Help needed to understand failed auto take off

I have a situation where auto take off does not succeed fully. What happens is shown in this video:

We see that the plane flies horizontally at the exit of the catapult, although the desired pitch is 20 degrees. We also hear that right after exiting the catapult the throttle is reduced from 80% (max throttle) to about 40% for an unknown reason. The observation is compliant with the log as shown on this screenshot:

My parameters values related to auto take off are :

ARSP_FBW_MIN : 9 m/s
ARSP_FBW_MAX : 22 m/s
THR_MIN : 0%.
THR_MAX : 80%

Any explanation(s) why auto take off fails ?


Can you post the colpleate log please.
Also it would help to post your mission.

Here is a link to download the log file :

The link remains valid for a week

Hi all

Any luck with this. I’m in a similar problem. Auto take of limits throttle to 90% and unable to take. Any ideas? ESC has been calibrated. testing throttle in FBW shows 100% throttle output. And I can feel the power is much more than that of auto. What is going on???

@Hugues Throttle cut is due to the aircraft being well above cruise airspeed (which is what it wants during takeoff). The reality though is that it should have been pitching up for the climb out though which would have helped bleed off speed anyways.

Out of curiosity are you confident in what the airpseed was reading? Looking at it it is noticeably quite noisy looking on the ground.

I’m not sure at the moment why it didn’t succeed, as it clearly shows that it was looking for a 20 degree pitch up. Maybe someone with more experience with looking at logs from things that uses multiple tail surfaces could take a look? @MagicRuB @tridge any ideas?

WickedShell, I get mine 90% on bench test (auto) which should read 0m/s airspeed, would it still do that throttle cut?

Not if the airspeed is 0, it should be looking for cruise airspeed (well first looking for minimum pitch up, then cruise airspeed, then max pitch (while at cruise airspeed if possible). Could you post a dataflash log from a bench test?

Hi @Hugues,

I took a look at your log, got a copy from @WickedShell since the link expired.

have you flown this aircraft before? I’m thinking that it’s too heavy and your servo trims need more gain.

The takeoff shows:

  • Bungee launch throws you to 18 m/s, desired pitch is 20deg
  • cruise airspeed is set for 12 m/s, so we slow down
  • pitch is not able to be reached, the pitch integrator grows to compensate but there’s not enough time to make a difference

This is indicative of an overweighted aircraft and/or a cruise speed set too low.
PTCH2SRV_I = 0.1125, I suggest pushing that up. Try the default of 0.3
PTCH2SRV_P = 1.35. that is very low, I suggest pushing that up to the default of 6.

These can be fixed with an autotune, have you done that yet? I would not suggest performing auto-takeoff until your aircraft is tuned.

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