Twitchy Tail

[aclysma]

Don't think so but just in case I don't understand your question here's a picture. Let me know if you want me to check anything else.

[spence46]

No your's looks good.

If you have the pinion to low on the shaft it will rub the bearing housing and make noise/ cause vibrations.

[aclysma]

I can't get the bearings out without a press and I don't own a press. I am pretty sure the inner bearing is 12mm outside, 8mm inside, and i don't know how wide it is. There may be 2 bearings there but I can't see well enough to know for sure.

The outer bearing I'm not sure on. It's definitely 8mm inside but outside it looked like 15 or 16mm. I can't find my calipers so I used a hardware sizing card which is less precise.

[BaconRaygun]

I have a slight tail twitch too, but I attribute it to the slop in my tail slider bushing...

[Chris Sexton]

I apologize if this was said before, but did you try replacing the servo? Does the servo have gear train slop? If this was discussed before, I am sorry, I didn't have time to re-read, but after having some gear train issues with my other savox servos, I thought I would throw it out there.

I know you have a video of the servo dancing, but maybe what you are seeing is servo issue?

Just trying to think outside of the box man.

[aclysma]

I have the motor apart otherwise I'd check since it'd be very quick and easy to do. I don't have a 600 size tail servo to test yet but if it's on a bench with nothing attached I could probably use any quick servo. Once the motor is back together I will check this. But given that the motor makes a high pitched whine, that I can feel the vibrations by hand on the frame, and that the sk720 is known for being more sensitive to vibrations than others, I'm positive that a motor bearing change would fix the problem. Thanks for the idea though - I will definitely confirm this when I have the motor together and attached again.

[aclysma]

Progress Update: I got the bearings out. The measurements are:

One 8x16x5 bearing
Two 8x12x3.5 bearings

Planning to get the rubber sealed bearings from boca bearings. I'm assuming that I will not need to lubricate them since they are sealed? I'm really not sure.

[aclysma]

And I should add that the bearings feel glassy smooth.. I still think that's the first thing to try replacing though.

[Slyster]

Thanks for the info.. I tucked the bearing sizes in the Fusion 50 bible for anyones future reference.

[BaconRaygun]

Here is the entire bearing list for those who need it.

Fusion 50 (FBL/FB)

Main Grips:
· 8 x 14 x 4mm (x4)
· Thrust- 6 x 14 x 5mm (x2)

Tail Grips:
· 5 x 9 x 3mm (x4)
· Thrust- 4 x 9 x 4mm (x2)
Frame:
· Elevaitor control rod- 5 x 10 x 4mm, flanged (x2)
· Elevator A-arm- 3 x 6 x 2.5mm, flanged (x2)
Drivetrain:
· Main shaft- 10 x 19 x 5mm (X3)
· Pinion support- 5 x 13 x 4mm >or< 6 x 13 x 4 (more popular)
· Oneway hub bearing- 12 x 21 x 5mm
· Oneway bearing- 14 x 20 x 16mm
· Bevel gears (front and rear)- 12 x 18 x 4mm (x4)
· Tail rotor shaft- 5 x 11 x 4mm (x2)
· Counter gear assembly- 4 x 9 x 4mm (x2)
· Torque Tube- 8 x 14 x 4mm (x2)
Tailcase:
· Bell crank lever- 2 x 5 x 2.3mm, flanged (x2)
Flybarred head: (mixing arms, etc... )
· See Saw- 4 x 7 x 2.5 (x4)
· Flybar rod holder (inner see-saw)- 3 x 6 x 2.5mm (x2)
· Upper and Lower mixing arms- 3 x 8 x 3mm (x8)
· Radius arms- 2 x 5 x 2.3mm, flanged (x4)
Flybarrless head:
· Swash driver leveler arms- 3 x 7 x 3mm (x4)
· Radius arms- 2 x 5 x 2.3mm, flanged (x4)

[Slyster]

Thanks.. adding that to the 50 bible too!

In case people want to bookmark it.. it's just a collection of info like the above list.. I have to work on it more though.

http://www.slyster.com/heli/fusion50bible.htm

[aclysma]

I replaced the motor bearings with Abec 5 steel bearings. This did not resolve the problem, although the vibes on the helicopter have reduced significantly. I left the motor disengage from the drive train.

During the first part of the video I check servo response and apply pressure to the servo to confirm that it is acting properly.

Then I plug the motor in and run it up and down several times. During the run-ups I do not touch the rudder.
- There are significantly less vibes that I can feel when I touch the frame
- I the high pitched whine is still present
- The tail servo still twitches when the high pitched whines "peak" - some sort of resonant frequency?

I still think something is mechanically wrong. I think I may need to find a different motor to attach to potentially prove the motor is doing something that the sk720 doesn't like.

That all said I am thinking if I put a beast x unit on this heli, the problem will likely go away.

YouTube

YouTube

Any ideas?

[aclysma]

Three more things I did:
- I removed the bearing block that supports the motor shaft.
- I put my hand on the frame near where the skookum is mounted and did not feel vibration during the high pitched "peak" that tends to cause the sk720 to twitch the servo.
- I put my finger across the servo wire terminals to feel if there were vibrations coming from the wires. I could feel no more vibration than I did on the wires and I didn't feel any excessive vibrations during the high pitched "peak" where the servo twitches.

[aclysma]

One last thing. I went ahead and pulled the gyro mounting plate and let the plate with sk720 attached hang down by the wires. As expected, the tail servo did not twitch.

This demonstrates that it must be the sk720's response to vibes - no shorts in cables, funky emf messing with pwm signals on servo wires, etc.

[SimStream]

Sounds like your picking up some interference. Try a different position for the Sk

[aclysma]

FYI I cross posted in the sk720 thread to hopefully get some opinions from people familiar with the sk720. I have some vibration log files and screenshots uploaded there.

http://helifreak.com/showthread.php?t=413224

[aclysma]

EDIT: Meant to post what was here in that other thread. Oops!

But while I'm bumping the thread, so far I have not found a solution to get the sk720 to work. I put a beastx on and it's flying very well. But, I really want the sk720 to work so that I can use the governor.

[Chris Sexton]

Originally Posted by aclysma

EDIT: Meant to post what was here in that other thread. Oops!

But while I'm bumping the thread, so far I have not found a solution to get the sk720 to work. I put a beastx on and it's flying very well. But, I really want the sk720 to work so that I can use the governor.

Could it be a defective 720? The beastx is not that forgiving for vibs. Every problem I have ever had with a beastx heli was due to vibs. If it not present with the beastx I think I would question the fbl unit.


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