Add notes on throttle and radio transmitter throttle stick configurat…

…ion.

Co-authored-by: Hwurzburg <hwurzburg@yahoo.com>

common/source/docs/common-radio-control-calibration.rst

@@ -39,10 +39,13 @@ Check the Transmitter's Setup
   - A 3 or 6 position switch (to control the flight mode) should be setup to control Channel 5 (default,if using Copter) or Channel 8 (default, if using Rover or Plane).  This channel can be moved by setting the :ref:`FLTMODE_CH <FLTMODE_CH>` parameter in Plane or Copter, or :ref:`MODE_CH<MODE_CH>` in Rover.
   - On Copter, a tuning knob should control Channel 6
   - On Copter and Rover, any remaining two or three position switches can be setup to control auxiliary functions by mapping them to channels 7 to 12
-- Move the transmitter's roll, pitch, throttle and yaw sticks and ensure the green bars move in the correct direction:
+- Note that for reversing motor controls on Plane and Rover, the throttle range is range is normally divided into two parts:
+  - Reverse throttle control: From ``RCx_TRIM`` (typically 1500us) to ``RCx_MIN``, where x is usually "3" for normal RC throttle input channel. This corresponds to motor outputs from ``SERVOx_TRIM`` to ``SERVOx_MIN``, where "x" is the output controlling the motor, and assumes the ``SERVOx_REVERSE`` bit is not set.
+  - Forward throttle control: From ``RCx_TRIM`` (typically 1500us) to ``RCx_MAX``, where x is usually "3" for normal RC throttle input channel. This corresponds to motor outputs from ``SERVOx_TRIM`` to ``SERVOx_MAX`, where "x" is the output controlling the motor, and assumes the ``SERVOx_REVERSE`` bit is not set. Using a spring-loaded stick may be most convenient, as it naturally returns to the neutral (stopped) position at ``RCx_TRIM``.
+
+  .. note:: For Plane, it is possible to use an AUX RC switch to provide easie reverse thrust control using the entire throttle stick range on non-sprung throttle sticks. Also, many set up a switch controlled mix in their RC transmitter to map throttle low stick position to output ``RCx_TRIM`` after calibration and either output RC as the stick is raised for reverse or forward operation depending on the TX switch position, giving full throttle stick range to forward or reverse operation.
 
-  - For roll, throttle and yaw channels, the green bars should move in the same direction as the transmitter's physical sticks.
-  - For pitch, the green bar should move in the **opposite** direction to the transmitter's physical stick. **This is not the default for many transmitters.**
+- Move the transmitter's roll, pitch, throttle and yaw sticks and ensure the green bars move in the correct direction:
   - If one of the green bars moves in the incorrect direction reverse the channel in the transmitter itself.  If it is not possible to reverse the channel in the transmitter you may reverse the channel in ArduPilot by checking the "Reversed" checkbox (Plane and Rover only).  If the checkbox is not visible it is possible to reverse the channel by directly changing the ``RCx_REVERSED`` parameter (where "x" is the input channel from 1 to 4). Again, always get the bar movement directions correctly setup using the transmitter's channel reversing feature, if possible, rather than the ``RCx_REVERSED`` parameter for the channel.
 
 .. figure:: ../../../images/mp_radio_calibration.png
@@ -76,6 +79,8 @@ Calibration
    .. figure:: ../../../images/radi-calib-results.png
       :target: ../_images/radi-calib-results.png
 
+- If when testing your throttle stick you notice that the motor turns in a single direction, check in the full parameters list that ``RCx_TRIM`` for the throttle channel is set to the motor controller's neutral value (usually about 1500 us). This should be detected and set automatically during calibration if you followed the procedure without forgetting to **center** your throttle in the last calibration step instead of the normal **low** position.
+
 Mode1 and Mode2 Transmitters
 ----------------------------