A common alternative to a traditional elevator and rudder is a V-Tail, or an ATail (an upside down V-Tail).
A V-Tail aircraft has the same functionality as a standard aircraft, but it requires special configuration of the servo outputs. Do not use V-Tail mixing on the transmitter. Although you may choose custom channels, the typical V-Tail setup uses channel 2 and 4 for the servo outputs.
Remove the propeller from the aircraft before starting the setup process.
Configuration & Setup¶
The most important step to setting up the plane is having the correct inputs, outputs, and reversals. Inputs are covered on the RC input setup page. After the RC inputs are configured, configure the outputs.
Make sure the AHRS_ORIENTATION is set correctly for the autopilot. If it is incorrect, this setup will fail, and the plane may crash upon entry into any stabilize mode.
Servo cables can be connected to any output of the autopilot, but using the default channels 1-4 listed below is recommended. Set the SERVOn_FUNCTIONS to the appropriate values.
Servo Function & Reversal¶
The next step is to correct the V-Tail functions and reversals. Both the function and reversal go hand-in-hand, so modifying one may partially change the behavior of the other. Connect the battery (with propeller removed) and turn on the RC transmitter. Switch to FBWA mode using the function switch or a ground station command, and disable the safety switch (if fitted).
When the plane is level, the servos should be near their trim values. Move the plane and leave the transmitter sticks centered while monitoring the control surfaces to determine if the function and reversals are correct. See the table for the correct control surface response to the movements. In each instance, the plane should move its control surfaces to level itself.
|Roll Plane Right||Left aileron moves up and right aileron moves down|
|Roll Plane Left||Left aileron moves down and right aileron moves up|
|Pitch plane up||Both tail surfaces move down|
|Pitch plane down||Both tail surfaces move up|
|Roll Plane Right||Both tail surfaces move left|
|Roll Plane Left||Both tail surfaces move right|
If the ailerons do not respond correctly, reverse the output by changing the corresponding SERVOn_REVERSED setting (from 0 to 1, or from 1 to 0).
If the V-tails do not respond correctly, change parameters using the following table to enable the correct behavior. Work on one channel at a time to avoid confusion.
|Control Surface Response||Corrective Action|
|Correct for 1 movement (pitch or roll), but not the other||Change the function (from 79 to 80; or 80 to 79)|
|Incorrect for both movements (pitch and roll)||Change the reversal of that channel|
Confirm RC Transmitter Input¶
Keep the plane level in FBWA mode and command the following inputs:
|Roll Right||Right aileron moves up and left aileron moves down|
|Roll Left||Left aileron moves up and right aileron moves down|
|Pitch up||Both tail surfaces moveup|
|Pitch down||Both tail surfaces move down|
|Yaw right||Both tail surfaces move right|
|Yaw left||Both tail surfaces move left|
Double check MANUAL mode for the inputs as well. If everything is setup correctly, the plane should be almost ready to fly.
With “A-Tail” planes (an inverted V-Tail), the control surface movements referenced above should still be the same directions. It is likely that the servo reversal or function will be opposite from a similar V-Tail setup.
Switch back to MANUAL mode in order to adjust the servo trim values. The servo trim is in the SERVOn_TRIM parameters.
Adjust the trim values so that the servo is centered when the transmitter sticks are centered. If the trim value is not between 1450 and 1550 PWM, mechanical trim adjustment is recommended.
Finally adjust the servo throws (range of movement for each of the servos).
Check any instructions that came with the 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 suggested throw values are not found, then choose a throw that doesn’t cause the servos to “bind” (often indicated by a high pitched sound when servos stall).
To adjust the throw, change the SERVOn_MIN and SERVOn_MAX values. The defaults are 1100 to 1900. On many aircraft, more throw may be desired. Changing throws to 1000 to 2000 or beyond is normal. Make sure that the servos are still moving when nearing the extrememe values.
To get to maximum throw on V-Tail control surfaces, command pitch and yaw at the same time in MANUAL mode.
The MIXING_GAIN parameter is critical for vtail aircraft. It is the gain used in mixing between yaw and pitch output and the vtail movement. For example, if MIXING_GAIN is 0.5, then the following outputs are used:
- LEFT_VTAIL = (yaw+pitch)*0.5
- RIGHT_VTAIL = (yaw-pitch)*0.5
Adjusting the MIXING_GAIN controls the percentabe of throws from pitch vs yaw.