I've been chasing a weird oscillation problem for close to a year now. It isn't visible by line-of-sight, but makes FPV really unpleasant. The oscillation is in the pitch-axis, and appears whenever there is horizontal motion.
I've exhausted the obvious (to me) possibilities, like motor/prop vibration, PID tuning, flight mode, airframe resonances, damped mass (gimbal) resonances, bad hardware, and video sync. The aircraft has evolved considerably since I first noticed the issue, but the problem persists. The only parts that I haven't changed are the FC vibration damping, and the motors/ESCs.
The vibration damping just occurred to me today, so that will be my next test.
So I'm now forced to consider some possibilities that I hadn't before, or didn't want to. I may have to replace the ESCs and motors. I really want to be able to use the set that I have. They are KDE products and set me back quite a bit. The possibility that I want to ask the forum about is whether or not there are any FC parameter changes that might help. I'm using a 3DR Pixhawk, Rev2.1, and Copter 3.4.6.
I've included a link to a goggle DVR recording and a recent dataflash log.
I would be most grateful if there is someone out there who can look over the log for anything unusual. I've spent some time checking it out, but I'm not expert enough to spot a problem. One thing that you might notice is that my STAB gains are pretty high. I don't know why it tunes this way, but I do know that I can reduce the STAB_P gain until the aircraft is extremely mushy and the problem persists.
The aircraft is a TBS Disco Pro, with 5200mAh/4s, KDE 20A ESCs, KDE XF2315-885kV motors, minimOSD, ImmersionRC 600mW/5.8GHz VTX, DragonLink Micro RX, 3DR GPS/Compass, my own barometer mod (tried that too), Runcam Swift FPV cam.