So, another flight today with my Pixracer-equipped QR400, with the goal of improving Acro performance and determining how good it can be. Results: much better than on first attempt, yet still a long way to go until it's anywhere near Clean-/Betaflight acro performance. And since PID tuning is quite troublesome (see below), for now, I'm switching to APM to try and see what the "other side" currently has to offer. Some more detailed feedback on the day:
First flight: using the new default values for the "generic 250 frame" as posted by Lorenz above, + ATT_BIAS_MAX = 0.0. More lock-in, but the PIDs are too high for my motor setup - audible oscillations even in Stabilize, and bad oscillations in Acro. LogMuncher log here: http://logs.uaventure.com/view/keBUJTk9ZvpC3NR2ZMpR2T, but safe to say, this setup wasn't flyable in Acro anywhere above hover throttle.
After landing, I tried using the ESP8266 Wifi link to my Android tablet (with freshly updated QGC) to adjust PIDs. However, it simply didn't work. It tried establishing a connection, but reported "connection lost... connection regained" every 5 seconds or so, despite the WiFi icon on the tablet showing full bars. After 10 minutes trying that and still failing to establish a full QGC connection (green connection bar never reached the right edge), I gave up on WiFi and tried USB (with an OTG cable). As opposed to before, now Android actually asked me "would you like to launch QGC for this device", to which I confirmed. QGC launched and didn't do anything - no connection being established. Tried replugging, with and without battery power, with QGC running and not running (force-quit) - no results. Trying to manually configure the serial link in QGC on Android still doesn't work, either - the serial port selection dropdown is empty.
Finally, I gave up on the tablet altogether and used my trusted laptop with QGC (Windows) and a normal USB connection to adjust the PIDs somewhat, reducing roll/pitch P and D values a little. And then went into the second flight:
Second flight: much better than before. No audible oscillations, and woohoo, Acro was actually usable now. I flew around for a couple minutes getting a feel for the copter, and tbh, it wasn't really good. While the lock-in was a lot better than on my first attempt a while back, it was still pretty bad, with occasional random drifts on both roll and pitch axis, and with sluggish recovery from large control inputs (extreme case being recovery from a 360° roll into level flight). The copter was nowhere near the precision I'd need to fly an FPV gate course. I did try navigating between a few trees, but that was already rather challenging compared to flying a Betaflight racer in the same area. LogMuncher: http://logs.uaventure.com/view/zgccLTJ22PQcsr6mtbAMgg, and I'm uploading a DVR video from my goggles from this flight as well, will add a link when it's up.
So... yeah. There's some progress, but I'm not convinced yet. I've tried acro with APM firmware on a Pixhawk back in APM 3.2.1 days (late last summer), and that was already quite a bit better than what I'm seeing here. They say they've improved it significantly in 3.3 and then again in 3.4, so that's next on the list of what I'll try. And if that fails... I might have to remove the Pixracer altogether, and install a SPRacingF3 or something like that to just run Cleanflight, maybe the INav branch that seems to have reliable PH and RTH lately.