Tuning for copter with slung load

ArduCopter Copter 4.0
1

I’m not sure if anyone has experience with this, but I’m curious if anyone has flown with a slung load, and if they have any insights on tuning issues associated with it. I have a medium sized quad, about 2.5kg ready to go with 15" props, but I’m flying with a 30m tether with a hefty weight on the end of it. The tether is attached close to CG. The tether itself weighs around 200g, and the weight on the end is around 600g. Intuitively, it seems like the slung load shouldn’t affect the pitch/roll/yaw tuning significantly, since it does not modify the angular inertia of the quad at all, but I expect it does have a significant affect on throttle tuning, as the hover throttle will change pretty dramatically when the weight is on or off the ground. Perhaps there is a part of the PID tuning related to throttle position, since the thrust response to throttle is not completely linear.

I’m curious if there is an appropriate strategy for tuning throttle in this case, and perhaps pitch/roll/yaw as well. Is it better to set hover throttle and acceleration terms based on the lightest configuration, or the heaviest? I would guess the former, but perhaps others have some useful feedback here.

2

You are correct, the copter shouldnt be affected much. They are nice to watch :slight_smile:
If the payload becomes significant then adjust you Pitch and Roll Accel values 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)
But usually you only change this for a fixed payload - you might be able to leave them unchanged even for significant tethered payloads.

3

Thanks for the feedback Shawn. I found another thread that indicated to tune throttle params based on lightest weight, which makes sense. So I’ll stick with that for now.

4

Yeah, if you stick to the tuning guide you cant go wrong
https://ardupilot.org/copter/docs/tuning-process-instructions.html

5

Yeah, that’s what I generally use for initial setup of new copters. If alt-hold and pos-hold work OK, then I generally spend some time tuning PIDs until it feels good. The unknowns were the influence of the slung load and variable total mass. Thanks again for your help.