Altitude Loss X8 Quadcopter

Hi all,
I’m having challenges with Pixhawk altitude sensor. First of all I want to inform about my vehicle.

Type: X8 Quadcopter
Motor: XRotor Pro X8
Propeller: Foldable 30 inch
Battery: 2x 22000mah (serial connected so 50.2V and 22000 mah total power)
Weight: 20kg

My vehicle, which has X8 chassis type, loses height when it moves on the pitch and roll axes. Especially in the pitch axis, that is, in forward and backward movements, the vehicle loses height from 7 meters to almost 1 meter. When I look at the flight time from the mission planner interface, I see that it is not aware of losing altitude because it always shows that it is at 7 meters. In autonomous flights, when it is ready to move to the next waypoint, it realizes this while correcting itself and corrects the altitude.

I’m getting the same trouble even though I’ve done all sorts of advanced calibrations. Currently, with my team, we are investigating the pressure change created on the chassis due to the propellers when the vehicle makes pitch and roll movements. I tried to use an external barometer and position it in other places, but the same problem continues.

Since we won’t be flying very high, I want to use lidar as the altitude sensor. In addition, I use Here 3 GPS and RTK GPS in the system, I want to make GPS the priority sensor for altitude nearby, but I have some reservations because I haven’t tried it yet, I really need your advice and opinions.

I have read many of your articles and answers and I am aware that you are knowledgeable in your field, maybe you can help me with this. Thanks in advance.

@UAVSkies @Eosbandi @Naterater @smartdave

Get a dataflash .bin log of the altitude loss happening and upload to a filesharing site, then provide the link here.
There could be a bit of tuning required, plus it could be caused by prop wash and the flight controller would need better protection.

This is Baro Alt Graphic in crash flight

Gps Alt Graphic


This is the autonomous flight plan. At this time of flight, the barometer was used as a priority sub-sensor. You can see from the flight plan that the altitude is always fixed at 7 meters, but the barometer sometimes even saw 12 meters, which is an almost 40% error. Because of this, vehicle reduces thrust and tries to bring itself to 7 meters, but it is not even aware of the actual height.