Strange Vibrations (SOLVED)

[RyanW]

Update- problem solved, not TDR related, see page two.

Hello Gents, I have racked my brain trying to chase down what is making my TDR want to try to destroy itself when I go into Idle Up 2, which is around 1850.

The video below (very poor quality, I am sorry) shows what it is doing. If you fast forward to 1:20 you can see what happens when it is put in the 1850+ range. This also happens to a lesser degree all the way down to 1750. I recommend make the video full screen and full quality, as it appears YouTube compressed it heavily.

So far I have gone through each sub assembly and the only thing I found after a very thorough investigation was that the replacement (non OEM) thrust bearings had too large OD for a free sliding fit inside the grips. I measured all standard 8x16x5 thrust bearings and came to the conclusion Jan must special order smaller OD bearings. He confirmed this morning that he does in fact turn them down on a lathe. This is good information, as 8x16x5 thrust bearings are available in many places, but they might bind if they aren't OEM from Jan. I modified (turned down) some new Mikado units I had to the same specs as Jan's. I thought the problem was solved. No so (video is from after the thrust bearings were changed again).

I went through the steps to check the vibrations with the VBar unit. With blades off I had the following:

1150 rpm- 106 max
1500 rpm- 119 max
1850 rpm- 191 max

I thought it might be the blades, so I balanced them and tried two different main blades and three different tail blades. All of the gear meshes are set correctly and the system is very smooth (thank you DryFluids) and spins forever after landing.

I double checked all of the bearings in the torque tube system.

I swapped the white bushing for black, thinking this might be more tolerant of vibration resonances.

When it first did this (with the white bushings and the thrust bearings where they could lock up) I had a HUGE vibration spike that resulted in the VBar giving two warnings (this is when it effectively 'blanked out')- Extreme Vibration and Elevator Sensor Value out of Range.

I haven't downloaded the VBar from today's flight yet, but it was doing the same thing with an added tail blow out.

On my list now is to try a Mini VBar in place of the standard one (even though the standard one was new with my Logo 700 and has worked perfectly).

I don't really see the FBL unit being a cause, but rather effect from severe vibration. I am just stumped at what would kick in at 1800+ to the point that it completely throws the FBL sensors out of range.

[JRAY]

Have you checked for any issue with tail servo linkage or the servo itself? I'm no expert at all, but IMHO/experience, that is either caused by the FBL(that could be the incorrect servo frequency or frame rate), binding tail linkage, a bad servo, or low head speeds also, which is obviously not the case here. My TDR which maxes at 1950 rpm, doesn't do that, and holds perfectly when I go up to idle 2. I'm running an AR7200BX, with rpm settings of 1600, 1800, and 1950, by the way.

[Sealord511]

Since I dont have a VBar I dont know what kind of vibration values are still good. Could the vibration come from the motor? Torque Tube Bearings would be also a possibility, but you wrote you already checked those.

How about trying a test without the complete tail and without blades? That could at least eliminate the tail assembly including the torque tube as the reason...

[RyanW]

Motor is on my list. I have access to a very old Pyro I might try.

The vibration levels are very low for VBar I believe. The tests were with no blades, as I had to spin it up to get the live measurements.

Here are some screen shots of the vibration analyzer:

[RyanW]

One more piece to the puzzle... it appears my headspeed ranges are off. I just did another run up, but monitored the Kosmik live view as well as optical tach and the region that I thought was 1500 is actually measuring 1350 and the region I expected to be 1800 measured 1670. That reminds me of guys saying there is a resonating frequency around 1650 to 1680. The only thing is that is supposed to clear up and when I increased it to what I now have measured as 1710 it isn't getting better.

I will increase these well beyond 1800 and try again tomorrow... with my helmet

[helicraze]

Ive had faulty motor and esc wire cut into frame cause weird stuff! Also severe vibes from tail

[laserfish]

I had similar problems , after maiden flight.
I had not tightend the grub screw on feathering shaft enough it allowed the shaft to slip, when blade grips pulled in and out there was slightly more playing one side.
I replaced the damper o rings and added optional shim
between blade grip and black bush. see manual!
Ended up at 1830 rpm for super smooth vibe figures.
Hope this helps.

[Mercuriell]

Well there is obvious unwanted cyclic servo movement when switching to 1800 - increased rpm per se will casue increased vibes and there may be a resonance phenomenon.

CHange the IU2 to a 100 rpm either way and see if it still does it - if so not a resonance phenomenon but an increased amolitude of vibes with increased HS.,

On the bench jiggle servo wires from servo to plug into VBAR module.

Tap the VBAR module live and watch for servo misbehaviour

Hook up VBAR to PC and look on the gyro indicator bars - watch for any sudden defections - could be bad sensor - most common elevator.

Post up the VBAR log.


I'd put money on the VBAR sensor.

[RyanW]

What is the consensus on white bushings- with or without shims?

Two more tests today, then trying my mini VBar.

It could definitely be a sensor issue, but there is a vibration activating this scenario. I want to make sure I f ind and fix the root cause and not just mask it.

[mkovalcson]

Originally Posted by RyanW

What is the consensus on white bushings- with or without shims?

Two more tests today, then trying my mini VBar.

It could definitely be a sensor issue, but there is a vibration activating this scenario. I want to make sure I f ind and fix the root cause and not just mask it.

I've heard a number of people have their VBar units fail. If that is the root cause you are fighting a loosing battle.

[Sealord511]

On the white bushings you habe to use the shims or you would have unwanted play in the system. I tried the rigid head with white bushings one flight and went back to the normal wobble head with black bushings. The helo didnt felt well and I think it had some drift too, but not that extreme like yours. Same for me, 1400 was fine and 1700 and up got worse.
But could also be the liwer temperatures on that flight or the worn out nut on the rotor head/main shaft connection. Lower temps seem to have a great impact on the rigid head, some guys over here have problems at lower temps too.

With the wobble head my TDR flies great between 1350 and 2000 rpm.

[wblombard]

I fully agree with John here! Def the vbar.

Originally Posted by Mercuriell

Well there is obvious unwanted cyclic servo movement when switching to 1800 - increased rpm per se will casue increased vibes and there may be a resonance phenomenon.

CHange the IU2 to a 100 rpm either way and see if it still does it - if so not a resonance phenomenon but an increased amolitude of vibes with increased HS.,

On the bench jiggle servo wires from servo to plug into VBAR module.

Tap the VBAR module live and watch for servo misbehaviour

Hook up VBAR to PC and look on the gyro indicator bars - watch for any sudden defections - could be bad sensor - most common elevator.

Post up the VBAR log.


I'd put money on the VBAR sensor.

[JRAY]

Originally Posted by Sealord511

With the wobble head my TDR flies great between 1350 and 2000 rpm.

Same here.

Good luck, Ryan. Hope you get it figured out.

[RyanW]

Did another set of tests today. Checked tail thrust bearings, went through various headspeed ranges, more blades and tail gains. Same symptoms.

I did get it to stay somewhat stationary at one rpm, but when I moved the tail quickly and let it stop hard it sent the entire machine into an oscillation that caused the forward flipping tendency.

Next on the list is to leave everything as-is and do the same test with the mini-VBar I have.

Here is a little video of a run up I did at the field with no blades to do a sensor check and see if the swash starts 'dancing' at any rpm. No problems on the bench and the vibration levels during this run up were relatively low (65 to 150 on the VBar live view).

For some reason my videos are very grainy on YT...

[CACN]

Are you using the Toroidal rings on your ESC cables?

[firemedic295]

CACN.
+1.

[mkovalcson]

Originally Posted by CACN

Are you using the Toroidal rings on your ESC cables?

I flew my TDR for a year without the ferrite rings on my SK-720 with no issues. I don't know if the VBar is more sensitive.

If this were mechanical the helicopter would be tearing itself to shreds rather than doing involuntary flips. I'm betting the VBar is doing very bad things.

I'd definitely put the mini-VBar on there as a reality check!

[CACN]

Helijive and Powerjive won't likely have problems without ferrite rings. However, for Kosmik it is highly recommended.

Read here: https://www.helifreak.com/showthread...hlight=ferrite

[Vinger]

Correct me if I wrong, you tried both the full size and mini VBar? If the vibration stayed, then it is obviously NOT the VBar. Have been flying my VBar TDR since 2010 with ZERO hassles.

Another source of vibes is the TT, check that the shaft is not to long, been a common problem, take about 1-2mm of the length and make sure it is centered to not bottom out on either side.

[RyanW]

Ferrite rings- yes... on both master and slave leads.

Mini VBar is up next. If that isn't it, then I will check the TT. Thanks for the info Vinger!

I think I mentioned before, but if not the live view on the VBar vibration log (no blades) showed the vibrations, which were not bad, actually went down with increased headspeed.