I just made a large 7kg weight hexacopter with 6x Gartt ML5008 400kV engines, 6x Hobbywing XRotor Pro 40A ESC with DEO ON, 6x 15" carbon fiber propellers. I have on board Tattu 16000mAh battery and it lasts for 8 minutes of hovering flight! I have a latest Ardupilot firmware on my pixhawk 4.
Does anyone have an idea what could be the reason for this and how to improve it?
You are using a a very small props for those motors. I have a simmilar motor from Tmotor 5008, and I user a 18" prop. Anyway, I have found this chart from your motor, and it say it can handle a 17" porps. This will give you more flight time.
In your current configuration, the chart it say if every motor must move 1.2kg (7 kg aprox total from your hexa), you will draw 5.8Amp for each motor, for a 35 amp aprox. So may be your current and voltage system are not calibrated, or your battery is a quite old. what is your landing voltage? what kind of voltage and current system are you using?
and share your log file
I ran your configuration through eCalc, which indicates the drone should be burning about 8 amps per engine at hover, so 48 amps for 6 engines. I assumed a 6s battery. All the other parameters look good. Calculated thurst/weight is 2.5 so your propeller choice is adequate. If you run 17 inch props, eCalc predicts two more minutes of flight time, but at max power you’d exceed the amp rating of your ESCs and push up the engine temp.
The only thing I can think of is your battery wasn’t fully charged or it’s old as suggested by AMV.
You likely won’t get ~85% of pack capacity if you are flying to 3.6-3.8V/cell and 50-52A at hover is right. You got what you built. Depending on your prop pitch I see a hover throttle a bit higher than desired. I used 1555. 1755, if they would fit your frame, would produce a good target hover throttle. Or lighter weight of course.
This craft is flying pretty well. The hover thrust commanded outputs are not bad and tuning is pretty good. The Rate Filters look a bit low for a15" craft but as mentioned tuning is good so perhaps keep it.
I think at this weight you will just have to live with the flight time you are seeing. Determine what a good low battery voltage to FS from by mah consumed (80-85% of pack capacity) and fly to that.
Set these parameters: