Tethered flight: EKF attitude estimation

[BroilerCompiler]

Hi everyone,
i am trying to control the crazyflie in a tethered setup, similar to https://forum.bitcraze.io/viewtopic.php?t=409.
I notice that when the tether is in tension, the attitude estimate drifts to zero.
However, when the drone is not flying, the attitude estimate seems to be fine.
In the attached screenshot i held the drone with my hand at ~15 degrees pitch.
When the rotors are off, the attitude estimate is stable.
When i turn the rotors on (press 'Takeoff' in cfclient) and hold the drone (i am applying a force with my hand to keep the drone in place), the estimate drifts back to zero

Can you help me understand why that is?
Or even give me pointers on how to change that behaviour s.t. the attitude estimate stays correct despite the disturbing force of my hand / the tether?
Thanks!

[gemenerik]

The Kalman filter behaves differently when flying vs. not flying. We feed the boolean quadIsFlying to the Kalman prediction step, which relies on the supervisor to determine if the drone is flying. The supervisor considers the drone to be flying if one or more motors are above "idle" thrust.

The flying mode of the Kalman filter expects the drone to be airborne, which is why it doesn't handle being held in a specific orientation or experiencing large external forces well; it wasn't designed for that.

The non-flying version of the Kalman filter was specifically designed to be compatible with manually moving the drone around by hand. You could try forcing the non-flying Kalman filter if you want to apply large external forces, but I'm not sure what kind of flight performance you can expect in your tethered setup. It might work better to just use the flying version.