Large Octocopter X8 Configuration unable to maintain altitude


I am a project student ad this is my first time trying out arducopter.
Currently, our octocopter is unable to maintain altitude even with full throttle and seems to be bouncing after awhile.

Can it be due to any configuration that i may have set up wrongly? Like PID etc…

The set up is as shown below:
Motor: T-Motor U3 700kv x 8
Propeller: 12 x4 CF
ESC: Turnigy Plush 30A
Battery: 10,000mah 4S1P 14.8V
Flight Controller: APM 2.7

Flight video:

Thanks in advance for all the help!

Hi Jonathan.

A few things may help.

How heavy is it all up weight ?

Can you download a log ? (although APM logs are pretty sparse for information it may help.If you can do a couple of test flights like that with different log options it may help more.Do a hover test like that with IMU enabled.Then disable that and do another test with RC_OUT enabled and IMU turned off.

The total AUW is 6.3kg
Sure i will do a flight test tmr and upload the log.
Thanks Jagger

It is too heavy for 12" props at 700kv. Are you using different props for the top and the bottom?

Both the top and bottom propeller are of the same size and pitch

Agreed.It looked like it was struggling a bit.I run a hexacopter @ 610 and 12 inch props around 3.6Kg.I run a bigger one with 18 inch props and 274Kv motors at about 7.8Kg.And that’s pushing it bravely.

What voltage does it run at ? Which motors ? I could ask questions all day. Perhaps a more detailed specification would help.But it does look heavy.And seriously good fun too I may add.

More volts may do it some favours but may produce smoke.

14.8V 4 cells lipo
T-Motor U3
I think the only way to for me to try out with a 13 x 4.4 propellers, it’s the maximum size and pitch that is recommended by the motor that i bought.
However, do i continue with 2 identical propeller on top and below or should i for example use a 13 x 4 on top and maybe a 12 x 4.5 propeller on the bottom?

Right.Limited to 4S then.

8 X 1600 = 12.8 Kg of thrust maximum on those props but reduce that total due to power losses in the X8 configuration.Think along the lines of 10Kg thrust lifting 6.3Kg at maximum throttle.and you’d get your video.

13 inch would give a theoretical boost too 14.4Kg but again X8 losses reduce that.But it would lift a bit more.

I see wheels.So it’s a self propelled octocopter ? And,most importsntly to save new hardware costs,can it lose a fair bit of weight ?

This project is to create a flying car so the wheels are for the car, basically an RC car and a drone combined into one.

I have change the material of the brackets from mild steel to aluminium which will actually save me around 300 grams.
Pretty significant. This will bring the AUW down to 6KG.

Btw, will spray painting a strip on the propeller affect the performance? It is currently on the propeller for aesthetic purposes.

The paint may affect the balance slightly.It can be used for balancing in fact but tape is easier and quicker drying.

Try it with 13 inch on the top and 12 on the bottom,increasing the pitch if possible.It can only be more fun than 12 inchers all round. :grin:

Oh yeah.Got a log ? You don’t get a lot from an APM log but I can scan the parameters and do a bit of prodding.If you can do the same test flight with RCOUT enabled it would be good.That would show graphically what the motors are trying to do.It could be the bottom motors are maxing out and dragging the top ones down with them thus losing more lift.Not sure.I’m guessing.

I think this project is a good engineering exercise because it shows how delicate a balance there is, in autonomous flight, between weight, energy storage and efficiency.

Makes me want to drag my DYS X8 out. :grin:

this is a good resource to get you in the ball park with engine and prop specifications.

This what happened when i attempted to get the log… :frowning:

Don’t use the terminal.Use the dataflash logs tab under the HUD on flight data page.You can select the download target under Config/Tuning /Planner.

I manage to do a test with 13 x 4.4 propellers from T-Motor and also 13 x 4 propellers from Quanum. With the 13 x 4.4 on top and bottom, the drone was able to maintain in the air before it ran out out battery and started descending.
However, with the 13 x 4 propellers on top and bottom, the drone was once again struggling to maintain a level flight.
The only difference other than the extra 0.4 in pitch, the blade on the 13 x 4.4 has a bigger surface area compared to the 13 x 4 propeller. Could it be the surface area of the propeller that made such a big difference?
I will carry on with more flight test soon as I ran out of batteries while doing these tests today.

Below is the video of the flight with 13 x 4.4 propellers.

That sort of backs up what we were saying about lack of lift.The 13x4.4s produce just enough to do a bit of actual flying but it still isn’t enough.About the only thing I can suggest is some even more aggressive props,both top and bottom,and keep a close eye on motor temperatures.Say 13 x 5 on the top and 12 x 6 underneath.That will be close to,or over,maximum for those motors I’m guessing.It depends on how long and far you want it to fly I suppose.

With that hardware you need ~15x4.5 props and 40a ESC;s. Or higher kv motors with smaller props.You can’t be getting much flight time out of that configuration.

Alright i will probably try 13 x 5 on top and 12 x 6 underneath and see how it goes