Stabilizing a Canard Delta with AS3X/SAFE
A number of modern fighter jets are deltas with elevons for pitch and roll and with
canards for airflow control and to provide exceptional maneuverability. Examples
include the Eurofighter, Rafale, Gripen, J-10 and J-20. If you want to use AS3X/SAFE
with one of these and have the stabilization applied to the elevons and canards (and
maybe thrust vector nozzles too) you basically have two options: use a separate mixer
for the elevons (the FMS Rafale comes like this) or use Forward Programming (FP) to
set up your receiver to do this. The way to set up with FP is not immediately obvious
and I have not found it described elsewhere (apologies if this is a repeat). I have tested
this on the AR630, AR631 and AR8360T, and it works with the AR637T as well.
First, you need access to a transmitter that provides an airplane wing type “Elevon” or
“Elevon B” and tail type “Elevon with 1 canard servo”. To the best of my knowledge
only the DX18G2, DX20 and iX20 have this at this time. However, once the receiver is
set up you can re-bind to a different transmitter such as a DX9, DX8G2, NX8, etc. and
the receiver will remember the stabilization settings. So as long as you can borrow the
iX20, DX20 etc. for the initial set-up you can still use your own transmitter for
flying provided you use the same channels for the same control surfaces. You also
need a receiver with an adequate number of channels for the model and that can be
Forward Programmed.
The steps I took are as follows, and describe how I did it with an iX20. The steps will
basically be the same with the DX18G2 or DX20.
1. Select Wing Type “Elevon” or “Elevon B” as appropriate for your model to
ensure the elevons respond correctly to the control sticks.. Channel Assign will
tell you which channels the elevons are assigned to.
2. Select Tail Type “Elevons and 1 canard servo”. Do this even if you have 2
canard servos. By default the iX20 assigns the canards to channel 9 (AUX4)
and calls it “Elevator”. Again, check this by looking at the channel and port
assignments. Don’t worry if your receiver only has 6 or 8 channels, just leave
this assignment alone (see item 9 below).
3. On the port assignment page of Channel Assignment, set the actual channel(s)
that your canards (or VT nozzle, or both) are plugged into, to “Elevator”. On an
AR630/631/637T this typically will be channel 6. On my AR8360T I used both
channels 6 and 7 for the two canards and channel 8 for the thrust vectoring nozzle.
You can go into Servo Setup to make sure that everything is working in the
correct direction, set travel limits, etc.
4. Enter Forward Programming. If your receiver has previously been forward
programmed you should do a Factory Reset. The receiver will now recognize
every channel you assigned to be a flight control as a channel to be stabilized:
elevons, rudder, canard channel(s) (which the Tx calls “elevator”), VT channel
(also called “elevator”).
5. Go through all the regular steps to set up SAFE and AS3X, including choosing a
gain channel and receiver flight mode channel. Do not choose any channels you
assigned to “Elevator” for these functions. Set your gains, etc. as usual.
6. Test to make sure that everything works and all stabilization corrections are
made in the correct direction. Remember that AS3X doesn’t become active until
the throttle has been advanced.
7. If you borrowed a DX18G2, DX20 or iX20 to do the set-up, you can rebind to
your own transmitter without destroying the stabilization settings. Just make
sure that all the channel assignments for the elevons, canards, VT (if any),
gains, flight mode, are the same as on the Tx you used for the initial setup. If
you can import the SPM file from the setup Tx then everything should already be
on the correct channel. Otherwise you will have to use a mix to get everything in
the right place.
8. If for any reason you do a Factory Reset or Relearn Servos under Forward
Programming, you will likely lose the settings for the channels assigned to
“Elevator” unless you are using the transmitter you originally used to do the
setup.
9. If you plan to mix any other functions into the canards (such as roll, or
compensation for crow, etc) then do not inhibit the default assignment of
“elevator” to Ch9 (AUX4) until you have made and tested the mix, or it won’t
work – see item 2 above.
JSK 04/2021