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?

You need to be a member of diydrones to add comments!

Join diydrones

Email me when people reply –

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? 

This reply was deleted.

Activity

DIY Robocars via Twitter
RT @a1k0n: Did I get rid of hand-tuned parameters? Yes. Am I still hand-tuning more parameters? Also yes. I have a few knobs to address the…
yesterday
DIY Robocars via Twitter
RT @a1k0n: I'm not going to spoil it, but (after charging the battery) this works way better than it has any right to. The car is now faste…
yesterday
DIY Robocars via Twitter
RT @a1k0n: Decided to just see what happens if I run the sim-trained neural net on the car, with some safety rails around max throttle slew…
Monday
DIY Robocars via Twitter
Saturday
DIY Robocars via Twitter
RT @SmallpixelCar: @a1k0n @diyrobocars I learned from this. This is my speed profile. Looks like I am too conservative on the right side of…
Saturday
DIY Robocars via Twitter
RT @a1k0n: @SmallpixelCar @diyrobocars Dot color is speed; brighter is faster. Yeah, it has less room to explore in the tighter part, and t…
Saturday
DIY Robocars via Twitter
RT @a1k0n: I'm gonna try to do proper offline reinforcement learning for @diyrobocars and throw away all my manual parameter tuning for the…
Friday
DIY Robocars via Twitter
RT @circuitlaunch: DIY Robocars & Brazilian BBQ - Sat 10/1. Our track combines hairpin curves with an intersection for max danger. Take tha…
Thursday
DIY Robocars via Twitter
RT @SmallpixelCar: Had an great test today on @RAMS_RC_Club track. However the car starts to drift at 40mph. Some experts recommended to ch…
Sep 11
DIY Robocars via Twitter
RT @gclue_akira: 世界最速 チームtamiyaのaiカー https://t.co/1Qq2zOeftG
Sep 10
DIY Robocars via Twitter
RT @DanielChiaJH: Always a good time working on my @diyrobocars car at @circuitlaunch. Still got some work to do if I’m to beat @a1k0n howe…
Sep 10
DIY Robocars via Twitter
RT @SmallpixelCar: My new speed profile for @RAMS_RC_Club track https://t.co/RtLb7TcgIJ
Sep 10
DIY Robocars via Twitter
RT @SmallpixelCar: Practiced at @RAMS_RC_Club today with my new @ARRMARC car https://t.co/AEu2hCx89T
Aug 28
DIY Robocars via Twitter
Aug 24
DIY Robocars via Twitter
RT @gclue_akira: 柏の葉で走行させてるjetracerの中身 #instantNeRF #jetracer https://t.co/giVvuE4hP7
Jul 4
DIY Robocars via Twitter
Cool web-based self-driving simulator. Click save when the AI does the right thing https://github.com/pncsoares/self-driving-car
Jul 4
More…