I have a quadcopter based on APM 3.1.5. It flies just fine in stabilize mode. It flew also great in auto, loiter and alt hold modes - everything worked perfectly. I have now changed the cheap plastic props for some CFK props (which I balanced first of course) and my old ESCs against Afro ESCs with SimonK firmware. It now still flies great in stabilize mode, even better than before. However, in loiter mode it will no longer hold altitude correctly. It will suddenly start to gain altitude once I set it into loiter mode. Sometimes it will also fall out of the sky (i.e., lose altitude rapidetly) in loiter mode.
I have no idea what is wrong. I have foam on my barometer and the vibrations seem to be ok. As I said, before changing the props and ESCs everything worked great. I have also reset the thr_mid parameter to 680 in order to reflect the new configuration of the quad.
I'm attaching a log which shows the misbehaviour. I have enabled IMU loggin in the log, so you can also take a look at the vibrations.
When plotting the BarAlt parameter, one can clearly see that the quadcopter is gaining and losing height during loiter:
I don't understand the ThrOut during Loiter. Once it even goes down to around 200, which explains the rapid loss of altitude. I do not understand why the APM would do that, since it knows the current altitude as shown above in the BarAlt graph.
As you can see in the attached log, the vibrations AccX and AccY seem to be well within tolerable parameters. The AccZ vibrations are (I think?) mostly ok as well, mostly withing -5 to -15, with some few exceptions.
I'm totally lost on why this previously great flying quad now behaves like it does. As I said, I only changed ESCs and propellers (which I balanced before).
Many thanks and all the best,
to make a long story short, it's been vibrations. Apparently those new great CFK props which I sanded in order to balance them perfectly still emit more vibrations than my old cheap GemFan plastic props. Who would've thought?
Anyways, I mounted my APM on some moongel (similar to alpha gel) and everything works fantastic again now.
All the best,