First step on tracking down the yaw problems is to check that all the blades are spinning in the right direction and at the same speed for a given PWM pulse width. So I added it little test to the code to step through each motor in turn, spinning them up to well under take-off speed. I started off with 10% and one motor did sound a little underpowered, but once I’d upped this to 20%, all motors were humming the same tune. They were also all spinning in the correct direction (CW or CCW) for their given location, and they are all using the same props, and were all blowing air downwards, so I really struggle to see how the yaw can be caused by any of these parts. Hmmm – I think I’ll have to put this on the back-burner until I come up with another great idea.