A common alternative to an elevator and rudder is a VTail, or sometimes an ATail (which is an upside down VTail).
A VTail aircraft has the same functionality as a normal four channel plane, but needs special care for the setting up of the VTail servos.
A typical VTail setup is
- servo output 1 is aileron
- servo output 2 is left vtail servo
- servo output 3 is throttle
- servo output 4 is right vtail servo
You don’t have to use this ordering if it isn’t convenient for your plane.
You should remove the propeller from your aircraft before starting your setup.
Setting Up Your Plane¶
After you have setup your RC inputs, the next step is to setup your 4 outputs.
You can connect your 4 servo cables to any output of your autopilot, although using the defaults listed above is recommended.
Next check that the SERVOn_FUNCTION values are correct. The following table shows the right settings for the default output ordering for a VTail plane.
The next step is to get the reversals right. You should connect the battery (with propeller removed) and turn on your RC transmitter. Now switch to MANUAL mode and disable the safety switch (if fitted).
At this point your RC transmitter should have control of your 3 control surfaces (aileron, left-vtail and right-vtail). You should now adjust the reversal of the 3 outputs so that in MANUAL mode the surfaces move in the right direction.
|Right Roll||Left aileron goes down and right aileron goes up|
|Left Roll||Right aileron goes down and left aileron goes up|
|Pull back on pitch||Both vtail surfaces go up|
|Push forward on pitch||Both vtail surfaces go down|
|Right Yaw||Both vtail surfaces go right|
|Left Yaw||Both vtail surfaces go left|
If any of the directions are incorrect then you need to change the corresponding SERVOn_REVERSED setting. So for example if your ailerons move the wrong way then you should change SERVO1_REVERSED to 1.
Confirm Servo Reversal¶
The above servo reversal test in MANUAL mode assumes your RC inputs have been correctly setup. As it is so easy to get that wrong, you should also do a stabilisation check.
Switch the plane to FBWA mode and with the transmitter sticks centered move the plane as follows:
|Roll plane right||Right aileron goes down and left aileron goes up|
|Roll plane left||Left aileron goes down and right aileron goes up|
|Pitch nose up||Both vtail surfaces go down|
|Pitch nose down||Both vtail surfaces go up|
Finally you should double check vtail rudder direction. This one is particularly easy to get wrong. First check that ground steering is disabled (by checking that GROUND_STEER_ALT is zero) and that you have a non-zero rudder gain in KFF_RDDRMIX.
Now check the following:
|Roll plane right||Both vtail surfaces go left|
|Roll plane left||Both vtail surfaces go right|
Now switch back to MANUAL mode in order to adjust the servo trim values. The servo trim is in the SERVOn_TRIM parameters.
You should adjust the trim values so that the servo is centered when your transmitter sticks are centered. If you find you need to adjust the trim value by more than 50 PWM from the default of 1500 then it is recommended that you instead adjust the trim mechanically.
Finally you should adjust your servo throw. The throw is the range of movement for each of your servos.
Check any instructions that came with your plane for suggested throw values. These are often specified in millimeters or inches of movement of the trailing edge of the control surface close to the fuselage. If your aircraft doesn’t come with any suggested throw values then choose a throw that doesn’t cause your servos to “bind” (which is indicated by a high pitched sound when your servos move too far).
To adjust the throw, change the SERVOn_MIN and SERVOn_MAX values. The defaults are 1100 to 1900. On many aircraft you will want more throw than that, and can change to a throw of 1000 to 2000.
The MIXING_GAIN parameter is critical for vtail aircraft. It is the gain used in mixing between yaw and pitch output and your vtail movement.
For example, if your MIXING_GAIN is 0.5, then the following outputs are used:
- LEFT_VTAIL = (yaw+pitch)*0.5
- RIGHT_VTAIL = (yaw-pitch)*0.5
By adjusting the MIXING_GAIN you can quickly setup the right throws of your vtail aircraft.