I am in the process of setting up a custom "H" configuration quad, designed for high speed. It uses 4x MN5212 motors with t-motor 18x6.1 props. AUW is about 4.5Kg, so this thing has the thrust/weight ratio of almost 4:1.
log file here:
Conditions were quite calm. winds about 2m/s at ground level.
I configured this system to tilt the motors forward 15 degrees, and used SENS_BOARD_Y_OFF to correct this. In this way it hovers at 0 degrees pitch, but with the props "out of plane". The goal of this effort was to present a smaller cross-section to airflow at high speed.
I successfully flew a simple "out and back" mission at up to 20m/s. As speed increased a strange thing started to happen when reaching the last (upwind in this case) waypoint. When stopping at this point a large roll and yaw error would be present. I could tell from the ground the system was fighting HARD to correct this, but for whatever reason couldn't manage it. After a few seconds it would sort itself out and be fine, but this is behavior is quite disturbing. The same issue was not observed when stopping at the downwind waypoint.
I flew a second similar system without the motor tilt, and it seemed fine, but had it's own set of issues (more on this below). Could this tilt angle be to blame? If so I would have expected to see the same problem on the downwind leg.
A different system than with the first issue. No motor tilt angle, but otherwise the same.
When transitioning from the last waypoint back to the first (re-starting the mission) very high "spikes" in roll angle are commanded. Why is this? I spread the initial and final waypoints apart a little and the problem went away.
It also seems that a considerable roll error is present at certain times. The tuning seemed good, I turned down the stock gains just a little to prevent oscillations, and overall it felt pretty good for a large system.
I conducted 6 flights in total on this day. Other logs show similar issues, I can provide them if required. Please also look at the vibration levels on each system. I have the autopilots mounted on silicone dampers that normally work quite well, so I'm not sure what is going on, or if this is fine.