Pitch up on Transitions to Fixed Wing

Hello,

I have built a VTOL from a X-UAV Clouds Airframe with 4 lifter motors and one pusher at the tail. The plane flies well in MC mode and also Fixed Wing mode. Some transitions went well, some didn’t. I am only concerned about transitions to fixed wing flight. Back transitions always work without a problem.

The problem with forward transitions is that the plane always pitches up when I flick the switch. When it’s windy (and the nose is pointed into the wind) it can even end up in half loop. In the log files I have found the following:

You can see the offset between Estimated and Setpoint Pitch. I always have this offset, but the plane flies perfectly level in MC mode. When I flick the switch for the transition, the green line shoots up to zero the Pitch Setpoint. But the Pitch angle already was actually zero, which means the plane pitches up. Then the wind catches it and flips it over. This one ended in a very soft crash without any damage in high grass.

Here is another graph from a transition that didn’t end up on the ground but also showed a significant pitch-up tendency:


My gut feeling tells me I need to get rid of the offset between the green and red line which means working on the Level Adjustment.

Btw, in a level multicopter hover with absolutely no wind, the artifical horizon in QGC is not level. The plane hovers level in Manual and Position mode without stick inputs (as long as there is no wind).

Please help, I am a bit out of ideas at the moment.

Thank you
Wolfgang

I’m just going to ping @sfuhrer here, hopefully he can chime in.

What do you mean? What’s the pitch attitude?

  • Would you mind sharing your log and params?
  • Try to be in Stabilize or Altitude mode for transition.
  • What’s your settings for:
    • VT_F_TRANS_DUR If = 0 or 1, maybe you can increase it to about 3 or 4.
    • VT_F_TRANS_THR If =1, maybe put it somewhere half way or so between that and your FW_THR_CRUISE setting.

Good luck.

@wov Hello, I have encountered the same problem as you, sometimes very good, sometimes not good, this problem will be more serious when there is wind, do you solve the problem?

Firmware:1.9.2 stable.
Is this the reason?