Normal/Elevon/VTail Mode & Reversing Servos

Every aircraft is different, and as people familiar with RC know, you’ve got to tell your RC equipment which way the servos go and how that moves the control surfaces, a process that involves “reversing” channels if needed. The same goes for an autopilot. This section will walk you through this process.

Overview

For a traditional aileron+elevator+rudder aircraft you need to follow the procedure outlined below to ensure the control surfaces move in the right direction. Note that you must get the channel reversals right both in your transmitter and in the ArduPilot parameters.

First, make sure your control surfaces are going the right way in Manual Mode. In this mode, the RC controls are sent straight through to the servos (possibly with elevon or vtail mixing). If any control surface is going in the wrong direction when you move your RC sticks, use your transmitter’s channel reverse function to reverse it.

Second, switch into FBWA mode and check that when the aircraft is rolled or pitched that the control surfaces move the right way to correct the attitude error. This means you need to check that

  • When you roll the aircraft to the right that the right aileron goes down and the left aileron goes up.
  • When you roll the aircraft to the left that the right aileron goes up and the left aileron goes down.
  • When you pitch the aircraft up that the elevator goes down
  • When you pitch the aircraft down that the elevator goes up

If any of these are incorrect then you should change the reversal for that channel. See the table below. Also note that if you have changed your RCMAP settings then the reversals will be on different channels. See RCMAP Input Channel Mapping.

If you are using MissionPlanner or a similar modern ground station than you can conveniently see the channel reversal setup and change the settings from one screen

../_images/mpreverse.png

Standard (non-elevon) reversal setup

For non-elevon setups (ie. setups with ELEVON_MIXING set to 0 and ELEVON_OUTPUT set to 0), you have 4 parameters that control the servo reversals, one for each channel you can reverse.

The 4 parameters are:

RC1_REV aileron reversal set to -1 for reversal defaults to 1 (meaning no reversal)
RC2_REV elevator reversal set to -1 for reversal defaults to 1 (meaning no reversal)
RC3_REV throttle reversal set to -1 for reversal (for some gas planes)
RC4_REV rudder reversal set to -1 for reversal defaults to 1 (meaning no reversal)

Getting the rudder direction right

A very common mistake is to have the rudder reversal set incorrectly. If you have it set incorrectly then your plane can fly very badly, and may not be able to navigate at all.

To check your rudder reversal you need to do the following:

Ensure the KFF_RDDRMIX parameter is set to a non-zero value. For this test you should set it to a high value (such as 0.8) to ensure the rudder movement is large. Remember to reset it back to a lower value afterwards.

You also need to disable ground steering if you have it enabled. Check the GROUND_STEER_ALT parameter. You can re-enable ground steering after getting the rudder direction right.

  • put the plane into FBWA mode
  • with no stick input (hands off the transmitter) roll the aircraft to the right. The rudder should turn towards the left as it tries to correct the roll.
  • Now roll the aircraft to the left. The rudder should turn to the right as it tries to correct the roll.

If the rudder moves in the wrong direction you should change the RC4_REV parameter. A value of 1 means no reversal. A value of -1 means to reverse the rudder.

New style Elevon mixing setup (ELEVON_OUTPUT option)

As of Plane 2.73 there is a new ELEVON_OUTPUT option. This option allows you to setup your transmitter for normal aileron/elevator control and for Plane to add elevon mixing on the output of channels 1 and 2. Using ELEVON_OUTPUT has a big advantage over ELEVON_MIXING that your inputs won’t saturate the roll/pitch control in FBWA mode, which means you can get better control of your plane.

Using this setup method it doesn’t matter which channel you have plugged into which aileron.

Note that you cannot use the ELEVON_OUTPUT option on an APM1 board if you have the flight mode channel (FLTMODE_CH) set to 8, as on the APM1 this will lead to hardware pass through of controls when in manual, which means your transmitter must be setup for elevon mixing. If using an APM2 or PX4 this is not a problem, and the ELEVON_OUTPUT option is recommended.

The ELEVON_OUTPUT option is designed to operate exactly as a hardware elevon mixer would operate. To set it up follow these steps:

  • Setup your transmitter with no elevon mixing
  • Set both RC1_REV and RC2_REV to 1 and ELEVON_MIXING to 0
  • Start by setting ELEVON_OUTPUT to 1. In later steps you may adjust this to 2, 3 or 4.
  • Put your APM into FBWA mode
  • Roll the plane to the right and observe what happens to the elevons
  • If the two elevons move in the same direction, then change ELEVON_OUTPUT to 2, and try again.
  • If the elevons move in opposite directions, but the APM is correcting the roll in the wrong direction (the left elevon is going down and the right one is going up) then change RC1_REV to -1
  • Next try pitching your plane up. If the elevons move in the wrong direction then change RC2_REV to -1
  • Now change to MANUAL mode and adjust your transmitter reversals for channels 1 and 2 to produce the right movement in MANUAL mode.

Please make sure that you do careful ground testing after setting these parameters!

Please also see the note about the MIXING_GAIN parameter below.

Old style elevon mixing setup (ELEVON_MIXING option)

Note: We STRONGLY suggest you use the new style mixing above rather then this old style.

For old elevon based setups where you have set ELEVON_MIXING to 1, you have 3 different parameters to setup. They are:

ELEVON_REVERSE reverse the sense of the elevon mixing set to 1 to reverse, defaults to 0
ELEVON_CH1_REVERSE reverse channel 1 elevon set to 1 to reverse, defaults to 0
ELEVON_CH2_REVERSE reverse channel 2 elevon set to 1 to reverse, defaults to 0

To select elevon mode or reverse elevon channels, use the elevon checkboxes at the bottom:

Roll to the right illustrated below.

../_images/mavelevon1.png

It takes a little trial-and-error to set up elevons on any particular aircraft, but here are the basic steps:

  1. First, set it up in manual mode by setting up elevon mixing on your RC transmitter. It matters which elevon is plugged into which channel! Shown above, the left wing aileron is plugged into Ch1 and the right wing into Ch2.
  2. Still in manual mode, check to see if you have to reverse any channels on your RC transmitter to ensure the control surfaces move the way they should in both pitch and roll.
  3. Now that it’s working in manual, connect to your APM board with the Mission Planner. Go through the regular setup process. When calibrating your RC input, don’t just move the elevator and aileron sticks to the normal up down, left right positions. Instead, you must move the stick to the CORNERS or the calibration will be wrong and the servos will try to move too far. This is because now that you’ve switched your RC transmitter into elevon mode, the elevator and airelon inputs are added when the stick is in the corner (full left and full up as an example).
  4. While still in the MP RC setup screen, switch into FBWA Mode. Move move the plane around to test and watch the control surfaces. When you tip the nose of the plane down, the two elevons should go up and vice versa. Likewise with roll; when you roll the plane, the elevons should move to counteract that and return the plane to level. You’ll probably have to reverse something with the check boxes on that screen for correct motion. Just change one thing at a time!
  5. If you just can’t seem to find the right combination that works, try swapping your servo cables, so that Right is in Output 1 and Left is Output 2. This is something of a last resort, because you’ll have to start the setup from the top of this list again.

Please make sure that you do careful ground testing after setting these parameters. Also remember that your RC transmitter must be set up to do elevon mixing, too!

Note

It is possible to configure differential spoilers with old style elevon mixing, although the feature is not widely used and not well tested. Differential spoilers cannot currently be configured with the new type elevon mixing.

Setting up a VTAIL plane

To setup a VTAIL plane, you can enable a software VTAIL mixer using the VTAIL_OUTPUT option. The VTAIL_OUTPUT option works the same way as the ELEVON_OUTPUT option, except that it operates on the elevator and rudder output channels (channels 2 and 4).

Note that you cannot use the VTAIL_OUTPUT option on an APM1 board if you have the flight mode channel (FLTMODE_CH) set to 8, as on the APM1 this will lead to hardware pass through of controls when in manual, which means your transmitter must be setup for vtail mixing. If using an APM2 or PX4 this is not a problem, and the VTAIL_OUTPUT option is recommended for vtail planes. On an APM1 use a hardware vtail mixer instead.

The VTAIL_OUTPUT option is designed to operate exactly as a hardware vtail mixer would operate. To set it up follow these steps:

  • Setup your transmitter with no vtail mixing
  • Set both RC2_REV and RC4_REV to 1 and KFF_RDDRMIX to 0.5
  • Start by setting VTAIL_OUTPUT to 1. In later steps you may adjust this to 2, 3 or 4.
  • Put your APM into FBWA mode
  • Pitch up the nose of the plane observe what happens to the vtail
  • If the two vtail segments move in opposite directions, then change VTAIL_OUTPUT to 2, and try again.
  • If the two vtail segments move in the same direction, but the APM is correcting the pitch in the wrong direction (both segments are moving up) then change RC2_REV to -1
  • Next try rolling your plane to the right. The two vtail segments should move to try to turn the plane left (to correct for the right roll). If they move in the wrong direction then set RC4_REV to -1
  • Now change to MANUAL mode and adjust your transmitter reversals for channels 2 and 4 to produce the right movement in MANUAL mode.
  • Finally adjust the KFF_RDDRMIX to a value that gives the right amount of rudder movement for coordinated turns on your plane. This may require some inflight tuning. A initial guess of around 0.5 is likely to work for most planes.

Here’s a V-Tail movement diagram courtesy of Miami Mike:

../_images/v-tail-300x200.gif

Please make sure that you do careful ground testing after setting these parameters!

Please also see the note about the MIXING_GAIN parameter below.

Using MIXING_GAIN to control mixing throws

If you use the ELEVON_OUTPUT or VTAIL_OUTPUT options, you may find the MIXING_GAIN parameter useful to control the gain of the mixer.

The default is a gain of 0.5, which ensures that over the full range of the mixer both inputs have authority (it can’t saturate). That also means that if you have one input of the mixer (eg. aileron on an elevon plane) at full range, and the other input neutral, then the output is only 1750. That may not be enough roll authority for some planes.

If you change the MIXING_GAIN to 1.0 then you will get the full range of output from a single channel, although if you have full aileron deflection and full elevator at the same time you will saturate the mixer. It will clip output outside of the valid range of 900 to 2100 microseconds.

So if you have found ELEVON_OUTPUT doesn’t have enough authority then try raising the MIXING_GAIN.

Important notes

  • Whenever you change your firmware your parameter (EEPROM) settings will revert to the defaults if the new firmware has an incompatible parameter (EEPROM) format. The release notes for a release will contain a note if this happens. The developers are careful to try to minimise the number of times this is needed. Please use the APM mission planner or your ground control station to save your settings, and carefully check them after any firmware change.
  • make sure you always do ground tests before every flight to ensure your channel mixing and reversals are all correct. Be careful to check that not only are your transmitter controls correct, but that the APM responds correctly to attitude changes in the plane when in FBWA mode.