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.
If a simple change can increase stability, I think it's really worth looking at and addressing.
The original person who posted that information did not mention anything about swapping the motor positions; only the type of prop and reversing direction. I have not tried this myself yet.. trying to get some sort of confirmation if this is indeed the best configuration
This is a most interesting thread for those of us with a Y6. Anyone that tries this out please post your findings.
Note that Oliver DID say the following: "exchange ESC outputs for those two motors and reverse spinning directions". This implies that motor 4 and 6 were switched in his scheme.
Later discussions in that group made it sound like the Wiki was going to be changed to reflect this new configuration. As of yet it hasn't happened. I'm at that point in my build where I need to decide what to do about motors 4 and 6 so perhaps we can get an update from Oliver?
thanks for clarifying that! I think this group is a bit hidden away, but I do hope Oliver stumbles upon it
It'll be really interresting what 3DR could say about this - anyone?
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.
I understand neither of you work for 3DR, and really appreciate your input. The problem here is we would be troubleshooting a fundamental element of 3DR's Y6 product. The reason I made this thread is to hopefully drum up enough interest so 3DR *must* address and come with a clear answer.
I'm not sure about others, but I really would rather not make my copter a test-dummy. A simple mistake can mean my copter will flip over or fly erratically. I'd rather 3DR perform the testing and respond with exact instructions on how to properly setup the tail.
not to stop anyone from trying it... but you know what I mean.
We had additional discussion about this how to deal with this issue on the dev call a few weeks ago but it will not make it to 3.1
We will be doing Y6a and a Y6b motor configuration firmware so that this can be resolved in software without having to re-wire the ESC connections and will allow people to transition to the new prop arrangement
So, is 3DR sort of confirming the issue?
And: I'd really like to get an answer on the "why do we have a different prop arrangement on the rear arm for the..." question
>>> why do we have a different prop arrangement on the rear arm
It's just historical and not optimum. The trick for us how to manage the transition to a new motor configuration. Currently the Frame Type is ignored on the Y6. Our plan is to start using it in 3.1 and have a Y6A and Y6B configuration to allow people to configure their vehicles with the new motor config. We probably need to do the same with the X8s too.