Hello,
I ran across this thread over at google code drones discussion and would like to bring attention to it, hoping 3DR chimes in and can clarify if indeed the rear blades need to be swapped (and documentation updated to reflect this). You will notice on your Y6 that if you yaw, it induces a pitch change. Here is a portion of the discussion where a solution is found:
Problem definitely solved : it was caused by the rear arm propellers arrangement (reversed compared to the front arms).
Here is how i did solve it :
- put a pusher prop on the rear arm upper position (instead of the normal one)
- put a normal prop on the rear arm lower position
- exchange ESC outputs for those two motors and reverse spinning directions
So now i have an Y6 with the 3 pusher props in upper location on the three arms.
No more Yaw / Pitch coupling at all.
This is confirming what i thought, a thrust asymmetry is induced if a propeller tandem is reversed on a arm.
The probable reason :
On a coaxial setup the two propellers of a same arm do not have the same efficiency. During yawing, we are accelerating three propellers, and slowing down the three other ones.
The problem is that the vertical thrust result is not the same if one arm has a different prop arrangement : rising the speed of the lower prop is more efficient than rising the speed of the upper prop.
So if two arms of the Y6 frame have pusher props in the up location, and the other arm have the pusher prop in the lower location, then when yawing one side, we produce a different vertical thrust on the rear arm compared to the front arms, because we are not speeding up the same arm propeller. This is causing a pitch derivative coupling.
The coupling is only derivative because the induced pitch unleveling is compensated by the APM accelerometers as soon as it is detected but with a small delay.
So the definitive solution is to put the three pusher props in the same location on the arms. That's it.
I think that reintroducing a top bottom ratio is not the solution, at least not without a more complex motor mixing to avoid axis coupling.
Olivier.
If a simple change can increase stability, I think it's really worth looking at and addressing.
Replies
Tried Y6b configuration with top and bottom 10x4.5 with HP2212-100kv - copter flies great, much better than before! I still need to solve a bit unstable loitering problem, but stabilize and alt-hold modes work very good now.
Thanks and happy new year!
Thanks phillip! That was the issue - flying very well now after re-configuring the motors. only thing I do notice with this new config is that if you are hovering then give a high throttle, it seems to yaw slightly - otherwise no complaints! I would definitely suggest making the change to any Y6 users.
Looks like the new Y6 configuration was included in the 3.1 release which is great news. Has anyone tried this yet on 3.1? or have been flying the new configuration (motor mapping with all top props CW, bottom pros CCW)? any feedback would be helpful
lots of proper tests here.. http://www.pulsosystem.com/u39m-280.html
For some reason after this change my Y6 that was flying more or less well, started to behave strange. It slowly spins (yaw) while it is in the alt hold mode. I did compass calibration and as well as compassmot (5-8%) - none of this helped. All props are 10x4.5.
Any update to this?
One key factor that no one has mentioned is that the Y6 uses differential propeller sizes on the upper and lower rotors to achieve greater efficiency (+10%-20%). If you use all of the same props, swapping the directions could make sense to remove any pitch/roll coupling.
However, none of the Y6 copters ship this way. What happens if you have three smaller upper rotors and three larger lower rotors? Well, you end up with a torque differential. In an equal power output state to the 6 motors, the aircraft will have a tendency to yaw in the direction of the smaller motors. That causes the smaller rotors to speed up, and the larger rotors to slow down to maintain yaw stability. You will also have difficulty yawing in one direction and overpowered yaw in the other. This also upsets the co-axial flow of air through each arm if they are matched for efficiency, which could remove the efficiency benefits of differential propellers in co-axial arrangements.
This change could potentially induce very complex and detrimental control interactions in pitch roll and yaw that may or may not be able to be solved in software. I would be very careful in making changes like this using the more efficient prop differential configuration.
Guys, what you see in that email is basically the entire conversation, as far as I know. The problem is clear, and the solution is easy. Neither Olivier nor I work for 3DR. I don't even own a Y6 so haven't tested it myself. I'm not sure if 3DR has noticed the problem yet or not, but the ball is sort of in their court.
One problem is that this is the type of change that is difficult to roll-out effectively. If we just go ahead and change the wiki without any discussion or announcement, it will lead to confusion and maybe an incident.
At this point, I would suggest you try following Olivier's instructions. If it works out, then maybe having some confirmation of the fact it does work better will lend weight to the need for implementing this change.
It'll be really interresting what 3DR could say about this - anyone?
This is a most interesting thread for those of us with a Y6. Anyone that tries this out please post your findings.
Regards
Greg
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