Urgent help needed. Big altitude drops in forward flight

900mm quad hovers great, but in forward flight (side to side or backwards as well) the quad can’t maintain altitude. To the point where it starts to lose stability. Normally I’d think this was insufficient thrust, but the rc out on rc1-4 never peak out or even close. I varied the gain from 1 to 2 with no difference. Anyone else run into this? Wouldn’t be such a big deal, but my main mapping crash. This is a unit I built in the last 24 hours and I’m supposed to work today lol! Please, if you have the time see what you think of the log : )

Note this log starts with an autotune which I disengaged after I noticed the altitude changes.

Pixhawk running 3.21
900mm quad at 4.9kg
Foxtech 5010 motors and DYS40a esc. (went with 5010 because I had sync issues with the Foxtech 6135’s)

Ruled out low pressure bubble in forward flight. As alt drops the pressure increases. Odd thing is the recorded altitude drops graph a lot less than reality. what shows as 1 meter is really 5 to 10 times that. If I’d continue forward flight it would crash into the ground. There’s a bit of correlation between Z axis vibs and alt drop but nothing extreme.

Freaking autopilot! So alt hold or loiter it falls out of the sky. Fly in Stab and all it takes is a click of up and it rips in forward speed. With I could work this out. Fly at 20amps is stab at 10m/s only consumes about 20 amps with a 6s22ah batter and a Sony A6000 camera. So for mapping this would rock…well if it didn’t fall like one in alt hold and loiter (auto)!

One more oddity, general stability. Even 4 m/s forward in alt hold or loiter and not only will it drop but it will start to wobble uncontrollably. In Stabilize at 10 m/s, no alt loss and zero wobble.

Everything looks ok, Once you selected loiter it held it’s altitude. If anything your radio is a little off. Normally middle is about 1500 pwm and yours is at 1396.


It holds alt in loiter only if it’s not moving. Anything over a could m/s and it drops. In the log, 1 or 2 meters is actually more like 5 or 10. Pulling my hair out lol!

the vibrations on z are on the high side, that won’t help. Although in your log the spds don’t get that high, but does seem to look worse when moving.