Throttle Dropping Issue


We have three quadcopters built on the DJI F450 platform using a Pixhawk flight controller with APM/Arducopter version 3.5.4. We are using DJI’s E800 propulsion system with the E800 motor and 620s ESC. All three quads exhibit a weird phenomenon: with no props, we arm in stabilize mode, then give constant throttle and after about 2 minutes, the rc3_out value (the value that Pixhawk is giving to ESCs) is dropping, even though our rc3_in (the value from the controller) is constant. After rc3_out drops, higher rc3_out values cannot be obtained, even with max throttle.

We have had several crashes where the quads are unable to stay in the air after 2 or 3 minutes so this issue is reproducible with and without props. Following are some tests without props in stabilize mode: (1) half throttle case; and (2) full throttle case. As you can see from the plots, ch3out value starts decreasing all of sudden and never recovers back as ch3in points.

Case 1: half throttle

In the beginning of half throttle case. The blue line is ch3out and the red line is ch3in.

Value of ch3_out started decreasing.

Even though I increased throttle, it didn’t go up as you can see from the blue line.

Case 2: full throttle

Full throttle case. The blue line is ch3out and the red line is ch3in. No offset in the beginning but ch3out started decreasing right away. Then, as time goes, ch3out kept decreasing.

Interestingly when I decreased throttle to the mid position, I wasn’t able to recover ch3out as you can see in the first drop. However, when I decreased throttle to the bottom and increased it all the way up, ch3out catched with ch3in. Does anybody know what is happening?

I have a one more additional question regarding this issue. We also saw that ch3out was not only affected by the throttle stick but also affected by roll and pitch sticks. We are guessing that ch3out might be something either that Pixhawk trying to give, or that motors are taking, but we are not sure about that yet. What exactly does ch3out show?

The followings are google shared links for log bin files:
(1) half throttle case:

(2) full throttle case:

Thanks a lot!

CH3 only means that the motor 3 or say RC3out rather the throttle out!!! It just reflects rc3’s output like the picture.

The The average of ch1out ch2out ch3out ch4out means the throttle.

You should provide these flight logs. It doesn’t make sense to test on the ground.

@Chinese66 is correct here. All this is showing is your I term is growing on one of your axis and dropping the output to one of your motors.

As stated, provide a log of the aircraft in flight and we will let you know what is happening.