What do these do? How can these be tuned? Are they useful? I can’t find any information about them. Based on @rmackay9’s videos, the FF is very important in rovers, but even the manual tuning section of the plane wiki says to just change P, I, and D gains.

Thank you in advance!

PTCH2SRV_FF “This is the gain from demanded rate to elevator output”

Guess that is useful only if your elevator movement is too much or not enough to follow desired pitch.

Maybe “demanded rate” is referring to ACRO mode. I also read the descriptions, but it doesn’t appear to imply much.

The short answer: You probably don’t want to use them. The autotune system is excellent, and it seems that it never uses them. (Someone let me know if I’m wrong about this?)

The long answer: “What does an FF term do?” is in the details of how PID control works.

If you don’t know how PID control works, google to find a tutorial… hopefully one of them will explain a feed-forward term (FF) which is an option.

If you already know how PID control works, some of the confusion may comes from the fact that ailerons/elevators can be thought of as commanding a roll-rate/pitch-rate. In some modes, like FBWA, using the PID control we instead map the TX input-stick to roll-angle/pitch-angle. Inside the PID controller, the desired rate is calculated from the input angle (which the pilot specifies, using the TX sticks) This generates confusing terminology for what “rate” means… as a roll-rate controller would call roll-rate the “quantity” and the “rate” of it is actually “roll-rate-rate” which is angular acceleration.

Does that help? If not, I’d be glad to help more… maybe you could ask a detailed question explaining what you already understand and where the confusion is?

Yes, I understand how PID’s work, and Leonard’s video prompted me to ask this question.

I have done quite a bit of autotuning, and you’re right, it doesn’t use them. The planes fly very well right now.

I totally understand what FBWA is doing and how it works, however I’m wondering if there is any additional benefit to tuning the _FF terms. Since I primarily fly in manual, FBWA, and auto mode, I am doubting that there is any reason to, but I thought that I would at least ask the question.

In addition, if I do add gains to the _FF terms, should I re-run an autotune? My understanding leads me to believe that the _FF gain may also have an effect on the other PID responses.

I don’t know the answers, but now I agree with you, I’d like to know! I’ll watch this thread for answers.

If you ever do some testing yourself, I’d love to hear what you think and what you find out from it.

I need cheaper test units for that reason. haha.