Can anyone comment on how the apm code handles X8 configurations? (Haven't delved into the code). More specifically, does AC 3.x send the exact same signal to the coax bottom and top motors on each arm, or does it somehow handle them differently?

Asking because I am thinking of playing with a "double X4", by just "Y'ing"  two motors to one ESC and configuring the copter as an X4 quad. I know I'll loose the redudancy that comes with separate ESCs , but I just happen to have some spare ESC's that have double the capacity required ...

And it seems that as long as one of the two motors can handle double the required thrust, loosing one motor in flight would still avoid a crash so I'd still have at least motor redundancy ...

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ESCs measure signals from the motor they are driving to control timing. I think connecting two motors to one esc would not work well.

Don't do it :)

Thanks for your replies.  Scott, I agree that in theory one would think each motor feedback to the ESC may be different so this would not work. Yet it does work on the bench, (two identical  motors on one esc, bypassing  fc) .   Some have  successfully flown that set-up on other controllers .

In general, I was wondering what, if any, the FC does in an X8 config that does not exactly duplicates  the signal to two motors on each arm, instead of one in X4 configuration. Seems like some aerodydnamic considerations on coax set-ups could be taken into consideration and taken advantage of by the fc  ( e.g. different pitches on top and bottom propellers, or code handling failure of one motor) yet don't know what the scoop is ...

Well you guys are  right as far as using one esc with two motors. Although it can  work, I saw some weird starts on the bench and sooner or later I am guessing smoke will probably appear, I am not going to continue to confirm :)

So scratching that idea ... Efficiency would also probably be terrible ...

Haven't tried Y'ing signal wires from two esc's on each arm to an FC, I've seen reports of this working fine with no difference from X8 (on DJI Naza though, I am using APM)  ..

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