F450 Oscillations with heavy load

[timhmn]

I have a bit of a home-brew F450:

• F450 frame with Naza M Lite/GPS
• Turnigy D2830/11 1000Kv Motors
• Turnigy Plush 30A esc
• 9047 props
• 2200 3s (20-30c) battery

Generally, it flies excellent. I'm not the greatest pilot in the world by any means, and I've not been flying that long, but that setup has worked well.

Until I added a GoPro / Tarot gimbal combination with a second 2200 lipo (I didn't want to power the whole thing off one 2.2 knowing the flight time would be pretty limited). What is happening with this load is that I'm getting a lot of roll (left / right to the direction of flight) oscillation. Sometimes it starts out smooth then gradually increases in frequency after a few seconds, but then hits a frequency and stays there. I've been able to keep it under control (flying in GPS mode), but its obviously not flying properly and wouldn't be able to get usable video.

I've messed around with the gain settings for the Naza, but its not making any difference. I suspect that its a mismatch of overall weight, props, motors, batteries, ESCs, but i'm not sure what is the right thing to change / experiment with. 4s batteries? Bigger or smaller props? Different Kv motors?

Any ideas appreciated...

 

[maxwelltub]

how much does it weigh, and how much thrust are you generating? You want double the thrust relative to your weight if possible. You can find your thrust specs by looking at the chart that is listed where you picked out your motors. It does seem like a gain setting, make sure you are adjusting your basic gains first.

 

[timhmn]

All up weight is right about 1700g - i don't have it exact, this is me adding up the component weight and adding a bit extra. The motor specs chart lists thrust at 890g (so x4 is 3560g) or more than double total weight.

I adjusted basic gains in roll and pitch from 70% to 170% (i had been running these at 150% before adding the camera/gimbal) without really seeing any noticeable change in the oscillation.

 

[econfly]

Did the center of gravity change with the heavier payload? If so, I would check the GPS offset values -- could be the issue particularly if the problem is only showing up in GPS mode.

 

[timhmn]

Did the center of gravity change with the heavier payload? If so, I would check the GPS offset values -- could be the issue particularly if the problem is only showing up in GPS mode.

This was a good suggestion - the CG did change quite a bit. I mounted all the stuff the way i wanted it, then checked the CG and added a little weight where needed and moved the second battery a bit to get the CG back to where i want it. I don't always fly with the camera, so i would like to keep the CG in the same place regardless (as opposed to remeasuring the position of the GPS relative to the CG with camera).

From there, I reset the gains to the values I was using (the ones in the table at the end of the Naza manual) which were likely too high, and began to work downward in a methodical way. Originally, pitch and roll basic gains were at 150%, at about 90% it started to settle down, and at 85% got pretty smooth. It was really breezy today so the next time its calmer I'll try to tune it further. For example, I'm not sure if my vertical gain is still off or if it was bobbing a bit because of the gusty conditions.

 

[RobertsUp]

There's a possible alternative explanation. The Pendulum Effect. (google it)

It happens when the weight carried is too low below the craft. I saw the phenomenon clearly when I dangled a fishing weight underneath my Blade mQX. The mQX began oscillating rhythmically side to side as the weight swung. The little fc couldn't compensate and the craft continued oscillating.