aclysma]

I generally don't like to cross post but some sk720 wisdom would be much appreciated. Rather than repeating everything please see this thread:

https://www.helifreak.com/showthread.php?t=408052

The short of it is that with just the tail servo, motor, and sk720 mounted, and the motor disengaged from all gears and bearings, the tail servo twitches. I've replaced the motor bearings so I don't have any other ideas but to replace the motor entirely or use a different FBL unit that is less sensitive to vibration.

[Cascade]

Are you running the gov? Too high gov gain can cause this.
If you use one of your BeastX units without figuring out why the tail is twitching and still have high vibes you're simply ignoring the problem which could lead to catastrophic results if it's a mechanical issue.
To clarify, the SK isn't more sensitive to vibration than any other FBL unit, if you used a different controller on your heli with no vibration logging you'd be running in circles trying to find the problem.
A log file would be useful here.

[aclysma]

I'm not running the sk720 governor. I am running the cc esc governor with gain set to 12 (pretty low)

I realize that fixing the symptom rather than the problem is not good. I've spent over a month, many hours, and a non trivial amount of money trying to fix this. I know for a fact that the vibration levels are lower now and when I spin up the motor and touch the frame or sk720 I do not feel excessive vibration. The only bearings turning are brand new boca bearings I replaces in the motor. All I can figure at this point is funky emf as someone suggested in the other thread or something about the motor can causing a problem.

I will pull a vibration log and upload it in a bit. Earlier logs indicated fore-aft and left-right vibration (which aligns with vibrations coming from the motor.)

[aclysma]

Here are some vibration logs. They stay at around 0.5 or lower. As before the logs show mostly lateral vibration and significant fore-aft vibration (well.. they are a significant component of the <0.5 total vibes). Virtually no up-down vibration.

[georgi UK]

=] hi, sounds like you are having a nightmare, phew!

ok, and not to make more work for you but can you run a playback log so we can view the accelerometer logging, the latest rev 3.11 offers better acc. filters.

did you try Tx curves with the CC gov off?

[aclysma]

I will follow up with that log and try turning off the CC governor later today. Thanks for the help!

[aclysma]

I'm at the field now. I would like to know if there is a way to update the firmware here. I have 3.11 on my laptop. 3.02 firmware. I would expect there is a way to do this without Internet access since I have the latest software installed.

[georgi UK]

=] not possible without a www connection, it comes from the SK server.......

[aclysma]

Here is a vibe and flight log from this afternoon. The castle governor was off. The logs are from sequentially running the motor from zero throttle to full throttle back to zero again. I will upload youtube links of both of these runs later today.

Also I am hoping the firmware update just fixes the problem. I have internet now so I'm going to try that. The logs that I'm uploading now are firmware 3.02.

[aclysma]

The new firmware did not resolve the tail twitch, so I'm pretty much stuck unless anyone has further suggestions.

[georgi UK]

=] no playback log? this has to be selected in the software.....

[aclysma]

These videos correspond to the two logs that are in the zip file above.

Vibration log video: [ame]http://www.youtube.com/watch?v=whMevl6wUuc[/ame]

Flight log video: [ame]http://www.youtube.com/watch?v=wpPf9FW691A[/ame]

[aclysma]

Look in the zip file. If you don't see it let me know. I think I mean to say "playback log" where I said flight log.

[georgi UK]

=] you need to get another servo on that toot sweet, I think the pot can't handle the vibes?

[aclysma]

Here are some observations that I think are relevant:
- I think there is a legitimate vibration that causes the twitch at a very particular RPM. There tends to be a high pitched noise that coincides with the twitch. This twitch happens for a fraction of a second at about the same motor RPM on every spool up.
- It may be that this vibration is at a high enough frequency that I cannot see it or hear it (or that's the high pitched noise I'm hearing!)
- When I turned off the governor and went 0%-100% using the soft start I tended to get one clean twitch rather than occasional twitching, particularly as the ESC begins to oscillate/converge on the desired RPM.
- I flew this sk720 unit with these settings mounted in the same place as it is now on this helicopter with this motor for about 15 7 minute flights the week I received it and don't recall any problems. At the time I had the castle ESC set up with too low a gear ratio such that I was unintentionally running at 1700-1800 rpm (LOL). I am now running at 2000-2100.

So now I'm thinking this problem has been around from day 1 and I didn't see it before because I was not running anywhere near the "problem" rpm. But with the corrected gear ratio my motor RPM is bouncing around that problem RPM.

My conclusion for now is that this is a vibration causing the sk720 to twitch the servo, but it is well within the "acceptable" range and is not hazardous to fly with. Perhaps the vibration's amplitude is accelerating quickly but never actually becomes significant?

I plan to set up a beastx and fly it like that if I can't find the problem tonight (I'll leave the skookum mounted and powered through the bec to collect more logging data.

[aclysma]

If I plug in the servo to a servo tester to hold neutral while the helicopter spins up I do not see this problem. Only when it is hooked to the sk720. I have also run the servo cable far around the heli to eliminate the possibility of emf from the motor interfering with the pwm signal.

[rdarby777]

just my two cents and maybe im really far off but arent vibes below 1 in magnitude really good??? I run mine around two and three and your log shows vibes below .5

[aclysma]

Yes. It's unrealistically good. The only reason I am getting .5 is because there is nothing but the motor spinning.. it is disengaged from all the gears. And even so it seems like it is over-reacting.

[aclysma]

I now have a beastx mounted and everything works perfectly on the bench (without the blades of course). Vibration was still less than 1 although I'm not sure I was completely spun up. The sk720 is still mounted and receiving power so I can measure vibrations during flight tomorrow.

[rdarby777]

Well I have the twitchy tail syndrome as well. Mine is fine until the governor takes the head speed up to 2700(I have a trex 500). I want the high head speed so im dealing with the twitching. Try backing the head speed down and see if it goes away. Also my cyclic gain was too high and by reducing that as we helped correct the tail at lower rpms. I'm no expert here just dealing with similar problems so I figured I would let you know what I have tried so far. Hope u get it figured out cause I know how frustrating it can be.