Strong oscillation on altitude


I have refitted my old SplashDrone (amphibic quadrocopter) with a HolyBro Pixhawk 4 Mini and did my first test flights today.
When in Stabilized mode all was ok.
When taking off in Altitude mode it was very difficult to control altitude. The copter bounced several times back to ground and was not able to keep a stable altitude. Even when trying to land the copter touched ground and took off again. I had to immediately disarm on ground or go back to Stabilized mode.
When in Position mode position accuracy was ok, but altitude accuracy was same as bad as in Altitude mode before.

I have uploaded two logs here:

If the barometer not working correctly or why don’t I see the oscillation there but on accelerometer of Z axis?
For the first log the copter was not sealed, so air pressure has reached the Pixhawk without dampening
For the second log I have sealed the copter. Air pressure reaches the Pixhawk through a special seal which is permeable for air.

What should I do to fix this?

Regards, Gerd


I had a similar case with a Tarot Sport 650 four Tarot 4114 engines (Pixhawk 4 Holybro PX4). During the first flights, I found many vertical oscillations of the altitude. In fact, I think the problem was caused by the bad fixing of one of the engines.

Regards Serge

The motors are mounted rock-solid and this copter was in use for 3 years already with a XAircraft MiniX flight controller without oscillations.

Regards, Gerd

As there seems to be no idea for fixing this oscillation and because of massive connection problems via USB, both to PC and Android as soon as the FC is connected to the copter (although all peripherals are powered by a BEC directly to avoid voltage drop for FC), I pulled the emergency brake and re-flashed the PixHawk Mini to the latest version of ArduPilot.

I just come in from short test flights within my garden. Stabilise mode felt identical to PX4. On the first flight Altitude mode seemed to have the same oscillation problem as with PX4 before, but oscillation quickly decreased and was gone after some time in air. And it didn’t come back in further flights. Is there some automatic adaption or self-learning implemented in ArduPilot?

Then I fully sealed my waterproof copter, so the baro sensor gets changes only through a pressure-leveling valve and altitude hold still worked. Finally I risked switching to Loiter mode and even that worked already fine with default parameters.

And last but not least, I don’t have connection problems to QGroundControl anymore, neither via USB nor via the meanwhile added HC-06 Bluetooth module, both to PC and Android devices.

So I must finally say bye-bye to the PX4 stack.

Regards, Gerd