Add FW auto-trimming doc

en/config_fw/trimming_guide_fixedwing.md

@@ -31,11 +31,30 @@ The correct order to set the above parameters is:
 1. Fly in stabilized mode at cruise speed and set the pitch setpoint offset (`FW_PSP_OFF`) to desired angle of attack.
    The required angle of attack at cruise speed corresponds to the pitch angle that the airplane needs to fly at in order to keep constant altitude during wing-leveled flight.
    If you are using an airspeed sensor, also set the correct cruise airspeed (`FW_AIRSPD_TRIM`).
-1. Look at the actuator controls in the log file (upload it to [Flight Review](https://logs.px4.io) and check the _Actuator Controls_ plot for example) and set the pitch trim (`TRIM_PITCH`).
-   Set that value to the average offset of the pitch signal during wing-leveled flight.
+1. Run [auto-trimming](#auto-trimming) during a flight to set `TRIM_ROLL`, `TRIM_PITCH` and `TRIM_YAW`.
 
-Step 3 can be performed before step 2 if you don't want to have to look at the log, or if you feel comfortable flying in manual mode.
-You can then trim your remote (with the trim switches) and report the values to `TRIM_PITCH` (and remove the trims from your transmitter) or update `TRIM_PITCH` directly during flight via telemetry and QGC.
+   Alternatively, if you feel comfortable flying in [Manual mode](../flight_modes_fw/manual.md), you can also set the trim values manually:
+   1. Disable auto-trimming by setting [FW_ATRIM_MODE=0](../advanced_config/parameter_reference.md#FW_ATRIM_MODE).
+   1. Fly in manual mode and trim your remote (with the trim switches).
+   1. Report the values to `TRIM_ROLL/PITCH/YAW` (and remove the trims from your transmitter), or update `TRIM_ROLL/PITCH/YAW` directly during flight via telemetry and QGC.
+
+### Auto-trimming
+
+<Badge type="warning" text="main (PX4 v1.15)" />
+
+Auto-trimming estimates the "center trim" at cruise speed (usually), and is enabled by default.
+On first flight (in "Calibration" mode), auto-trimming estimates the current trim of the aircraft, and then updates the general trim parameters (`TRIM_ROLL`, `TRIM_PITCH` and `TRIM_YAW`) with calibrated values after landing.
+The trim estimate is only updated when the aircraft flies with a bank angle of less than 10 degrees for more than 7 seconds.
+So for the first calibration flight you should fly large circles or several phases of straight and level flight.
+
+On subsequent flights (in "Continuous" mode), auto-trimming continuously estimates the trim, reducing the constant load on the integrator of the angular rate control loop if the trim calibration was imperfect or if the trim has changed.
+In this mode the stored trim parameters are updated gradually — the maximum change to the trim parameters following each flight is 0.05.
+
+Auto-trimming is enabled using [FW_ATRIM_MODE](../advanced_config/parameter_reference.md#FW_ATRIM_MODE).
+This parameter is set to `(1) Calibration` (trim parameters are replaced by the trim estimate on landing) for the first flight and is then automatically changed to `(2) Continuous` mode (trim estimate is used to slowly update the existing trim parameters).
+
+The trims can be reset by setting the `TRIM_ROLL`, `TRIM_PITCH` and `TRIM_YAW` back to `0` and `FW_ATRIM_MODE` to `1`.
+This should not normally be necessary as the trims are continuously corrected. 
 
 ## Advanced Trimming
 
@@ -48,6 +67,6 @@ For this purpose, a bilinear curve function of airspeed and a pitch trim increme
 ![Dtrim Curve](../../assets/config/fw/fixedwing_dtrim.png)
 
 <!-- The drawing is on draw.io: https://drive.google.com/file/d/15AbscUF1kRdWMh8ONcCRu6QBwGbqVGfl/view?usp=sharing
-Request access from dev team. -->
+Request access from dev team.  -->
 
 A perfectly symmetrical airframe would only require pitch trim increments, but since a real airframe is never perfectly symmetrical, roll and yaw trims increments are also sometimes required.

en/releases/main.md

@@ -78,6 +78,14 @@ Please continue reading for [upgrade instructions](#upgrade-guide).
 ### Fixed-wing
 
 - Improvement: Fixed-wing auto takeoff: enable setting takeoff flaps for hand/catapult launch. [PX4-Autopilot#23460](https://github.com/PX4/PX4-Autopilot/pull/23460)
+- [Simplified airspeed sensor configuration](../config_vtol/vtol_without_airspeed_sensor.md): 
+  Replacef parameter `CBRK_AIRSPD_CHK` with [SYS_HAS_NUM_ASPD](../advanced_config/parameter_reference.md#SYS_HAS_NUM_ASPD) and renamed parameter `FW_ARSP_MODE` to [FW_USE_AIRSPD](../advanced_config/parameter_reference.md#FW_USE_AIRSPD).
+  To be able to arm without an airspeed sensor set `SYS_HAS_NUM_ASPD` to 0.
+  To not use the airspeed sensor data in the controller, set `FW_USE_AIRSPD` to 0.
+  Added to PX4 in [PX4-Autopilot#22510](https://github.com/PX4/PX4-Autopilot/pull/22510).
+- [Auto-trimming](../config_fw/trimming_guide_fixedwing.md#auto-trimming) (enabled by default).
+  Automatically calibrates trim parameters and then continuously adjusts trim of the aircraft during flight.
+  Added to PX4 in [PX4-Autopilot#22668](https://github.com/PX4/PX4-Autopilot/pull/22668).
 
 ### Rover