Thrust spike seconds after takeoff


I’m doing flight tests and tuning on a small/medium size UAV platform (Quadcopter, MTOW: 1.5Kg, 11inch propellers, 5s battery, etc). And I am experiencing strange behavior on all the test flights I am doing on the platform.

In all flights a few seconds after takeoff (approx. 4s) a sudden spike in the “vhicle_thrust_setpoint” occurs. This spike in “vhicle_thrust_setpoint” makes the motor outputs jump to the maximum for an instant, and then go back down almost to the minimum to counteract the ascent. In many cases this sudden jump causes a momentary loss of control, which is quickly recovered and has never caused an accident in all the testing I have done. After this event, the rest of the flight persists in a regular way without any similar events.

This event is very regular and has happened to me in all the test flights I have done so far (more than 20). Always approximately 4s after takeoff.

Attached are screenshots and links to logs.These flights correspond to the last series of flights that I have made, but I have a multitude of previous flights where this same event also occurs:

Fligth Logs:

Log 106:
Log 107:
Log 108:
Log 109:

I would like someone to help me understand why this is happening. And find out how to solve it so that it stops happening after takeoffs.

Thanks for your help.

Wow, that’s fascinating. Thanks for sharing the logs. I can see what you mean but I don’t yet understand why it happens.

Let me tag @MaEtUgR hoping that he might have an idea, or at least find it interesting as well :slight_smile: