The easy-ish part of installing a gasser is outside of the fuselage, mainly because there is plenty of room to work and move things around. Once you get to the inside of a finished fuselage you're trying to stuff 20 pounds of stuff in a 10 pound bag!
So the engine is done (other than just plugging in a fuel line) on the outside. The plug wire is in place and the pickup wires are routed inside as well. But that's the easy-ish stuff.
Inside I need to install a gas tank and do a little wiring before I can test-fire the engine for the first time. It's used, but from a reputable source, so I'm not too worried about it, but I still want to make sure it runs properly before finalizing all the inside stuff.
This engine uses an electronic ignition instead of the old-school magneto. It's supposedly more reliable, efficient, whatever, but it also comes with more required hardware and more headaches. Personally, I like the old magneto engines for the simplicity. With any gas engine, a real concern is the ability to kill the engine from the transmitter or with signal loss. Programming a failsafe or a switch to either close the throttle completely or to choke the engine (or both) can accomplish this task, but if your receiver dies and the electronic ignition box still has power the engine will keep on running, which makes a bad situation much worse. Especially on larger planes! When it comes to electronic ignitions, an optical kill switch is the answer.
The optical switch is a pretty cool piece of technology. It is connected to an auxiliary channel on the receiver and simply looks for a signal to be on or off. If it's getting the signal from the receiver it assumes everything is good and allows the engine to keep running. If the receiver dies, loses power, or if you flip the "kill" switch on the transmitter the signal goes "off" and the optical switch then kills all power to the electronic ignition. So if you fly out of range and lose signal, turn off the transmitter, have a receiver battery die, or simply turn off the battery for the receiver the engine dies. Some fields require this for all gas engines. To help determine if power is on for the ignition the optical switch has an LED which you mount somewhere it can be seen. When the LED is on, there is power going to the electronic ignition. With the LED off the ignition is dead. Here's the basic optical switch:
This is a basic diagram showing how the optical switch is used, and how it'll be installed in the Citabria. All of the excess wiring, servos, etc is removed for clarity. My setup will use a 6 volt battery for the electronic ignition and the 5 volt for the receiver and servos. I'll also have separate physical switches on the side of the plane to kill power to the ignition and/or the receiver. That will let me test servos or check programming without sending power to the ignition, another level of safety.
As mentioned, I'm adding switches to the fuselage side to kill power to the individual batteries. There are various styles and sizes available, but I've used this type before and really like it. It's a bit bigger and more obvious than others, but I can live with that. This isn't a highly detailed and scale plane to begin with...! The switch assembly also has charge ports built in so the batteries can be charged easily without running extra cables into the fuselage or removing the wing.
Right now it looks like I'll be on track to at least fire the engine tomorrow, although I had to order a couple pieces that are needed before I can officially say it's ready for flight. Oh well, no rush!