fw pos ctrl: set idle throttle in landed conditions

also set the default idle throttle to zero as most PX4 applications use electric motors

src/modules/fw_pos_control_l1/FixedwingPositionControl.cpp

@@ -1722,7 +1722,7 @@ FixedwingPositionControl::control_auto_landing(const hrt_abstime &now, const flo
 
 		/* set the attitude and throttle commands */
 
-		// TECS has authority (though constrained) over pitch during flare, throttle is killed
+		// TECS has authority (though constrained) over pitch during flare, throttle is hard set to idle
 		_att_sp.pitch_body = get_tecs_pitch();
 
 		// flaring is just before touchdown, align the yaw as much as possible with the landing vector
@@ -1731,7 +1731,9 @@ FixedwingPositionControl::control_auto_landing(const hrt_abstime &now, const flo
 		// enable direct yaw control using rudder/wheel
 		_att_sp.fw_control_yaw = true;
 
-		_att_sp.thrust_body[0] = 0.0f;
+		// idle throttle may be >0 for internal combustion engines
+		// normally set to zero for electric motors
+		_att_sp.thrust_body[0] = _param_fw_thr_idle.get();
 
 	} else {
 
@@ -1791,7 +1793,7 @@ FixedwingPositionControl::control_auto_landing(const hrt_abstime &now, const flo
 		// enable direct yaw control using rudder/wheel
 		_att_sp.fw_control_yaw = false;
 
-		_att_sp.thrust_body[0] = (_landed) ? 0.0f : get_tecs_thrust();
+		_att_sp.thrust_body[0] = (_landed) ? _param_fw_thr_idle.get() : get_tecs_thrust();
 	}
 
 	_tecs.set_equivalent_airspeed_min(_param_fw_airspd_min.get()); // reset after TECS calculation

src/modules/fw_pos_control_l1/fw_pos_control_l1_params.c

@@ -338,8 +338,11 @@ PARAM_DEFINE_FLOAT(FW_THR_MIN, 0.0f);
  *
  * This is the minimum throttle while on the ground
  *
- * For aircraft with internal combustion engine this parameter should be set
- * above desired idle rpm.
+ * For aircraft with internal combustion engines, this parameter should be set
+ * above the desired idle rpm. For electric motors, idle should typically be set
+ * to zero.
+ *
+ * Note that in automatic modes, "landed" conditions will engage idle throttle.
  *
  * @unit norm
  * @min 0.0
@@ -348,7 +351,7 @@ PARAM_DEFINE_FLOAT(FW_THR_MIN, 0.0f);
  * @increment 0.01
  * @group FW L1 Control
  */
-PARAM_DEFINE_FLOAT(FW_THR_IDLE, 0.15f);
+PARAM_DEFINE_FLOAT(FW_THR_IDLE, 0.0f);
 
 /**
  * Climbout Altitude difference