Hi Olli,
This is an extremely important parameter for tuning aircraft. Most ESC’s work on a principle that the commanded input is approximately proportional to the average voltage used to drive the motor. This results in a strongly non linear thrust response for each propeller. I used measurements of approximately 100 propellers to come up with the default MOT_THST_EXPO value of 0.65. The basic principle is the larger the prop the higher the value. So a 30" propeller would be closer to 0.8 for a normal ESC and a 5" prop would be more like 0.5. Coax propellers also seem to be lower so I use 0.5 for most coaxial designs if I don’t have measure thrust characteristics.
However, some ESC’s appear to do a compensation on the commanded input to attempt to linearise the thrust value. Depending on the ESC and propeller the MOT_THST_EXPO can vary from -0.3 to 0.5.
The other parameter that changes the value of MOT_THST_EXPO is the MOT_SPIN_MIN value. So if you increase the MOT_SPIN_MIN then the perfect MOT_THST_EXPO will need to be reduced.
So the only way to define this parameter properly is to measure the thrust curve and calculate the MOT_THST_EXPO. However for most ESC and propeller combinations the defaults will work well.
So how do you tell if this value is wrong?
You get oscillations when you go full throttle or you have very poor control at low throttle.
You find that you can’t change the take off weight of the aircraft without redoing the tuning.
Both these assume you have a near perfect tune at hover (and I mean my idea of a near perfect tune). You should also have the voltage compensation features set in the MOT_ parameters.
So to give you some examples from aircraft I have tuned. I have aircraft with hover throttles of less than 15% that will not show any signs of oscillation at full throttle with no adjustment or compromise of the PIDs at low throttle settings. I have aircraft whose take off weight can vary from 12.5 to 50 kg without adjusting the PID values.
I hope that answers your question.