FW Pos Control params: improve description of FW_AIRSPD_TRIM

Signed-off-by: Silvan Fuhrer <silvan@auterion.com>

src/modules/fw_pos_control_l1/FixedwingPositionControl.cpp

@@ -141,19 +141,26 @@ FixedwingPositionControl::parameters_update()
 
 	landing_status_publish();
 
+	int check_ret = PX4_OK;
+
 	// sanity check parameters
-	if ((_param_fw_airspd_max.get() < _param_fw_airspd_min.get()) ||
-	    (_param_fw_airspd_max.get() < 5.0f) ||
-	    (_param_fw_airspd_min.get() > 100.0f) ||
-	    (_param_fw_airspd_trim.get() < _param_fw_airspd_min.get()) ||
-	    (_param_fw_airspd_trim.get() > _param_fw_airspd_max.get())) {
-
-		mavlink_log_critical(&_mavlink_log_pub, "Airspeed parameters invalid");
-
-		return PX4_ERROR;
+	if (_param_fw_airspd_max.get() < _param_fw_airspd_min.get()) {
+		mavlink_log_critical(&_mavlink_log_pub, "Config invalid: Airspeed max smaller than min");
+		check_ret = PX4_ERROR;
 	}
 
-	return PX4_OK;
+	if (_param_fw_airspd_max.get() < 5.0f || _param_fw_airspd_min.get() > 100.0f) {
+		mavlink_log_critical(&_mavlink_log_pub, "Config invalid: Airspeed max < 5 m/s or min > 100 m/s");
+		check_ret = PX4_ERROR;
+	}
+
+	if (_param_fw_airspd_trim.get() < _param_fw_airspd_min.get() ||
+	    _param_fw_airspd_trim.get() > _param_fw_airspd_max.get()) {
+		mavlink_log_critical(&_mavlink_log_pub, "Config invalid: Airspeed cruise out of min or max bounds");
+		check_ret = PX4_ERROR;
+	}
+
+	return check_ret;
 }
 
 void

src/modules/fw_pos_control_l1/fw_pos_control_l1_params.c

@@ -461,11 +461,12 @@ PARAM_DEFINE_FLOAT(FW_AIRSPD_MIN, 10.0f);
 PARAM_DEFINE_FLOAT(FW_AIRSPD_MAX, 20.0f);
 
 /**
- * Cruise Airspeed (CAS)
+ * Trim/ Cruise Airspeed (CAS)
  *
- * The trim CAS (calibrated airspeed) of the vehicle. If an airspeed controller is active,
- * this is the default airspeed setpoint that the controller will try to achieve if
- * no other airspeed setpoint sources are present (e.g. through non-centered RC sticks).
+ * This is the default cruise airspeed setpoint (calibrated airspeed) used by the system in assisted
+ * and autonomous control modes if no other airspeed setpoint is given.
+ * It is also used for control surface effectiveness scaling.
+ * (scaling = FW_AIRSPD_TRIM / calibrated_airspeed).
  *
  * @unit m/s
  * @min 0.5