Four Channel Planes

The most common sort of fixed wing plane is a 4 channel plane. As the name implies, it has 4 output servos, most commonly in this order:

  • servo output 1 is aileron
  • servo output 2 is elevator
  • servo output 3 is throttle
  • servo output 4 is rudder

These are the default outputs for APM:Plane as it is such a common setup. It is commonly referred to as an AETR setup.


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 AETR output ordering.


Servo Reversal

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, elevator and rudder). You should now adjust the reversal of the 3 outputs so that in MANUAL mode the surfaces move in the right direction.

Right RollLeft aileron goes down and right aileron goes up
Left RollRight aileron goes down and left aileron goes up
Pull back on pitchElevator goes up
Push forward on pitchElevator goes down
Right YawRudder goes right
Left YawRudder goes 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 rightRight aileron goes down and left aileron goes up
Roll plane leftLeft aileron goes down and right aileron goes up
Pitch nose upElevator goes down
Pitch nose downElevator goes up

Finally you should double check 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 rightRudder goes left
Roll plane leftRudder goes right

Servo Trim

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.

Servo Throw

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 leading 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.

Final Setup

After completing the above you should move onto the final setup of your aircraft.