Just came across this post, and would like to add on my experience. I thought I was the only one encountering this problem with extremely, extremely violent oscillation. I never posted about this problem because I don’t have a log.
I don’t have an airspeed indicator, but do have a gps/compass. I believe compass is calibrated correctly but sometimes the airspeed/groundspeed/wind speed just don’t make sense at all. Sometimes. The violent oscillation happened on my second and third flight after doing autotune. It tends to happen when the “displayed airspeed” get too low. You just have to push full throttle, dive down, and let the airspeed build up again to more normal value. This is before I turn the synthetic airspeed on!
After turning on the synthetic airspeed, the problem persist and seems to get worse. As soon as your displayed airspeed goes below minimum airspeed, there is a very high chance the plane would enter into extremely violent oscillation. I shouldn’t have continued to fly, but I did because I know I can get out of it by diving hard and let airspeed build up again. To continue the troubleshooting, I halve all the PIDs but problem still exists. (maybe not enough?). Then the final mistake: I tried to turn off EKF2. AHRS_EKF_TYPE=0. In my twenty years of RC experience, I’ve never had a complete hull loss. But thinking back I am just glad nobody on ground got killed as it happened 300 meters over a lake. Again, same problem, pitch momentarily went up and airspeed momentarily went below min. speed. Started shaking. The more violent it shakes the lower the displayed airspeed. The lower the display airspeed the more violet the shake. vicious cycle. Lost 300 meters in matter of seconds. I tried to give full down elevator, diving straight for the ground to get out of the shake, but airspeed never get picked up again this time (maybe EKF2 off) and plane plunged into the lake.
@tridge apologies again I don’t have a log, but I wish someone can really look at this issue. This problem of extremely violent uncontrollable oscillation could be a big safety hazard for the hobby. In the old days when planes are made out of balsa, there is no chance the plane can survive fractions of this oscillation . Thank you.