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.

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In addition to the motor direction shown here, the Pixhawk treats the X8 configuration the same as an X4 configuration, and sends identical signals to the motor pairs on each arm if I'm not mistaken.  

I do not think so, it must spin top and bottom props differently for yaw.

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.

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.

Yes, duh I should have realized.  In forward backward and side to side flight without yaw, the motors do act in pairs.  Just looked at a few logs to confirm.  In yaw they split up.

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

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

Accompanying 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.

Hi all,

The X8 is set up the way it is to ensure that all axis have equal authority and are equally affected by the induced flow. That is why the X8 is basicly a H quad on top of an X quad.

The Y6 is setup to minimise coupling between the control axis. The original Y6 would not auto tune because this effect is so pronounced.

The main difference between the Y6 and the X8 is the Y6 will have throttle output separated between the upper and lower motors to balance the yaw torque. The X8 has the upper and lower motors always equal. This difference means the relationship between the upper and lower props and the induced flow is very different.

"The X8 has the upper and lower motors always equal."

Well except for yaw behavior, yes?

Thanks Leonard. I'm building a 3DR X8 frame and that's good to know.

Hi Stephen,

Yes, If there is an acceleration in Yaw the upper and lower motors in the X8 will diverge to achieve the unbalanced torque. However, when in a hover the upper and lower motors should be equal. This simplifies the problem of optimising the performance because you can assume pwm output to the upper and lower motor will be equal.

In contrast, when in a stationary hover the upper and lower motors of the Y6 are not equal and this difference is taken up by the I term build up in the rate Yaw controller. If you change the combination of propellers the relative output to each propeller is dependent on the relative drag of each propeller. For this reason the best combination of upper and lower propeller for the Y6 will be different to that of the X8. It is also much more complicated to find the best combination as you have to simultaneously optimise both the power/velocity/flow and torque of both upper and lower propeller.

I just want to highlight:

The upper and lower motors of the X8 will be approximately equal when hovering. When applying yaw the pwm output to the upper and lower motors will diverge.

I am sorry I didn't make this clear in my previous statement. (thanks Stephen)

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