Z Axis Vibrations

Hey out there in Arducopter land.
Hope everyone is good. I got one of my large birds out today for some testing after upgrading to the Pixracer r15 to a pro. I was hoping the internal dampening on the pro would help on the vibration front.
I have the machine flying reasonably well now, knocking down one issue after another but vibrations I just can’t seem to beat and so looking to the community for any thoughts.
Its clear to me that the vibrations increase with speed, the logs show that in detail.
All the props are balanced, the wires are short and flexible as they connect to the FC. The motors are props are up off the arms.
The vibration dampener works great on other drones so I assume it does here as well.
Yet I am plagued with high vibes. While I see an increase in vibes on all axis with speed, Z is the worst.
Any thoughts would be appreciated.

I never use damping on the PRP or other mRo FC on the mid size frames. Could be worth a try to remove the damping and fly it.

you get a little air flow as well.

Really hard mounting the pixracer pro. Never considered that.

I talked to Phil a few year ago and we seem to agree that vibration dampeners can add a secondary oscillation or has the ability to.

Well that’s interesting.
I can give it a try.

See if it helps. Here is the x2.1 777 mid sized frame 10" props hard mounted.

well I am shocked. Now my machine is bigger 18 inch props…but I am willing to give it a go.

We use vibration dampening in the motor mounts. It removes a lot of vibration at the source of the problem.

What do you mean at the motor mounts…what are you using.

Hard mounting an FC with an isolated IMU seem to work OK if the frame has high rigidity. Based on plenty of anecdotal evidense on this forum a blanket “must be hard mounted” isn’t always the most effective. I do agree that it’s a worthy goal.

I am in the hard mount camp for years now. I do recall notes back in the Cube Black days said No to vibration dampeners. A little 3M tape is all we needed for the black. However once you go to a large frame all bets are off.

Rubber vibration absorbers between the motors and the arms

You mean like rubber washers or a a flat piece of rubber between the motor and mount. Do you have any photos. I have a pack of silicon rubber washers that I could put between the motor and mount. They are 3mm ID.

This is the drone I am having issues with.
I don’t know if the answer is soft mounting the motors and hard mounting the FC.
But I confess I will try anything.
While I am not having a problem with clipping, I fear a vibration compensation accident and so want to knock this down.

You have an added complication with that sub PCB board. You may recall our late friends efforts on his X8 to reduce Z-vibes with several different attempts at mounting the FC to the PCB and the PCB to the main frame. The best result came when he got rid of the PCB and mounted the FC directly to the frame plate. It wasn’t a pleasant choice because it really messed up what was very clean wiring. I still have those logs… Not suggesting what you have to do here just some additional info. Bottom line was the PCB had a mode of it’s own.

Yes I recall his challenges.
I have been pondering the PCB of late. It is on soft mounts. I have been kicking the idea around of moving it to hard mounts. It’s not a hard things to do just a bit of time. Was thinking that I would do that first before going to any change in the FC mount. I need to slowly rule out causes. However the PCB did make me wonder. I confess I am not about to get rid of the board as it cleans the build up big time. But I will explore hard mounting it,

Back to the shop/.

Ok I moved the PCB to Hard mounts this morning. Will see about getting it airborne before I move on to FC changes.

I’ve tried that exact propeller style over the years. 12, 14 and 15-inch. No cookie.
I’ve settled to less pitchy props, mainly b/c Z-vibes.

Ok so I am making a list of suggestions then
Hard mount the FC
Hard mount my PCB - done
Soft mount the motors.
Change the props for less pitch versions.

I am going to slowly implement each suggestion and see which has the best result.

thanks @ThePara for the suggestion.

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At least check the tracking of the propellers, the Z vibration is usually relative to propeller tracking not balance. Most out of the package props come balanced well enough to use with out balancing yourself however tracking is not. I have had bad props out of the box balance fine but track poorly.

Also most propellers have a resonance rpm area that when a specific rpm is reached the propeller will vibrate. You may notice the vibration change as rpm changes. Sometimes the propeller will flutter at this rpm and if the rpm changes the vibration goes away. Its difficult to know this without testing as, most RC hobby propellers don’t go to these kind of testing and or post these findings. Its very common in general aviation. I usually try another manufacture of prop, as slight differences in material or composite can be drastically different in performance in regards to vibration.

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