Hexacopter is difficult to fly

[sirch]

I'm new to the self-build game and have built myself a hexacopter on a Tarot 680 frame, so it's a bit of a beast. I am using a Pixhawk 4 flight controller but it is difficult to fly and I am wondering if this is normal or if I haven't got it set up correctly. I have only flown it a few times and have wrecked several props and the undercarriage, it seems incredibly difficult to land, it is either hurtling up in the air or crashing into the ground, there doesn't seem to be much in between. I have been launching it in stabilised mode and have once got it into hold/loiter which worked fine. It also yaws/spins at any opportunity. Is it possible to change the scaling on the sticks to make them less sensitive? Or do I just need a lot more flying practice?

[ched]

I found the bigger the quad the easier it was to fly as it responds slower.
I assume you have done basic checks like motors and props spinning in correct directions on all arms?
I assume you are flying in a stabilised mode? If not fly in a stabilised mode as acro/rate mode is very difficult to fly.
I assume that you have calibrated the esc, setup the receiver to the correct ranges (1000 = low 1500 = middle 2000 = hi).
How did you setup the PIDs? PIDs might be way off which could be hampering your flights or tx rates. Rates are a sort of scaling of your sticks, you can put in a curve that makes the sticks less sensitive in the middle and very sensitive at the ends (not sure on Pixhawk but that's how Betaflight works).

I found using a computer simulator was good for getting my fingers better able to make the small movements you need on the sticks. I use FPVFreerider and Velocidrone with my tx plugged into a laptop. They are both more for acro/rate fpv flying but they might help getting your fingers working better? FPVFreerider has a free version (full only about £8) that you can try. Josh Bardwell has a good series on youtube using FPVFreerider to learn to fly, this will be in acro/rate mode though but FPVFreerider does do stabilised mode.

So good luck, keep us posted and any picys of your 680?

[sirch]

Thanks for the reply ched999uk, yes it's in stabilised mode and I have checked all the prop rotations many times, it actually gets of the ground OK but then shoots skywards an yaws/rotates and is hard to get back to correct orientation.

This -  https://docs.px4.io/en/config_mc/pid_tuning_guide_multicopter.html suggests that the default PIDs should be OK so I haven't done any set up for them.

I'll try the simulator software and see if that is similar in handling to the real thing


This thread is where it all began

Here's a photo of it early in the build
[attachment id=0 msg=214283]

[ched]

I remember the thread 
If it's zooming off and yawing that suggests that maybe it needs calibrating. I have not used a Pixhawk but normally you need to calibrate all esc, calibrate tx to rx to FC (Pixhawk), calibrate the accelerometers and make sure gps is set correctly.

The sim will not be similar in handling in any way to your t680 but it might help your fingers get used to small movements needed.

[sirch]

Yeah, done all that calibrating several times, it didn't make any difference. Hopefully since this is in the PixHawk thread someone will be along to share expereince.

[UrbanFPV]

With Pixhawk, you will need to calibrate all sensors in the sensors page on QGroundControl.  But normally it won't let you arm if they're not calibrated.

Check the mixer is right for the Hex ie: Hex + or X mixer.

https://docs.px4.io/en/airframes/airframe_reference.html

Yawing suggests a magnetometer issue.  There have been magnetometer initialization issues with particular pixhawk boards (PixMini).

I presume you're using PX4/Dronecode firmware as opposed to Ardupilot?

I've not had a problem setting up quads or fixed wing with Pixhawk, but did have issues trying to get a Coax Octo to fly correctly which I'm sure was down to the mixer not being right for the setup.

[sirch]

I'm fairly sure I've got PX4 firmware but I'll check and I know I have X hexacopter frame set up correctly.

The thing is that it flies, lioters, etc it is just massively sensitive, breathe on the sticks and zings of in one direction or another, it's like the sticks could do with being 5 times less sensitive.

[ched]

It's always worth running al calibrations again when you have issues as sometimes they have not saved properly.

Just having a look at the documentation and I found this : https://docs.px4.io/en/advanced_config/parameter_reference.html
Look at MPC_XY_MAN_EXPO and MPC_YAW_EXPO. They seem to be setting to give a bit of exponential curve to the controls. This link isnt anything about px4 but it shows what an exponential curve is https://oscarliang.com/rc-roll-pitch-yaw-rate-cleanflight/
I am surprised that there is not an easier way, maybe there is?
As I said I have NO experience with px4 or any Pixhawk  Flight Controllers.

Hope that gives you some ideas?

[sirch]

Friday was the first time I have had to have another go with the drone. I have reinstalled the firmware and gone through all the calibrations, etc. again but perhaps the biggest change (or most likely to have had an effect) is to set some curves (expo) for the sticks on the transmitter. As mentioned above it all just felt to sensitive and I set some pretty dramatic curves reduces that sensitivity.

So after charging the batteries, and doing final checks it was into the garden to give it a go


https://youtu.be/Igd5PeCPrPg

The fact that I could fly it with one hand and video it with the other speaks for itself