Is direction of rotation of motor critical in a single copter?

Will a single copter function properly if the motor spins CW instead of CCW when looking from top of the copter?

Arducopter documentation suggests (or requires?) motor to spin CCW and to be connected to output 5.
However it is not always possible, especially with a gasoline engine mounted above propeller which is a standard practice in single copters. Most gasoline engines cannot reverse their direction of rotation and so would spin their propeller CW in that configuration.

If CW direction does not affect its function, should the motor be connected to servo output 5 or to output 6 like the lower motor in a coax copter that spins CW?

I think you would need to reverse all the servo movement in the control software and also ensure that the auto stabilization movement of the servos are reversed compared to when using a CCW propeller.

I think it doesn’t change anything. In the end, your servos acts on the flow of air, this doesn’t change with how the prop spins.
The thing that will change is the angular momentum of your prop. When accelerating the prop, the copter will yaw the other way, and when rotating the copter, the gyroscopic precession will add a pitch or yaw offset by + or - 90° to your input. But Ardupilot does not considere those phenomenon, it would need a feed forward in the control loop and I never saw it. The PID alone is enough to compensate for it.

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I’m quite certain 2lian that you are correct. I have tried CW and CCW rotating props since I posted my question, and I could not notice any difference. This was good news because I built one larger copter without paying attention to the direction of rotation. In that copter the direction could not be reversed and it was opposite to what Arducopter instructed, hence my original question.

I was also concerned that the software anticipates propeller inertia and precession but I was not able to examine the code. Had to find it out the hard way.
I’m again quite impressed by your skills.

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