Hello. A friend of mine built a VTOL aircraft with an MTOW of 25 kg. Unfortunately, he isn’t familiar with ArduPilot or Mission Planner, so I handled the basic VTOL setup. We conducted the flight tests as follows:
- We usually hover in QHOVER and then switch to QLOITER. If the aircraft hovers stably, we attempt the first fixed-wing flight in FBWA. In our case, hovering was perfectly fine.
- For the initial fixed-wing flight, we performed a vertical takeoff in QLOITER, switched to FBWA for the transition, and flew several circuits.
- After that, we switched from FBWA back to QLOITER for the reverse transition and then landed in QLOITER.
The problem is that immediately after switching from FBWA to QLOITER (reverse transition), the aircraft shows severe pitch oscillations and quickly loses stability. (Please see the video and logs at the link below.) I understand the initial pitch-up right after reverse transition—it’s essentially a hard braking action—but I can’t make sense of the subsequent loss of stability.
After discussing with friends, we came up with the following possible causes:
- Parameter settings may be incorrect.
- The aircraft may be too lightly loaded for the motor/propulsion output. (Although the system was selected for 25 kg MTOW, the actual weight—while not yet measured—is estimated to be around 10 kg.)
Some friends even suggest that the flight controller might be “saving” or enforcing an incorrect flight mode/behavior, which sounds odd to me. If anyone has insight into this, I’d appreciate your advice. I also need to clear up a growing misunderstanding that I misconfigured the setup. Thank you.
The oscillations begin after 5:40.
