Quad Loiter poor performance

Need help to tune the ALT-HOLD and LOITER performance

Hi All,
My build specs,
Frame size - 800 mm (motor to motor)
Controller - Pixhawk 4
Code - 3.6.7
Battery - 6S LiPo
Motors - T-Motor MN501-S 360kv
Propeller - 18*5.5 APC
AUW - 5.43 Kg
T/W - 3.6

Hover throttle is 30% and MOT_THST_EXPO - 0.4 based on thrust rig data
Quad flew well in Stabilize mode and in ALT-HOLD - copter bounces during fast pitch breaking, performed AUTOTUNE in Roll / Pitch / Yaw axes separately
Still alt-hold bounce is seen during flight.
in LOITER mode quad oscillates and over corrects to winds of 3 m/s and also on quick release of stick
Did AUTO mission, quad climbed and navigated smoothly in tail wind, whereas in head winds the copter started to oscillate in pitch axis, soon i have triggered LOITER to take control but that made it worsen the oscillations of around 80 deg in roll, again on switching back to AUTO quad stabilized but with oscillations in pitch it returned home and landed.

Can someone here help me to identify the cause and solve it?, link for data flash log of AUTOTUNE as well as AUTO mission is been added below


What ESC are you using to drive those MN501’s ?

Your GYROs and ACCs look decent, but the Vibes are high and it starts clipping the values… Drivetrain seems to not be able to sharply cope up with the autopilot demands.

Hi @ThePara thanks for the reply.
Esc is T-Motor AIR 40A
I tried all sorts of dampening isolated controller from touching, use of foam, used rubber padding for motors as well but still the vibrations are not coming down, One thing to note is the winds are gusty upto 8 m/s when I was flying.

Vibrations are killing AltHold.

Post some pictures of your rig so we can see how the FC is mounted and how you have the interconnection wires laid out.

What does it mean when the vibes drop to 0 and go flat?

Hi All,
@ThePara and @OldGazer, pixhawk is touching the side walls, now i have re adjusted the mounting and flew yesterday, looks like the vibrations reduced, but still there in Z-axis, i guess due to winds, please provide your views also, winds during yesterday’s flight were higher than my previous flights. please take a look at the new log https://drive.google.com/open?id=1tYIxv2TTSnszPjcMLMMaxKtoS0tO9QqV
And now my focus is to get a better performance in LOITER in terms of holding the position, the vehicle slips into oscillations on getting disturbed by winds.
Can someone suggest me how to tune loiter in 3.6.7 please.

If you can’t follow my instructions, there is nothing I can do to help you. Post pictures that show how your aircraft is built, how you have the wires routed and how you have the flight controller mounted.

In my experience, oscillations are induced by two factors: wrong PID values or lagging drivetrain.

I’ve tried a lot of stuff progressively on 4S, 6S and 12S. I found out it’s a big no-no going with props larger then 16 inches on 6S, and best results come from 15 inches. That is, if you want to fly regardless of whatever nature throws at you below gale wind speeds.
Otherwise, on a calm, flat day, you can wallow around on 24 inch props on low-kV motors, at no more then 3 m/s, but if someone sneezes, it will start oscillating :smiley:

I know those MN501’s are pretty big, and an 18 inch prop fits them, on paper, but you’ll have to try driving them with some serious silicon. Those 40 Amp AIR’s won’t do.