OK my plane flew - albeit over-powered and very twitchy!
I'm using a Spektrum DX8E transmitter and a Spektrum AR620 receiver. On the receiver, I have the ESC plugged into port 1 of the receiver, the right aileron servo into port 2, elevator into port 3, rudder into port 4, and left aileron into port 5.
When I choose "Dual Ail" for Wing in the Aircraft Type menu on the transmitter, all controls work as expected including the ailerons. Under Channel Input Config, channels 1-4 are N/A, Channel 5 (labelled Gear) shows Inh, as do channels 6-8. How the receiver "knows" that port 5 is the left aileron I have no idea, but I have checked there is no mix programmed for RA->LA in the transmitter, or any other mix involving ailerons.
Conversely, when I choose "Flaperon" for Wing type, all controls work as expected *except* the ailerons; only the right aileron moves. When I assign flap movement to a switch under "Flap system", only the right aileron moves when I operate the switch. When I look at channel 5 under Channel Input Config, there is no option available for Left Aileron, only Aileron; if I choose Aileron, then the left aileron moves correctly but it doesn't react to the flap switch. I have tried to program a Right Aileron -> Left Aileron mix manually, but I run into the same problem that I can't select Left Aileron.
So somehow the built-in Flaperon option seems to be assuming a setup different from what I have, but I don't know what that setup is.
Litterbug, if you read this, can you explain how I would do a custom mix to achieve the same result as flaperons, i.e. downward movement of both ailerons triggered by a switch?
Many thanks in advance for any advice.