by 13brv3 » Fri Jan 17, 2014 3:19 pm
Yaw is an opposite reaction to the work a prop is doing to accelerate the air. The coaxial yaw imbalance comes from the fact that the bottom props don't usually do as much work as the top props, so the goal would be to make the top and bottom props do the same amount of work. There are a few ways to do this.
First, the yaw parameter is not really what you need to change, so I'd put those back to 100.
The top prop takes air that's essentially stationary, and accelerates it. This is just like any normal multirotor prop, so nothing special about this. Now consider the bottom prop. I has fast moving air above it from the top prop, so to do any work, it has to accelerate that air to a faster speed than the top prop did. Obviously, if it's turning the same RPM as the top prop, it will just be coasting along, and not really doing much work, so most of the thrust and yaw will be coming from the top motors.
The goal will be to make the bottom prop work as hard as the top one. If you're using the same props on top and bottom, you can simply turn the bottom prop faster. This can be done by using slightly higher KV motors on the bottom, or you can set the thrust parameter in the mixer table to a higher value for the bottom motor than the top motors.
The other way to equalize the work is to increase the prop size, but pitch is far more important than diameter for this. I personally like to keep the same diameter, then increase the pitch of the bottom prop. Unfortunately, the selection of counter-rotating props is not good enough to allow a lot of options for this. Increasing the diameter would work, but only if you go up significantly in size, and with no less pitch. I would bet that you're current prop selection is doing more harm than good, and you'd be better off just using the same props.
Hopefully some of that was coherent, since I'm still low on caffeine this morning.
Cheers,
Rusty