Heavy Lift Octorotor Loss Altitude

[chris_pilot (Chris)]

Hello everyone.
I have a problem about climbing to waypoint altitude.

frame : Octoquad
motors: T-motor u8ii 150kv
ESC: T-motor alpha 60A LV
prop: 28 inch
battery : 22000mah 25C x 2(Parallel) (6S)
pixhawk : CUBE
Drone total weight with payload : 18Kg

Hover current : 80 - 90 ampers

I can takeoff with ALTHOLD mode and i fly to auto climbing point manually but when i started a mission at this point to 150 meters altitude, the OCTO the starts to descend suddenly. finally I hit to ground. Also I started RTL in another flight but drone hit to ground very fast.

I tried this test three times but same problem happened.
I tuned drone with autotuning PID succesfully no problem also my DESroll and roll logs is good.
I have no problem about GPS jamming and Baro altitude.
I checked all cables and batteries no problem no heating after flight.

here is log:
log

Waiting yours helping
thank you so much …

[dkemxr (Dave)]

Access request required for that log.

[chris_pilot (Chris)]

Sorry, I edited the link

[dkemxr (Dave)]

Too heavy of a lift and the motor imbalance is bad. The average commanded output at hover is ~1750us.and one motor is commanded at max. Overweight/under powered.

[chris_pilot (Chris)]

thank you for your answer.
Yes i saw that. But i couldnt any reason for that. I measured all of motors angle. motors are same angle.
motors are new.

U8ii is a much more powerful for this total weight. (18 kg total weight / one motor : 2250 gram)

[dkemxr (Dave)]

If it was performing per this table it wouldn’t be drawing 85A at hover. 85A (~10.5/motor) roughly jives with the ~70% throttle command your log is showing. Are you sure the total (All-Up-Weight) is 18kg?

eCalc suggests closely what your log is saying with an even higher current draw at hover. ~72% throttle at hover, 1.4:1 thrust/weight. Perhaps your coaxial configuration is loosing more efficiency than typical between flat and co-axial.

[chris_pilot (Chris)]

Yes i am sure that 18 kg
Because Octo frames are 20 percent less efficient from flat frames. i guess this is not problem because this motors need to lift more weight according to t-motor specs.
There is a another problem.
When i installed arducopter 4.0 to pixhawk, frame can climb to 150 meters with auto mission ( need to climb 300 meters) but at this altitude , drone starting to descend very fast .
When i installed arducopter 3.7 to pixhawk, frame can climb to 10 meters but at this altitude , drone starting to descend very fast .
Why problem is changing at different firmwares.

Also this frame has proven itself.
Here is link

https://www.foxtechfpv.com/d130-x8-v2-a2-6s.html

[xfacta (Shawn)]

One thing I’m not sure about, but it could have a big effect, is the ESC type you are using and this parameter:
MOT_THST_EXPO,0.79
It’s normally this value for those sized props, 28inch.
Those ESCs may linearise the thrust, and if they do you’ll need to set:
MOT_THST_EXPO,0.2 (from 0.15 to 0.25)
EDIT: according the advertising blurb on the T-Motor Alpha web pages they have “Linear throttle” but there’s no specifics, so I’d probably give 0.2 a test and see how it changes the flight.

There’s definitely some motor imbalance there, and it got worse mid-flight too

…as if weight shifted to left rear - battery moved towards motor 6.

You dont have battery and current monitoring set up. Configure the current/voltage monitor type then reboot and set these:
BATT_ARM_VOLT,22.1
BATT_CRT_VOLT,21
BATT_LOW_VOLT,21.6
MOT_BAT_VOLT_MAX,25.2
MOT_BAT_VOLT_MIN,19.8
BATT_FS_CRT_ACT,1
BATT_FS_LOW_ACT,2

Also some of your tuning parameters seem off for this size craft, I’d set these:
ACRO_YAW_P,1.2
ATC_ACCEL_P_MAX,24600
ATC_ACCEL_R_MAX,24600
ATC_ACCEL_Y_MAX,10800
ATC_RAT_PIT_FLTD,9
ATC_RAT_PIT_FLTE,0
ATC_RAT_PIT_FLTT,9
ATC_RAT_RLL_FLTD,9
ATC_RAT_RLL_FLTE,0
ATC_RAT_RLL_FLTT,9
ATC_RAT_YAW_FLTD,0
ATC_RAT_YAW_FLTE,2
ATC_RAT_YAW_FLTT,9
INS_ACCEL_FILTER,18
INS_GYRO_FILTER,18
…but you’ll need to go over the tuning guide some more, then run Autotune.
For example the MOT_BAT parameters will definitely help with tuning and stability (in conjunction with correct PIDs and all) and are often overlooked.

[chris_pilot (Chris)]

Thank you for your answer.
I tuned drone as looking at tuning page.
https://ardupilot.org/copter/docs/tuning-process-instructions.html
here is expo graph:

I’m confused now about MOT_THST_EXPO . I did 0.8 because my prop is 28inch. Yes, you may right T-motor ESC characteristic feature may be different.

EDIT: When drone hit to ground my payload dropped at this time : (Also you can see at current graph)

what about you say about ATC_THR_MIX_MAX parameter.

AC3.4-RC5 Hard Landing - Loss of Altitude / Throttle Control

Thanks Mirko, didn’t catch that at first. I wonder why CTUN.ThO would bottom out at 1000 for the same period of time? That suggests to me that the flight controller deliberately cut throttle to all motors. The quad remains upright during the rapid descent and maintains maneuverability - but hits really hard when it lands. I would think that a motor failure on C3 would result in a death spiral or flip of death. The weather is pretty rotten this afternoon but will try to get a video demonstra…

I will try battery set up.

[chris_pilot (Chris)]

rmackay9
Leonardthall
please help.

[xfacta (Shawn)]

I’m not sure what made me think you didnt have voltage and current monitoring set up - I took a fresh look at your log and that seems OK.

A few things still stand out though:
Motors at times are going to maximum, or near maximum. Underpowered/overweight - at least until the payload detaches.
I still think you should investigate or try: MOT_THST_EXPO,0.2
PIDs seem a bit aggressive or strange to me, but then again every craft is different.

[chris_pilot (Chris)]

I will try to test MOT_THST_EXPO,0.2.
do you have an idea about ATC_THR_MIX_MAX ?

[Daveemtb (David E)]

I’m just building a Foxtech D130 X8 Titan (this version is 12S has different motors and props) so I’m interested in the advice in this thread.
The motors you have are 150KV, and Foxtech suggests 170KV for 6S, 100KV for 12S. That’s a reasonable difference. Are you using the 28x8" props that Foxtech recommend with 170KV or the 28x9.2" that T-motor use in the motor data?

Given the motor imbalance, is there a problem with the CG? Motors 1 and 7 are the lowest load, and are on the same axis, and motors 3 and 8 are the highest loaded and are on the same axis, opposite 1 and 7. Perhaps the CG is closer to 1 and 7 and further from 3 and 8? Or perhaps there is an issue with one of the motors/ESCs on the 3/8 arm?

[chris_pilot (Chris)]

Hello again friends.
I am sad because changing parameters didnt work. Even OCTO can not climb 2 meters.
I tried all MOT_THST_EXPO values. But doesnt respond.
I beg your helps

xfacta
rmackay9
Leonardthall

Here my logs :

MOT_THST_EXPO : 0.3
https://drive.google.com/file/d/1WvKYhMFuFYLpYKLp-zMeQhC1mTjoUoHR/view

MOT_THST_EXPO : 0.5
https://drive.google.com/file/d/1tRM6TJpNkBTcQJxJSaIJAhYAdBZAqlb7/view

MOT_THST_EXPO : 0.8
https://drive.google.com/file/d/1BjPBEGnX1pH-ojmQEV6pqtHetr7iWZYL/view

MOT_THST_EXPO : 0.9
https://drive.google.com/file/d/1xJQWCgpEfmD0uECCS7_dDy_JZkX7L4Fc/view

[dkemxr (Dave)]

@chris_pilot

Even OCTO can not climb 2 meters.

Of course it can’t and no amount of parameter manipulation will improve a fundamental problem Let’s review your new log. Hey, it looks as bad as the previous log graphed several posts up…

[chris_pilot (Chris)]

Yes you are right but i stucked. I can not find any solution. i will check all esc’s firmware.

[dkemxr (Dave)]

If your current sensor is calibrated the current draw is inline with what is being seen here. You are at 100A at hover. Using the chart you posted above at 6S and those props that’s close to 80% throttle just what shows here. I don’t think there is any mystery and it won’t be fixed with parameters or settings. . Rebuild it for 12S power. Or call it a Not So Heavy Lift Octorotor.