I am not sure exactly what you are referring to but I will try to clarify this.
The expo curve moves between a y = x straight line to a y = x^2 line based on the expo value.
The idealised propeller model and motor model produces a y = x^2 thrust curve but in real systems this curve is flattened out due to the various losses in the system.
When we use this thrust in a multirotor we can’t use the zero thrust point because this requires the propellers to stop spinning and esc’s don’t like trying to start again from zero rpm under load. So we set the minimum rpm and minimum static thrust using the MOT_SPIN_MIN parameter. This keeps our motors and esc’s spinning safely at our lowest thrust level.
So now the expo fitting line must match the thrust curve from some higher thrust point that the system will treat as zero thrust.
As we increase the MOT_SPIN_MIN parameter to increase the lowest point on the curve, the curve becomes closer and closer to a straight line. This means that the expo used to fit to this curve becomes closer and closer to zero (y=x).
So to get expo correct we need to move the MOT_SPIN_MIN to (0,0) before fitting the expo. If we increase our MOT_SPIN_MIN the expo will need to reduce a little to match.
Keep in mind that while our expo formulation has a very solid theoretical justification based on ideal propeller model, the real world is much more complicated. For example coax propellers increase losses and tend to drop the expo. Smaller propellers distort under load and appear to move to a lower effective pitch, flattening out the upper thrust curve. Then we start thinking about how this curve starts changing with negative inflow at high speed and … well it gets hard to know where to start.
- Get your expo close but don’t get too hung up on getting it perfect. (we hope an approach similar to the one above will get “close enough”)
- Get your frame built well and propellers aligned properly.
- Set your MOT_SPIN_MIN carefully to ensure you get good low thrust but without stalling the esc’s under load.
- Make sure your battery voltage range is setup in the MOT parameters.
- Take your time to read, understand and follow the tuning instructions.