X8 Motor Output, shoud it be balanced?

Just built a large/heavy X8 config running 3.3.3 - 12 pounds and 17.5" props. Tiger 80A ESC and KDE 360 kV motors.

On the initial flight test the X8 would yaw left without input and even at full yaw input would barely rotate to the right. The log files show a pretty large difference in mean output between the CCW motors (outputs 1,3,5,7) and the CW motors (outputs 2,4,6,8) (see attached). I can't find conclusive information on this, but logically it seems like this should not be occurring. I'd expect all 8 motors to have output levels about the same when at a steady hover, is this accurate?


 If so, what could be causing this? Obviously I've checked and rechecked the motor connections, spin direction and prop direction.

x8 motors.jpg

2016-03-22 11-06-24.bin

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          • Jimmy may be correct for the x4 and x8.

            My understanding is that yaw is created by a difference in power to the clockwise vs counter clockwise props.  Not top and bottom.

            The Y6b has all clockwise props on top and all counter clockwise props on bottom, so in this case it is tops vs bottoms.

            • T3

              Jimmy mentioned X8.  As per the image I linked, X8 is not all one direction on top and bottom.  Even if it were, the top and bottom motors could not share the same PWM signal because they are different CW vs CCW directions and need to spin differently to yaw.

      • Interesting that the Y6b is set up for all the top props to be clockwise rotation and all the bottom props counter clockwise.

        Each motor will never have the same output as any other motor in any arrangement due to the continuous changing environment.  The outputs to each motor are whatever is needed to keep the craft stable.  This is a given absolute truth.  So even on the bench I see no reason for the FC to ever have equal outputs to each motor.  

        Delta gives a good explanation of the induced flow of the Y6, X8 configuration, its not a problem, just use different props on the bottom.  I have been using 15x4 top props with 14x5.5 on bottom.  Bottom props still pull less current than the top props.  However, I have not observed the Yaw issue discribed by climr.  Still I'm considering moving up to 14x6 on bottom.  

        Have you tried a higher pitch on bottom props, or lower pitch on top props?

        • My 2c

          I have a Sky Hero Spy 750 Y6 and a big box of props :)

          I use T-Motor or Xoar 15x5 top and Xoar 14x7/14x8 bottom. Here's the RCCOUT signature from top to bottom in heavy wind flying the other day:

          Xoar 15x5 top  Xoar 14x7 bottom short flight from last year on 3.3 RC1

          TRngiwi.png

          A short hover from a few days ago in 20+ mph winds.  T-Motor 15x5 top Xoar 14.7 bottom

          EgNHxic.pngAccompanying video.  It took a while to figure it all out, but IMO smaller props on bottom with 2-3+ pitch vs top results in extremely stable flight and pos hold is like the copter is cemented in the air. VRS is near non-existent with consistent auto landing. Note also the axial distance from top to bottom changes flight characteristics as well.

          When the bottom props spin faster it is less efficient the further the spread in RCOU is. They are basically freewheeling. Take two square house fans and line them up; same concept.

          Compare above video with this hover video test using same size/pitch props top/bottom. The copter wanders more and is less stable. A few reasons for this: 1) props are low quality causing flexing 2) bottom props are not "working" 3) LEA-6H GPS may be contributing to some of the horizontal movement but as I recall the wind was mostly controlling this. 

          • With 15x5 top/bottom T-Motor props, which are much better than "clones" (Tarots were the absolute worst), the spread is pretty good, but the bottoms are still free wheeling somewhat. Switching the bottom to 14x7 changed the way the copter handles immensely, particularly in windier conditions. Xoar props are nice, but not quite as efficient as the T-Motors. All Xoars produced the most stable hovering.

            OZ3D3BS.png

      • Ah I see...

        Very strange, induced flow is still present, no matter what.

        The configuration I described is still easier on the flight control loop, as far as I know...

  • Now ban Craig Issod and we can all live in peace. He has so many rules violations at RCG I can't remember the last time his posts aren't public. Every single post is rabid 3DR bashing. The guy really is disgusting.

    • Developer

      Discussion about banning people don't create a great atmosphere in any forum, so lets keep it to a minimum.

      Thanks to the moderators, let's work to keep this thread on track and help Climr with the issue at hand.

      Thanks all!!

  • Many years following and this is the first time that something like this occurs in this forum, really strange during changing process :(

  • r&d reports are generally published in open,

    especially in case, they may refer to public safety in open airspace.

    In theory RPM badly matched performace vs. ESC PWM input

    can be compensated by Pixhawk run APM

    but in practice badly matched performance, power of motors may result in fly aways, flipping, burnouts, power supply issues, power shut down in midair.

    Tell me about your credentials and send abstract and 2-3 demo pages with charts for review by aircraft experts.

    If I am satisfied with quality of your report and your study can be verified by third parties and price is ok, I would like to deliver your report to FAA for free.

    FAA is today home for all personal drones subject to registration.

    By declaration, you, as a pilot, make declaration on airthworthiness of your personal drones, you fly.

    Some may claim otherwise, but since you fly an aircraft as a pilot, your personal drone is subject to standard aircraft airthworthiness examination procedure (vide website of  FAA).

    If you buy personal drone from a shop, so this shop is responsible for airthworthiness of your aircraft.

    If you assembly your personal drone at home from Drone Certified Parts (declaration by seller, manufacturer, distributor) + certified drone pilot software, you should follow airthworthiness examination procedures and standards by FAA.

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