Oscillation in Roll and Pitch Resulting in Crash

Hey all,

Been trying to determine the problem with my Aircraft. I had this problem a while ago where it would being flying fine and then randomly go into a death wobble (osscillations in pitch and roll) where RCOUT(rc9-12) maxs out.
The log I will share is after some pretty vigorous testing after the last tune that I gave it.

This problem was also happening on stock tune, previous tune and now this tune.
Some points to help:

  1. its happened at various voltages (23.2, 22, 21.7)
  2. batteries are in good health
  3. Aircraft flys fine before this happens (no oscillation in desRoll/despitch and roll/pitch)
  4. this log shows that motors are commanded to power up but RPM doesnt increase

Aircraft specs:

Your help would be appreciated.



Change these, very safe to do so

That quad is almost a bit overweight.

I see what you mean about RPM not necessarily matching the commanded output. There might be some sort of limiting action going on with the ESCs. What are all your BLHELI settings? A screenshot is OK.
On the other hand, a commanded output might not produce as much RPM change as we’d expect.

Descent stability is an issue, you might want to experiment with a lower MOT_THST_EXPO and see if that makes any difference - probably after we sort out the whole motor/RPM/thrust loss thing.

If this stuff can be fixed then another Autotune would be in order.
I’ll keep looking and call out if I spot something.

1 Like

Thanks for the quick Reply!
Wont be able to get back out to the field until tuesday, but here is the BLHeli settings

Motor Timing = Auto ← let BLHELI do it’s thing
Temperature Protection = 100
Low RPM Power Protect = Off ← this is the main one

I use Sine Modulation too, because it sounds like a really good idea when you read the doco, never had an issue because of it. A lot of people dont use it, maybe just because it’s optional.

After that do a hover test in AltHold just with gentle movements, no aerobatics or missions
Let’s see that log

Also set
since there’s low risk of fly away.

Question: it would be fairly easy to set these all by default using dshot commands. Would that be useful do you think? Or too dangerous?

Hi Andy,
that’s a good idea, I’d like to consider it more before making a call.
I suspect even small quads using BLHELI would be OK with Low RPM Power Protect = Off at least. I know it’s an essential setting for anything over say 9 inch props. You would know a lot more about the smaller quads than I - what BLHELI settings do you use on small quads?

I think I’d even drop the temp to 90 degrees C. I just looked at a long log from todays test flight of a QuadX8 (6S, 13inch props) and none of the 8 ESCs got over about 65 peak, and averaged around 45 to 50 degrees C. BLHELI starts limiting output gradually from this temperature and above, it’s not like there’s a hard cut off.

I do also turn off the low voltage protection - it may not be appropriate to set this for everyone since some people may depend on it. I’m a bit of a voltage nut and go to lengths to ensure the power brick is calibrated accurately for voltage and current, and I rely on that rather than the ESCs.

Hey I havnt been able to get to the field due to weather. Hopefully Thursday.
is the KV maybe a bit Low for this size of aircraft? My thought process is that the autopilot is asking for certain output from the motors that the motors cant get to fast enough, causing the instability.
Looking at all my crash logs is does seem to appear more often towards the tail end of a battery (~22V)?