Mysterious behavior of TECS

Hello all,

I recently flew a powered-sailplane with PX4 V1.13.3 and made the following observations:

  1. The aircraft was flown in mission flight mode in rectangular circuits in moderate winds (4-5 m/s). The control laws (TECS) maintained a constant airspeed throughout the flight with very small deviations from the setpoint.

2.Groundspeeds in the headwind and tailwind legs were also found to differ which is sensible given the winds. However, the aircraft was trimming with two different combinations of pitch and throttle in headwind and tailwind.

  1. The aircraft trimmed with a low pitch and high throttle in headwind, and at high pitch and lower throttle in tailwind.

  2. There has not been any airspeed sensor failure, and therefore TECS was using the airspeeds coming off of the airspeed_validated topic.

  3. Moreover, flight logs indicate differential pressure (hence IAS) being constant throughout the flight. But the change in trim settings of the aircraft in headwind and tailwind legs indicate otherwise. There is clearly a disconnect between what the data says and how the aircraft behaved.

  4. It was also evident that throttle increased as the aircraft approached certain waypoints, and reduced at others. Is there a guidance logic which is responsible for this behavior?

Multiple rectangular circuits were flown during which the aforementioned observations were consistently observed.

Has anyone witnessed this behaviour?

Thank you for taking your time to read through. Any help would be highly appreciated.