As most of you may or may not know, I've been a big proponent of using PPM Sum (aka CPPM) with the Ardupilot since I first figured out how to make it work. The simplicity of just running a single wire from the Rx to the APM is very attractive. It eliminates 7 possible failure modes (losing the wire connecting channels 2-8) which could result in loss of control. There's only 1 wire to get right, but also one wire to get wrong. Fortunately, the APM can handle complete loss of signal well, and if you set your failsafe to handle complete signal loss, all is good.
I was using this on my HK450GT FBL helicopter, along with a FrSky D8R-SP Rx. Again, as anybody who knows me is aware, I'm a big fan of FrSky radio equipment. It is affordable, and high performance offering true diversity on the Rx, easy to use CPPM (just requires a jumper on two pins), the Rx is very tolerant of low voltage (it'll keep going LONG after the APM would have shut down, down below 3V). And rebooting and reacquiring the Tx signal takes less than 1 second.
All of these features are superior to the big name brands, Futaba, Spektrum, etc.
And the telemetry features offered by FrSky is just the icing on the cake. RSSI (signal strength), Rx voltage, external voltage (flight battery), and even the option for a telemetry hub with all kinds of goodies This is somewhat negated by using an APM, but still, you can put one of these on your Tx and have all the info right at your fingertips.
Anyway, back to my little 450 heli with a HUGE Rx mounted on it, the D8R-SP.
I discovered that FrSky offer a new Rx, the D4Fr which is a small, yet full-range Rx. And while it only has 4 channels out, it can be configured to do 8-channel CPPM with the simple addition of a jumper! Brilliant, and there's no reason why any modern compact receiver shouldn't be able to do this. This Rx does not have as many telemetry features as it's big brother. It can only report RSSI and Rx Voltage, it does not have connectors for the telemetry hub or external voltage. But that's fine, as the APM can be used to report voltage anyway. So I ordered one and it arrived Friday.
I started working on setting it up, when the wheels started turning in my brain. The "case" is just a flimsy plastic thing, almost useless anyway. And I'm only going to have a single cable going from the Rx to the APM, and it's so small. And the APM2 has those stand-off pins onto which the daughterboard is soldered. And those pins are kinda long... And I've got this piece of empty prototype board sitting around...
Pretty soon I had de-cased the Rx and stripped all the input pins off the APM2.
I soldered the proto-board to the daughter board standoffs, and then glued the Rx board onto the proto-board. I soldered jumpers on both the Rx and APM2 in order to set both into CPPM mode. And then I soldered a jumper wire for the CPPM signal, power and ground. Done.
The complete unit is compact. There's no wires to get in the way of whatever vibration-damping mount you are using. There are no wires to come disconnected. I suspect this setup is at least an order of magnitude more reliable than a standard 8-channel APM2 setup.
Even better, it's compact, and easy to move between vehicles.