I would be grateful if someone could explain what ATC_ACCEL_P_MAX does. The default settings is 110000 but after autotune I get a value of 57000 which according to the documentation this is lower than the slow value. I had set the autotune’s aggressiveness to 0.1.
Thanks
David Ardis
The ATC_ACCEL parameters define maximum angular acceleration, i.e., how fast roll/pitch/yaw rate can change. It therefore has a very noticeable effect on how the copter feels to fly - a low acceleration will feel docile or even sluggish, while a high acceleration will feel snappy or twitchy. Larger craft are better served with lower acceleration values because of their greater inertia; a big copter may not be able to physically achieve a high acceleration setting.
Generally, you can change these values independently of the PIDs without affecting stability as long as it is a reasonable number. Just do so carefully when increasing it higher than what autotune set it. It is often worth doing several autotunes to see what range of values it will set.
Rick
I the value was too high would it appear twitchy in high winds say 20mph .
Did an Autotune on an 5kg X8 yesterday and got a value of 13000 the original was 110000 which was way to low started to rock back and forward in the air ended up having to dump it onto then ground breaking the undercarriage no big deal.
Twitchiness is generally associated with the PIDs rather than the acceleration setting, although it might be possible. I haven’t done much testing of setting the acceleration too high or too low.
But 13k definitely sounds too low, I’m surprised that autotune would set it that low on a 5kg copter. For an aircraft of that size, I generally find that 50k-120k works, depending on how tight you want the controls. The oscillating may have been a combination of very low acceleration and I term windup.
Where can we find explanations for all these parameters? I was perusing MissionPlanner/ConfigInitialParams.cs at eb764d07d71202edbbfaa2087f4d4fab6c8b0e63 · ArduPilot/MissionPlanner · GitHub
And saw this param, and could not find any description of it in the whole repo, neither in GitHub - ArduPilot/ardupilot: ArduPlane, ArduCopter, ArduRover, ArduSub source
Searching the docs Mission Planner Home — Mission Planner documentation turns up nothing as well.
ATC_ACCEL_P_MAX and ATC_ACCEL_R_MAX can be thought of as closely related to your prop size. In reality it is more about what your aircraft can actually achieve for pitch and roll angular acceleration. There’s also Yaw of course.
The thinking is multirotors with smaller props also have smaller mass, so ATC_ACCEL MAX could be higher. Bigger props are used on bigger wheelbase heavier multirotors, so ATC_ACCEL MAX should be lower to be achievable and not stress motors and cause desyncs or other nasty effects.
Usually Autotune will find what can be achieved and set this value, and you can manually lower it to suit a payload as per:
ATC_ACCEL_P_MAX x (min_TOW / max_TOW)
ATC_ACCEL_R_MAX x (min_TOW / max_TOW)
ATC_ACCEL_Y_MAX x (min_TOW / max_TOW)
It would be very unusual to increase the ATC_ACCEL_P_MAX and ATC_ACCEL_R_MAX beyond what Autotune can achieve with minimum take off weight.
Good starting values can be found using the MissionPlanner Alt A plugin, and now also in the Setup tab.
This spreadsheet is the source of the plugin
which is in turn derived from the Tuning Guide
https://ardupilot.org/copter/docs/tuning-process-instructions.html
Thanks. I searched the Mission Planner Docs, but not the copter docs: Copter Home — Copter documentation
Which turns up everything I need.
Reading through the docs is good but a long road 
This will be useful
Copter Complete Parameter List
But the crucial thing is to set up your multirotor using the Initial Parameters plug-in (in MissionPlanner) or the spreadsheet if you like to do things manually. Then follow the tuning guide!
EDIT:
After initial test flights, it’s worth setting up the Harmonic Notch Filter as soon as possible.
Start your own thread if you need help.