Severe vibration issues

[hogster]

Hi all, it's me again

I'm pleased to say my AQ-equipped X8 left the ground under its own power today, and nothing caught fire, or smoked, or did anything untoward!

That is apart from the enormous vibration levels my platform seems to suffer from - see attached.

I've heard that normal levels of vibrations are in the region of +/- 1G ... not 10!

I'll post a picture of the frame in the next day or so, but it basically looks like this:

http://shop.carboncore.co.uk//cortex

with the AQ on the top, like this:



I ran up each prop in turn and added tape where necessary to make them run buttery smooth, so I'm not quite sure where +/- 10G of vibration is coming from!

Unfortunately the level of vibration present was passing very noisy demands through to the motors, so it was virtually unflyable in this condition

Any pointers as to where I could start looking to isolate / remove this vibration would be much appreciated. Is it worth seeing how much G is created by just one prop spinning up on its own?

Believe me, I can't wait until I can start creating new threads with stories of successful flights rather than problems!

Many thanks for your help David

[chschmid]

How does your AQ box look inside? Touching the board?
Are your props balanced?
How rigid are your motomounts and armholders?
AQ likes a very rigid frame, well balanced drives and nylon standoffs for the FC.

Cheers
Christof

[hogster]

Hi Christof,

Thanks for the reply. I also wondered if the box was adversely affecting the vibration, so I did another quick flight without the box ... and it seems exactly the same (see attached).

I noticed that the more throttle I gave, the rougher the motors sounded, as if they were being driven by very noisy signals. So I did a test where I created a motor mixing table without any Pitch, Roll or Yaw components (just throttle), and I throttled up all the props on their own, then just the top props, then just the bottom props – all until the platform started to become light on its skids (see attached).

So with no noisy motor demands (the motors sounded very smooth during these tests), there's already +/- 4G of vibration when both top and bottom props are spinning, and that's well before reaching take-off power.

I imagine a coaxial setup will always have more vibration than a non coaxial setup due to the interfering air between each pair of props? But +/- 10G still sounds far too much!

Should I look at prop balance again, or look to mount the AQ in a more vibration absorbent manner?

Sorry, to answer your earlier questions Christof, I did balance each prop in turn by attaching it to a motor and spinning it up on the bench. Some were very smooth, others needed a little bit of tape added to make them as smooth as the others. The carbon fibre motor mounts and oval carbon-fibre arms are very stiff I would say.

Many thanks for your help, David

[Max]

Well, one obvious question is why are the bottom-only plots more noisy than the top-only? Did you try running each motor separately and compare those outputs?

I think the primary source of extra vibes on a coax are mostly due to the arm stiffness. As per Angel's vibration analysis study, except there's twice the force acting on the arms, so any frequency issues are also multiplied (exponentially?). I'm guessing you have at least some of that going on since the combined total ACC level is greater than either of the other 2 tests.

What's the ID, OD, and length of those arm tubes?

-Max

[chschmid]

Hi David

Give Max the tube info. I guess he will check your frequencies.

Meanwhile you could statically balance your props. You can use such a tool:

If you use tiger Props, you should be fine. They should be dynamically balanced.
Make a "vib log" without props to make sure your mots are fine.


Cheers
Christof

[hogster]

Thank you for your quick replies guys

Max - I'm not sure why the bottom plots would be different, other than that they're different size and pitch props – the top props are 15 x 5 and the bottom props are 16 x 5.4 (T-Motor).

The motor arms are 19mm wide, 24mm high and have a wall thickness varying from about 1.34mm on the 'flat' faces of the tubes going down to maybe 0.5mm on the radii ... The tubes are a fraction over 23cm long, and from the inner end to the centre of the motor is 26cm.

I think my tests are a little flawed in that the props were running at three different speeds for each section – I was simply throttling up as much as I could before the craft became too light on its skids. However, it does quite clearly show that there is a lot of vibration present in the frame when the props are running.

Maybe I shouldn't have tried balancing the props myself by just adding pieces of tape here and there? It seemed to help with the rotational speeds I was running on the bench, but maybe I was introducing resonances at higher RPMs that I wasn't aware of. I'll try removing all my tape and running the tests again.

Christof - Yes that would be worth doing. I actually have a magnetic prop balancer but I've found the centring cones are mis-shaped, and so depending on how they're positioned, they will give different results! I think I need to invest in a better balancer ...

Many thanks again for your help guys David

[Max]

[hogster]

Hi Max, thanks so much for your detailed reply

Yes they're the same motors top and bottom, and at the moment I'm using the attached motor mixing table. I think I remember reading that it's worth dropping the values on the lower props because they're larger and higher pitch? Do all the values need to be reduced?

Sorry yes it is the CarbonCore Cortex. Unfortunately this is how the Cortex is shipped (with 15x5 on top and 16x5.4 on the bottom). I believe this is the generally-accepted way of setting up a coaxial platform, to have a higher pitch prop on the bottom (although not necessarily larger diameter, but you don't have a choice with the T-Motor props). They seem to have no particular problems with this setup, by the looks of their videos anyway:

http://shop.carboncore.co.uk//videos

That's not to say all their platforms are perfectly smooth and vibration free ... It's just I'd rather not drop another ~£120 on a set of 15x5 clockwise props for the bottom if the prop combination is already tried and tested.

The motor mounts look flimsy but actually they feel incredibly rigid ... but then I've got nothing to compare their rigidity against If I grab the tips of the upper prop and gently try to twist the motor arm (and prop) there is naturally a little bit of flex, but it doesn't seem inordinate given the size of the prop and the length of the motor arm.

Good point about setting a limit to the throttle mix for doing the tests, I hadn't thought of that! I will try to repeat my tests with the tape removed from the props, and also whilst filming the motors / props from the side. To be honest from my bench testing, and these last tests in the garden, flex in the motor arms definitely wasn't something that sprang to my attention.

I've actually used that DuBro at my last company, and used it to balance all the props for my Hexacopter, where the magnetic balancer failed! I just need to buy my own now as I no longer work at the company

Many thanks again for your help

David

[Max]

[Max]