The proposed change in code would allow APM to address every custom quad copter layout (scope limited to fixed vertical rotors). I'm asking for input before doing a pull on code to make this happen. So please speak now or ....
Your quad may be custom if:
o the front motors are spaced different than the side motors (not square).
o the aft rotors are spaced different than the front rotors.
o prop spin is reversed.
Such layouts are typically done to accommodate a wider field of view (FOV) for a FPV or video/photo camera. Sometimes the change is made just because it looks cool.
1) Changing the aspect of the length to width of the copter changes the relative motor factors for pitch and roll. This is not always understood and is not always addressed by custom settings for decoupled roll and pitch PIDs.
2) Making the aft width different than the front width induces coupling between yaw and roll requiring a fix by using non-unity yaw factors for two of the rotors.
Solution: The proposed process for setting up custom quads within APM is explained in the following video.
Please let me know if you have input or support doing this.