X8 coax thrust induced vibration

Hi all, 

Since moving to high-end motors and props I have had very low vibes in all my craft, so I was surprised to run into vibration issues in a recent build, my first X8. I did all the usual things of (re)checking all my motors and props for balance (both hub-wise and length-wise), checking prop tip alignment, etc, but still had the issue.

I thought it could be the turbulent air between the coax props that was causing it, and this thing has plenty of thrust to fly with only one prop in each pair so I did a test whereby I flew exactly the same load with fresh packs three times:

1) with both coax props

2) with only the lower props

3) with only the upper props

And voila... I see the usual low vibes with single props and much higher Z-axis vibes when running the coax props.

Coax Props (I flew this twice to be sure):

3691183513?profile=original

3691183615?profile=original

Lower Props Only:

3691183530?profile=original

Upper Props Only:

3691183244?profile=original

I'm still not sure what the general solution would be, but for now I've at least got the vibes within reason by adding damping at the motor mounts and additional damping under the Pixhawk.

When I get some time I'll try a modified motor config to spin the lower props in the same direction rather than contra-rotating, I suspect that might help (someone else suggested this would lessen the impact from pressure waves)

But for now I just wanted to point out my finding in case anyone else was struggling with the same issue.

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Replies

  • MR60

    Your vibrations levels are perfect (+1/-1). So what is the issue really ?

  • How stiff are your props?

    Perhaps you can try stiffer ones, or smaller diameter 3 bladed ones.

    • 18" CF props from TMotor, they don't get much stiffer than that! Being larger slower turning props I'm guessing this is a frequency thing that you wouldn't get on smaller faster props. 

      There's approx 10cm spacing between the props, btw.

      Stuart: the booms are very thin (10mm wide box aluminium, 25mm tall) so they don't block much. I tried adding aerodynamic fillets to the top+bottom in the rotor area in case the box shape was causing additional vibration from turbulent flow but that didn't make any appreciable difference

  • I know designers of D-27 engine and SV-27 propfan for Antonov An-70 had some problems with vibrations and extra noise. After initial testing program they put propellers bit far from each other. But you should admit that SV-27 propfan utilizes 8-bladed front propfan and 6-bladed rear propfan. So front and rear propfans thrust definitely nonequals. Maybe you should work on optimal front/rear rpm ratio wich I think is rpm dependent.

    Picture of D-27 with SV-27 attached

    image.jpg

    https://storage.ning.com/topology/rest/1.0/file/get/3701945583?profile=original
  • Another possibility over your idea is a resonant frequency. the single props setups will probably be revving higher than the coax setup to get the same overall thrust.

    I changed my quad to X8 and didn't see what you have seen, both about the same.

    Interesting seeing your upper and lower only though...in theory lower is better in regards to beating doe to the air flow being blocked by the boom. Yours looks pretty close.

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