I am not sure I quite follow you. Don't you need the mixer to give the FC a reference of which motor is doing what? My understanding is that is what the mixer is doing. I admit there is not much info on the topic in the wiki other then explaining if the motor is right of the Y axis you assign a negative value and positive if it is on the left. For tilt you give a positive if the motor is above the X axis and negative below. All counter clockwise motors get a negative value and all clockwise a positive. They explain that the values go from 0 to 64. They say 64 is 100%. But then there is an example where they have 71 as a value for all bottom motors. Is that because this assumes both bottom and top props are the same size and they want the bottoms to have more response than the top, where as if I leave them all at +_64 respectively I make the bottom props smaller so that aerodynamically they are more responsive because of less drag on the motors? If so I had my assumptions wrong. I assumed that the 71 value was factoring in a smaller prop on the bottom. There does not seem to be a lot of insight on this.
Of course the mix has to be right. However with a coax setup you also have to choose the right prop combination. Some examples:
- using same size and pitch top and bottom and same mixer value for both, can cause the lower prop to almost freewheel in the top prop's stream. This way it wont contribute much to overall lift.
- rising the mixer value for the lower prop makes it rotate faster, thus providing more lift. However you will also increase the drag, making the prop less efficient.
- the wash from a prop is slightly conical. So compared to the top prop you need a slightly smaller prop below. This way the prop will operate in 1 uniform airspeed across the prop. Operating in more than one airspeed causes the prop to vibrate, and we really dont want that.
So, what do we need...? We need high diameter, low pitch on top, and slightly lower diameter but higher pitch on the lower prop.
Why do we need higher pitch? Because low pitch top, and high pitch below?
Because low pitch is Best for static thrust (like in a hover) and high is Better for air already moving (recieving the wash from the top prop, and accelerate it further)
One thing not mentioned yet is yaw.
As a MR uses torque for rotation. Using higher mixing values can cause unwanted yaw.
End the end though, it's about getting the 2 motors to share the load!
(written from mobile, so there might be some typos
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