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I have a HobbyKing KK2.0 Flight Controller (FC) on a quadcopter. I have been trying to work out the correct mixer settings for a new H configuration I'm trying however the documentation isn't that great and if I follow it the craft is uncontrollable.

In the standard X configuration with the Centre of Gravity (COG) in the dead centre of the craft, I understand the default settings:

Default configuration

Everything is keyed off "100" (assuming these are abstract units and 100 simply means 100%), with the rudder set to +/-100 since the 100 refers to the arm distance with the centre of yaw the dead centre of the craft, while the ailerons and elevator need to be set to +/-71 since the distance from the pitch/roll centre is 71% of the arm length according to Pythagoras' theorem. All good so far.

My problem is that my new configuration has the centre of COG quite far forward since I've moved the battery pack to the front of the craft (in front of the front arm). The FC itself is positioned roughly at the new COG (it's maybe 5-10cm further forward in case it makes a difference).

The documentation in the manual states this about the mixer settings:

Mixer Editor: This menu lets you adjust where and how much signal the motors gets from the sticks and sensors. This makes you able to make any configuration possible, with up to 8 motors or servos. To change between the output channels 1-8, press CHANGE when the upper right number is highlighted.

-"Throttle" item: Amount of throttle command. Usually 100% if the output channel is connected to a ESC.

-"Aileron" item: Amount of aileron/roll command. Use positive value for motors on the right side of the roll axis, and negative for the left side of the roll axis. The value is given by the motor's distance from the roll axis. More is further away

-"Elevator" item: Amount of elevator/pitch command. Use positive value for motors on the front side of the pitch axis, and negative for the back side of the pitch axis. The value is given by the motor's distance from the pitch axis. More is further away.

-"Rudder" item: Amount of rudder/yaw command. Usually 100%. Use a positive value for a CW spinning propeller, and negative for a CCW spinning propeller.

So the manual implies that the FC relies on the difference in distance between the FC and the motors, rather than anything to do with weight. However, I feel like there is an assumption here that the FC is also positioned at the COG.

I initially tried modifying the settings with everything based off distances, but the craft was uncontrollable. I scrapped that idea and have also tried basing everything off weight (and a lot of trial and error) and have come up with these settings:

New configuration

The green is the actual frame and arms, with the numbers on the diagram indicating distances in centimetres. The blue "arms" don't physically exist - they are there just to show distances from the COG to the motors to show what the equivalent X layout would look like.

However, these settings still don't produce a stable, controllable craft:

  • It still feels "tail heavy"
  • It still yaws unpredictably and correcting yaw makes it also want to roll in the opposite direction
  • Roll control doesn't feel right at all

I really want to understand how the mixer works, and also get my craft flying correctly. My questions are:

  1. Does the FC always need to be positioned at the COG?
  2. What are the correct calculations to take into account having the FC not at the COG?
  3. Does COG play no part in the calculations in the FC - does it only rely on distances?
  4. What are the correct settings for my FC given the above measurements, using the calculations in 2?

Thanks very much.

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