Autotune sudden unstable and blow up

ArduCopter Copter 3.6
1

Hi guys, something really unexpected happened today. This quad has a very powerful motor prop combo, with an expected thrust to weight ratio of 3.2 when it is flying.

I’ve set the PID filter rates to 10 Hz, because the prop size is 22 inch, which I think is considered as a big copter.

I started the autotune process with the default PID gains, for just the roll axis only. Immediately after flipping autotune mode, the copter went unstable and just blew up. I suspect its the default P gain being too large a value, but any inputs please?

Log file is attached here. Thanks a lot!

000000001-explode.BIN (316 KB)

2

Please tell me more about how it “just blew up”. Like it blasted into pieces? The way you say ‘went unstable and just blew up’ makes it sound like the instability and the blowing up were two sequential events…

3

With such a high power to weight ratio you will need to do more than just the filters.

Go through the tuning page here

If you have a look at your motor outs you can see that the FC just could not reduce power sufficiently to maintain stability.

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Screen Shot 2019-11-12 at 8.03.24 am.png2320×893 315 KB

Another interesting thing is that the VIBES go way up, including a lot of clipping, as soon autotune started.
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Screen Shot 2019-11-12 at 8.02.57 am.png2325×891 240 KB

The IMU data looked good until you started autotune at which time it increased dramatically.
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Screen Shot 2019-11-12 at 8.05.08 am.png2312×885 400 KB

So I would also look at your frame and motor setup for any aeroelastic effects.
It doesn’t look like the frame is handling the power very well.

4

Oh i meant the controls blew up, as in it became unstable

5

I also noticed the vibrations started as soon as I flipped the autotune on. The cube pixhawk is bolted on the frame securely so thats why I am so puzzled. Do you think there is something wrong with the IMUs?

As for the frame I believe it is rigid enough. I’ll rebuild the aircraft and follow this first and see what happens.

6

I noticed you said “expected thrust to weight ratio of 3.2.”

Was the aircraft Take Off Weight (TOW) any where near your “expected” TOW?