Quadcopter constantly gains height during Loiter

Hi all,

I have a problem with my Quad which uses APM 3.1.5 as flight controller. Yesterday I exchanged cheap plastic props agains proper CFK props. Before the change, the quad flew excellent. Alt hold, Loiter, Auto mode - everything perfect. I only changed the props against CFK props (after properly balancing them) and now I’m suddenly facing strange problems:
In loiter mode the quad will no longer hold altitude. It will stay at the same position but constantly gain altitude (about 10 metres) after which it suddenly will rapidly lower altitude again only to repeat the process. This worked flawlessly before - before the change in props, it would hold altitude without any problems. I have one of the old APM boards which did not came with a case yet. However, I have properly protected the barometer and it worked flawlessy before changing the props.

I only have this problem with the altitude, everything else works great and it flies much better with the balanced CFK props than before with the cheap (although still balanced plastic props).

This is very strange and I’m wuite lost why this should happen. I have attached a log.

Maybe anyone can help? This would be greatly appreciated.

Please help :frowning:

Many thanks and all the best,
Michael

…said that, I did some more analysis and I think I know what might be the reason. The old props (cheap plastic slow-fly props where 9x4.7), the new cfk props are 9x5. Still, with the cfk props I need considerably more throttle to hover. Thus, I think the thr_mid value might simply no longer be correct and thus the copter raises when I’m going into loiter, since I’m out of the dead band.

Does that make sense?

All the best,
Michael

enable IMU logging
most likely, you have too much vibrations in Z axis - log and plot IMU.AccZ ( X and Y are worth checking out too)

Hi Andre,

many thanks. I will consider this as well. The vibrations were perfectly fine before and now with the new cfk props (balanced of course) they seem to be even less. But should it not me the reason stated above, I will enable IMU logging to see if the vibrations have changed.

I will report back.

Many thanks and all the best,
Michael

Hi all,

the thr_mid settings did not work. Could it be that the new CFK props exhibit more vibrations (although I have balanced them) than the old cheap plastic props?

I’m attaching a log file with IMU logging enabled. It shows that the barometer correctly logs the altitude. It also shows that the APM exhibits very strange Thro_Out parameter during loiter.

Could vibrations be the issue? AccX and AccY seem to be well controlled, only AccZ seems to something go out of the -5 to -15 band. Can you take a look?

I’m very lost as to what the reason could be. The quad flew perfectly before changing the props and in stabilized mode it flies much better now with the CFK props than before.

Please help.

Many thanks and all the best,
Michael

Hi all!

to make a long story short, it’s been vibrations - as Andre assumed. 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,

Michael