Different rpm for bottom engines at coax possible?

HI!

That's my first post, although I'm reading this forum for years now. Hello to everybody!

I'm struggling with a big coax X8 machine with 29" props, based on the Pixhawk. It's flying nicely, but from what I can tell from the telemetry data, I think it would be possible to get it a bit more efficient by running the bottom four engines a bit faster.

With a coax octo you should use props with bigger pitch on the lower engines, since the air coming from the top props is already moving quite fast. Since there are not too many 29" props available and I'm therefore using the same props on all engines, I would like to increase the rotations of the lower engines instead. But I can't find a mixer table or something similar in the latest firmware. 

It would be possible to flash the bottom escapes with a firmware with a different signal-speed-characteristic, but it would be easier to do that in the main controller instead. 

Any suggestions?

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Replies

  • I see the coax differential speed thing is logged as an issue, but it looks like nobody has got to it yet:

    https://github.com/diydrones/ardupilot/issues/1467

    Typically you want the lower props 10-15% faster than the top, and to run at a higher pitch. Whether you want the lower prop to be a different size depends on the distance between the props and whether you're purely hovering or moving (if you fly fast the lower prop will never be entirely within the upper prop's thrust).

    When hovering, the airflow from the top prop is initially sucked in to a smaller diameter (you can see this with vortex tip condensation on manned helis hovering in high humidity), so if the lower prop is close then you want it to be smaller to stay within this uniform airflow. But that airflow quickly expands again to become even larger than the top prop, so depending on the distance between them you either want the lower prop smaller, the same, or larger. Hard to say which is best without testing on a thrust stand.

    But for sure the differential top/bottom speed issue needs to be supported for optimal coax performance, other platforms have supported this for a while and people report big improvements.

  • T3
    29" inch props, that's amazing. Do you have a picture you can share? With regards to your issue, have you thought about moving your battery to center the weight? These days people are keep the same size top and bottom.
    • Sorry, I can't share a picture at the moment, see PM.

      Well, batteries are as close to the center as possible, but the main payload of about 7kg is in the center, take-off weight with a very sturdy but heavy frame is about 19-20 kg. 

      I can see in the telemetry that the top engines are using quite a bit more amps than the bottom ones (edit: this is not because of a torque problem or off-axis engines - I'm flying "one quad above another") . Currently I am hovering with 88-89 A at 6s. I would like to come down to 80 amps, which should be possible be equalizing the load on both engine levels. 

  • Nobody? 

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